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Geotechnical Engineering Report
Fort Collins, Colorado
December 6, 2024 1 Terracon Project No. 20245055
Prepared for:
Mountain 300, LLC
262 E. Mountain Avenue
Fort Collins, Colorado 80524
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December 6, 2024
Mountain 300, LLC
262 E. Mountain Avenue
Fort Collins, Colorado 80524
Attn: Mr. Colin Vito
P: (970) 490-2626
E: colin@bohemiancompanies.com
Re: Geotechnical Engineering Report
New Office Building
300 E. Mountain Avenue
Fort Collins, Colorado
Terracon Project No. 20245055
Dear Mr. Vito:
Terracon Consultants, Inc. (Terracon) has performed the scope of Geotechnical
Engineering services for the project referenced above in general accordance with
Terracon Proposal No. P20245055 (Revised) dated October 17, 2024. This report
presents the findings of the subsurface exploration and provides geotechnical
recommendations concerning earthwork and the design and construction of foundations
and floor systems for the proposed project.
We appreciate the opportunity to be of service to you on this project. Materials testing
and construction observation services are provided by Terracon as well. We would be
pleased to discuss these services with you. If you have any questions concerning this
report or if we may be of further service, please contact us.
Sincerely,
Terracon �oSPpO L1,-
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Alec N. Strassburg, P.E. Eric D. Bernhardt, P.E.
Geotechnical Department Manager Regional Geotechnical Manager
Geotechnical
Geotechnical Engineering Report
New Office Building i Fort Collins, Colorado ierracon
December 6, 2024 i Terracon Project No. 20245055
Table of Contents
Introduction.................................................................................................... 1
ProjectDescription.......................................................................................... 1
SiteConditions................................................................................................ 3
Geotechnical Characterization ......................................................................... 3
Subsurface Profile..................................................................................... 3
Groundwater Conditions............................................................................. 4
SeismicSite Class............................................................................................ 5
Corrosivity ...................................................................................................... 5
GeotechnicalOverview .................................................................................... 6
Existing, Undocumented Fill........................................................................ 6
Groundwater............................................................................................ 6
Expansive/Collapsible Soils and Bedrock....................................................... 7
DifficultExcavation ................................................................................... 7
Foundation and Floor System Recommendations............................................ 8
Earthwork....................................................................................................... 8
Demolition ............................................................................................... 8
SitePreparation........................................................................................ 9
ExistingFill .............................................................................................. 9
Excavation.............................................................................................. 10
Subgrade Preparation ............................................................................... 11
Subgrade Stabilization .............................................................................. 11
FillMaterial Types.................................................................................... 13
Fill Placement and Compaction Requirements ............................................... 13
Utility Trench Backfill................................................................................ 14
Gradingand Drainage............................................................................... 15
Exterior Slab Design and Construction......................................................... 16
Earthwork Construction Considerations ....................................................... 16
Construction Observation and Testing .........................................................17
ShallowFoundations ..................................................................................... 17
Reinforced Mats - Design Recommendations ................................................ 17
Shallow Foundation Construction Considerations...........................................20
Construction Adjacent to Existing Building ...................................................20
FloorSystems ............................................................................................... 21
Below-Grade Structures................................................................................. 21
Elevator Pit - Design Recommendations ......................................................21
LateralEarth Pressures ................................................................................. 22
Design Parameters...................................................................................22
GeneralComments ........................................................................................ 24
Facilities I Environmental I Geotechnical I Materials i
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Figures
GeoModel
Attachments
Exploration and Testing Procedures
Site Location and Exploration Plans
Exploration and Laboratory Test Results
Supporting Information
Note: This report was originally delivered in a web-based format. Blue Bold text in the
report indicates a referenced section heading. The PDF version also includes hyperlinks
which direct the reader to that section and clicking on the Wferracon logo will bring you
back to this page. For more interactive features, please view your project online at
client.terracon.com.
Refer to each individual Attachment for a listing of contents.
Facilities I Environmental I Geotechnical II Materials ii
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Introduction
This report presents the results of our subsurface exploration and Geotechnical
Engineering services performed for the proposed office building to be constructed at 300
E. Mountain Avenue in Fort Collins, Colorado. The purpose of these services was to
provide information and geotechnical engineering recommendations relative to:
■ Subsurface soil and rock conditions
■ Groundwater conditions
■ Seismic site classification per IBC
■ Site preparation and earthwork
■ Demolition considerations
■ Dewatering considerations
■ Foundation design and construction
■ Floor system design and construction
■ Lateral earth pressures
The geotechnical engineering Scope of Services for this project included the
advancement of test borings, laboratory testing, engineering analysis, and preparation
of this report.
Drawings showing the site and boring locations are shown in the Site Location and
Exploration Plans section of this report. The results of the laboratory testing
performed on soil and bedrock samples obtained from the site during our field
exploration are included on the boring logs and as separate graphs in the Exploration
Results section.
Project Description
Our final understanding of the project conditions is as described in the following table.
Item Description
The project information described below is based on the
following:
Information ■ Email and phone communication with Jensen LaPlante
Provided Development, VFLA Architecture + Interiors (VFLA), KL&A
Engineers and Builders, and National Inspection Services
■ Draft Site Plan 7-8-2024 provided by VFLA
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Geotechnical Engineering Report
New Office Building Fort Collins, Colorado Terracon
December 6, 2024 i Terracon Project No. 20245055
Item Description
We understand the project includes demolition of an existing
structure and construction of a new office building at the project
site. The footprint area of the building is estimated to be about
3,500 square feet (approximate width of the building is about
771/2 feet and approximate length of the building is about 881/2
feet). The building is anticipated to be a two-story, steel frame
Project structure. We understand a basement level extending to a depth
Description of about 10 to 12 feet below existing grades is also planned. We
anticipate the structure will be supported on a large, reinforced
concrete, mat foundation system. An elevator is planned within
the building.
No new parking areas or driveways are planned on the site.
There is existing on-street parking which will service the
building.
The elevation of the first floor was not provided at the time of
Finished Floor this report. However, based on our communication with the
Elevation project team, we understand the first floor elevation is not more
than 1 to 11/2 feet from existing sidewalk elevations.
In the absence of information provided by the design team, we
will use the following loads in our engineering analysis based on
our experience with similar projects.
Maximum Loads 0 Columns: 80 kips or less
■ Walls: 1 to 4 kips per linear foot (klf)
■ Slabs: 150 pounds per square foot (psf)
Grading plans were not provided to Terracon at the time of this
Grading/Slopes proposal. However, we anticipate cuts and fills on the order of
10 to 12 feet will be required to achieve proposed grades due to
the planned basement.
Below-Grade We understand a basement and elevator pit are planned below
Structures the proposed building extending approximately 10 to 12 feet
below the ground surface.
Free-Standing Retaining walls are not expected to be constructed as part of
Retaining Walls site development to achieve final grades.
Pavements No new pavements are planned as part of this project.
Building Code 2021 International Building Code (IBC)
Terracon should be notified if any of the above information is inconsistent with the
planned construction, especially the grading limits, as modifications to our
recommendations may be necessary.
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Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado Terracon
December 6, 2024 i Terracon Project No. 20245055
Site Conditions
The following description of site conditions is derived from our site visit in association
with the field exploration.
Item Description
The project site is located at 300 E. Mountain Avenue in Fort
Parcel Collins, Colorado.
Information Latitude/Longitude (approximate): 40.587281 N, 105.073481 W
(see Site Location).
An existing single-story building located on the site was already
demolished.
Existing
Improvements Another 2-story structure is located adjacent to the project site
boundary. Terracon does not know whether the adjacent
building has a basement or not.
Surrounding developments consist mainly of commercial and
retail developments with some light industrial. Existing paved
Surrounding roadways and sidewalks are adjacent to the northwest and
Developments south sides of the site.
The Cache la Poudre River is located about 1,100 feet east of
the project site.
The site was partially covered by the existing building. However,
Current Ground the ground surface is currently bare earth now that the previous
Cover building was demolished. The existing roadways bordering the
site are paved with asphalt and the sidewalks are concrete.
Existing
Based on our site visit, the site is relatively level.
Topography
Geotechnical Characterization
Subsurface Profile
We have developed a general characterization of the subsurface conditions based upon
our review of the subsurface exploration, laboratory data, geologic setting, and our
understanding of the project. This characterization, termed GeoModel, forms the basis of
our geotechnical calculations and evaluation of the site. Conditions observed at each
exploration point are indicated on the individual logs. The individual logs can be found in
the Exploration Results and the GeoModel can be found in the Figures attachment of
this report.
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Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
As part of our analyses, we identified the following model layers within the subsurface
profile. For a more detailed view of the model layer depths at each boring location, refer
to the GeoModel.
Model Layer Layer Name General Description
1 Surface Layer About 2 to 6 inches of recycled asphalt.
Existing Fill —
Consisted primarily of lean clay soils with variable
2 Clayamounts of sand and gravel; brown, light brown,
dark brown, gray.
Existing Fill — Consisted primarily of sand and gravel soils with
3 Sand and variable amounts of silt and clay; brown, tan,
Gravel reddish brown.
4 Bedrock Very hard sandstone bedrock with clay seams; olive,
olive gray, gray, light brown, brown.
Stratification boundaries on the boring logs represent approximate locations of changes
in material types; in situ, the transition between materials may be gradual. Further
details of the borings can be found on the boring logs in Exploration Results.
Based on the results of the laboratory testing and our experience with similar materials,
we anticipate the site soils and sandstone bedrock to have low expansive potential.
Groundwater Conditions
The boreholes were observed while drilling and shortly after completion of drilling for the
presence and level of groundwater. Groundwater was not encountered in the boreholes
at these times. These observations represent relatively short-term groundwater
conditions at the time of and after completion of the field exploration and may not be
indicative of other times or at other locations. Long-term groundwater monitoring was
outside Terracon's scope of services for this project.
Based on our discussions with the project team, we understand 3 monitoring wells
(designated as Borehole Numbers P1 through P3) were previously installed in the
southeast corner of the project site by National Inspection Services on September 24
and 25, 2024. The wells were installed to depths of approximately 14 to 141/2 feet below
existing grades. Based on the water level readings taken in these wells on October 2,
2024, groundwater was encountered at depths of about 12.3 to 12.4 feet below existing
grades.
Groundwater level fluctuations occur due to seasonal variations in the water levels
present in nearby water features (such as the Cache la Poudre River), amount of rainfall,
runoff, subsurface dewatering systems of nearby structures (if any), and other factors
not evident at the time the borings were performed. Therefore, groundwater levels
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New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
during construction or at other times in the life of the structure may be higher or lower
than the levels indicated on the boring logs. The possibility of groundwater level
fluctuations should be considered when developing the design and construction plans for
the project.
Seismic Site Class
The seismic design requirements for buildings and other structures are based on Seismic
Design Category. Site Classification is required to determine the Seismic Design
Category for a structure. The Site Classification is based on the upper 100 feet of the
site profile defined by a weighted average value of either shear wave velocity, standard
penetration resistance, or undrained shear strength in accordance with Section 20.4 of
ASCE 7 and the International Building Code (IBC). Based on the soil and bedrock
properties observed at the site as described on the exploration logs and laboratory test
results, our professional opinion is a Seismic Site Classification of C be considered for
the project. Subsurface explorations at this site were extended to a maximum depth of
approximately 24 feet. The site properties below the boring depth to 100 feet were
estimated based on our experience and knowledge of geologic conditions of the general
area. Additional deeper borings or geophysical testing may be performed to confirm the
conditions below the current boring depth.
Corrosivity
The table below summarizes the results of laboratory soluble sulfate, soluble chloride,
sulfides, electrical resistivity, Red-Ox, and pH testing performed on a select sample of
site soils. The values may be used to estimate potential corrosive characteristics of the
on-site soils with respect to contact with the various underground materials which will be
used for project construction.
Corrosivity Test Results Summary
Boring ID Soluble Soluble Total Electrical Red-
& Sample Sulfate Chloride Sulfides Salts Resistivity Ox pH
Depth (%) (%) (%) (S2-cm) 7 (mV)
B-2 at
1 to 5 feet 0.0008 0.0073 Nil 0.099 1,900 +249 7.83
1. Laboratory electrical resistivity testing was performed on a saturated sample.
Results of water-soluble sulfate testing indicate Exposure Class SO according to ACI
(American Concrete Institute) 318 - Building Code Requirements for Structural
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Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Concrete. Therefore, ASTM Type I, II, or IL portland cement or approved alternative can
be specified for all project concrete on and below grade. Foundation concrete can be
designed for low sulfate exposure in accordance with the provisions of the ACI 318.
Numerous sources are available to characterize corrosion potential to buried metals
using the parameters above. ANSI/AWWA is commonly used for ductile iron, while
threshold values for evaluating the effect on steel can be specific to the buried feature
(e.g., piling, culverts, welded wire reinforcement, etc.) or agency for which the work is
performed. Imported fill materials may have significantly different properties than the
site materials noted above and should be evaluated if expected to be in contact with
metals used for construction. Consultation with a NACE certified corrosion professional is
recommended for buried metals on the site.
Geotechnical Overview
Based on subsurface conditions encountered in the borings, the site appears suitable for
the proposed construction from a geotechnical point of view provided certain precautions
and design and construction recommendations described in this report are followed and
the owner understands the inherent risks associated with construction on sites underlain
by low potential expansive soils and bedrock. We have identified several geotechnical
conditions that could impact design, construction and performance of the proposed
structure and other site improvements. These included existing, undocumented fill,
groundwater, expansive/collapsible soils and bedrock, and potentially difficult
excavation. These conditions will require particular attention in project planning, design
and during construction and are discussed in greater detail in the following sections.
Existing, Undocumented Fill
Existing, undocumented fill was encountered to depths of up to about 10 feet in the
borings drilled at the site. Existing fill could exist at other locations on the site and
extend to greater depths. We do not possess any information regarding whether the fill
was placed under the observation of a geotechnical engineer. Therefore, the fill is
considered undocumented. Undocumented fill can present a greater than normal risk of
post-construction movement of site improvements supported on or above these
materials. A low risk alternative is complete removal of existing fill below foundations,
slabs, and other site improvements and replacement with newly compacted engineered
fill. Discussion regarding alternatives to complete removal of existing fill are presented
in the Existing Fill section of Earthwork.
Groundwater
As previously discussed, groundwater was not encountered in Boring Nos. B-1 and B-2
at the time of drilling or shortly after completion of drilling. However, three monitoring
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New Office Building I Fort Collins, Colorado Terracon
December 6, 2024 1 Terracon Project No. 20245055
wells previously installed at the project site by National Inspection Services in
September 2024 indicate groundwater levels at depths of about 12 to 121/2 feet below
existing grades in October 2024. We understand a basement level and elevator pit are
planned for the new building and are anticipated to extend to depths of about 10 to 12
feet below existing grades. Terracon recommends maintaining a separation of at least 3
feet between the bottom of proposed below-grade foundations and measured
groundwater levels during construction and post construction, if possible. Where below-
grade structures extend below groundwater levels (if any) and permanent dewatering is
not planned, these below-grade structures will need to be designed as water-tight and
will need to resist buoyant forces. Final site grading should be planned and designed to
avoid cuts where shallow groundwater is known to exist, and in areas where such
grading would create shallow groundwater conditions. If deeper cuts are unavoidable,
temporary construction dewatering, installation of a subsurface drainage system, and/or
development of a sufficient dewatering plan by a dewatering contractor may be needed.
Expansive/Collapsible Soils and Bedrock
Expansive/collapsible soils and bedrock are present on this site; however, our
experience in the area and laboratory swell test results indicate site soils and bedrock
are generally slightly compressible or low swelling. This report provides
recommendations to help mitigate the effects of soil shrinkage and expansion. However,
even if these procedures are followed, some movement and cracking in the structure,
and adjacent flatwork is possible. The severity of cracking and other damage such as
uneven floor slabs and flatwork will probably increase if modification of the site results in
excessive wetting or drying of the expansive/collapsible clayey soils and the sandstone
bedrock. Eliminating the risk of movement and cosmetic distress is generally not
feasible. It is imperative the recommendations described in the Grading and Drainage
section of Earthwork be followed to reduce potential movement.
Difficult Excavation
Gravels and possibly cobbles are locally present within the site soils. These conditions
can complicate and increase difficulty of excavation and additional effort may be
necessary to extract larger-sized materials, particularly in deeper narrow excavations,
such as utility trenches. Relatively hard sandstone bedrock was also encountered at the
site at a depth of about 10 feet below existing grades. We anticipate the planned
basement level for the new building to be installed to a depth at or near the bedrock
elevation. Excavation penetrating the bedrock may require the use of specialized heavy-
duty equipment, together with ripping or jack-hammering to advance the excavation and
facilitate rock break-up and removal. Consideration should be given to obtaining a unit
price for difficult excavation in the contract documents for the project.
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Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Foundation and Floor System Recommendations
Based on our understanding of the proposed construction at the site and the
encountered conditions in the borings, the new building can be supported on a
reinforced concrete, mat foundation as planned. We anticipate the foundation will bear
at/near the encountered bedrock elevation and recommend the foundation bear directly
on the very hard sandstone bedrock. To provide a relatively level working platform on
top of the bedrock, we recommend a relatively thin layer (about 4 to 6 inches thick) of
compacted Colorado Department of Transportation (CDOT) Class 5 or 6 aggregate or
recycled concrete base course material, or a lean concrete mud mat be installed between
the bottom of the mat foundation and the top of bedrock. Design recommendations for
foundations for the proposed structure and related structural elements are presented in
the Shallow Foundations section of this report.
Since a basement level is planned under the entire footprint area of the building, we
anticipate the first level floor will be designed as a structural floor system which will be
connected to the building structure and supported by the building foundation. Therefore,
specific design recommendations for a floor system for the proposed structure are not
presented in this report. If a portion of the building will include a floor system which will
have subgrade soil below it (such as a slab-on-grade system), then we can provide
additional recommendations for floor systems upon request.
The recommendations contained in this report are based upon the results of field and
laboratory testing (presented in the Exploration Results), engineering analyses, and
our current understanding of the proposed project. The General Comments section
provides an understanding of the report limitations.
Earthwork
Earthwork is anticipated to include demolition, site preparation, excavations, subgrade
preparation, soil stabilization (if needed), and engineered fill placement. The following
sections provide recommendations for use in the preparation of specifications for the
project. Recommendations include critical quality criteria, as necessary, to render the
site in the state considered in our geotechnical engineering evaluation for foundations
and floor slabs.
Demolition
Demolition of any existing development at the project site should include complete
removal of all above-grade structures/improvements, foundation systems, below-grade
structural elements, pavements, and exterior flatwork within the proposed construction
area. This should include removal of any utilities to be abandoned along with any loose
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Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
utility trench backfill or loose backfill found adjacent to existing foundations. All
materials derived from the demolition of existing structures and pavements should be
removed from the site.
Consideration could be given to re-using the asphalt and concrete produced from the
demolition of any existing improvements, pavements, or flatwork provided the materials
are processed and uniformly blended with the on-site soils. Asphalt and/or concrete
materials should be processed to a maximum size of 2 inches and blended at a ratio of
30 percent asphalt/concrete to 70 percent of on-site soils.
Site Preparation
Prior to placing new fill, existing vegetation, topsoil, and root mats (if any) should be
removed. Complete stripping of the topsoil (if necessary) should be performed in the
proposed building area. As previously stated, we also recommend complete removal of
existing, undocumented fill within proposed building area. Existing fill was encountered
in our borings extending to a depth of about 10 feet below existing site grades. Stripped
organic materials should be wasted from the site or used to re-vegetate landscaped
areas after completion of grading operations. Prior to the placement of fills, the site
should be graded to create a relatively level surface to receive fill, and to provide for a
relatively uniform thickness of fill beneath the proposed structure.
Although no evidence of underground facilities (such as septic tanks, cesspools,
basements, and utilities) was observed during the exploration and site reconnaissance,
such features could be encountered during construction. If unexpected fills or
underground facilities are encountered, such features should be removed, and the
excavation thoroughly cleaned prior to backfill placement and/or construction.
Existing Fill
As noted in Geotechnical Characterization, existing fill was encountered in both
borings to a depth of about 10 feet below the ground surface at the time of our field
subsurface exploration. Existing fill could exist at other locations on the site and extend
to greater depths. We do not possess any information regarding whether the fill was
placed under the observation of a geotechnical engineer. Therefore, the fill is considered
undocumented. Undocumented fill can present a greater than normal risk of post-
construction movement of site improvements supported on or above these materials. A
low risk alternative is complete removal of existing fill below foundations, slabs, and
other site improvements and replacement with newly compacted engineered fill. Since
the proposed building is planned to have a basement level extending to depths of about
10 to 12 feet below surface grades, we anticipate all existing fill will be removed when
the basement excavation is performed. No existing fill should be left below the new
reinforced concrete, mat foundation planned to support the new structure and the
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New Office Building I Fort Collins, Colorado Terracon
December 6, 2024 1 Terracon Project No. 20245055
foundation should be constructed directly over a relatively thin layer of imported
granular fill overlying the sandstone bedrock as discussed in Foundations and Floor
Systems Recommendations section of Geotechnical Overview and in the Shallow
Foundations section.
Excavation
We anticipate excavations for the proposed construction can be accomplished with
conventional earthmoving equipment. Excavations into the on-site soils could encounter
loose/weak zones and/or saturated soil conditions with possible caving conditions.
Gravels and possibly cobbles are also locally present within the site soils. These
conditions can complicate and increase difficulty of excavation and additional effort may
be necessary to extract larger-sized materials, particularly in deeper narrow
excavations, such as utility trenches. The bottom of excavations should be thoroughly
cleaned of loose/disturbed materials prior to backfill placement and/or construction.
Excavation penetrating the bedrock may require the use of specialized heavy-duty
equipment, together with ripping or jack-hammering to advance the excavation and
facilitate rock break-up and removal. Consideration should be given to obtaining a unit
price for difficult excavation in the contract documents for the project.
Any existing building foundations exposed during the excavation for the new foundation
should be examined and evaluated by Terracon to determine the need for any shoring or
underpinning. Excavations should not extend into the stress influence zone of the
existing foundations without prior evaluation by Terracon. The stress influence zone is
defined as the area below a line projected down at a 1H:1V (Horizontal:Vertical) slope
from the bottom edge of the existing foundation. Excavations within the influence zone
of existing foundations can result in loss of support and can create settlement or failure
of the existing foundations. While the evaluation of existing foundations and the design
of a shoring system are beyond the scope of this study, we can perform these tasks as a
separate study.
Depending upon depth of excavation and seasonal conditions, surface water infiltration
and/or groundwater may be encountered in excavations on the site. We anticipate
pumping from sumps may be utilized to control water within excavations. Well points
may be required for significant groundwater flow, or where excavations penetrate
groundwater to a significant depth. Groundwater seepage should be anticipated for
excavations approaching the level of bedrock.
The subgrade soil conditions should be evaluated during the excavation process and the
stability of the soils determined at that time by the contractors' Competent Person as
defined by OSHA. Slope inclinations flatter than the OSHA maximum values may have to
be used. The individual contractor(s) should be made responsible for designing and
constructing stable, temporary excavations as required to maintain stability of both the
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December 6, 2024 1 Terracon Project No. 20245055
excavation sides and bottom. All excavations should be sloped or shored in the interest
of safety following local, and federal regulations, including current OSHA excavation and
trench safety standards. If any excavation, including a utility trench, is extended to a
depth of more than 20 feet, it will be necessary to have the side slopes and/or shoring
system designed by a professional engineer.
As a safety measure, we recommend all vehicles and soil piles be kept a minimum
lateral distance from the crest of the slope equal to the slope height. The exposed slope
face should be protected against the elements.
Subgrade Preparation
After site preparation, removal of existing, undocumented fill, and completion of any
required undercuts, the top 10 inches of the exposed ground surface should be scarified,
moisture conditioned, and compacted to at least 95 percent of the maximum dry unit
weight as determined by ASTM D698 before any new fill, foundations, slabs, and other
site improvements are placed or constructed. Scarification and compaction of bedrock is
not required if bedrock is encountered at the base of the excavation.
Large areas of prepared subgrade should be proof rolled prior to new construction. Proof
rolling is not required in areas which are inaccessible to proof rolling equipment.
Subgrades should be proof rolled with an adequately loaded vehicle such as a fully-
loaded tandem-axle dump truck. Proof rolling should be performed under the
observation of the Geotechnical Engineer or representative. Areas excessively deflecting
under the proof roll should be delineated and subsequently addressed by the
Geotechnical Engineer. Such areas should either be removed and replaced with new
engineered fill or scarified, moisture conditioned, and compacted. Excessively wet or dry
material should either be removed and replaced with engineered fill, or scarified,
moisture conditioned, and compacted.
After the bottom of the excavation has been prepared as recommended above,
engineered fill can be placed to bring the building pad to the desired grade. Engineered
fill should be placed in accordance with the recommendations presented in subsequent
sections of this report.
Subgrade Stabilization
Methods of subgrade stabilization/improvement, as described below, could include
scarification, moisture conditioning and compaction, removal of unstable materials and
replacement with granular fill (with or without geosynthetics), and chemical treatment.
The appropriate method of improvement, if required, would be dependent on factors
such as schedule, weather, the size of area to be stabilized, and the nature of the
instability. More detailed recommendations can be provided during construction as the
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December 6, 2024 1 Terracon Project No. 20245055
need for subgrade stabilization occurs. Performing site grading operations during warm
seasons and dry periods would help reduce the amount of subgrade stabilization
required.
If the exposed subgrade is unstable during proof rolling operations, it could be stabilized
using one of the methods described below.
■ Scarification and Compaction - It may be feasible to scarify, dry, and compact
the exposed soils. The success of this procedure would depend primarily upon
favorable weather and sufficient time to dry the soils. Stable subgrades likely
would not be achievable if the thickness of the unstable soil is greater than about
1 foot, if the unstable soil is at or near groundwater levels, or if construction is
performed during a period of wet or cool weather when drying is difficult.
■ Crushed Stone - The use of crushed stone or crushed concrete is a common
procedure to improve subgrade stability. Typical undercut depths would be
expected to range from about 6 to 18 inches below finished subgrade elevation.
Crushed stone and/or concrete can be tracked or "crowded" into the unstable
subgrade until a stable working surface is attained. The use of high modulus
geosynthetics (i.e., geotextile or geogrid) could also be considered after
underground work such as utility construction is completed. Prior to placing the
geosynthetic, we recommend all below-grade construction, such as utility line
installation, be completed to avoid damaging the geosynthetic. Equipment should
not be operated above the geosynthetic until one full lift of crushed stone fill is
placed above it.
■ Chemical Treatment - Improvement of subgrades with portland cement, lime,
or fly ash could be considered for improving unstable soils. Chemical treatment
should be performed by a pre-qualified contractor having experience with
successfully treating subgrades in the project area on similar sized projects with
similar soil conditions. Results of chemical analysis of the chemical treatment
materials should be provided to the Geotechnical Engineer for review prior to use.
The hazards of chemicals blowing across the site or onto adjacent properties
should also be considered. Additional testing would be needed to develop specific
recommendations to improve subgrade stability by blending chemicals with the
site soils. Additional testing could include, but not be limited to, determining the
most suitable chemical treating agent, the optimum amounts required, the
presence of sulfates in the soil, and freeze-thaw durability of the subgrade.
Further evaluation of the need and recommendations for subgrade stabilization can be
provided during construction as the geotechnical conditions are exposed.
Facilities I Environmental I Geotechnical I Materials 12
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Fill Material Types
Fill for this project should consist of engineered fill. Engineered fill is fill that meets the
criteria presented in this report and has been properly documented. On-site soils free of
deleterious materials or approved granular and low plasticity cohesive imported
materials may be used as fill material. Bedrock excavated during site development and
construction can be reused as fill provided the material is broken down and thoroughly
processed to a "soil-like" consistency, with no particles greater than 2 inches in size. The
earthwork contractor should expect significant mechanical processing and moisture
conditioning of the site soils and/or bedrock will be needed to achieve proper
compaction.
Imported fill materials (if required) should meet the following material property
requirements. Regardless of its source, compacted fill should consist of approved
materials that are free of organic matter and debris. Frozen material should not be used,
and fill should not be placed on a frozen subgrade.
Gradation Percent Finer by Weight (ASTM C136)
3" 100
1" 70-100
No. 4 Sieve 30-100
No. 200 Sieve 10-60
Soil Properties Values
Liquid Limit 35 (max.)
Plasticity Index 15 (max.)
Aggregate or recycled concrete base course used below the new foundation as a leveling
course should meet CDOT requirements for Class 5 or 6 aggregate base course
materials.
Other import fill material types may be suitable for use on the site depending upon
proposed application and location on the site and could be tested and approved for use
on a case-by-case basis.
Fill Placement and Compaction Requirements
Engineered fill should be placed and compacted in horizontal lifts, using equipment and
procedures that will produce recommended moisture contents and densities throughout
the lift.
Facilities I Environmental I Geotechnical I Materials 13
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New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Item Description
9 inches or less in loose thickness when heavy, self-propelled
Maximum Lift compaction equipment is used.
Thickness 4 to 6 inches in loose thickness when hand-guided equipment
(i.e., jumping jack or plate compactor) is used.
Engineered Fill: At least 95% of the maximum dry unit weight
as determined by ASTM D698.
Minimum Engineered Fill 8 Feet Thick or Greater: At least 98% of the
Compaction maximum dry unit weight as determined by ASTM D698 for the
Requirements entire depth of fill in areas receiving 8 feet of fill or greater.
Aggregate or Recycled Concrete Base Course: At least 95% of
maximum dry unit weight as determined by ASTM D1557 when
base course is used below foundations.
Water Content Cohesive (clay): -1% to +3% of optimum moisture content
Range 2,3 Granular (sand): -3% to +3% of optimum moisture content
1. We recommend engineered fill be tested for moisture content and compaction during
placement. If the results of the in-place density tests indicate the specified moisture or
compaction limits have not been met, the area represented by the test should be
reworked and retested as required until the specified moisture and compaction
requirements are achieved.
2. Moisture conditioned clay materials should not be allowed to dry out. A loss of moisture
within these materials could result in an increase in the material's expansive potential.
Subsequent wetting of these materials could result in undesirable movement.
3. Specifically, moisture levels should be maintained low enough to allow for satisfactory
compaction to be achieved without the fill material pumping when proof rolled.
Utility Trench Backfill
Any loose, soft, or unsuitable materials encountered at the bottom of utility trench
excavations should be removed and replaced with engineered fill or bedding material in
accordance with public works specifications for the utility to be supported. This
recommendation is particularly applicable to utility work where settlement control of the
utility is critical. Utility trench excavation should not be conducted below a downward
1H:1V (Horizontal:Vertical) projection from existing foundations without engineering
review of shoring requirements and geotechnical observation during construction.
On-site materials are considered suitable for backfill of utility and pipe trenches provided
the material is free of organic matter and deleterious substances.
Utility trench backfill should be placed and compacted as discussed earlier in this report.
Compaction of initial lifts should be accomplished with hand-operated tampers or other
lightweight compactors. Flooding or jetting for placement and compaction of backfill is
Facilities I Environmental I Geotechnical I Materials 14
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
not recommended. If utility trenches are backfilled with relatively clean granular
material, they should be capped with at least 18 inches of cohesive fill in non-pavement
areas to reduce the infiltration and conveyance of surface water through the trench
backfill.
For low permeability subgrades, utility trenches are a common source of water
infiltration and migration. Utility trenches penetrating beneath the building should be
effectively sealed to restrict water intrusion and flow through the trenches, which could
migrate below the building. The trench should provide an effective trench plug that
extends at least 5 feet from the face of the building exterior. The plug material should
consist of cementitious flowable fill or low permeability clay. The trench plug material
should be placed to surround the utility line. If used, the trench plug material should be
placed and compacted to comply with the water content and compaction
recommendations for engineered fill stated previously in this report.
All underground piping within or near the proposed structure should be designed with
flexible couplings, so minor deviations in alignment do not result in breakage or distress.
Utility knockouts in foundation walls should be oversized to accommodate differential
movements.
We recommend a representative of the Geotechnical Engineer provide full-time
observation and compaction testing of trench backfill within building areas.
Grading and Drainage
All grades must provide effective drainage away from the proposed building and nearby
existing buildings during and after construction and should be maintained throughout the
life of the structure. Water retained next to the building can result in soil movements
greater than those discussed in this report. Greater movements can result in
unacceptable differential floor slab and/or foundation movements, cracked slabs and
walls, and roof leaks. The roof should have gutters/drains with downspouts that
discharge onto splash blocks at a distance of at least 10 feet from the building.
Exposed ground should be sloped and maintained at a minimum 5% away from the
building for at least 10 feet beyond the perimeter of the building, if possible. Locally,
flatter grades may be necessary to transition ADA access requirements for flatwork.
After building construction and landscaping have been completed, final grades should be
verified to document effective drainage has been achieved. Grades around the structure
should also be periodically inspected and adjusted, as necessary, as part of the
structure's maintenance program.
Flatwork and pavements will be subject to post-construction movement. Maximum
grades practical should be used for paving and flatwork to prevent areas where water
can pond. In addition, allowances in final grades should take into consideration post-
Facilities I Environmental I Geotechnical I Materials 15
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado Terracon
December 6, 2024 1 Terracon Project No. 20245055
construction movement of flatwork, particularly if such movement would be critical.
Where paving or flatwork abuts the structure, care should be taken that joints are
properly sealed and maintained to prevent the infiltration of surface water.
Planters located adjacent to the structure (if any) should preferably be self-contained.
Sprinkler mains and spray heads should be located a minimum of 5 feet away from the
building line(s). Low-volume, drip style landscaped irrigation should be used sparingly
near the building.
Exterior Slab Design and Construction
Exterior slabs-on-grade, exterior architectural features, and utilities founded on, or in
backfill or the site soils will likely experience some movement due to the volume change
of the material. Subgrade soils below new fill should be scarified to a depth of at least
10 inches, moisture conditioned, and compacted prior to placement/construction of new
engineered fill, aggregate base course, or pavement/flatwork materials. Potential
movement could be reduced by:
■ Minimizing moisture increases in subgrade soils and new fill;
■ Controlling moisture-density during subgrade preparation and new fill placement;
■ Using designs which allow vertical movement between the exterior features and
adjoining structural elements; and
■ Placing control joints on relatively close centers.
Earthwork Construction Considerations
Upon completion of filling and grading, care should be taken to maintain the subgrade
water content prior to construction of grade-supported improvements such as floor slabs
and flatwork. Construction traffic over the completed subgrades should be avoided. The
site should also be graded to prevent ponding of surface water on the prepared
subgrades or in excavations. Water collecting over or adjacent to construction areas
should be removed. If the subgrade freezes, desiccates, saturates, or is disturbed, the
affected material should be removed, or the materials should be scarified, moisture
conditioned, and recompacted prior to floor slab construction.
Construction site safety is the sole responsibility of the contractor who controls the
means, methods, and sequencing of construction operations. Under no circumstances
shall the information provided herein be interpreted to mean Terracon is assuming
responsibility for construction site safety or the contractor's activities; such
responsibility shall neither be implied nor inferred.
Excavations or other activities resulting in ground disturbance have the potential to
affect adjoining properties and structures. Our scope of services does not include review
Facilities I Environmental I Geotechnical I Materials 16
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New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
of available final grading information or consider potential temporary grading performed
by the contractor for potential effects such as ground movement beyond the project
limits. A preconstruction/precondition survey should be conducted to document nearby
property/infrastructure prior to any site development activity. Excavation or ground
disturbance activities adjacent or near property lines should be monitored or
instrumented for potential ground movements that could negatively affect adjoining
property and/or structures.
Construction Observation and Testing
The earthwork efforts should be observed by the Geotechnical Engineer (or others under
their direction). Observation should include documentation of adequate removal of
surficial materials (vegetation, topsoil, and existing flatwork), evaluation and
remediation of existing fill materials, subgrade stabilization, as well as proof rolling and
mitigation of unsuitable areas delineated by the proof roll. Each lift of compacted fill
should be tested, evaluated, and reworked, as necessary, as recommended by the
Geotechnical Engineer prior to placement of additional lifts.
In areas of foundation excavations, the bearing subgrade should be evaluated by the
Geotechnical Engineer. If unanticipated conditions are observed, the Geotechnical
Engineer should prescribe mitigation options.
In addition to the documentation of the essential parameters necessary for construction,
the continuation of the Geotechnical Engineer into the construction phase of the project
provides the continuity to maintain the Geotechnical Engineer's evaluation of subsurface
conditions, including assessing variations and associated design changes.
Shallow Foundations
If the site has been prepared in accordance with the requirements noted in Earthwork,
the following design parameters are applicable for shallow, reinforced concrete, mat
foundations bearing on the very hard sandstone bedrock.
Reinforced Mats — Design Recommendations
Item Description
Maximum Net Allowable Bearing
Pressure ' 5,000 psf
Facilities I Environmental I Geotechnical I Materials 17
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Item Description
Very Hard Sandstone Bedrock
with a thin layer (about 4 to 6 inches or
Required Bearing Stratum 2 less) of CDOT Class 5 or 6 aggregate or
recycled concrete base course or a lean
concrete mud mat on top
Anticipated Foundation Bearing About 10 to 12 feet below
Depth existing grades
■ 77 feet and 6 inches (south boundary
along E. Mountain Avenue)
Anticipated Foundation Dimensions 0 gg feet and 5 inches (east boundary)
■ 117 feet and 6 inches (northwest
boundary along Chestnut Street)
On-site or import clay soils:
Active, Ka = 0.42
Passive, Kp = 2.37
Lateral Earth Pressure Coefficients 3 At-rest, Ko = 0.59
On-site or import granular soils:
Active, Ka = 0.31
Passive, Kp = 3.25
At-rest, Ko = 0.47
Aggregate base course or intact sandstone
Sliding Resistance bedrock:
p = 0.54 (ultimate)
Moist Soil Unit Weight On-site or import clay or granular soils:
y = 120 pcf
Minimum Embedment Below
Finished Grade 4 30 inches
Facilities I Environmental I Geotechnical I Materials 18
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New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Item Description
kl = 300 psi/in.
B+1 Z ()
K(Bxe) = k1(2B (sands)
K(BxB) = kl B (clays)
K(BxB)�1 + 0.5 * (B)
K(BxG)
Where:
Subgrade Modulus ki = coefficient of subgrade reaction of
foundations measuring 1 ft. x 1 ft.
K(BxB) = coefficient of subgrade modulus
for a square foundation having
dimensions B (ft.) x B (ft.).
K(BXL) = coefficient of subgrade modulus
for a rectangular foundation having
dimensions B (ft.) x L (ft.).
Estimated Total Movement 6 About 1 inch or less
Estimated Differential Movement 6 About 1/2 to 3/4 of total movement
1. The maximum net allowable bearing pressure is the pressure in excess of the minimum
surrounding overburden pressure at the footing base elevation. Values assume exterior
grades are no steeper than 20% within 10 feet of structure. The design bearing
pressure applies to a dead load plus design live load condition. The design bearing
pressure may be increased by one-third when considering total loads that include wind
or seismic conditions.
2. Unsuitable or soft/loose materials, if encountered should be over-excavated and
foundation extended deeper to suitable bearing materials. We anticipate the layer of
aggregate or recycled concrete base course between the base of the foundation and the
bedrock to be on the order of 4 to 6 inches thick.
3. Use of lateral earth pressures require the foundation forms be removed and compacted
engineered fill be placed against the vertical footing face. Assumes no hydrostatic
pressure. The lateral earth pressure coefficients are ultimate values and do not include a
factor of safety. The foundation designer should include the appropriate factors of
safety.
4. Embedment necessary to minimize the effects of frost and/or seasonal water content
variations. For sloping ground, maintain depth below the lowest adjacent exterior grade
within 5 horizontal feet of the structure.
5. The estimated movements presented above assume the maximum foundations are no
larger than the anticipated dimensions presented above. Larger foundation footprints
will likely require reduced net allowable soil bearing pressures to reduce risk for
potential settlement.
Foundations should be proportioned to reduce differential foundation movement.
Proportioning on the basis of equal total settlement is recommended; however,
proportioning to relative constant dead-load pressure will also reduce differential
Facilities I Environmental I Geotechnical I Materials 19
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado Terracon
December 6, 2024 1 Terracon Project No. 20245055
movement between adjacent foundations. Foundations should be reinforced as
necessary to reduce the potential for distress caused by differential foundation
movement.
Foundation excavations should be observed by Terracon. If the material conditions
encountered differ significantly from those presented in this report, supplemental
recommendations will be required.
Any excavation that extends below the bottom of foundation elevation should extend
laterally beyond all edges of the foundations at least 8 inches per foot of excavation
depth below the foundation base elevation. The excavation should be backfilled to the
foundation base elevation in accordance with the recommendations presented in this
report.
Shallow Foundation Construction Considerations
As noted in Earthwork, foundation excavations should be evaluated under the
observation of the Geotechnical Engineer. The base of all foundation excavations should
be free of water and loose soil, prior to placing concrete. Concrete should be placed soon
after excavating to reduce bearing soil disturbance. Care should be taken to prevent
wetting or drying of the bearing materials during construction. Excessively wet or dry
material or any loose/disturbed material in the bottom of foundation excavations should
be removed/reconditioned before foundation concrete is placed.
If unsuitable bearing materials are observed at the base of the planned foundation
excavation, the excavation should be extended deeper to suitable materials.
Foundation elements should be reinforced as necessary to reduce the potential for
distress caused by differential foundation movement.
Unstable subgrade conditions encountered in foundation excavations should be observed
by Terracon to assess the subgrade and provide suitable alternatives for stabilization.
Typical methods of stabilization/improvement are presented in the Subgrade
Stabilization section of Earthwork.
Construction Adjacent to Existing Building
Care should be taken during excavation adjacent to existing foundations to avoid
disturbing existing foundation bearing soils. New foundations should bear at or near the
bearing elevation of immediately adjacent existing foundations. Depending upon their
locations and current loads on the existing foundations, foundations for the new building
could cause settlement of adjacent walls. The structural capacity of existing foundations
should be evaluated by a licensed structural engineer, where increases in loading are
possible.
Facilities I Environmental I Geotechnical I Materials 20
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado Terracon
December 6, 2024 1 Terracon Project No. 20245055
Floor Systems
Since a basement level is planned under the entire footprint area of the building, we
anticipate the first level floor will be designed as a structural floor system which will be
connected to the building structure and supported by the building foundation. Therefore,
specific design recommendations for a floor system for the proposed structure are not
presented in this report. If a portion of the building will include a floor system which will
have subgrade soil below it (such as a slab-on-grade system), then we can provide
additional recommendations for floor systems upon request.
Below-Grade Structures
We understand the new building will be constructed with a basement level and elevator
pit. The basement and elevator pit are anticipated to extend to depths of about 10 to 12
feet below existing grades. We recommend the below-grade areas be designed as water-
tight and to resist buoyant forces. They should also be designed to resist lateral earth
pressures. Design parameters for lateral earth pressures are presented in the Lateral
Earth Pressures section.
Elevator Pit - Design Recommendations
We understand an elevator pit will be included in the interior of the building. The
elevator pit will likely consist of reinforced concrete walls with a concrete base slab.
Based on our experience with this type of structure, we anticipate the base slabs will be
about 2 to 4 feet below the basement level floor slab.
Subsurface conditions at this site at/near the elevator pit excavation are generally
anticipated to consist of very hard sandstone bedrock. Groundwater levels observed at
the site in the National Inspection Services monitoring wells were at depths of about 12
to 121/2 feet below existing site grades in October 2024. Therefore, we anticipate
groundwater levels during construction at the project site could be similar to the levels
measured in the monitoring wells. Also, groundwater levels can and should be expected
to fluctuate over time.
Depending upon final site grades and elevator pit elevations, groundwater could impact
the performance of the pit base slab. If the pit slab is constructed at or within about 4
feet of the level of groundwater, the pit/slab should be designed and constructed to
resist hydrostatic pressures and uplift due to the effects of buoyancy or it should be
protected by an underdrain system for permanent dewatering. "Water-proofing" of the
pit will also be needed if permanent dewatering is not used. Terracon can evaluate the
groundwater level within the elevator pit area prior to or during construction.
Facilities I Environmental I Geotechnical I Materials 21
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New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Reinforced concrete elevator pit walls should be designed for lateral earth pressures
and/or combined hydrostatic and lateral earth pressures at least equal to those stated
previously in the Lateral Earth Pressures section for above and/or below groundwater
conditions.
The lateral earth pressures presented previously do not include a factor of safety. As
such, appropriate factors of safety should be applied to these values. Furthermore, the
lateral earth pressures do not include the influence of surcharge, equipment, or floor
loading, which should be added.
Lateral Earth Pressures
Design Parameters
Structures with unbalanced backfill levels on opposite sides should be designed for earth
pressures at least equal to values indicated in the following table. Earth pressures will be
influenced by structural design of the walls, conditions of wall restraint, methods of
construction, and/or compaction and the strength of the materials being restrained. Two
wall restraint conditions are shown in the diagram below. Active earth pressure is
commonly used for design of free-standing cantilever retaining walls and assumes wall
movement. The "at-rest" condition assumes no wall movement and is commonly used
for basement walls, loading dock walls, or other walls restrained at the top. The
recommended design lateral earth pressures do not include a factor of safety and do not
provide for possible hydrostatic pressure on the walls (unless stated).
S Surcharge For active pressure movement
�(0.002 H to 0.004 H)
S For at-rest pressure
-No Movement Assumed
Horizontal
Finished
Grade
H
Horizontal
Finished Grade
I+--p2 p, Retaining Wall
Facilities I Environmental I Geotechnical Materials 22
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New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Lateral Earth Pressure Design Parameters
Earth Coefficient Equivalent Surcharge Earth Pressures, p2
Pressure for Backfill Fluid pressure 3 (psf) 2,1
Condition 1 Type 2 Density s
(pcf) Pi (Psf) Unsaturated s Submerged
Granular - 40 (0.31)S (40)H (80)H
0.31
Active (Ka)
Fine Grained
- 0.42 50 (0.42)S (50)H (90)H
Granular - 60 (0.47)S (60)H (90)H
0.47
At-Rest (Ko)
Fine Grained
- 0.59 70 (0.59)S (70)H (100)H
Granular - 390
3.25
Passive (Kp)
Fine Grained
- 2.37
285 --- ---
---
1. For active earth pressure, wall must rotate about base, with top lateral movements
0.002H to 0.004H, where H is wall height. For passive earth pressure, wall must move
horizontally to mobilize resistance. Fat clay or other expansive soils should not be used
as backfill behind the wall.
2. Uniform, horizontal backfill, with a maximum unit weight of 120 pcf for cohesive soils and
120 pcf for granular soils.
3. Uniform surcharge, where S is surcharge pressure.
4. Loading from heavy compaction equipment is not included.
5. To achieve "Unsaturated" conditions, drainage behind walls should be incorporated into
the design. "Submerged" conditions are recommended when drainage behind walls is not
incorporated into the design.
Backfill placed against structures should consist of granular soils or low plasticity
cohesive soils. For the granular values to be valid, the granular backfill must extend out
and up from the base of the wall at an angle of at least 45 and 60 degrees from vertical
for the active and passive cases, respectively.
To control hydrostatic pressure behind walls, we typically recommend a drain be
installed at the base of walls with a collection pipe leading to a reliable discharge.
However, we understand no below-grade drainage system is planned for the proposed
building. Therefore, the below-grade walls of the new building should be designed to
resist combined hydrostatic and lateral earth pressures. The hydrostatic and lateral
earth pressures should be calculated for fine grained or granular backfill using the
submerged equivalent fluid pressures provided in the above table. These pressures do
not include the influence of surcharge, equipment, or floor loading, which should be
added. Heavy equipment should not operate within a distance closer than the exposed
height of retaining walls to prevent lateral pressures more than those provided.
Facilities I Environmental I Geotechnical I Materials 23
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado Terracon
December 6, 2024 1 Terracon Project No. 20245055
Foundations, floor slabs, or other loads bearing on backfill behind walls may have a
significant influence on the lateral earth pressure. Placing foundations within wall backfill
and in the zone of active soil influence on the wall should be avoided unless structural
analyses indicate the wall can safely withstand the increased pressure.
The lateral earth pressure recommendations given in this section are applicable to the
design of rigid retaining walls subject to slight rotation, such as cantilever, or gravity
type concrete walls. These recommendations are not applicable to the design of modular
block - geogrid reinforced backfill walls (also termed MSE walls). Recommendations
covering these types of wall systems are beyond the scope of services for this
assignment. However, we would be pleased to develop a proposal for evaluation and
design of such wall systems upon request.
General Comments
Our analysis and opinions are based upon our understanding of the project, the
geotechnical conditions in the area, and the data obtained from our site exploration.
Variations will occur between exploration point locations or due to the modifying effects
of construction or weather. The nature and extent of such variations may not become
evident until during or after construction. Terracon should be retained as the
Geotechnical Engineer, where noted in this report, to provide observation and testing
services during pertinent construction phases. If variations appear, we can provide
further evaluation and supplemental recommendations. If variations are noted in the
absence of our observation and testing services on-site, we should be immediately
notified so that we can provide evaluation and supplemental recommendations.
Our Scope of Services does not include either specifically or by implication any
environmental or biological (e.g., mold, fungi, bacteria) assessment of the site or
identification or prevention of pollutants, hazardous materials or conditions. If the owner
is concerned about the potential for such contamination or pollution, other studies
should be undertaken.
Our services and any correspondence are intended for the sole benefit and exclusive use
of our client for specific application to the project discussed and are accomplished in
accordance with generally accepted geotechnical engineering practices with no third-
party beneficiaries intended. Any third-party access to services or correspondence is
solely for information purposes to support the services provided by Terracon to our
client. Reliance upon the services and any work product is limited to our client and is not
intended for third parties. Any use or reliance of the provided information by third
parties is done solely at their own risk. No warranties, either express or implied, are
intended or made.
Facilities I Environmental I Geotechnical I Materials 24
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New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Site characteristics as provided are for design purposes and not to estimate excavation
cost. Any use of our report in that regard is done at the sole risk of the excavating cost
estimator as there may be variations on the site that are not apparent in the data that
could significantly effect excavation cost. Any parties charged with estimating excavation
costs should seek their own site characterization for specific purposes to obtain the
specific level of detail necessary for costing. Site safety and cost estimating including
excavation support and dewatering requirements/design are the responsibility of others.
Construction and site development have the potential to affect adjacent properties. Such
impacts can include damages due to vibration, modification of groundwater/surface
water flow during construction, foundation movement due to undermining or subsidence
from excavation, as well as noise or air quality concerns. Evaluation of these items on
nearby properties are commonly associated with contractor means and methods and are
not addressed in this report. The owner and contractor should consider a
preconstruction/precondition survey of surrounding development. If changes in the
nature, design, or location of the project are planned, our conclusions and
recommendations shall not be considered valid unless we review the changes and either
verify or modify our conclusions in writing.
Facilities I Environmental I Geotechnical I Materials 25
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Figures
Contents:
GeoModel
Facilities I Environmental I Geotechnical I Materials
New Office Building
300 E. Mountain Avenue I Fort Collins, Colorado so ierracon
Terracon Project No. 20245055
1901 Sharp Point Or Ste C
Fort Collins, CO
GeoModel
4,978
B-2
B
4,976 ...........-1................................. .0.5..............................................................................................................
4,974 ....... ............................. 2 ...............................................................................................................
2 3
.. 4,972 ....... ............................. ...............................................................................................................
N 4
.�, 4,970 ....... . ............................. 3 ...............................................................................................................
J
4,968 ....... 3 :............................. . ,...............................................................................................................
p4,966 ....... �o............................................................................................................
►+ 10
j4,964 ....... ............................. :...............................................................................................................
w
J
W ::: ....:
w
::..
X 4,960 ....... .....
X
0 4,958 ....... 4 :::::............................. ...............................................................................................................
a
IL
a4,956 ....... :::::............................. �:...............................................................................................................
4,954 ....... ..................................
:.... .:
4,952 ....... :: ............................. :;::-24.3..........................................................................................................
24.1
4,950
This is not a cross section.This is intended to display the Geotechnical Model only. See individual logs for more detailed conditions.
Model Layer Layer Name General Description Legend
Recycled Asphalt Sandy Lean Clay
1 Surface Layer About 2 to 6 inches of recycled asphalt. with Gravel
Consisted primarily of lean clay soils with variable IDISilty Clayey Sand Sandstone
2 Existing Fill-Clay amounts of sand and gravel; brown, light brown,dark Poorly-graded
brown,gray. "Gravel with Silt and
Existing Fill-Sand & Consisted primarily of sand and gravel soils with variable Sand
3 Gravel amounts of silt and clay; brown,tan, reddish brown.
4 Bedrock Very hard sandstone bedrock with clay seams; olive, olive
gray, gray, light gray, brown.
NOTES:
Layering shown on this figure has been developed by the
geotechnical engineer for purposes of modeling the subsurface
conditions as required for the subsequent geotechnical engineering
for this project.
Numbers adjacent to soil column indicate depth below ground
surface.
Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
Attachments
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Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado Terracon
December 6, 2024 1 Terracon Project No. 20245055
Exploration and Testing Procedures
Field Exploration
Number of Borings Approximate Boring Location
Depth (feet)
2 24.1 to 24.3 Within the planned
(Boring Nos. B-1 and B-2) building area
Boring Layout and Elevations: Terracon personnel provided the boring layout using
handheld GPS equipment (estimated horizontal accuracy of about ±15 feet) and
referencing existing site features. Approximate ground surface elevations were obtained
by interpolation from a publicly available USGS topographic map
(https://apps.national map.gov/viewer/). If ground surface elevations and a more precise
boring layout are desired, we recommend the borings be surveyed.
Subsurface Exploration Procedures: We advanced the borings with a truck-mounted,
rotary drill rig using continuous-flight, solid-stem augers. Sampling was performed using
standard split-barrel and modified California barrel sampling procedures. Bulk samples of
auger cuttings from the upper approximately 5 feet of each borehole were also collected
for laboratory testing. In the split-barrel sampling procedure, a standard 2-inch outer
diameter split-barrel sampling spoon was driven into the ground by a 140-pound
automatic hammer falling a distance of 30 inches. The number of blows required to
advance the sampling spoon the last 12 inches of a normal 18-inch penetration is
recorded as the Standard Penetration Test (SPT) resistance value. The SPT resistance
values, also referred to as N-values, are indicated on the boring logs at the test depths.
In the modified California barrel sampling procedure, a 21h-inch outer diameter split-
barrel sampling spoon is used for sampling. Modified California barrel sampling
procedures are similar to standard split spoon sampling procedure; however, blow
counts are typically recorded for 6-inch intervals for a total of 12 inches of penetration.
Modified California barrel sampler blow counts are not considered N-values. The samples
were placed in appropriate containers and taken to our soil laboratory for testing and
classification by a Geotechnical Engineer.
We also observed the boreholes while drilling and at the completion of drilling for the
presence of groundwater. Groundwater was not observed at these times in the
boreholes.
Our exploration team prepared field boring logs as part of the drilling operations. The
sampling depths, penetration distances, and other sampling information were recorded
on the field boring logs. These field logs included visual classifications of the materials
observed during drilling and our interpretation of the subsurface conditions between
samples. Final boring logs were prepared from the field logs. The final boring logs
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Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ferracon
December 6, 2024 1 Terracon Project No. 20245055
represent the Geotechnical Engineer's interpretation of the subsurface conditions at the
boring locations based on field data, observation of samples, and laboratory test results.
We backfilled the borings with bentonite pellets after completion of drilling. Auger
cuttings were drummed and left on site for disposal by others. Our services did not
include repair of the site beyond backfilling the boreholes and drumming of the auger
cuttings. Because backfill material often settles below the surface after a period, we
recommend checking boreholes periodically and backfilling, if necessary.
Laboratory Testing
The project engineer reviewed the field data and assigned laboratory tests. The
laboratory testing program included the following types of tests:
■ Moisture Content
■ Dry Unit Weight
■ Atterberg Limits
■ Grain Size Analyses
■ One-dimensional Swell
■ Corrosivity
The laboratory testing program included examination of soil samples by an engineer
and/or geologist. Based on the results of our field and laboratory programs, we
described and classified the soil samples in accordance with the Unified Soil
Classification System. A brief description of this classification system as well as the
General Notes can be found in the Supporting Information section.
Laboratory test results are indicated on the boring logs and are presented in depth in
the Exploration Results section. Laboratory tests are performed in general accordance
with applicable local standards or other acceptable standards. In some cases, variations
to methods are applied as a result of local practice or professional judgement.
Rock classification was conducted using locally accepted practices for engineering
purposes; petrographic analysis may reveal other rock types. Rock core samples
typically provide an improved specimen for this classification.
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Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado ,� ierracon
December 6, 2024 1 Terracon Project No. 20245055
Site Location and Exploration Plans
Contents:
Site Location Plan
Exploration Plan
Note: All attachments are one page unless noted above.
Facilities I Environmental I Geotechnical I Materials
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Geotechnical Engineering Report
New Office Building I Fort Collins, Colorado rerraeon
December 6, 2024 1 Terracon Project No. 20245055
Exploration Plan
Approldmate Boring Location �+
L
40
w
4b 9
x .
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3
�` r —. i,Ler�sui� �rp�r•1.� �+_J 1rr�furn �'ra'� JIrr.J�ir.�,
DIAGRAM IS FOR GENERAL LOCATION ONLY,AND IS NOT INTENDED FOR CONSTRUCTION PURPOSES MAP PROVIDED BY MICROSOFT BING MAPS
Facilities I Environmental I Geotechnical Materials
Exploration and Laboratory Results
Contents:
Boring Logs (B-1 and B-2)
Atterberg Limits
Grain Size Distribution
Consolidation/Swell (2 pages)
Corrosivity
Note: All attachments are one page unless noted above.
New Office Building lFrerracon
300 E. Mountain Avenue I Fort Collins,Colorado 1901 Sharp Point Or Ste C
Terracon Project No. 20245055 Fort Collins,CO
Boring Log No. B-1
o, Location: See Exploration Plan ° �� Atterberg
a) �. N a N N a a u Limits
Latitude:40.58740 Longitude: -105.07340 7aiL J m y N °o ' c = u
U
y E °7 v ° 3 p LL-PL-PI a"
CU
Depth Ft. Elevation:4975(Ft.)+/-
RECYCLED ASPHALT,very dark gray,about 2 inches
thick
FILL-SANDY LEAN CLAY WITH GRAVEL, brown to
i� light brown, with gray
2
11-18 6.4
29/12"
4.0 4971
FILL-SILTY CLAYEY SAND(SC-SM),with variable
amounts of gravel, brown to tan,with reddish brown 5 8-30-50/4" 3.9
3
30-50/4" 3.2 19-15-4 30
becomes gravelly below about 9 feet 30-50/4" 2.8
:'10.0 4965 1
SANDSTONE,with clay seams,olive to olive gray,
very hard
50/2" <-0.1/ 12.8 110
1000
1
4 ::
gray to light gray, with brown below about 19 feet 50/2" 15.2
Z
r..24.1 4950.9 50 1" 14.3 112
Boring Terminated at 24.1 Feet
See Exploration and Testing Procedures for a description of field and laboratory procedures used and Water Level Observations Drill Rig
additional data(If any). Groundwater not encountered while drilling or at CME 55
See Supporting Information for explanation of symbols and abbreviations. completion of drilling
Elevation Reference:Elevations were interpolated from a publicly available USGS topographic map. Hammer Type
Classification of rock estimated from disturbed samples. Core samples and petrographic analysis Automatic;Hammer
may reveal other rock types. Efficiency=68/o
Driller
Terracon Fort Collins
Notes Advancement Method Logged by
4-inch diameter,continuous-Flight,solid-stem augers P.Agudelo
Boring Started
10-30-2024
Abandonment Method Completed
g Co
Boring backfilled with bentonite pellets upon completion. Boring
Co
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New Office Building lFrerracon
300 E. Mountain Avenue I Fort Collins,Colorado 1901 Sharp Point Or Ste C
Terracon Project No. 20245055 Fort Collins,CO
Boring Log No. B-2
o, Location: See Exploration Plan ° �� Atterberg
a) �. N a N N a a u Limits
Latitude:40.5872°Longitude: -105.0735°7aiL J m y N °o ' c = u
U
y E °7 v ° 3 p LL-PL-PI a"
C� 0 '0 U) � N J U �
Depth Ft. Elevation:4976(Ft.)+/-
7 0.5 RECYCLED ASPHALT,very dark gray,about 6 inches 4975.5
thick
FILL-SANDY LEAN CLAY WITH GRAVEL, brown to
2 dark brown, with gray
3.0 4973 10 9 1
N=
FILL-SILTY CLAYEY SAND(SC-SM),with variable 10
amounts of gravel, brown to tan,with reddish brown
10-30 500/ 9.5 113 24-18-6 29
Jr _ 40/12"
3
50/5" 2.2
`9.0 4967
FILL-POORLY GRADED GRAVEL WITH SILT AND 50/6" 2.1 125 NP 7
10.0 SAND(GP-GM), brown to reddish brown 4966 1
SANDSTONE,with clay seams,olive to olive gray,
very hard
50/4" 14.8
1
4
gray to light gray, with brown below about 19 feet 50/1" 14.8 106
Z
24.3 4951.7 50 3" 14.3
Boring Terminated at 24.3 Feet
See Exploration and Testing Procedures for a description of field and laboratory procedures used and Water Level Observations Drill Rig
additional data(If any). Groundwater not encountered while drilling or at CME 55
See Supporting Information for explanation of symbols and abbreviations. completion of drilling
Elevation Reference:Elevations were interpolated from a publicly available USGS topographic map. Hammer Type
Classification of rock estimated from disturbed samples. Core samples and petrographic analysis Automatic;Hammer
may reveal other rock types. Efficiency=68/o
Driller
Terracon Fort Collins
Notes Advancement Method Logged by
4-inch diameter,continuous-Flight,solid-stem augers P.Agudelo
Boring Started
10-30-2024
Abandonment Method Completed
g Co
Boring backfilled with bentonite pellets upon completion. Boring
Co
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New Office Building ■ rerracon
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Terracon Project No. 20245055 Fort Collins,CO
Atterberg Limit Results
ASTM D4318
60 / 000,
50
/ Oi
40
c /
30
u
/ Ov
a /
20
MH ir OH
/
/
10
/
7
a ML cr OL
0
10 16 20 30 40 50 60 70 80 90 100 110
Liquid Limit
Boring ID Depth (Ft) LL PL PI Fines USCS Description
B-1 7-8 19 15 4 30.1 SC-SM SILTY, CLAYEY SAND
M B-2 4-5 24 18 6 28.7 SC-SM SILTY, CLAYEY SAND
B-2 9-9.5 NP NP NP 7.3 GP-GM POORLY GRADED GRAVEL with SILT and SAND
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Grain Size Distribution
ASTM D422 / ASTM C 136
U.S. Sieve Opening in Inches U.S. Sieve Numbers Hydrometer
6 4 3 2 1.5 1 3/4 1/23/8 3 4 6 810 1416 20 30 40 50 60 100140200
0
100
95
90 10
85
80 20
75
70 30
65
60 40
v
m
0 55
r'
50 50 n
o
a
iic 45
Q
v 40 60 �
v
m
a 35 rt
30 70
25
20 80
15 INA
10 90
5
p 100
100 10 1 0.1 0.01 0.001
Grain Size(mm)
Cobbles Gravel Sand Silt or Clay
coarse fine coarse medium fine
Boring ID Depth (Ft) USCS Classification USCS AASHTO LL PL PI Cc Cu
B-1 7-8 SILTY, CLAYEY SAND SC-SM A-2-4(0) 19 15 4
m B-2 4-5 SILTY, CLAYEY SAND SC-SM A-2-4(0) 24 18 6
♦ B-2 9-9.5 POORLY GRADED GRAVEL with SILT and GP-GM A-1-a(0) NP NP NP 0.49 101.23
SAND
Boring ID Depth (Ft) Dioo D60 D30 Dlo %Cobbles %Gravel %Sand %Fines %Silt %Clay
B-1 7-8 19 0.675 0.0 14.7 55.2 30.1
B-2 4-5 12.5 0.618 0.082 0.0 8.7 62.6 28.7
♦ B-2 9-9.5 37.5 12.777 0.893 0.126 0.0 50.2 42.5 7.3
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One-Dimensional Swell or Collapse
2.0
1.5
1.0
0.5
010
-0.5
0
-1.0
X
Q
1.5
-2.0
-2.5
-3.0
-3.5
-4.0
100 1,000 10,000
Pressure (psf)
Boring ID VDepth(Ft) Description USCS '4(pcf) WC(%)
• B-1 14- 14.3 SANDSTONE 111 12.8
Notes: Sample exhibited less than 0.1 percent compression upon wetting under an applied pressure of 1,000 psf.
FaciHhes p rnvironanemka@ p Geotechnical I Materials
New Office Building Wrerracon
300 E. Mountain Avenue I Fort Collins,Colorado 1901 Sharp Point Or Ste C
Terracon Project No.20245055 Fort Collins,CO
One-Dimensional Swell or Collapse
2.0
1.5
1.0
0.5
010
-0.5
0
-1.0
16
Q
-1.5
-2.0
-2.5
-3.0
-3.5
-4.0
100 1,000 10,000
Pressure (psf)
Boring ID Depth (Ft) Description USCS ^fd(pcf) WC(%)
• B-2 4-5 SILTY, CLAYEY SAND(SC-SM) SC-SM 113 9.5
Notes: Sample exhibited 0.2 percent swell upon wetting under an applied pressure of 500 psf.
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rracon
Explore with us
Client Project
Mountain 300 LLC New Office Building -300 E. Mountain Avenue
Fort Collins, CO 20245055
Date Received: 11/11/2024
Results from Corrosion Testing
Sample Location B-2
Sample Depth (ft.) 1.0'-5.0'
pH Analysis, AASHTO T289 7.83
Water Soluble Sulfate, ASTM C1580, (mg/kg) 8
Sulfides, AWWA 4500-S D, (mg/kg) Nil
Chloride, ASTM D512, (mg/kg) 73
Red-Ox, ASTM G200, (mV) +249
Total Salts, AWWA 2520 B, (mg/kg) 990
Resistivity(Saturated), ASTM G57, (ohm-cm) 1900
Analyzed By: ChrisAnne Ross
Staff Geologist
The tests were performed in general accordance with applicable ASTM and AWWA test methods. This report is exclusively for the use of the client
indicated above and shall not be reproduced except in full without the written consent of our company. Test results transmitted herein are only applicable to
the actual samples tested at the location(s)referenced and are not necessarily indicative of the properties of other apparently similar or identical materials.
Supporting Information
Contents:
General Notes
Unified Soil Classification System
Note: All attachments are one page unless noted above.
New Office Building
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Terracon Project No. 20245055
1901 Sharp Point Dr Ste C
General Notes Fort Collins,CO
Sampling Water Level Field Tests
N Standard Penetration Test
Water Initially Resistance(Blows/Ft.)
Modified Encountered
Auger California (HP) Hand Penetrometer
Cuttings Ring Water Level After a
Sampler Specified Period of Time
Standard Water Level After (T) Torvane
Penetration V a Specified Period of Time
Test om Cave In (DCP) Dynamic Cone Penetrometer
Encountered
Water levels indicated on the soil boring logs are the UC Unconfined Compressive
Strength
levels measured in the borehole at the times
indicated. Groundwater level variations will occur over (PID) Photo-Ionization Detector
time. In low permeability soils, accurate
determination of groundwater levels is not possible
with short term water level observations. (OVA) Organic Vapor Analyzer
Descriptive Soil Classification
Soil classification as noted on the soil boring logs is based Unified Soil Classification System. Where sufficient laboratory data exist to classify the
soils consistent with ASTM D2487"Classification of Soils for Engineering Purposes"this procedure is used.ASTM D2488"Description and
Identification of Soils(Visual-Manual Procedure)" is also used to classify the soils, particularly where insufficient laboratory data exist to classify the
soils in accordance with ASTM D2487. In addition to USCS classification,coarse grained soils are classified on the basis of their in-place relative
density,and fine-grained soils are classified on the basis of their consistency. See"Strength Terms"table below for details.The ASTM standards
noted above are for reference to methodology in general. In some cases, variations to methods are applied as a result of local practice or
professional judgment.
Location And Elevation Notes
Exploration point locations as shown on the Exploration Plan and as noted on the soil boring logs in the form of Latitude and Longitude are
approximate. See Exploration and Testing Procedures in the report for the methods used to locate the exploration points for this project. Surface
elevation data annotated with +/-indicates that no actual topographical survey was conducted to confirm the surface elevation. Instead,the surface
elevation was approximately determined from topographic maps of the area.
Strength Terms Jll� J
Relative Density of Coarse-Grained Soils Consistency of Fine-Grained Soils Bedrock ml
(More than 50%retained on No.200 sieve.) (50%or more passing the No.200 sieve.)
Density determined by Standard Penetration Consistency determined by laboratory shear strength testing,field
Resistance visual-manual procedures or standard penetration resistance
Standard Unconfined Standard Standard Ring
Relative Density Penetration Sampler Compressive Penetration Sampler Penetration or Sampler
y or N-Value er Consistency cY Strength or N-Value er Consistency y N-Value
(Blows/Ft.) (Blows/Ft.) Qu (psf) (Blows/Ft.) (Blows/Ft.) (Blows/Ft.) (Blows/Ft.)
Very Loose 0-3 0- 5 Very Soft less than 500 0- 1 < 3 Soft < 20 < 24
Loose 4-9 6- 14 Soft 500 to 1,000 2-4 3- 5 Firm 20-29 24-35
Medium Dense 10-29 15-46 Medium Stiff 1,000 to 2,000 4-8 6- 10 Medium Hard 30-49 36-60
Dense 30-50 47-79 Stiff 2,000 to 4,000 8- 15 11 - 18 Hard 50-79 61 -96
Very Dense > 50 > 80 Very Stiff 4,000 to 8,000 15-30 19-36 Very Hard >79 > 96
Hard >8,000 > 30 > 36
Relevance of Exploration and Laboratory Test Results
Exploration/field results and/or laboratory test data contained within this document are intended for application to the project as described in this
document. Use of such exploration/field results and/or laboratory test data should not be used independently of this document.
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Unified Soil Classification System
Criteria for Assigning Group Symbols and Group Names Using Soil Classification
Laboratory Tests A Group Symbol Group Name B
Gravels: '•Clean Gravels Cu>_4 and 1<_Cc<_3 E GW Well-graded gravel F
More than 50% of Less than 5% fines c Cu<4 and/or[Cc<1 or Cc>3.0] E GP Poorly graded gravel F
coarse fraction
retained on No. 4 Fines classify as ML or MH GM Silty gravel F, "
Gravels with Fines:
Coarse-Grained Soils: sieve More than 12% fines c Fines classify as CL or CH GC Clayey gravel F,G-"
More than 50% retained
on No. 200 sieve Clean Sands: Cu>_6 and 1<_Cc<_3 E SW Well-graded sand I
Sands: Less than 5% fines ° E I
50% or more of Cu<6 and/or[Cc<1 or Cc>3.0] SP Poorly graded sand
coarse fraction Fines classify as ML or MH SM Silty sand e," I
passes No. 4 sieve Sands with Fines:
More than 12% fines ° Fines classify as CL or CH SC Clayey sand e,",I
PI > 7 and plots above"A"line' CL Lean clay K,c,"'
Silts and Clays: Inorganic:
PI < 4 or plots below"A"line ML Silt K,c,M
Liquid limit less than
50 LL oven dried Organic clay K,c,M,N
Fine-Grained Soils: Organic: <0.75 OL
LL not dried Organic silt K,L,M,o
50% or more passes the
No. 200 sieve PI plots on or above"A"line CH Fat clay K,L,M
Silts and Clays: Inorganic: K,�,M
Liquid limit 50 or PI plots below"A"line MH Elastic silt
more LL oven dried Organic clay K,L,M,P
Organic: LL not dried Organic
0.75 OH Organic silt K,L,M,Q
Highly organic soils: Primarily organic matter, dark in color, and organic odor PT Peat
A Based on the material passing the 3-inch (75-mm) sieve. "If fines are organic, add "with organic fines"to group name.
e If field sample contained cobbles or boulders, or both, add"with I If soil contains >_ 15% gravel, add"with gravel"to group name.
cobbles or boulders, or both"to group name. ' If Atterberg limits plot in shaded area, soil is a CL-ML, silty clay.
c Gravels with 5 to 12%fines require dual symbols: GW-GM well- K If soil contains 15 to 29% plus No. 200, add"with sand"or
graded gravel with silt, GW-GC well-graded gravel with clay, GP-GM "with gravel,"whichever is predominant.
poorly graded gravel with silt, GP-GC poorly graded gravel with clay. L If soil contains >_ 30% plus No. 200 predominantly sand, add
° Sands with 5 to 12%fines require dual symbols: SW-SM well- "sandy"to group name.
graded sand with silt, SW-SC well-graded sand with clay, SP-SM M If soil contains >_ 30% plus No. 200, predominantly gravel, add
poorly graded sand with silt, SP-SC poorly graded sand with clay. "gravelly"to group name.
E Cu = D6o/Dio Cc = (D,o)' "PI >_ 4 and plots on or above"A"line.
0 PI < 4 or plots below"A"line.
Duo x Deo P PI plots on or above"A"line.
F If soil contains >_ 15% sand, add"with sand"to group name. Q PI plots below"A"line.
G If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM.
60
For classification of fine-grained
soils and fine-grained fraction
50 —of coarse-grained soils
Equation of°A"-line
Horizontal at PI=4 to LL=25.5.
X 40 then PI=0.73(LL-20) 0�
pEquation of"Ll" line °t
z Vertical at LL=16 to PI=7, G
30 — then PI=0.9(LL-8)
V i
Q 20 (�
_J MH or OH
D-
10 — Z,
4 GL-ML ML or OL
0 i _
0 10 16 20 30 40 50 60 70 80 90 100 110
LIQUID LIMIT LL
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