HomeMy WebLinkAboutREDTAIL RESIDENTIAL - FDP - 26-01A - SUBMITTAL DOCUMENTS - ROUND 3 - VARIANCE REQUEST (3)Geotechnical Engineering Supplemental Report Terracon
Retail — Proposed Mixed Use Development
Fort Collins, Larimer County, Colorado
Project No. 20015260
Page 6
and dewatering requirements are the responsibility of others. In the event that changes in the nature,
design, or location of the project as outlined in this report are planned, the conclusions and
recommendations contained in this report shall not be considered valid unless Terracon reviews the
changes and either verifies or modifies the conclusions of this report in writing.
We appreciate the opportunity to be of service to you on this phase of your project. If you have any
questions concerning this report, or if we may be of further service to you, please do not hesitate to contact
us.
Sincerely,
�Z
David A. Ri
Geotechnical ' rtment Manager
Enclosures: Schematic Site Plan of Proposed Roadway Alignment from North Star Design, Inc.
Schematic Cross -Sectional Diagram of Proposed Roadway from North Star Design, Inc.
Figure No. 1 — Test Boring Location Diagram — Terracon Initial Report dated July 2, 2002
Revised Site Plan — Figure No. 1 Illustrating New Layout
Test Boring No. 1 — Drilled December 4, 2001 and Piezometer No. 5 Drilled July 8, 2002
Copies: Addressee (3)
North Star Design (2): Patricia Kroetch, P.E.
Weeks and Associates (1): Mr. Gary Weeks, P.E.
Geotechnical Engineering Supplemental Report
Retail — Proposed Mixed Use Development
Fort Collins, Larimer County, Colorado
Project No. 20015260
Page 5
Terracon
Fill against grade beams and retaining walls should be compacted to densities specified in Earthwork.
Compaction of each lift adjacent to walls should be accomplished with hand -operated tampers or other
lightweight compactors. Overcompaction may cause excessive lateral earth pressures, which could result in
wall movement. For retaining walls exceeding 8-feet in heights, Terracon should be notified so additional
recommendations can be provided, if necessary.
Retaining Wall Drainage
To reduce hydrostatic loading on retaining walls, a subsurface drain system should be placed behind the
wall, The drain system should consist of free -draining granular soils containing less than five- percent fines
(by weight) passing a No. 200 sieve placed adjacent to the wall. The free -draining granular material should
be graded to prevent the intrusion of fines or encapsulated in a suitable filter fabric. A drainage system
consisting of either weep holes or perforated drain lines (placed near the base of the wall) should be used to
intercept and discharge water, which would tend to saturate the backfill. Where used, drain lines should be
embedded in a uniformly graded filter material and provided with adequate clean -outs for periodic
maintenance. An impervious soil should be used in the upper layer of backfill to reduce the potential for
water infiltration. As an alternative, a prefabricated drainage structure, such as geocomposite, may be used
as a substitute for the granular backfill adjacent to the wall.
GENERAL COMMENTS
Terracon should be retained to review the final design plans and specifications so comments can be made
regarding interpretation and implementation of our geotechnical recommendations in the design and
specifications. Terracon also should be retained to provide testing and observation during excavation,
grading, foundation and construction phases of the project_
The analysis and recommendations presented in this report are based upon the data obtained from the
borings performed at the indicated locations and from other information discussed in this report. This
report does not reflect variations that may occur between borings, across the site, or due to the modifying
effects of weather. The nature and extent of such variations may not become evident until during or after
construction. If variations appear, we should be immediately noted so that further evaluation and
supplemental recommendations can be provided.
The scope of services for this project does not include either specifically or by implication any
environmental assessment of the site or identification of contaminated or hazardous materials or
conditions. If the owner is concerned about the potential for such contamination, other studies should be
undertaken.
This report has been prepared for the exclusive use of our client for specific application to the project
discussed and has been prepared in accordance with generally accepted geotechnical engineering
practices. No warranties, either express or implied, are intended or made. Site safety, excavation support,
Geotechnical Engineering Supplemental Report
Retail — Proposed Mixed Use Development
Fort Collins, Larimer County, Colorado
Project No. 20015260
Page 4
Terracon
The geotechnical engineer should observe foundation excavations to verify compliance with the project
specifications. If the soil conditions encountered differ significantly from those presented in this report,
supplemental recommendations will be required.
Lateral Earth Pressures — (for wall less than 8-feet in height)
For soils above any free water surface, recommended equivalent fluid pressures for unrestrained foundation
elements are:
Active:
Cohesive soil backfill (on -site clay) ....................................................... 50 psf/ft
Cohesionless soil backfill (imported granular material) ........................ 35 psf/ft
On -site bedrock materials .........................................not recommended for use
Pc,ssive:
Cohesive soil backfill (on -site clay).....................................................250 psf/ft
Cohesionless soil backfill (imported granular material) ...................... 350 psf/ft
Where the design includes restrained elements, the following equivalent fluid pressures are recommended:
• At rest:
Cohesive soil backfill (on -site clay) ....................................................... 65 psf/ft
Cohesionless soil backfill (imported granular material) ........................ 50 psf/ft
On -site bedrock materials ......................................... not recommended for use
The lateral earth pressures presented above do not include any factor of safety and are not applicable for
submerged soils/hydrostatic loading. For submerged conditions the following lateral earth pressures should
be applied:
• Active: Submerged Soils/Hydrostatic Loading
Cohesive soil backfill (on -site clay) ....................................................... 90 psf/ft
Cohesionless soil backfill (on -site or imported granular material)........ 80 psf/ft
Passive: Submerged Soils/Hydrostatic Loading
Cohesive soil backfill (on -site clay).....................................................125 psf/ft
Cohesionless soil backfill (imported granular material) ...................... 175 psf/ft
• At rest: Submerged Soils/Hydrostatic Loading
Cohesive soil backfill (on -site clay).......................................................95 psf/ft
Cohesionless soil backfill (imported granular material)........................90 psf/ft
r
Geotechnical Engineering Supplemental Report Terracon
Retail — Proposed Mixed Use Development
Fort Collins, Larlmer County, Colorado
Project No. 20015260
Page 3
jurisdictional City of Fort Collins roadway. Therefore, after completion of the embankment fill, and "rough -
final subgrade" preparation a follow up geotechnical engineering exploration should be completed in general
accordance with the Larmer County Urban Area Street Standard (LCUASS) Pavement Design Criteria. For
estimating/planning or budget purposes, the recommendations for "main -traffic" corridors contained in our
original report could be used, which indicated a minimum of 4-inches of Hot Mix Asphalt (HMA) underlain by
a minimum of 7-inches of aggregate base course. Proofrolling and recompacting the subgrade is
recommended immediately prior to placement of the aggregate road base section. Soft or weak areas
delineated by the proofrolling operations should be undercut or stabilized in -place to achieve the
appropriate subgrade support.
General "earthwork" recommendations for the project pertaining to site and subgrade preparation, imported
materials, placement and compaction criteria and excavation construction procedures are included in
original report under the "Earthwork" section on page 11 through 13.
Slope Protection
The slopes of these two embankments should be protected from wave action, wind erosion and general
weathering. It is recommended the face of the embankments be rip -rapped, consisting of a durable rock
adequately sized or an equivalent such as a fabric map to reduce erosion. The dprap should be properly
bedded with gravel or separated from the embankment by filter fabric to minimize the washing out of fines
below the riprap. If elected, the fabric mat should be installed in accordance with the manufacture's
recommendations. The top of the embankment slope should be provided with a minimum of 6 inches of
gravel base course surfacing for future truck access.
Foundation Systems - Spread Footings or Pads for Box Culvert, Wing Walls and Retaining Walls
Based on the anticipated loads for the proposed concrete box culvert structure and associated wing walls,
and any retaining wall for this portion of the project, it is our opinion the use of a spread footing/pad
foundation system bearing upon approved embankment fill material may be utilized to support these
structural elements. Footings should be placed on a minimum 3-foot layer of approved fill material placed
and compacted as previously described. The subsurface soils at or near the groundwater interface may be
soft or loose, and compressible or settlement prone, exhibiting a relatively low bearing capacity and
potentially unstable characteristics. Therefore, it may be necessary to stabilize the bearing stratum with
either 1-1/2-inch clean graded aggregate or 3-inch minus pit -run material.
All footings bearing on a minimum 3-foot layer of approved engineered fill material could be design using a
maximum net allowable bearing pressure of 2,000 psf. Footings should be proportioned to reduce differential
foundation movement. Proportioning on the basis of equal total movement is recommended; however,
proportioning to relative constant dead -load pressure will also reduce differential settlement between
adjacent footings. Total movement resulting from the assumed structural loads is estimated to be on the
order of 1-inch. Exterior footings should be placed a minimum of 30 inches below finished grade for frost
protection and to provide confinement for the bearing soils. Finished grade is the lowest adjacent grade for
perimeter footings.
Geotechnical Engineering Supplemental Report
Retail — Proposed Mixed Use Development
Fort Collins, Larimer County, Colorado
Project No. 20015260
Page 2
Terracon
was encountered at approximate depths of '/2 to 7-1/2-feet below site within these two (2) locations or
approximate mean sea level elevation of 4984 to 4987.
The stratification boundaries shown on the enclosed bon ng/piezometer logs represent the approximate
locations of changes in soil types; in -situ, the transition of materials may be gradual. The soil profile for this
study is generally consistent with the soil and bedrock conditions presented in our previous subsurface
exploration report.
GROUNDWATER CONDITIONS
Groundwater was encountered in these two borings/piezometers at approximate depths of 6 to 7-1/2-feet
below existing site grades or approximate elevations of 4983 to 4984. These observations represent only the
current groundwater conditions, and may not be indicative of other times or at other locations. Groundwater
levels can be expected to fluctuate with varying seasonal and weather conditions, as well as volume of each
pond.
GEOTECHNICAL ENGINERING RECOMMENDATION
• Earthwork Embankment Recommendations for Roadway Alignment Within Pond In -Fill
The civil engineering consultants for the project, North Star Design, and the project developer, Lagunitas
Companies, have requested Terracon to provide supplemental geotechnical engineering recommendations
for earthen embankments to be installed/constructed within the northwestern pond as depicted in the
attached schematic sent to Terracon via electronic mail from North Star Design. The embankment fill
materials should consist of relatively homogeneous material which is placed and compacted in uniform lifts,
moisture conditioned to plus or minus 2 percent of optimum moisture content and mechanically compacted to
at least 98 percent of Standard Proctor Density (SPD) ASTM D698.
Temporary dewatering and stabilization of the underlying soft/compressible soils within the existing pond will
be necessary. For stabilization purposes, we suggest excavating approximately 2-feet into the underlying
bedrock formation to allow for placement of additional fill material. After temporary dewatering methods are
in -place and the excavation into the bedrock has been completed we recommend placement of a 12 to 18-
inch layer of granular fill material such as 3-inch minus pit run or 1-1/2-inch washed aggregate as a
stabilization mechanism. It may also be necessary to place a geogrid geotextile in combination with the
structural fill/stabilization material. The need for geotextle as well as the stabilization zone will need to be
determined based upon field conditions; however, the contract documents should contain a budget provision
if this procedure is necessary.
The embankment fill material should consist of an approved fill material and should be placed in uniform 8 to
12-inch lifts; moisture conditioned to plus or minus 2 percent of optimum moisture content, and mechanically
compacted to 98 percent of SPD ASTM D698. During the placement and compaction procedures field
density tests should be taken to verify compliance. The proposed embankments should be constructed on
minimum slopes of 2-1/2:1 (horizontal to vertical) or flatter on both the upstream and downstream faces.
Additional pavement recommendations for this portion of the site have been provided in the original
geotechnical engineering report on Page 8. It is our understanding the internal roadway will be a
August 12, 2005 Irerracon
Consulting Engineers & Scientists
Terracon Consultants, Inc
Lagunitas Company 301 North Howes
3944 JFK Parkway —Suite B200 Fort Collins. Colorado 80521Phone 970.484,0359
Fort Collins, Colorado 80525 Fax 970.484,0454
www.terracon.com
Attn: Mr. Jon Prouty
Re: Geotechnical Engineering Report — Addendum No. 2
Retail - Proposed Mixed Use Development - Cameron Park 2nd Filing
Supplemental Recommendations for Realignment of Roadway — Northwest Portion of Site
Fort Collins, Larimer County, Colorado
Terracon Project No. 20015260
Terracon conducted a geotechnical engineering exploration for Retail Property, a proposed mixed -use
development project, located west of the Cameron/Fossil Creek Office Complex and College Avenue/US
Highway 287, south of Fairway Lane, and east of the Burlington Northern & Santa Fe Railroad tracks in
south Fort Collins, in December of 2001. The primary purpose of the initial study was to evaluate the site
for soil, bedrock and groundwater conditions; as well as to provide foundation, pavement, and earthwork
design recommendations based on the subsurface conditions encountered. For further information and
findings thereof, please refer to our "Geotechnical Engineering Report' December 20, 2001, Project No.
20015260. Terracon also provided a supplemental report dated July 26, 2002, in which additional
information in regards to groundwater conditions relative to the adjacent ponds were addressed as well as
recommendations for earthen embankments for a waterline and a roadway through the central portion of
the site were provided. On July 23, 2003, we provided an addendum report which provided additional
information regarding proposed finished floor and foundation grade elevations relevant to the groundwater
level encountered throughout the site.
We have recently been requested to provide supplemental geotechnical engineering design criteria for the
proposed roadway alignment to be constructed along the western perimeter of the site which also
traverses through an existing detention pond. The proposed intent is to earthen fill the central portion of
this pond, creating two ponds connected by an approximate 3-foot by 5-foot concrete box culvert.
SUBSURFACE CONDITIONS
During our initial subsurface exploration activities, two (2) test borings were drilled in the general vicinity of
the proposed "pond in -fill' roadway alignment area. Boring No. 1, drilled on December 4, 2001 to an
approximate depth of 14-1/2-feet below site grade was located on the north side of the pond, while
Boring/Piezometer No. 5, drilled on July 8, 2002, to an approximate depth of 12-feet below site grades was
located to the south of pond. The subsurface soils encountered within these two related borings or
piezometers generally consisted of an approximate 3-foot layer of existing fill material at the surface of Boring
No. 1, underlain by the native sandy lean clay which extended to the bedrock below. At the surface of
Piezometer No. 5 was an approximate 6-inch layer of silty topsoil underlain by the weathered sandstone
bedrock below. Weathered sandstone bedrock transitioning to competent bedrock with increased depths
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August 11, 2005
Ms. Susan Joy
City of Fort Collins Engineering
281 North College Avenue
PO Box 580
Fort Collins, Colorado 50522-0580
Re: Redtail Request for Variance #7 — Side slope in ROW
Proj : 173-03
Dear Susan:
The following is a request for variance from Section 7.7.2 — Grading within Right -of -Way,
Paragraph A — Slopes, in the "Larimer County Urban Area Street Standards, October 1, 2002".
This variance request is to allow the use of a side slope of 2.7:1 on the east and west sides of
Conejos Road where it crosses the existing pond.
The construction of Conejos Road will split the existing pond into two halves. The water surface
area of the pond lost with the construction of this road crossing is required to be mitigated on a
1:1 ratio. A portion of the water surface area lost will be regained by enlarging the west half of
the existing pond to the north. In order to achieve adequate water surface area to meet the
mitigation requirement, the side slopes must be placed at a maximum of 2.7:1.
The following precautions will be taken to mitigate any safety concerns caused by the increase
side slope:
• Along Conejos Road where it crosses the existing pond a wall and a structural handrail
will be installed which will act as a guardrail for vehicular traffic.
• Access to this area will be deterred by the construction of the wall and by the planting of
short junipers near the top of the side slope and wetland grasses in the remaining areas.
This variance from the above Standard will not be detrimental to the public health. Issues that
may compromise public safety and welfare have been addressed to the greatest extent possible.
Please call me with any questions or additional information that you may need for the approval of
this variance. , A
Sincerely,
Troy Spra
North Stai
700 Automation Drive, Unit I Windsor, Colorado 80550
970-686-6939 Phone • 970-686-1 1 88 Fax