HomeMy WebLinkAboutFOSSIL CREEK ESTATES PUD FIRST FILING FINAL - 50 92F - SUBMITTAL DOCUMENTS - ROUND 1 - GEOTECHNICAL (SOILS) REPORTi
GEOTECHNICAL ENGINEERING REPORT
FOSSIL CREEK ESTATES PHASE I
SOUTH SHIELDS STREET
FORT COLLINS, COLORADO
ELI PROJECT NO. 20945036
A Division of
The Terracon Companies, Inc.
Terracon
Fossil Creek Partners
ELI Project No. 20945036
Mapping completed by the Colorado Geological Survey ('Hart, 1972), indicates the site in an area
of "Moderate Swell Potential". Potentially expansive materials mapped in this area include
bedrock, weathered bedrock and colluvium (surficial units).
Soil and Bedrock Conditions: As presented on the Logs of Boring, the subsurface soils were
encountered in order of increasing depths as follow:
• Silty Toosoil: The majority of the site is overlain by a 6-inch layer of silty topsoil. The
topsoil has been penetrated by root growth and organic matter.
• Pavement: Four and one-half (4'%) to 5 inches of asphalt underlain by 11 to 18 inches of
gravel base course were encountered in the two borings drilled through South Shields
Street along .the east edge of the property.
• Fill Material: A layer of fill underlies the pavement in Borings 9 and 10 and extends to
depths of 3'/2 to 6 feet. The fill consists of a mixture of clayey sand with minor amounts
of gravel. The fill is moist and loose to medium dense.
• Lean Clay with Sand: This stratum underlies the topsoil in Borings 1 through 8 and below
the fill in Borings 9 and 10 and extends to the depths explored and/or the bedrock below.
The lean clay with sand is generally moist and medium to very stiff in consistency.
• Sandstone-Siltstone-Claystone Bedrock: The bedrock was encountered in Borings 1, 2, 4
and 6 through 10 at depths of 4'/z to 12'/z feet and extends to greater depths. The upper
2 to 4'/z feet of the bedrock is highly weathered; however, the underlying sandstone and
siltstone is firm to hard.
Laboratory Test Results: Laboratory test results indicate that the clay subsoils at shallow depth
have low to moderate expansive potential. The siltstone-claystone bedrock exhibits moderate to
high expansive potential. The clay soils at anticipated foundation bearing depth have moderate
bearing capacities and the bedrock has high bearing capacity.
Groundwater Conditions: Groundwater was not encountered in the test borings at the time of
field exploration, nor when checked one day after drilling. Borings 9 and 10 were filled in
immediately after drilling for safety reasons. These observations represent only current
groundwater conditions, and may not be indicative of other times, or at other locations. The
'Hart, Stephen S., 1972, Potentia9y Swelling Sal and Rock in the Front Range Urban Corridor, Colorado, Colorado
Geological Survey, Environmental Geology No. 7.
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Fossil Creek Partners Terracon
ELI Project No. 20945036
preliminary report prepared for the site on August 28, 1992 indicates groundwater was
encountered in a portion of the site at depths of 2 to 5'/z feet below the surface. Groundwater
levels can be expected to fluctuate with varying seasonal and weather conditions.
Based upon review of U.S. Geological Survey Maps (2Hillier, et al, 1983), regional groundwater
beneath the project area predominates in colluvial, landslide or windblown materials, or in fractured
weathered consolidated sedimentary bedrock located at a depth near ground surface. Seasonal
variations in groundwater conditions are expected since the aquifer materials may not be
perennially saturated. Groundwater is generally encountered at depths ranging from 5 to 20 feet
below ground surface; depth to seasonal groundwater is generally 10 feet -or less.
Zones of perched and/or trapped groundwater may also occur at times in the subsurface soils
overlying bedrock, on top of the bedrock surface or within permeable fractures in the bedrock
materials. The location and amount of perched water is dependent upon several factors, including
hydrologic conditions, type of site development, irrigation demands on or adjacent to the site,
fluctuations in water features, seasonal and weather conditions.
Fluctuations in groundwater levels can best be determined by implementation of a groundwater
monitoring plan. Such a plan would include installation of groundwater monitoring wells, and
periodic measurement of groundwater levels over a sufficient period of time.
The possibility of groundwater fluctuations should be considered when developing design and
construction plans for the project.
CONCLUSIONS AND RECOMMENDATIONS
Site Development Considerations: The site appears suitable for the proposed construction.
Potentially expansive soils and bedrock will require particular attention in the design and
construction.
Because of variations in the engineering properties of the on -site soils, foundation bearing levels,
structural loads, and possible final grades, the following foundation systems were evaluated for
use on the site:
'Hillier, Donald E.; Schneider, Paul A., Jr.; and Hutchinson, E. Carter, 1983, Depth to Water Table 11979) in the Boulder -Fort
Collins -Greeley Area, Front Range Urban Corridor. Colorado, United States Geological Survey, Map 1-855-I.
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Terracon
Fossil Creek Partners
ELI Project No. 20945036
• spread footings and/or grade beams bearing on undisturbed soils;
• spread footings and/or grade beams bearing on engineered fill;
• grade beams and straight -shaft piers drilled to the bedrock.
Design criteria for alternate foundation systems is subsequently outlined. Use of the alternative
foundation systems outlined in this report should be determined prior to construction.
Slab -on -grade construction, for basement or garage areas, is considered acceptable for use when
subgrade soils consist of the on -site clays, provided that design and construction
recommendations are followed. Given the engineering characteristics of the bedrock stratum,.
consideration should be given to use of structural floor systems where the slabs are placed in or
within 3 feet of the bedrock stratum. Slabs placed in or within 3 feet of the bedrock should be
provided with a complete dewatering system.
Foundation Systems: Due to the presence of low- to moderate -swelling soils on the site, spread
footing and/or grade beam foundations bearing upon undisturbed subsoils and/or engineered fill
are recommended for support for proposed structures placed a minimum of 3 feet above the
bedrock stratum. The footings may be designed for a maximum bearing pressure of 2,000 psf.
In addition, the footings should be sized to maintain a minimum dead -load pressure of 500 psf.
The design bearing capacities apply to dead loads plus design live load conditions. The design
bearing capacity may be increased by one-third when considering total loads that include wind or
seismic conditions. Exterior footings should be placed a minimum of 30 inches below finished
grade for frost protection. Interior footings should bear a minimum of 12 inches below finished
grade.
Existing fill on the site should not be used for support of foundations without removal and
recompaction.
Footings should be proportioned to minimize 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 settlement between adjacent footings. Total or
differential settlement resulting from the assumed structural loads are estimated to be on the order
of 3/4 inch or less, provided that foundations are constructed as recommended.
Additional foundation movements could occur if water from any source infiltrates the foundation
soils; therefore, proper drainage should be provided in the final design and during construction.
For foundations adjacent to slopes, a minimum horizontal setback of five (5) feet should be
maintained between the foundation base and slope face. In addition, the setback should such that
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an imaginary line extending downward at 45 degrees from the nearest foundation edge does not
intersect the slope.
Foundations and masonry walls should be reinforced as necessary to reduce the potential for
distress caused by differential foundation movement. The use of joints at openings or other
discontinuities in masonry walls is recommended.
Foundation excavations should be observed by the geotechnical engineer. If the soil conditions
encountered differ significantly from those presented in this report, supplemental recommendations
will be required.
Alternate Foundation Systems: Structures founded in or within 3 feet of the bedrock stratum
should be supported by a grade beam and drilled pier foundation system. Straight shaft piers,
drilled a minimum of 5 feet into firm or harder bedrock, with a minimum shaft length of 10 feet
are recommended.
For axial compression loads, piers may be designed for a maximum end -bearing pressure of
15,000 pounds per square foot (psf), and skin friction of 1,500 psf for the portion of the pier in
firm or harder bedrock.
Required pier penetration should be balanced against potential uplift forces due to expansion of
the subsoils and bedrock on the site. For design purposes, the uplift force on each pier can be
determined on the basis of the following equation:
l/P = 20 x D
Where: UP = the uplift force in kips, and
D = the pier diameter in feet
Uplift forces on piers should be resisted by a combination of dead -load and pier penetration below
a depth of 5 feet and in the bearing strata.
All piers should be reinforced full depth for the applied axial, lateral and uplift stresses imposed.
The amount of reinforcing steel for expansion should be determined by the tensile force created
by the uplift force on each pier, with allowance for dead -load. Minimum reinforcement of at least
one percent of the cross -sectional area of each pier should be specified.
To reduce potential uplift forces on piers, use of long grade beam spans to increase individual pier
loading, and small diameter piers are recommended. For this project, use of a minimum pier
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ELI Project No. 20945036
diameter of 10 inches is recommended. A minimum 4-inch or greater void space should be
p provided beneath grade beams between piers. The void material should be of suitable strength
H to support the weight of fresh concrete used in grade beam construction, and to avoid collapse
when foundation backfill is placed.
Drilling to design depths should be possible with conventional single flight power augers.
' Groundwater conditions indicate that temporary steel casing may be required to properly drill and
clean piers prior to concrete placement. Groundwater should be removed from each pier hole prior
to concrete placement. Pier concrete should be placed immediately after completion of drilling and
cleaning. If pier concrete cannot be placed in dry conditions, a tremie should be used for concrete
placement. Due to potential sloughing and raveling, foundation concrete quantities may exceed
%> calculated geometric volumes.
If casing is used for pier construction, it should be withdrawn in a slow continuous manner
maintaining a sufficient head of concrete to prevent infiltration of water or the creation of voids
in pier concrete. Pier concrete should have a relatively high fluidity when placed in cased pier
holes or through a tremie. Pier concrete with slump in the range of 5 to 7 inches is recommended.
Free -fall concrete placement in piers will only be acceptable if provisions are taken to avoid striking
the concrete on the sides of the hole or reinforcing steel. The use of a bottom -dump hopper, or
an elephant's trunk discharging near the bottom of the hole where concrete segregation will be
minimized, is recommended.
To provide increased resistance to potential uplift forces, the sides of each pier should be
mechanically roughened in the bearing strata. This should be accomplished by. a roughening tooth
placed on the auger. Pier bearing surfaces must be cleaned prior to concrete placement. A
representative of the geotechnical engineer should inspect the bearing surface and pier
configuration.
Basement Construction: Groundwater was not encountered on the site to a maximum depth of
15 feet below existing grade at the time of the site exploration. However, groundwater was
encountered at shallow depths at the time when the preliminary geotechnical report was prepared
in August of 1992. This appears to indicate that water levels are influenced by irrigation and
seasonal variations. Full -depth basement construction is considered feasible on the site provided
that basement subgrade is placed 3 feet above the bedrock and/or groundwater.
Z Perched groundwater may occur at times since the subsurface soils are relatively impermeable and
tend to trap water. Completion of site development, including installation of landscaping and
11 irrigation systems, will likely lead to perched groundwater development.
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Fossil Creek Partners
ELI Project No. 20945036
To reduce the potential for perched groundwater to impact foundation bearing soils and enter the
basement of the structure, installation of a perimeter drainage system is recommended. The
drainage system should be constructed around the exterior perimeter of the basement foundation,
and sloped at a minimum 1 /8 inch per foot to a suitable outlet, such as a sump and pump system.
The drainage system should consist of a properly sized perforated pipe, embedded in free -draining
gravel, placed in a trench at least 12-inches in width. Gravel should extend a minimum of 3-inches
beneath the bottom of the pipe, and at least 2 feet above the bottom of the foundation wall and
extending at least to the edge of the backfill zone. The gravel should be covered with drainage
fabric prior to placement of foundation backfill.
Construction of area drains below or adjacent to sewers will help lower water levels at the site.
Area drains should be designed for the subsurface conditions, have a suitable outlet and be
approved by the City of Fort Collins.
Seismic Considerations: The project site is located in Seismic Risk Zone I, of the Seismic Zone
Map of the United States as indicated by the Uniform Building Code. Based upon the nature of
the subsurface materials, a seismic site coefficient, "s" of 1.0 should be used for the design of
structures for the proposed project (Uniform Building Code, Table No. 23-J).
Floor Slab Design and Construction: The variability of the existing soils at approximate slab
subgrade elevation could result in differential movement of floor slab -on -grade should expansive
siltstone-claystone bedrock become elevated in moisture content. Use of structural floor systems
where slabs are placed in or within 3 feet of the siltstone-claystone bedrock, structurally
supported independent of the subgrade soils, is a positive means of eliminating the potentially
detrimental effects of floor movement.
If slab -on -grade is utilized, the subgrade soils should be prepared as outlined in the "Earthwork"
section of this report.
Additional floor slab design and construction recommendations are as follows:
• Positive separations and/or isolation joints should be provided between slabs and
all foundations, columns or utility lines to allow independent movement.
• Contraction joints should be provided in slabs to control the location and extent of
cracking. Maximum joint spacing of 15 to 20 feet in each direction is
recommended.
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Fossil Creek Partners
ELI Project No. 20945036
• A minimum 2-inch void space should be constructed above, or below non -bearing
partition walls placed on the floor slab founded in or within 3 feet of the bedrock
stratum. Special framing details should be provided at door jambs and frames
within partition walls to avoid potential distortion. Partition walls should be isolated
from suspended ceilings.
• Interior trench backfill placed beneath slabs should be compacted in accordance
with recommended specifications outlined below.
• In areas subjected to normal loading, a minimum 4-inch layer of clean -graded gravel
should be placed beneath interior slabs.
• A minimum 8-inch layer of free -draining gravel should be placed beneath basement
floor slabs in conjunction with the underslab drainage system.
• If moisture sensitive floor coverings are used on interior slabs, consideration should
be given to the use of barriers to minimize potential vapor rise through the slab.
• Floor slabs should not be constructed on frozen subgrade.
• Other design and construction considerations, as outlined in the ACI Design Manual,
Section 302.1 R are recommended.
Pavement Design and Construction: Design of pavements for the project have been based on the
procedures outlined in the 1986 Guideline for Design of Pavement Structures by the American
Association of State Highway and Transportation Officials (AASHT0). Areas within proposed
pavements on the site will be divided into two categories based upon anticipated traffic and usage.
Traffic criteria provided for pavement thickness designs include 18-kip equivalent single axle loads
(ESAL's) of 36,500 for residential streets, 182,500 for collector streets, and 1,095,000 for South
Shields Street, which is an arterial street.
Based upon AASHTO criteria, Colorado is located within Climatic Region VI of the United States.
This region is characterized as being dry with hard ground freeze and spring thaw. The spring
thaw condition typically results in saturated or near -saturated subgrade soil moisture conditions.
The AASHTO criteria suggests that these moisture conditions are prevalent for approximately 12-
1/2% of the annual moisture variation cycle.
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ELI Project No. 20945036
Local drainage characteristics of proposed pavements areas are considered to vary from fair to
good depending upon location on the site. For purposes of this design analysis, fair drainage
characteristics are considered to control the design. These characteristics, coupled with the
approximate duration of saturated subgrade conditions, results in a design drainage coefficient of
1.0 when applying the AASHTO criteria for design.
For flexible pavement design, terminal serviceability indices of 2.0 for residential streets and 2.5
for collector and arterial streets were utilized along with an inherit reliability of 70% for residential
streets, 85% for collector streets, and 90% for arterial streets. Using the correlated design R-
values of 5 for internal residential and collector streets and 9 for South Shields Street, appropriate
'+ ESAL/day, environmental criteria and other factors, the structural numbers (SN) of the pavement
sections were determined on the basis of the 1986 AASHTO design equation.
In addition to the flexible pavement design analyses, a rigid pavement design analysis was
completed, based upon AASHTO design procedures. Rigid pavement design is based on an
evaluation of the Modulus of Subgrade Reaction of the soils (K-value), the Modulus of Rupture of
the concrete, and other factors previously outlined. The design K-value of 100 for the subgrade
)V soil was determined by correlation to the laboratory tests results. A modulus of rupture of 650
psi (working stress 488 psi) was used for pavement concrete. The rigid pavement thicknesses
for each traffic category were determined on the basis of the AASHTO design equation.
Recommended alternatives for flexible and rigid pavements, summarized for each traffic area, are
as follows:
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Fossil Creek Partners
ELI Project No. 20945036
Recommended Pavement Section Thickness (inches)
Asphalt
Concrete
Surface
Aggregate
Base
Course
Select
Subbase
Plant Mix
Bituminous
Base
Portland
Cement
Concrete
Total
Traffic
Area
Alter-
native
A
3"
8„
11"
B
2"
4"
6"
Residential
Streets
C
6"
6"
A
4"
8"
5"
17"
B
4"
4"
8"
Collector
Streets
C
6,/2 "
6'/x "
A
4"
8"
11"
23"
B
4"
6"
10"
Arterial
Streets
C
8"
8"
Overlay
A
2%z"
2Y="
Where grades allow, the existing pavement section of South Shields Street will require a 2'/2 inch
overlay of asphalt concrete. Prior to placement of the overlay, the existing pavement should be
thoroughly cleaned, the joints sealed and a tack coat applied.
Each alternative should be investigated with respect to current material availability and economic
conditions. In view of the subgrade soil conditions and projected traffic, either full -depth asphalt
or rigid concrete pavement sections should be considered in areas of main traffic corridors, drive
bays or truck access. Rigid concrete pavement is recommended at the location of dumpsters
where trash trucks will park and load.
Aggregate base course (if used on the site) should consist of a blend of sand and gravel which
meets strict specifications for quality and gradation. Use of materials meeting Colorado
Department of Transportation Class 5 or 6 specifications is recommended. Select subbase shall
meet Colorado Department of Transportation Class 1 specifications.
In addition, the base course material should be moisture stable. Moisture stability is determined
by R-value testing which shows a maximum 12 point difference in R-values between exudation
pressures of 300 psi and 100 psi. Aggregate base course material should be tested to determined
compliance with these specifications prior to importation to the site.
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ELI Project No. 20945036
Aggregate base course should be placed in lifts not exceeding six inches and should be compacted
to a minimum of 95% Standard Proctor density (ASTM D698), within a moisture content range
of 2 percent below to 2 percent above optimum. Where base course thickness exceeds 6 inches,
the material should be placed and compacted in two or more lifts of equal thickness.
Asphalt concrete should be obtained from an approved mix design stating the Hveem properties,
optimum asphalt content, job mix formula, and recommended mixing and placing temperatures.
Aggregate used in asphalt concrete should meet a particular gradation. Use of materials meeting
Colorado Department of Transportation Grading C or CX specification is recommended. The mix
design should be submitted prior to construction to verify its adequacy. The asphalt materials
should be placed in maximum 3-inch lifts and should be compacted to a minimum of 95% Hveem
density (ASTM D 1561).
Plant -mixed bituminous base course should be composed of a mixture of aggregate, filler and
additives if required and approved bituminous material. The bituminous base should conform to
an approved mix design stating the Marshall or Hveem properties, optimum asphalt content, job
mix formula, and recommended mixing and placing temperatures. Aggregate used in plant -mixed
bituminous base course should meet a particular gradation. Use of aggregates meeting Colorado
Department of Transforation Grading G or C specifications is recommended. The mix design
should be submitted prior to construction to verify it adequacy. The asphalt material should be
placed in maximum 3-inch lifts, and should be compacted to a minimum of 95% Hveem density
(ASTM D 1561).
Where rigid pavements are used, the concrete should be obtained from an approved mix design
with the following minimum properties:
• Modulus of Rupture @ 28 days ........................ 650 minimum
• Strength Requirements ................................ ASTM C94
• Minimum Cement Content ......................... 5.5 sacks/cu. yd.
• Cement Type .................................... Type I Portland
• Entrained Air Content ................................... 6 to 8%
• Concrete Aggregate ................. ASTM C33 and CDOT Section 703
• Aggregate Size ................................. 1 inch maximum
• Maximum Water Content ....................... 0.49 lb/lb of cement
• Maximum Allowable Slump ............................... 4 inches
Concrete should be deposited by truck mixers or agitators and placed a maximum of 90 minutes
from time the water is added to the mix. Other specifications outlined by the Colorado
Department of Transportation should be followed.
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GEOTECHNICAL ENGINEERING REPORT
FOSSIL CREEK ESTATES PHASE I
SOUTH SHIELDS STREET
FORT COLLINS, COLORADO
ELI PROJECT NO. 20945036
Prepared for.
FOSSIL CREEK PARTNERS
363 WEST DRAKE ROAD
FORT COLLINS, COLORADO 80526
ATTN: MR. CARTER EWING
Empire Laboratories, Inc.
A Division of the Terracon Companies, Inc.
Fossil Creek Partners Terracon
ELI Project No. 20945036
Longitudinal and transverse joints should be provided as needed in concrete pavements for
expansion/contraction and isolation. The location and extent of joints should be based upon the
final pavement geometry and should be placed (in feet), at roughly twice the slab thickness (in
inches), on center in either direction. Sawed joints should be cut within 24-hours of concrete
placement, and should be a minimum of 25% of slab thickness plus 1/4 inch. All joints should
be sealed to prevent entry of foreign material and dowelled where necessary for load transfer.
Where dowels cannot be used at joints accessible to wheel loads, pavement thickness should be
increased by 25 percent at the joints and tapered to regular thickness in 5 feet.
Future performance of pavements constructed on the clay soils at this site will be dependent upon
several factors, including:
• maintaining stable moisture content of the subgrade soils; and,
• providing for a planned program of preventative maintenance.
Since the clay soils on the site have shrink/swell characteristics, pavements could crack in the
future primarily because of expansion of the soils when subjected to an increase in moisture
content to the subgrade. The cracking, while not desirable, does not necessarily constitute
structural failure of the pavement.
The performance of all pavements, but in particular the recommended asphalt sections, can be
enhanced by minimizing excess moisture which can reach the subgrade soils. The following
recommendations should be considered at minimum:
0 Site grading at a minimum 2% grade away from the pavements;
• Compaction of any utility trenches for lands aped areas to the same criteria as the
pavement subgrade;
• Sealing all landscaped areas in or adjacent to pavements to minimize or prevent
moisture migration to subgrade soils;
• Placing compacted backfill against the exterior side of curb and gutter; and,
• Placing curb, gutter and/or sidewalk directly on subgrade soils without the use of
base course materials.
Preventative maintenance should be planned and provided for through an on -going pavement
management program in order to enhance future pavement performance. Preventative
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ELI Project No. 20945036
maintenance activities are intended to slow the rate of pavement deterioration, and to preserve
the pavement investment.
Preventative maintenance consists of both localized maintenance (e.g. crack sealing and patching)
and global maintenance (e.g. surface sealing). Preventative maintenance is usually the first priority
when implementing a planned pavement maintenance program and provides the highest return on
investment for pavements.
Recommended preventative maintenance policies for asphalt and jointed concrete pavements,
based upon type and severity of distress, are provided in Appendix D. Prior to implementing any
maintenance, additional engineering observation is recommended to determine the type and extent
of preventative maintenance.
Earthwork:
• General Considerations: The conclusions contained in this report for the proposed
construction are contingent upon compliance with recommendations presented in this
section.
Although fills or underground facilities, such as septic tanks, cesspools, basements, and/or
utilities, were not observed during site reconnaissance, such features might be encountered
during construction.
• Site Clearing:
1. Strip and remove existing vegetation, debris, and other deleterious materials from
proposed building and pavement areas. All exposed surfaces should be free of
mounds and depressions which could prevent uniform compaction.
2. If unexpected fills or underground facilities are encountered during site clearing,
such features should be removed and the excavation thoroughly cleaned prior to
backfill placement and/or construction. All excavations should be observed by the
geotechnical engineer prior to backfill placement.
3. Stripped materials consisting of vegetation and organic materials should be wasted
from the site or used to revegetate exposed slopes after completion of grading
operations. If it is necessary to dispose of organic materials on -site, they should be
placed in non-structural areas and in fill sections not exceeding 5 feet in height.
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ELI Project No. 20945036
Terracon
4. Sloping areas steeper than 3:1 (horizontal:vertical) should be benched to reduce the
potential for slippage between existing slopes and fills. Benches should be level and
wide enough to accommodate compaction and earth moving equipment.
5. The site should be initially graded to create a relatively level surface to receive fill
and to provide for a relatively uniform thickness of fill beneath proposed building
structures.
6. All exposed areas which will receive fill, once properly cleared and benched where
necessary, should be scarified to a minimum depth of twelve inches, conditioned
to near optimum moisture content, and compacted.
• Excavation:
1. It is anticipated that excavations for the proposed construction can be accomplished
with conventional earthmoving equipment.
2. Excavation penetrating the hard bedrock may require the use of specialized heavy-
duty equipment, such as a track -mounted backhoe.
3. Depending upon depth of excavation and seasonal conditions, groundwater may be
encountered in excavations on the site. 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.
• Slab Subgrade Preparation:
1. Where existing clay soils will support floor slab, the soils should be scarified,
moisture -conditioned and compacted to a minimum depth of 8 inches.
2. A minimum 4-inch layer of clean -graded gravel should be placed beneath slabs.
3. A minimum 8-inch layer of free -draining gravel should be placed beneath basement
floor slabs in conjunction with the underslab drainage system.
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• Pavement Subgrade Preparation:
1. The subgrade should be scarified, moistened as required, and recompacted for a
minimum depth of 8 inches prior to placement of fill and pavement materials.
2. On -site clay soils may pump or become unstable or unworkable at high water
contents. Workability may be improved by scarifying and drying. Overexcavation
of wet zones and replacement with granular materials may be necessary.
Lightweight excavation equipment may be required to reduce subgrade pumping.
Use of lime, fly ash, kiln dust, cement or geotextiles could also be considered as a
stabilization technique. Laboratory evaluation is recommended to determine the
effect of chemical stabilization on subgrade soils prior to construction.
• Fill Materials:
1. Clean on -site soils or approved imported materials may be used as fill material for
the following:
• general site grading • exterior slab areas
• foundation areas • pavement areas
• interior floor slab areas • foundation backfill
2. On -site bedrock materials are not recommended for use beneath structural ares of
the site or as backfill. Should bedrock materials be used for general site grading,
placement in fills at non-structural locations on the site is recommended.
3. Select granular materials should be used as backfill behind walls which retain earth.
4. Frozen soils should not be used as fill or backfill.
5. Imported soils (if required) should conform to the following:
• Gradation (ASTM C136):
percent finer by weight
6.. ................................................... 100
31................................................. 70-100
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ELI Project No. 20945036
No.4 Sieve .......................................... 50-100
No. 200 Sieve ....................................... 35 (max)
0 Liquid Limit ......................................... 35 (max)
• Plasticity Index ...................................... 15 (max)
6. Aggregate base should conform to Colorado Department of Transportation Class 5
or 6 specifications.
• Placement and Compaction:
1.
2.
3.
4.
Place and compact fill in horizontal lifts, using equipment and procedures that will
produce recommended moisture contents and densities throughout the lift.
Uncompacted fill lifts should not exceed 10 inches loose thickness.
No fill should be placed over frozen ground.
Materials should be compacted to the following:
Material
On -site soils:
- Minimum Percent
Compaction (ASTM D698)
Beneath foundations ............................ 95
13.mnnth QiahQ ............................ 95
Beneath pavements ............................
95
Utility trenches below building & paved areas ..........
95
Utility trenches below grassed areas .................
90
Imported fill:
Ranaath foundations ............................ 95
Beneath slabs ................................
95
Beneath pavements ..... .......................95
Utility trenches below building & paved areas ..........
95
Utility trenches below grassed areas .................
90
Miscellaneous backf ill ................................
90
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ELI Project No. 20945036
5. If a well defined maximum density curve cannot be generated by impact compaction
in the laboratory for any fill type, engineered fill should be compacted to a minimum
of 80 percent relative density by determined by ASTM D4253, D4254.
6. On -site clay soils should be compacted within a moisture content range of optimum
moisture to 2 percent above optimum. On -site clay below paved areas and
imported soils should be compacted within a moisture range of 2 percent below to
2 percent above optimum.
• Slopes:
1. For permanent slopes in compacted fill areas, recommended maximum
configurations for on -site materials are as follows:
Material
Maximum Slope
Horizontal:Vertical
Cohesive soils (clays) ........................... 2Y2:1
Bedrock ...................................... 2:1
If steeper slopes are required for site development, stability analyses should be
completed to design the grading plan.
2. The face of all slopes should be compacted to the minimum specification for fill
embankments. Alternately, fill slopes can be over -built and trimmed to compacted
material.
• Compliance: Recommendations for slabs -on -grade, foundations and pavement elements
supported on compacted fills or prepared subgrade depend upon compliance with
"Earthwork" recommendations. To assess compliance, observation and testing should be
performed under the direction of the geotechnical engineer.
• Excavation and Trench Construction: Excavations into the on -site soils will encounter a
variety of conditions. Excavations into the clays and bedrock can be expected to stand on
relatively steep temporary slopes during construction. However, caving soils may also be
encountered. The individual contractor(s) should be made responsible for designing and
constructing stable, temporary excavations as required to maintain stability of both the
19
Fossil Creek Partners Terracon
ELI Project No. 20945036
excavation sides and bottom. All excavations should be sloped or shored in the interest
RT of safety following local, and federal regulations, including current OSHA excavation and
U1 trench safety standards.
The soils to be penetrated by the proposed excavations may vary significantly across the
site. The preliminary soil classifications are based solely on the materials encountered in
m widely spaced exploratory test borings. The contractor should verify that similar conditions
•t exist throughout the proposed area of excavation. If different subsurface conditions are
encountered at the time of construction, the actual conditions should be evaluated to
determine any excavation modifications necessary to maintain safe conditions.
As a safety measure, it is recommended that all vehicles and soil piles be kept to a
minimum lateral distance from the crest of the slope equal to no less than the slope height.
The exposed slope face should be protected against the elements.
The contractor should retain a geotechnical engineer to monitor the soils exposed in all
excavations and provide engineering services for slopes. This will provide an opportunity
to monitor the soil types encountered and to modify the excavation slopes as necessary.
It also offers an opportunity to verify the stability of the excavation slopes during
construction.
Drainage:
• Surface Drainage:
1. Positive drainage should be provided during construction and maintained throughout
the life of the proposed residences. Infiltration of water into utility or foundation
excavations must be prevented during construction. Planters and other surface
features which could retain water in areas adjacent to the building or pavements
should be sealed or eliminated.
2. In areas where sidewalks or paving do not immediately adjoin the structure, we
recommend that protective slopes be provided with a minimum grade of
approximately 10 percent for at least 10 feet from perimeter walls. Backfill against
footings, exterior walls, and in utility and sprinkler line trenches should be well
compacted and free of all construction debris to reduce the possibility of moisture
infiltration.
20
Terracon
Fossil Creek Partners
ELI Project No. 20945036
3. Downspouts, roof drains or scuppers should discharge into splash blocks or
extensions when the ground surface beneath such features is not protected by
exterior slabs or paving.
4. Sprinkler systems should not be installed within 5 feet of foundation walls.
Landscaped irrigation adjacent to the foundation system should be minimized or
eliminated.
• Subsurface Drainage: Free -draining, granular soils containing less than five percent fines
(by weight) passing a No. 200 sieve should be placed adjacent to walls which retain earth.
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.
Additional Deshqnand Construction Considerations:
• Exterior Slab Design and Construction: Exterior slabs -on -grade, exterior architectural
features, and utilities founded on, or in backfill may experience some movement due to the
volume change of the backfill. Potential movement could be reduced by:
• minimizing moisture increases in the backfill
• controlling moisture -density during placement of backfill
• using designs which allow vertical movement between the exterior features
and adjoining structural elements
• placing effective control joints on relatively close centers
• allowing vertical movements in utility connections
o' • Corrosion Protection: Results of soluble sulfate testing indicate that ASTM Type I Portland
cement is suitable for all concrete on and below grade. However, if there is no, or minimal
cost differential, use of ASTM Type II Portland cement is recommended for additional
sulfate resistance of construction concrete. Foundation concrete should be designed in
17
J accordance with the provisions of the ACI Design Manual, Section 318, Chapter 4.
21
Fossil Creek Partners
ELI Project No. 20945036
GENERAL COMMENTS
Terracon
It is recommended that the Geotechnical Engineer be retained to provide a general review of final
design plans and specifications in order to confirm that grading and foundation recommendations
have been interpreted and implemented. In the event that any changes of the proposed project
are planned, the conclusions and recommendations contained in this report should be reviewed
and the report modified or supplemented as necessary.
The Geotechnical Engineer should also be retained to provide services during excavation, grading,
foundation and construction phases of the work. Observation of footing and/or pier excavations
should be performed prior to placement of reinforcing and concrete to conform that satisfactory
bearing materials are present and is considered a necessary part of continuing geotechnical
engineering services for the project. Construction testing, including field and laboratory evaluation
of fill, backfill, pavement materials, concrete and steel, should be performed to determine whether
applicable project requirements have been met. It would be logical for Empire Laboratories, Inc.
to provide these additional services since we are most qualified to determine consistency of field
conditions with those data used in our analyses.
The analyses and recommendations in this report are based in part upon data obtained from the
field exploration. The nature and extent of variations beyond the location of test borings may not
become evident until construction. If variations then appear evident, it may be necessary to
reevaluate the recommendations of this report.
Our professional services were performed using that degree of care and skill ordinarily exercised,
under similar circumstances, by reputable geotechnical engineers practicing in this or similar
localities. No warranty, express or implied, is made. We prepared the report as an aid in design
of the proposed project. This report is not a bidding document. Any contractor reviewing this
report must draw his own conclusions regarding site conditions and specific construction
techniques to be used on this project.
This report is for the exclusive purpose of providing geotechnical engineering and/or testing
information and recommendations. 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.
22
Empire Laboratories, Inc.
A Division of The Terracon Companies, Inc.
P.O. Box 503 • 301 No. Howes
Fort Collins, Colorado 80522
(303) 484-0359
FAX No. (303) 484-0454
Chester C. Smith, P.E.
Neil R. Sherrod, C.P.G.
March 24, 1994
Fossil Creek Partners
363 West Drake. Road
Fort Collins, Colorado 80526
Attn: Mr. Carter Ewing
Re: Geotechnical Engineering Report, Fossil Creek Estates Phase I
South Shields Street, Fort Collins, Colorado
ELI Project No. 20945036
Empire Laboratories, Inc. (ELI) has completed a geotechnical engineering exploration for the
proposed project to be located on South Shields Street in southwest Fort Collins, Colorado. This
study was performed in general accordance with our proposal number D2094025 dated January
21, 1994.
The results of our engineering study, including the boring location diagram, laboratory test results,
test boring records, and the geotechnical recommendations needed to aid in the design and
construction of foundations and other earth connected phases of this project are attached.
The subsurface exploration indicated conditions which are typical of soils commonly found in the
southwest Fort Collins area. The subsurface, soils at the site consisted of lean clay with sand
underlain by claystone-siltstone-sandstone bedrock. Clayey sand fill was encountered below the
existing pavement at the site. The information obtained by the results of field exploration and
laboratory testing completed for this study indicates that the soils at the site have low to moderate
expansive potential, and the siltstone-claystone bedrock at the site has moderate to high
expansive potential. The soils at anticipated foundation bearing depth have moderate load bearing
capability, and the bedrock has high load bearing capability.
Based on the geotechnical engineering analyses, subsurface exploration and laboratory test results,
we recommend that the proposed structures founded in the upper soil a minimum of 3 feet above
the bedrock be supported on spread footing and/or grade beam foundation systems. Structures
placed in or within 3 feet of the bedrock should be supported by grade beams and straight -shaft
piers foundation systems. Slab -on -grade maybe utilized for the interior floor system provided that
care is taken in the placement and compaction of the subgrade soil.
Offices of The Terracon Companies, Inc. Geotechnical, Environmental and Materials Engineers
Arizona: Tucson ■ Colorado: Colorado Springs, Denver, Ft. Collins, Greeley, Longmont ■ Idaho: Boise ■ Illinois: Bloomington,
Chicago, Rock Island ■ Iowa: Cedar Falls, Cedar Rapids, Davenport, Des Moines, Storm Lake ■ Kansas: Lenexa, Topeka,
Wichita ■ Minnesota: St. Paul ■ Missouri: Kansas City ■ Nebraska: Lincoln, Omaha ■ Nevada: Las Vegas
■ Oklahoma: Oklahoma City, Tulsa ■ Texas: Dallas, Fort Worth ■ Utah: Salt Lake City ■ Wyoming: Cheyenne
QUALITY ENGINEERING SINCE 1965
Fossil Creek Partners
ELI Project No. 20945036
Terracon
Given the engineering characteristics of the claystone-siltstone bedrock in areas where slabs are
placed in or within 3 feet of the bedrock, consideration should be given to use of structural floor
systems if no movement can be tolerated.
Other design and construction details, based upon geotechnical conditions, are presented in the
report.
We have appreciated being of service to you in the geotechnical engineering phase of this project,
and are prepared to assist you during the construction phases as well. If you have any questions
concerning this report or any of our testing, inspection, design and consulting services, please do
not hesitate to contact us.
Sincerely,
EMPIRE LABORATORIES, INC.
J A Division of The Terracon Companies, Inc.
7
eil R. d
Senior Engineering Geologist
Reviewed by:
Chester C. Smith, P.E.
Division Manager
NRS/CCS/cic
Copies to: Addressee (2)
' Shear Engineering - Mr. Brian Shear (2)
0
Terracon
Fossil Creek Partners
ELI Project No. 20945036
TABLE OF CONTENTS
Page
No.
Letter of Transmittal...................................................
ii
INTRODUCTION.....................................................
1
PROPOSED CONSTRUCTION ............................................
1
1
SITE EXPLORATION ..................................................
Field Exploration
2
7
Laboratory Testing ..............................................
2
SITE CONDITIONS ...................................................
3
SUBSURFACE CONDITIONS ............................................
3
Geology.
3
Soil and Bedrock..tuckConditions .......................................
4
Laboratory Test Results ..........................................
4
Groundwater Conditions ..................::::::
4
CONCLUSIONS AND RECOMMENDATIONS ......... : ::::::: : ::::: : :::
5
Site Development Considerations ....................................
5
'
Foundation Systems .............................................
6
Alternate Foundation Systems ...
7
Basement Construction ...........................................
8
`
Seismic Considerations
9
Floor Slab Design and Construction ..................................
9
Pavement Design and Construction .................................
10
Earthwork ...................................................
15
J
General Considerations
15
Site Clearing ............................................
15
Excavation
16
Slab Subgrade Preparation .
16
Pavement Subgrade Preparation ...............................
17
Fill Materials ...............................:............
17,
Placement and Compaction
18
19
Slopes................................................
19
,._
Compliance ...................................
I
Excavation and Trench Construction . ....... ..
19
20
Drainage....................................................
Surface Drainage
20
Subsurface Drainage .......................................
21
Additional Design and Construction Considerations ......................
21
Exterior Slab Design and Construction ....:..:.:...:............
21
Corrosion Protection
21
Fossil Creek Partners
ELI Project No. 20945036
TABLE OF CONTENTS (Cont'd)
Terracon
Page No.
GENERAL COMMENTS ............................................... 22
APPENDIX A
Figure No.
SITEPLAN .... ...............:...................... ..............
. 1
Logs of Borings .........................................
Al thru A10
APPENDIX B
Consolidation Test ........................................
B1 thru B2
Hveem Stabilometer Curves .................................
83 thru B4
Summary of Test Results .........................................
B5
APPENDIX C: GENERAL NOTES
Drilling & Exploration ............................................
C1
Unified Soil Classification .........................................
C2
Bedrock Classification, Sedimentary Bedrock ............................ C3
Laboratory Testing, Significance and Purpose ........................... C4
ReportTerminology ............................................
C5
APPENDIX D
Recommended Preventative Maintenance -Asphalt Concrete Pavements .........
D1
Recommended Preventative Maintenance -Jointed Concrete Pavements .........
D2
GEOTECHNICAL ENGINEERING REPORT Terracon
FOSSIL CREEK ESTATES PHASE I
SOUTH SHIELDS STREET
FORT COLLINS, COLORADO
ELI PROJECT NO. 20945036
MARCH 24, 1994
INTRODUCTION
This report contains the results of our geotechnical engineering exploration for the proposed
project to be located on South Shields Street in southwest Fort Collins, Colorado. The site is
located in the Southeast 1 /4 of Section 10, Township 6 North, Range 69 West of the 6th Principal
Meridian.
The purpose of these services is to provide information and geotechnical engineering
recommendations relative to:
• subsurface soil and bedrock conditions
• groundwater conditions
• foundation design and construction
• basement construction
• floor slab design and construction
• pavement design and construction
• earthwork
• drainage
The conclusions and recommendations contained in this report are based upon the results of field
and laboratory testing, engineering analyses, and experience with similar soil and structural
conditions.
PROPOSED CONSTRUCTION
The project as we understand it will be the construction of 32 residential lots. Residential and
collector streets will be constructed through the project area, and improvements will be made to
South Shields Street along the east edge of the site. Minor amounts of site grading are
anticipated.
SITE EXPLORATION
The scope of the .services performed for this project included site reconnaissance by an
engineering geologist, a subsurface exploration program, laboratory testing and engineering
analyses.
Fossil Creek Partners Terracon
ELI Project No. 20945036
Field Exploration: A total of 10 test borings were drilled to depths of 10 to 15 feet at the
locations shown on the Site Plan, Figure 1. Five borings were drilled within the areas of the
proposed residences, and five borings were drilled in the area of proposed and existing streets.
Previous geotechnical investigations were prepared for the northeast portion of the site by Empire
Laboratories, Inc. dated August 28, 1992. An additional preliminary report for the Fossil Creek
Estates project was prepared by Empire Laboratories, Inc. on September 21, 1993. All borings
were advanced with a truck -mounted drilling rig, utilizing 4-inch diameter solid stem auger.
The location of borings were positioned in the field by measurements from existing property lines
and/or existing site features. Elevations were interpolated from a topographic map provided to
Empire Laboratories, Inc. The accuracy of boring locations and elevations should only be assumed
to the level implied by the methods used to determine each.
Continuous lithologic logs of each boring were recorded by the engineering geologist during the
drilling operations. At selected intervals, samples of the subsurface materials were taken by
means of pushing thin -walled Shelby tubes, or by driving split -spoon samplers. Representative
bulk samples of subsurface materials were obtained from pavement borings.
Penetration resistance measurements were taken with each sampling with the split -spoon by
driving the sampler with a 140-pound hammer falling 30 inches. When properly interpreted, the
penetration resistance is a useful index to the consistency, relative density or hardness of the
materials encountered.
Groundwater conditions were evaluated in each boring at the time of site exploration, and one day
after drilling.
Laboratory Testing: All samples retrieved during the field exploration were returned to the
laboratory for evaluation by the project geotechnical engineer, and were classified in accordance
with the Unified Soil Classification System described in Appendix C. Samples of bedrock were
classified in accordance with the general notes for Bedrock Classification. At that time, the field
descriptions were confirmed or modified as necessary, final boring logs prepared, and an applicable
laboratory testing program was formulated to determine engineering properties of the subsurface
materials. Boring Logs for the project are presented in Appendix A.
Selected soil and bedrock samples were tested for the following engineering properties:
Fossil Creek Partners
ELI Project No. 20945036
• Water content
• Dry density
• Consolidation
• Compressive strength
Terracon
• Expansion
• Plasticity
• R-Value
• Soluble sulfate content
The significance and purpose of each laboratory test is described in Appendix C. Laboratory test
results are presented in Appendix B, and were used for the geotechnical engineering analyses, and
the development of foundation and earthwork recommendations. All laboratory tests were
performed in general accordance with the applicable ASTM, local or other accepted standards.
SITE CONDITIONS
The site consists of gently -rolling, irrigated pasture land. The Scenic Knolls ditch forms the
southwest property boundary. The property slopes to the north-northeast and has positive
drainage in these directions. A small ridge trending east -west is located in the central portion of
the site. Fences surround the property on all sides. The site is bordered on the north and west
by open land, to the east by South Shields Street, and to the south by existing residences. A
wetlands area is located to the north, and cottonwood trees are located in the southwest portion
of the site.
SUBSURFACE CONDITIONS
Geology: The project area is located within the Colorado Piedmont section of the Great Plains
physiographic province. The Colorado Piedmont, formed during Late Tertiary and Early quaternary
time (approximately 2,000,000 years ago), is a broad, erosional trench which separates the
Southern Rocky Mountains from the High Plains. Structurally, the site lies along the western flank
of the Denver Basin. During the Late Mesozoic and Early Cenozoic Periods (approximately
70,000,000 years ago), intense tectonic activity occurred, causing the uplifting of the Front Range
and associated downwarping of the Denver Basin to the east. Relatively flat uplands and broad
valleys characterize the present-day topography of the Colorado Piedmont in this region. The site
is underlain by the Cretaceous Pierre Formation. The Pierre shale underlies the majority of the site
at depths of 4%z to approximately 20 feet below the surface. The Pierre shale is overlain by
residual and alluvial soils of Pleistocene and/or Recent Age.
Cl