HomeMy WebLinkAbout1.26 Geotech Study
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, CO
PRELIMINARY GEOTECHNICAL STUDY
PROPOSED RESIDENTIAL DEVELOPMENT
BATTLEMENT MESA, HALF MOON PARCEL
NORTHSTAR TRAIL
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-732
OCTOBER 27, 2021
PREPARED FOR:
RISING TIDES ENTERPRISES, LLC
ATTN: TODD BARTON
2399 46-1/2 ROAD
DEBEQUE, COLORADO 81630
toddebarton@msn.com
Kumar & Associates, Inc. ® Project No. 21-7-732
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
FIELD EXPLORATION ............................................................................................................ - 2 -
SUBSURFACE CONDITIONS ................................................................................................. - 2 -
ENGINEERING ANALYSIS ..................................................................................................... - 3 -
DESIGN RECOMMENDATIONS ............................................................................................ - 3 -
FOUNDATIONS .................................................................................................................... - 3 -
FOUNDATION AND RETAINING WALLS ....................................................................... - 4 -
FLOOR SLABS ...................................................................................................................... - 5 -
UNDERDRAIN SYSTEM ..................................................................................................... - 5 -
SITE GRADING ..................................................................................................................... - 6 -
SURFACE DRAINAGE ......................................................................................................... - 6 -
PAVEMENT SECTION ......................................................................................................... - 7 -
LIMITATIONS ........................................................................................................................... - 8 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 – LEGEND AND NOTES
FIGURES 4 THROUGH 6 – SWELL-CONSOLIDATION TEST RESULTS
FIGURES 7 AND 8 - GRADATION TEST RESULTS
TABLE 1 – SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 21-7-732
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for the proposed residential development to be
located on Battlement Mesa, Half Moon Parcel, Northstar Trail, Garfield County, Colorado. The
project site is shown on Figure 1. The purpose of the study was to develop recommendations for
the subdivision design. The study was conducted in accordance with our proposal for
geotechnical engineering services to Rising Tides Enterprises, LLC dated August 16, 2021.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the field
exploration were tested in the laboratory to determine their classification, compressibility or
expansion potential and other engineering characteristics. The results of the field exploration
and laboratory testing were analyzed to develop recommendations for foundation types, depths
and allowable pressures for the proposed building foundation and subgrade conditions for
pavement section design. This report summarizes the data obtained during this study and
presents our conclusions, design recommendations and other geotechnical engineering
considerations based on the proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
Development plans were preliminary at the time of our study. The development plan for
proposed 47 single family lots and access street off Northstar Trail is shown on Figure 1. In
general, the proposed residences are assumed to be one and two-story structures. Ground floors
could be structural above crawlspace or slab-on-grade. Grading for the structures and access
street is assumed to be relatively minor with cut depths between about 2 to 10 feet. Foundation
loadings for this type of construction are assumed to be relatively light.
If building development plans change significantly from those described above, we should be
notified to re-evaluate the recommendations presented in this report.
SITE CONDITIONS
The property was vacant of structures at the time of our study. The ground surface slopes gently
down to the west with moderate slopes beyond the north-northeast sides of the property down to
a dry gully. The south and west sides are bordered by existing residential development and
Northstar Trail as shown on Figure 1. Vegetation consists of moderately thick grass and weeds.
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Kumar & Associates, Inc. ® Project No. 21-7-732
FIELD EXPLORATION
The field exploration for the project was conducted on September 24, 2021. Eight exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
The borings were advanced with 4-inch diameter continuous flight augers powered by a truck-
mounted CME-45B drill rig. The borings were logged by a representative of Kumar &
Associates.
Samples of the subsoils were taken with 1⅜ inch and 2-inch I.D. spoon samplers. The samplers
were driven into the subsurface materials at various depths with blows from a 140 pound
hammer falling 30 inches. This test is similar to the standard penetration test described by
ASTM Method D-1586. The penetration resistance values are an indication of the relative
density or consistency of the subsoils. Depths at which the samples were taken and the
penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The
samples were returned to our laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils encountered, below a thin topsoil layer, consist of sandy silt and clay (loess deposit) in
most borings above very stiff to hard, sandy silty clay with scattered gravel and cobbles
underlain by medium dense to dense, clayey sandy gravel with cobbles and scattered boulders
and sandy clay with gravel zones. Drilling in the coarse granular soils with auger equipment was
difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit at
some of the borings.
Laboratory testing performed on samples obtained from the borings included natural moisture
content, density, gradation analyses and liquid and plastic limits. Results of swell-consolidation
testing performed on the silt and clay soils, shown on Figures 4 through 6, generally indicate low
compressibility under existing moisture conditions and nil to minor expansion potential when
wetted under light loading. The samples of sandy silty clay, shown on Figures 5 and 6, had
moderate expansion potential when wetted. Results of gradation analyses performed on small
diameter drive samples (minus 1½-inch fraction) of the coarse granular subsoils are shown on
Figures 7 and 8. The laboratory testing is summarized in Table 1.
Free water was not encountered in the borings at the time of drilling and the soils were typically
slightly moist.
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Kumar & Associates, Inc. ® Project No. 21-7-732
ENGINEERING ANALYSIS
There are no geologic conditions of significance which would make development of the property
infeasible. The natural sandy silt and clay soils typically encountered below the topsoil are
suitable for support of lightly loaded shallow spread footings with low bearing capacity.
Settlement potential is expected to be relatively minor under relatively light loadings. The
underlying clay soils could possess excessive expansion potential and need mitigation such as
sub-excavation and replacement with structural fill but we should evaluate the expansion
potential at the time of excavation and when building configurations and foundation depths and
loadings have been determined. Structural fill consisting of granular soil can be used to
reestablish design bearing level if needed. Below grade areas of the structures should be
protected against groundwater impacts with an underdrain system. The following
recommendations are made primarily for construction on the natural low or non-expansive soils
or structural fill. Site specific subsoil information should be developed for foundation design of
each building.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the buildings be founded with spread footings bearing
on the natural low or non-expansive soils or compacted structural fill.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural low or non-expansive soils or
structural fill should be designed for an allowable bearing pressure of 1,500 psf to
3,000 psf. Based on experience, we expect settlement of footings designed and
constructed as discussed in this section will be up to around 1 inch. Additional
differential settlement of around ½ to 1 inch could occur if the bearing soils are
wetted.
2) The footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
area.
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Kumar & Associates, Inc. ® Project No. 21-7-732
4) Continuous foundation walls should be reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 12 feet.
Foundation walls acting as retaining structures should also be designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of this report.
5) The topsoil, expansive clay soils and loose disturbed soils should be removed and
the footing bearing level extended down to the firm natural soils. The exposed
soils in footing areas should then be moisture adjusted to near optimum and
compacted. Structural fill should be compacted to at least 98% of standard
Proctor density at near optimum moisture content and extend laterally beyond the
footing edges a distance at least one-half the depth of fill below the footing.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 50 pcf for backfill consisting
of the on-site soils. Cantilevered retaining structures which are separate from the building and
can be expected to deflect sufficiently to mobilize the full active earth pressure condition should
be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight
of at least 40 pcf for backfill consisting of the on-site granular soils. Backfill should not contain
organics, debris or rock larger than about 6 inches.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at near optimum moisture content. Backfill placed in pavement and
walkway areas should be compacted to at least 95% of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
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Kumar & Associates, Inc. ® Project No. 21-7-732
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill.
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.30 for the silt and clay soils and 0.45 for the granular soils.
Passive pressure of compacted backfill against the sides of the footings can be calculated using
an equivalent fluid unit weight of 300 pcf. The coefficient of friction and passive pressure
values recommended above assume ultimate soil strength. Suitable factors of safety should be
included in the design to limit the strain which will occur at the ultimate strength, particularly in
the case of passive resistance. Fill placed against the sides of the footings to resist lateral loads
should be compacted to at least 95% of the maximum standard Proctor density at a moisture
content near optimum.
FLOOR SLABS
The natural on-site sandy silt and granular soils, exclusive of topsoil, are suitable to support
lightly loaded slab-on-grade construction. The clay soils may have potential to heave floor slabs
and the subgrade condition should be further evaluated at the time of excavation to assess the
need for sub-excavation of expansive clay soils and replacement with structural fill. To reduce
the effects of some differential movement, floor slabs should be separated from all bearing walls
and columns with expansion joints which allow unrestrained vertical movement. Floor slab
control joints should be used to reduce damage due to shrinkage cracking. The requirements for
joint spacing and slab reinforcement should be established by the designer based on experience
and the intended slab use. A minimum 4-inch layer of free-draining gravel should be placed
beneath interior slabs to facilitate drainage. This material should consist of minus 2-inch
aggregate with at least 50% retained on the No. 4 sieve and less than 2% passing the No. 200
sieve.
All fill materials for support of floor slabs should be compacted to at least 95% of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
onsite predominantly granular soils devoid of vegetation, topsoil and oversized rock placed at
near optimum moisture content.
UNDERDRAIN SYSTEM
Free water was not encountered in the exploratory borings during our exploration but it has been
our experience in the area and where there are clay soils that local perched groundwater can
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Kumar & Associates, Inc. ® Project No. 21-7-732
develop during times of heavy precipitation or seasonal runoff. Frozen ground during spring
runoff can create a perched condition. We recommend below-grade construction, such as
retaining walls, crawlspace and basement areas, be protected from wetting and hydrostatic
pressure buildup by an underdrain system. Shallow crawlspace areas (4 feet or less deep) may
not need to be protected from wetting which should be evaluated by the lot specific subsurface
conditions.
Where subdrains are provided, the drains should consist of drainpipe placed in the bottom of the
wall backfill surrounded above the invert level with free-draining granular material. The drain
should be placed at each level of excavation and at least 1 foot below lowest adjacent finish
grade and sloped at a minimum ½% to a suitable gravity outlet. Drywells for drain water outlet
will probably have limited capacity due to the silty clayey matrix of the granular soil. Free-
draining granular material used in the underdrain system should contain less than 2% passing the
No. 200 sieve, less than 50% passing the No. 4 sieve and have a maximum size of 2 inches. The
drain gravel backfill should be at least 1½ feet deep.
SITE GRADING
The risk of construction-induced slope instability at the site appears low provided the building
and utility trench excavations are dry and cut and fill depths are limited. We assume the cut
depths for below grade levels and utilities will not exceed about 10 feet. Fills should be limited
to about 8 to 10 feet deep. Structural fills should be compacted to at least 95% of the maximum
standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade
should be carefully prepared by removing all vegetation and topsoil and compacting to at least
95% of the maximum standard Proctor density. The fill should be benched into slopes that
exceed 20% grade. Permanent unretained cut and fill slopes should be graded at 2 horizontal to
1 vertical or flatter and protected against erosion by revegetation or other means. This office
should review site grading plans for the project prior to construction.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after each building has been completed:
1) Inundation of the foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the maximum standard Proctor density in landscape areas.
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Kumar & Associates, Inc. ® Project No. 21-7-732
3) The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 6 inches in the first 10 feet in unpaved areas and a minimum slope of
2½ inches in the first 10 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with at least 2 feet of the on-site finer graded
soils to reduce surface water infiltration.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at least
5 feet from foundation walls. Consideration should be given to use of xeriscape
to reduce the potential for wetting of soils below the building caused by irrigation.
PAVEMENT SECTION
A pavement section is designed to distribute concentrated traffic loads to the subgrade.
Pavement design procedures are based on strength properties of the subgrade and pavement
materials assuming stable, uniform subgrade conditions. Certain soils such as the fine-grained
silt and clay soils and clayey matrix soils encountered on this site, are frost susceptible and could
impact pavement performance. Frost susceptible soils are problematic when there is a free water
source. If those soils are wetted, the resulting frost heave movements can be large and erratic.
Therefore, pavement design procedures assume dry subgrade conditions by providing proper
surface and subsurface drainage.
Subgrade Materials: The upper silt and clay soils encountered at the site are mainly low
plasticity which are considered a fair support for pavement materials. The classification tests on
the soil indicate an Hveem stabilometer 'R' value of around 10 for asphalt pavements and a
modulus of subgrade reaction of 50 pci for rigid (portland cement) pavements. The silt soils are
considered moderately susceptible to frost action.
Pavement Section: Since anticipated traffic loading information was not available at the time of
report preparation, an 18 kip equivalent daily load application (EDLA) of 10 was assumed for
combined automobile and truck traffic areas. This loading is typical of a local street with
occasional service vehicles and should be checked by the project civil engineer. If the pavement
will support significant construction traffic, we should be contacted for evaluation of the
additional loading and section thickness. A Regional Factor of 1.5 was assumed for this area of
Garfield County based on the site terrain, drainage and climatic conditions.
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Kumar & Associates, Inc. ® Project No. 21-7-732
Based on the assumed parameters, the pavement section in areas of combined automobile and
truck traffic should consist of 6 inches of CDOT Class 6 aggregate base course and 4 inches of
asphalt surface. An alternate section of 9 inches of CDOT Class aggregate base course and
3 inches of asphalt surface can be used.
As an alternative to asphalt pavement and in areas where truck turning movements are
concentrated, the pavement section can consist of 6 inches of portland cement concrete on
4 inches of CDOT Class 6 aggregate base course.
The section thicknesses assume structural coefficients of 0.14 for aggregate base course, 0.44
for asphalt surface and design strength of 4,500 psi for air entrained portland cement concrete.
The material properties and compaction should be in accordance with the project specifications.
Subgrade Preparation: Prior to placing the pavement section, the entire subgrade area should
be stripped of topsoil, scarified to a depth of 8 inches, adjusted to a moisture content near
optimum and compacted to at least 95% of the maximum standard Proctor density. The
pavement subgrade should be proof-rolled with a heavily loaded pneumatic-tired vehicle.
Pavement design procedures assume a stable subgrade. Areas which deform excessively under
heavy wheel loads are not stable and should be removed and replaced with structural granular
material such as CDOT Class 2 base course to achieve a stable subgrade prior to paving.
Drainage: The collection and diversion of surface drainage away from paved areas is extremely
important to the satisfactory performance of pavement. Drainage design should provide for the
removal of water from paved areas and prevent wetting of the subgrade soils. Uphill roadside
ditches should have an invert level at least 1 foot below the road base.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area at this time. We make no warranty either express or implied.
The conclusions and recommendations submitted in this report are based upon the data obtained
from the exploratory borings drilled at the locations indicated on Figure 1, the proposed type of
construction and our experience in the area. Our services do not include determining the
presence, prevention or possibility of mold or other biological contaminants (MOBC) developing
in the future. If the client is concerned about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions identified at the exploratory borings and variations in the subsurface
conditions may not become evident until excavation is performed. If conditions encountered
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-732 Page 1 of 2
SAMPLE LOCATION NATURAL MOISTURE CONTENT
NATURAL DRY DENSITY
GRADATION
PERCENT PASSING NO. 200 SIEVE
ATTERBERG LIMITS AASHTO CLASSIFICATION SOIL TYPE BORING DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%)
(ft) (%) (pcf) (%) (%)
1 2½ 4.5 109 Sandy Clay and Silt
10 and 15
combined 10.8 35 26 39 Sandy Clay and Gravel
2 2½ 9.5 88 Calcareous Sandy Silty
Clay
5 8.1 37 20 43 Sandy Clay and Gravel
3 2½ 5.1 108 91 Slightly Sandy Silt
5 10.6 103 Sandy Silty Clay
4 2½ 6.0 99 Sandy Clay and Silt
10 8.7 35 28 37 Sandy Clay and Gravel
5 5 7.4 112 Sandy Silty Clay
10 11.1 64 45 28 A-7-6 (13) Sandy Clay with Gravel
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-732 Page 2 of 2
SAMPLE LOCATION NATURAL MOISTURE CONTENT
NATURAL DRY DENSITY
GRADATION
PERCENT PASSING NO. 200 SIEVE
ATTERBERG LIMITS AASHTO CLASSIFICATION SOIL TYPE BORING DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%)
(ft) (%) (pcf) (%) (%)
6 2½ 5.2 102 89 Sandy Clayey Silt
10 9.0 29 33 38 Sandy Clay and Gravel
7 3 6.5 79 29 10 A-4 (6) Sandy Silty Clay
8 1 6.3 87 29 10 A-4 (8) Sandy Silty Clay