HomeMy WebLinkAboutSubsoil 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, Colorado
PRELIMINARY SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED HOUSE, ADU/STORAGE AND BARN
BUCK POINT RANCH
723 COUNTY ROAD 121
GARFIELD COUNTY, COLORADO
PROJECT NO. 21-7-291
MAY 24, 2021
PREPARED FOR:
SGJD, LLC
ATTN: JIM DETTERICK
4894 MEADOW LANE
VAIL, COLORADO 81657
jim.detterick@mac.com
Kumar & Associates, Inc. ® Project No. 21-7-291
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
SUBSIDENCE POTENTIAL ..................................................................................................... - 2 -
FIELD EXPLORATION ............................................................................................................ - 2 -
SUBSURFACE CONDITIONS ................................................................................................. - 3 -
FOUNDATION BEARING CONDITIONS .............................................................................. - 3 -
DESIGN RECOMMENDATIONS ............................................................................................ - 4 -
FOUNDATIONS .................................................................................................................... - 4 -
FOUNDATION AND RETAINING WALLS ....................................................................... - 5 -
FLOOR SLABS ...................................................................................................................... - 6 -
UNDERDRAIN SYSTEM ..................................................................................................... - 7 -
PAVEMENT SECTION ......................................................................................................... - 7 -
SURFACE DRAINAGE ......................................................................................................... - 8 -
LIMITATIONS ........................................................................................................................... - 8 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURES 3 AND 4 - SWELL-CONSOLIDATION TEST RESULTS
FIGURES 5 AND 6 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 21-7-291
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed house, ADU and storage
building and barn to be located at Buck Point Ranch, 723 County Road 121, Garfield County,
Colorado. The project site is shown on Figure 1. The purpose of the study was to develop
recommendations for the foundation design. The study was conducted in accordance with our
agreement for geotechnical engineering services to SGJD, LLC dated March 22, 2021.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock obtained during
the field exploration were tested in the laboratory to determine their classification,
compressibility or swell 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 foundations. 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
The proposed house will be a one-story structure over walkout basement level with attached
garage. The proposed ADU and storage building will be a tall single-story structure with garage
door access along the front of the building. The proposed barn will be a single-story structure.
The proposed ground floors will be structural over crawlspace for the living areas of the house
and slab-on-grade for the garage, barn and ADU/storage building. Grading for the structures is
assumed to be relatively minor with cut depths between about 3 to 8 feet. We assume relatively
light foundation loadings, typical of the proposed type of construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The subject site was vacant at the time of our field exploration. Several 2-track roads cross the
site and were used for access. The ground surface generally slopes moderately down to the south
at a grade of about 10 percent. A grassy and marshy area is on the west side of the site and an
irrigation ditch runs approximately 150 feet west of the proposed house. Vegetation consists of
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Kumar & Associates, Inc. ® Project No. 21-7-291
grass, weeds and sagebrush with stands of oak brush and cottonwood trees around the marsh
area.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the subject site. These rocks
are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some massive beds
of gypsum and limestone. There is a possibility that massive gypsum deposits associated with
the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the gypsum under certain
conditions can cause sinkholes to develop and can produce areas of localized subsidence.
During previous work in the area, several sinkholes were observed scattered throughout Garfield
and Eagle Counties. These sinkholes appear similar to others associated with the Eagle Valley
Evaporite in areas of the Eagle Valley.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities
was encountered in the subsurface materials; however, the exploratory borings were relatively
shallow, for foundation design only. Based on our present knowledge of the subsurface
conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of
future ground subsidence at Buck Point Ranch throughout the service life of the proposed
residence, in our opinion, is low; however, the owner should be made aware of the potential for
sinkhole development. If further investigation of possible cavities in the bedrock below the site
is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on April 28, 2021. Five 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, Inc.
Samples of the subsoils were taken with 1⅜-inch and 2-inch I.D. spoon samplers. The samplers
were driven into the subsoils 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 and hardness of the bedrock. 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.
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Kumar & Associates, Inc. ® Project No. 21-7-291
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils consist of about ½ to 2 feet of topsoil overlying dense, clayey sandy gravel down to
between 8 and 23 feet deep where very stiff to hard, sandy clay was encountered down to
between 14 and 31 feet deep. In the area of the house, dense silt sand was encountered in
Boring 2 from 14 to 19 feet deep overlying weathered siltstone/sandstone bedrock down to
31 feet deep. In the area of the barn in Boring 4, very stiff, expansive clay soil was encountered
below the gravel from 8 to 18 feet deep overlying dense, silty sandy gravel down to 26 feet deep.
In the ADU/storage building area, in Boring 5, very stiff clay was encountered below the topsoil
from 1½ to 4½ feet deep underlain by dense, clayey sandy gravel from 4½ to 23 feet deep and
very stiff, sandy clay from 23 to 31 feet deep.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, Atterberg limits and gradation analyses. Results of swell-consolidation
testing performed on relatively undisturbed drive samples, presented on Figures 3 and 4, indicate
low to moderate compressibility under existing low moisture conditions and light loading and a
nil to high expansion potential when wetted under constant light surcharge. Results of gradation
analyses performed on small diameter drive samples (minus 1½-inch fraction) of the coarse
granular subsoils are shown on Figures 5 and 6. The laboratory testing is summarized in
Table 1.
Free water was encountered in Boring 2 at a depth of 15 feet and in Boring 5 at a depth of
30½ feet at the time of drilling and the upper subsoils were slightly moist to moist.
FOUNDATION BEARING CONDITIONS
The bearing conditions were variable across the site and we recommend additional borings be
drilled to verify the soil conditions at each structure once building elevations have been
determined. For preliminary planning purposes, the residence can be founded on the upper
gravel soils. We believe the garage can be founded on spread footings bearing on the natural
granular soils with a risk of movement. We recommend the barn be founded on spread footings
bearing on the natural granular soils. We recommend the ADU/storage building be supported on
spread footings bearing on the natural granular soils below the clay soils or compacted structural
fill placed on the natural gravel soils.
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Kumar & Associates, Inc. ® Project No. 21-7-291
PRELIMINARY DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we believe the buildings can be founded with spread footings bearing
on the natural granular soils or compacted structural fill with a risk of movement. Once building
bearing elevations have been developed, additional borings will need to be drilled at each
structure to verify bearing conditions.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural granular soils or compacted structural
fill should be designed for an allowable bearing pressure of 1,500 to 3,000 psf.
Based on experience, we expect movement of footings designed and constructed
as discussed in this section will be about 1 inch or less. Additional, post
construction, movement could occur if the bearing soils become wetted. The
magnitude of additional movement would depend on the depth and extent of
wetting but could be on the order of about 1 inch.
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.
4) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14 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) In the house foundation excavation, the deeper footing areas may need to be
sub-excavated a minimum of 4 feet below proposed footing level. The exposed
subgrade should be moisture conditioned and compacted prior to placing
structural fill. Structural fill can consist of the onsite granular materials devoid of
organics and oversized (plus 6-inch) rock, or imported granular material such as
CDOT Class 6 road base. Structural fill should be compacted to 98 percent of
maximum standard Proctor density at a moisture content near optimum.
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Kumar & Associates, Inc. ® Project No. 21-7-291
Structural fill should extend laterally beyond the edges of footings at a slope of
½ horizontal to 1 vertical.
6) Topsoil, clay soils and any loose disturbed soils should be removed and the
footing bearing level extended down to the relatively dense natural granular soils
in the barn and ADU/Storage buildings. The exposed soils in footing area should
then be moistened and compacted. If water seepage is encountered, the footing
areas should be dewatered before concrete placement.
7) 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, more granular soils. Cantilevered retaining structures which are separate from the
buildings 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 45 pcf for backfill consisting of the on-site granular soils.
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 a moisture content near optimum. 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
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
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Kumar & Associates, Inc. ® Project No. 21-7-291
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.45. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 400 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 a granular material compacted to at least
95% of the maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The on-site clay soils possess an expansion potential and slab heave could occur if the subgrade
soils were to become wet. Slab-on-grade construction may be used provided precautions are
taken to limit potential movement and the risk of distress to the building is accepted by the
owner. A positive way to reduce the risk of slab movement, which is commonly used in the
area, is to construct structurally supported floors over crawlspace.
To reduce the effects of some differential movement, nonstructural floor slabs should be
separated from all bearing walls and columns with expansion joints which allow unrestrained
vertical movement. Interior non-bearing partitions resting on floor slabs should be provided with
a slip joint at the bottom of the wall so that, if the slab moves, the movement cannot be
transmitted to the upper structure. This detail is also important for wallboards, stairways and
door frames. Slip joints which will allow at least 1½-inches of vertical movement are
recommended. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. Slab reinforcement and control joints 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 immediately beneath basement
level slabs-on-grade. This material should consist of minus 2-inch aggregate with less than 50%
passing the No. 4 sieve and less than 2% passing the No. 200 sieve. The free-draining gravel
will aid in drainage below the slabs and should be connected to the perimeter underdrain system.
Required fill beneath slabs can consist of the on-site gravelly soils or a suitable imported
granular material, excluding topsoil and oversized rocks. The fill should be spread in thin
horizontal lifts, adjusted to at or above optimum moisture content, and compacted to at least 95%
of the maximum standard Proctor density. All vegetation, topsoil and loose or disturbed soil
should be removed prior to fill placement.
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Kumar & Associates, Inc. ® Project No. 21-7-291
The above recommendations will not prevent slab heave if the expansive soils underlying slabs-
on-grade become wet. However, the recommendations will reduce the effects if slab heave
occurs. All plumbing lines should be pressure tested before backfilling to help reduce the
potential for wetting.
UNDERDRAIN SYSTEM
Although free water encountered during our exploration was below the lowest proposed grade, it
has been our experience in the area and where clay soils are present that local perched
groundwater can 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.
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 1% to
a suitable gravity outlet. 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. In areas
where clay soils are present, an impervious membrane such as 20 mil PVC should be placed
beneath the drain gravel in a trough shape and attached to the foundation wall with mastic to
prevent wetting of the bearing soils.
PAVEMENT SECTION
We understand a gravel section is proposed for the access driveway. We were provided
preliminary designs from SGM for review. The proposed gravel section consists of 4 inches of
CDOT Class 6 road base placed on 8 inches of CDOT Class 2 subbase with 12 inches of
compacted native material below. For the granular subgrade soils encountered in Boring 3 in the
new driveway area, we believe this gravel section is sufficient for the proposed driveway. In
areas where fine-grained subgrade soils are encountered, we recommend increasing the gravel
section to 6 inches of road base placed on 12 inches of subbase for the proposed driveway.
Required fill to establish design subgrade level can consist of the on-site granular soils or
suitable imported granular soils such as CDOT Class 2 structural fill. Prior to fill placement the
subgrade should be scarified to a depth of 8 inches, adjusted to near optimum moisture and
compacted to at least 95% of standard Proctor density. In soft or wet areas, the subgrade may
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Kumar & Associates, Inc. ® Project No. 21-7-291
require drying or stabilization prior to fill placement. A geogrid and/or sub-excavation and
replacement with aggregate base soils may be needed for the stabilization. The subgrade should
be proofrolled. Areas that deflect excessively should be corrected before placing pavement
materials.
SURFACE DRAINAGE
Proper surface drainage may be critical to the long-term performance of the proposed buildings.
The following drainage precautions should be observed during construction and maintained at all
times after the buildings have been completed:
1) Inundation of the foundation excavations and underslab areas should be avoided
during construction. Drying could increase the expansion potential of the soils.
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.
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 12 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inches in the first 10 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site 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
10 feet from foundation walls.
LIMITATIONS
This preliminary 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. We recommend that additional borings be
drilled at each building location once the foundation elevations have been determined.
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 21-7-291
SAMPLE LOCATION NATURAL MOISTURE CONTENT
NATURAL
DRY DENSITY
GRADATION
PERCENT PASSING NO.
200 SIEVE
ATTERBERG LIMITS EXPANSION PRESSURE EXPANSION SOIL TYPE BORING DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%)
(ft) (%) (pcf) (%) (%) (psf) (%)
1 10 23.5 96 50 30 Clay
15 17.3 112 -- -- Sandy Clay
2 3 & 5
combined 6.2 50 35 15 Clayey Very Sandy Gravel
15 19.8 106 67 Sandy Silt
25 14.7 119 59 Very Sandy Silt
30 15.5 117 55 Sand and Silt
3 5 3.2 44 40 16 Clayey Gravel and Sand
4 3 & 5
combined 4.9 45 38 17 Clayey Gravel and Sand
10 15.4 112 19,000 3.5 Sandy Silt
20 7.7 115 16 Silty Gravel and Sand
5 3 11.3 111 12,000 5.5 Sandy Clay
10 & 15
combined 10.7 35 39 26 Clayey Gravel and San