HomeMy WebLinkAboutSubsoils Report for Foundation DesignKumar & Associates, Inc.
'( Geatechnical and Materials Engineers 5020 County Road 154
and Environmental Scientists Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwoodpkumarusa.com
An Employ®® Owned Company a ww.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT H 25, FILING 7, ASPEN GLEN
345 SADDLEBACK ROAD
GARFIELD COUNTY, COLORADO
PROJECT NO.24-7-576
OCTOBER 11, 2024
PREPARED FOR:
TAMI YORK
1441 WAZEE STREET, APARTMENT 401
DENVER, COLORADO 80202
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TABLE OF CONTENTS
PURPOSEAND SCOPE OF STUDY........................................................................................ I
PROPOSED CONSTRUCTION .......................... ..... ..........•. I -
SITECONDITIONS .............................................. ......................................................................
I -
SUBSIDENCEPOTENTIAL...................................................................
............................... I -
FIELDEXPLORATION .........................................................................................................
2-
SUBSURFACECONDITIONS..................................................................................................
2-
FOUNDATION BEARING CONDITIONS...............................................................................
3-
DESIGN RECOMMENDATIONS .................................................
...... .......... 3 -
FOUNDATIONS.................................................................................................................-
3 -
FOUNDATION AND RETAINING WALLS........................................................................
4-
FLOORSLABS.......................................................................................................................
4 -
UNDERDRAINSYSTEM.....................................................................................................-
5 -
SURFACEDRAINAGE.........................................................................................................
- 5-
LIMITATIONS...........................................................................................................................-
6-
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS
TABLE I- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 24-7-576
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot H25, Filing 7, Aspen Glen, 345 Saddleback Road, 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 Tami York dated October 2, 2024.
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
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 building foundation. 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
Plans for the propsoed residence were conceptual at the time of our field exploration. The
proposed residence is assumed to be a two story structure with attached garage. Ground floors
could be structural over crawlspace or slab -on -grade. Grading for the structure is assumed to
be relatively minor with cut depths between about 2 to 6 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. The ground surface was
relatively level with a gentle slope down to the north at a grade estimated at 2 percent. A pond
borders the lot to the north. Vegetation consists of grass and weeds.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen Subdivision.
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 the Aspen Glen Subdivision. These sinkholes appear similar to others
associated with the Eagle Valley Evaporite in areas of the Eagle Valley.
Kumar & Associates, Inc. 0 Project No. 24-7-576
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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 on Lot H25 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 October 8, 2024. Two 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 CNM-45B drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
Samples of the subsoils were taken with I %-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 testis 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 about '/z foot of topsoil consist of stiff to very stiff, sandy silty clay
between 3% and 6 feet deep overlying dense, silty sandy gravel and cobbles with possible
boulders to the maximum drilled depth of 8 feet. 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.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and gradation analyses. Results of swell -consolidation testing performed
on a relatively undisturbed drive sample, presented on Figure 4, indicate low to moderate
compressibility under conditions of loading and wetting. Results of percent finer than sand size
gradation analyses performed on a small diameter drive sample (minus 1 %-inch fraction) of the
sandy silty clay subsoils show that the silt and clay content is 89 percent. The laboratory testing
is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
Kumar & Associates, Inc. 0 Project No. 24-7-576
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FOUNDATION BEARING CONDITIONS
The upper silty clay soils encountered in the borings possess low bearing capacity and low to
moderate settlement potential especially when wetted. The underlying gravel soils possess
moderate bearing capacity and typically low settlement potential. Spread footings placed on the
upper silty clay soils will have a risk of long-term foundation settlement especially if the bearing
soils become wetted. We recommend the proposed residence be founded with spread footings
bearing on the underlying dense gravel soils. If the risk of settlement is understood and accepted
by the owner the residence can be founded on the upper silt soils with a reduced bearing
capacity. Provided below are recommendations for spread footings bearing on the underlying
gravel soils. If recommendations for bearing on the upper silt soils are desired, we should be
contacted to provide them.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural granular soils below the upper silt soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed entirely on the undisturbed natural gravel soils below the upper
silty clay soils should be designed for an allowable bearing pressure of 2,500 psf.
Based on experience, we expect settlement of footings designed and constructed
as discussed in this section will be about 1 inch or less.
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 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) Topsoil, the upper silty clay soils and any loose disturbed soils should be removed
and the footing bearing level extended down to the relatively dense natural
granular soils. 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.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
Kumar & Associates, Inc. 0 Project No. 24-7-576
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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 55 pcf for backfill consisting
of the on -site soils. Cantilevered retaining structures which are separate from the residence 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 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. Backfill should not contain organics, debris or rock larger
than about 6 inches.
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 0.45. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 375 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 soils, exclusive of topsoil, are suitable to support lightly loaded slab -on -grade
construction. 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
Kumar & Associates, Inc. 0 Project No. 24-7-576
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the designer based on experience and the intended slab use. A minimum 4-inch layer of free -
draining gravel should be placed beneath slabs at grade 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
on -site granular soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area 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,
if any, be protected from wetting and hydrostatic pressure buildup by an underdrain system.
The drains should consist of rigid perforated PVC 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 1/2% 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 I1/2 feet deep.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence 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.
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
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 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.
Kumar & Associates, Inc. 0 Project No. 24-7-576
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
during construction appear different from those described in this report, we should be notified
so that re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on -site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer
Respectfully Submitted,
Kumar & Associates,
Daniel E. Hardin, P.E.
JHP/kac
Kumar & Associates, Inc. 0 Project No. 24-7.576
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24-7-576 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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24-7-576
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LOGS OF EXPLORATORY
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LEGEND
® TOPSOIL; CLAY, SANDY, FIRM, SLIGHTLY MOIST, BROWN.
CLAY (CL); SANDY, STIFF TO VERY STIFF, SLIGHTLY MOIST, BROWN.
GRAVEL AND COBBLES (GM); SILTY, SANDY, DENSE, SLIGHTLY MOIST, BROWN AND GRAY.
DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE.
I
DRIVE SAMPLE, 1 3/8—INCH I.D. SPLIT SPOON STANDARD PENETRATION TEST.
1
17/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 17 BLOWS OF A 140—POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
PRACTICAL AUGER REFUSAL WHERE SHOWN ABOVE BOTTOM OF BORING, INDICATES THAT
I MULTIPLE ATTEMPTS WHERE MADE TO ADVANCE THE HOLE.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON OCTOBER 8, 2024 WITH A 4—INCH—DIAMETER
CONTINUOUS —FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE NOT MEASURED AND THE LOGS OF
THE EXPLORATORY BORINGS ARE PLOTTED TO DEPTH.
4. THE EXPLORATORY BORING LOCATIONS SHOULD BE CONSIDERED ACCURATE ONLY TO THE
DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (pcf) (ASTM D2216);
—200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140).
24-7-576 1 Kumar & Associates I LEGEND AND NOTES I Fig. 3
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