HomeMy WebLinkAboutSubsoils Study for Foundationrc,liiçlffi#fffi1iiiå*"'
Ân Employoç Owncd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com
Office l,ocations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit Count¡r, Colorado
October I1,2022
Steve & Kimberly Ochko
4362Cowty Road 100
Garfield County, Colorado 81623
kim@ gouldconstruction. com
Project No.22-7-595
Subject: Subsoil Study for Foundation Design, Proposed ADU Residence,4362 County
Road 100, Garfield County, Colorado
Dear Steve & Kim:
As requested, Kumar & Associates, Inc. performed a subsoil study for design of foundations at
the subject site. The study was conducted in accordance with our agreement for geotechnical
engineering services to you dated August 19,2022. The data obtained and our recommendations
based on the proposed construction and subsurface conditions encountered are presented in this
report.
Proposed Construction: The proposed ADU residence will be two story wood frame structure
over a crawlspace located below an irrigation ditch in the southern part of the property. Ground
floor will be structural over crawlspace. Cut depths are expected to range between about 2 to
6 feet. Foundation loadings for this type of construction are assumed to be relatively light and
typical of the proposed type of construction.
If building conditions or foundation loadings are significantly different from those described
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The site consists of pinyon/juniper forest and slopes down to the west at 10 to
I5Yo grade. the buildin g area and part of the driveway had been cleared of trees. An old
backfilled irrigation ditch is located just uphill of the building site. There is a buried irrigation
pipe in the alignment (per owner). Basalt rocks were observed on the ground surface.
Subsurf¿ce Conditions: The subsurface conditions at the site were evaluated by excavating two
exploratory pits on the north and south sides of the proposed house. The logs of the pits are
presented on Figure 1. The subsoils encountered, below about 1 foot oftopsoil, consist of
medium stiff, sandy silt. Results of moisture/density and percent finer than sand size gradation
testing performed on relatively undisturbed samples of the sandy silt, presented on Table 1,
indicate low compressibility under existing moisture conditions and light loading and a low
"|
expansion potential when wetted. Results of a percent finer than sand size gradation analysis
performed on samples of sandy silt obtained from the site indicated the silt soils had 18 to
20 percent sand. No free water was observed in the pits at the time of excavation and the soils
were slightly moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings
placed on the undisturbed natural soil designed for an allowable soil bearing pressure of
1,500 psf for support of the proposed residence. The soils may compress if wetted and there
ìñff6Gne post-construction foundation settlement on the order of up to one inch. Footings
should be a minimum width of 18 inches for continuous walls and2 feet for columns. Loose and
disturbed soils encountered at the foundation bearing level within the excavation should be
removed and the footing bearing level extended down to the undisturbed natural soils. Exterior
footings should be provided with adequate cover above their bearing elevations for frost
protection. Placement of footings at least 36 inches below the exterior grade is typically used in
thisarea.Continuousforrndation*ul1,.hffiforcedtopandbottomtospanlocal
anomalies such as by assuming an unsupported length of at least 12 feet. Foundation walls
acting as retaining structures should be designed to resist a lateral earth pressure based on an
equivalent fluid unit weight of at least 55 pcf for the on-site soil as backfill.
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the areathat 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 and crawlspaces
deeper lhan 4 feet, 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 as needed andat least 1 foot below lowest adjacent finish grade and sloped at a
minimum IYo to a suitable gravity outlet. Free-draining granular material used in the underdrain
system should contain less than 2Yo 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 lYzfeet
deep. 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.
Kumar & Agsociates, lnc.6 Project No. 22-7-595
-3-
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the ADU has been completed:
1) Inundation ofthe foundation excavations andunderslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95"/o of the maximum standard Proctor density in pavement and slab areas
and to at least 90Yo of the maximum standard Proctor density in landscape areas.
Free-draining wall backfill (if any) should be capped with about 2 feet of the on-
site, finer graded soils to reduce surface water infiltration.
3) The ground surface surounding 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 pavement and walkway areas. A swale may be
needed uphill to direct surface runoff around the ADU residence.
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 the building. Consideration should be given to the use of xeriscape to
limit potential wetting of soils below the foundation caused by irrigation.
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 pits excavated at the locations in the building area
and to the depths shown 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
fìndings include interpolation and extrapolation of the subsurface conditions identified at the
exploratory pits 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 at once so 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
Kumar & Associates, lnc. @ ProjectNo. 22-7-595
-4-
should provide continued consultation and field services during construction to review and
monitor the implernentation of our recommendations, and to veriff that the recommeridations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the rec¡mmendations 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.
If you have any questions or if we may be of further assistance, please let us know.
Respectfully Submitted,
Kumar & Associates,lnc.
Daniel E. Hardin,
DEHlkac
attachments
Table I -of Laboratory Test Results
Pits
Kumar & Associates, lnc.8'Project No. 22-7-5gs
I
PIT 1
SOUTH SIDE OF
BUILDING AREA
PIT 2
NORTH SIDE OF
BUILDING AREA
0 0
F
UJt¡l
LL
I-FfLtt¡o
WC=6.9
DD=78
-200=80
Ftd
L¡l
LL
I-t-fLt¡lô
5
WC=4.6
DD=96
-200=80
WC=5.0
DD=82
-2OO=78
5
LEGEND
TOPSOIL: ORGANIC SANDY SILT, WITH ROOTS, ORGANICS, SOFT, SLIGHTLY MOIST, DARK
BROWN.
SILT (ML): SANDY, MEDIUM STIFF, SLIGHTLY MOIST, BROWN.
F
HAND DRIVE LINER SAMPLE.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON AUGUST 26, 2022
2. THE ELEVATIONS OF THE EXPLORATORY PITS WERE NOT MEASURED AND THE LOGS OF THE
EXPLORATORY PITS ARE PLOTTED TO DEPTH. PIT 1 WAS ABOUT 2 FEET HIGHER THAN PIT 2.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORAÏORY PIT LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
4. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF EXCAVATION. PITS WERE
BACKFILLED SUBSEQUENT TO SAMPLING.
5. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSITY (PCt) (ISTV D 2216);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1 140)
22-7 -595 Kumar & Associates LOGS OF EXPLORATORY PITS Fig. 1
l*rtg;,ffif;ffifffi1'r'å;å**TABLE 1SUMMARY OF LABORATORY TEST RESULTS1PIT244Jfft)DEPTHSAMPLE LOCATION5.04.66.9(%)NATURALMOISTURECONTENT829678NATURALDRYDENSITYlocf)r:/"1GRAVEL$tSANDGRADATION788080PERCENTPASSING NO.200 stEvEPlolLIQUID LIMITf%)PLASTICINDEXATTERBERG LIMITSlosflUNCONFINEDCOMPRESSIVESTRENGTHSandy SiltSandy SiltSandy SiltSOIL TYPENo. 22.7.595