HomeMy WebLinkAboutSubsoil Study for Foundation Design 03.15.2022I (+rt iiçlfi#nfffiniTiå*"'
RECEIVED
APR ü 6 2Û22
GARFIELD COUNTY
COMMUNITY DEVELOPMENT
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
www.kumarusa.comAn Employcc otrncd Compony
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
March 15,2022
Dan Gruenefeldt
P.O. Box 1910
Basalt, Colorado 81621
dan@gruenefeldt.com
Project No. 22-7- 186
Subject: Subsoil Study for Foundation Design and Septic System, Proposed Residenceo
219 Shore Drive, Lot3, Riverview Ranch, Garfield County, Colorado
Dear Dan:
As requested, Kumar & Associates, Inc. performed a subsoil study for foundation design and
septic disposal feasibility at the subject site. The study was conducted in accordance with our
agreement for geotechnical engineering services to you dated February 16,2022. The data
obtained and our recommendations based on the proposed construction and subsurface
conditions encountered are presented in this report.
Proposed Construction: The proposed residence will be a 1200 square foot ADU over a 3-bay
garage located on the site in the area of Pits I and? as shown on Figure 1. Ground floors are
proposed to be slab-on-grade. Cut depths are expected to range between about 3 to 4 feet.
Foundation loadings for this type of construction are assumed to be relatively light to moderate
and typical of the proposed type of construction. The septic disposal system is proposed to be
located southwest of the residence in the area of Profile Pits I and2.
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 proposed building area is relatively flat and was covered with 6 inches of
snow at the time of our site visit on February 28,2022. Vegetation consists of grass and weeds.
The building area is located on the northern edge of a relatively flat topographic bench. North of
the building area, there is a moderately steep snow-covered slope down to the Colorado River,
which is significantly lower than the proposed building area. Pits I and2 were open and the
septic profile pits were being excavated at the time of our site visit.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two
exploratory pits in the building area and two profile pits in the septic disposal area at the
approximate locations shown on Figure l. The logs ofthe pits are presented on Figure 2. The
subsoils encountered, below a thin layer of topsoil, consist of loose to medium dense, slightly
clayey silty sand. Results of swell-consolidation testing performed on relatively undisturbed
samples of the silty sand, presented on Figures 3 and 4, indicate low compressibility under
existing moisture conditions and light loading and a low to moderate collapse (settlement under
constant load) potential when wetted. Results of a USDA gradation analysis performed on a
sample of silty sandy loam obtained from the site are presented on Figure 3. The laboratory test
results are summarized in Table l. No free water was observed in the pits at the time of
excavation and the soils were 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 tend to compress after wetting and
there could be some post-construction foundation settlement. Footings should be a minimum
width of 24 inches. Loose and disturbed soils encountered at the foundation bearing level within
the excavation should be compacted or removed and the footing bearing level extended down to
the undisturbed natural soils. We should observe the completed foundation excavation to confirm
suitable bearing conditions. 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 this area. 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 be designed to resist a lateral
earth pressure based on an equivalent fluid unit weight of at least 50 pcf for the on-site soil as
backfill, excluding organics.
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 forjoint spacing and slab reinforcement should be
established by the designer based on experience and the intended slab use. A minimum 4 inch
layer of 3A -inch road base gravel should be placed beneath slabs for slab support. This material
should consist of minus 2 inch aggregate with less than 50o/o passing the No. 4 sieve and less
than l2o/o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site soils devoid of vegetation and topsoil.
Kumar & Associates, lnc. o Project No.22-7-186
-3-
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, be protected from wetting and hydrostatic pressure buildup by an underdrain
system.
If needed, 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 I foot below lowest adjacent finish grade and
sloped at a minimum lYo 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 IYz 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 ofthe 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/o 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 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 l0 feet in unpaved areas and a minimum slope of
3 inches in the first l0 feet in pavement and walkway areas.
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 building caused by inigation.
Septic System: Two profile pits were dug at the locations shown on Figure l. The soils
exposed in the Profile Pits shown on Figure 2 consist of silty sandy loam. Based on the
subsurface conditions encountered, the site should be suitable for a conventional infiltration
septic disposal system.
Kumar & Associates, lnc. o Project No.22-7-186
-4-
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 indicated on Figure l,
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. Ifthe client is concerned about MOBC, then a professional in
this special field ofpractice should be consulted. Our findings include interpolation and
extrapolation of the subsurface conditions identified at the exploratory pits and variations in the
subsurface conditions mày 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. Vy'e 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 veriry 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.
If you have any questions or if we may be of further assistanceo please let us know.
Respectfully Submitted,
Kumar & Associates,
Daniel E. Hardin, P
DEH/kac
attachments PitsFigure I
Figure 2 Pits
cc:
Figures 3 and 4 - Swell-Consolidation Test Results
Figure 5 * USDA Gradation Test Results
Table I - Summary of Laboratory Test Results
All Service Septic - Carla Ostberg (cgt,ta.asÉÊ{g.l@eud1lçam)
Kumar & Associates, lnc. @ Project No, 22-7.186
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PIT 1 Ptl 2 PROFILE PIT f PROFILE PIT 2
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LEGEND
TOPSOIL, ORGANIC, SANDY CLAY, SILT, FROZEN, DARK BROWN.
SAND
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(su); SllrY, SLIGHTLY CLAYEY, LooSE TO MEDIUM DENSE, SLIGHTIY MOIST
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HAND DRIVE SAMPLE.
DISTURBED BULK SAMPLE.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A MINI-EXCAVATED ON FEBRUARY 28,
2022.
2, THE EXPLORATORY PITS WERE LOCATED BY CLIENT.
3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE NOT MEASURED AND THE LOGS OF
THE EXPLORATORY PITS ARE PLOTTED TO DEPTH.
1. TIIE EXPLORATORY PIT LOCATIONS SHOULD BE CONSIDERED ACCURATE ONLY TO THE
DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT LOGS REPRESENT
THE APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSIÏIONS MAY
BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT ÏHE ÏIME OF EXCAVATION
PITS WERE BACKFILLED SUBSEQUENÏ TO SAMPLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
D0 = DRY DENSTTY (pct) (lSrU D 2216);
GRAVEL = PERCENT RETAINED ON NO. 10 SIEVE;
SAND = PERCENT PASSING NO. 10 SIEVE AND RETAINED ON NO. 325 SIEVE;
SILT = PERCENT PASSING NO. 325 SIEVE TO PARTICLE SIZE .002MM;
CLAY = PERCENT SMALLER THAN PARTICLE SIZE .002MM.
22-7 -186 Kumar & Associates LOGS OF EXPLORATORY PITS lig. 2
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SWELL-CONSOLIDATION TEST RESULTS Fig. 322-7-186 Kumar & Associates
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22-7-186 Kumar & Associates Fig. 4SWTLL-CONSOLIDATION TIST RESULTS
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HYDROMETER ANAL ANALYSIS
24HR. 7HR l MIN.
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DIAMETER OF PARÏCLES IN MILLIMETERS
CLAY coBBtÊs
GRAVEL 1 %SAND 58 o/o SILT 29 %CLAY 12 %
USDA SOILTYPE: Silty Sandy Loam FROM: PP1 @ 5'-6'
SILT JM coAFsE lu {msÉt SMALL MEDIUM LARGE
22-7 -186 Kumar & Associates USDA GRADATION TEST RESULTS Fig. 5
rcrf åmf;mfffii:ifü.*"TABLE ISUMMARY OF LABORATORYTEST RESULTSNo.22-7-186Siþ SandSilty Sandy LoamSOILWP€Siþ SandctÁY(%)2ISILT(Y")2958sÆ,tD(%)(%)GRAVELISILT&CLAY('/6)SAND(%)GRADATION(Y")GRAVELNATURALDRYDENSIÍY(pct)98895.5I¡ATURAtMOISfURECONTEI,¡Tl'tù3.0I445to6(fr)DEPTI{JI2PP-1SAtrlPtE LOCATIONPIT