HomeMy WebLinkAboutSubsoil StudyI(+rtiit*,ffin#sn$iå*"'
An Employcc Ovrncd Compony
5020 County Road 154
Glenrvood 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
March 12,2021
Last Ridge Development
Attn: Gary Manchester
1720Wazee,#5D
Denver, Colorado 80202
gmanchester@lastrid gedev.com
Project No. 2l-7-164
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot26, Stirling
Ranch, 0678 Schooner Lane, Garfield County, Colorado
Dear Gary:
As requested, Kumar & Associates performed a subsoil study for design of foundations at the
subject site. The study was conducted in accordance with øur agreement for geotechnical
engineering services to Last Ridge Development dated February 3,2021. The data obtained and
our recommendations based on the proposed construction and subsurface conditions encountered
are presented in this report.
Proposed Construction: The residence will be located on the lot as shown on Figure 1. The
building will be a single-story wood frame structure over a walkout basement level with an
attached master bedroom wing at the main level also over a walkout basement level. Ground
floors will be slab-on-grade. Cut depths are expected to range between about 3 to 9 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 lot was vacant and the ground surface appeared mostly natural at the time
of our field exploration. The temain is relatively flat and moderately steeply sloping down to the
south/southeast at grades from about 12to25%o in the buildingarea. Elevation difference across
the proposed building foot-print is about l5 feet. Vegetation consists of pinyon and juniper trees
with sage brush, grass and weeds. There are basalt cobbles and boulders on the ground surface.
a
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating
three exploratory pits at the locations shown on Figure 1. The proposed building corners had
been staked in the field by others.
The logs of the pits are presented on Figure 2. The subsoils encountered, below about I to
lYzfeet of topsoil, consisted of nilto about2 feet of stiff, sandy silty clay overlying relatively
dense, basalt gravel, cobbles and boulders in a sandy clayey silt matrix that extended down to the
maximum depth explored of 8 feet. The near surface silty clay soils were only encountered at
Pit 2. Results of swell-consolidation testing performed on relatively undisturbed samples of the
silty clay and the sandy clayey silt matrix soils, presented on Figures 3 and 4, indicate low
compressibility under existing moisture conditions and light loading and a low hydro-
compression potential when wetted. The samples showed moderate compressibility when loaded
after wetting. Results of a gradation analysis performed on a sample of the coarse granular soils
(minus S-inch fraction) obtained from the site are presented on Figure 5. The laboratory testing
is summarized in Table l. No groundwater was observed in the pits at the time of excavation
and the soils were slightly moist below the moist topsoil layer.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings
placed entirely on the undisturbed natural coarse granular soils designed for an allowable bearing
pressure of 2,000 psf for support of the proposed residence. The footings should be a minimum
width of 1 8 inches for continuous walls and 2 feet for columns. All topsoil, predominantly clay
soils and any loose 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 coarse granular soils. The subgrade should then be moistened and compacted. Any
voids from large cobble or boulder removal below footing areas should be backfrlled with lean
concrete or well graded sand and gravel, such as base course, compacted to at least 98Yo standard
Proctor density at a moisture content near optimum.
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 well reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least 72 feet. Foundation walls
Kumar & Associates, lnc. o Project No. 21-7-164
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acting as retaining structures should be designed to resist alateral earth pressure based on an
equivalent fluid unit weight of at least 50 pcf for the on-site granular soils, excluding topsoil and
oversized (plus 6-inch) rocks, as backfill.
Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded
slab-on-grade construction. There could be some slab settlement where underlain by the silty
clay soils if the subgrade were to become wetted. Removal and replacement of the silty clay
soils with compacted structural fill could be done to reduce the risk of settlement.
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 controljoints 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 minimurn 4 inch layer of free-draining gravel
should be placed immediately beneath basement level slabs to facilitate drainage. This material
should consist of minus Z-inch aggregate with less than 50o/o passing the No. 4 sieve and less
than2o/o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at leasf 95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site granular soils devoid of topsoil and oversized (plus 6-inch) rocks.
Underdrain System: Although groundwater was not encountered during our exploration, it has
been our experience in the area and where clayey soils are present that local perched
groundwater can develop during times of heavy precipitation or seasonal runoff. Frozen ground
during spring runoff can also 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 4 inch diameter PVC drainpipe placed in the bottorn of the wall
backfill surrounded above the invert level with free-draining granular material. The drain should
be placed ateach level of excavation and at least 1 foot below lowest adjacent finish grade and
sloped at a minimum %o/o to a suitable gravity outlet. Free-draining granular material used in the
underdrain system should contain less than 2o/o passing the No. 200 sieve, less than 50% passing
Kumar & Associates, lnc. @ Project No. 21-7-164
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the No. 4 sieve and have a maxlmum stze of 2 inches. The drain gravel backfill should be at
least lV, feet deep and be covered by filter fabric such as Mirafi 140N or 160N.
Surface Drainage: The following drainage precautions should be observed during construction
and maintained at all times after the residence has been completed:
l) 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"/o of the maximum standard Proctor density in pavement and slab areas
and to at least 90o/o of the maximum standard Proctor density in landscape areas.
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 l0 feet in pavement and walkway areas. A swale may be
needed uphill to direct surface runoffaround the residence.
4) Roof downspouts and drains should discharge wellbeyond the limits of all
backfill.
5) Landscaping which requires regular heavy inigation, such as sod, and lawn
sprinkler heads 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 irigation.
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 wananty 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 1
and to the depths shown on Figure 2, 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 pits and variations in the subsurface conditions may not become evident until
Kumar & Associates, lnc. @ Project No. 21-7-164
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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
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to verifu 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
ofexcavations and foundation bearing strata and testing ofstructural fill by a representative of
the geotechnical engineer.
If you have any questions or if we may be of fuilher assistance, please let us know.
Respectfully Submitted,
Kurnar &
David A. Young,
DAY/kac
attachments
Figure 2 - Logs of Exploratory Pits
Figures 3 and 4 - Swell-Consolidation Test Results
Figure 5 - Gradation Test Results
Table I - Summary of Laboratory Test Results
Stryker Brown Architects - David Brown (david@strykelbrown.com)
Figure 1 Exploratory Pits
cc:
Kumar & Associates, lnc. €'Project!lo. 2l-7-164
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APPROXIMATE SCALE-FEET
21 -7 -1 64 Kumar & Associates LOCATIONS OF EXPLORATORY PITS Fig. 1
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PIT 1
ELEV. 975,
Ptr 2
ELEV. 971'
PIT 3
ELEV. 986'
0 0
WC=10.1
DD=92
-200=68r-tJ
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WC=11.0
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LEGEND
ToPSolL; SANDY SILT AND CLAY WITH GRAVEL AND COBBLES, ORGANICS, LOOSE, MOIST,
DARK BROWN,
CLAY (CL); SILTY, SANDY WITH GRAVEL AND CoBBLES, VERY STIFF, SLIGHTLY MolST,
BROWN, CALCAREOUS.
GRAVEL AND COBBLES (GM_GC
SANDY CLAYEY MATRIX, SLIGHT
CALCAREOUS.
)
L
; CONSISTING OF BASALT FRAGMENTS To BoULDER SIZE, lN A
Y CLAYEY, MEDIUM DENSE, SLIGHTLY MOIST, LIGHT BROWN,
F
t
HAND DRIVEN 2-INCH DIAMETER LINER SAMPLE
I
I
)
I
DISTURBED BULK SAMPLE.
DIGGING REFUSAL WITH LIGHT DUTY MINI-EXCAVATOR.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON FEBRUARY 9,2021
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY PIT LOCATIONS AND ELEVATION 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 TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE IIME OF EXCAVATION
7, LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216):
DD = DRY DENSITY (pcf) (ASTM D 2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D 422);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D 1 1 4O);
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21 -7 -1 64 Kumar & Associates LOGS OF EXPLORATORY PITS Fig. 2
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SAMPLE OF: Sondy Siliy Cloy
FROM:Pit2 E^2.5'
WC = 10.1 %, DD = 92 pcf
-2OO = 68 %
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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21-7-164 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 3
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:
SAMPLE OF: Sondy Cloyey Mcfrix
FROM: Pit 2@ 4,5'
WC = 11.0 %, DD = 95 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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frð
1
JJ
IJ
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ED PRESSURE _ KSF IO 100
21 -7 -1 64 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig.4
HYDROMETER ANALYSIS SIEVE ANALYSIS
IIME READINêS
24 HRS 7 HRS
U.S. STANDARD SERIES CLEAR SQUARE OPENINGS
\/Eâa/a'11/tr
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100
90
ao
70
60
50
10
30
20
10
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to
20
30
40
50
60
70
ao
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loo
.oo2 .oo5 .oo9 .019 .o37 .125 2.O
DIAMETER OF PARTICLES IN MILLIMETERS
t52
CLAY TO SILT COBBLES
GRAVEL 59 % SAND 35 %
LIQUID LIMIT - PLASTICIÏY INDEX
SAMPLE oF: S¡lty Sondy Grovel wilh Cobbles
SILT AND CLAY 6 %
FROM: Pi+ 5 @ 4' lo 5'
Th€s€ l€sl resulls opply only lo lh6
sompl€s whlch w€rs l€sled. Th6
l6sllng 16port sholl nol b€ reproduced,
exc€pl ln tull, wlthoul lhe wr¡ll6n
opprovol of Kumor & Assoclolos, lnc.
Slov6 onolysls losllng ls p€rformêd lñ
occordoncô wllh ASIM D6913, ASÍM 07928,
ASIM Cl36 ond/or ASTM 01140.
SAND GRAVEL
FINE MEDTUM lCOanSe FINE COARSE
21 -7 -1 64 Kumar & Associates GRADATION TEST RESULTS Fig.5
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TABLE I
SUMMARY OF LABORATORY TEST RESULTS
No.21.7.164
GRÂDÀTIONSAMPLE LOCATION
SAND
tf/")
PERCENT
PASSING NO.
200 stEVE
LIQUID LIMIT
to/"\tol"ì
PLASTIC
INDEX
losfì
UNCONFINED
COMPRESSIVE
STRENGTH SOIL TYPEPIT
(ftì
DEPTH
(o/"\
NATURAL
MOISTURE
CONTENT
NATURAL
DRY
DENSIÌY
lþcfì
GRAVEL
t:h\
Sandy Silty Clay22%10.1 92 68
Sandy Clayey Silt Matrix4YrI1.0 93
Silty Sandy Gravel with
Cobbles34-5 59 35 6