HomeMy WebLinkAboutSubsoil Study for Foundation Design 11.07.16H-P*KUMAR
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5CI2û cor¡nty Hoad 154
Glenwood Springs, Ce 81691
Phone: {970} 945-79Ê8
Fax {e70) S4S.8454
Email hpkglenwood@ kumarusa.com
Offico Locations: Parker, Glenwood Springs, and $ilvprihorne, Colorado
November 7,2ü16
Tim Rafferty
365 Rabbit Road
Carbondale, Colorado 81623
lTmrj1522@gmail.comi
Project No.l6-?484
Subject:Subsoil Study for Foundation Design, Proposed Residence, Parts of Lots 4 and ?,
Rimledge Ranch, West of County Road 100, Garfield County, Colorado
Dear Mr. Rafferty:
As requested, H-PlKumar performed a subsoil study for design of foundations ât the subject site.
The study was conducted in accordance with our agreement for geotechnical engineering
services to ytu dated September 27,2016. 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 one story wood frame construction
above a walkout basement. Ground floor will be slab-on-grade. Cut depths are expected to
range between about 3 to g 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 Ccnditions¡ The vacant property is vegetated by sage brush, grass and weeds with a stand
of scrub oak west of the building area. The site is located on a rolling upland mesa. The ground
surface slopes moderately to strongly down to tire northwest at a grade of about l? percent in the
building area.
Subsidence Potential: Bedroek of the Pennsylvanian Age Eagle Valley Evaporite underlies the
Rimledge Ranch and the site is located within the Ca¡bondale Collapse Center (Tweto and
Others (t978), Kirkham and Scott (2002). These rocks âre a sequence of gypsiferious shale,
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fine-grained sandstone/siltstone and limestone with some massive beds of gypsum. There is a
possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie
portions of the property. 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 broad subsidence areas and sinkholes have been observed. These sinkholes appear
similar to others associated with the Eagle Valley Evaporite in areas of the lower Roaring Fork
Valley.
No evidence of subsidencs or sinkholes were observed on the property or encCIuntered in the
subsurface materials, however, the exploratory pits were relatively shallown for foundation
design only. Based on our present knowledge of the subsurface conditions at the site, it can not
be said for certain that sinkholes wiil not develop. The risk of ft¡ture ground subsidence at the
sitc throughout the service life of the structure, in our opinion is low, however the owner should
be 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.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two
exploratory pits at the approximate locations shown on Figure l. The logs of the pits are
presented on Figure 2. The subsoils encountered, below about clne foot of topsoil, consist of
stratified slightly gravelly silty eand and sandy silt overlying basalt cobbles and boulders in a
sandy silt matrix at Pit l. A thin sandy clay layer was observed above the sand and silt soils at
Pit 2. Results of swell-consolidation testing performed on relatively undisturbed samples of
sandy silt, presented on Figures 4 and 5, indicate low to moderate compressibility under existing
moisture conditions and light loading and wetting. No free water was observed in the pits at the
time of excavation and the soils were slightly moist to moist.
Foundation Bearing Conditions: The subsurface conditions åt the site are variable. Basalt
cobbles and boulders were encountered in the uphill exploratory pit above assumed excavation
depth. The stratified sand and silt sails encountered at typical shallow foundation depth tend to
settle when they become wetted. A shallow foundation placed on the sand and silt soils will
have a risk of settlement if the soils become wetted and care should be taken in the surface and
subsurface drainage around the house to prevent the soils from becoming wet. It will be critical
to the long term performance of the structure that the recommendations for surface drainage and
subsurface drainage contained in this report be followed. The amount of settlement, if the
bearing soils become wet, will be related to the depth and extent of subsurface wetting.
Mitigation methods such as deep compaction, or a heavily reinforced mat foundation, on the
order of 2 feet thick, and designed by the structural engineer can be used to support the proposed
house with a lower risk of settlement. If a deep foundation or mat foundation is desired, we
should be contacted to provide further design recommendations.
H-P + KUMAR Projecl No. 1S-7-484
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Foundation Recommendations¡ Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed constn¡ction, \rye 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-co¡¡struction foundation settlement. Footings should be a minimum width of
l8 inches for continuous walls and 2 feet for columns. Loose and disturbed soils and existing fill
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 42 inches below the exterior grade is typically used in this area. Continuous
foundation walls should be reinforced top and bottom to spaÍ local anomalies such as by
assuming an unsuppCIrted length of at least 12 feet. Foundation walls acting as retaining
structures should be designed to resist a lateral earth pressure based rn aÍ equivalent fluid unit
weight of at least 50 pcf for the on-site soil as backfill.
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 movemenq 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 darnage due
to shrinkage cracking. The requirements for joint spacing and slab reinforcêment 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 beneath basement level slabs to facilitate drainage.
This materiai should consist of minus 2 inch aggregate with less than 507¿ passing the No. 4
sieve and less than 27o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least gi%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site soils devoid of vegetation, topsoil and oversized rock.
Underdrain Systemr Although free water $/as 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. Vfe recommend below-grade construction, such ås retaining walls and basement
arÊas, be protected from wetting and hydrostatic pressure buildup by an underdrain system.
The drains should consist of drainpipe placed in rhe bottom of the wall backfill sunoundEd 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 l7o to
H-P= KUMAR Project No. 16-7-¿184
4
a suitable gravity outlet. Free-draining granular material used in the underdrain sysfem should
contain less than ZVo passing the No. 20Û sieve, less than 507o passing the No. 4 sieve and have a
maximum size of 2 inches. The drain gravel backfill should be at least l% feet deep.
Surface Drainage: The following drainage precautions should be observed during construction
and mainlained at all times after the residence has been completed:
l) Inundation of the foundation excavations and underslab areas should be avoided
during constn¡ction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95Vo cf the maximum standard Proctor density in pavement and slab areas
and to at least 90Vo of the maximum standard Proctor density in landscape areas.
Free-draining wall backfill should be capped with about 2 feet of the on-site, finer
graded soils to reduce surface water infiltration.
3) The ground surlace sunounding the exterior of the building should be sloped to
drain away from the foundation in all directions. lVe recommend a minimum
slope of l2 inches in the fïrst l0 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 runoff around the residence.
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy inigation should be located at least 10
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 indicated on Figure I
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
exploratCIry pits and variations in the subsurface conditions may not become evident until
excavation is performed. If conditions encountered during construction âppeff different from
those described in this report, we should be notifïed at once so re*evaluation of the
recommendâtions may be made.
II-F* KUMAR Froject Nû. 1ô.7-484
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This repo* 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 recommendatirns, and to verify that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. 1üfe 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,
H-P*KUMAR
Louis E. Eller
Reviewed by:
Daniel E. Hardin, P.E,
DEHlksw
attachments Figure I * Location of Exploratory Pits
Figure 2 * Logs of Exploratory Pits
Figure 3 * Legend and Notes
Figures 4 and 5 * Swell-Consolidation Test Results
References:
Kirkham, R.M. and Others, 2OA2. Evaporìte Tectonism ín the Lo¡ver Roaring Fork Ríver Valley,
West*Centrel Colorado, in Kirkham, R.M., Scott, R.B. and Judkins, T.W. eds.. Iale
Cenoxoíc Evaporite Tectonism and Volcanism ín West Central Colorado. Geological
Society of America Special Paper 366, Boulder, Colorado.
Tweto, Ogden and Others, 1978. Geology Map af the Leadville I" X 2o Quadrangle,
Northwestem Colorado. U.S. Geological Survey Map l"9gg.
H-F* KUMAR Project No. 16-7-484
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16-7-484 H-Pvl<UMAR LOCATION OF EXPLORATORY PITS Fig. 1
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PIT 2EL 7A21'
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16-7-484 H-P*IruVîAR
. 'TJaa .-."1.,, ,.1.,: :; j LOGS OF EXPLORATORY PITS Fig. 2
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TOPSOILI OR0ANIC SANDY SILT AND ClåY, FIRM, SLIGHTLY l/OtST, f¡ÂRK BROWN
CLAY (CL); SANDY, SILTY, STIFF, SLIGHTLY MOIST, REDDISH BROWI|
q!!-I ANo SAND (HL-SM); STRAT¡FTED srLry SAND AND sAN0y srLT, sLtGHTLy GRAVELLy,sTtFF, MO|5T, LTGHT BROyfN, CALCAREoUS.
BASALT COESLTS AND BOULOERS (GC)¡ IN Å SANDY SILTY CI.ÀY MATRIX, DENSE, SLIGHTLY
MOIST, LIGHT SROWN, CALCAREOUS.
HAND DRIVEN UNER SAMPLE.
PRACNCAL OIGGING REFUSAL.
NOTES.
l. THË EXPLORATORY FITS WERE EXCAVATSD W|TH A SACKHOE ON SEPTEMBER 27, 2:016.
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APFROXII'!ÅTELY BY PACING FROM
FEATURES SHOTVN ON THE SITE PI.AN PROVII}ED.
3. ÎHË ELEVATIONS OF THE EXPLORATORY PITS WERE OETAINED gY INTERPOLqïON BETWEEN
CONÎOURS ON THE SITE PI.AN PRCIVIOEO.
4. THE EXPLORATORY PIT LOCATIONS ANO ELËVATIONS SHSULD EE CONS¡DERED ACCURATE ONLYTO T¡{E OEGRÉE IMPUED BY THE IIETHOO USEI}.
5. TI{E LINES BETì,VEEN MATSRIALS SHOWN ON THË EXPLORATORY PIT LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY 8E GfiAOUAL.
6. GROUNDI,VATEF WAS NOT ENCOUNTERED ¡N THE P¡ÎS AT THE TIMË OF EXCAVATING.
7. LABORATORY TESï RESULTS¡
WC = WATER CONTENT (X) (ASTM Ð 2216)i
DD g IIRY DENSITY (PCt) (ISTM D 2216);
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a.:+t:.'.. :,.. ::-..., :.:LEGEND AND NOTES Fis. 3
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SAMPLE OF: Sondy Slll
FROM:P|I 2 O 3'
WC = 13.2 X, Oû = 81 Pêf
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ADDITIONAL COMPRESSION
UNÐER CONSTANT PRESSURE
DUE TO WETTING
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1 6-7-484 H-P+KUMAR.a ::,'.. a !.SWELL-CONSOLIDATION TEST RESULT Fis. 4
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SÂMPLE OF: Sondy Slll
FROM:P|i2O4'
WC = lO.5 ll, DD = 8û Pcl
AODITIONAL COMPRËSSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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16-7-484 H.P*I(UMAR SWELL-CONSOLIDATION TEST RESULT Fig. 5