HomeMy WebLinkAboutSubsoil Study for Foundation Design 04.12.2006Hepworth-Pawlak Geotechnical, inc.
5020 County Roacl 154
Clcnwood Springs, Colorado 81601
Phoner 970-945-7988
HEPWORTH - PAWLAK GEOTECH NICAL Fax: 970-945-8454
email: hpgeo@l'rpgeotech. com
Apnl12,2006
Caribou Construction
Attn: Fred Cooke
734}/la¡n Street
Silt, Colorado 81652
Job No.106 0291
Subject:Subsoil Study fo¡ F.oundation Design, Proposed Residønce, Lot20, Fiiing
6, Elk Springs Subdivision, Garfieid County, Colorado
Dear Mr. Cooke:
As requested, Hepworf;h-Pawlak Geotechnical,lnc. pe.rformed a subsoil study for design
of foundations at the subject site. The study was condlcted in accordance with our
agreement for geotechnical engineering services to Caribou Construction dated }ld:arch23,
2006. The data obtained and our lecommendations based on the proposed construction
and subsurface conditions encountered are presented in this re,port. Hepworth-Pawiak
Geotechnical, Inc., previously perfonned a preliminary geotechnical study for Filings 6
through 9, Eik Springs {formerly Los Amigos Ranch PLID) and reported our findings on
February 14,1997, Job Ncj, L97 617.
Proposed Construction: Theproposed residence wil1be one story wood frame
construction above a basement with an attac}red garage and located on the site as shown
on Figure 1. Basement and garage floors will be siab-on-grade. Cut depths are expected
to range between about 3 to 8 feet. Foundation loadings for this ty¡re of construction are
assumed to be relatively light and t5lpical of the proposed fire of construction.
If building conditions or foundation loadings are significantly different frorn those
deseribed above, we should be notified to re-evaluate the recommendations presented in
this report.
SÍte Conditions: Lot 20 is located on the west (downhíll) side of Wood Nympth Lane on
a rolling mesa. Vegetation consists of sage brush grass and weeds and scaftered pinion
and junipertees. The ground surface slopes gently down to the southwest ai a grade of
about 11 percent in the building area. Se¿ittered basalt cobbles are visible on the ground
surface.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating two exploratory pits at tlle approximate locations shown on Figure 1. The
logs of the pits are presented on Figure 2. ^Ihe subsoils encountered, below about one
foot of topsoil, consist ofbasalt cobbles and boulders in a silt and sand rnatrix. Results of
swell-consolidation testing performed on a relatively undisfurbed sampie of the silt and
sand matix soils, presented on Figure 3, indicate low to moderate compressibility under
conditions of loading and wetting. Results of a gradation analysis performed on a sample
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of gravel (minus 5 inch fraction) obtained from the site are presented on Figure 4. No
free water was observed in the pits at the time of excavation and the soils were slightly
moist to moist.
Foundatíon Recommendations: Considering the subsoil conditions encountered in thc
exploratory pits and the nafure of the proposed construction, we recommend spread
footings placed on the undisturbed natural soil designed for an allowabie soil bearing
pîessure of 2,50A psf for support of the proposed residence. The matrix soils tend to
compress after wettin g and there could be some post-construction foundation settlement.
Foofings should be a minimum width of i6 inches for continuous walls and 2 feet for
colurnns. Utility trenches and deep cut areas may require rock excavating teclrniques
such as chipping or blasting. Loose and disturbed soils encountered at the foundation
bearing level within the excavation should be removecl and the footing bearing level
extended down to the undisturbed natural soils. Voids created from boulder removal at
footing grade should be filled with a structural material such as road base compacted to
gS.percent standard Proctor density at a moisture content near optimurn. Exte¡ior footings
should be provided with adequate cover above theirbearing 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 anornalies such as by assuming an unsupported length of at least 10 feet.
Foundation walls acting as retaining structures should be designed to resist a lateral earth
pïessur'Ê based on an equivalerit fluid unit weight of at least 50 pcf for the on-site soil as
backfili.
Floor Slabs: The nafural on-site soils, exclusive of topsoil, are suitable to support lightly
to moderately 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 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
material should consist of minus 2 :r;rch aggregate with less thart SÙYo passing the No. 4
sieve and less than ZYopassingthe No. 200 sieve-
AII fiil 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 fil1can
consist of the on-site soils devoid of vegetation, topsoil and oversized rock.
Underdrain System: Although free water was not encountered during our exploratio4 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.
Job No.106 0291
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The drails should ccnsist of drainpipe piaced in the bottom af the wali backfill
surrounded above tire invert level with ûee-drai¡ring granular material. The drain should
be placed at each lavel of excavation ønd at least I fuot below lowest a,âjacent finish
grade arid sloped at a minimum 1o/ata a suítable gravity outlet. Free-draining granular
matenalused in the underdrain system shoukl contain less fhan 2o/opassingthe No. 20t)
sieve,lessthan50%passingthe No.4sieve andhaveamaxirnum sizeof2 inches. The
drain gravel backfill shouldbe at least 7kteetdeep.
Surface Drninage: The following drainage precautions should be abserved during
construction and maintained at afl times aftcr the residence has been completed:
l) lnunclation of the foundation excavations and underslab areas should be
avoidcd cluring co¡rstruction.
2) Exterior baekfill should be acljusted to near optirnum m*isture and
compacted to at least 95o/o af the maximum sta::dard Proctor density in
pavÐment and slab areas and to at least 9û% of the maximum stanclard
Proctor density in landscapc âreas. Free-draining rvall backfiil should bs
capped with about 2 faet of the on-site, finer graded soils to reduce surfhce
water infiltration.
3) The grouncl surfacs surrounding the exted*r of the building should be
slcped to drain away frorn the foundation in all directions. 'We
recomrnend a rninirnum slope of 12 inches in the first 10 fest in unpaved
areas and a mjnimurn slope of 3 inchss in the first 10 feet in pavement and
walkway arsas. A swale maybeneeded uphill to dirsct surfäce runoff
around the tesidence.
4) Roof downspouts and drains should diseharg* ¡¡'ell beyond the limits of all
backfill.
timitations: This sfudy has been conducted in accordance with geneially accepted
geotechnical engineering princþies and practices in this areaat this tirne. We make no
warranty either express or implied. The conciusions and recommendations submitted in
this report are based upon the data obtained from tire exploratory pits excavated at the
iocations indicated on Figure 1 and to the depths shown on Figure 2, fhe proposed lype of
oolskuction, ând our experience in the area. Our services do not inciude determining the
presetrce, prevention or possibility of mold or other biological contaminants (MOBC)
ãeveloping in the future. If the client is c,oncemed about MOBC, then a professional in
this special field of practice should be consulted. Our {indings include interpoiation and
extrapolation of the subsurface conditions identified at the exploratory pits and variations
in the subsurface conditions may not becorne evident untii excavation is performed. If
conditions encountered during consh'uction appeår different liom those described in this
report, we should be natified at once so re-evaluation of the recomrnendations may be
rnade.
This report has been prepared for thc exclusive use by aur client for clesign purposes. We
are not responsibie for technjcal interpretations by others of aur inforrnation. As the
project evolves, we should provide continued consultation and field services dur.ing
constructicn to review and monitor the implementation of our recommendations, and to
verify that the recommendations have been appropriately intcrpreiecl. Sigaificant design
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changes may require additional analysis or modífications to the recommendations
presented herein. We recommend on-site obselvation 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 maybe of fi¡rther assistance, please let us know
Respectfully Submitted,
HEP\AiORTH . PA'WLAK GEOTEC}INICAL, INC.
Louís E. E1ler
Reviewed by:
Daniel E. Hardin, P-E.
LEE/ksw
attachments Figure I - Location of Exploratory Pits
Figure 2 *Logs of Exploratory Pits
Figure 3 - Swell-Consolidation Test Results
Figure 4 - Gradation Test Results
Job No.l06 0291
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APPROXIMATE SCALE
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LOT 19
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FIESIDENCE - -
LOCATION OF EXPLORATORY PITS1 06 0291 Figure 1
PIT 1
ELEY.= 7072'
PIT 2
ËLEV.:7074'
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WC=29.8
DD=80
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WC=25.8
+4=61
-200=18
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TOPSOIL; sandy silt and c¡ay, organic, mo¡st, dark brown
BASALT COBBLES AND BOULDERS (GM); in a silt and sand matr¡x, dense, slightly mo¡st to moist, light brown
to white, calcareous.
2" Diameter hand driven liner sample.
Disturbed bulk sample.
Practical digging refusal with backhoe,
NOTES:
1. Exploratory pits were excavated on April 3, 2006 with a Cat 4200 backhoe
2. Locations of exploratory pits were measured approximately by pacing from features shown on the site plan
provided.
3. Elevations of exploratory pits were oblajned by interpolation between contours shown on the site plan provided and
checked by instrument level.
4. The exploratory pit locations and elevations should be considered accurate only to the degree implied by the method
used.
5. The lines beiween materials shown on the exploratory pit logs represeni the approximate boundaries between
material types and transitions may be gradual.
6. No free water was encountered in the pits at the time of excavating. Fluctuation in water level rnay occur with time.
7. Laboratory Testing Results:
WC = Water Content (7d
DD: Dry Ðensity (pcl)
*4 : Percent retained on the No. 4 sieve
-200 : Percent passing No. 200 sieve
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106 0291 (3¿
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LOGS OF EXPLOHATORY PITS Figure 2
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TIME READINGS7HF15 MlN. 60MlNl9MlN.4 MtN.l MfN, #200 #100U.S. STANDARD SERIES#50 #30 #16.005 .oo9 .019 .037 .o74 .150 .300 .600 1.18 2.36DIAMETEF OF PARTICLES IN MILLIMETEHSCLEAF SOUABE OPENINGS24045#8 #4 318' 314" 1 112" 3u 5'ô" 8'1009070602ñØù50 r--zl¿¡(Jt¡J40L6t¡l 40zaf-I¡JÉþ50zhlC)uKJfL 6030809010,2A0,001 .002ct¡YtosLTGRAVEL 61 O/"SAND 21 %LIOUID LlMlï o/oSAMPLË OF: Gravel and Cobbles with S¡lt and SandMatrix. COEBLÃìSILTANDCLAY 1B %PLASTICITY INDEX o/oFROM: Pit 2 at 3lo 4/z FeeT4.75 9.5 19.0 37.512.576.2 152127GåHEPYvoÌrni.PAr1 06 0291GRADATION TEST RESULTSFigure 4t