HomeMy WebLinkAboutSubsoils Report for Foundation DesignH
H E PWORTH.PAWLAK G EOTECH N I CAL
I lcpworth-Pawlak Ceotcchnical, Inc.
5020 County Road 154
Olcnrvood Springs, Colorado 81601
Phonc: 970"945-7988
ljax: 970-945-8454
Dmail : hpgco@hpgcotech.conr
iltN .ig
August 12,2015
Jay Blackstock
4601 South Dasa Drive
Cherry Hills, Colorado 80111
0 avb Iackstoc k(.rlmac. conr)
Job No.I l5 343r\
Subject;Subsoil Study for Foundation Design, Proposed Residence, Lot IS-18,
Wildflower Laneo Aspen Glen Subdivisiono Garfield County, Colorado
Dear Mr. Blagkstock:
As requested, I-Iepworth-Pawlak Geotechnical, Inc. perfbrmed 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 July 27,2015. 'fhe data
obtained and our recommendations based on the proposed construction and subsurface
conditions encountered are presented in this report. Chen-Northern, Inc. previously
conducted a preliminary geotechnical engineering study for development of Aspen Glen
and a geotechnical engincering study for preliminary plat design (Chen-Northem, 1991
and 1993).
Proposcd Construction: The proposed residence will be two story wood frame
construction above a crawlspace and with an attached gaxage. The property is shown on
Figure 1. Garage floor will be slab-on-grade. Cut depths are cxpected to range between
about 3 to 4 feet. Foundation loadings for this type of construction are assumed to bs
relatively light and typical of the proposed type of construction.
If building conditions or foundation loadings are significantly different from those
described aboveo we should be notified to re'evaluate the recoffImendations presented in
this report.
Site Conditions: The vacant lot is located at thc cul-de-sac of Wildflower Lane.
Vegetation consists of grass and weeds in the front (north) portion and cottonwood trees
with bnrsh grass zurd weeds in the rear part of the site. 'l'he topography consists of
relatively flat upper and lower teffaces separated by a steep slope. The upper bench was
graded during subdivision development and about 3 feet of fill material was encountered
in the upper expioratory pit.
Sutrsidcnce Potential: Aspen Glen is underlain by Pennsylvania Age Eagle Valley
Evaporite bedrock. The cvaporite contains gypsum deposits, Dissolution of the gypsum
under certain conditions can causc sinkholes to dcvelop and can produce areas of
,iljtT
Parker 303-841-7119 Colorado Springs 719-633-5562 Silverthorne 970-468-1989
localized subsidence. During previous studies in the area by Chen-Northern, several
broad subsidence areas and smaller sizc sinkholes were observed scattered throughout the
Aspen Glen development. Thcsc sinkholes were primarily located on the east side of the
Roaring Fork River and appear sirnilar to others associated with the Eagle Valley
Uvaporite in arcas of thc Roaring Fork l{ivcr vallcy. 'l'he nearest mapped sinkhoie is
about 1,000 fcet wcst of this site. Based on our present knowledge of the site, it cannot
be said for certain that sinkholcs will not develop. In our opinion, the risk of grclund
subsidence at Lot IS-i8 is low and similar to other lots in the area but thc owner should
be aware of the potential for sinkhole development.
Subsurfacc Conditions: -fhc subsurfacc conditions at thc sitc were evaluated by
excavating two exploratory pits at the approxirnate locations shown on Figure l. The
logs of the pits arc prcsented on l?igurc 2. 'Ihe subsoils encountered, below about 2%to 3
leet of fill (which includes about I foot of topsoil) consist of sandy silty clay down to the
bottom of I'it 1 ar 14 fcet and I 0 f'cct in Pit 2 where sandy gravel with cobbles was
encountered. Rcsults of swell-consolidation testing performed on relatively undisturbed
sanplcs uf sandy silty clay, plescnted on Figule 3, indicatc lowto modcratc
comprcssibility undcr loading and wetting. Free water was observed in Pit 1 at 14 feet
and Pit 2 at 10lbct at thc time of excavatins and the upper soils wsre moist.
Foundation Bearing Conditions: 'l'he sandy silty clay soils appear to possess a
moderate settlemcnt potential when wettod and under light loads which could result in
movemcnt of footings bcaring on thc soils particularly if they become wetted. Surface
runoff, lanclscape irrigation, and utility leakage arc possiblc sourccs of water which could
cause wetting. An alternative with a lower risk of settlement would be to remove and
replace a certain depth of the silty clay soils with compacted structural fill. 'l'he lowest
risk of settlement would bc to place the foundation entirely on thc underlying relatively
dense granular soil with a deep foundation system such as helical piers or micropiles. If a
deep foundation down to dense gravel is proposed, we should be contacted for additional
recomrnendations. The subgrade should be observed for bearing conditions and further
evaluated for settlement potential at thc time of construction.
Foundation llecommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread
footings placcd on the undisturbed natural sandy silty clay soil designed for an allowable
soil bearing pressure of 1,200 psf for support of the proposed residence. Footings placed
on at least 3 fcet of structural fill which will bear on the undisturbed natural soils can be
designed for an allowablc bealing prcssure of 1,500 psf. Based on experience, we expect
initial settlement of footings designed and oonstructcd as ciiscussed in this section will be
qhnrrf 1 inch nr lpcc. 'T'herp r.nrrld hp erlditinnel seftlcment if the siltv clav soils hecome
wetted. The movement could be differential and could be about t/zto 1% inches for a
wetted depth on the order of 10 feet below structural fill bearing level. Footings should
be a minimum width of 20 inches for continuous walls and 2 feet for columns. Loose and
disturbed soils or existing fill encountered at the foundation bearing level within thc
excavation should be removed and the lboting bearing level extended down to the
undisturbed natural soils. Compactcd structural fill should be a granular material such as
T4 inch road base (CDOT Class 6) and compacted to at least 98Yo of standarcl Proctor
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density at a moisture content near optimum. The fill should cxtend laterally beyond the
footing a distance at least equal to half the depth of fill below the footing. Structural fill
placed to reduce the settlement risk should be at least 3 feet deep below the footings.
Exterior footings should be provided with adequate cover above their bearing elevations
for frost protection. Placement of ibotings at least 36 inches below the exterior grade is
typically used in this area. Continuous foundation walls should bc reinforced top and
bottom to span local anomalies such as by assuming an unsupported length of at least 12
feet, Foundation walls aating 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. A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
F.loor Slabs: The natural on-site soils, exclusive of fill and topsoil, are suitable to
supporl lightly to rnoderately loaded slab-on-grade construction. 'fo reduce the effects of
some differential movement, floor slabs should be separated from all bearing walls and
columns with expansion joints which allow uruestrained vertical movement. Floor slab
controljoints should be used to reducc damage clue to shrinkage cracking. The
requirements for joint spacing and slab reinforcemcnt should be established by the
designer based on experience and the intended slab usc. A minimum 4 inch layer of free-
draining gravel should be placed beneath basement level slabs to facilitate drainage. This
nraterial should consist of minus 2 inch aggregate with less than 50% passing the No. 4
sieve and less than 2% passing the No. 200 sicve.
All fill 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 fill can
consist of the on-site soils devoid of vegetation, topsoil and oversized rock.
Underdrain System: Although free watcr was enoountered below expected excavation
depth 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 basements, retaining walls and deep crawlspaoe areas, be protected
from wetting and hydrostatic pressure buildup by an underdrain system. Shallow
crawlspace (lcss than 4 f'eet) and slabs-on-grade should not need an underdrain system.
If installed, the drains should consist of drainpipe placed in the bottom of the wall backfill
surrounded above the invert level with frec-draining granular material. 1'he drain should
be placed at each level of excavation and at least I tbot below lowest adjacent finish
grade and sloped at a minimum 1% to a suitable gravity outlet or sump and pump. Free-
draining granular rnaterial uscd in the underdrain systern should contain less than 2%
passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a maximum
size of 2 inches. 'fhe drain gravel backfill should bc at lcast l% feet 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 prevcnt wetting of the
bearing soils.
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Surface Drainage: The following drainage prccautions should be observed during
construction and maintained at all times after the residence has been completed:
l) Inundation of the foundation excavations and underslab areas should be
avoided during construction.
2) Extcrior backtill should be adjusted to near optimum muislure altd
conrpactcd to at lcast 9596 of thc maximum standard Prootor density in
pavemcnt and slab areas and to at least 90% ol'thc maxirnum 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 surface surrounding the exterior of the building should be
sloped to drain away llom the foundation in all directions. We
recommend a minimum slope of 6 inches in the first 10 feet in unpaved
arcas and a minimum slope of 3 inches in the first l0 feet in pavement and
walkway areas.
4) ILoof downspouts and drains should discharge well beyond the limits of all
backfill,
5) l,anclscaping which requircs rcgular heavy inigation should be located at
least 5 feet frorn the building. C<lnsideration should be given to the use of
xeriscape to limit potential wotting of soils below the foundation caused by
irrigation.
Limitltions: 'fhis study has bcen conducted in accordancc with gcncrally accepted
geotcchnical engineering principles and practices in this area at this timc. We make no
warranty either cxprcss or implied. 'l'he conclusions and rccommendations submitted in
this report are based upon the data obtained fiom the exploratory pits excavated at the
looations 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, prevcntion or possibility of mold or other biological contaminants (MOBC)
developing in the future. If the client is concemed 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 subsurfhce conditions rnay 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 recolrunendations may be
made.
This report has been prcpared for the exclusive use by our client for design purposes. We
are not responsibie for te chnicai inteqpretations by others of our information. As the
^rnipof e.r^l.rps rrre chnrrlrl -rnrrirle nnnfinrrr.rl r.nncrrltalinn and field servic,es drtrinrtyr\rJvvr vvvrvvur "v "rrvsrv
construction to review and monitor the implementation of our recommendations, and to
verify that thc 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 reprcsentative of the geotechnical
engineer.
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If you have any questions or if we may be of further assistance, please let us know
Respectfully Submitted,
HEPWORTH - PAWLAK GEO'TECHNICAI,, INC.
Louis Eller
Reviewed by:
Daniel E. I{ardin, P.E.
LEE/ksw
attachmenis Figure I -Location Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Figure 3 - Swell-Consolidation'fest Results
Table I - Summary of Laboratory Testing
Refererrces
Chen-Northern, Inc., 1991, Preliminary Geotechnical Engineering Study, Proposed
Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen
Company, dated December 2A, 1991, Job No. 4 I12 92.
Chen-Northern, Inc., 1993, Geotechnical Engineering Studyfor Preliminary Plat Design,
Aspen Glen Development, Garfield County, Colorado, prepared for Aspen Glen
Company, dated May 28, 1993, Job No. 4 ll2 92.
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1153434
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LOCATION OF EXPLORATORY PITS Figure 1
PIT 1 PIT 2
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WC=19.3
DD='103 5
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LEGEND
FILL; about 12 inches of topsoil overlying sandy silty clay withh scattered cobbles and boulders, stiff/medium
dense, slightly moist, ffrixed brown,
CLAY (CL); sandy, silty, stiff, moist to wet with depth, brown.
GRAVEL AND COBBLES (GM); sandy, slightly silty, probable boulders, dense, wet, brown.
F
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2" Diameter hand driven liner sample.
Free water level in pit at time of excavating.
NOTES:
1. Exploratory pits were excavated on August 6, 2015 with a Cat 4168 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 not measured and the logs of exploratory pits are drawn to depth. Pit 2 is about 2
feet lower than Pit 1.
4. The exploratory pit locations and elevations should be considered accurate only to the degree implied by the method
used.
5, The lines between nraterials shown on the exploratory pit logs represent the approxinrate boundaries between
material types and transitions may be gradual.
6. Water level readings shown on the logs were macle at the time and under lhe condiiions irrdicated. Fluctuations in
water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pct)
1153434 &
Hsoworth-Powlok Gsotochnlcol
L.OGS OF EXPLORATORY PITS Figure 2
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APPLIED PRESSURE - ksf
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APPLIED PRESSURE - Ksf
Moisture Content: 19.9
Dry Donoity - 95
Sample of: Sandy Silty Clay
From: Boring 1 at 8 Feet
percent
pcf
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(Compression
upon
"wetting
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Moisture Content: 19.3
Dry Density : 103
Sample of: Silty Sandy Clay
From: Boring 2 at 3 /z Feel
percent
pcf
-No movement
upon
wetting
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o,1 1.0 100
115 3434 SWELL.CONSOLI DATION TEST RFSLJLTS Figure 3
H EPWORTH-PAWLAK GEOTECH NICAL, INC.
TABLE 1
SUMMARY OF IABORATORY TEST RESULTS
Job No. 115 343A
SOIL TYPE
Sandy Silty Clay
Sandy Silty Clay
UNCONFINED
COMPRESSryE
STRENGTH
PLASNC
INDEK
{o/o)
LIQUID
LIMTT
(o/o\
PERCENT
PASSING
NO,200
SIEVE
PIT DEPTH
NATURAL
MOISTURE
CONTENT
NATURAL
DRY
DENSITY
GMVEL SAND
(o/o)(o/o)
95
103
19.9
19.3
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ASPEN GLEN F.ILING NO. 2
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