HomeMy WebLinkAboutObservation of Excavation and Soil Boring Report 05.31.2012~tech
HEPWORTH · PAWLAK GEOTECHN ICAL
May 31, 2012
C lint & Susan Omang
19 B lu e Creek Trai l
Carbondale, Co lorad o 8 1623
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Job No. 11 2 l 19A
Subject: Observation of Excavation a nd Soil Boring, Proposed Res idence, Lot 20
Wooden D eer Subdivis ion , 262 Wooden Deer , Missour i Heights, Garfield
County, Colorado
Dear Mr. & Mrs. Omang:
As requested, a represent ative of H epworth-Pawlak Geotechn ica l, Inc . observed the
excavation at the subj ect s ite on May 8, 20 12 to evaluate the so ils exposed for foundation
support. The findings o f our o b servat io ns and re co mmendations for the foundation
design ar e present ed in thi s report. The services were performed in acco rd ance w ith our
agreement for profess ional engineering services to you, dated May 4, 2012 . We were
pro vid ed a r eport of a sub so il study performed by CTC Geotek, Inc dated Ja nu ar y 24,
2000, Project 992043 for a res id ence at the s ubject s ite.
Th e propost:<l res idence w ill be a one story woo d fr a me structure over a walko u t
basement. Fo undations were designed b ased on an a llowab le soil bearing pressure of
2,000 psf based o n the previous soi ls r eport.
Observation of Excavation: At the time of o ur v is it to the s ite, the foundation
excavation had been cut in two levels fro m 2 to 10 feet below the adjacent gro und
surface. The soils exposed in the bottom of the excavat ion consisted mo s tl y of medium
st iff, sand y s ilt and c lay. Results of swell-co nso lid at io n t esting per fo nned on s il t a nd clay
samples t aken from the s it e, shown o n F igure 1, indicate the soi ls are highly co ll apsible
(settlement und er constant load) when wett ed and were highly compressible under
increased loadin g after wetting. H ighl y weath ered s ilt stone b edrock o f the Eagle Vall ey
Evaporite Fo nnation was observed in the deepest excavation areas o n th e uph ill , north
s id e of the excavation . No free water was encount ered in the excavation and the soils
wer e s lightly mois t to moi st.
Due to the large s ettlement potential of the s ilt and clay bearing so il s based on the testing
result s, we r eco mmende d that at least o ne bo rin g be drilled on th e do wnhill , so u t h s ide of
the house to assess the depth of sett lement-pro ne so il s and eva luat e potential settlem ent s
in the event that the so il s underling the house became wet.
Subsurface Boring : A boring was dri ll ed ne ar the southeast comer of the hou se w ith a
CME-45B tru ck-mount e d drill ri g to a d epth o f21 feet o n May 15 , 20 12. Med ium s tiff to
Pa rk er 303 -84 1-7 l l 9 • C n lnradn Springs 7 19-63 3-5562 • S ih-erthnrn e 9 7 0 -4 6~-l 9~9
Clint & Susan Omang
May 31, 2012
Page 2
stiff, sandy silt and clay were encountered to a depth ofl4 feet (about 11 feet below
footing grade) overlying hard siltstone bedrock down to the bottom of the boring.
Samples of the soil and bedrock were taken from the boring with relatively undisturbed
sampling methods for laboratory testing. Results of swell-consolidation testing
performed on silt and clay samples taken from the boring, shown on Figures 2 and 3,
indicate the soils have a low collapse potential when wetted and were moderately
compressible under increased loading after wetting. No free water was encountered in the
boring at the time of drilling.
Conclusions: The sandy silt and clay soils underlying the house have variable settlement
potential especially when wetted. The underlying siltstone bedrock is not settlement
prone. The depth of silt and clay below footing grade varies from nil at the northwest
comer of the house to about 11 feet at the southeast corner. Our laborat01y test results
indicated a variable amount of potential settlement. Based on the test results and our
experience with similar soils, we expect the overall settlement potential to be about 2 to
3% of the wetted depth of the clay and silt soils below footing grade. In the event that the
bearing soils became wet, the resulting settlement could be 2 to 3 inches and likely result
in building distress. The settlement would be differential between the north side of the
house, which would not settle since it is on the siltstone bedrock, and the south side of the
house, where there is about I 0 feet of settlement prone soil below footing grade. In a
discussion with Steve Millard, structural engineer, he stated that the wood frame house
could probably tolerate up to I Yi inches of differential settlement but 2 to 3 inches was
not tolerable. We discussed several foundation options for mitigating the settlement risk
with Steve Millard and Clint Omang. The options included helical piles, drilled piers,
geopiers (gravel columns), complete and partial subexcavation of the settlement prone
soils and placement of compacted fill or "flow fill" to re-establish design footing grade.
Consideration was given to potential cost of the alternatives and whether a specialty
contractor would be required. In general, the deep foundation alternatives, such as helical
or drilled piers which would have less risk of settlement, would also likely have greater
cost. Partial excavation and lowering the footings could be mostly done by the
homeowner and would cost less, but would still have some settlement risk.
Settlement Mitigation Options: Based on discussions with Clint Omang, two options for
mitigating the potential settlement of the silt and clay soils were considered: drilled piers
using a local drilled pier driller, and partial subexcavation of the silt and clay soils and
lowering of the footing grade.
Footings: Considering the conditions exposed in the excavation and the nature of the
proposed construction, spread footings placed on the undisturbed natural soil designed for
an allowable soil bearing pressure of2,000 psf can be used for support of the proposed
residence. The exposed sandy silt and clay soils tend to compress when wetted and there
could be some post-construction settlement of the foundation ifthe bearing soils become
wet. In order to reduce potential settlement to a tolerable level, we recommend that the
footing grade in the southern third of the building footprint be lowered at least 5 feet and
Job No. 112l19A
Clint & Susan Omang
May 31, 2012
Page 3
the footing grade extended down to that level. In the middle third of the building area,
the footing grade should be lowered at least 2Y, feet. The current footing grade in the
northern third of the building footprint can remain at its cutTent level. We expect that
settlements on the order of 1 to 2 inches could occur in the event of wetting of the bearing
soils. Footings should be a minimum width of20 inches for continuous walls and 2 feet
for columns. Loose and disturbed soils in footing areas should be removed. The
exposed, undisturbed natural soils should then be moistened to near optimum and
compacted. Exterior footings should be provided with adequate soil cover above their
bearing elevations for frost protection. 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 also be designed to
resist a lateral earth pressure based on an equivalent fluid unit weight of at least 50 pcf for
on-site soil as backfill. A perimeter foundation drain should be provided to prevent
temporary buildup of hydrostatic pressure behind the basement walls and prevent wetting
of the lower level. An impervious membrane, such as 20 mil PVC should be provided
below the drain gravel in a trough shape and attached to the foundation wall with mastic
to prevent wetting of the bearing soils. Structural fill placed within floor slab areas can
consist of the on-site soils compacted to at least 95% of standard Proctor density at a
moisture content near optimum. Backfill placed around the structure should be
compacted and the surface graded to prevent ponding within at least 10 feet of the
building. Landscape that requires regular heavy irrigation, such as sod, and sprinkler
heads should not be located within 10 feet of the foundation.
Drilled Piers: Considering the subsoil conditions encountered in the explorat01y boring
and the nature of the proposed construction, straight shaft piers drilled into the underlying
bedrock should provide a lower risk alternative for building support. The design and
construction criteria presented below should be observed for a straight-shaft drilled pier
foundation system.
1) The piers should be designed for an allowable end bearing pressure of
8,000 psf and a skin friction of!,000 psf for that pottion of the pier
embedded in bedrock. Pier penetration through the upper sandy silt and
clay deposit should be neglected in the skin friction calculations.
2) All piers should have a minimum total embedment length of 15 feet and a
minimum penetration into the bedrock of 5 feet.
3) The pier holes should be properly cleaned prior to placement of concrete.
The natural sandy silt and clay soil is generally stiff which indicates that
casing of the holes should not be required. Some caving and difficult
drilling may be experienced in the bearing soils due to scattered rocks or
hard bedrock. Placing concrete in the pier hole immediately after drilling
is recommended.
4) The pier drilling contractor should mobilize equipment of sufficient size to
achieve the design pier sizes and depths.
5) Free water was not encountered in the borings made at the site and it
appears that dewatering will probably not be needed.
JobNo.112119A
Clint & Susan Omang
May3 l ,2012
Page 4
6) A representative of the geoteclmica l engineer should observe pi er drilling
operations on.a full-time basi s.
Th e recommendations submitted in thi s lett er are based on our observation of the soils
exposed within the building excavation and the bo1ing drilled near the so utheast corner of
the house. This study is based on the assumption that soils beneath the footings have
equal or better supp011 than those exposed. The risk of foundation movement may be
g reater than indicated in this report because of po ssible var iations in the subsurface
conditions. Our services do not include determining the presence, prevention or
possibility of mold or other biological contamin ants (MOBC) deve loping in the future. If
the client is concerned abo ut MOBC, then a professional in this s peci a l field of practice
should be co nsult ed.
If you have any questions or need further assistance, please call our o ffice .
Sincerely,
Reviewed by: SLP
DEH/ljg
attachments Figures l to 3 -Swell-Conso lid ation Test Results
cc: Rako wski Millard Engineers -Attn: Steve Millard
Job No. 11 2 I1 9A
~tech
0
' Moisture Content -7.8 percent
2 Dry Density = 89 pcf
Sample of: Sandy Silt and Clay
From: Footing Grade at SW Corner
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APPLIED PRESSURE ( ksf)
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Moisture Content = 11.1 percent
2 Dry Density = 87 pct
Sample of: Sandy Silt and Clay
From: Footing Grade near NE Corner
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APPLIED PRESSURE ( ksf)
112119A ~tech SWELL-CONSOLIDATION TEST RESULTS FIGURE 1
HEPWORTH-PAWLAK GEOTECHNICAL
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112119A
Moisture Content = 4.9 percent
Dry Density = 100 pct
Passing No. 200 Sieve = 83 percent
Sample of: Sandy Silt and Clay
From: Boring 1 at 3 Feet ------
h._ )
~ / Compression
upon
wetting
\
b .
1.0 10 100
APPLIED PRESSURE ( ksf)
Moisture Content = 6.8 percent
Dry Density = 101 pct
Passing No. 200 Sieve = 85 percent
Sample of: Sandy Silt and Clay
From: Boring 1 at 5 Feet
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'a . ...___ r-.. No movement
upon wetting
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1.0 10 100
APPLIED PRESSURE ( ksf)
~tech SWELL-CONSOLIDATION TEST RESULTS FIGURE 2
HEPWORTH-PAWLAK GEOTECHNJCAL
Moisture Content = 9.8 percent
Dry Density = 106 pcf
Passing No. 200 Sieve = 57 percent
Sample of: Very Sandy Silt and Clay
From: Boring 1 at 10 Feet
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APPLIED PRESSURE ( ksf)
112119A ~tech SWELL-CONSOLIDATION TEST RESULTS FIGURE 3
HEPWORTH-PAWLAK GEOTECHNICAL