HomeMy WebLinkAboutObservation of Excavation 04.15.2021Kdt H
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April 15,2021
Marcelino Holguin
274Lanat Road
Silt, Colorado 81652
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Subject
Project No. 21-7-330
Observation of Excavation, Proposed Residence,920 County Road 218, Silt,
Colorado
Dear Marcelino:
As requested, a representative of Kumar & Associates observed the excavation at the subject site
on April 8,2021 to evaluate the soíls exposecl for foundation support. The finelings of our
observations and recomrnenclations for the founclation supporl are presented in this roport. The
services were perfonnecl in accordance with our agreetnent tbr professional engineering serices
to you dated April 8, 2021. i
T'he proposed residence will be a one-story wood fiarne structurc over crawlspace with attachecl
garage. The garage floor rvill be slab-on-grade. Foundations rvere designed based on an
assulned allowable soil bearing pressure of I,500 psf.
At the time of our visit to the site, the foundation excavation had been cut in one level from 3 to
4k feet below the adjacent ground surfàce. The soiis exposecl in the bottotn of the excavation
consisted of loose to me<iium dense, silty sand. Results of rnoisture/density ancl percent finer
than sancl size graclation testing perfonned on a samplc takeu fì'orn the site, iirdicate the silty sand
s<lils have a silt content of 28% and a dry density of 90 pcf at a moisture content of 1.9%. No
free water was encoullterecl in the excavatiori and thc soils were slightly rrioist. Two pits,
previously excavatecl about 30 f-eet south of tlie house, showed silty sand ancl gravel soils. The
gravel soils were nat visible in the house excavation.
Based on our otrservations and laboratory test results, the subsoils exposed at footing subgrade in
the fìrundation excavation are low density silty sand which will tencl to settle whcn wettecl. We
recommclldcd that sorne pits be excavated in the area of the house to evaluate the depth of the
silty sand soils below f-ooting grade. Three pits were dug on April l5tl' in thc house excavation
which showed there was about I foot of silty sand overlying the gravcl soils in the northwestem
(garage) part of the excavation and about lYr. feet o1'silty sand in thc soutlieastem part of the
excavation, Wirile the natural silty sand soils have settlement potential, the li¡nitecl depth of i2
to I B inches below ftroting gracle shoulcl lirnit the setl.leinent to less than I inch.
'fhe laboratory test results indicate that the silty sand soils ivill tcncl to settlc if they become wet.
Considerjng the conditions exposed in the excavatiorl and the nature of tlie proposed
Marcelino Holguin
Apnl20,2A21
Page 2
construction, spread fcrotings placed on the undisturbed natural soil designed for an allowable
soil bearing pressure of 1,500 psfcan be used for support ofthe proposed residence provided
settlement on the order of I inch can be tolerated. Footings should be a rninimurn width of
18 inches for continuous walls and 2 feet for columns. Loose and disturbed soils in footing areas
should be moistened and compacted or removed and the bearing level extended down to the
undisturbecl natural gravel soils. Exterior f-ootings should be provided with adequate soil cover
above their bcaring elevatiotts for fiost protection. Continuous f'oundation walls should be
teinforced top and bottorn to sparl local anomalies such as by assurning an unsupported length of
at least 15 tbet. Foundation walls acting as retaining structures should also be designed to resist
a lateral earth pressure based o¡1 an equivalent fluid unit weight of at least 50 pcf for on-site soil
as backlill. A perimeter foundatic¡n drain should be provicled to prevent temporary buildup of
hydrostatic pressure behinel the baserncnt walls and prcvent wetting of the crawlspace level, An
impervious membrane, such as 20 rnil PVC shoulcl be provided below the drain gravel in a
trough shape and attached to the fbundation wall witir mastic to prevent wetting of the bearing
soils. Structural fill placed within tloor slab areas carl consist of the on-site soils cornpacted to at
least 95% of standard Proctor density at a moisture cc¡ntent near optirnum. Backfill placed
around the strucfure 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, ancl
sprinkler hearls should not bc located within 10 feet of the foundation.
Tire recommendations subrnitted in this letter are based on our clbseruation of the soils exposed
within the founclation excar,ation and do not include subsurfàce exploration to evaluate the
subsurface conditions within the loaded depth of fbundation influence. This study is basecl on
the assutnption that soils beneath the footings have equal or better support than those exposed.
The risk of fbundation movement may be greater than indicated in this report because of possible
variations in the subsurface conditions. In order to reveal the nature and extent of variations in
the subsurface conditions below the excavation, clrilling would be required. It is possiblc tlic
Cata cbtained by subsurface exploration could'change the reconÌmendations confainecl in this
Ietter. Our services c1o not include detennining the presence, prevention or possibility of mold or
otlier biological contarninants (MOBC) developing in the future. If the client is concemed about
MOBC, then a professional in this special ficld, cf prtcil;c tho;lC b; c;;::;l:;J.
If you have any questious or uced furthe¡ assistarice, pleasc call our o{fice.
Sincereiy,
Daniel E. I{
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