HomeMy WebLinkAboutObservation of Excavation 06.04.2020K+A
Kumar .& Lv,t darm, Inc.'
G atechhnical and Materials Engineers
and Environmental Scientists
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employee Owned Company www.kumarusa.com
Office locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
June 4, 2020
Steve Spangler
P. O. Box 382
Silt, Colorado 81652
si ltbum441u; yahoo. com
Project No. 17-7-578.A
Subject: Observation of Excavation, Proposed Residence, Lot 3, Cedar Ridge Subdivision,
River Bend Drive, Garfield County, Colorado
Dear Steve:
As requested, a representative of Kumar & Associates observed the excavation at the subject site
on May 26 and 28, 2020 to evaluate the soils exposed for foundation support. The findings of
our observations and recommendations for the foundation design are presented in this report.
The services are supplemental to H-P/Kumar (now Kumar & Associates) agreement for
professional engineering services to you dated July 27, 2020.
We previously conducted a subsoil study for design of foundations for a previous proposed barn
at the site and presented our findings in a report dated August 31, 2017, Project No. 17-7-578.
The proposed residence will be located on the moderately steep, southwesterly facing hillside
about 150 to 200 feet northeast of and above the barn site. The residence will be single -story
wood frame construction over a walkout basement level. Ground floor will be slab -on -grade.
It is desired to found the residence on spread footings.
At the time of our May 26 site visit, the foundation excavation had been cut in one level up to
about 8 feet deep and daylighting to the southwest. The soils exposed in the bottom of the
excavation consisted of stiff, very sandy silty clay transitioning to fill along the southwest side of
the excavation where the walkout basement area footing trench had not yet been dug. We
returned to the site on May 28 to observe and sample a backhoe pit (Pit 1) that had been
excavated to the southwest side of the excavation. The soils exposed in the pit were similar to
those exposed in the bottom of the foundation excavation and consisted of stiff, very silty sand
down to the pit depth of 91/2 feet. Results of swell -consolidation testing performed on samples
taken from the site, shown on Figure 1, indicate the soils are moderately compressible under
conditions of loading and wetting with a low to moderate hydro -compression potential. The
sample from the pit was likely disturbed due to the sampling process. No free water was
encountered in the excavation or the backhoe pit and the soils were slightly moist.
Considering the conditions exposed in the excavation and the backhoe pit, and the nature of the
proposed construction, spread footings placed on the undisturbed natural soil designed for an
allowable bearing pressure of 1,500 psf can be used for support of the proposed residence with
some risk of settlement. The risk of settlement is primarily if the bearing soils were to become
wetted, and precautions should be taken to prevent wetting. A lower risk of settlement would be
to remove at least 3 feet of the existing soils below the footing areas and replace in a moistened
and well compacted condition. Any fill below footing areas should be compacted to at least 98%
standard Proctor density at a moisture content with about 2% of optimum.
Steve Spangler
June 4, 2020
Page 2
The footings should be a minimum width of 18 inches for continuous walls and 2 feet for
columns. All existing fill and any loose disturbed soils in footing areas should be removed and
the bearing level extended down to the undisturbed natural soils, and the subgrade moistened and
compacted. Exterior footings should be provided with adequate soil cover above their bearing
elevations for frost protection. Continuous foundation walls should be heavily reinforced top
and bottom to span local anomalies and better withstand the effects of some differential
settlement such as by assuming an unsupported length of at least 14 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 55 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. The underdrain system should include at
least 4 inches of drain gravel below the basement slab. Structural fill placed within floor slab
areas can consist of the on -site soils compacted to at least 95% of standard Proctor density (SPD)
at a moisture content near optimum. Backfill placed around the structure should be compacted to
at least 90% SPD and the surface graded to prevent ponding within at least 10 feet of the
building. This may require a swale to divert surface water along the uphill side 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.
The recommendations submitted in this letter are based on our observation of the soils exposed
within the foundation excavation and the limited subsurface information from the backhoe pit.
The risk of foundation 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, drilling would be required. It is possible the
data obtained by subsurface exploration could change the recommendations contained in this
letter. 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.
If you have any questions or need further assistance, please call our office.
Sincerely,
Kumar & Associates, Inc.
1
David A. You
DAY/kac
attachments Figure.
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ell -Consolidation Test Results
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Kumar & Associates, Inc.'
Project No.17-7-578.A
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SAMPLE OF: Very Sandy Silty Clay
FROM: Bottom of Northwestern Portion
of Excavation
WC = 13.6 %, DD = 103 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
1.0 APPLIED PRESSURE - KSF 10 100
Kumar & Associates
SWELL —CONSOLIDATION TEST RESULT
CONSOLIDATION - SWELL
CONSOLIDATION - SWELL
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SAMPLE OF: Very Sandy Silty Clay
FROM: Bottom of South —Eastern Portion
of Excavation
WC = 6.8 %, DD = 101 pcf
I ADDITIONAL COMPRESSION
i I UNDER CONSTANT PRESSURE
I _ _ _ . T
! r.•r
I _ DUE TO WETTING
1.0 APPLIED PRESSURE — KSF 10
100
SAMPLE OF: Very Sandy Silty Clay
FROM: Pit 1 ® 9'
WC = 11.2 %, DD = 106 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
1
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1.0 APPLIED PRESSURE — KSF
Kumar & Associates
DUE TO WETTING
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SWELL —CONSOLIDATION TEST RESULT
100
Fig. 2