HomeMy WebLinkAboutObservation of Exvacations 10.01.2016H-PKUMAR
Geotechnical Engineering J Engineering Geology
Materials Testing Environmental
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Fax: (970) 945-8454
Email: hpkglenwood®kumarusa.com
October 4, 2016
Westhoff Construction
1338 Grand Avenue, #127
Glenwood Springs, Colorado 81601
andrewwesthoff@gmail.com
Office Locations: Parker, Glenwood Springs, and Skverthome, Colorado
OCT 2 11016
Project No. 16-7489
Subject: Observation of Excavations, Proposed Additions, 481 Stage Coach Lane, Ranch
at Roaring Fork, Garfield County, Colorado
Gentlemen:
As requested, a representative of HP/Kumar observed the excavations at the subject site on
September 29, 2016 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 were performed in accordance with our agreement for professional engineering services
to Westhoff Construction, dated September 27, 2016.
An addition above crawlspace is proposed to the front, west side of the residence and 6 deck
piers are proposed along the east and south sides of the residence. Footings sized for an
allowable bearing pressure of 1,500 psf were designed for the building addition and deck pier
support. The existing residence is above crawlspace and supported on shallow spread footings.
At the time of our visit to the site, the building addition excavation had been cut in one level
from 3 to 4 feet below the adjacent ground surface. The soils exposed in the bottom of the
perimeter footing excavation mainly consisted of stiff, silty sandy clay. Probing the bottom of
the excavation indicated the clay was between about 6 inches deep in the north part and up to
about 2 feet deep in the south part of the footing excavation. The interior footing trench was
based on old backfill soils. Two pier excavations outboard and west of the footing excavation
were based in clay soils about 6 inches deep. The northern pier excavation appeared to be on the
edge of the waterline trench backfill possibly another 2 feet deep. The underlying soils appeared
to consist of dense, sandy gravel and cobbles (river terrace alluvium). Results of swell -
consolidation testing performed on a sample of the clay taken from the south footing line, shown
on Figure 1, indicate the soils are slightly to moderately compressible under conditions of
loading and wetting. The deck pier excavations had similar profiles with only the north pier
excavation based in dense river gravel and the remaining pier excavations based in clay soils at
the cut depths of 3 to 31/2 feet. No free water was encountered in the excavations and the soils
were slightly moist to moist. The bottom of existing footings were exposed in the building
addition excavation.
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
Westhoff Construction
October 4, 2016
Page 2
sail bearing pressure of 1,500 psf should be adequate for support of the proposed building
addition and pier footings. The clay soils tend to compress when wetted and there could be some
post -construction settlement of the foundation differential to the existing foundation. Footings
should be a minimum width of 16 inches for continuous walls and 2 feet for columns. Loose
disturbed soils and existing fill in footing areas should be removed and the bearing level
extended down to the undisturbed natural soils. The new interior wall footing could be
redesigned to span the old backfill by deepening the footing down to the bearing level of the
existing building. The bearing soils should be protected against frost and concrete should not be
placed on frozen soils. 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 not be needed provided the perimeter wall backfill
is well compacted and has a positive slope away from the foundation. Structural fill placed
within slab -on -grade areas can consist of the on-site soils or road base 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.
The recommendations submitted in this letter are based on our observation of the soils exposed
within the foundation excavation and do not include subsurface exploration to evaluate the
subsurface conditions within the loaded depth of foundation influence. This study is based on
the assumption that soils beneath the footings have equal or better support than those exposed.
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,
, r ' .
Steven L. Pawlak, I'.E.� 1B2221
SLP/ksw a a1 l d/'fifb. `12
1.05"4:.
•
ion Test Results
Attachment: Figure 1— S
H -P ' KUMAR
Project No. 16-7-489
Compression %
0
1
2
3
4
Moisture Content = 17.6 percent
Dry Density = 107 pcf
Sample of: Sandy Silty Clay
From: Bottom of Excavation
r
1
No movement
upon
wetting
Y
01
1.0
APPUED PRESSURE - kst
10
100
16-7-489
H -P -KUMAR
SWELL -CONSOLIDATION TEST RESULTS
Figure 1