HomeMy WebLinkAboutObservation of ExcavationC~t ......
HEPWORTH· PAWLAK GEOTECHNICAL
June 21, 2016
Crystal Mugford
512 County Road 225
Rifle, Colorado 81650
mu gfordcr ystal @gmai1.com
Hcp\H>rth ·P.ml.1k Gcoi ... · ... hni • ,1(, In ...
5020 Counf) Ro:iJ 154
GknwnoJ Srrm ~;, G•J ,>r.1do 81601
Phom. 970·945 7958
F.1x 970-945 8454
cm .iii hpi.:cn@hr)!t.:(>ll!'-h .com
Job No. 116 253A
Subject: Observation of Excavation, Proposed Residence, County Road 259, north
of Silt, Garfield County, Colorado
Dear Crystal:
As requested, a representative of Hepworth-Pawlak Geotechnica1, Inc. observed the
excavation at the subject site on June 17, 2016 to evaluate the soils exposed for
foundation support. The findings of our observations and recommendations are presented
in this report. The services were performed in accordance with our agreement for
professional engineering services to you, dated June 16, 2016.
The residence will be a single story, premanufactured wood frame structure over
crawlspace. The building has been designed to be supported on spread footings assuming
an allowable soil bearing pressure of 1,500 psf. We understand the building will be
constrncted with jacks for leveling of the structure, and releveling as needed.
At the time of our site visit, the foundation excavation which was essentially complete
had been cut in one level from about 3 to 3V2 feet below the adjacent ground surface. The
soils exposed in the bottom of the excavation consisted of medium dense, silty sand. The
footings for the foundation had been formed and reinforcement was in place. It appeared
the soil below the footing areas had been compacted. Results of swell-consolidation
testing performed on samples taken from the site, shown on Figures 1 and 2, indicate the
soils are moderately to highly compressible under conditions of loading and wetting with
a moderate hydro-compression potential. The samples may have been partly disturbed
from the sampling process. No free water was encountered in the excavation and the soils
were slightly moist.
The soils at the site possess low bearing capacity and moderate to possibly high
settlement potential when wetted. Spread footings bearing on the natural soils may be
feasible for foundation support of the building provided the risk of settlement is
acceptable. A boring should be drilled to better evaluate the settlement potential and
depth to underlying less compressible soils and/or bedrock. Spread footings placed on a
Parker 303-841-7119 • Colorado Springs 719-633-5562 • Silverthorne 970-468-1989
Crystal Mugford
June 21, 2016
Page 2
depth of structural fill (typically about 4 feet) or helical piers that extend down to less
compressible soils or bedrock would provide a lower risk of foundation settlement.
Provided below are recommendations for spread footings bearing on the natural soils. If
recommendations for structural fill below the footings or a helical pier foundation are
desired, we should be contacted.
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 bearing pressure of 1,500 psf can be used for support of the proposed residence
with a risk of settlement. The risk of settlement is primarily if the bearing soils were to
become wetted. The magnitude of the settlement would depend on the depth and extent
of the wetting but may be on the order of l to 2 inches or more. Precautions to prevent
wetti ng of the bearing soils such as adequate compaction of foundation backfill, positive
surface slope away from foundation walls and limiting irrigation near foundation walls
should be observed.
Footings should be a minimum width of 18 inches for continuous walls and 2 feet for
columns. All loose and 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 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 50 pcf for on·site soil as backfill. A perimeter foundation drain around
the shallow crawlspace is not needed provided there is adequate compaction of
foundation backfill and positive surface slope away from foundation walls and limiting
irrigation near foundation walls. Backfill placed around the structure should be
compacted to at least 90% (95 % in pavement areas) of standard Proctor density at a
moisture content near optimum 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 l 0 feet of the foundation. Consideration
should be given to the use of xeriscape near the residence .
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
Job No. 116 253A
Crystal Mugford
June 21, 2016
Page 3
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,
HEPWORTH -PAWLAK GEOTECHNICAL, INC. c
cc : Garfield County-Andy Schwaller (aschwaller@ e arfteld-count y.com)
Job No . 116 253A
Moisture Content = 7 .6 percent
Dry Dens ity = 100 pcf
Sample of : Si lty Sand
! From : Bottom of Southwest corner of Excavation,
(6 -17-16)
0 ---~ ~}
1
I
2
D
I( Compression
upon
?fl. 3 wetting
c::
ii 0 "iii \ en
Q.) 4 .... \ a.
E
0
(.)
5
6 \
7
8 \
\ I 9
)
10 \
\ 11
12 \
' 'b
13
01 1.0 10 100
APP UE D PR ESSUR E • ksf
116 253A ~
HEPWORTit-PAWLAK GEOTECHNICAL
SWELL·CONSOLIDATION TEST RESULTS Figure 1
Moisture Content = 6.1 percent
Dry Density = 94 pcf
Sample of: Silty Sand
From : Bottom of Northeast corner of Excavation,
(6-17-16)
0 --r-r-. :--....
1
...... ~
11.l
2
Compression (
~r-.. upon
?fl. 3 wetting
-~ c
0 ·u;
(/)
(!) 4 ....
0.
E
0
(.)
I) 5 I\ 6
7
\
8
9 I\
\ 10 I )
11 \
\
12
\ 13
p
14
01 10 10 100
APPLIED PRESSURE -ksl
116 253A ~
H E PWORTH·PAWLAK G E O'TECHNICAL
SWELL-CONSOLIDATION TEST RESULTS Figure 2