HomeMy WebLinkAboutSoils Report for Foundation Design & Perc 07.13.2001Gtech
July 13, 2001
David Johnston Architects
Attn: James Harris
758 Main Street
Carbondale, Colorado 81623
Hepworth-PawIi k Geotechnical, Inc.
5020 County Ruud 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
hpgeo@hpgeotech.com
Job No. 101 489
Subject: Subsoil Study for Foundation Design and Percolation Test, Proposed
Residence, Lot 2, Wooden Deer Subdivision, East of County Road 103,
Missouri Heights, Garfield County, Colorado
Dear Mr. Harris:
As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study and
percolation test for foundation and septic disposal designs at the subject site. The study
was conducted in accordance with our agreement for geotechnical engineering services
to David Johnston Architects dated June 26, 2001. The data obtained and our
recommendations based on the proposed construction and subsurface conditions
encountered are presented in this report.
Proposed Construction: The proposed residence will be a one story wood frame
structure over a crawlspace with an attached garage located on the site as shown on
Fig. 1. The garage floor is proposed to be slab -on -grade. Cut depths are expected to
range between about 3 to 5 feet. Foundation loadings are assumed to be relatively light
and typical of the assumed type of construction. The septic disposal system is proposed
to be located northwest of the proposed residence.
If building conditions or foundation loadings are significantly different from those
described above, we should be notified to re-evaluate the recommendations presented in
this report.
Site Conditions: The lot was vacant at the time of our field work and is located at the
end of Wooden Deer Road. The building area is situated on a knoll at the west end of a
west trending ridge and is relatively flat in the area of proposed construction. The
ground surface slopes gently down from the proposed building site to the east. Steeper
slopes exist down to the west and south. The lot is vegetated with pinon and sagebrush.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating two exploratory pits in the building area and one profile pit in the assumed
septic disposal area at the approximate locations shown on Fig. 1. The logs of the pits
are presented on Fig. 2. The subsoils encountered, below up to about 11/2 feet of
topsoil, consist of relatively dense basalt cobbles and boulders in a sandy silt matrix.
David Johnston, Architects
July 13, 2001
Page 2
Results of swell -consolidation testing performed on a relatively undisturbed sample of
the sandy silt matrix, presented on Fig. 3, indicate low compressibility under existing
moisture conditions and light loading and a low collapse potential (settlement under a
constant load) when wetted. The sample showed moderate compressibility after wetting
and increased loading. Results of a gradation analysis performed on a sample of silty
sand with gravel and cobbles (minus 5 inch fraction) obtained from the site are
presented on Fig. 4. No free water was observed in the pits at the time of excavation
and the soils were slightly moist to moist.
Foundation Recommendations:
Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread
footings placed on the undisturbed natural soil designed for an allowable soil bearing
pressure of 2,000 psf for support of the proposed residence. The matrix soils tend to
compress after wetting and there could be some post -construction foundation settlement.
Footings should be a minimum width of 16 inches for continuous walls and 2 feet for
columns. Loose and disturbed soils encountered at the foundation bearing level within
the excavation should be removed and the footing bearing level extended down to the
undisturbed natural soils. Excavation may be difficult due to the basalt boulders and
may require special excavation techniques such as rock splitting or chipping. Voids
created by removal of boulders should be backfilled with compacted road base or
concrete. Exterior footings should be provided with adequate cover above their bearing
elevations for frost protection, Placement of footings at Ieast 36 inches below the
exterior grade is typically used in this area. 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 be
designed to resist a lateral earth pressure based on an equivalent fluid unit weight of at
least 50 pcf for the on-site soil as backfill devoid of oversized rock. An underdrain
should not be needed provided that positive surface drainage as described below is
maintained around the residence.
Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support
lightly loaded slab -on -grade construction. The matrix soils tend to compress after
wetting and there could be some post -construction settlement. To reduce the effects of
some differential movement, garage floor slabs should be separated from all bearing
walls and columns with expansion joints which allow unrestrained vertical movement.
Floor slab control joints should be used to reduce damage due to shrinkage cracking.
The requirements for joint spacing and slab reinforcement should be established by the
designer based on experience and the intended slab use. A minimum 4 inch layer of
free -draining gravel should be placed beneath floor slabs to facilitate drainage. This
material should consist of minus 2 inch aggregate with less than 50% passing the No. 4
sieve and less than 2% passing the No. 200 sieve.
H -P GEOTECH
David Johnston, Architects
July 13, 2001
Page 3
All fill materials for support of floor slabs should be compacted to at Ieast 95 % of
maximum standard Proctor density at a moisture content near optimum. Required fill
can consist of the on-site soils or imported road base.devoid of vegetation, topsoil and
oversized rock.
Surface Drainage: The following drainage precautions should be observed during
construction and maintained at all times after the residence has been completed:
1) Inundation of the foundation excavations and underslab areas should be
avoided during construction.
2) Exterior backfill should be adjusted to near optimum moisture and
compacted to at Ieast 95 % of the maximum standard Proctor density in
pavement and slab areas and to at least 90 % of the maximum standard
Proctor density in landscape areas.
3) The ground surface surrounding the exterior of the building should be
sloped to drain away from the foundation in all directions. We
recommend a minimum slope of 12 inches in the first 10 feet in unpaved
areas and a minimum slope of 3 inches in the first 10 feet in pavement
and walkway areas.
4) Roof downspouts and drains should discharge well beyond the limits of
all backfill.
5) Landscaping which requires regular heavy irrigation should be located at
least 5 feet from the building.
Percolation Testing: Percolation tests were conducted on July 6, 2001 to evaluate the
feasibility of an infiltration septic disposal system at the site. One profile pit and three
percolation holes were dug at the locations shown on Fig. 1. The test holes (nominal 12
inch diameter by 12 inch deep) were hand dug at the bottom of shallow backhoe pits
and were soaked with water one day prior to testing. The soils exposed in the
percolation test holes are similar to those exposed in the Profile Pit shown on Fig. 2 and
below about 1 foot of topsoil, consist of 21/2 feet of stiff silt and sand overlying dense
basalt cobbles and boulders in a sandy silt matrix. Percolation test results indicate
infiltration rates between 36 and 90 minutes per inch with an average of 57 minutes per
inch. The percolation test results are presented in Table II:
Limitations: This study has been conducted in accordance with generally accepted
geotechnical engineering principles and practices in this area at this time. We make no
warranty either expressed or implied. The conclusions and recommendations submitted
in this report are based upon the data obtained from the exploratory pits excavated at the
locations indicated on Fig. 1, the proposed type of construction, percolation test results
and our experience in the area. Our findings include interpolation and extrapolation of
the subsurface conditions identified at the exploratory pits and variations in the
H -P GEOTECH
David Johnston, Architects
July 13, 2001
Page 4
subsurface conditions may not become evident until excavation is performed. If
conditions encountered during construction appear different from those described in this
report, we should be notified at once so re-evaluation. of the recommendations may be
made.
This report has been prepared for the exclusive use by our client for design purposes.
We are not responsible for technical interpretations by others of our information. As
the project evolves, we should provide continued consultation and field services during
construction to review and monitor the implementation of our recommendations, and to
verify that the recommendations have been appropriately interpreted. Significant design
changes may require additional analysis or modifications to the recommendations
presented herein. We recommend on-site observation of excavations and foundation
bearing strata and testing of structural fill by a representative of the geotechnical
engineer.
If you have any questions or if we may be of further assistance, please let us know.
Sincerely,
HEPWORTH - PAWLAK GEOTECHNICAL, INC.
Trevor L. Knell
Reviewed by:
Daniel E. Hardin, P.E.
TLK/ksw
attachments
H -P GEOTECH
EASEMENT
LOT 1
I
i
l EXISTING WATER
STORAGE
y TANK
1
1
LOT 2
\
PIT 1
P-1
Q P-2
P-3 A
0
PROFILE
PIT
l PROPOSED
RESIDENCE
1
101 489
HEPWORTH-PAWLAK
GEOTECHNICAL, INC_
/
PROPERTY
BOUNDARY
i
r
�!'8ENCHMARK: GROUND AT
PROPERTY PIN, 100.0' AS
PROVIDED
LOT 3
APPROXIMATE SCALE
1"=60'
CONTOUR INTERVAL = 10'
LOCATION OF EXPLORATORY PITS
AND PERCOLATION TEST HOLES
Fig. 1
Depth — Feet
- 0
- 5
10
LEGEND:
[i]
T
PIT 1
ELEV.=113.0'
r
WC -12.3
_ a +435
-200223
WC -14.2
DD -71
-200=55
PIT 2
ELEV.=113.0'
PROFILE PIT
ELEV.-107.1'
TOPSOIL; silty sand, organic, firm, moist, black, basalt cobbles at pit 1 and 2.
0
5
10
Depth — Feet
SAND AND SILT (SM—ML); slightly clayey, with scattered basalt fragments, stiff, slightly moist,
tan, calcareous.
BASALT COBBLES AND BOULDERS (GM); in a sandy silt matrix, dense, slightly moist, tan, calcareous.
2" Diameter hand driven liner sample.
Disturbed bulk sample.
Practical backhoe refusal to digging on basalt boulders.
NOTES:
1. Exploratory pits were excavated on July 5, 2001 with o Cat 416C backhoe.
2. Locations of exploratory pits were measured approximately by pacing from features on the site plan
provided.
3. Elevations of exploratory pits were measured by instrument level and refer to the Bench Mark shown
shown on Fig. 1.
4. The exploratory pit locations and elevations should be considered accurate only to the degree Implied
by the method used.
5. The lines between materials shown on the exploratory pit logs represent the approximate boundaries
between material types and transitions may be gradual.
6. No free water was encountered in the pits at the time of excavating. Fluctuations in water level may
occur with time.
7. Laboratory Testing Results:
WC = Water Content ()
DD = Dry Density ( pcf )
+4 = Percent retained on No. 4 sieve
—200 = Percent passing No. 200 sieve
101 489
HEPWORTH-PAWLAK
GEOTECHNICAL, INC.
LOGS OF EXPLORATORY PITS
Fig. 2
Compression X
0
1
2
3
4
5
6
Moisture Content = 14.2 percent
Dry Density — 71 pcf
Sample of: Very Sandy Silt Matrix
From: Pit 1 at 6 Feet
___,c
.../
Compression
upon
wetting
c\
r
r
w
0.1
101 489
.0 10
APPLIED PRESSURE — ksf
HEPWORTH-PAWLAK
GEOTECHNICAL, INC.
SWELL CONSOLIDATION TEST RESULTS
100
Fig. 3
H10ROAE1E11 ANALYSIS
TF1E RFADINYS
SIEVE ANALYSIS
24151. 7NR
p 45 USL 10 11111. 50IOL 11111111. 4 Y!L 1 ION.
10
20
30
40
50
60
70
ea
50
100
.001 .002
11.. STANDARD sExes
ADO
CLEAR SOU AK O EPOWIS
416 /6 44 3/0" 3h' 1 1/2' r 6•e' r lea
. I
1
{
t
t
1-
J
J
i
a
r
t
1
1 - E
1-
i
.006 .005 .010 .037 .074 .150 .300 .600 1.16 2.35 4.75
DIAMETER OF PARTICLES IN MILOMETERS
0.51
1110 37.5 76.2127 62 203
MAY TO SILT
Fwc i
GRAVEL 35 %
SAND 42 X SILT AND CLAY 23 X
LIQUID UMIT X PLASTICITY INDEX X
SAMPLE OF: Silty Sand with Gravel FROM: Pit 1 at 4 thru 5 Feet
and Cobbles
101 489
HEPWORTH—PAWLAK
GEOTECHNICAL. INC.
GRADATION TEST RESULTS
90
60
70
e0
50
40
30
20
10
0
' RCENT PASS1 t
Fig. 4
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE 1
PERCOLATION TEST RESULTS
JOB NO. 101 489
HOLE NO.
HOLE DEPTH
(INCHES)
LENGTH OF
INTERVAL
(MIN)
WATER DEPTH
AT START OF
INTERVAL
(INCHES)
WATER DEPTH
AT END OF
INTERVAL
IINCHESI
P-1
31
15
water added
4%
4 '4
DROP IN
WATER
LEVEL
(INCHES)
,A
4'%
4
,4
4
3 'A
3Y2
3
A
,4
'/2
4
3%
'4
3'
3'A
'4
3 4
3
'4
3
2'
%
AVERAGE
PERCOLATION
RATE
(MIN./INCH)
36
P-2
28
15
5%
5 '/4
5 Y4
5
'4
5
4°
1/4
4 ,4
4 %
'4
4 '4
4 '/4
14
4 '/4
4 '/4
0
4 '/4
4
'/4
4
3%
'/4
90
P-3
31
15
7
6 '/4
6 '/4
6
Y4
6
5 'h
%
5'
5 Y
'A
5 '/4
4°
'/t
4 7
4%
'/4
4'
4 '/4
'/4
4 '/4
3 74
45
Note: Percolation test holes were dug and soaked on July 15, 2001. Percolation test was
performed on July 6, 2001. The average percolation rates were based on the last three
readings of each test. There was a Targe rock on the side of P-1 and there was a large
rock on the bottom of P-2.