HomeMy WebLinkAboutSoils Study for Foundation Design & Perc Test 05.24.2001May 24, 2001
Beulah Wilson Estate
Attn: Virginia Sterrett
6235 County Road 109
Carbondale, Colorado 81623
Hepwartls-Pawlak Geotechnical, Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970-945-8454
hpgeo@hpgeotech.com
Job No. 101 338C
Subject: Subsoil Study for Foundation Design and Percolation Test, Proposed
Residence, Lot C, Beulah Wilson Subdivision Exemption, County Road
109, Garfield County, Colorado
Dear Ms. Sterrett:
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 the Beulah Wilson Estate dated May 3, 2001. The data obtained and our
recommendations based on the proposed construction and subsurface conditions
encountered are presented in this report. Evaluation of potential geologic hazard
impacts on the site are beyond the scope of this study.
Proposed Construction: Plans for the residence are conceptual at this time and the
report was prepared for purchase of the lot. One to two story wood frame construction
above a basement or crawlspace is typical of the area. Cut depths are expected to range
between about 3 to 8 feet. Foundation loadings for this type of construction are
assumed to be relatively light and typical of the proposed type of construction. The
septic disposal system is assumed to be located downhill and northeast of the building
area.
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.
Subsidence Potential: Lot C is underlain by Pennsylvania Age Eagle Valley Evaporite
bedrock. The evaporite contains gypsum deposits. Dissolution of the gypsum under
certain conditions can cause sinkholes to develop and can produce areas of localized
subsidence. Sinkholes were not observed in the immediate area of the subject lot. The
exploration pits were relatively shallow, for foundation design only. Based on our
present knowledge of the site, it cannot be said for certain that sinkholes will not
Beulah Wilson Estate
May 24, 2001
Page 2
develop. In our opinion, the risk of ground subsidence at Lot C is low but the owner
should be aware of the potential for sinkhole development.
Site Conditions: Lot C is located on the west side of County Road 109 and bordered to
the west by a very steep east -facing slope. The ground surface in the building area is
relatively flat with a gentle slope down to a large irrigation ditch which crosses the lot
below the bu' ding area. A low area created by filling the County Road is located
between the irrigation ditch and the County Road. The building area is vegetated with
sage brush, grass and weeds. The remaining part of the site and steep hillside are
vegetated with a pinion and juniper forest. There is an existing well near the middle of
the lot.
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 septic
disposal area at the approximate locations shown on Fig. 1. The logs of the pits are
presented on Fig. 2. The subsoils encountered in the proposed building area, below
about 2 feet of topsoil, consist of sandy silty clay with occasional gravelly layers to the
maximum depth explored, 13 feet. Results of swell -consolidation testing performed on
relatively undisturbed samples of the sandy clay, presented on Figs. 3 and 4, indicate
low compressibility under existing moisture conditions and light loading and a low
collapse potential (settlement under constant load) when wetted. ' The samples were
moderately to highly compressible under increased loading after wetting. 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 1,200 psf for support of the proposed residences. The soils tend to compress
after wetting and there could be some post -construction foundation settlement. Footings
should be a minimum width of 18 inches for continuous walls and 2 feet for columns.
Loose and disturbed soils and existing fill encountered at the foundation bearing level
within the excavation should be removed and the footing bearing level extended down to
the undisturbed natural soils. Exterior footings should be provided with adequate cover
above their bearing elevations for frost protection. Placement of footings at least 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
H -P GEOTECH
Beulah Wilson Estate
May 24, 2001
Page 3
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 backf ll.
Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support
lightly loaded slab -on -grade construction. To reduce the effects of some differential
movement, floor slabs should be separated from all bearing walls and cob,rnns 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 basement level 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.
All fill materials for support of floor slabs should be compacted to at least 95 % of
maximum standard Proctor density at a moisture content near optimum. Required fill
can consist of the on-site soils devoid of vegetation, topsoil and oversized rock.
Underdrain System: Although free water was not encountered during our exploration,
it has been our experience in the area that Local perched groundwater can develop
during times of heavy precipitation or seasonal runoff. Frozen ground during spring
runoff can create a perched condition. We recommend below -grade construction, such
as retaining walls, crawlspace and basement areas, be protected from wetting and
hydrostatic pressure buildup by an underdrain system.
The drains should consist of drainpipe placed in the bottom of the wall backfill
surrounded above the invert level with.free-draining granular material. The drain
should be placed at each level of excavation and at least 1 foot below lowest adjacent
finish grade and sloped at a minimum 1% to a suitable gravity outlet. Free -draining
granular material used in the underdrain system should contain less than 2% passing the
No. 200 sieve, less than 50% passing the No. 4 sieve and have a maxinnim size of
2 inches. The drain gravel backfill should be at least 11/2 feet deep. An impervious
membrane such as 20 mil PVC should be placed beneath the drain gravel in a trough
shape and attached to the foundation wall with mastic to prevent wetting of the bearing
soils.
Surface Drainage: The following drainage precautions should be observed during
construction and maintained at all times after the residence has been completed:
H -P GEOTECH
Beulah Wilson Estate
May 24, 2001
Page 4
1) Inundation of the foundation excavations and underslab areas should be
avoided during construction. Drying could increase the expansion
potential of the soils.
2) Exterior backfill should be adjusted to near optimum moisture and
compacted to at least 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. Free -draining wall backfill should be
capped with about 2 feet of the on-site, finer graded soils to reduce
surface water infiltration.
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. A swale may be needed uphill to direct surface
runoff around the residence.
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 10 feet from the building Consideration should be given to the use
of xeriscape to limit potential wetting due to irrigation. .
Percolation Testing: Percolation tests were conducted on May 9, 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 holes are similar to those exposed in the Profile Pit shown on Fig. 2 and
consist of 11/2 feet of topsoil and 41 feet of sandy silty clay overlying silty sand gravel
to the pit depth, 8 feet. The percolation test results are presented in Table 11. Based on
the subsurface conditions encountered and the percolation test results, the tested area
should be suitable for a conventional infiltration septic disposal system.
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 constriction and our experience in
the area. Our findings include interpolation and extrapolation of the subsurface
H -P GEOTECH
3
Beulah Wilson Estate
May 24, 2001
Page 5
conditions identified at the exploratory pits and variations in the 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 recoiniuend 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.
Louis E. Eller
Reviewed by:
Daniel E. Hardin, P.E.
LEE/ksw
attachments
H -P GEOTECH
APPROXIMATE SCALE
1" = 60'
L
1
PIT 2
■
PIT 1
PARCEL B
BENCH MARK GROUND AT PROPERTY
CORNER; ELEV. = 100.0', ASSUMED.
P 3
P 2 A
A• PROFILE
P 1
PIT
A
PARCEL C
PARCEL D
EXISTING
IRRIGATION
DITCH
1
1
0
0
2.
1
73
0
c
0
(o
101 338C
HEP WORTH-PAWLAK
GEOTECHNICAL, INC.
LOCATION OF EXPLORATORY PITS
AND PERCOLATION TEST HOLES
Fig. 1
0
5
Le -
C• L
o 10
15'
LEGEND:
ti
1/a
PIT 1
ELEV.= 108.6'
We -12.2
00=104
-200=45
LLr3
P1.41
WC -12.5
00=109
PIT 2 PROFILE PIT
ELEV.= 111.2 ELEV.= 106.5'
WC -13.4
DD -00
WC -15.0 .
00=101
-200-50
TOPSOIL; sandy silt and clay. organic. firm, moist, dark brown
5
10
15 —
CLAY (CL);silty. sandy with occasional gravelly zones and scattered cobbles, stiff,
slightly moist to moist. brown.
GRAVEL (GM); silty. sandy, dense to very dense. moist. Tight brown, subrounded
rock.
Hand driven liner sample.
Disturbed bulk sample.
_J
NOTES:
1. Exploratory pits were excavated on May 8, 2001 with a backhoe.
2. locations of exploratory plts 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. Loge are drawn to depth.
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 (X ) —200 = Percent passing No. 200 sieve
DD = Dry Density ( pcf ) LL = Liquid Limit ( % )
+4 = Percent retained on No. 4 sieve Pi = Plasticity Index ( % )
101 338C
HEPWORTH—PAWLAK
GEOTECHNICAL. INC.
LOGS OF EXPLORATORY PITS
Depth — Feet
Fig: 2
Moisture Content = 12.5 percent
Dry Density = 109 pcf
Sample of: Silty Sandy Clay
From: Pit 1 at 4.5 Feet
'Compression
upon
wetting
•
•
i
.-..
I
,
...•
..
w
(7°A
\ IL
,
e...,........>„..."Yer.......>:„...........).
1
'
I
1
I
I
O r tV M1 11'1 td
% UOf889JdWO3
n
o)
CONSOLIDATION TEST RESUL
O
C.)
co
M
0
�1! r•
1)
l 1
HEPWORTH—PAWLAK SWELL CONSOLIDATION TEST RESULTS I
GEOTECHNICAL, INC_
0
1
2
4
5
6
Moisture Content = 13.4 percent
Dry Density = 99 pcf
Sample of: Silty Sandy Clay
From: Pit 2 at 4.5 Feet
Compression
upon
wetting
411,
r
0.1
1.0 10
APPLIED PRESSURE — ksf
100
•
101 338C
Fig. 4
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE I
SUMMARY OF LABORATORY TEST RESULTS
JOB NO. 101 3380
SAMPLE LOCATION'
NATURAL
MOISTURE
CONTENT
(961
NATURAL
DRY
DENSITY
IPm1
GRADATION
PERCENT
PASSING
NO. 200
SIEVE
ATTERBBRG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
(PSFI
SOIL OR
BEDROCK TYPE
PIC
DEPTH
(feet)
GRAVEL
(%1
SAND
(961
LIQUID
LIMIT
(96I
PLASTIC
INDEX
(%)
1
2%
12.2
104
45
23
' 8
Very Clayey Sand
4%2
12.5
109
Silty Sandy Clay
2
434
13.4
99
Silty Sandy Clay
7
15.0
101
50
r
Silty Sand and Clay
•
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE II
PERCOLATION TEST RESULTS
JOB NO. 101 338C
HOLE NO.
HOLE DEPTH
(INCHES)
LENGTH OF
INTERVAL
(MIN)
WATER DEPTH
AT START OF
INTERVAL
(INCHES)
WATER DEPTH
AT END OF
INTERVAL
.(INCHES)
DROP IN
WATER
LEVEL
(INCHES)
AVERAGE
PERCOLATION
RATE
(MIN./INCH)
P-1
32
15
9
8
•
1
45
8
7 A
%
7 34
6 /4
%4
6%
61
%
61/4
5%
74
534
5
'/4
5
4 %
14
4 /4
4 1:
14
P-2
36
15
10
8 /4
1 14
30
8 I4
8
%
8
7 14
74
714
6% •
1/a
6 14
6 '/4
1/4
614
5344
14
53.
514
/
5 14
4 /4
P-3
40
15
12
11 -
1
30
11
10'/4
%
10 Y2
93'4
%
9 /4
9 14
3
9 1/4
9
14
9
83
1/a
83
8
14
8
7 1z
14
Note: Percolation test holes were hand dug and soaked on May 8, 2001. Percolation testing
was performed on May 9, 2001. The average percolation rate was based on the last
three readings of each test.