HomeMy WebLinkAbout1.03 GeotechMay 23, 2001
Beulah Wilson Estate
Attn: Virginia Sterrett
6235 109 Road
Hepworth•Pawlal, Geotechnical; Inc.
5020 County Road 154
Glenwood Springs, Colorado 81601
Phone: 970.945-7988
Fax: 970-945.8454
hpgeo@lipgeotech.com
Cartiondale, Colorado 81623 Job No. 101 338A
Subject: Subsoil Study for Foundation Design and Percolation Testing, Proposed
Residence, Lot A, 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 testing 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 proposed residence are conceptual at this time
and this report was prepared for the purchaselsale 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 proposed to be Iocated downhill and
northwest 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.
Site Conditions: Lot A is located on the west side of County Road 109. The ground
surface in the building area is gently rolling with moderate to steep slopes down to the
Beulah Wilson Estate
May 23, 2001
Page 2
east. The lot is vegetated by a pinon and juniper forest with ground cover of grass,
weeds and cactus. A large irrigation ditch parallel to County Road 109 crosses through
the site below the building area.
Subsurface Conditions: The subsurface conditions at the site were evaluated by
excavating one exploratory pit 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, below about 2 feet of topsoil, consist of
silty sandy gravel with cobbles and small boulders. Results of a gradation analysis
performed on a sample of sandy gravel (minus 6 inch fraction) obtained from the site
are presented on Fig. 3. 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 2,500 psf for support of the proposed residence. 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. 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 10 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 45 pcf for the on-site soil as backfill.
Floor SIabs: 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 columns with
expansion joints which allow unrestrained vertical movement. Floor slab control joints
GEOTECH
Beulah Wilson Estate
May 23, 2001
Page 3
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 mountainous areas 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 maximum size of
2 inches. The drain gravel backfill should be at least 11/2 feet deep.
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 least 95 % of the maximum standard Proctor density in
H -P GEOTECH
Beulah Wilson Estate
May 23, 2001
Page 4
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 fust 10 feet in pavement
and walkway areas. A swale will be needed uphill to direct surface
runoff around the residence.
4) Roof downspouts and drains should discharge well beyond the limits of
all backfill.
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 about 11/2 to 2 feet of topsoil overlying silty sandy gravel with cobbles. The
percolation test results are presented in Table I. 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 construction 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 subsurface conditions
may not become evident until excavation is performed. If conditions encountered
H -P GEOTECH
Beulah Wilson Estate
May 23, 2001
Page 5
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.
Louis E. Eller
Reviewed by:
Daniel E. Hardin, P.E.
LEE/ksw
attachments
H -P GEOTECH
HEPWORTH-PAWLAK GEOTECHNICAL, INC.
TABLE I
PERCOLATION TEST RESULTS JOB N0,101 338A
HOLE NO. HOLE DEPTH
(INCHES)
P-1 28
LENGTH OF
INTERVAL
(MIN)
15
water added
water added
WATER DEPTH
AT START OF
INTERVAL
(INCHES)
6 +%
WATER DEPTH
AT END OF
INTERVAL
(INCHES)
5
DROP IN
WATER
LEVEL
(INCHES)
1 Y4
AVERAGE
PERCOLATION
RATE
(MINJINCH)
20
8
7
1
7
6
1
6
5
1
6
4 %4
%
4'/4
3 A
%
2
6 744
5 +%
'/4
P-2
28
15
water added
6 %=
5 '/4
1 %4
26
5 '/4
4 %z
4 A
3 '/4
%
3 %
3
%
6 A
5 Y:
/4
5 %:
5
5
4
4 %4
3 '/a
'/:
P-3
30
15
water added
6 %
5
1 %
23
6
5 '/4
'/4
5%
4%_
%
4 %z
3 '/4
'/4
5 '/4
4 /
Y.
4 '%
4
%4
4
3 '/4
3 114 - _ —
2 '/4
'/z
Note: Percolation test holes were hand dug in the bottom of backhoe pits and soaked on May
81 2001. Percolation tests were conducted on May 9, 2001. The average percolation
rates are based on the last three readings of each test.
PIT 1
PROFILE PIT
ELEV.= 120.9'
ELEV.= 115.3'
0
0
f
,
LL
5 u
ry
5 I
•. 1
•. 1
t
.P
❑
+4-76
- —20o-4
10
1D
LEGEND:
TOPSOIL; sandy silt with scattered cobbles and small boulders, organic, firm,
moist, dark brown.
° GRAVEL (GP—GM); sandy, slightly silty, with cobbles and small boulders,
4 dense to very dense, moist, light brown, subrounded rock.
Disturbed bulk sample.
NOTES:
1. Exploratory pits were excavated on May 8, 2001 with a 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. Logs 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 an 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:
+4 = Percent retained on No. 4 sieve
—200 = Percent passing No. 200 sieve
1 101 338A I GEOTEOC NICAL,W NC. I LOGS OF EXPLORATORY PITS I Fig. 2
APPROXIMATE SCALE
PARCEL B 1' = 60'
BENCH MARK: GROUND AT PROPERTY
CORNER: ELEV. = 100.0', ASSUMED.
1
1
P-1
P-3 0 no
G
®� P-2
PROFILE �o
PIT yo
PIT 1 'oma
LOT A
1
EXISTING
IRRIGATION
DITCH `
LOT B
101 338A HEPWORTH-PAWLAK LOCATION OF EXPLORATORY PITS Fig. 1
GEOTECHNICAL, INC_