HomeMy WebLinkAbout03363 H P Hepworth- Pewlak Geotechnical, Inc. 154
5020 �� �V Vh Glenwood County Road Co
Glemvood Springs, Colorado 81601
Phone: 970-945-7988
Fax: 970. 945 -8454
hpgeo @hpgeotech.com
April 24, 2000
Jeremy Henderson
831 Latigo Loop
Carbondale, Colorado 81623 Job No. 100 295
Subject: Subsoil Study for Foundation Design and Percolation Test, Proposed
Residence, Lot 1, Los Adobes, Garfield County, Colorado
Dear Mr. Henderson:
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 you dated March 30, 2000. The data obtained and our recommendations based on
the proposed construction and subsurface conditions encountered are presented in this
report. Potential geologic hazard impacts on the proposed development are beyond the
scope of our study.
Proposed Construction: The proposed residence will be a single story wood frame
structure over a walkout basement located on the site as shown on Fig. 1. Ground
floors are proposed to be slab -on- grade. Cut depths are expected to range between
about 3 to 12 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 located about 80 feet downhill to the south 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 is located on the eastern side of the Los Adobes Subdivision.
The site was vacant the time of our field work. The ground surface in the building area
slopes moderately steep down to the south at grades up to about 30 %. There is about
10 to 12 feet of elevation difference in the proposed building area. The terrain becomes
steeper to the west of the building envelope at grades up to about 65%. An ephemeral
drainage is located to the east of the building envelope. An existing road provides
access into the lot. The lot is vegetated with pinon trees, sparse brush, grass and weeds.
Jeremy Henderson
April 24, 2000
Page 2
Red rock of the Maroon Formation sandstone outcrops on the hillside to the west of the
building site.
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, below about ' foot of topsoil, consist
of stiff, dense slightly clayey sandy silt with scattered gravel and occasional cobbles.
The west end of the proposed building extends to the base of the steep hillside where
bedrock may be at relatively shallow depth. Results of swell - consolidation testing
performed on relatively undisturbed samples of the silt, presented on Figs. 3 and 4,
indicate low compressibility under existing moisture conditions and light loading and a
low to moderate collapse potential (settlement under constant load) when wetted. The
samples showed high compressibility upon increased loading after wetting. No free
water was observed in the pits at the time of excavation and the soils were slightly
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 residence. The soils tend to compress
after wetting and there could be some post- construction foundation settlement. The
amount of settlement would depend on the depth and extent of subsurface wetting and
could be 1 to 2 inches or more. The settlement would likely be differential especially if
bedrock were encountered at foundation bearing level. Precautions should be taken to
prevent post construction wetting of the bearing soils. Footings should be a minimum
width of 20 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.
The exposed soils should then be moistened and compacted. 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 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.
H-P GEOTECH
Jeremy Henderson
April 24, 2000
Page 3
Floor Slabs: The natural on -site soils, exclusive of topsoil, are suitable to support
lightly loaded slab -on -grade construction. The silt soils are compressible when wetted.
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 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 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 1 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:
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
H -P GEOTECH
Jeremy Henderson
April 24, 2000
Page 4
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 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.
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 April 11, 2000 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 1/2 foot of topsoil overlying slightly clayey sandy silt with scattered
gravel and occasional cobbles. The percolation test results are presented in Table II.
The percolation test results indicate an infiltration rate between 20 and 23 minutes per
inch with an average of 22 minutes per inch. 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
•
Jeremy Henderson
April 24, 2000
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 GEOT `" :‘,. INC.
2• /
Jordy Z. Adams° Jr., P. P In 2970 �} Q
Reviewed by: 1 Ask z4l ..
Steven L. Pawlak, P.E.
JZA /ksm
attachments
cc: Kirk, Watkins & Associates, LLC - Attn: John Watkins
H -P GEOTECH
PIT 1 PIT 2 PROFILE PIT
ELEV. = 6288' ELEV. = 6281' ELEV. = 6270'
0 �� 0
M pi ,
I _ - � 5
5 o w0=6.1 t
0 _ WC-5.4
DD -06 , _ 0
200=61
10 10
LEGEND:
is TOPSOIL; clayey sandy silt, slightly organic, loose, slightly moist, reddish brown.
r
7 SILT (ML); sandy, slightly clayey, scattered gravel, occasional cobbles, stiff, slightly
.2 moist, reddish brown, slightly porous, slightly calcareous.
I SI 2" Diameter hand driven liner sample.
[:: Disturbed bulk sample.
NOTES:
1. Exploratory pits were excavated on April 10, 2000 with a bockhoe.
2. Locations of exploratory pits were measured approximately by taping from features
on the site plan provided.
3. Elevations of the exploratory pits were obtained by interpolation between contours on the site plan
provided. 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 on the exploratory pit logs represent the approximate
boundaries between material types and transitions may be gradual.
6. No free woter 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 )
-200 = Percent passing No. 200 sieve RTH 100 295 I H E WO C NI P WI LAK LOGS OF EXPLORATORY PITS I Fig. 2 C.
Moisture Content = 5.4 percent
Dry Density = 96 pcf
Sample of: Sandy Silt
From: Pit 1 at 6 feet
D
1
Compression
upon
2 wetting
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APPLIED PRESSURE — ksf
100 295 I H — P A LA SWELL— CONSOLIDATION TEST RESULTS Fig. 3
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