HomeMy WebLinkAboutGeotechnical Study - Foundation Design & Perc Test 12.05.2018December 5, 2018
Dave Poulsen
193 Red Bluff Vista
Glenwood Springs, Colorado 81601
ciavepoul;cnu`L inti
H-P�KUMAR
Geotechnical Engineering I Engineering Geology
Materiels Testing I Environmental
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Fax: (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado
RECEIVED
GARFIELD COUNTY
COMMUNITY DEVELOPMENT
Project No.18-7-633
Subject: Geotechnical Study for Foundation Design and Percolation Testing, Proposed
Residence, Lot 14, Pinyon Peaks Subdivision, Maroon Mesa Road, Garfield
County, Colorado
Dear Dave,
As requested, H-P/Kumar performed a geotechnical study and percolation testing for foundation
and septic disposal designs at the subject site. The study was conducted in accordance with our
proposal for geotechnical engineering services to you dated October 10, 2018. 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 has not been designed but will be located on
the site in the area of Pits 1 and 2, as shown on Figure 1. Ground floors may be structural over
crawlspace or slab -on -grade. Cut depths are expected to relatively shallow and range between
about 2 to 5 feet. Foundation loadings are assumed to be relatively light and typical of
residential type of construction. The septic disposal system is proposed to be located downhill to
the south of the 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 site is located off the north end of the Maroon Mesa Road cul-de-sac in
open pinyon forest. The building site slopes down to the south at grades of about 5 to 10
percent. Below the building area, the grade is about 15 percent down to the south. There is a
low ridge of Maroon Formation sandstone/siltstone bedrock located just to the north of the
proposed building area.
-2-
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two
exploratory pits in the building area and two profile pits in the septic disposal area at the
approximate locations shown on Figure 1. The logs of the pits are presented on Figure 2. The
subsoils encountered, below about 1 foot of root zone, highly weather bedrock, silty sand with
rock fragments, consist of weathered Maroon Formation sandstone/siltstone bedrock down to
practical digging refusal to the baekhoe at 4 to 4'%z feet. The bedrock bedding is relatively flat.
No free water was observed in the pits at the time of excavation and the weathered rock was
slightly moist.
Foundation Recommendations: Considering the subsurface conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend spread footings
placed on the undisturbed natural weathered bedrock designed for an allowable soil bearing
pressure of 4,000 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 bedrock
fragments encountered at the foundation bearing level within the excavation should be removed
and the footing bearing level extended down to the undisturbed natural weathered bedrock.
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. Shallow, frost -protected footings can also be used. 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 weathered bedrock (minus 5 inch in size) as backfill.
Floor Slabs: The natural on -site weathered bedrock is 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 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 (3/,-inch screened rock).
H-P%KUMAR
Project No. 18-7-633
-3-
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 on -site
processed soils and rocks devoid of vegetation, topsoil and oversized rock.
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience where bedrock is shallow that local perched groundwater can develop
during times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can
also 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 (3/-inch screened rock). The drain gravel backfill should be at least
1'h 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 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 weathered bedrock 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.
H-P*KUMAR
Project No 18-7-633
-4-
4) Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Percolation Testing:
Percolation tests were conducted on October 18, 2018 to evaluate the
feasibility of an infiltration septic disposal system at the site. Two profile pits and adjacent
shallow percolation holes were dug at the locations shown on Figure 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. The soils exposed in the percolation holes are similar to those exposed
in the Profile Pits shown on Figure 2 and consist of about 1 foot of root zone over weathered
Maroon Formation bedrock. Results of a USDA gradation analysis performed on a sample of
excavated weathered bedrock (minus 1'/2 inch fraction) obtained from the site are presented on
Figure 3. The percolation test results ate presented in Table 1. Based on the shallow bedrock,
the tested area is probably not suitable for a conventional infiltration septic disposal system. We
expect that a mounded system with a sand bed filter will be needed. An engineer should design
the 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 Figure 1,
the proposed type of construction and our experience in the area. 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. 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 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
H-P%KUMAR
Project No. 18-7-633
-5-
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.
Respectfully Submitted,
H-PKUMAR
Daniel E. Hardin, P.
Reviewed by
Steven L. Pawlak, P.E.
DEH/kac
attachments Figure 1 — Location of Exploratory Pits
Figure 2 — Logs of Exploratory Pits
Figure 3 - USDA Gradation Test Results
Table 1 — Percolation Test Results
H-P%KUMAR
Project No 18-7-633
LOT 15
ciN
Neighbor's
0'it
Driveway
Found /5 Rebor
with 1 1/4"
Orange Plastic
Cop (illegible)
S 5722 J3 W,
a 93'
50 0 50 100
APPROXIMATE SCALE —FEET
LOT 14
Found /5 Rebor with 1
1/4' Orange Plastic
Cop L S /15710
S 2724 56" W, 0.66'
Edge of Pavement
LOT 13
18-7-633
H-P--A5KUMAR
LOCATION OF EXPLORATORY PITS
Fig. 1
PIT 1 PIT 2
PROFILE PIT 1 PROFILE PIT 2
L__5
LEGENP
0
1-
- W
- GRAVEL=15 _
- 1SAND=53
SILT=26 EL
CLAY=6 0
TOPSOIL; ROOT ZONE, HIGHLY WEATHERED BEDROCK, SILTY SAND WITH ROCK FRAGMENTS,
FIRM. MOIST, REDDISH BROWN.
f WEATHERED BEDROCK; SANDSTONE/SILTSTONE, HARD, SLIGHTLY MOIST, RED, FLAT BEDDING,
MAROON FORMATION.
DISTURBED BULK SAMPLE.
PRACTICAL DIGGING REFUSAL.
NOTES
5 -
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON OCTOBER 18, 2018.
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE NOT MEASURED AND THE LOGS OF THE
EXPLORATORY PITS ARE PLOTTED TO DEPTH.
4. THE EXPLORATORY PIT LOCATIONS 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 THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF DIGGING. PITS WERE
BACKFILLED SUBSEQUENT TO SAMPLING.
7. LABORATORY TEST RESULTS:
GRAVEL = PERCENT RETAINED ON NO. 10 SIEVE;
SAND = PERCENT PASSING NO. 10 SIEVE AND RETAINED ON NO. 325 SIEVE;
SILT = PERCENT PASSING NO. 325 SIEVE TO PARTICLE SIZE .002MM;
CLAY = PERCENT SMALLER THAN PARTICLE SIZE .002MM.
18-7-633
H-P KUMAR
LOGS OF EXPLORATORY PITS
Fig. 2
H-PKUMAR
TABLE 1
PERCOLATION TEST RESULTS
PROJECT NO. 18-7-633
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
--
28
_____
28
10
Water added
5
4%
'/:
40
41/2
4
'4
4
3% _
'/4
3%
3%
1/2
51/2
5
%
5
4%
%
4%
41/2
'/,
41/2
41/4
%
P-2
10
Water added
6
5'
%
17
51/4
41/2
'
41/2
3%
%
3'
3
3/,
6'
6
'
6
5'/s
1
51/2
5
'/s
5
4'/.
24
Note: Percolation test holes were hand dug in the bottom of backhoe pits. Percolation
tests were conducted on October 18, 2018. The average percolation rates were
based on the last three readings of each test.