HomeMy WebLinkAboutFoundation Design 06.18.2015June 18. 2015
Michael Egan and Pauli Hayes
928 Branch Road
Scottsville, Virginia 24590
(,. eft ' )
Job No. 115 069A
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot 13,
Filing 2, Elk Springs, 480 Pinon Point, Garfield County, Colorado
Dear Mike and Pauli:
As requested, Hepworth-Pawlak Geotechnical, Inc. performed a subsoil study for design
of foundations at the subject site. We also evaluated the planned septic disposal site. The
study was conducted in general accordance with our agreement for geotechnical
engineering services to you dated February 26, 2015. The data obtained and our
recommendations based on the proposed construction and subsurface conditions
encountered are presented in this report.
Background Information: l-lepworth-Pawlak Geotechnical previously performed a
preliminary geotechnical study for Filings 6 through 9, Elk Springs (formerly Los
Amigos Ranch PUD) and reported our findings on February 14, 1997, Job No. 197 617.
We also previously performed a subsoil study and percolation test at the subject lot and
reported our findings in a report dated June 28, 1996, Job No. 196 285. Information from
the 1996 report has been incorporated into this report as needed including the profile pit
excavated and percolation testing performed at the septic disposal site.
Proposed Construction: The residence will be a single story wood franc structure over
a walkout basement lei el located on the lot as shown on Figure 1. The basement and
garage floors will be slab -on -grade. Cut depths are expected to range between about 3 to
5' feet. Foundation loadings fir this type of construction are assumed to be relatively Iight
and typical of the proposed type of construction. The septic disposal field is planned
dou nhi11 and to the south/southwest of residence.
-2
If building conditions or foundation Ioadings 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 on a rolling upland mesa above the Roaring Fork
River Valley. The lot is vacant and vegetated with a pinion and juniper forest with a
ground cover of grass and weeds. The ground surface on the lot appears mostly natural
and slopes down to the southwest at a grade of about 20 percent. Elevation difference
across the proposed building is about 8 or 9 feet. Numerous basalt cobbles and boulders
are visible on the ground surface.
Subsurface Conditions: The subsurface conditions at the site for the current study were
evaluated by excavating three exploratory pits (Pits 3 and 4 and Profile Pit 2) at the
approximate locations shown on Figure 1. The approximate locations of the three
previous exploratory pits on the site from the 1996 study (Pits 1 and 2 and Profile Pit 1)
are also shown on Figure 1. The logs of the current pits are presented on Figure 2. The
logs of the previous pits are provided in Appendix A as Figure 2. Note that the elevations
shown on the 1996 pit logs are not consistent with the current topography survey shown
on Figure 1.
The subsoils encountered, below about % to 2 feet of topsoil, consisted of relatively
dense, basalt gravel, cobbles and boulders in a sandy silt and clay matrix that extended
down to the maximum depth explored of 8 feet. Digging in the dense basalt rocks was
difficult and digging refusal was encountered in the deposit. Results o f swell -
consolidation testing performed on a relatively undisturbed sample of the sandy silt and
clay matrix soils, presented on Figure 3, indicate low compressibility under existing
moisture conditions and light loading, and moderate compressibility when loaded after
wetting. Results of gradation analyses performed on disturbed bulk sample of subsoils
(minus 3 to 5 inch fraction) obtained from the site are presented on Figures 4 and 5. The
laboratory testing is summarized in Table 1. Results of a gradation analyses performed
on a bulk sample of the subsoils from the previous study (minus 5 inch fraction) are
provided in Appendix A as Figure 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 granular soil designed for an allowable bearing
pressure of 2,500 psf for support of the proposed residence. Utility trenches and deep cut
areas below about 5 feet may require rock excavating techniques such as chipping or
blasting.
Job No. 115 069A
Gittech
-3 -
Footings should be a minimum width of 18 inches for continuous wails 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. Voids created from boulder removal at footing grade should be
filled with a structural material such as road base compacted to 98% standard Proctor
density at a moisture content near optimum or with concrete. 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 well 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 also be designed to resist a lateral earth
pressure based on an equivalent fluid unit weight of at Ieast 50 pcf for the on-site granular
soil, excluding topsoil and oversized (plus 8 inch) rocks, as backfill.
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 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 grave[ should be placed beneath slabs
for support and to facilitate drainage. This material should consist of minus 2 inch
aggregate with Iess than 50% passing thc No. 4 sieve and less than 2% passing the No.
200 sieve.
Underdrain System: Although free water was not encountered during our exploration, it
has been our experience in the area and where clayey soils are present 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 thc 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 and be covered by filter fabric such
as Mirafi 140N or 160N.
Job No. 115 069A
HPtech
-4 -
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 filter fabric and 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 5 feet from the building.
Septic Disposal Site: The current and previous studies are the basis for our evaluation of
the proposed septic disposal site for treatment of the on-site wastewater. Profile Pits 1
and 2 were excavated at the locations shown on Figure 1 the subsoils encountered and
consisted of Very Gravelly Sand (per USDA classification system) to the maximum depth
explored of 8 feet with no free water encountered. The previous percolation test results at
the site are provided on Table 2 in Appendix A. The percolation test results indicate rates
from 20 to 60 minutes per inch (mpi) with an average rate of about 37 mpi. 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 express 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 and to the depths shown on Figure 2, 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
Job No. i 15 069A
-5 -
conditions encountered during construction appear different from those described in this
report, we should be notified at once so re-evaluation of the recommendations inay be
made.
-Phis 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.
Respectfully Submitted,
HEPWORTH,- PAWLAK GEOTECHNICAL, INC.
Louis E. Eller
Reviewed by:
L3 = -\\, +
-
:n I
David A. Young, P.E.°�,tj : C."
reA 616/-1C:44:
LEE/ksw err �' "`•=,.t.+, V+ice
attachments Fiygurc 1 = Location of Exploratory Pits
Figure 2 — Logs of Exploratory Pits
Figure 3 — Swell -Consolidation Test Results
Figure 4 — Gradation Test Results
Figure 5 — USDA Gradation Test Results
Table 1 - Summary of Laboratory Testing
Appendix A -- 1996 Report Subsoil and Percolation Test Information
Joh No 1 1 i169A
Gegtech
APPROXIMATE SCALE
1' = 60'
LOT 12
okr P-1
P-2A
PROFILE PIT 1
p_3 PROPOSED
SEPTIC FIELD
LOT 14
LEGEND:
■ EXPLORATORY PIT FOR CURRENT STUDY.
EXPLORATORY PIT FOR PREVIOUS STUDY,
❑ DATED JUNE 28, 1996, JOB NO. 196 285.
Q PERCOLATION TEST HOLE FOR PREVIOUS STUDY,
DATED JUNE 28, 1996, JOB NO. 196 285.
115 069A
LOCATION OF EXPLORATORY PITS
Figure
0
5
10
LEGEND:
...•
PIT 3
ELEV.= 1019'
-200=26
WC=13.5
D0=79
PIT 4
ELEV.— 1009'
WC=17.0
DD -91
-200=92
J
PROFILE PIT 2
ELEV.= 1001'
GRAVEL=51
_ _ SAND=24
SILT=16
CLAY=9
TOPSOIL; organic sandy silt and clay with basalt cobbles and boulders, slightly moist, dark brown.
5
10
BASALT GRAVEL, COBBLES AND BOULDERS (GUI -GC); in a sandy silt and clay matrix, dense, slightly
moist, light brown, calcareous.
2" Diameter hand driven finer sample.
Disturbed bulk sample.
TPractical digging refusal.
NOTES:
1. Exploratory pits were excavated on April 20, 2015 with a John Deere 50D mini -excavator.
2. Locations of exploratory pits were measured approximately by pacing from features shown on the site plan
provided.
3. Elevations of exploratory pits were obtained by interpolation between contours shown on the site plan provided. Pit
Togs 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. Fluctuation in water level may occur with time.
7. Laboratory Testing Results:
WC = Water Content (%)
DD = Dry Density (pcf)
4 = Percent retained on the No. 4 sieve
-200 = Percent passing No. 200 sieve
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 then particle size ,002mm
115 069A
H
Hepworth—PawlaeCh
k Geatachnical
LOGS OF EXPLORATORY PITS
Figure 2
Compression
0
3
4
5
0.1
Moisture Content = 13.5 percent
Dry Density 79 pcf
Sample of: Sandy Silt and Clay Matrix
From: Pit 3 at 3 Feet
10
APPLIED PRESSURE - ksf
Compression
upon
wetting
10 100
115 069A
Hepworth—Pawtak Geotachntcal
SWELL -CONSOLIDATION TEST RESULTS
Figure 3
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DIAMETER OF PARTICLES IN MILLIMETERS
37.5
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127
CLAY TO SIT
FAE
GRAVEL 34 %
LIQUID LIMIT %
SAH) J GRAIL
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SAND 40 % SILT AND CLAY 26 %
PLASTICITY INDEX %
FROM: Pit 3 at 2 to 3 Feet
SAMPLE OF Silty Clayey Sand and Gravel with
115 069A
Cobbles
H
He. worth—Puy/kik Geotechnical
GRADATION TEST RESULTS
`' ERCENT PASSI
Figure 4
I
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CLAY
DIAMETER OF PARTICLES IN MILLMETE.RS
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SILT 16 %
CLAY 9 %
60
50
40
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20
10
0
FROM; Profile Pit 2 at 2 to 3 Feet
agaa 7WII
115 069A
GaZtech
HEPWORTH•PAWLAK GEOTECHNFCAL
USDA GRADATION TEST RESULTS
Figure 5
Job No. 115 069A
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APPENDIX A
1996 Report Subsoil
and
Percolation Test Information
Job No. 115 069A
0
5
10
LEGEND:
TOPSOIL; sandy silty clay, scattered gravel, organics, slightly moist to moist, brown.
NOTES:
PIT 1
ELEV. = 999'
J
WC 12.3
+4-41
-200 - 30
U. 37
PI - 12
PIT 2
ELEV. = 988'
PROFILE PIT
ELEV. = 980'
GRAVEL and COBBLES (GC ); sandy siity clay matrix, with boulders up to 4 feet diameter,
medium dense, slightly moist to moist, fight brown to white, subangular to angular basalt
rock, calcareous.
Disturbed bulk sample.
0
5
10
1. Exploratory pits were excavated on June 17,1996 with a backhoe.
2. Locations of exploratory pits were measured approximately by pacing from features shown on
the site plan provided.
3. Elevations of exploratory pits were obtained by interpolation between contours on the site plan provided.
Logs of exploratory pits 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 (%) LL = Liquid Limit (%)
+4 = Percent retained on No. 4 sieve. PI = Plasticity Index (`Yo)
-200 = Percent passing No. 200 sieve.
196 285
HEPWORTH - PAWLAK
GEOTECHNICAL, INC.
LOGS OF EXPLORATORY PITS
Fig. 2
24 HR.
45 MRI.
100
70
7RR.
15 MOL W ASN. I9M9L 1400E 1 MK 1200 1100
IINOROMETE71ANALYSIS
TIMENEADIN69
50
40
30
10
0
D01
ELS. STANDARD, 5E111E3
140 1.35 110 M
SI EVE ANALYSIS
1
CLEAR SOUME OPE/13142S
14 318' 1/2' 3.44' 1 1R Y 5' E'
1
1
}
r
4 F
t-
1-
r.
1
1
.00R Das .119 .015 .077 .074 .150 .300 .800 1.18 2.7E 4.71
DIAMETER OF PARTICLES IN MILLIMETERS
880 19.0 378 752 102
12.5 477
0
10
20
30
40
50
40
70
100
707
CAYTO 547
SAND GRAVEL
1 MFOglM 1 COARSE 1 Fjj� 1 COARSE
GRAVEL 41 % SAND 29 % SILT AND CLAY 30 %
LIQUID LIMIT 37 % PLASTICITY INDEX 12 %
SAMPLE OF; Silty Clayey Sandy Gravel FROM: Pit 1 from 4 to 6 Feet
i-
196 285
HEPWORTH - PAWLAK
GEOTECHNICAL, INC_
GRADATION TEST RESULTS
Fig. 3
HEPWORTH-PAWLAK GEOTECHNICAL„ INC.
TABLET.
PERCOLATION TEST RESULTS
JOB NO. 196 285
HOLE NO.
P-1
P-2
P-3
HOLE DEPTH
(INCHES)
46
44 1/2
45
LENGTH OF
INTERVAL
(MIN)
15
refill
refill
15
refill
15
refill
refill
WATER DEPTH
AT START OF
INTERVAL
(INCHES)
7 1/4
WATER DEPTH
AT END OF
INTERVAL
(INCHES)
6 1/4
DROP IN AVERAGE
WATER PERCOLATION
LEVEL RATE
(INCHES) (MIN.IINCHI
1
6 1/4
5 1/4
1
7 3/4
6 314
1
6 3(4
6 1/4
1/2
6 1/4
5 314
1/2
7 3/4
7 1/4
1/2
7 114
6 3/4
1/2
6 3/4
8 1/2
6 1/4
7 3/4
1/2
3/4
7 3/4
7
3/4
7
6 1/2
1/2
6 1/2
6
1/2
6
5 3/4
1/4
8 314
8 1/2
1/4
8 1/2
8 1/4
114
8 114
8 1/4
8
6 1/2
1/4
1 3/4
6 1/2
5 1/4
1 114
8 1/2
7 1/4
1 1/4
7 1/4
6 1/4 1
6 1/4
5 114 1
8 1/2
7 3/4 3/4
7 3/4
7 3/4
7
6 1(4 3/4
30
60
20
Note: Percolation test holes were hand dug in the bottom of backhoe pits and soaked on June 17, 1996.
Percolation tests were conducted on June 1 8, 1996.