HomeMy WebLinkAboutSoils Report 02.05.2018H-PI(UMAR
Geotechnical Engineering 1 Engineering Geology
Materials Testing 1 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
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 81, SPRING RIDGE RESERVE
ELIC RIDGE DU.IVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 18-7-102
FEBRUARY 5, 2018
PREPARED FOR:
>i EITH WITTENBERG
470 HIDDEN VALLEY DRIVE
GLENWOOD SPRINGS, COLORADO 81601
(,I<<iggs09@ yahoo.conI)
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 1 -
ROCKFALL HAZARD - 2 -
FIELD EXPLORATION - 3 -
SUBSURFACE CONDITIONS - 3 -
DESIGN RECOMMENDATIONS - 4 -
FOUNDATIONS - 4
FOUNDATION AND RETAINING WALLS - 5 -
FLOOR SLABS - 6 -
UNDERDRAIN SYSTEM - 6 -
SURFACE DRAINAGE - 7 -
LIMITATIONS - 7 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL -CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
H-P*KUMAR
Project No 18-7-102
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located at Lot
81, Spring Ridge Reserve, Elk Ridge Drive, Garfield County, Colorado. The project site is
shown on Figure 1. The purpose of the study was to develop recommendations for the
foundation design. The study was conducted in general accordance with our proposal for
geotechnical engineering services to Keith Wittenberg dated December 19, 2017. Hepworth-
Pawlak Geotechnical previously performed a preliminary geotechnical study for the subdivision
development and reported their findings February 26, 2001, Job No. 101 126 and updated the
study in a report dated June 22, 2004.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock obtained during
the field exploration were tested in the laboratory to determine their classification,
compressibility or swell and other engineering characteristics. The results of the field
exploration and laboratory testing were analyzed to develop recommendations for foundation
types, depths and allowable pressures for the proposed building foundation. This report
summarizes the data obtained during this study and presents our conclusions, design
recommendations and other geotechnical engineering considerations based on the proposed
construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
The proposed residence will be one and two-story, wood frame construction with an attached
garage at the upper, main level and a walkout lower level located as shown on Figure 1. Ground
floors will slab -on -grade. Grading for the structure is assumed to be relatively minor with cut
depths between about 3 to 8 feet. We assume relatively light foundation loadings, typical of the
proposed type of construction.
If building loadings, location or grading plans change significantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The residence will be located in the lower, western part of the building envelope near Elk Ridge
Drive as shown on Figure 1. Vegetation consists of grass and weeds with scattered stands of
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Project No 18-7-102
_2 -
scrub oak above the building site. The ground surface slope is down to the west at about 10%
across the building area and steepens somewhat in the upper lot area. An abandoned irrigation
ditch crosses through the lot just below the building area.
ROCKFALL HAZARD
The subject lot slopes up to the east-southeast at about 8 to 10 percent across the proposed
building portion of the lot. An abandoned, north trending irrigation ditch with a small berm
bordering the ditch, is located west of the building area in the northwest part of the lot. Above
the building area, the terrain generally slopes up at about 20 to 25 percent with localized slopes
of up to about 30 to 40 percent near the crest of the slope. The slope is about 300 feet in length
from the building area to the crest. At the time of our site visit, this building areas was clear of
snow and snow cover ranged up to about 8 inches in depth on the slope above the lot. Surficial
soils on the hillside above the proposed residence area consist of relatively shallow colluvium,
comprised of silty, clayey sand with gravel, cobbles and small boulders, overlying sandstone
bedrock of the Maroon Formation. Scattered, mostly flat (tabular) shaped rock fragments,
typically up to about 1 foot in size, are exposed on the hillside above the residence area. An area
of cobbles, larger boulders and the remnants of a bedrock outcrop occurs near the crest of the
ridge, with boulders ranging from 1 foot to several feet in size. The boulders are generally
tabular in shape, and typically partially embedded in the overburden soil. Drainage on the slope
above the proposed residence appears to be primarily by sheet flow.
Based on our observations and professional experience, we conclude that the rockfall hazard to
the proposed construction is low, and rockfall mitigation measures are not warranted.
Although
low in probability, down-slope rock movement, if it occurs, will likely originate from the steeper,
rocky zone located near the crest of the slope. Cobbles and boulders in the rocky zone are
typically tabular in shape and not prone to movement. Scattered cobbles and boulders with sub-
angular to sub -rounded shapes were also observed and will be more prone to movement down-
slope. Storm and seasonal events, resulting in erosion of surficial soils, may eventually cause
instability of rock fragments on the hillside. If the owner is concerned with the potential for
down-slope rock movement, we recommend periodic observation by the owner of the slope to
identify potential rocks prone to movement, and we should be contacted if further investigation
is desired at that time. If alterations to the slope above the residence are planned in the future,
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Project No 18-7-102
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we should be consulted to review the planned alterations for potential slope stability and rockfall
hazards.
FIELD EXPLORATION
The field exploration for the project was conducted on January 10, 2018. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
The borings were advanced with 4 -inch diameter continuous flight augers powered by a truck-
mounted CME -45B drill rig. The borings were logged by a representative of H-P/Kumar.
Samples of the subsoils were taken with a 2 -inch I.D. spoon sampler. The sampler was driven
into the subsurface materials at various depths with blows from a 140 pound hammer falling 30
inches. This test is similar to the standard penetration test described by ASTM Method D-1586.
The penetration resistance values are an indication of the relative density or consistency of the
subsoils and hardness of the bedrock. Depths at which the samples were taken and the
penetration resistance values are shown on the Logs of Exploratory Borings, Figure 2. The
samples were returned to our laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils, below about six inches of topsoil, consist of about 8 to 13 feet of stiff to hard, silty
sandy clay to very sandy clay with depth, underlain by hard to very hard, siltstone/sandstone
bedrock to the drilled depth of 16 to 21 feet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and percent finer than sand size gradation analyses. Results of swell -
consolidation testing performed on relatively undisturbed drive sample of clay soils, presented
on Figure 4, generally indicate low to moderate compressibility under conditions of loading and
wetting. The sample from Boring 2 at 21/2 feet showed a low expansion potential when wetted
under light loading. The laboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils and bedrock
were slightly moist.
H-PkKUMAR
Project No 18-7-102
soils should be further evaluated at the time of excavation i
-4 -
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings, the nature of the
proposed construction and our experience in this area, we recommend the building be founded
with spread footings bearing on the natural soils.
The expansion potential of the clay sample
from Boring 2 at 21/2 feet appears to be an anomaly and the expansion potential of the exposed
or possible mitigation measures.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils should be designed for an
allowable hearing pressure of 2,000 psf.
Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be less than 1 inch. Additional differential movement of about 1/2 to 1 inch could
occur if the bearing soils are wetted and precautions should be taken to keep the
bearing soils dry.
2) The footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least 36 inches below exterior grade is typically used in this
area.
4) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 14 feet.
Foundation walls acting as retaining structures should also be designed to resist
lateral earth pressure as described below in the "Foundation and Retaining Walls"
section.
5) All topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to the undisturbed native soils. The exposed soils in
footing area should then be moistened and compacted as needed.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
H-P-KUMAR
Project No 18-7-102
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FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structures which are laterally supported and can be expected to
undergo only a slight amount of deflection should be designed for a lateral earth pressure
computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting
of the on-site soils. Cantilevered retaining structures which are separate from the residence and
can be expected to deflect sufficiently to mobilize the full active earth pressure condition should
be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight
of at least 45 pcf for backfill consisting of the on-site soils.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and a horizontal
backfill surface. The buildup of water behind a wall or an upward sloping backfill surface will
increase the lateral pressure imposed on a foundation wall or retaining structure. An underdrain
should be provided to prevent hydrostatic pressure buildup behind walls.
Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at near optimum moisture content. Backfill placed in pavement and
walkway areas should be compacted to at least 95% of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even if the material is placed correctly, and could result in distress to
facilities constructed on the backfill.
The lateral resistance of foundation or retaining wall footings will be a combination of the
sliding resistance of the footing on the foundation materials and passive earth pressure against
the side of the footing. Resistance to sliding at the bottoms of the footings can be calculated
based on a coefficient of friction of 0.35. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 300 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suitable factors of safety should be included in the design to limit the strain which will
occur at the ultimate strength, particularly in the case of passive resistance. Fill placed against
H-P--14KUMAR
Project No 18-7-102
-6 -
the sides of the footings to resist lateral loads should be compacted to at least 95% of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab -on -grade
construction. The expansion potential and need for sub -excavation to remove expansive clay
soils should be evaluated at the time of excavation. 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 at least
50% retained on 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 or imported relatively well graded granular 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 and where bedrock is shallow 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
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Project No 18-7-102
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maximum size of 2 inches. The drain gravel backfill should be at least 11/ feet deep. An
impervious membrane, such as 20 mil PVC should be placed below the drain gravel in a trough
shape and attached to the foundation wallwith 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 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.
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 paved areas. Free -draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site finer graded
soils to reduce surface water infiltration.
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 foundation walls.
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 borings drilled 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
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Project No 18-7-102
-8 -
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions identified al the exploratory borings 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 so
that 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.
Respectfully Submitted,
H -P
KU MAR
Steven L. Pawlak, P.
Reviewed by:
„. e
Daniel E. Hardin, P.E.
SLP/kac
1
Cc: Don Pettygrove (clgpengineeringllc@gmail.com)
H-P�KUMAR
Project No 18-7-102
W
C7
—J
41
OPEN SPACE
25 0 25 5C'
APPROXIMATE SCALE—FEET
LOT 80
NOTE:
CONTOURS LINES IN THE BUILDING AREA
SHOWN ARE MODIFIED FROM THE EXISTING
MOSTLY NATURAL CONTOUR LINES.
18-7-102
H -P-` KUMAR
LOCATION OF EXPLORATORY BORINGS
Fig. 1
18-7-102
UJ
w
w
I
z
0
1-
>
w
J
w
BORING 1
EL.6338'
BORING 2
EL.6330'
6340 6340
- 6335
6330
6325
6320
6315
44/12 6335
/
/ f] 9/12
WC=10.3
/ DD=117
//
f
16/12
/ WC=8.7
DD=121
-200=58
50/4
50/3
/ 30/12
WC=5.3
DD=117
/ /] 20/12
/ WC=7.4
DD=117
/ -200=57
/
/-;
50/5
WC=8.2
DD=124
/ 150/4
6330
6325
6320
6315-
6310
315-
6310 6310 -
H-P-� KUMAR
ELEVATION -FEET
LOGS OF EXPLORATORY BORINGS
Fig. 2
LEGEND
•
•
/
7
TOPSOIL; ORGANIC SANDY SILTY CLAY, FIRM, DARK, BROWN.
CLAY (CL); SILTY, SANDY TO VERY SANDY AND SCATTERED GRAVEL WITH DEPTH, STIFF TO
HARD, SLIGHTLY MOIST, RED, LOW PLASTICITY.
pSILTSTONE/SANDSTONE BEDROCK, HARD TO VERY HARD WITH DEPTH, SLIGHTLY MOIST, RED,
STEEP BEDDING DIP. MAROON FORMATION.
RELATIVELY UNDISTURBED DRIVE SAMPLE; 2—INCH I.D. CALIFORNIA LINER SAMPLE.
44/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 44 BLOWS OF A 140—POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE CALIFORNIA SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 10, 2018 WITH A 4—INCH DIAMETER
CONTINUOUS FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY BORING 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 BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSITY (pcf) (ASTM D 2216);
—200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140).
18-7-102
H -P- KUMAR
LEGEND AND NOTES
Fig. 3
CONSOLIDATION SWELL
CONSOLIDATION - SWELL
1
0
1
2
2
1
0
1
2
.1
10 APPLIED PRESSURE - KSF
10
100
SAMPLE OF: Sandy Silty Clay
FROM: Boring 1 @ 5'
WC = 10.3 %, DO = 117 pcf
I
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
•-,.<
n
Tal
These lest results apply only to the
samples tested. T e testing apart
shall not be reproduced. enc pt in
full, without the w itten appr vel of
Kumar and Associates, Inc. Swell
Consolidation tasting performed in
accordance wilh ASTM 0-4546
.1
10 APPLIED PRESSURE - KSF
10
100
1 0 APPLIED PRESSURE - KSF
10
100
18-7-102
H -P- KUMAR
SWELL -CONSOLIDATION TEST RESULTS
Fig. 4
SAMPLE OF: Sandy Silty Clay
FROM: Boring 2 @ 2.5'
WC = 5.3 %, DD = 117 pcf
I
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
n
�11
These lest results apply only to the
samples tested. T e testing apart
shall not be reproduced. enc pt in
full, without the w itten appr vel of
Kumar and Associates, Inc. Swell
Consolidation tasting performed in
accordance wilh ASTM 0-4546
1 0 APPLIED PRESSURE - KSF
10
100
18-7-102
H -P- KUMAR
SWELL -CONSOLIDATION TEST RESULTS
Fig. 4
PKU MAR
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 18-7-102
SAMPLE LOCATION I
NATURAL
MOISTURE
CONTENT
(%) I
GRADATION
ATTERBERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
(PSF)
SOIL TYPE
BORING
DEPTH
(ft)
NATURAL
DRY
DENSITY(%)
(pcf)
GRAVEL
SAND
(%)
PERCENT
PASSING ',
NO. 200
SIEVE
LIQUID
LIMIT
(%)
PLASTIC
INDEX
(%) 1
1
5
10.3
117
Sandy Silty Clay
10
8.7
121
58
Very Sandy Silty Clay
2
21/2
5.3
117
Sandy Silty Clay
5
7.4
117
57
Very Sandy Silty Clay
10
8.2
124
Weathered
Siltstone/Sandstone