HomeMy WebLinkAboutSubsoil Study for Foundation Design 02.03.2020IC A
KumsarAngolan, In.*
Geotechnical and Materials Engineers
and Environmental Scientists
5020 County Road 154
Glenwood Springs, CO 8160]
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood(0),kumarusa.com
An Employee Owned Company www.kumarUsa.com
Office Locations: Denver (1IQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT H-26, THE HOMESTEAD
ASPEN GLEN
36 SADDLEHORN COURT
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-122
FEBURARY 3, 2020
PREPARED FOR:
MIKE BODROGI
998 HUEBINGER DRIVE
GLENWOOD SPRINGS, COLORADO 81601
mbodro .i1(a grnail.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY - 1 -
PROPOSED CONSTRUCTION - 1 -
SITE CONDITIONS - 1 -
SUBSIDENCE POTENTIAL - 2 -
FIELD EXPLORATION - 2 -
SUBSURFACE CONDITIONS - 3 -
FOUNDATION BEARING 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
FIGURES 4 & 5 - SWELL -CONSOLIDATION TEST RESULTS
FIGURE 6 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. Project No. 20-7-122
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Lot
H-26, The Homestead, Aspen Glen, 36 Saddlehorn Court, 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 accordance with our agreement for
geotechnical engineering services to Mike Bodrogi dated January 27, 2020.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils 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
At the time of our study, design plans for the residence had not been developed. The building is
proposed in the area roughly between exploratory boring locations shown on Figure 1. We
assume excavation for the building will be between 3 and 10 feet (if a basement is proposed)
below the existing ground surface. For the purpose of our analysis, foundation loadings for the
structure were assumed to be relatively light and typical of the proposed type of construction.
When building location, grading and loading information have been developed, we should be
notified to re-evaluate the recommendations presented in this report.
SITE CONDITIONS
The lot was vacant at the time of the field exploration. The terrain is gently sloping down to the
south. Vegetation consisted of grass and weeds. Two-story, single-family residences are to the
southeast and west, Saddlehorn Court is to the south, Saddleback Road is to the west, and Aspen
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Glen golf course is to the east. Eagle Valley Evaporite bedrock outcrops are visible in the valley
side to the west.
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen
development. These rocks are a sequence of gypsiferous shale, fine-grained sandstone and
siltstone with some massive beds of gypsum and limestone. There is a possibility that massive
gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the lot.
Dissolution of the gypsum under certain conditions can cause sinkholes to develop and can
produce areas of localized subsidence. During previous work in the area, several sinkholes were
observed scattered throughout the development, mostly east of the Roaring Fork River. These
sinkholes appear similar to others associated with the Eagle Valley Evaporite in areas of the
Roaring Fork River valley.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities
was encountered in the subsurface materials; however, the exploratory borings were relatively
shallow, for foundation design only. Based on our present knowledge of the subsurface
conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of
future ground subsidence on Lot H26 throughout the service life of the proposed residence, in
our opinion, is low; however, the owner should be made aware of the potential for sinkhole
development. If further investigation of possible cavities in the bedrock below the site is desired,
we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on January 28, 2020. 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 Kumar &
Associates.
Samples of the subsoils were taken with 13/8 inch and 2 inch I.D. spoon samplers. The samplers
were driven into the subsoils 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.
Kumar & Associates, Inc., Project No, 20.7.122
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The penetration resistance values are an indication of the relative density or consistency of the
subsoils. 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 consist of about 1 foot of topsoil overlying 4%2 feet of very stiff to hard, silty, slightly
sandy to sandy (with depth) clay with scattered gravel (with depth), underlain by dense, silty
sand and gravel with cobbles and scattered boulders. Drilling in the dense granular soils with
auger equipment was difficult due to the cobbles and boulders.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density, percent fines (percent passing the No. 200 sieve), and gradation analyses.
Results of swell -consolidation testing performed on relatively undisturbed drive samples of the
clay soils, presented on Figures 4 and 5, indicate low compressibility under existing low
moisture content and light loading and moderate to high compressibility after wetting. Results of
gradation analyses performed on a small diameter drive sample (minus 1'A -inch fraction) of the
coarse granular subsoils are shown on Figure 6. The laboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The upper natural clay soils possess a relatively low bearing capacity and a low to moderate
settlement potential mainly when wetted. The underlying natural sand and gravel soils possess a
moderate bearing capacity and a low settlement potential. We recommend that the building be
supported on spread footings bearing on the natural granular soils to achieve a low settlement
risk foundation. Shallow spread footings could also be founded on a minimum 2 feet of
compacted structural fill in clay soil areas.
Kumar & Associates, Inc. `' Project No. 20-7-122
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DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural granular soils or a minimum 2 feet of compacted structural fill.
The design and construction criteria presented below should he observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural granular soils should be designed for
an allowable bearing pressure of 3,000 psf. Footings placed on properly
compacted structural fill in clay soil areas should be designed for an allowable
bearing pressure of 1,500 psf. Based on experience, we expect settlement of
footings designed and constructed as discussed in this section will be about 1 inch
or less. Additional settlement may occur if the clay soils below the structural fill
become wetted.
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 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
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of this report.
5) The topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to the relatively dense natural granular soils.
Structural fill (if used) in clay soil areas should be at least 2 feet deep, extend to at
least one foot beyond footing edges and be compacted to a minimum of 98% of
the standard Proctor density at a moisture content near optimum. The exposed
soils in footing area should be moistened and compacted prior to concrete or
structural fill placement.
Kumar & Associates, Inc. r' Project No. 20-7.122
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6) A representative of the geotechnical engineer should test compaction of structural
fill (if any) and observe all footing excavations prior to concrete placement to
evaluate bearing conditions.
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. Backfill should not contain organics,
debris or rock larger than about 6 inches.
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 a moisture content near optimum. 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 for clay soils and 0.50 for granular soils. Passive
pressure of compacted backfill against the sides of the footings can be calculated using an
Kumar & Associates, Inc. Project No, 20-7-122
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equivalent fluid unit weight of 400 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 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, are suitable to support lightly loaded slab -on -grade
construction with an accepted risk of settlement in clay soil areas. Structural floors over
crawlspace should be used in areas where the risk of settlement cannot be tolerated. The
movement risk of slabs on clay soils can be evaluated at the time of construction and use of
structural fill to reduce the risk.
To reduce the effects of some differential movement, non-structural 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 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 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.
Kumar & Associates, Inc. Project No. 20.7-122
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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 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
5 feet from foundation walls. Consideration should be given to use of xeriscape
to reduce the potential for wetting of soils below the building caused by irrigation.
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
Kumar & Associates, Inc, Project No. 20-7-122
8
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 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,
Kumar & Associates, Inc.
,dt
Shane J. Robat, P.E.
Reviewed by:
Steven L. Paw
SJR/kac
Kumar & Associates, Inc.
Project No. 20-7-122
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DD=109
-200=22
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BORING 2
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DD=91
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20-7-122 20-7-122
Kumar & Associates
LOGS OF EXPLORATORY BORINGS
Fig. 2
LEGEND
- 1'
f
TOPSOIL; SANDY CLAY WITH GRAVEL, FIRM, SLIGHTLY MOIST, BROWN, SLIGHTLY ORGANIC.
CLAY (CL); SILTY, SLIGHTLY SANDY TO SANDY WITH DEPTH, SCATTERED GRAVEL WITH DEPTH,
VERY STIFF TO HARD, SLIGHTLY MOIST, BROWN. SLIGHTLY POROUS AND CALCAREOUS.
SAND AND GRAVEL (SM -GM, SP—SM); SILTY WITH COBBLES AND SCATTERED BOULDERS,
DENSE, SLIGHTLY MOIST, BROWN, ROUNDED ROCK.
DRIVE SAMPLE, 2—INCH I.D. CALIFORNIA LINER SAMPLE.
DRIVE SAMPLE, 1 3/8—INCH I.D. SPLIT SPOON STANDARD PENETRATION TEST.
16/12 DRIVE SAMPLE BE OW COUNT. INDICATES THAT 16 BLOWS OF A 140—POUND HAMMER
FALLING 30 'NCI IL:, WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 28, 2020 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 MEASURED BY HAND LEVEL AND REFER
TO THE BENCHMARK ON FIG. 1.
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 D2216);
DD = DRY DENSITY (pcf) (ASTM D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
—200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140).
20-7-122
Kumar & Associates
LEGEND AND NOTES
Fig. 3
CONSOLIDATION - SWELL
P
2
0
—2
—4
—6
—8
— 10
—12
— 14
TMww fuA ru.eAr on* nAY t. tly
. or.pI f4Ird. TM fowling eywrt
• I, I ncf be. erprodoced, orerpl In
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kpn,m wed AnnwYma. l c. Sre11
r .911ib11wn toiling wk. -mall in
eec«dere• .dh AS1u P-4&14.
.
SAMPLE OF: Silty Sandy Clay
FROM: Boring 1 0 2.5'
WC = 9.1 %. DD = 88 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
1,0 APPLIED PRESSURE - KSF
10 100
20-7-122
Kumar & Associates
SWELL—CONSOLIDATION TEST RESULTS
Fig. 4
CONSOLIDATION - SWELL
2
0
— 2
—4
— 6
- 8
—10
SAMPLE OF: Silty Sandy Clay
FROM: Boring 2 0 2.5'
WC = 8.2 %, DD = 91 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
Von, netult.7.1,:tax: to Mt"
.hdf eos w npeaduot4ora.1In
h4..ouw5 U. wwltt n op rwN of
Knnvy old
aiaoswnp.cInt. .sd
Ennvdow.ir01p. �—l6W6.Ml
1,0 APPLIED PRESSURE - KSF 10 100
20-7-122
Kumar & Associates
SWELL -CONSOLIDATION TEST RESULTS
Fig. 5
100
90 •
e0
70
60
00
HYDROMETER ANALYSIS
7114E AEA01N05
0.1 11A5 7 HRS
45 KW 1.5_MI11 60MIN ISNMN 4MIN IN IN
401- — 1 —
30
20
10
0•
.001 .602
L
.11
SIEVE ANALYSIS
U.S. STANDARD SERIES
J4,
III D_ a.
I I
.I_ . I
I
I
I I 1 1 1 I I 11 1 11111 1 11 I 1111 I I !I. Ll tf.l I 1 1 1 11.1 II 100
.606 .000 .010 .0:17 .073 .143 .300 1 .000 1.16 12.36 4.75 .0 10 66.1 76.2 127 200
.423 2.0 162
DIAMETER OF PARTICLES IN MILLIMETERS
SAND GRAVEL
COMMONCLEAR SOUARE COMMONN
3.0T 3/4'
1
10
20
30
40
5°
60
70
00
00
CLAY TO SILT
FINE
MEDIUM )COARSE
GRAVEL 30 % SAND 51 X
LIQUID LIMIT PLASTICITY INDEX
SAMPLE OF: SII1y Sand and Gravel
FINE
COARSE COBBLES
SILT AND CLAY 19 X
FROM: Boring 2 0 10'
These feet reecho apply only to the
samples whtch were teefed. Tits
feeling report shall not be reproduced,
creep' In full, without the written
approval of Kumar & Associates, Inc.
Sieve analyale testing Is performed In
accordance wllh ASTM 06013, ASTM 07020,
ASTM 0136 and/or ASTM 61140.
20-7-122
Kumar & Associates
GRADATION TEST RESULTS
Fig. 6
Komar & Associates, Inc.'"
Geared and serials Eg n -; •
arid EnviTawn +tel Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 20.7.122
SAMPLE LOCATION
NATURAL
MOISTURE
CONTENT
(%)
NATURAL
DRY
DENSITY
(PF)
GRADATION
1
PERCENT
PASSING NO.
200 SIEVE
ATTERBERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH SOIL TYPE
(Psfl
BORING
DEPTH
(R)
GRAVEL
(%)
SAND
(%)
LIQUID LIMIT
(%)
PLASTIC
INDEX
(%)
1
21/2
9.1
88
Silty Sandy Clay
5
4.2
109
22
Silty Sand and Gravel
2
21/2
8.2
9.1
Silty Sandy Clay
10
1.9
30
51
19
Silty Sand and Gravel