HomeMy WebLinkAboutSubsoils Report for Foundation DesignI(+A 5020 County Road 154
Geotechnical and Materials Engineers Glenwood Springs, CO 81601
and Environmentalscientists phone: (970) 945-7ggg
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employcc orncd compony www.kumarusa.com
Of{ice Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 67, SPRTNG RrDGE RESERVE
HIDDEN VALLEY DRTVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-167
MARCH 19,2020
PREPARED FOR:
MODFIN DESIGN + BUILD, LLP
ATTN: KEITH WITTENBBRG
243ELKRIDGE DRIVE
GLENWOOD SPRTNGS, COLORADO 81601
shagss09@Yahoo.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION ..
SITE CONDITIONS
GEOLOGY
FTELD EXPLORATION
SUB SLIRFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOTINDATIONS
FOUNDATION AND RETAINING WALLS.
FLOOR SLABS
UNDERDRAIN SYSTEM
SITE GRADING..........
SURFACE DRAINAGE...............
LIMITATIONS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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...- 2 -
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I
-2-
-3-
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-5-
-6-
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.....- 6 -
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FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
Kumar & Associates, lnc.Project No 20-7-167
PT]RPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Lot
67, Spring Ridge Reserve, Hidden Valley Drive, Garfield County, Colorado. The project site is
shown on Figure 1. The purpose of the study was to develop recommendations for foundation
design. The study was conducted in accordance with our agleement for geotechnical engineering
services to Modfin Design + Build, LLP, dated March 3,2024.
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, comprossibility 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, recommendations and other geotechnical engineering
considerations based on the proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
The proposed single-family residence will be single-story structure above crawlspace with an
attached slab-on-grade garage located as shown on Figure 1. We assume excavation for the
building will be cut about 2 to 5 feet below the existing ground surface. Foundation loadings for
the structure were assumed to be relatively light and typical of the proposed type of construction.
If building loadings, location or grading plans are significantly different from those described
above, we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The property was vacant with patchy snow cover at the time of our field exploration. The site is
vegetated with grass and weeds. The ground surface slopes moderately down to the west with
Kumar & Associates, lnc.Project No 20-7-167
around 5 feet of elevation difference in the general building area. Maroon Formation sandstone
is exposed on the hillside to the west of the lot.
GEOLOGY
According to the Geologic Map of the Cattle Creek Quadrangle, Garfield County, Colorado, by
Krikham, Steufert, Hemborg, and Stelling, dated 20l4,the site is underlain by alluvium and
colluvium deposits of the Holocene age overlying Maroon Formation.
FIELD EXPLORATION
The field exploration for the project was conducted on March 4,2020. Two exploratory borings
were drilled at the locations shown on Figure I to evaluate the subsurface conditions. The
borings were advanced with 4-inch diameter continuous flight auger powered by a truck-
mounted CME-458 drill rig. The borings were logged by a representative of Kumar &
Associates.
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 profiles encountered at the site are shown on Figure 2. Below
about I foot of organic topsoil, the subsoils consist of very stiff, sandy silty clay down to about
25 feetunderlain by stiff/medium dense, silt and sand with scattered gravel underlain by hard to
very hard sandstone bedrock at depths ofabout 36 to 41 feet.
Laboratory testing performed on samples obtained during the field exploration included natural
moisture content and density and percent silt and clay (percent passing the No. 200 sieve).
Swell-consolidation testing was performed on relatively undisturbed drive samples of the clay
Kumar & Associates, lnc.Project No 20-7-167
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soils. The swell-consolidation test results, presented on Figures 4 and 5, indicate low
compressibility under relatively light surcharge loading and minor to moderate expansion
potential when wetted under a constant light surcharge. The laboratory testing is summarized in
Table 1.
No free water was encountered in the borings at time of drilling and the subsoils were slightly
moist to moist with depth.
FOUNDATION BEARING CONDITIONS
The subsoils encountered at the site possess variable low to moderate movement potential mainly
when wetted. The expansion potential measured in the samples from depths of 5 to l0 feet
generally appears to be anomalously high and the expansion potential should be further
evaluated at the time of excavation. Sub-excavation to 3 feet below footing bearing level and
placement of structural fill could be used to help mitigate movement potential. Surface runoff,
landscape irrigation, and utility leakage are possible sources of water which could cause wetting.
Footings placed on the natural soils can be used for foundation support with the accepted risk of
movement. Deep foundations, such as drilled piers or micro-piles, can be used if the risk of
movement cannot be tolerated. We should be contacted if deep foundation recommendations are
desired.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, the residence can be founded with spread footings placed on
undisturbed natural soils with a risk of movement mainly if the bearing soils are wetted.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils can be designed for an allowable
bearing pressure of 1,500 psf. Based on experience, we expect initial settlement
of footings designed and constructed as discussed in this section will be up to
about I inch. Additional movement could be around I to I% inches depending on
the depth and extent of wetting.
Kumar & Associates, lnc.Project No 20-7-167
2)
3)
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The footings should have a minimum width of 16 inches for continuous footings
and24 inches for isolated pads.
Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies and limit the risk of differential movement. One method of
analysis is to design the foundation wall to span an unsupported length of at least
12 feet. Below grade level are not currently planned. If a basement level is
planned, the foundation walls acting as retaining structures should also be
designed to resist alateral earth pressure as discussed in the "Foundation and
Retaining Walls" section of this report.
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 the exterior grade is typically used in this
area.
Prior to the footing construction, the topsoil and loose or disturbed soils should be
removed and the footing bearing level extended down to competent bearing soils.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
4)
5)
6)
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 strucfures 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.
Kumar & Associates, lnc.Project No 20-7-167
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Backfill should be placed in uniform lifts and compacted to at least 90Yo of the maximum
standard Proctor density at near optimum moisture content. Backfill placed in pavement and
walkway areas should be compacted to at least 95o/o 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 0.40. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 350 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, can be used to support lightly loaded slab-on-grade
construction. There could be diflerential settlement potential from wetting of the bearing soils
similar to that described above for footings. 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 relatively well graded sand and gravel such as road base should
be placed beneath slabs for support. This material should consist of minus 2-inch aggregate with
at least 50Yo retained on the No. 4 sieve and less than l2o/o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo of maximum
standard Proctor density at a moisture content near optimum. Required filI can consist of the on-
site soils devoid of vegetation, topsoil and oversized (plus 6-inch) rock.
Kumar & Associates, lnc.Project No 20-7-167
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UNDERDRAIN SYSTEM
Although groundwater was not encountered during our exploration, it has been our experience in
the area and where clay soils are present that local perched groundwater can develop during
times of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a
perched condition. Therefore, we recommend below-grade construction, such as crawlspace and
basement areas (if provided), be protected from wetting by an underdrain system. The drain
should also act to prevent buildup of hydrostatic pressures behind foundation walls.
The underdrain system should consist of a drainpipe surrounded by free-draining granular
material placed at the bottom of the wall backfill. The drain lines should be placed at each level
of excavation and at least I foot below lowest adjacent finish gtade, and sloped at a minimum
1o/o grade to a suitable gravity outlet. Free-draining granular material used in the drain system
should consist of minus 2-inch aggregate with less than 50o/o passing the No. 4 sieve and less
than2o/o passing the No. 200 sieve. The drain gravel should be at least l% feet deep. An
impervious liner such as 20 mil PVC should be placed below the drain gravel in a trough shape
and attached to the foundation wall with mastic to keep drain water from flowing beneath the
wall and to other areas of the building.
SITE GRADING
The risk of construction-induced slope instability at the site appears low provided cut and fill
depths are limited. We assume cut and fill depths for foundation construction will not exceed
about 5 to 6 feet. Embankment fills should be compacted to at least 95o/o of the maximum
standard Proctor density near optimum moisture content. Prior to fill placement, the subgrade
should be carefully prepared by removing all vegetation and topsoil and compacting to at least
95o/o of the maximum standard Proctor density. The fill should be benched into the portions of
the hillside exceeding 20o/o grade. Permanent unretained cut and fill slopes should be graded at
2 horizontal to I vertical or flatter and protected against erosion by revegetation or other means
This office should review site grading plans for the project prior to construction.
SURFACE DRAINAGE
Providing proper surface grading and drainage will be critical to prevent wetting of the bearing
soils and limiting building sefflement and distress. The following drainage precautions should be
observed during construction and maintained at all times after the residence has been completed:
Kumar & Associates, lnc.Project No 20-7-167
n
1)Excessive wetting or drying of the foundation excavations and underslab areas
should be avoided during construction.
Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95o/o of the maximum standard Proctor density in pavement areas and to at
least 90olo of the maximum standard Proctor density in landscape afeas.
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 l0 feet in paved areas.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Landscaping which requires regular heavy irrigation should be located at least l0
feet from foundation walls. Consideration should be given to use of xeriscape to
prevent wetting of bearing soils from landscape irrigation.
2)
3)
4)
LII\4ITATIONS
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
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 to be 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 pu{poses. 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
5)
Kumar & Associates, lnc.Project No 20-7-167
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monitor the implementation of our recommendations, and to veriff that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
or modifications of 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,
Kunrar & Associates, Inc.
Steven L.
Reviewed by:
Daniel E. Hardin, P
SLPlkac
15222
Kumar & Associates, lnc.Project No 20-7-167
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LOT 68
LOT 69
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20-7 -167 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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EL. 1 00
1 BO R ING 2
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0 0
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WC=5.'l
DD= 1 03
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13/ 12
5 24/ 12
WC=8.7
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DD=118
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-200=65
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-200=50
35 35
40 4030/5,10/o
50/2.545 45
2A-7 -1 67 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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LEGEND
N
TOPSOIL; ORGANIC SANDY SILT AND CLAY, MOIST, BROWN
CLAY (CL); SANDY, SILTY, VERY sTlFF, SLIGHTLY MOIST TO MOIST wlTH DEPTH, RED-BROWN,
POROUS AND CALCAREOUS TRACES.
SILT AND SAND (ML_SM); SCATTERED GRAVEL, STIFF/MEDIUM DENSE, MOIST, RED.
SANDSTONE BEDROCK; HARD TO VERY HARD, SLIGHTLY MOIST' RED. MAROON SANDSTONE.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
16/ 12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 16 BLOWS OF A 14o_POUND HAMMER
FALLING 50 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON MARCH 4, 2O2O WITH A 4-INCH-DIAMETER
CONTINUOUS_FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY TAPING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFER
TO BORING 1 AS EL. = 100', ASSUMED.
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 (PCt) (ASTU D2216);
-2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D1140).
20-7 -1 67 Kumar & Associates LEGEND AND NOTES Fig. 5
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SAMPLE OF: Sondy Silty Cloy
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EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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EXPANSION UNDER CONSTANT
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20-7 -167 Kumar & Associates SWELL-CONSOLIDATION TEST RTSULTS Fig. 4
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SAMPLE OF: Sondy Silty Cloy
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20-7 -1 67 Kumar & Associates SWELL_CONSOLIDATION TTST RESULTS Fig. 5
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
2
1
BORING
30
20
10
5
30
20
1 0
2%
(ft1
DEPTH
SAMPLE LOCATION
I aJ I
t5.2
12.0
5 I
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NATURAL
MOISTURE
CONTENT
6.6
r7.8
12.0
8.7
110
r20
113
119
115
118
103
(ocf)
NATURAL
DRY
DENSITY
I12
ffit
GRAVEL
(%)
SAND
GRADATION
63
PERCENT
PASSING NO.
200 srEVE
50
t%l
LIQUID LIMIT
t%l
PLASTIC
INDEX
ATTERBERG LIMITS
{psf)
UNCONFINED
COMPRESSIVE
STRENGTH
Silt and Sand
Sandy Silt and Clay
Sandy Silty Clay
Sandy Silty Clay
Sandy Silt with Gravel
Sandy Silt and Clay
Sandy Silty Clay
Sandy Silty Clay
SOIL TYPE
No.20-7-167