HomeMy WebLinkAboutSubsoil Study
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
www.kumarusa.com Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 17-A, MINEOTA RIDGE ESTATES
MINEOTA DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-744
FEBRUARY 26, 2021
PREPARED FOR:
SIGI MARIONI
198 COMANCHERO TRAIL
NEW CASTLE, COLORADO 81647
sigi.marioni@gmail.com
Kumar & Associates, Inc. ® Project No. 20-7-744
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
FIELD EXPLORATION ............................................................................................................ - 2 -
SUBSURFACE CONDITIONS ................................................................................................. - 2 -
FOUNDATION BEARING CONDITIONS .............................................................................. - 3 -
DESIGN RECOMMENDATIONS ............................................................................................ - 3 -
FOUNDATIONS .................................................................................................................... - 3 -
FOUNDATION AND RETAINING WALLS ....................................................................... - 4 -
FLOOR SLABS ...................................................................................................................... - 5 -
UNDERDRAIN SYSTEM ..................................................................................................... - 5 -
SURFACE DRAINAGE ......................................................................................................... - 6 -
SEPTIC DISPOSAL AREA ................................................................................................... - 6 -
LIMITATIONS ........................................................................................................................... - 7 -
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS AND PITS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS AND PITS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 6 - GRADATION TEST RESULTS
FIGURES 7 and 8 – UDSA GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 20-7-744
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 17-A, Mineota Ridge Estates, Mineota 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 accordance with our agreement for geotechnical
engineering services to Sigi Marioni dated December 2, 2020.
A field exploration program consisting of exploratory borings and pits 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 a single-story structure located between the exploratory borings
shown on Figure 1. Ground floor will be structural over crawlspace. Grading for the structure is
assumed to be relatively minor with cut depths between about 3 to 6 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 lot was vacant with a rough-cut driveway leading to the building site (marked by corner
stakes) at the time of our field exploration. The ground surface is moderately sloping down to
the southeast with around 5 feet of elevation difference across the designated building area.
Vegetation consists of juniper trees, sage brush, grass and weeds. About 1 to 2 inches of snow
covered the ground at the time of our field exploration.
- 2 -
Kumar & Associates, Inc. ® Project No. 20-7-744
FIELD EXPLORATION
The field exploration for the project was conducted on December 15, 2020. Two exploratory
borings were drilled and 2 profile pits were dug 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 pits were dug with a backhoe
provided by the client. The borings and pits were logged by a representative of Kumar &
Associates.
Samples of the subsoils in the borings were taken with 1⅜ inch and 2-inch I.D. spoon samplers.
The samplers were 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. Samples of the subsoils in
the pits were taken by disturbed sampling methods. Depths at which the samples were taken and
the penetration resistance values of the boring samples are shown on the Logs of Exploratory
Borings and Pits, 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 1 foot of topsoil, consist of calcareous, very stiff to hard, sandy silty clay
to depths of about 2½ to 6 feet overlying calcareous, medium dense, mixed gravel and clay. At
depths of about 8 to 12 feet in the borings, hard claystone bedrock was encountered to the drilled
depths of 21 feet. Digging in the upper soils with backhoe equipment was difficult due to the
hard calcareous cemented condition and digging refusal was encountered in the deposit at Profile
Pit 1.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and gradation analyses. Results of swell-consolidation testing performed on
relatively undisturbed drive samples, presented on Figures 4 and 5, indicate low compressibility
under existing low moisture condition. The upper clay soil sample showed a low expansion
potential and the underlying claystone sample showed a high expansion potential when wetted
under relatively light loading. Results of gradation analyses performed on a small diameter drive
sample (minus 1½-inch fraction) and disturbed bulk samples of the clay and granular soils are
shown on Figures 6, 7 and 8. The laboratory testing is summarized in Table 1.
- 3 -
Kumar & Associates, Inc. ® Project No. 20-7-744
No free water was encountered in the borings or pits and the soils and bedrock were slightly
moist.
FOUNDATION BEARING CONDITIONS
The upper clay and mixed gravel soils have low to moderate bearing capacity and can be used
for support of lightly loaded spread footings with relatively low movement potential, mainly
under wetted conditions. The expansion potential measured on the sample of clay appears to be
an anomaly but the expansion/compression potential should be further evaluated at the time of
excavation. The underlying claystone bedrock has a high expansion potential and could cause
excessive heave of lightly loaded foundations under wetted conditions. Shallow footings should
have a bearing level around 6 feet or greater above the top of claystone. If a basement level is
proposed, we should be contacted for additional evaluation and alternative foundation design
recommendations such as for drilled piers designed to mitigate foundation heave potential.
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 upper natural soils at least 6 feet above the top of bedrock.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the upper natural soils should be designed for an allowable
bearing pressure of 2,000 psf. Based on experience, we expect initial settlement
of footings designed and constructed as discussed in this section will be about
1 inch or less. Additional settlement/heave up to about 1 inch could occur if the
bearing soils are wetted depending on the depth and extent of wetting.
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 12 feet.
- 4 -
Kumar & Associates, Inc. ® Project No. 20-7-744
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 natural soils. The exposed soils in footing
area should then be moistened and compacted.
6) A representative of the geotechnical engineer should 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 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
- 5 -
Kumar & Associates, Inc. ® Project No. 20-7-744
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 325 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 a risk of movement if the bearing soils are wetted. The subgrade soils should
be evaluated for expansion potential at the time of construction and the need for subexcavation
and replacement with structural fill. 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 subgrade support. This material should consist of minus 2-inch aggregate with at least 50%
retained on the No. 4 sieve and less than 12% 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 devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
It is our understanding the proposed finished floor elevation at the lowest level is at or above the
surrounding grade. Therefore, a foundation drain system is not required. 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 and basement
areas, be protected from wetting and hydrostatic pressure buildup by an underdrain and wall
drain system. A foundation drain is not recommended for typical shallow crawlspace around
3 feet deep and slab-on-grade garage areas to help keep the bearing soils dry.
- 6 -
Kumar & Associates, Inc. ® Project No. 20-7-744
If the finished floor elevation of the proposed structure is revised to have a floor level below the
surrounding grade, we should be contacted to provide recommendations for an underdrain
system. All earth retaining structures should be properly drained.
SURFACE DRAINAGE
Keeping the bearing soils dry will be critical to limiting potential building movement and
distress. 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 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. Consideration should be given to use of xeriscape
to reduce the potential for wetting of soils below the building caused by irrigation.
SEPTIC DISPOSAL AREA
The subsoil conditions in the planned septic disposal area were evaluated by digging 2 profile
pits (PP-1 and PP-2) at the locations shown on Figure 1. The subsurface profiles encountered are
shown on Figure 2 with results of USDA gradation tests performed on samples of the upper,
fine-gained soils and more granular soils shown on Figures 7 and 8. Based on these findings, the
tested area appears suitable for an infiltration septic disposal system. A civil engineer should be
engaged to design the septic disposal system.
Kumar & Associates
Kumar & Associates
Kumar & Associates
Kumar & Associates
Kumar & Associates
Kumar & Associates
1 MIN.
4 MIN.19MIN.15 MIN.60MIN.#325 #140 3/4"3/8"1 1/2"3"5"6"8"
DIAMETER OF PARTICLES IN MILLIMETERS
U.S. STANDARD SERIES CLEAR SQUARE OPENINGS
SIEVE ANALYSIS
TIME READINGS
HYDROMETER ANALYSIS
#4#10#18#35#60
7 HR
45 MIN.
24 HR.
0
10
20
30
40
50
100
90
80
70
60
50
60
70
80
90
100
0
10
20
30
40
20315276.237.519.09.54.752.001.00.500.025.106.045.019.009.005.002.001
SILT COBBLESLARGE
GRAVEL
MEDIUMCOARSEMEDIUMV. FINE
SANDCLAY FINE V. COARSE SMALL
USDA SOIL TYPE:
GRAVEL %SILT %CLAY %
FROM:PP-1 @ 3'-4'
1 37 23SAND %39
Loam
Kumar & Associates
1 MIN.
4 MIN.19MIN.15 MIN.60MIN.#325 #140 3/4"3/8"1 1/2"3"5"6"8"
DIAMETER OF PARTICLES IN MILLIMETERS
U.S. STANDARD SERIES CLEAR SQUARE OPENINGS
SIEVE ANALYSIS
TIME READINGS
HYDROMETER ANALYSIS
#4#10#18#35#60
7 HR
45 MIN.
24 HR.
0
10
20
30
40
50
100
90
80
70
60
50
60
70
80
90
100
0
10
20
30
40
20315276.237.519.09.54.752.001.00.500.025.106.045.019.009.005.002.001
SILT COBBLESLARGE
GRAVEL
MEDIUMCOARSEMEDIUMV. FINE
SANDCLAY FINE V. COARSE SMALL
USDA SOIL TYPE:
GRAVEL %SILT %CLAY %
FROM:PP-2 @ 4'-5'
42 44 4SAND %10
Gravelly Loamy Sand
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 20-7-744
SAMPLE LOCATION NATURAL MOISTURE CONTENT
NATURAL DRY DENSITY
GRADATION USDA SOIL TEXTURE SOIL TYPE BORING DEPTH GRAVEL SAND SILT&CLAY GRAVEL SAND SILT CLAY
(ft) (%) (pcf) (%) (%) (%) (%) (%) (%) (%)
1 1 10.6 99 Sandy Silty Clay
10 7.2 129 Claystone
2 4 6.5 17 32 51 Sandy Silty Clay and
Gravel
Profile
Pit
1 3-4 1 37 39 23 Loam
2 4-5 42 44 10 4 Gravelly Loamy Sand