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 ADDITION TO EXISTING RESIDENCE
1452 COUNTY ROAD 259
BETWEEN SILT AND RIFLE
GARFIELD COUNTY, COLORADO
PROJECT NO. 22-7-441
SEPTEMBER 29, 2022
PREPARED FOR:
URIEL MELLIN
144 CLIFFROSE WAY
GLENWOOD SPRINGS, COLORADO 81601
uriel.mellin@hotmail.com
Kumar & Associates, Inc. ® Project No. 22-7-441
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ....................................................................................... - 1 -
PROPOSED CONSTRUCTION ................................................................................................ - 1 -
SITE CONDITIONS ................................................................................................................... - 1 -
FIELD EXPLORATION ............................................................................................................ - 2 -
SUBSURFACE CONDITIONS ................................................................................................. - 2 -
FOUNDATION BEARING CONDITIONS .............................................................................. - 2 -
DESIGN RECOMMENDATIONS ............................................................................................ - 3 -
FOUNDATIONS .................................................................................................................... - 3 -
FOUNDATION AND RETAINING WALLS ....................................................................... - 3 -
FLOOR SLABS ...................................................................................................................... - 4 -
UNDERDRAIN SYSTEM ..................................................................................................... - 5 -
SURFACE DRAINAGE ......................................................................................................... - 6 -
LIMITATIONS ........................................................................................................................... - 6 -
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
Kumar & Associates, Inc. ® Project No. 22-7-441
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed addition to the existing
residence to be located at 1452 County Road 259, Garfield County, Between Rifle and Silt,
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
agreement for geotechnical engineering services to Uriel Mellin, dated June 21, 2022.
An exploratory boring was drilled 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,
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 addition had not been developed. The addition is
proposed on the south side of the existing residence near the exploratory boring location shown
on Figure 1. We assume excavation for the building will have a maximum cut depth of one
level, about 4 feet 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 subject site was developed with a one-story modular residence at the time of our field
exploration. The ground surface was moderately sloping down to the west at an estimated grade
of 8 percent. A small irrigation pond is located east of the existing residence. Vegetation
consists of grass and weeds.
Kumar & Associates, Inc. ® Project No. 22-7-441
- 2 -
FIELD EXPLORATION
The field exploration for the project was conducted on July 7, 2022. One exploratory boring was
drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The boring was
advanced with 4-inch diameter continuous flight auger powered by a truck-mounted CME-45B
drill rig. The boring was logged by a representative of Kumar & Associates, Inc.
Samples of the subsoils were taken with a 2-inch I.D. spoon sampler. The sampler was 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. 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 Log of Exploratory Boring, Figure 2. The samples were returned to our laboratory
for review by the project engineer and testing.
SUBSURFACE CONDITIONS
A graphic log of the subsurface profiles encountered at the site is shown on Figure 2. Below
about ½ foot of organic topsoil, the subsoils consist of stiff to medium stiff, very sandy silty clay
to the boring depth of 31 feet. The clay portions of these soils above the groundwater level can
possess an expansion potential when wetted.
Laboratory testing performed on samples obtained during the field exploration included natural
moisture content and density and finer than sand grain size analyses. Swell-consolidation testing
was performed on a relatively undisturbed drive sample of the very sandy silty clay subsoils.
The swell-consolidation test results, presented on Figure 3, indicate low compressibility under
relatively light surcharge loading and a low expansion potential when wetted under a constant
light surcharge. The laboratory testing is summarized in Table 1.
Free water was encountered in Boring 1 at a depth of 17¾ feet deep at time of drilling. The
subsoils were slightly moist to moist above the water.
FOUNDATION BEARING CONDITIONS
The subsoils encountered at the site possess low expansion potential when wetted. The
expansion potential is low and can probably be neglected in foundation design. We should be
contacted at the time of excavation to evaluate the soils exposed in the excavation for expansion
potential and the need for mitigation. Surface runoff, landscape irrigation, and utility leakage are
possible sources of water which could cause wetting.
Kumar & Associates, Inc. ® Project No. 22-7-441
- 3 -
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we recommend the addition be founded with spread footings placed
on undisturbed natural soils.
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 with a risk of movement if the bearing soils become
wetted. Based on experience, we expect settlement or heave of footings designed
and constructed as discussed in this section will be up to about 1 inch. There
could be some additional movement if the bearing soils were to become wet.
3) The footings should have a minimum width of 18 inches for continuous footings
and 24 inches for isolated pads.
4) 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
14 feet. Foundation walls acting as retaining structures should also be designed to
resist a lateral earth pressure as discussed in the "Foundation and Retaining
Walls" section of this report.
5) 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.
6) Prior to the footing construction, any existing fill, topsoil and loose disturbed soils
should be removed and the footing bearing level extended down to undisturbed
natural soils. If water seepage is encountered in the excavation, the footing areas
should be dewatered before concrete placement.
7) 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
Kumar & Associates, Inc. ® Project No. 22-7-441
- 4 -
of the on-site soils. Cantilevered retaining structures which are separate from the residence and
addition 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 a moisture content slightly above optimum. Backfill placed in
pavement 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 375 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 on-site soils possess an expansion potential and slab heave could occur if the subgrade soils
were to become wet. Slab-on-grade construction may be used provided precautions are taken to
limit potential movement and the risk of distress to the building is accepted by the owner. A
positive way to reduce the risk of slab movement, which is commonly used in the area, is to
construct structurally supported floors over crawlspace.
Kumar & Associates, Inc. ® Project No. 22-7-441
- 5 -
To reduce the effects of some differential movement, nonstructural floor slabs should be
separated from all bearing walls and columns with expansion joints which allow unrestrained
vertical movement. Interior non-bearing partitions resting on floor slabs should be provided with
a slip joint at the bottom of the wall so that, if the slab moves, the movement cannot be
transmitted to the upper structure. This detail is also important for wallboards, stairways and
door frames. Slip joints which will allow at least 1½-inches of vertical movement are
recommended. Floor slab control joints should be used to reduce damage due to shrinkage
cracking. Slab reinforcement and control joints 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 immediately beneath basement
level slabs-on-grade. This material should consist of minus 2-inch aggregate with less than 50%
passing the No. 4 sieve and less than 2% passing the No. 200 sieve. The free-draining gravel
will aid in drainage below the slabs and should be connected to the perimeter underdrain system.
Required fill beneath slabs should consist of a suitable imported granular material, excluding
topsoil and oversized rocks. The fill should be spread in thin horizontal lifts, adjusted to at or
above optimum moisture content, and compacted to at least 95% of the maximum standard
Proctor density. All vegetation, topsoil and loose or disturbed soil should be removed prior to
fill placement.
The above recommendations will not prevent slab heave if the expansive soils underlying slabs-
on-grade become wet. However, the recommendations will reduce the effects if slab heave
occurs. All plumbing lines should be pressure tested before backfilling to help reduce the
potential for wetting.
UNDERDRAIN SYSTEM
An underdrain is not required for relatively shallow excavations up to 4 feet. If deeper
excavations are proposed, an underdrain should be installed. We recommend below-grade
construction, deeper than 4 feet, 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 1 foot below lowest adjacent finish grade, and sloped at a minimum
1% 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 50% passing the No. 4 sieve and less
Kumar & Associates, Inc. ® Project No. 22-7-441
- 6 -
than 2% passing the No. 200 sieve. The drain gravel should be at least 1½ feet deep and covered
with filter fabric such as Mirafi 140N or 160N. Void form below the foundation can act as a
conduit for water flow. 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 above the void form with
mastic to keep drain water from flowing beneath the wall and to other areas of the building.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the addition has been completed:
1) Excessive wetting or drying of the foundation excavations and underslab areas
should be avoided during construction. Drying could increase the expansion
potential of the soils.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement areas and to at
least 90% of the maximum standard Proctor density in landscape areas. Free-
draining wall backfill should be covered with filter fabric and capped with about
2 to 3 feet of the on-site 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 paved areas.
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 boring drilled at the location 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
Kumar & Associates
Kumar & Associates
Kumar & Associates
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 22-7-441
SAMPLE LOCATION NATURAL MOISTURE CONTENT
NATURAL DRY DENSITY
GRADATION
PERCENT PASSING NO. 200 SIEVE
ATTERBERG LIMITS UNCONFINED COMPRESSIVE STRENGTH SOIL TYPE BORING DEPTH GRAVEL SAND LIQUID LIMIT PLASTIC INDEX (%) (%)
(ft) (%) (pcf) (%) (%) (psf)
1 2½ 7.9 114 55 Very Sandy Silty Clay
5 7.2 121 55 Very Sandy Silty Clay
10 8.4 110 51 Very Sandy Silty Clay
15 20.4 104 63 Sandy Silty Clay