HomeMy WebLinkAboutSubsoil Study for Foundation Design 10.21.2024
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 SHOP BUILDING
PARCEL NO. 212733200374, COUNTY ROAD 233
RIFLE, COLORADO
PROJECT NO. 24-7-583
OCTOBER 21, 2024
PREPARED FOR:
JILL & MIKE CHRISTIANSON
2601 COUNTY ROAD 233
RIFLE, COLORADO 81650
jamchristianson@msn.com
Kumar & Associates, Inc. ® Project No. 24-7-583
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ........................................................................... - 1 -
PROPOSED CONSTRUCTION .................................................................................... - 1 -
SITE CONDITIONS ....................................................................................................... - 1 -
FIELD EXPLORATION ................................................................................................ - 1 -
SUBSURFACE CONDITIONS ..................................................................................... - 2 -
FOUNDATION BEARING CONDITIONS .................................................................. - 2 -
DESIGN RECOMMENDATIONS ................................................................................ - 2 -
FOUNDATIONS ........................................................................................................ - 2 -
FLOOR SLABS .......................................................................................................... - 3 -
UNDERDRAIN SYSTEM ......................................................................................... - 4 -
SURFACE DRAINAGE ............................................................................................. - 4 -
LIMITATIONS ............................................................................................................... - 4 -
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
- 1 -
Kumar & Associates, Inc. ® Project No. 24-7-583
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed shop to be located on parcel
212733200374 along County Road 233, adjacent to 2601 County Road 233, Rifle, 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 Jill and Mike Christianson dated October 7,
2024.
A field exploration program consisting of one exploratory boring 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 shop will be a single-story structure with a slab-on-grade ground floor. Grading
for the structure is assumed to be relatively minor with cut depths between about 2 to 3 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 subject lot was mostly undeveloped at the time of our field exploration. The site appears to
have undergone some minor cut grading for equipment and trailer storage purposes. The ground
surface in the proposed shop area slopes moderately down to the southwest at grades between 5
to 10%. The vegetation consists of grass, weeds, sage brush, and juniper trees.
FIELD EXPLORATION
The field exploration for the project was conducted on October 9, 2024. One exploratory
borings was drilled at the location shown on Figure 1 to evaluate the subsurface conditions. The
boring was 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, Inc.
- 2 -
Kumar & Associates, Inc. ® Project No. 24-7-583
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 and hardness of the bedrock. 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 conditions encountered at the site are shown on Figure 2. Below
about ½ foot of topsoil, the subsoils consist of about 16½ feet of loose to medium dense silt and
sand overlying very hard sandstone bedrock that extended down to the maximum explored depth
of 20 feet. Drilling in the hard bedrock with auger equipment was difficult and drilling refusal
was encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and gradation analyses. Results of swell-consolidation testing performed on relatively
undisturbed drive samples, presented on Figure 3, indicate moderate compressibility under
conditions of loading and wetting. The laboratory testing is summarized in Table 1.
No free water was encountered in the boring at the time of drilling and the subsoils were slightly
moist.
FOUNDATION BEARING CONDITIONS
The silt and sand soils encountered at typical shallow foundation depth possess low bearing
capacity and moderate settlement potential. Spread footings bearing on the natural soils should
be feasible for support of the proposed shop with a risk of post-construction settlement. The risk
of settlement is primarily if the bearing soils become wetted and care should be taken in the
surface grading and drainage around the shop to prevent the soils from becoming wet. A lower
risk of settlement would be to remove a depth (typically 2 to 3 feet) of the soils and replace with
moisture controlled and compacted structural fill below footings.
Presented below are recommendations for spread footings bearing on the natural soils. If
recommendations for spread footings bearing on compacted structural fill are desired, we should
be contacted.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory boring and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural soils.
- 3 -
Kumar & Associates, Inc. ® Project No. 24-7-583
The design and construction criteria presented below should be 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 1,200 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less. There could be some additional post-construction
settlement on the order of 1 to 2 inches, primarily if the bearing soils were to
become wetted.
2) The footings should have a minimum width of 18 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 a
lateral earth pressure corresponding to an equivalent fluid unit weight of at least
55 pcf.
5) All existing fill, topsoil and any loose or disturbed soils should be removed and
the footing bearing level extended down to firm natural soils. The exposed soils
in footing area should then be moistened and compacted. If water seepage is
encountered, the footing areas should be dewatered before concrete placement.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction. The soils are typically compressible when wetted. 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 should be placed
beneath floor slabs for 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.
- 4 -
Kumar & Associates, Inc. ® Project No. 24-7-583
UNDERDRAIN SYSTEM
It is our understanding that 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 mountainous areas 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 (if any) be
protected from wetting and hydrostatic pressure buildup by an underdrain and wall drain system.
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
The following drainage precautions should be observed during construction and maintained at all
times after the shop 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 6 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
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
5 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 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. 24-7-583
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 2 2.1 101 41 Silt and Sand
4 3.6 97 Sandy Silt
9 5.8 101 Sandy Silt