HomeMy WebLinkAboutSoils Report 08.06.2020It -A
Kumar & Associates, Inc.®
Geotechnical and Materials Engineers
and Environmental Scientists
An Employee Owned Company
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
August 6, 2020
Taani Martinez
406 County Road 265
Rifle, Colorado 81650
taanim@outlook.com
Project No.20-7-410
Subject: Subsoil Study for Foundation Design, Proposed Addition, 406 County Road 265
(Prefontaine Avenue), Rifle, Colorado
Dear Ms. Martinez:
As requested, Kumar & Associates, Inc. performed a subsoil study for design of foundations at
the subject site. The study was conducted in accordance with our agreement for geotechnical
engineering services to you dated July 24, 2020. The data obtained and our recommendations
based on the proposed construction and subsurface conditions encountered are presented in this
report.
Proposed Construction: The proposed addition will be a 2 -story structure attached to the north
side of the existing residence shown on Figure 1. Ground floor will be slab -on -grade. Cut
depths are expected to range between about 2 to 3 feet. Foundation loadings for this type of
construction are assumed to be relatively light and typical of the proposed type of construction.
If building conditions or foundation loadings are significantly different from those described
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The existing structure is a 2 -story home located just west of Prefontaine
Avenue. The proposed addition will extend into the gravel surfaced driveway/parking area. The
ground surface is relatively flat and gently sloping down to the south in the building area.
Vegetation consists of grass and scattered mature trees.
Subsurface Conditions: The subsurface conditions at the site were evaluated by observing one
exploratory pit which had been dug with a backhoe prior to our site visit at the approximate
location shown on Figure 1. The log of the pit is presented on Figure 2. The subsoils
encountered, below about one foot of various fill soils, consist of stiff, sandy to very sandy silt to
the pit depth of about 5 feet. Results of swell -consolidation testing performed on relatively
undisturbed samples of the silt soil, presented on Figures 3 and 4, indicate low compressibility
under existing low moisture condition and light loading and a low collapse potential (settlement
under constant load) when wetted and high compressibility under additional loading. The
laboratory test results are summarized in Table 1. No free water was observed in the pit at the
time of our site visit and the soils were slightly moist to moist with depth.
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Foundation Recommendations: The subsoils encountered at the site are relatively low density
and compressible, even under light loading and especially when wetted. Based on these
conditions and the nature of the proposed construction, lightly loaded spread footings placed on
the undisturbed natural soil and designed for an allowable soil bearing pressure of 1,000 psf can
be used for support of the proposed addition with a risk of settlement. The soils are compressible
after wetting and post -construction foundation settlement differential to the existing structure
could cause building distress. Footings should be a minimum width of 20 inches for continuous
walls and 2 feet for columns. Loose disturbed soils and existing fill encountered at the
foundation bearing level within the excavation should be removed and the footing bearing level
extended down to the undisturbed natural soils. We should observe the completed foundation
excavation to evaluate the exposed soils for bearing conditions. Exterior footings should be
provided with adequate cover above their bearing elevations for frost protection. Placement of
footings at least 36 inches below the exterior grade is typically used in this area. Continuous
foundation walls should be heavily reinforced top and bottom to span local anomalies such as by
assuming an unsupported length of at least 14 feet. Foundation walls acting as retaining
structures (if any) should be designed to resist a lateral earth pressure based on an equivalent
fluid unit weight of at least 50 pcf for the on-site soil as backfill.
Floor Slabs: The natural on-site soils, exclusive of topsoil and old fill soils, can be used to
support lightly loaded slab -on -grade construction with a risk of settlement if the bearing soils are
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 such as CDOT Class 6 base course should be placed beneath slabs for
support. This material should consist of minus 1 -inch aggregate with less than 50% passing 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 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 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. If a crawlspace is provided, an underdrain should not be placed around
the foundation to help protect the bearing soils from surface water infiltration.
Kumar & Associates, Inc. ° Project No. 20-7-410
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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: Proper surface grading and drainage will be critical to keeping the bearing
soils dry and limiting potential differential settlement and distress. The following drainage
precautions should be observed during construction and maintained at all times after the addition
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 pavement and walkway 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
10 feet from the building. Consideration should be given to the use of xeriscape
to limit potential wetting of soils below the foundation 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 pit excavated at the location indicated on Figure 1
and to the depth shown on Figure 2, 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
pit 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 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 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
Kumar & Associates, Inc. ° Project No. 20-7-410
4
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.
If you have any questions or if we may be of further assistance, please let us know.
Respectfully Submitted,
Kumar & Associates, Inc.
Steven L. Pawtak;
Reviewed by:
Daniel E. Hardin, P.E.
SLP/kac
Attachments: Figure 1 — Location of Exploratory Pit
Figure 2 — Log of Exploratory Pit
Figures 3 and 4 — Swell -Consolidation Test Results
Table 1 — Summary of Laboratory Test Results
cc: JH Design — Jeff Halferty (Thdesign@sopris.net)
Glenwood Structural and Civil — Dolf Gorra (gsc@,sopris.net
Kumar & Associates, Inc. 4s Project No. 20-7-410
■
PIT 1
v
PROPOSED
ADDITION
0
EXISTING
BUILDING
5 0 5 10
APPROXIMATE SCALE—FEET
ORIGIHAL CABIN
20-7-410
Kumar & Associates
LOCATION OF EXPLORATORY PIT
Fig. 1
ed Addlilon—Prefontaine Ave\
DEPTH -FEET
0
5
10
PIT 1
WC=9.0
DD=89
— 200=81
WC=12.6
DD=95
— 200=85
LEGEND
•
•
FILL; DRIVEWAY GRAVEL SURFACE.
FILL; SANDY SILT AND CLAY, RED—BROWN.
SILT (ML); SANDY TO VERY SANDY, SLIGHTLY CLAYEY WITH
DEPTH, STIFF, SLIGHTLY MOIST TO MOIST WITH DEPTH, LIGHT
BROWN.
HAND DRIVEN 2" DIAMETER LINER SAMPLE.
NOTES
1. THE EXPLORATORY PIT WAS OBSERVED ON JULY 27, 2020 AND
WAS DUG WITH A BACKHOE.
2. THE LOCATION OF THE EXPLORATORY PIT WAS MEASURED
APPROXIMATELY BY TAPING FROM THE NORTHEAST CORNER OF THE
EXISTING BUILDING.
3. THE ELEVATION OF THE EXPLORATORY PIT WAS NOT MEASURED
AND THE LOG OF THE EXPLORATORY PIT IS PLOTTED TO DEPTH.
4. THE EXPLORATORY PIT LOCATION SHOULD BE CONSIDERED
ACCURATE ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT
LOG REPRESENT THE APPROXIMATE BOUNDARIES BETWEEN MATERIAL
TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT OBSERVED IN THE PIT AT THE TIME OF
OUR SITE VISIT.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D 2216);
DD = DRY DENSITY (pcf) (ASTM D 2216);
—200 = PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140).
20-7-410
Kumar & Associates
LOG OF EXPLORATORY PIT
Fig. 2
0
—2
— 4
z
0
1-
J
—5
O
N
O
° —6
— 7
— 8
— 9
These test results apply only to the
samples tested. The testing report
shall not be reproduced, except in
full, without the written approval of
Kumar and Associates, Inc. Swell
Consolidation testing performed in
accordance with ASTM D-4546.
SAMPLE OF: Sandy Silt
FROM: Pit 1 ® 3'
WC = 9.0 %, DD = 89 pcf
—200 = 81
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
1.0 APPLIED PRESSURE - KSF
10
100
20-7-410
Kumar & Associates
SWELL -CONSOLIDATION TEST RESULTS
Fig. 3
0
SAMPLE OF: Sandy Silt
FROM: Pit 1 ® 5'
WC = 12.6 %, DD = 95 pcf
—200 = 85
— 2
Z
O
—4
— 5
— 6
— 7
These test results apply only to the
samples tested. The testing report
shall not be reproduced, except in
full, without the written approval of
Kumar and Associates, Inc. Swell
Consolidation testing performed in
accordance with ASTM D-4546.
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
1.0 APPLIED PRESSURE — KSF
10
100
20-7-410
Kumar & Associates
SWELL—CONSOLIDATION TEST RESULTS
Fig. 4
I(+A
Kumar & Associates, Inc®
Geotechnical and Materials Engineers
and Environmental Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
Project No. 20-7-410
SAMPLE LOCATION
NATURAL
MOISTURE
CONTENT
(%)
NATURAL
DRY
DENSITYPASSING
(pcf)
GRADATION
PERCENT
NO.
200 SIEVE
ATTERBERG LIMITS
UNCONFINED
COMPRESSIVE
STRENGTH
(psf)
SOIL TYPE
PIT
DEPTH
(ft)
GRAVEL
SAND
LIQUID LIMIT
(%)
PLASTIC
INDEX
(%)
1
3
9.0
89
81
Sandy Silt
5
12.6
95
85
Sandy Silt