HomeMy WebLinkAboutSoils Report 03.27.2020CTL I THOMPSON
1 N C O R P.,.O t A_ T. e b
March 27, 2020
G. H. Daniels & Associates, Inc.
140 G. H. Daniels Boulevard
Gypsum, CO 81637
Attention: Mr. Jody Daniels
Subject: Geotechnical Engineering Investigation
Modular Residence
Lot 7, Quicksilver Court Subdivision
Garfield County, Colorado
Project No. GS06392.001-120
CTL I Thompson, Inc. performed a geologic evaluation and preliminary ge-
otechnical investigation for the development previously referenced as Dan-
iels/Hasenberg Subdivision (Project No. GS05135-115; report dated January 11,
2008) to identify geologic conditions at the site and provide preliminary mitigation
for construction of single-family residence on Lots 1 - 7 of the development. Our
opinions and recommendations in the report were based on seven exploratory
borings drilled at the site (one on each of the seven Tots) and pertinent laboratory
testing. Subsequently, we utilized our field and laboratory data obtained for our
report to provide a soils and foundation investigation for the seven Tots in the sub-
division (Project No. GS05135-120; letter dated February 14, 2008). However,
building plans for residences on Lots 1 — 7 were not developed at the time of our
letter.
The information, conclusions, and recommendations presented in our re-
port and letter were based upon consideration of many factors including, but not
limited to, the type of structures proposed, the geologic setting, and the subsur-
face conditions encountered. Standards of practice continuously change in the
area of geotechnical engineering. Our recommendations are appropriate for
about three years. Because residences were not constructed at the site within
this time period, and because building plans were not known at the time of our
soils and foundation investigation, we recommended that our geotechnical engi-
neering letter be updated. This letter provides our geotechnical engineering
opinions and recommendations for the modular residence proposed on Lot 7,
Quicksilver Court Subdivision.
234 Center Drive I Glenwood Springs, Colorado 81601
Telephone: 970-945-2809 Fax: 970-945-7411
Site Conditions
The subject site is Lot 7, Quicksilver Court Subdivision in Garfield County,
Colorado. Lot 7 is an approximately 10-acre parcel north of Quicksilver Court and
generally west of Quicksilver Way. No structures were present on the subject lot
at the time of our investigation. An aerial photograph of the site is included as
Figure 1. Ground surface at the building site slopes gently down to the north-
east. Natural vegetation in the building area consists of sparse grasses with
scattered sage, juniper and pinyon.
Proposed Construction
A one-story, modular residence is planned for Lot 7. A cast -in -place foun-
dation is proposed to support the residence. Some interior column pads are ex-
pected. The perimeter foundation walls will form a crawl space below the modu-
lar unit. Foundation Toads are expected to be Tess than 3,000 pounds per lineal
foot of foundation wall. Column pads load will likely be less than 50 kips. We do
not know if concrete flatwork, such as a patio or driveway, will be constructed ad-
jacent to the building.
Subsurface Condition
Subsurface conditions in the proposed area of the residence were investi-
gated by excavating two exploratory pits (TP-1 and TP-2) with a trackhoe at the
approximate locations shown on Figure 1. Exploratory excavation operations
were directed by our representative who logged the soils encountered and ob-
tained samples for laboratory testing.
Subsurface conditions encountered in our exploratory pits were about 6
inches of sandy clay topsoil, underlain by clayey gravel with cobbles, boulders,
and lenses of clayey sand to the total explored depth of 9 feet. Free ground wa-
ter was not encountered in our exploratory pits. The pits were backfilled immedi-
ately after exploratory excavation operations were completed. Graphic logs of
the soils encountered in our exploratory pits are shown on Figure 2.
Samples obtained from our pits were returned to our laboratory where
they were visually classified and typical samples were selected for testing. One
sample selected for gradation analysis contained 30 percent gravel, 24 percent
sand, and 46 percent silt and clay (passing the No. 200 Sieve). Gradation test
results do not include rocks larger than 5 inches, which are present in the in -situ
soils. Engineering index testing on one soil sample indicated the sandy clay ma-
trix soils at the site have a liquid limit of about 32 percent and plasticity index of
G. H. DANIELS & ASSOCIATES, INC. 2
LOT 7, QUICKSILVER COURT SUBDIVISION
PROJECT NO. GS06392.001-120
about 14 percent. Gradation test results are shown on Figure 3. Laboratory test
results are summarized on Table I.
Foundation
We judge that a footing foundation supported by the undisturbed, natural
clayey gravel soil is appropriate for the residence proposed on Lot 7. The recom-
mendations in the Surface Drainage section of this letter will be critical for perfor-
mance of the foundation. Design and construction criteria for footings are pre-
sented below.
1 Footing foundations can be supported on the undisturbed, natural
clayey gravel soil. Soils loosened during excavation or the forming
process for the footings should be removed or the soils can be re -
compacted prior to placing concrete.
2. Footings on the undisturbed, natural clayey gravel should be de-
signed for a maximum soil bearing pressure of 3,000 psf.
3. Continuous wall footings should have a minimum width of at least 16
inches. Foundations for isolated columns should have minimum di-
mensions of 24 inches by 24 inches.
4. Grade beams and foundation walls should be well -reinforced, top
and bottom. We recommend reinforcement sufficient to span an un-
supported distance of at least 10 feet.
5. The soils under exterior footings should be protected from freezing.
We recommend the bottom of footings be constructed at a depth of
at least 36 inches below finished exterior grades for frost protection.
The Garfield County building department should be consulted to ver-
ify the required depth.
Slab -on -Grade Construction
We do not know if exterior concrete slabs, such a patio or driveway, will be
constructed adjacent to the residence. Areas which will support concrete slabs
should be stripped of vegetation, organic soils and debris. The on -site soils free of
rocks larger than 3 inches in diameter, organic matter, and debris are suitable for
use as structural fill below slabs. Fill should be placed in loose lifts of 10 inches
thick or less, moisture -conditioned to within 2 percent of optimum moisture content,
and compacted to at least 98 percent of standard Proctor (ASTM D 698) maximum
G. H. DANIELS & ASSOCIATES, INC.
LOT 7, QUICKSILVER COURT SUBDIVISION
PROJECT NO. GS06392.001-120
3
dry density. Moisture content and density of fill should be checked by a repre-
sentative of our firm during placement. Observation of the compaction procedure
is necessary.
Exterior slabs should be separated from the building foundation with slip
joints which allow free vertical movement of the slabs. These slabs should be well -
reinforced to function as independent units. Frequent control joints should be pro-
vided, in accordance with American Concrete Institute (ACI) recommendations, to
reduce problems associated with shrinkage and curling. Recommendations in the
Surface Drainage section will be critical for slab performance.
Below -Grade Walls
The modular residence will be supported on a cast -in -place foundation that
forms a crawl space below the residence. Foundation walls which extend below -
grade should be designed for lateral earth pressures where backfill is not present
to about the same extent on both sides of the wall, such as in crawl spaces. For
a very rigid wall where negligible or very little deflection will occur, an "at -rest" lat-
eral earth pressure should be used in design. For walls which can deflect or rotate
0.5 to 1 percent of wall height (depending upon the backfill types), lower "active"
lateral earth pressures are appropriate.
Our experience indicates that typical below -grade walls in residences can
deflect or rotate slightly under normal design loads, and that this deflection results
in satisfactory wall performance. Thus, the earth pressures on the walls will likely
be between the "active" and "at -rest" conditions. If the on -site soils are used as
backfill, we recommend design of below -grade walls using an equivalent fluid den-
sity of at least 45 pcf for this site. This equivalent density does not include allow-
ances for sloping backfill, surcharges or hydrostatic pressures.
Foundation Wall Backfill
Properly -placed backfill adjacent to foundation wall exteriors is important to
reduce infiltration of surface water and subsequent consolidation. This is espe-
cially important for backfill that will support exterior concrete flatwork, such as a
patio or driveway. The on -site excavated soils can be used as foundation wall
backfill. The backfill should be free of organic matter, debris, and rocks larger than
3 inches in diameter. Backfill should be moisture conditioned to within 2 percent
of optimum moisture content and compacted to at least 95 percent of standard
Proctor (ASTM D 698) maximum dry density.
Subsurface Drainage
Water from rain, snow melt, and surface irrigation frequently flows through
relatively permeable backfill placed adjacent to a residence and collects on the
G. H. DANIELS & ASSOCIATES, INC.
LOT 7, QUICKSILVER COURT SUBDIVISION
PROJECT NO. GS06392.001-120
4
surface of relatively undisturbed soils at the bottom of the excavation. This can
cause wetting of foundation soils after construction, which results in movement of
foundations and floor slabs. It can also result in standing water and/or damp con-
ditions in below -grade areas, such as crawl spaces. To mitigate these concerns,
we recommend provision of a foundation drain around the crawl space below the
residence.
The foundation drain should consist of a 4-inch diameter, slotted pipe en-
cased in free draining gravel. A prefabricated drainage composite should be
placed adjacent to foundation walls. Care should be taken during backfill opera-
tions to prevent damage to drainage composites. The drain should lead to a pos-
itive gravity outfall, or to a sump pit where water can be removed by pumping.
Gravity outlets should not be susceptible to clogging or freezing. Installation of a
clean -out along the drain pipe is recommended. The foundation drain concept is
shown on Figure 4.
Surface Drainage
Surface drainage is critical to the performance of foundations, and concrete
flatwork. Surface drainage should be designed and constructed to provide rapid
runoff of surface water away from the residence. Proper surface drainage and irri-
gation practices can help control the amount of surface water that penetrates to
foundation levels and contributes to settlement or heave of soils and bedrock that
support foundations and slabs -on -grade. Positive drainage away from the founda-
tion and avoidance of irrigation near the foundation also help to avoid excessive
wetting of backfill soils, which can lead to increased backfill settlement and possi-
bly to higher lateral earth pressures, due to increased weight and reduced strength
of the backfill. We recommend the following precautions be observed during con-
struction and maintained at all times after the residence is completed:
1. The ground surface surrounding the exterior of the residence should
be sloped to drain away from the residence and exterior slabs. We
recommend providing a slope of at least 12 inches in the first 10 feet
around the residence, where possible.
2. Backfill around the exterior of foundation walls should be placed in
accordance with recommendations in the Foundation Wall Backfill
section.
3. The residence should be provided with roof gutters and downspouts.
Roof downspouts and drains should discharge well beyond the limits
of all backfill. Splash blocks and downspout extensions should be
provided at all discharge points.
4. Landscaping should be carefully designed to minimize irrigation.
Plants used near foundation walls should be limited to those with low
G. H. DANIELS & ASSOCIATES, INC.
LOT 7, QUICKSILVER COURT SUBDIVISION
PROJECT NO. GS06392.001-120
5
moisture requirements; irrigated grass should not be located within 5
feet of the foundation. Sprinklers should not discharge within 5 feet
of the foundation and should be directed away from the building.
5. Impervious plastic membranes should not be used to cover the
ground surface immediately surrounding the residence. These
membranes tend to trap moisture and prevent normal evaporation
from occurring. Geotextile fabrics can be used to control weed
growth and allow some evaporation to occur.
Geotechnical Risk
The concept of risk is an important aspect of any geotechnical evaluation.
The primary reason for this is that the analytical methods used to develop ge-
otechnical recommendations do not comprise an exact science. The analytical
tools which geotechnical engineers use are generally empirical and must be tem-
pered by engineering judgment and experience. Therefore, the solutions or rec-
ommendations presented in any geotechnical evaluation should not be consid-
ered risk -free and, more importantly, are not a guarantee that the interaction be-
tween the soils and the proposed structure will perform as desired or intended.
What the engineering recommendations presented in the preceding sections do
constitute is our estimate, based on the information generated during this and
previous evaluations and our experience in working with these conditions, of
those measures that are necessary to help the building perform satisfactorily.
The information, conclusions, and recommendations presented herein are
based upon consideration of many factors including, but not limited to, the type of
structure proposed, the geologic setting, and the subsurface conditions encoun-
tered. The conclusions and recommendations contained in this letter are not valid
for use by others. Standards of practice continuously change in the area of ge-
otechnical engineering. The recommendations provided in this letter are appro-
priate for about three years. If the proposed project is not constructed within
three years, we should be contacted to determine if we should update this letter.
Limitations
Our exploratory pits provide a reasonable estimate of subsurface condi-
tions in the area of the proposed residence. Actual conditions below the building
will vary. Our representative should be called to observe subsurface conditions
exposed in the completed foundation excavation to confirm that conditions are as
anticipated from our geotechnical engineering investigation.
G. H. DANIELS & ASSOCIATES, INC.
LOT 7, QUICKSILVER COURT SUBDIVISION
PROJECT NO. GS06392.001-120
6
This geotechnical engineering investigation was performed in a mariner
consistent with that level of care and skill ordinarily exercised by geotechnical en-
gineers currently practicing under similar conditions in the locality of this project.
No warranty, express or implied, is made. We are available to discuss the contents
of this letter. If you have questions or need additional information, please call
Very Truly Yours
CTL 1 THOMPSON, INC.
Ryan W. DeMars, E.I.T.
Staff Engineer
RWD:JDK:ac
G. H. DANIELS 8 ASSOCIATES, INC.
LOT 7, QUICKSILVER COURT SUBDIVISION
PROJECT NO. GS06392.001.120
Reviewed By:
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SCALE: 1" = 200'
LEGEND:
TP-1 APPROXIMATE LOCATION OF
EXPLORATORY PIT
APPROXIMATE PROPERTY BOUNDARY
FROM GARFIELD COUNTY GIS
NOTE: IMAGE FROM GOOGLE EARTH
G. H. Daniels & Associates, Inc.
Lot 7, Quicksilver Court Subdivision
Project No. GS06392.001-120
Aerial
Photograph
FIg. 1
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TP-1
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TP-2
15 15
G. H. DANIELS & ASSOCIATES, INC.
LOT 7, QUICKSILVER COURT SUBDIVISION
PROJECT NO. GS06392.001-120
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LEGEND:
NOTES:
TOPSOIL, CLAY, SANDY, MOIST, BROWN,
RUST.
GRAVEL, CALYEY, COBBLES AND
BOULDERS, LENSES OF CLAYEY SAND,
MEDIUM DENSE TO DENSE, SLIGHTLY
MOIST TO MOIST, TAN. (GC, CL)
INDICATES BULK SAMPLE FROM
EXCAVATED SOILS.
1. EXPLORATORY PITS WERE EXCAVATED
WITH A TRACKHOE ON FEBRUARY 26, 2020.
PITS WERE BACKFILLED IMMEDIATELY
AFTER EXPLORATORY EXCAVATION
OPERATIONS WERE COMPLETED.
2. FREE GOUNDWATER WAS NOT FOUND IN OUR
EXPLORATORY PITS AT THE TIME OF
EXCAVATION.
3. LOCATIONS OF EXPLORATRY PITS ARE
APPROXIMATE.
4. EXPLORATORY PITS ARE SUBJECT TO THE
EXPLANATIONS, LIMITATIONS AND
CONCLUSIONS CONTAINED IN THIS
REPORT.
Summary Logs of
Exploratory
Pits
FIG. 2
HYDROMETER ANALYSIS I SIEVE ANALYSIS
25 HR, 7 HR. TIME READINGS U.S. STANDARD SERIES CLEAR SQUARE OPENINGS
45 MIN. 15 MIN. 60 MIN. 19 MIN. 4 MIN. 1 MIN. *200 '100 *50 '40 *30 '16 *10 *8 *4 3/8" 3/4" 1%" 3" 5"6" 8"
PERCENT PASSING
0 0 0 0 0 0 0 0 0 C
—r
i 0 0 0 0 cn 0 .11.
0 0 0
PERCENT RETAINED
i
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.001 0 002 .005 .009 .019 .037 .074 .149 .297 .590 1 19 2.0 2.38 4 76 9.52 19.1 36.1 76.2 127 200
0.42 152
DIAMETER OF PARTICLE IN MILLIMETERS
CLAY (PLASTIC) TO SILT (NON -PLASTIC)
SANDS
GRAVEL
FINE I MEDIUM I COARS
FINE I COARSE I COBBLES
Sample of GRAVEL, CLAYEY (GC)
From TP - 1 AT 6 - 7 FEET
GRAVEL 30 % SAND 24 %
SILT & CLAY 46 % LIQUID LIMIT
PLASTICITY INDEX
HYDROMETER ANALYSIS I SIEVE ANALYSIS
25 HR. 7 HR. TIME READINGS U.S. STANDARD SERIES CLEAR SQUARE OPENINGS
45 MIN. 15 MIN. 60 MIN. 19 MIN. 4 MIN. 1 MIN. '200 "100 *50 '40 '30 *16 *10 '8 *4 3/8" 3 4" 1%" 3" 5"6" 8"
100 n
N W J ERCCNT PPSSINJC
3 0 0 0 0 0 0 0 0 0
fO 03 J 0 N A W N
D O O O O O 00 00
PERCENT RETAINED
.001 0 002 .005 .009 .019 .037 .074 .149 .297 .590 1 19 2 0 2.38 4.76 9.52 19.1 36.1 76.2 127 200
0.42 152
DIAMETER OF PARTICLE IN MILLIMETERS
CLAY (PLASTIC) TO SILT (NON -PLASTIC)
SANDS
GRAVEL
FINE I MEDIUM I COARS
FINE I COARSE I COBBLES
Sample of
From
G. H. Daniels & Associates, Inc.
Lot 7, Quicksilver Courst Subdivision
PROJECT NO. GS06392.001-120
GRAVEL
SILT & CLAY
PLASTICITY INDEX
% SAND
OA
% LIQUID LIMIT
0/0
Gradation
Test Results
FIG. 3
Drain-5 Reference
NOTE:
DRAIN SHOULD BE AT LEAST 2 INCHES
BELOW BOTTOM OF FOOTING AT THE
HIGHEST POINT AND SLOPE DOWNWARD
TO A POSR VE GRAVITY OUTLET OR TO
A SUMP WHERE WATER CAN BE
REMOVED BY PUMPING.
SLOPE
PER REPORT
BACKFILL
MIRADRAIN G2O0N
OR EQUIVALENT
ATTACH PLASTIC SHEETING
SLOPE TO FOUNDATION WALL
PER
OSHA
COVER ENTIRE WIDTH OF
GRAVEL WITH NON -WOVEN
GEOTEXITLE FABRIC (MIRAFI
140N OR EQUIVALENT).
id If •• . •
r''
sLopE DRApiJ 2" MINIMUM
8" MINIMUM --"— }
OR BEYOND
1:1 SLOPE FROM
BOTTOM OF FOOTING
(WHICHEVER IS GREATER)
4-INCH DIAMETER PERFORATED DRAIN PIPE. THE
PIPE SHOULD BE PLACED IN A TRENCH WITH A
SLOPE OF AT LEAST 1/8-INCH DROP PER FOOT
OF DRAIN.
STRUCTURAL FLOOR
G.H. Daniels & Associates, Inc.
Lot 7, Quicksilver court Subdivision
Project No. GS06392.001 -1 20
r— CRAWL SPACE —�
FOOTING OR PAD
"MUD SLAB" OR
VAPOR BARRIER
Foundation
Wall Drain
Concept
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