HomeMy WebLinkAboutSubsoil Study for Foundation Design 05.22.2024l(1 f.:ffi[ffi[H1If;,**
An Emdoycc Orncd Compony
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 Colling Glenwood Springs, and Summit County, Colorado
SUBSOIL STTJDY
FOR FOT]NDATION DESIGN
PROPOSED RESIDENCE
LOT 20, RAPTDS ON THE COLORADO
RAPIDS VIEW LANE
GARFIELD COUNTY, COLORADO
PROJECT NO.24-7-240
MAY 22,2024
PREPARED FOR:
DAVE AI\IDRUS
433 RIVER VIEW DRIVE, #1605
NEW CASTLE, COLORADO 81647
dmandruslT@smail.com
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TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS
..............- 2 -
.-4-
.-5-
.-5-
.-5-
1
3-
6-
FIELD E)PLORATION ........
SUBSURFACE CONDITIONS ......
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOUNDATIONS..
FOIINDATTON AND RT'IAINING WALLS
FLOOR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE...
LIMITATIONS
FIGURE 1 . LOCATION OF DPLORATORY BORINGS
FIGURE 2 .T,OGS OF DGLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL.CONSOLIDATION TEST RESULTS
FIGURE 5 - GRADATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
-', -
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Kumar & Associateg, lnc, o Project No. 2+7-24A
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot20, Rapids on the Colorado, Rapids View Lane, 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 Dave Andrus dated April 15,2024.
A field exploration program consisting of exploratory borings was conducted 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, design recommendations and other geotechnical
engineering considerations based on the proposed construction and the subsurface conditions
encountered.
PROPOSED CONSTRUCTION
Plans for the proposed residence were conceptual at the time of our study. The proposed
residence is assumed to be a one- or two-story structure with an attached garage. Ground floors
could be structural over crawlspace or slab-on-grade. Grading for the structure is assumed to be
relatively minor with cut depths between about 2to 4 feet. We assume relatively light
foundation loadings, 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 vacant at the time of our field exploration. The ground surface was gently
sloping down to the north at grades estimated at around 5 percent.. The Colorado River is north
ofthe lot as shown on Figure 1. Vegetation consists of grass and weeds.
FIELD EXPLORATION
The field exploration for the project was conducted on April24,2024. Two exploratory borings
were drilled at the locations shown on Figure I 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 borings were logged by a representative of Kumar &
Associates,Inc.
Kumar & Associates, lnc, @ Project No. 24-7-240
a
Samples of the subsoils were taken with l%-inch and 2-inch I.D. spoon samplers. The samplers
were 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-l586.
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 Logs of Exploratory Borings, 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 encountered below about 1 foot of topsoil consist of silty sandy clay to between 4
and 8 feet deep overlying dense, silty sandy gravel and cobbles to the maximum drilled depth
of 13 feet. A layer of loose silty sand was encountered from 8 to l1 feet deep in Boring I
between the clay and gravel soils. Drilling in the coarse granular soils with auger equipment was
difficult due to the cobbles and boulders and drilling refusal was encountered in the deposit.
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 sample of the upper clay soil, presented on Figure 4, indicate low to
moderate compressibility under conditions of loading and wetting. Results of gradation analyses
performed on a small diameter drive sample (minus lt/z-inchfraction) of the coarse granular
subsoils are shown on Figure 5. The laboratory testing is summarized in Table l.
Free water was encountered in Boring lat a depth of l0% feet at the time of drilling and the
upper soils were slightly moist to moist with depth.
FOUNDATION BEARING CONDITIONS
The upper silty sandy clay soils encountered in the borings possess low bearing capacity and
low to moderate settlement potential especially when wetted under load. The underlying coarse
granular soils possess moderate bearing capacity and typically low settlement potential. At
assumed excavation depths, we expect the exposed subsoils to consist primarily of silty sandy
clay. The proposed residence can be supported on spread footings bearing on the natural soils
with a risk of differential settlement due to the variable bearing conditions of the clay soils and
possibly the gravel subsoils. A lower risk option would be to sub-excavate foundation areas to
expose the underlying gravel soils (where feasible) and extend the bearing level down to the
dense gravel or backfill the sub-excavated depth with compacted structural fill up to design
bearing level.
Kumar & Associates, lnc. o Projec{ No. 2+7-24A
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DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature
of the proposed construction, the building can be founded with spread footings bearing on the
natural soils or compacted structural fill with a risk of settlement.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
l) Foatings placed on the undisturbed natural soils shotrld be designed for an
allowable bearingpressure of 1,500 psf. Footings placed entirely on the
underlying gravel soils or compacted structural fill can be designed for an
allowable bearing pressure ol}$lf. Based on experience, we expect initial
settlement of footings designed and constructed as discussed in this section will
be about I inch or less. Additional post construction sefflement could occur for
footings placed on the clay soils if the bearing soils become wetted. The
magnitude of additional settlement would depend on the depth and extent of
wetting and could be on the order of % to I inch.
2) The footings should have a minimum width of 18 inches for continuous walls and
2 feet for isolated pads.
3) Exterior footings and footings beneath unheated areas should be provided with
adequate soil cover above their bearing elevation for frost protection. Placement
of foundations at least$fglg below exterior grade is typically used in this
area.
4) Continuous foundation walls should be well reinforced top and bottom to span
local anomalies and resist differential movement such as by assuming an
unsupported length of at least 12 feet. Foundation walls acting as retaining
structures should also be designed to resist lateral earth pressures as discussed
in the "Foundation and Retaining Walls" section ofthis report.
5) Topsoil and any loose disturbed soils should be removed and the footing bearing
level extended down to the firm natural soils. If structural fill is used, footings
areas should be sub-excavated to expose the underlying gravel soils with a lateral
distance at least half the depth of fill below the footing. The exposed soils in
footing area should then be moistened and compacted. Structural fill can consist
of the onsite soils devoid of organics, topsoil and rock larger than about 4 inches
or a suitable imported granular soil, such as CDOT class 6 base course. Structural
fill should be moisture conditioned to near optimum moisture content and
compacted to at least 98 percent maximum proctor density.
Kumar & Associates, lnc. @ Project No. 2+7-240
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A representative ofthe geotechnical engineer should observe all footing
excavations and test structural fill prior to concrete placement to evaluate bearing
conditions.
FOUNDATION AND RETAINING WALLS
Foundation walls and retaining structuros 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.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, lraflic, construction materials and equipment.
The pressures recornmended 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%o of the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
walkway areas should be compacted to at least95Yo 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 coeffrcient of friction of 0.30. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 300 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 least95o/o of the
maximum standard Proctor density at a moisture content near optimum.
6)
Kumar & Associates, lnc. @ Project No. 24F7-244
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FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction. To reduce the effects of some differential movemento 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 free-
draining gravel should be placed beneath basement level slabs to facilitate drainage. This
material should consist of minus 2-inch aggregate with at least 50% retained on the No. 4 sieve
and less than2Yo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least95%o 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.
TINDERDRAIN SYSTEM
Although free water was encountered below the assumed excavation depth, it has been our
experience in the area that the groundwater level will seasonally fluctuate, and 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 system. Typical shallow crawlspace should not be provided with an
underdrain to help protect the bearing soils from wetting.
The drains should consist of drainpipe placed in the bottom of the wall backfill surrounded above
the invert level with free-draining granular material. The drain should be placed at each level of
excavation and at least 1 foot below lowest adjacent finish grade and sloped at a minimum IVoto
a suitable gravity outlet. Free-draining granular material used in the underdrain system should
contain less than 2Yo passing the No. 200 sieve, less than 50% passing the No. 4 sieve and have a
maximum size of 2 inches. The drain gravel backfill should be at least lYzfeetdeep. An
impervious membrane such as 20 mil PVC should be placed beneath the drain gravel in a trough
shape and attached to the foundation wall with mastic to prevent wetting of the bearing soils.
SURFACE DRAINAGE
Providing proper surface grading and drainage will be critical to help keep the bearing soils dry
and limit potential settlement and distress of the residence. The following drainage precautions
should be observed during construction and maintained at all times after the residence has been
completed:
Kumar & Associates, lnc. o Project No. 24-7-240
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1)
2)
3)
Inundation ofthe foundation excavations and underslab areas should be avoided
during construction.
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.
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 feetof the on-site soils to
reduc,e surface water infi ltration.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Landscaping which requires regular heavy inigation should be located at least
l0 feet from foundation walls. Consideration should be given to the use of
xeriscape to limit potential wetting of soils below the building caused by
irrigation.
4)
5)
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this areaatthis 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 borings drilled at the locations indicated on Figure l, 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 elient 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 borings 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
so that re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use hy our client for design pu{poses. 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
or modifications to the recommendations presented herein. We recommend on-site observation
Kumar & Associates, lnc. o Project No.2&7-240
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of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kumar & Associales,
James H. Parsons, P
Reviewed by:
/,Q*tL
Steven L. Pawlak, P.E.
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Kumar & Associates, lnc. @ Proiect No.24-7-244
APPROXIMATE SCALE-FEET
24-7-240 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
BORING 1
EL. 100'
BORING 2
EL. 98.5'
0 0
6/ 12
WC=7.4
DD=79
26/12
WC=8.0
DD=1 OO
5
11/12
WC= 1 0.4
DD=97
-2OO=72
30/6, 5A/5
5
F
LlJ
LrJ
LL
I
:E-o-
tlJo
7 /12tNC=27.O
DD=94
-200=59
FLItdt!
I-Fo-
lrJo
10 10
3s/ 12
*4=59
-200=1 1
15 15
24-7-240 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
5
I
LEGEND
N
TOPSOIL; SLIGHTLY SANDY, CLAYEY ORGANIC SILT, FIRM, SLIGHTLY MOIST, DARK BROWN.
CLAY (CL), SANDY, StLTy, MEDTUM STTFF TO STIFF, SLtcHTLy MOIST, LtcHT BRoWN.
SAND (SM), SlLTy, LOOSE, MOTST TO WET, DARK BROWN.
GRAVEL
TO TAN
(GM),
AND
SANDY, SILTY, DENSE TO VERY DENSE, VERY MOIST TO WET, MEDIUM BROWN
GRAY.
DRIVE SAMPLE, z-INCH I.D. CALIFORNIA LINER SAMPLE
DRIVE SAMPLE, 1 s/l-INCH r.D. SpLrT SPOON STANDARD PENETRATTON TEST.
nurc DRIVE SAMPLE BLOW COUNT. INDICATES THAT 6 BLOWS OF A 140-P0UND HAMMER-,'- FALLING 50 INGHES WERE REQUIRED To DRIVE THE SAMPLER 12 INCHES.
-=- DEPTH TO WATER LEVEL ENCOUNTERED AT THE TIME OF DRILLING.
_} DEPTH AT WHICH BORING CAVED.
PRACTICAL AUGER REFUSAL. WHERE SHOWN ABOVE BOTTOM OF BORING, INDICATES THAT
MULTIPLE ATTEMPTS WHERE MADE TO ADVANCE THE HOLE.
NOTEg
1. THE EXPLORATORY BORINGS WERE DRILLED ON APRIL 24, 2024 WITH A 4-INCH-DIAMETER
CONTINUOUS_FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY HAND LEVEL AND REFER
TO BORING 1 AS 1OO" ASSUMED.
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER LEVELS SHOWN ON THE LOGS WERE MEASURED AT THE TIME AND UNDER
CONDITIONS INDICATED. FLUCTUATIONS IN THE WATER LEVEL MAY OCCUR WITH TIME.
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSIW (pcf) (ASTM D2216);
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6915);
-200= PERCENTAGE PASSING No. 200 SIEVE (ASTM D1 140).
F
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t
24-7-240 Kumar & Associates LEGEND AND NOTES Fig. 3
SAMPLE OF: Silty Sondy Cloy
FROM:Boring2@2'
WC = 8.0 %, DD = 100 pef
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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24-7 -240 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
SOIL TYPE
Silty Sandy Clay
Silty Sandy Clay
Very Silty Sand
Silty Sandy Gravel
Silty Sandy Clay
{osfl
UNCONFINED
COMPRESSIVE
STRENGTH
PLASTIC
INDD(
lo/"1
ATTERBERG LIMITS
l"hl
LIQUID LIMIT
PERCENT
PASSING NO.
200 stEVE
72
39
11
SAND
ft
30
GRAI}ATION
(%)
GRAVEL
59
NATURAL
DRY
DENSTTY
{ocfl
79
97
94
100
lolol
NATURAL
MOISTURE
CONTENT
7.4
10.4
27.0
8.0
(ftt
DEPTH
2
4
9
T2
2
SAMPLE LOCATIOI{
BORING
1
2
No.24-7-24a