HomeMy WebLinkAboutSubsoil Study for Foundation Design 09.07.2022l(t iiçiffi*imF""
fir Employec Ownsd Compony
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
wwwkumarusa.com
Office Locations: DE¡rwsri(HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED GARAGE ADDITION
217 JB COURT
GARF'rELD COUNTY, COLORADO
PROJECT NO.22-7-424
SEPTEMBER7,2022
PREPARED FOR:
STIG SVEDBERG
277 JB COURT
GLDNWOOD SPRTNGS, COLORADO 81601
ssvedberg@,comcast.net
TABLN OF CONTENTS
PURPOSE AND SCOPE OF S'I'UDY
PROPOSED CON S'I'RUC'TION
SITE CONDITIONS.....
FLOOR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE................
I
I
-l-
FIELD EXPLORATION ..............- I .
SUBSURFACE CONDITIONS .....-2 -
FOUNDAT]ON tsEARING CONDITION S -2-
DESIGN RECOMMENDATIONS _') _
..-2 -
FOUNDATION AND RETAINING WALLS ................- 3 -
FOUNDATIONS
-4-
-4-
5LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORING
FIGURE 2 - LOG OF EXPLORATORY BORING
FIGURE 3 - GRADATION TEST RESULTS
Kumar & Associates, lnc. @ Project No. 22-7.424
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed garage addition to the residence
located at2t7 JB Court, Garfield County, Colorado. The project site is shown on Figure L 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 Stig
Svedberg dated June 14, 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 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 addition were not developed at the time of our study. The proposed
addition is expected to be a one-story wood framed structure with a slab-on-grade floor. Grading
for the structure is assumed to be relatively minor with cut depths of about 4 to 6 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 site was occupied with a 1- and 2-story wood frame house with an attached slab-on-
grade garage. The building is situated to the east of, and 6 to 8 feet above, Canyon Creek. The
boring location and site of the proposed garage addition was to the rear (east) of the existing
house at the base of a steep slope. The ground surface was relatively flat and vegetated with
scattered grass and weeds. An apparent shallow depth of fill materials is likely present in the
proposed addition area.
FIELD EXPLORATION
The field exploration for the project was conducted on June 21,2022. 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 augers powered by a truck-mounted CME-
458 drill rig. The boring was logged by a representative of Kumar & Associates, Inc.
Kumar & Assooiâtes, lnc. o Project No. n4lr42i4
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Samplcs of thc subsoils were taken with a 1%-inch I.D. spoon sampler. The sarnpler was driven
into the suhsoils at varior¡s clepths 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 ofthe relative density or consistency ofthe
subsoils. Depths at which the sarnples 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 is shown on Figure 2. The
subsoils consist of about I foot of topsoil and about I foot of stiff, silty santly cluy (possible fill)
overlying dense, sandy, silty to clayey gravel and cobbles with possible boulders down to the
maximum explored depth of 15 feet.
Laboratory testing performed on samples obtained from the boring included natural moisture
content and gradation analyses. Results of gradation analyses performed on a small diameter
drive sample (minus lYz-inch fraction) of the coarse granular subsoils are shown on Figure 3.
Free water was encountered in the boring at a depth of about 13 feet during drilling and at about
5Yzfeetwhen checked 1 day after drilling. The upper soils were moist to wet with depth.
FOUNDATION BEARING CONDITIONS
The natural granular soils encountered below the topsoil and clay layer are relatively dense and
have low settlement potential. Placing the foundation entirely on the natural granular soils
should provide a relatively low risk of foundation movement.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountercd in the exploratory boring and the nature of
the proposed construction, we recommend the addition be founded with spread footings hearing
on the natural granular soils.
The design and construction criteria presentcd bclow 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 2,500 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about 1 inch or less.
Kumar & Associates, lnc. @ Project No. 22.7-424
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4)
The footings should have a minimum width of 16 inches for continuous walls and
2 feet for isolated pads.
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.
Continuous foundation walls should be reinforced top and bottom to span local
anomalies such as by assuming an unsupported length of at least l0 feet.
Foundation walls acting as retaining structures should also be designed to resist
lateral earth pressure as described below in the "Foundation and Retaining Walls"
section.
Loose disturbed soils, existing fill soils and topsoil should be removed and the
footing bearing level extended down to the relatively dense natural granular soils.
The exposed soils in footing area should then be moistened and compacted. As
an alternative, design bearing level can be re-established with structural fill
compacted to at least 98% of standard Proctor densþ at a moistur,e content near
optimum. The fill can consist of on-site sand and gravel soils, sorted to remove
organics and oversized (plus 6-inch) rock or a suitable imported material such as
3/¿-inch base course. The fill should extend laterally beyond the edge of footings a
minimum distance of at least %the depth of fill below footings.
A representative ofthe geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
s)
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 50 pcf for backfill consisting
of the on-site granular soils. Cantilevered retaining structures which are separate from the
building 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 40 pcf for backfill consisting of the on-site granular 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 relatively level
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.
2)
3)
6)
Kumar & Associates, lnc. @ Project No. 22-7-424
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Backfill should be placed in uniform lifts antl compacted to at least 90% of the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
walkway areas should be compacted to at least 95Yo 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 exoessive lateral pressure on the wall. Some settlement of deep foundation wall
backfill should be expected, even ifthe material is placed correctly, and could result in distress to
facilities constructed on the backfill. Backfill should not contain organics, debris or rock larger
than about 6 inches.
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 0.45. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 400 pcf. The
coefftcient 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 a granular material compacted to at least
95Yo of the maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
Any existing fill, topsoil, and loose or disturbed soils should be removed from slab-on-grade
areas. The underlying natural granular soils are suitable to support lightly loaded slab-on-grade
construction. 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 free-
draining gravel should be placed beneath interior slabs to facilitate drainage. This material
should consist of minus 2-inch aggregate with at least 50% rctained on the No. 4 sieve and less
than2Yo passing the No. 200 sieve.
All ftll materials for support of floor slabs should be compacted to at least 95%o of maximum
standard Proctor density at a moisfure content near optimum.
TINDERDRAIN SYSTEM
Free water was encountered in the exploratory boring and it has been our experience in the area
that the groundwater level can rise and local perched groundwater can develop during times of
heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched
Kumar & Associates, lnc. o Project No. 22-7-424
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condition. With the proximity of Canyon Creek, the adjacent steep slope that could direct
surface runoff onto the site, and the presence of free water in the exploratory boring, we
recommend the garage addition foundation and any below-grade construction, such as retaining
walls, be protected from wetting and hydrostatic pressure buildup by an underdrain system.
Where required, the drains should consist of drainpipe placed in the bottom of the wall backfrll
surrounded above the invert level with free-draining granular material. The drain should be
placed at each level of excavation and at least I foot below lowest adjacent finish grade and
sloped at a minimum l%o to a suitable gravþ outlet. Free-draining granular material used in the
underdrain system should contain less than 2o/o passingthe 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 lYz feet deep and be covered with filter fabric.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the addition has been completed:
1) Inundation ofthe foundation excavations and underslab areas should be avoided.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95o/o of the maximum standard Proctor density in pavement and slab areas
and to at least 90o/o 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
covered with filter fabric and capped with about 2 feet of the on-site finer graded
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 submiffed 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, lnc. @ Project No. 22-7-424
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practice shmmbe consulted. Our findings inolude interpolation and extrapolation of the
subsurfacemditions identified at the exploratory boring and variations in the subsurface
conditionsmtSrnot become evident until excavation is performed. If conditions encountered
during conffiion appear different from those described in this reporto we should be notifiedw
that re-evaludñm of the recommendations may be made.
This reporthbeen prepared for the exclusive use by our client for design purposes. We âreM
responsible futechnical interpretations by others of our information. As the project evolveso wrc
should provüdecontinued consultation and field services during construotion to review and
monitor the@úementation of our recommendations, and to veriry that the recommendations
have been q4ropriately interpreted. Significant design changes may require additional analpüs
or modificdärr$ to the recommendations presented herein. We recommend on-site observatim
of excavatim md foundation bearing strata and testing of structural fill by a representative of
the geotechñcal engineer.
Respectfu$rSubmitted,
Kumar & Associates, Inc.
David A.l{dúoom, Staff Engineer
Reviewedþn
Steven L. Pawdalç P
SLP/kac
b
Kumar & Associates, lnc.ti Project No. 22-7-424
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APPROXIMATE SCALE-FEET
Fig. 1LOCATION OF EXPLORATORY BORINGKumar & Associates22-7 -424
BORING 1 LEGEND
0 ññ
T0PSO|L; ORGANIC SANDY SILT AND CLAY, FIRM, MOIST, DARK
BROIVN,
CLAY (CL); SILTY SANDY, STIFF, MOIST, RED-BR0WN.
50/3
5 30/12
WC=6.8
+4=46
-200=1 6
GRAVTL (GM-GC); COBBLES, POSSIBLE B0ULDERS, SANDY,
SILTY TO CLAYEY, DENSE, MOIST TO WET WITH DEPTH,
BROÌVN.
F-l¡l
l¡JlÀ
I-FfLl¡Jo
I DRTVE SAMPLE, 1 3/8-|NCH t.D. SpLtT Sp00N STANDARD
PENETRÀTION TEST.
10
37 /12 rnr11DR|VE SAMPLT BL0W COUNT. INDICATES THAT 30 BLOWS 0F
110-POUND HAMMER FALLING 30 INCHES WERE REQUIRED
TO DRIVE THE SAMPLER 12 INCHTS.
O,1 DEPTH To wATER LEVEL AND NUMBER OF DAYs AFTER= DRILLING MEASUREMENT WAS MADE.
15
23/12
.--> DEPTH AT WHICH BORING CAVED WHEN CHECKED ON
lvNE 22, 2022.
NOTES
THE EXPLORATORY BORING WAS DRILLED ON JUNE 21, 2022
WITH A 4-INCH DIAMETER CONTINUOUS FLIGHT POWIR AUGER.20
2. THE LOCATION OF THE IXPLORATORY BORING WAS MEASURED
APPROXIMATELY BY PACING FROM FEATURES SHOWN ON THE
SITE PI.AN PROVIDTD.
3. THE ELEVATION OF THE EXPLORATORY BORING WAS NOT
MEASURED AND THE LOG OF THE EXPLORATORY BORING IS
PLOTÏED TO DEPTH.
4, THE EXPLORATORY BORING LOCATION SHOULD BE CONSIDERED
ACCURATE ONLY TO THE DEGREE IMPLIED BY THE METHOD
USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY
BORING LOG REPRESENT THE APPROXIMATE BOUNDARIES
SETWEEN MATERIAL TYPES AND THE TRANSIIIONS MAY BE
GRADUAL.
6. GROUNDWATFR I FVFI SHOWN ON THE LOG WAS MEASURED AT
THE TIME AND UNDER CONDITIONS INDICATED. FLUCTUATIONS
IN THE WATIR LEVEL MAY OCCUR WITH TIME.
7. LABORATORY TEST RTSULTS:
VIC = WATER CONTENT (X) (ISTU D 2216);+4 = PTRCENTAGE RETAINED ON NO. 4 SIEVE
(ASTM D 6et3);
-200 = PERCENTAGE PASSING N0. 200 SIEVE
(ASTM D f110).
22-7-424 Kumar & Associates LOG OF EXPLORATORY BORING Fig. 2
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HYDROMETER ANALYSIS SIEVE ANALYSIS
ÍIYE RËADINGS
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U.S. SIANOARD SERIES
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CLüR SOUANE OPENINCS
at^. 7t^' 1 ttr-
2
a
100
90
a0
70
60
50
40
50
20
10
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to
20
30
40
50
60
70
ao
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100
.150 I
.125
DIAMETER OF IN MILLI
CLAY TO SILT COBBLES
GRAVEL 16 X SAND
LIQUID LIMIT
SAMPLE 0F: Sitþ Cloyey SondY Grovel
38%
PLASTICITY INDEX
SILT AND CIAY 16 %
FROM:Bor¡ngle4.5'
lhc!. lrll rcsull! opply only lo lhr
toñD|.. vhlch v!rc tc¡i.d. Thc
futl¡äo rôporl rholl not b. r.producod,
.rc.pl ln full, wtlhoul lha wrlll.ñ
opprovol ol Kumqr & Attocíqlct, lnc.
Silvr qnolyrk l.lllnq 13 p.rfomad ln
occordonc. slth ASTI¡ 069f3, ASTM D7928,
ASTI¡ C156 ond,/or ASÍY Dl l,ao.
SAND GRAVEL
COARSEFINEMEDTUM lCOlnSr FINE
22-7 -424 Kumar & Associates GRADATION TEST RESULTS Fis. 3