HomeMy WebLinkAboutSubsoils Report for Foundation Designl(+rt l(umr & AssoGiates. Inc.e
Geotechnical and Materials Engineers 5020 Countv Road 154
and Environmental Scientists Glenwood Springs. CO 8160!
phone: (970) 945-798t
fax: (970) 945-8454
email : kaglenwood@kumarusa.corp-
An EmplOyCC CnrnCd Cgmpgny www.kumafusa.co*!
Offrce Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOT]NDATION DESIGN
PROPOSED RESIDENCE
LOT 55, FTLTNG 8, ASPEN GLEN
589 SADDLEBACK ROAD
GARFTELD COUNTY, COLORADO
PROJECT NO.20-7-610
NOVEMBER12,2020
PREPARED FOR:
JORDAI\ ARCHITECTURE
ATTN: BRAD JORDAN
P.O. BOX 1031
GLENWOOD SPRINGS, COLORADO 81602
:...':
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY..
PROPOSED CONSTRUCTION
SITE CONDITIONS
SUBSIDENCE POTENTIAL
FIELD EXPLORATION...
SUBSURFACE CONDITIONS
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOUNDATIONS
FO{'NDATION AND RETAINING WALLS ...
FLOOR SLABS
UNDERDRAIN SYSTEM ..............
SURFACE DRAINAGE
LIMITATIONS
FIGURE I - LOCATION OF DGLORATORY BORINGS
FIGURE 2 - LOGS OF DGLORATORY BORINGS
PIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Asgociates, lnc. o Project No.20-7.610
PT]RPOSE AI\D SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot 55, Filing 8, Aspen Glen, 589 Saddleback Drive, 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 Jordan Architecture dated October 13, 2020.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples ofthe 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 analyzedto 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
Design plans for the residence had not been developed at the time of our study. In general, we
assume the proposed residence will be a two-story structure over crawlspace or basement level
with slab-on-grade garage floor. Grading for the structure is assumed to be relatively minor with
cut depths between about 4 to 8 feet. We assume relatively light foundation loadings, typical of
the proposed type of construction.
When building loadings, location and grading plans have been developed, we should be notified
to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The lot was vacant at the time of the field exploration. The terrain is relatively flat with a slight
slope down to the northeast with about 2 feet of elevation difference across the building area.
The ground surface is natural with minimal grading from road construction. Vegetation consists
of grass and weeds. Nearby buildings include one and two-story, single family residences.
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the subject site and the
nearby areas of the Aspen Glen Development. These rocks are a sequence of gypsiferous shale,
fine-grained sandstone and siltstone with some massive beds of gypsum and limestone. There is
a possibility that massive gypsum deposits associated with the Eagle Valley Evaporite underlie
portions of the property. Dissolution of the gypsum under certain conditions can cause sinkholes
to develop and can produce areas of localized subsidence. During previous work in the area,
sinkholes have been observed scattered throughout the Roaring Fork River valley. These
sinkholes appear similar to others associated with the Eagle Valley Evaporite in other areas of
the lower Roaring Fork River valley. The nearest mapped sinkhole is located about %mile
southeast of this lot.
Sinkholes were not observed in the immediate area of the subject lot. No evidence of cavities
was encountered in the subsurface materials; however, the exploratory borings were relatively
shallow, for foundation design only. Based on our present knowledge of the subsurface
conditions at the site, it cannot be said for certain that sinkholes will not develop. The risk of
future ground subsidence on Lot 55 throughout the service life of the proposed residence, in our
opinion, is low and similar to other lots in Aspen Glen; however, the owner should bc aware of
the potential for sinkhole development. If further investigation of possible cavities in the
bedrock below the site is desired, we should be contacted.
FIELD EXPLORATION
The field exploration for the project was conducted on October 16,2020. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
The borings were advanced with 4-inch diameter continuous flight augers powered by a truck-
mounted CME-458 drill rig. The borings were logged by a representative of Kumar &
Associates-
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-1586. 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.
Kumar & Associates, lnc. @ Projecl No.20-7.61c
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STIBSURFACE CONDITIONS
Graphic logs ofthe subsurface conditions encountered at the site are shown on Figure 2. T\e
subsoils encountered, below about Yz-foot of topsoil, consist of 10 to 12 feet of stiff to very stiff,
sandy silty clay overlying slightly silty to silty sand and gravel with cobbles down to the
maximum depth explored of 16 feet. Drilling in the coarse granular subsoils with auger
equipment was relatively difficult due to the cobbles and possible boulders in the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and finer than sand size gradation analyses. Results of swell-consolidation
testing performed on relatively undisturbed drive samples, presented on Figure 4,indicate low to
moderate compressibility under conditions of loading and a minor expansion potential when
wetted under a constant light surcharge. The laboratory testing is summarizedin Table 1.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The upper clay soils appear to possess a minor expansion potential when wetted but it has been
our experience with the developed lots on Saddleback Road that the soils can also be prone to
settlement when wetted. The variable expansion/compression potential could result in
movement of footings bearing on the soils if they become wetted. Surface runoff, landscape
irrigation, and utility leakage are possible sources of water which could cause wetting. A lower
risk alternative would be to place the foundation entirely on the underlying relatively dense
gravel soils or remove and replace a certain depth of the clay soils with compacted structural fill.
The subgrade should be observed for bearing conditions and further evaluated for
heave/settlement potential at the time of construction.
DESIGN RECOMMENDATIONS
FOI.JNDATIONS
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 at least
3 feet of compacted structural fill or on the natural gravel subsoils. If a deep foundation is
proposed, we should be contacted for additional recommendations.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
Kumar & Associates, lnc. @ Project No. 20-7-610
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r)Footings placed on at @er the clay soils should be
designed for an allowable bearing pry1g!!99!grf. Based on experience, we
expect initial settlement of footings designed and constructed as discussed in this
section will be about 1 inch or less. There is a heave/settlement potential for the
clay soils if they were to become wetted. The movement would be differential
and could be around an additional I inch for a wetted depth on the order of 8 feet
below footing bearing level. use of a full depth basement would reduce the
movement potential with less depth of clay soils below the foundation bearing
level.
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 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 should also be designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of this report.
The topsoil and any loose or disturbed soils should be removed to expose the
natural clay soils. The clay soils should be removed for 3 feet below footing
grade and the design bearing level re-established with compacted structural fill to
reduce settlementlheave potential. The fill can consist of the onsite excavated
soils or a relatively well graded granular material such as CDOT Class 6 (%-inch)
road base compacted to at least 98% of standard proctor density at a moisture
content near optimum. The fill should extend laterally beyond the footing a
distance at least equal to one-half the depth of fill below the footing.
A representative of the geotechnical engineer should test the structural fill during
placement for compaction and observe all footing excavations prior to concrete
placement to evaluate bearing conditions.
3)
4)
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
2)
s)
6)
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computed on the basis of an equivalent fluid unit weight of at least 55 pcf for backfill consisting
of the on-site fine-grained 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 fine-grained 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 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 90Vo 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 over-compact the backfill or use large equipment near the wall, since
this could cause excessive lateral pressure on the wall. Some sefflement 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 0.40. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The
coefficient of friction and passive pressure values tecommended 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 strenglh, particularly in the case of passive resistance. Fill placed against
the sides of the footings to resist lateral loads should be compacted to at least 95Yo of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-on-grade
construction with a risk of movement and distress if the bearing soils become wetted. The risk of
slab movement can be reduced by removing the clay soils and placing at least 2 feet of
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compacted structural fill, such as road base, below the slab. 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 controljoints 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 than 2% 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
onsite soils, %-inch road base or imported relatively well graded granular soil devoid of
vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
this area and where there are clay soils that local perched groundwater can develop during times
ofheavy precipitation or seasonal runoff. Frozen ground during spring runoffcan create a
perched condition. We recommend below-grade construction, such as retaining walls,
crawlspace and basement areas, be protected from wetting and hydrostatic pressure buildup by
an underdrain system.
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 I foot below lowest adjacent finish grade and sloped at a minimum lYoto
a suitable gravity outlet or sump and pump. Free-draining granular material used in the
underdrain system should contain less than 2% 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 LYz feet deep. 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.
STIRFACE DRAINAGE
Proper grading and surface drainage will be critical to limit potential for wetting below the
foundation and building distress. The following grading and drainage precautions should be
observed during construction and maintained after the residence has been completed:
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l)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 95Yo of the maximum standard Proctor density in pavement and slab areas
and to at least 90Vo 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 l0 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site soils to
reduce surface water infiltration.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Landscaping which requires regular heavy irrigation should be located at least
10 feet from foundation walls. Consideration should be given to use of xeriscape
to reduce the potential for wetting of soils below the building caused by inigation.
3)
4)
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 borings drilled at the locations 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
practice should be consulted. Our findings include interpolation and extrapolation ofthe
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 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 veriff that the recommendations
2)
s)
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have been appropriately interpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. We recommend on-site observation
of excavations and foundation bearing stata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kumar & Associates, fnc.
Steven L. Pawlak, P.E.
Reviewed by:
h !
Daniel E. Hardin,P.E'
SLPikae
Kumar & Associates, lnc. o Proiect I'1o.20-7-616
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(BASIS OF BFAR/NC)
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APPROXIMATE SCALE_FEET
LOCATION OF EXPLORATORY BORINGS Fig. 120-7 -il4 Kumar & Associates
-5
BORING 1
EL. 6060'
BORING 2
EL. 6058'
0 0
15/ 12
1s/ 12
WC=8.7
DD=1 O2
5 515/12
WC=6.7
DD=1 1 1
-2AO=82
12/12
WC=1 1.2
DD=1 14
-200=88
Flrl
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TLlrlo
10 10
FtrltdtL
I-F(L
Ld6
17/12
WC=8.5
DD=1 1 4
-200=80
12/6, 30/6
15 1550/2 50/6
20 20
20-7 -ilA Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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LEGEND
N
TOPSOIL; ORGANIC SANDY SILT AND CLAY, SLIGHTLY MOIST, BROWN
CLAY (CL); SILTY, SANDY, STIFF TO VERY STiFF, SLTGHTLY MOIST, BROWN, LOW PLASTiCITY,
CALCAREOUS TRACES.
SAND AND GRAVEL
BROWN, ROUNDED
(SM-GM); SLIGHTLY SILTY TO SILTY, COBBLES, DENSE, SLIGHTLY MO|ST,
ROCK
F
I
DRIVE SAMPLE. z-INCH I.D. CALIFORNIA LINER SAMPLE
DRTVE SAMPLE, 1 3/9-|NCH r.D. SPLIT SPOON STANDARD PENETRATION TEST.
.E r14 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 15 BLOWS 0F A 140-POUND HAMMER''l '' FALLTNG g0 INcHEs wERE REQUTRED To DRrvE THE SAMPLER 12 tNcHES.
NOTES
1 THE EXPLORATORY BORINGS WERE DRILLED ON OCTOBER 16,2020 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 INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
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 WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING.
7. LABORATORY TEST RESULTS;
WC = WATER CONTENT (%) (ASTM D2216):
DD = DRY DENSITY (pcf) (ASTM D2216);
-200= PERCENTAGE PASSING NO. 200 SIEVE (ASTM D1140).
2A-7 -610 Kumar & Associates LEGEND AND NOTES Fig. 3
SAMPLE OF: Sondy Silty Cloy
FROM:Borlngl@5'
WC = 6.7 %, DD = 111 pcf
-2OQ = 82 %
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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100t0
t0
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2
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SAMPLE OF: Sondy Sill ond Cloy
FROM:Boring2@2.5'
WC = 8.7 %, DD = 102 pcf
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NO MOVEMENT UPON
WETTING
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100t.0
20-7-610 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
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TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
SOIL TYPE
Sandy Silty Clay
Sandy Silty Clay
Sandy Silt and Clay
Slightly Sandy Silty Clay
losl)
TJNCONFINED
COMPRESSIVE
STRENGTH
P|-ASItC
INDB(
lol
ATTERBERG LIMITS
lo/"1
UQUID LIMIT
PERCEI{T
PASSING NO.
200 stEVE
82
80
88
l%t
SAND
(vt
GRAVEL
NATUML
DRY
DENSTTY
{ocfl
111
Lt4
t02
tr4.2l1
Pl
NATURAL
MOFTURE
CONTENT
6.7
8.5
8.7
0I
2%
5
{ftt
DEPTH
5
2
SAMPLI
BORING
1
No.20-7-610