HomeMy WebLinkAboutSubsoil StudylGrtiiffil['trni:ß:r*iiy;*"'
An Employcc Owncd 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 Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT IS-4, ASPEN GLEN, FILTNG 2
INDIAN PAINTBRUSH
GARFIELD COUNTY, COLORADO
PROJECT NO.20-7-665
DECEMBER 7,2020
PREPARED FOR:
RED HOUSE ARCHITECTURE
ATTN: BRUCE BARTH
815 BLAKE AVENUE
GLENWOOD SPRINGS, COLORADO 81601
bruce@,redhousearchitectu re. com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION ........
SITE CONDITIONS
SUBSIDENCE POTENTIAL
FIELD EXPLORATION
SUBSURFACE CONDITIONS
FOLTNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOUNDATIONS.
FOUNDATION AND RETAINTNG WALLS
FLOOR SLABS
LTNDERDRAIN SYSTEM
SURFACE DRAINAGE
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 . LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - GRADATION TEST RESULTS
I
I
I
1-L-
a
aJ-
....- 3 -
-?
a
-A
_{
..........- 6
..........- 6
Kumar & Associates, lnc, o Project No.20-7-665
PURPOSE AND SCOPE OF STUDY
This reportpresents the results ofa subsoil study for a proposed residence to be located on
Lot IS-4, Filing 2, Aspen Glen, Indian Paintbrush, 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 Red House Architecture dated October 30,2020.
A flreld 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 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
The proposed residence will be a 1 and 2-story structure with ground floors of slab-on-grade in
the garage and basement or structural above crawlspace in other living areas. Grading for the
structure is assumed to be relatively minor with cut depths between about 3 to 9 feet. We
assume relatively light foundation loadings, typical of the proposed type of construction.
If building location, grading or loading information is significantly different than described, we
should be notified to re-evaluate the recommendations presented in this report.
SITE CONDITIONS
The site was vacant and clear of snow cover at the time of our field exploration. The ground was
vegetated with grass and weeds. The site is located in the valley bottom with the terrain gently
sloping generally down to the north with elevation difference of around 2 feet in the building
footprint shown on Figure 1. The nearby lots are developed with 1 and 2-story, single family
residences.
Kumar & Associates, lnc. @ Project No.20-7-665
a-L-
SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen Subdivision.
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 Eag\e Valley Evaporite underlie portions of the lot. 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, several sinkholes were observed
scattered throughout Aspen Glen, mainly east of the Roaring Fork River. These sinkholes appear
similar to others associated with the Eagle Valley Evaporite in areas of the middle to lower
Roaring Fork River valley.
Sinkholes were not observed on 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 IS-4 throughout the service life of the proposed residence, in our opinion, is low; however,
the owner should be made 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 November 30,2020. 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-458 drill rig. The borings were logged by a representative of Kumar &
Associates, Inc.
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
Kumar & Associates, lnc. @ Project No. 20.7-665
-3-
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, below about one foot of topsoil, consist of 2% to 3 feet of very stiff/medium dense, silt
and sand underlain by dense, slightly silty sandy gravel and cobbles with probable boulders to
the depths explored of 9% to 1 1 feet. 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 gradation analyses. Results of gradation analyses performed on a small diameter
drive sample (minus l%-inch fraction) of the coarse granular subsoils are shown on Figure 4.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
FOUNDATION BEARING CONDITIONS
The natural silt and sand soils encountered in about the upper 3 feet of the borings possess low
bearing capacity and variable compressibility potential when loaded and wetted. The underlying
gravel soils possess moderate bearing capacity and typically low settlement potential. At
assumed excavation depths, the subgrade could expose either materials. Spread footings placed
on the natural soils should be feasible for foundation support of the residence. The
compressibility potential of the silt and sand soils should be further evaluated at the time of
excavation. To reduce the risk of differential movement due to the variable bearing conditions,
the footings could be extended down to the natural gravel soils.
DESIGN RECOMMENDATIONS
FOTINDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread lootings bearing
on the natural soils.
Kumar & Associates, lnc, @ ProJect No.20-7-665
-4-
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 2,000 psf. Footings placed entirely on the natural
dense granular subsoils can be designed for an allowable bearing pressure of
3,500 psf. Based on experience, we expect initial settlement of footings
constructed on the natural soils will be about 1 inch or less and could be
differential between the upper fine-grained soils and underlying gravel soils.
2) The footings should have a minimum width of 16 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 36 inches below exterior grade is typically used in this
area.
4) Continuous foundation walls should be reinforced top and bottom to span local
anomalies 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 of this report.
5) The topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to the firm natural soils. The exposed soils in
footing area should then be moistened and compacted.
6) A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
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 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
Kumar & Associates, lnc. @ Project No. 20-7-665
5-
of at least 40 pcf for backfill consisting of the on-site soils. Backfill should not contain organics,
debris or rock larger than about 6 inches.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, trafhc, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and a horizontal
backflrll 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% of the maximum
standard Proctor density at a moisture content near optimum. Backfrll placed in pavement and
walkway areas should be compacted to at least 95o/o 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 corectly, 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 coefficient of friction of 0.35 for silt and sand soils and 0.50 for gravel soils. Passive
earth pressure of compacted backfill against the sides of the footings can be calculated using an
equivalent fluid unit weight of 450 pcf for gravel backfill. 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 a granular material compacted to at least 95o/o of the maximum standard
Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, appear suitable to support lightly loaded slab-on-
grade construction with low settlement risk. To reduce the effects of some differential
Kumar & Associates, lnc, @ Project No.20-7-665
-6-
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 basement level
slabs to facilitate drainage. This material should consist of minus 2-inch aggregate with at least
50o/o retained on the No. 4 sieve and less than 2Yo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95o/o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site granular soils devoid of vegetation, topsoil and oversized rock.
LTNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, 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, 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 1 foot below lowest adjacent finish grade and sloped at a minimum l%oto
a suitable gravity outlet or drywell based in the underlying granular soils. Free-draining granular
material used in the underdrain system should contain less than 2o/o 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 l% feet deep.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
1) Inundation ofthe foundation excavations and underslab areas should be avoided
during construction.
Kumar & Associates, lnc. @ Project No.20-7-665
-7 -
3)
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 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 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.
Roof downspouts and drains should discharge well beyond the limits of all
backfill.
Landscaping which requires regular heavy irrigation should be located at least 5
feet from foundation walls.
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 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 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
2)
s)
Kumar & Associates, lnc. @ Project No, 20-7.665
-8-
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 strata and testing of structural fill by a rqtresentative of
the geotechnical engineer.
Respectfu lly Submitted,
Kumar & Associates, Inc.
Steven L. Pawlak, P
Reviewed by:
Daniel E. Hardin, P.E.
SLPlkac
/
û 15222
Kumar & Associates, lnc.'iu Project No, 20-7-665
-a
.E
I
!
i!f:
BENCHMARK:
UTILITY BOX BASE
ELEVATION 100,, ASSUMED
1
APPROXIMATE SCALE_FEET
20-7 -665 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
¡
r
€
BORING 1
EL. 99.5'
BORING 2
EL. 98'
0 0
21/12
38/12
5
1o/ 12 5
Ft¡l
L¡Jl!
ITt-fL
l¡Jâ
31/12 t--l¡J
l¿¡l¡-
I
:Ef-Èulô
60/ 12
WC=2.6
+4=47
-2AQ=1 1
10 1050/4.5
15 15
LOGS OF EXPLORATORY BORINGS Fig. 220-7 -665 Kumar & Associates
LEGEND
N
TOPSOIL; ORGANIC SAND AND SILT, SCATTERED GRAVEL, FIRM, BROWN.
n SAND AND SILT (SM-ML); MEDIUM DENSE/VERY STIFF, SLIGHTLY MOIST, RED
BROWN.
SAND, GRAVEL AND COBBLES (GM-GP); SLIGHTLY SILTY TO SILTY, MEDIUM
DENSE TO DENSE, SLIGHTLY MOIST, BROWN, ROUNDED ROCK.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
i DRTVE SAMPLE, 1 S/ï-|NCH r.D. SPLIT SPOON STANDARD PENETRATION TEST
ıa t.ı DRIVE SAMPLE BLOW COUNT. INDICATES THAT 21 BLOWS 0F A 14O-POUND HAMMERzt/ t¿ FALLTNG go TNcHES wERE REQUIRED To DRtvE THE SAMpLER t2 tNcHEs.
I enacrrcAL AUcER REFUSAL.
NOTES
I. THE EXPLORATORY BORINGS WERE DRILLED ON NOVEMBER 30, 2O2O WITH A 4-INCH DIAMETER
CONTINUOUS-FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY TAPING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
5. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFER
TO THE BENCHMARK ON FIG. 1.
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 ÏIME OF DRILLING
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM D2216)I
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
-2OQ= PERCENTAGE PASSING NO, 200 SIEVE (ASTM 01140).
20-7 -665 Kumar & Associates LEGEND AND NOTES Fig. 5
E
s
Ê
SIEVE ANALYSISHYDROMETER ANALYSIS
t1 1
CLEAR SOUARE OPENINOS
ttâ' zf/' t Itr'lMtx a2
å¡l HRS 7 HRSa5 vtN t5 ltN
ÎIYE RilDINGS
aovttr igutN ¿utN
I
I I /
I
i /
I
i
I
i
/
I L
--..1-
i
I
i
l
I
I
I
_{-
ø,
ã
H
t00
90
to
70
ao
50
10
t0
20
t0
0
fo
20
30
Æ
30
80
70
80
e0
t00
Ieh
EI
Ë
DIAMETER OF
CLAY TO SILT COBBLES
GRAVEL 47 % SAND
LIQUID LIMIT
SAMPLE OF: SÌlty Sondy Grovol
42
'(
PLASTICITY INOEX
SILT AND CLAY 11 X
FROM:Borlng1O7'
fhrlc lctl ralullr qpply only to lh!
somplcr whlch w€ro lcllld, Tha
lcallng r.porl rholl nol be rcproduccd,
oxcopl ln full, wllhoul lho wrllllnqpprcvol of Kumqr & Asloclqlls, lna,sl.v. onolysl! l.allng ls parformrd ln
qocordqnoe wlth ASTM D6915, ASTM 07928'
ASTM C'158 ond/or ASTM Dfl,lo.
GRAVELSAND
MEDIUM COARSE FINE COARSEFINE
20-7 -665 Kumar & Associates GRADATION TEST RESULTS Fig. 4