HomeMy WebLinkAboutSubsoil Study for Foundation Design 07.31.2020I (lrt ij,çiffiffüfnr'rÍå *"'
An Employcc Ownod GomPonY
5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax (970) 945-8454
email: kaglenwood@kumarusa. com
www.kumarusa.oom
Offrce Locations: Denvø (HQ), Parker, Colorado Springs, Fort Collhs, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUIIDATION DESIGN
PROPOSED RESIDENCE
LO't 73,SPRrNG RrDGE RESERVE PUD
TBD HIDDEN VALLEY DRIVE
GARFIELD COI-INTY, COLORADO
PROJECT NO. 20-7-3ss
JULY 3l,2o20
PREPARED FOR:
MICHAEL RICE
1120 WESTLOOK DRIVE
GLENWOOD SPRTNGS, COLORADO 81601
adventureau tosales @s uestoffi ce'net
l!
Iä.BLE ON'CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS
ROCKFALL
FTEI,D ÐGTORATION
SUBSURFACE CONDITIONS
DESIGN RECOMMENDATIONS
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
FLOOR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE
LIMITATIONS
FIGURE I - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF ÐGLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE I- SUMMARY OF LABORATORY TEST RESULTS
1
-1-
I
-2-
-J-
.-3 -
-3-
-4-
-5-
-6-
-6-
n
Kumar & Associates, lnc. o Projec{ No. 20-7-355
PTJRPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located at
Lot'13, Spring Ridge Reserve, TBD Hidden Valley Drive, Garfield County, Colorado. The
project site is shown on Figure l. The pu{pose 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 Michael Rice dated |lur:re 19,2020.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils and bedrock obtained dunng
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
The proposed residence is assumed to be a one to two-story structure. Ground floor will be slab-
on-grade. Grading for the structure is assumed to be relatively minor with cut depths between
about 5 to l0 feet. We ¿ßsume relatively light foundation loadings, typical of the proposed type
of construction.
When building location, grading and information have been we
notified to re-evaluate the recommendations in this
SITE CONDITIONS
The site was vacant and vegetated with grass and weeds, with scrub oak and junipertrees uphill
to the northeast. The lot down to the southwest at grades of l0 to 20 percent. Red
sandstone rock outcrops were observed on the hillside above the site. Nearby lots are developed
Kumar & Associates, lnc. o Projec't No. 20-7-355
*
-2-
withlto2story residences. An abandoned irrigation ditch crosses the site near the uphill side of
the building envelope.The slopes above the irrigntion ditüh are 30 (o 40 ¡rercent clown to the
southwest.
-
ROCKF'ALL
The hillside above the irrigation ditch consists of shallow, rocþ colluvium above sandstone
bedrock of the Maroon Formation. The ground surface slopes at about 40 to 45olo (22" to 24")
vúich roughly coinsidcs with thc bedding dip of the formation rock. The colluvium thickens at
the base of the hillside where the ditch is located. Scattered, mostly flat shaped rock fragments
typically up to I to 2 feet in size are exposed on the hillside to the top of ridge about 140 feet
above the building area.
No evidence of ro area was observed and of
inttre orooosed buil dins area, in our opinion, is not warranted.The existing abandoned
Pl^"þ
Naleditcha catchment area for small rock fragments that may slide down the
hillside by erosion and should be kept above the residence for a width of at least 10 feet.
F'IELD EXPLORATION
The field exploration f'or the project was conducted on Jwte22,2020. 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, Inc.
Sarnples of the su'osoils were taken wrth a 2 inch I.D. spoon sampler. The sampler was 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-I586. The
penetration resistance values are an indication of the relative density of the subsoils and hardness
of the bedrock. 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 retumed to our
laboratory for review by the project engineer and testing.
Kumar &Associates, lnc. o Projec't No. 20.7-355
-J-
SUBSTJRF'ACE COhIDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils consist of about lYz feet of topsoil overlying l2Yz to 16% feú of medium dense, silty
sand and sandy silt. Hard sandstone bedrock of the Maroon Formation was encountered at
depths of 14 to 18 feet dor¡¿n to the maximum depth explored of 22Yz feet where refusal to drill
augers was encountered. Drilling in the bedrock with auger equipment was difficult due to rock
hardness and drilling refusal wÍN encountered in the bedrock at Boring 1.
Laboratory testing performed on samples obtained from the borings included natural moisture
content, density and percent finer than sand size gradanon analyses. Results of swell-
consolidation testing performed on relatively undisturbed drive samples of the silty sand,
presented on Figure 4, indicate low to moderate compressibility under conditions of loading and
wetting with a low collapse potential vr¿hen wetted under light load. The laboratory testing is
summarized in Table 1.
No free wate¡ was encountered in the borings at the time of drilling and the subsoils were
slightly moist.
DESIGN RECOMMEIIDATIONS
FOI.INDATIONS
Considering the subsurface conditions encoturtered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural sand and silt soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
l) Footings nlaced on the rmdistu¡'bed natural snilc qhorrld he designed for an
allowable bearing pressure of 1,500 psf. Based on experience, we expect
settlement of footings designed and constructed as discussed in this section will
be about I inch or less. There could be additional settlement of around % to
I inch if the bearing soils are wetted.
Kumar & Associates, lnc. @ Project No. 20-7-355
2)
-4-
The footings shoulcl have a minimum width of l8 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-þches below exterior grade is typically used in this
areâ".
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 refaining sffuetr:res should also be designed to resist
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of this report.
All existing 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.
A representative of the geotechnical engineer shoulcl observe all footing
5)
6)
excavattons to to evaluate conditions.
FOIJNDATION 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 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 fi.ilI 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, traffic, construction materials and equipment. The
pressures recommended above assume drained oonditions behind the walls and a horizontal
backfill sur.f¿ce. The buildup ol water belùrd a w¿rll CIr är 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
4)
Kumar & Associates, lnc. o
buildup behind walls.
Project No. 20.7.355
-5-
Backfill should be placed in uniform lifts and compacted to at least 90% of the maxrmum
standard proctor density at a moisture content near optimum. Backfill in pavement and walkway
areas should be compacted to at leastg1Yoof 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 correctþ, 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 coefiïcient of friction of 0.3 5 . Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight o1325 pcf. The
coeffrcient 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 u*rich 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.
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 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 50% retained on the No. 4 sieve
and less than2Yopassing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at leastg5o/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.
Kumar & Associates, lnc. o Project No. 20-7-355
-6
We recommend retarders conform to at least the of ASTM 81745
Class C matori-al. Certain floor types âre more sensitivc to water vapor transmissiorr thsn others..æ-.'-¿
For floor slabs bearing oq_q4gular gravel or where flooring system sensitive to water vapor
transmission are utilized, wc rccommend a vaoor barrier be utilized conformins to the minimum
F1745 Class A mæerial. The vapor retarder should be installed in
accordance with the manufacturers' recommendations and ASTM F1643
UNDERDRAIN SYSTEM
Although free water \Ã/as not encountered during our exploration, it has been our expenence in
mountainous areas that local perched groundwater can develop during times of heavy
precipitation or seasonal runoff. Frozen ground during spring runoff can also create a perched
condition. We recommend below-grade construction, such as
æ4
basement be from 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 TYoto
a suitable gravity outlet. 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 lYzfeet 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.
SURFACE DRAI}{AGE
The following drainage precautions should be observed during construction and maintained at all
times afTer the residence has been completed:
1) Inundation of the foundation excavations and underslab areas should be avoided
cluring constnlcfion.
2) Exterior backfill should bc adjustcd to ncur optimunr mr:istulc 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 are¿N.
Kumar & Associates, lnc. o Proiect No. 20-7-355
-7 -
3) The ground surface surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend aminimum
4)
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
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. Consideration should be given to use of xeriscape
to reduce the potential for wetting of soils belowthe building caused by irrigation.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principles and practices in this area atthis 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 concemed 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 veri$' that the recommendations
have been appropriately interpreted. Significant design changes may require additional analysis
5)
Kumar & Associates, lnc. o Project No. 20-7-355
-8-
or morüfications to the recommendations presonted herein. We recommend on-site observation
of excavetions and foundation bearing strata and testing of struct¡ral fill by a representative of
the geotechnical engineer.
Respectfr rlly Submitted,
Kumar & A,ssociates, lnc"
Daniel E. Hardin, P
Reviewed by:
Steven L. Pawlak, P.E.
DEHlkac
Kumar & A¡eociatê*, lnc. Ê Project No. 20-7"355
,l
LOT 74
/
,'/'\.\
t
/,f
/
/t\../'
LOT 73
\o
BORING 1
\
,/'\\BORING 2
o\
\\
\\BENCHMARK:
MANHOLE RIM EL.
lOO' ASSUMED
\L
\
\
\
%,ó
\\
\
LOT 72
\\
30 50
APPROXIMATE SCALE_FEET
Fig. 1LOCATION OF EXPLORATORY BORINGSKumar & Associates20-7 -355
t
I
BORING 1
EL. 1 04'
BORINGtL. 111
2
0 o
13/12
5 5
13/ 12
WC=6.2
DD=1 15
12/12
WC=4.0
DD=l 1 I
-200=34
10 l0
F-
l¿l
LJIr
!It-ô-
t¡Jô
15/12
WC=5.5
DD=1 17
-200=5 1
11/12
WC=6.5
DD= 1 05
¡-t¡l
L'J
lL
I-t-û-
t¡Jô
15 1517/12
WC=E,0
DD=1 13
-200=69
30/12
2A 2050/3 s5/8, 1o/o
25 25
20-7-355 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fîg. 2
D
N ropsoru sAND AND stLT, oRGANtc MATTER, FIRM, sLIcHTLY Molsr, RED/BRowN
N
SAND (SM); SILTY TO SANDY SILT, MEDIUM DENSE, SLIGHTLY MOIST, RED/BROWN.
SANDSTONE/SILSTONE BEDROCK; HARD T0 VERY HARD, SLIGHTLY MOIST. RED.
DRIVE SAMPLE, 2_INCH I.D. CALIFORNIA LINER SAMPLE.
1s/1z PXli'^3^Si'-.i',1.'i"-?-I,åt?åEå'iã'JHJ.'i,P'3lu',lJ*ii iffiË8$D HAMMER
t PRACTICAL AUGER REFUSAL
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JUNE 22, 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'
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE MEASURED BY HAND LEVEL AND REFER
TO THE SEWER MANHOLE ON HIDDEN VALLEY DRIVE AS ELEVATION 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
AppRoxtMATE eoulo¡RlEs arrwrrN MATERTAL TYPES AND THE TRANSlrloNs MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE BORINGS AT THE TIME OF DRILLING'
7. LABORATORY TEST RESULTS:
WC = WATER CONTENT (%) (ASTM OZZIø);
DD = DRY DENSITY (pct) (asrv ozzta);
_2OQ= PERCENTAGE PÀSSINO NO.2OO SIEVE (ASTM D1140).
Fîg. 3LEGEND AND NOTESKumar & Associates20-7-355
E
¡
:d
è
SAMPLE OF: Very Silly Sond
FROM:Boringl@5'
WC = 6.2 %, ÐD = 'l 15 pcf
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
APPLIED PRESSURE - KSF
JJ
l¿¡
=1n
I
z.ot-
ô
I()
u'lz.o
J
J
t¡J
=t!',
I
o
F
ô
=olnzoo
1
0
-1
-2
-3
-4
1
0
-1
-2
-5
-4
1.0
1,0
t0
100
SAMPLE OF: Very Silty Sond
FROM:Borlng2@10'
WC = 6,5 %, DD = 105 pcf
h Ir
wlth È45S-
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETÏING
20-7-355 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
rcrf *mimäfÉ;xHfd-""TABLE ISUMMARY OF LABORATORY TEST RESULTSNo.20-7-355Sandy SiltSilty SandVery Silty SandSilt and SandVery Silty SandSOIL TYPE{psDUNCONFII¡EDcoütPRESstvESTRENGTHPLASTICINDEX(%)ATTERBERG LftIITS(o/o)LtoutD LtililT341569PERCENTPASSING NO.200 stEvEtf/"|SANDGRADATION(wGRAVEL118105tt7113(pciNATURALDRYDENSITYlt56.24.06.5558.0(o/olNATURALfi|OISTURECONTENT5010I5I(ñ)DEPTH52ISA¡IPLE LOCATIONBORING