HomeMy WebLinkAboutSubsoil Study for Foundation Design 01.08.2021I (+rt åifrli#lfÉffn i'iÍå
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An Employcc Owncd Compony
5020 Cowrty Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax (970) 945-8454
email: kaglorwood@liumarusa.com
www.kurnarusa.com
Offrce L¡cations: Denvø (HQ), Parker, Color¿do Springs, Fort Collins, Glenwood Spriugs" and Summit Coünty, Colorado
SUBSOIL STUDY
F'OR X'OT]I{DATION DESIGN
PROPOSED RESIDENCE
LOT I.6O SPRING RIDGE RESERVE
125 HIGHPOINT DRTVE
GARFIELD COUNTY, COLORADO
PROJECT NO. 20-7-759
JANUARY 8,2021
PREPARED F'OR:
TREVOR RUONAVAARA
160 SPRING RIDGE DRIVE
GLENWOOD SPRINGS, COLORADO 81601
tdinishes@omail.com
TABLE OF CONTENTS
PI]RPOSE AND SCOPE OF STI]DY
PROPOSED CONSTRUCTION
SITE CONDITIONS
FIELD ÐGLORATION
SUBSURFACE CONDITONS
FOUNDATION BEARING CONDITIONS
DESIGN REC OMMENDATIONS
FOUNDATIONS
FOI.]NDATION AND RETAINING WALLS
FLOOR SLABS......
UNDERDRAIN SYSTEM
SURFACE DRAINAGE
LIMITATIONS
FIGURE 1 - LOCATION OF ÐGLORATORY BORINGS
FIGURE 2 . LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS
TABLE I- SUMMARY OF LABORATORY TEST RESULTS
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Kumar &Associates, lnc. o Projec{ No. 20-7-759
PURPOSE AND SCOPE OF'STT]DY
This reportpresents the results ofa subsoil study for aproposed residenceto be located on
Lot 16, Spring Ridge Reserve, 125 Highpoint Drive, Garfield Counfy, Colorado. The project site
is shown on Figure l. The purpose of the study was to develop recommendations for the
for¡ndation design. The study was conducted in accordance with our agreement for geotechnical
engineering services to Trevor Ruonavaara dated December 11,2020.
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
The proposed residence will be a single story wood frame structure with attached garage.
Ground floor will be slab-on-grade for the garage and structural over crawlspace for the living
areas. Grading for the structure is assumed to be relatively minor with cut depths between about
2to 5 feet. We assume relatively light foundation loadings, typical of the proposed type of
constructlon.
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 vacant at the time of our fïeld exploration. The ground surface is gently
sloping dor¡¿n to the northeast at a grade of about 5 percent with about 4 feú of elevation
difference across the proposed building fooþrint. Vegetation consists of grass and weeds.
Kumar &Associates, lnc. o Projec't No. 20-7-759
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F'IELD EXPLORATION
The field exploration for the project was conducted on December 16, 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 wrth l% 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-I586.
The penetration resistance values are an indication of the relative density or consistency of the
subsoils and hardness of the bedrock. Depths at which the samples were taken and the
penetration resistance vah¡es are shown on the Logs of Exploratory Rorings, Figure 2. The
samples were retumecl to our laboratory for review by the project engineer ancl testing.
S U BS IJRX'AC E COND TT I O N S
Graphic logs of the subsurface conditions encountered at the site are shor,¡vn on Figure 2. The
subsoils consist of about 1 foot of topsoil overþing very stiff, sandy clay to between I I and
12 feet in depth further underlain by medium dense, clayey silty sand to a depth of between
24Yz and26 feet. Sandstone bedrock was encountered in Boring I at a depth of 37% feet and in
Boring 2 at24Yz feet deep. A layer of clay was encountered between the sand and sandstone in
Boring 1 from 26 to 37Vz feet deep.
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 perÊormed on relatively undisturbed drive samples, presented on Figures 4 and5, indicate
low to moderate compressibility under existing moisture conditions and light loading and a
minor to low expansion potential r¡¡hen wetted. The laboratory testing is summarized in Table 1.
No free water was encountered in the borings at the time of drilling ancl the subsoils were
slightly moist to moist.
Kumar & Associates, lnc. o Projec't No. 20-7.759
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F'OTJIIDATION BEARING CONDITIONS
The upper clay soils encountered in the borings possess a low bearing capacity and possibly a
low swell potential vrhen wetted. The low swell potential should not affect foundation design
but it should be evaluated at the time of excavation for possible mitigation such as sub-
excavation and placing structural fill. The underþing sand soils possess a low bea¡ing capacity
and typically low settlement potential, The sandstone bedrock encountered at depth in the
borings possesses a moderate to high bearing capacity and low settlement potential. A spread
footing foundation system placed on the upper clay soils can be used for support of the proposed
residence with a risk of foundation movement, mainly if the bearing soils become wetted. A
lower risk option would be to replace expansive clay soils with compacted structure fill if needed
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 with a risk of foundation movement.
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 1,500 psf. Based on experience, we expect
movement of footings designed and constructed as discussed in this section will
be about I inch or less.
2) The footings should have aminimum width of 16 inches for continuous walls and
2 feú 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
aÍe4".
4) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies such as by assuming an unsupported length of at least 12feet.
Foundation walls acting as retaining structures should also be designed to resist
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lateral earth pressures as discussed in the "-þoundation and Retaining Walls"
section ofthis report.
Topsoil and any loose disturbed soils should be removed and the footing bearing
level extended dor¡vn to the undisturbed natural soils. The exposed soils in footing
areas should then be moistened and compacted. Structural fill should consist of
3/¿-inch road base compacted to at least 98o/o of staridard Proctor density at near
optimum moisture content.
A representative ofthe geotechnical engineer should observe all footing
excavations prior to concrete placement to evalnate bearing conclitions
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 55 pcf for backfrll 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 footrngs, traffic, construction materials and equipment.
The pressures recommended above ¿ßsume drained conditions behind the walls and a horizontal
backfill surÊace. 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 hyclmstatiu pressure builtlup 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 slightly above optimum. Backfill placed in
pavement and walkway areas should be compacted to at leastgl%oof 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 settlemcnt of
deep foundation wall backfïll should be expected, even if the rnaterial is placed correctly, and
could result in distress to facilities constructed on the backfrll.
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Kumar & Associates, lnc. o Project No. 20-7-759
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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 0.3 5. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 350 pcf. The
coefiîcient 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 least 95%o of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural clay soils encountered on the site possess a low swell potential when wetted. The
low swell potential should be further evaluated at the time of construction for mitigation methods
such as sub-excavation and placing structural fill but'should not impact slab design.
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 relatively well
graded sand and gravel such as road base should be placed beneath slabs for support. This
material should consist of minus 2-inch aggregate with at least 50% retained on the No. 4 sieve
and less than 72%o passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least95Yo of maximum
standard Proctor density at a moisture content near optimum. Required fill should consist of
granular soils devoid of vegetation, topsoil and oversized rock.
UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area and lvhere there are clay soils that local perched groundwater can develop during times
of heavy precipitation or seasonal runoff. Frozen ground during spring runoffcan create a
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perched condition. We recommend below-grade construction, such as retaining walls and
crowlspoce orcos, bc protcctcd from wctting and hydrostotic prcssurc 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 minimumlYoto
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.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times a"fter the residence has been completed:
1) hrrurdation of the foundation excava[ions and turderslab areas should be avoided
during construction.
2) 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 90Yo of the maximum standard Proctor density in landscape are¿N.
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 12 inches in the first l0 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
capped with about 2 feet of the on-site 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
l0 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.
Kumar & Associates, lnc. o Projec't No. 20-7-759
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The conclusions and recommendations submitted in this report a¡e 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
presenre, 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 condítions 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 dwing construction to review and
monitor the implementation of our recommendations, and to veri$r 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
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Kunrar & Ass Inc
James H. Parsons, E.l.
Reviewed by:
Steven L. Pawlak,
JHPlkac
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Kumar &,Associates, lnc.Project No, 20-7-759
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20-7 -759 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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LEGEND
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TOPSOIL: CLAY, ORGANICS, ROOTS, FIRM, MOIST, BROWN
CLAY (CL): SANDY, VERY STIFF, SLIGHTLY MOIST, BROWN, LOW PLASTICITY, CALCAREOUS
STREAKS.
sAND (SC-SM): CLAYEy, StLTy, SCATTERED GRAVEL, MEDTUM DENSE, SLtcHTLy MO|ST, RED
BROWN.
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SANDSTONE BEDROCK: HARD TO VERY HARD, SLIGHTLY MOIST, RED. MAROON FORMATION
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
DRIVE SAMPLE, 1 5/8-|NCH t.D. SpLtT SPOON STANDARD PENETRATTON TEST
zc/t.> DRIVE SAMPLE BLOW COUNT. INDICATES THAT 52 BLOWS 0F A 14O-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON DECEMBER I6, 2O2O 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 ELEVÁTIONS 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)t
-200= PERCENTAGE PASSING N0.200 SIEVE (ASTM 01140).
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20-7-759 Kumar & Associates LEGEND AND NOTES Fig. 3
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20-7-759 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 5
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I(+rti5ffii,Hj#fffii*'""å;**'TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.20-7-759Sandy Silty ClaySOIL TYPESandy Silty ClaySandy Silty ClayVery Silty Clayey Sandwith GravelSandy Silty Clay(osñUNCONFINEDcofitPRESstvESTRENGTH(o/olPLASTtCINDEXATTERBERG LIfiIITSlo/olLIQUID LIÍIIIT4958PERCENTPASSING NO.200 stEVE('ôSANDGRADATION(wGRAVEL(ocflNATURALDRYDENSITYll0114r25113tt27.410.79.9I6.It2.z(o/olNATURALfitotsTuRECONTENT2%0I2050I(fr)DEPTH2SAÌIPLE LOCATIONBORINGI