HomeMy WebLinkAboutSubsoil Study for Foundation Design 10.19.2021lGrt
An Ëmployee Owned Compcny
5020 County Road 154
Glenwood Springs, CO 8tó01
phone: (970) 945-7988' fax: (970) 945-8454
email : kaglenwood@kumarusa.com
www.kumarusa.con't
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
RECEIVED
FEB 0 1 2t22
GARFIELd COUNTY
COMMUNITY DEVELOPMENl
SUBSOIL STT]DY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 55, SPRING RrDGE RESERVE
HIDDEN VALLEY DRIVE
GARFTELD COIINTY, COLORADO
PROJECT NO. 21-7-681
ocroBER 19,2021
PREPARED FOR:
TERRY AIYD HEIDI RUONAVAARA
160 SPRINGRIDGE DRTVE
GLnNWOOD SPRTNGS, COLORADO 81601
thruonavaara@msn.com
l(unar & Associates, lnc.@
Geotechniæl and Materials Engineers
and Environmental Scientists
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY........
PROPOSED CONSTRUCTION ......
SITE CONDITIONS....
FIELD EXPLORATION
SUBSURFACE CONDITIONS ..
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS
FOUNDATIONS
FOUNDATION AND RETAINING V/ALLS
FLOOR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE..........
LIMITATIONS.......
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2. LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURE 4 - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1- SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc. o Project No.21-7-681
PURPOSE AI\D SCOPE OF STT]DY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 55, Spring Ridge Reserve, Hidden Valley Drive, 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 Terry & Heidi Ruonavaara dated August 19,2021.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the fìeld
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 f'or the proposed building foundation. This report summarizes the data obtained during
this study and presents our conclusions, design recoÍlmendations and other geotechnical
engineering considerations based on the proposed construction and the subsurface conditions
encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a one or two-story wood-frame structure with attached garage.
Ground floor could be a combination of structural over crawlspace and slab-on-grade. Grading
for the structure is assumed to be relatively minor with cut depths between about 2 to 8 feet. \Me
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 CONDITTONS
The subject site was vacant at the time of our field exploration. The site lies at an elevation
between about 6,500 and 6,550 feet. The existing topography is represented by the contour lines
(l-foot contour interval) on Figure 1. The ground surface is sloping down to the west at a grade
of between 10 and 15 percent. Vegetation consists of grass, sagebrush and juniper trees.
GEOLOGY
According to the Geologic Map of the Leadville lox2o Quadrangle, Northwestern Colorado, by
Tweto, Ogden, Moench, R.H., and Reed, J.C., dated 1978,the site is underlain by Maroon
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Formation and Weber Sandstone covered by thin colluvium. The Maroon Formation is
described as maroon and grayish-red sandstone, conglomerate, and mudstone of the Permian and
Pennsylvanian periods. Weber Sandstone is described as yellow-gray sandstone of the Permian
and Pennsylvanian periods.
F'IELD EXPLORATION
The field exploration for the project was conducted on August 27,2021. 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 subsurface materials at various depths with blows from a 140-pound
hammer falting 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 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.
SUBSURF'ACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils consist of about I foot of topsoil overlying medium dense, clay and sand to between 4
and4Yz feet deep where hard, siltstone bedrock was encountered down to the maximum explored
depth of 14% feet deep. Drilling in the bedrock with auger equipment was difficult due to its
hardness and drilling refusal was encountered in the deposit.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and finer than sand grain size gradation analyses. Results of swell-
consolidation testing performed on a relatively undisturbed drive sample of the sand and clay
soil, presented on Figure 4, indicate low to moderate compressibility under existing low moisture
conditions and light loading anda low expansion potential when wetted under constant light
surcharge. The laboratory testing is summarizedinTable l.
No free water was encountered in the borings at the time of drilling and the subsoils and bedrock
were slightly moist.
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F'OUNDATION BEARING CONDITIONS
The natural clay and sand soils possess a low bearing capacity and variable expansion/
compression potential when wetted. The bedrock possesses moderate to high bearing capacity
and a low risk of settlement. Spread footings may span both soils and bedrock materials
depending on the design footing elevations. Lightly loaded spread footings transitioning
between the sand and clay soils and bedrock will have some potential for differential movement.
The risk of differential movement is due to the variable bearing conditions, especially at the
transition from natural soils to bedrock and if the soils were to become wetted. To reduce the
risk of differential settlement, spread footings could bear entirely on the underlying bedrock or
on compacted structural fill. The contractor should be prepared to excavate bedrock by splitting,
rock spade, blasting, or other rock excavation methods.
DESIGN RECOMMENDATIONS
FOUNDATIONS
Considering the subsurface conditions encountered in the exploratory borings and the nature of
the proposed construction, we recommend the building be founded with spread footings bearing
on the natural soils or bedrock or compacted structural fill in soil areas to reduce the risk of
differential movement.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the bedrock or properly compacted structural frll should be
designed for an allowable bearing pressure of 2,500 psf. Footings placed entirely
on bedrock should be designed for an allowable bearing pressure of 4,000 psf.
Structural fill should be compacted to a minimum of 98% of the standard Proctor
density. Based on experience, we expect settlement of footings designed and
constructed as discussed in this section will be about I inch or less.
2) The footings should have a minimum width of 16 inches for continuous walls and
2 leet 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
afea.
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 14fieet.
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s)
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, sand and clay soils and loose or disturbed soils and rock should be
removed and the footing bearing level extended down to the bedrock. The
exposed surface in footing areas should then be moistened and compacted.
Stnrctural fill can consist of the onsite soils excluding organics and rock larger
than 6 inches or imported granular soil such as base course and should extend to
at least one foot beyond footing edges.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions and test
structural fill for compaction.
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 suffrciently 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 soils. Backfill should not contain organics,
debris or rock larger than about ó inches.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffrc, 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 90% 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 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 pressrre on the wall. Some settlement of
deep foundation wall backfill should be expected, even if the material is placed correctly, and
could result in distress to facilities constructed on the backfill.
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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 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 350 pcf. The
coefficient of friction and passive pressure values recommended above assume ultimate soil
strength. Suit¿ble 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% of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils and bedrock, exclusive of topsoil, are suitable to support lightly loaded
slab-on-grade construction. Sub-excavation of expansive soils and replacement with structural
fill may be needed. 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 ptaced 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 than2o/o passing the No. 200 sieve.
All fïll 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 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 where bedrock is shallow 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 and
crawlspace 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
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excavation and at least I foot below lowest adjacent finish grade and sloped at a minimum lYoto
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 sizeof2 inches. Thedraingravelbackfillshouldbeatleast 1%feetdeep.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
l) Inundation of the foundation excavations and underslab 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 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 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 finer-graded
soils to reduce surface water infíltration.
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. Consideration should be given to the use of
xeriscape to limit potential wetting of soils below the foundation caused by
irrigation.
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 thç 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
Kumar & Associates, lnc. ô Proiect No. 2l-7-681
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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 recoÍrmendations, and to veri$ that the recommendations
have been appropriatelyinterpreted. Significant design changes may require additional analysis
or modifications to the recommendations presented herein. Vy'e recommend on-site observation
of excavations and foundation bearing strata and testing of structural fill by a representative of
the geotechnical engineer.
Respectfully Submitted,
Edtq;a¡.n¡" da Àäi;r"¡ci::{êtri" ã ¡¡{.
pr'-rw T"F¿'rM
James H. Parsons, P.E.
Reviewed by:
tl_
Steven L. Pawlak,
JHPlkac 1
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21 -7 -681 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig.1
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BORING 1
EL. 6504'
BORING 2
EL. 6516.5'
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22/12
WC=6.5
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WC=3.7
DD=1 15
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21 -7 -681 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
LEGEND
ñ
TOPSOIL; SILT AND SAND, ORGANICS' FIRM, SLIGHTLY MOIST, RED.
SAND AND CLAY (SC-CL); SILTY, SCATTERED GRAVEL, MEDIUM DENSE, SLIGHTLY MOIST, RED.
ffi
SILTSTONE BEDROCK; MICACEOUS, SCATTERED CALCAREOUS, VERY HARD, SLIGHTLY MOIST,
RED. MAROON FORMATION.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
i DRTVE SAMPLE, 1 S/ï-|NCH l.D. SPLIT SP00N STANDARD PENETRATION TEST.
ao t,q ORIVE SAMPLE BLOW COUNT. INDICATES THAT 22 BLOWS OF A 140-POUND HAMMER¿¿'/ t¿' FALLTNG so TNcHES wERE REQUIRED To DRtvE THE SAMpLER tz tNcHES.
I nnlcncAl AUcER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON AUGUST 27,2021 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.
3. 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 D2216h
-200= PERCENTAGE PASSING NO. 2CI0 SIEVE (ASTM D1140).
21 -7 -681 Kumar & Associates LEGEND AND NOTES Fig. 3
;
=
SAMPLE OF: Very Sondy Sllty Cloy
FROM:Boringl@2.5'
WC = 6.3 %, DD = 112 pcî
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EXPANSION UNDER CONSÏANT
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21-7-681 Kumar & Associates SWELL-CONSOLIDATION TEST RISULTS Fig. 4
I(t I åffifi',#,ffi'*''lË;'^**TABLE 1SUiIITIARY OF LABORATORY TEST RESULTSSOILTYPEVery Sandy Silty ClayVery Clayey Silt and Sand{psf}UNCOilFilEDcorPREssfvESiIREilGTHPt-ASTtCINDEXt%tAITERBERG IIMITSlol"lLnufr, LttffPERCEIITPASSIi¡G r¡O.200 stEt/E49(%)SAIIDGRADATIOI'I(%)GRAVEL115NATURAIDRYDENSÍÍY{oclltt2t%ìNATURALiIOISTURECONTENT6.33.t{frtDEPÌH2y,2Y,12BORINGNo. 21-7681