HomeMy WebLinkAboutSubsoil Studyr(trt l(umar & Associates, lnc. 5020 County Road 154
Geotechnical and Materials Engineers Glenwood Springs, CO 81601
and Environmentatscientists phone: (970) 945_7ggg
fax: (970)945-8454
email: kaglenwood@kumarusa.com
An Employcc Owncd Compony 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 550 TRONBRTDGE' PHASE rlr
TBD BLUE HERON DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO.21-7-145
FEBRUARY 19,2021
PREPARED FOR:
SCIB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD' SUITE 52018
ASPEN, COLORADO 81601
luke.gosda@sun riseco.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY..
PROPOSED CONSTRUCTION ...
SITE CONDITIONS
SUBSIDENCE POTENTIAL.
FIELD EXPLORATION.
SUBSURFACE CONDITIONS
FOLINDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ..
FOUNDATIONS
FLOOR SLABS
UNDERDRAIN SYSTEM ...........
SURFACE DRAINAGE.......
LIMITATIONS...
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FICURE 4 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 5 - GRADATION TEST RESUI-TS
TABLE 1 - SUMMARY OF LABORATORY TEST RESULTS
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Kumar & Associates, lnc.Project No.21.7.145
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot 55, Ironbridge, Phase III, TBD Blue Heron 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 SCIB, LLC dated January 20,2021.
A field exploration program consisting of exploratory borings was conducted to obtain
information on the subsurface conditions. Samples of the subsoils obtained during the 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
At the time of our study, design plans for the residence had not been developed. The building is
proposed within the building envelope shown on Figure 1. For the purposes of our analysis, we
assume the proposed residence will be a wood frame structure over a crawlspace with an
attached slab-on-grade garage. Grading for the structure is assumed to be relatively minor with
cut depths between about 3 to 5 feet. We assume relatively light foundation loadings, typical of
the proposed type of construction.
If building loadings, location or grading plans change signihcantly from those described above,
we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
The lot was vacant and appeared to have had some minor overlot grading, likely during the
subdivision development. The surface of the lot slopes gently down to the north with about
4 feet of elevation difference across the building envelope area. Vegetation consists of grass and
weeds with some sage brush. The lot was covered by about one foot of snow at the time of our
field exploration.
Kumar & Associates, lnc Project No.21-7-145
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SUBSIDENCE POTA,NTIAL
Bcdrock of the Pennsylvanian age Dagle Valley Evaporite underlies the Ironbridge development.
These rocks are a sequence of gypsiferous shale, finc-graincd sandstonc and siltstonc with somc
massive beds of gypsum and lirnestone. There is a possibility that massive gypsum deposits
associated with the Eagle Valley Evaporite underlie portions of the lot. Dissolution of the
gypsttm 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 the lronbridge development. These sinkholes appear similar to others
associated with the Eagle Valley Evaporite in areas of the Roaring Fork Valley.
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 the area of similar subsurface profiles; 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 January 25,2021. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
The borings \ilere 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 I% 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.Project No,21.7.145
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SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. In
Boring 1, below about Yzfoot of topsoil, the subsoils consist of very stiff, sandy silty clay down
to a depth of about 5 feet. This was underlain by dense, silty sand and gravel with cobbles down
to the drilled depth of 16 feet. In Boring 2, below about Yzfoot of topsoil, the subsoils consist of
very stiff, sandy silty clay down to a depth of about 11 feet. This was underlain by dense, silty
sand and gravel with cobbles down to the drilled depth of 14 feet. Drilling in the dense granular
soils with auger equipment was difficult due to the cobbles and probable boulders and drilling
refusal was encountered in Boring 2 inthe deposit at 14 feet.
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 of clay soils, presented on Figure 4,
indicate low to moderate compressibility under conditions of loading and wetting with a low
expansion potential on the sample from Boring 2 when wetted. The laboratory testing is
summarized in 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 clay soils encountered at foundation level have a low bearing capacity and tend to settle
when they become wetted. There is also a significant difference in the thickness of the clay soils
across the building area. A shallow foundation placed on the upper natural clay soils may have
significantly different depth of clay soil beneath the footing depending on the excavation depth.
V/etting of the clay soils creates a risk of differential movement of the foundation and building
distress. The amount of settlement or differential movement will be mainly related to the depth
of clay soils and extent of subsurface wetting. It will be critical to the long term performance of
the structure that the recommendations for surface drainage contained in this repofi be followed.
Recommended forms of settlement mitigation include: 1) deep compaction,2) a deep foundation
such as drilled piers or helical piers bearing on the underlying dense gravel and cobble soils, or
3) sub-excavation of the clay soils down to the natural granular soils and replacement with
compacted structural fill.
Kumar & Associates, lnc.Project No.2'l-7-145
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The footing bearing level on Lot 55 could also be deepened below existing ground surface
around 6 to 8 feet, to limit the depth of clay soils to around 6 fcct bclow thc bcaring level as a
foundation settlement mitigation measure. In sub-excavated areas, the on-site soils or road base
could be replaced compacted to reestablish design bearing level. For typical shallow footing
depth of 3 feet, the depth of stn¡ctural fill shoulcl be arouncl4 feet below footing bearing level.
Recommendations for a spread footing foundation are presented below. If a deep foundation or
other type foundation is desired, we should be contacted for additional analysis and
recommendations.
DESIGN RECOMMENDATIONS
FOIiNDATIONS
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 clay or granular soils or compacted structural fill with a settlement risk.
The design and construction criteria presented below should be observed for
r)Footings placecl on up to 6 feet of the undisturbed natural clay soils, compacted
structural fill, or natural granular soils should be designed for an allowable
bearing pressure of 1,500 psf, or 3000 psf if bearing completely on the natural
granular soils. Based on experience, we expect initial settlement of footings
designed and constructed as discussed in this section will be about 1 inch or less.
Additionsl diffcrcntial movcmcnt up to about 1 inch could occur if the clay
bearing soils are u'etted.
The footings should have a minimum width of 20 inches for continuous walls and
2 feet f'or 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
àrea.
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 a
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Kumar & Associates, lnc.Project No. 21.7.145
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lateral earth pressure conesponding to an equivalent fluid unit weight of at least
55 pcf for the on-site clay soil, or 45 pcf for the on-site granular soil, as backfill.
The
exposed soils in footing areas should then be moistened and compacted.
Structural fill should extend laterally beyond the footing edges a distance at least
Vzthe fill depth below the footing and be compacted to at least 98% of the
standard Proctor density atnear optimum moisture content. The soils should be
protected from frost and concrete should not be placed on frozen soils.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction with a risk of settlement if the bearing soils are wetted. We recommend at least
2 feef of compacted structural fill similar to that placed below footings be placed below the floor
slab to help mitigate the settlement potential in clay areas. 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 interior 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 I2o/o passing the No. 200
sieve.
All fill materials for support of floor slabs should be compacted to at least 95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site soils devoid of vegetation and topsoil or a suitable imported material such as road base.
LINDERDRAIN SYSTEM
It is our understanding the ground finished floor elevation of the residence is at or above the
surrounding grade. Therefore, a foundation drain system is not recommended. It has been our
experience in the arcathat local perched groundwater can develop during times of heavy
precipitation or seasonal runoff. Frozen ground during spring runoff can create a perched
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Kumar & Associates, lnc.Project No,21-7-145
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conclition. We recommend below-grade constn¡ction, such as retaining walls ancl basement
areas, if provided, be protected from wctting ancl hyclrostatic pressure builclup by an nnderclrain
and wall drain system. An underdrain is not recommended around the crawlspace area to help
limit the potential for wetting below the shallow footings.
If the finished floor elevation of the proposed structure has a floor level below the surrounding
grade, we should be contacted to provide recommendations for an underdrain system. All earth
retaining structures should be properly drained.
SURFACE DRAINAGE
It will be critical to the building performance to keep the bearing soils dry. 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
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95o/o of the maximum standard Proctor density in pavement and slab areas
and to at least 90Yo 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. W.e 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 10 feet in paved areas. Graded swales should have a
minimum slope of 3%.
4) Roof downspouts and drains should clischarge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at least
10 feet from founclation walls. Consicleration should be given to use of xeriscape
to reduce the potential for wetting of soils below the building caused by irrigation.
LIMITATIONS
This study has been conducted in accordance with generally accepted geotechnical engineering
principlcs 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
Kumar & Associates, lnc,Project No. 21-7-145
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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 irnplementation of our recommendations, and to verifo 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,
Ku Associate Inc.
David A. Noteboom, Staff Engineer
Reviewed by:
Steven L.
SLP/kac
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21-7 -1 45 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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LEGEND
TOPSO|L; ORGANIC SANDY SILT AND CLAY, FROZEN, BROWN
CLAY (CL); SILTY, SANDY, VERY STIFF, SLIGHTLY MOIST, BROWN
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GRAVEL
BROWN,
(GM); SILTY, SANDY, COBBLES, PROBABLE BOULDERS, DENSE, SLIGHTLY MOIST,
ROUNDED ROCK.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
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DRTVE SAMPLE, 1 3/ï-|NCH t.D. SpLtT SPOON STANDARD pENETRATTON TEST
13/ 12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT 13 BLOWS OF A 14o-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
1 PRACTICAL AUGER REFUSAL.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 25, 2021 WITH A 4-INCH DIAMETER
CONTIhIUOUS-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 MEASURED BY INSTRUMENT LEVEL AND
REFER TO BORING 1 AS 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
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 (PCt) (ASTV D2216);+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (aSrU OOS.IS);
-200= PERCENTAGE PASSING No. 200 SIEVE (ASTM D1lao);LL = LIQUID LIMIT (ASTM 04318);PI = PLASTICITY INDEX (ASTM D4318).
21 -7 -1 45 Kumar & Associates LEGEND AND NOTES Fig. 3
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EXPANSION UNDER CONSTANT
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21-7 -145 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
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HYDROMETER ANALYSIS SIEVE ANALYSIS
TIME READINOS
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PLASTICITY INDEX
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occordonco wlth ASIM D6913, ASTM D7928,
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FIN E MEDTUM lCOanSe FI NE COARSE
21-7 -1 45 Kumar & Associates GRADATION TTST RESULTS Fig.5
I (+rI iitilfi'ffill:ÉtrÉ'''nË; n *' *TABLE 1SUMMARY OF LABORATORY TEST RESULTSNo.21-7-145SOIL TYPESandy Silt and ClaySandy Silt and ClaySilty Sandy GravelSandy Silty ClaySandy Silty Clay(psflUNCONFINEDCOMPRESSIVESTRENGTH81PLASTICINDEXP/"134ATTERBERG LIMITS(ololLIQUID LIMIT85PERCENTPASSING NO.2()() SIEVE77I1SAND(%)3541GRADATIONGRAVEL(%){pcflNATURALDRYDENSITYt07II4r0.4(olrlNATURALMOISTURECONTENT6.27.51.81l47 and 10combined401(ft)DEPTH1SAMPLE LOCATIONBORING12
l(t tffirfiffiffiiiiiiå*"5020 Ceiunty Road 154
Glenwood Splings, CO 8i601
phone: (970) 945-7988
fax: (970) 945-8454
ernai I : kaglenwood@kumarusa.com
w¡v:ry,kg¡4ruSg¡s¡IAn Emplollca Chrned Compsny
Office Locations: Denver (I'lQ), Palker. Colorado Springs, Fort Colli¡s. Clenwood Springs, and Summit County, Colorado
August 9,2021
SCIB, LLC
Attn: Luke Gosda
0115 Boomerang Road, Suite 52018
Aspen, Colorado 81611
lut<e.gqsda@lsunrjs
Project No. 21-7-145
Subject: PotentialDifferential Settlement Addendum, Proposed Residence, Lot 55,
Ironbridge, Phase III, Blue Heron Drive, Garfield County, Colorado
Gentlemen:
As requested, we are providing clarification to recommendations and foundation settlement
discussion presented in our previous report for design offoundations at the site dated
February 19,2021, Project No. 2l-7-145.
We understand the residence is planned to have crawlspace in the living area and slab-on-grade
in the garage. A spread footing foundation placed on the natural soils or compacted structural
filI has tentatively been selected for the building support based on recommendations presented in
our previous report.
The soil conditions encountered at the site mainly consist of sandy silty clay. The laboratory
testing indicates the clay soils to have minor to low expansion potential when wetted under light
loading. It is our experience that the clay soils tend to settle when wetted under load and the
expansion potential can be ignored in the foundation design. The settlement potential of lightly
loaded spread footings placed on the natural clay soils up to 6 feet deep below the bearing level
or compacted structural fill is up to around 1 inch and additional differential movement could be
up to around I inch depending on the depth and extent of post-construction wetting of the
bearing soils. Isolated footing pads constructed separate from the building foundation walls
should be designed for the expected post-construction differential foundation movement.
If you have any questions or need further assistance, please call our office.
Sincerely,
Kumar & Associatesn Inc.
Steven L. Pawlak, P
SLP/kac
cc: Colorado S mik oradostru