HomeMy WebLinkAboutSubsoil Study for Foundation Design 01.04.2021lGrtåit*h:'åäfßtrnr'iiå*'"
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 18, CERISE RANCH
LARKSPUR DRIVE
GARFIELD COUNTY, COLORADO
PROJECT NO.20-7-722
JANUARY 4,2021
PREPARED FOR:
PETER DOLAN
270 SAM GRANGE COURT
CARBONDALE, COLORADO 81623
pdolanaspen@aol.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY ............... .... - I -
PROPOSED CON S'|RU C'I'ION
SITE CONDITIONS.....
SUBSIDENCE POTENTIAL......... .... - 2 -
FIELD EXPLORATTON ....-2 -
SUBSURFACE CONDTTIONS aJ
FOLINDATION BEARING CONDITIONS ^J-
DESIGN RECOMMENDATIONS ................ .... - 4 -
FOLINDATIONS .........-4-
FOI.INDATION AND RETAINING WALLS ..- 4 -
FLOOR SLABS
UNDERDRAIN SYSTEM
SURFACE DRAINAGE......................
................ - 6 -
................ - 6 -
.,,,........'...-7 -
_'7 _
FIGURE I - 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
Kumar & Associates, lnc. o Project No. 20-7-722
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot 18, Cerise Ranch, Larkspur Drive, Garfield County, Colorado. The project site is shown on
Figure 1. The purpose of the study was to develop recommendations for foundation design. The
study was conducted in accordance with our agreement for geotechnical engineering services to
Peter Dolan, dated November 23,2020.
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 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, recommendations and other geotechnical engineering
considerations based on the proposed construction and the subsurface conditions encountered.
PROPOSED CONSTRUCTION
The proposed residence will be a I story, wood frame structure over a walkout basement level
with an attached garage,located in the building envelope shown on Figure 1. The basement
floor and attached garage will be slab-on-grade. We assume grading for the structure will be
relatively minor with cut depths between about 3 to l0 feet below the existing ground surface.
For the purpose of our analysis, foundation loadings for the structure were assumed to be
relatively light and typical of the proposed type of construction.
If building loadings, location or grading plans are significantly different from those described
above, we should be notified to re-evaluate the recommendations contained in this report.
SITE CONDITIONS
Lot l8 was vacant at the time of our field exploration. Larkspur Drive borders the lot to the
north. The lot is moderately sloping down to the southwest at grades of 8 to 10 percent. The
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elevation difference across the lot is about 20 t-eet, and across the building envelope is about
12 t'eet. An abandoned irrigation ditch crosses the building envelope. An irrigation ditch located
to the south of the building envelope was in use at the time of our study. The vegetation on the
site consists of nativc grass and weeds. Eagle Valley Evaporite Formation bedrock is visible in
the valley hillside to the north.
SUBSIDENCE POTENTIAL
Bedrock of thc Pcnnsylvanian age Eagle Valley Evaporite underlies the Cerise Ranch
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 Eagle 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, sinkholes were
identified by CTl/Thompson within the Cerise Ranch Subdivision.
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 1 I throughout the service lifè 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 December 3,2020. 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 ancl 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.
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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.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface profiles encountered at the site are shown on Figure 2. In the
borings, below about 6 inches of organic topsoil, the subsoils consisted of stiff, sandy silty clay
with scattered gravel becoming more moist and medium stiff to soft with depth. Relatively
dense and wet clayey sandy gravel was encountered beneath the clay at a depth of about 20%to
23 feet down to the maximum explored depth of 26 feet. Drilling in the relatively dense gravel
soil with auger equipment was difficult due to the size and hardness of the cobbles encountered
and practical drilling refusal was encountered in the deposit.
Laboratory testing performed on samples obtained during the field exploration included natural
moisture content and density and finer than sand size gradation analysis. Swell-consolidation
testing was performed on relatively undisturbed drive samples of the clay subsoils. The swell-
consolidation test results, presented on Figure 4, indicate low to moderate compressibility under
conditions of loading and wetting. The laboratory testing is summarized in Table l.
Groundwater was encountered in the borings between depths of about l3Yz feet and l5Yz feet at
the time of drilling. The upper soils were typically moist to wet near or below the groundwater
level.
FOUNDATION BEARING CONDITIONS
Based on the subsoil conditions encountered in the borings, a spread footing foundation bearing
on the upper sandy silty clay soils appears feasible with some risk of differential settlement and
building distress. A deep foundation (such as driven piles) which extends down to the relatively
dense gravel subsoils could be used to provide a moderate load capacity and a low settlement
risk. We should be contacted if a deep foundation is proposed.
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DESIGN RECOMMENDATIONS
IOLINDATIONS
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 upper natural soils.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
l) Footings placed on the undisturbed natural soils should be designed 1'or an
allowable bearing pressure "l t,200 pffased 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 l8 inches for continuous footings
and 24 inches 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 the exterior grade is typically used in this
area.
4) Continuous foundation walls should be heavily reinforced top and bottom to span
local anomalies and limit the risk of differential movement such as by assuming
an unsupported length of at least l4 feet. Foundation walls acting as retaining
structures should also be designed to resist a lateral earth pressure 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 natural soils. The exposed soils in footing
areas should then be moistened and compacted.
6) A representative ofthe gcotcchnical cnginccr should obscrve 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
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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 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 soils. Backfill should not contain
vegetation, topsoil, 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, 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 near optimum. Backfill in pavement areas should
be compacted to at least 95Yo 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 correctly 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. Passive pressure of compacted backfill against the
sides of the footings can be calculated using an equivalent fluid unit weight of 300 pcf. 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 compacted to at least 95%o of the
maximum standard Proctor density at a moisture content near optimum.
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FLOOR SLABS
'l'he 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 Z-inch aggregate with at least 50% retained on the No. 4 sieve
and less than2Yo passing the No. 200 sieve.
All flill materials for support of floor slabs should be compacted to at least95%o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of
imported granular soils such as %-inch road base devoid of vegetation, topsoil, and oversized
rock.
LTNDERDRAIN SYSTEM
Although free water was encountered below the proposed basement level 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. Therefore, we recommend below-grade construction, such as crawlspace and
basement areas, be protected from wetting by an underdrain system. The drain should also act to
prevent buildup of hydrostatic pressures behind foundation walls.
The underdrain system should consist of a 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 ad.jacent interior finish grade
and sloped at a minimum lYo grade to a suitable gravity outlet or sump and pump. Free-draining
granular material used in the drain system should consist of minus 2-inch aggregate with less
than 50% passing the No. 4 sieve and less thanZYo passing the No. 200 sieve. 'l'he drain gravel
should be at least l% feet deep. An impervious membrane such as 30 mil PVC liner should be
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placed below the drain gravel in a trough shape and attached to the foundation wall with mastic
to prevent wetting of the bearing soils.
SURFACE DRAINAGE
The following drainage precautions should be observed during construction and maintained at all
times after the residence has been completed:
l) Inundation ofthe 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 areas and to at
least 90% 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 l0 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
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) Inigation sprinkler heads and landscaping which requires regular heavy irrigation
should be located at least 5 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.
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 identiflred at the exploratory borings and variations in
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the subsurface conditions may not become evident until excavation is performed. If conditions
encountered during construction appear to be different from those described in this report; we
should be notified at once so re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We me 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
or modifications of 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,
Kumar & Associates, Inc.
David A. Noteboom, Staff Engineer
Reviewed by:
Daniel E. Hardin, P.E.
DN/kac
Kumar & Associates, lnc. i)Projeci No. 20-7-722
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20-7 -722 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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BORING 1
El_EV. 6367
BORING 2
ELEV. 6576
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WC= 19.2
DD=1 07
-2OO=67
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DD=111
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20-7 -722 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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LEGEND
TOPSOIL. CLAY, SANDY, SILTY, ORGANICS, STIFF, SLIGHTLY MOIST, BROWN
CLAY (CL), SANDY TO VERY SANDY, SLIGHTLY SILTY WITH SCATTERED GRAVEL AND COBBLES,
STIFF TO SOFT WITH DEPTH, MOIST TO WET WITH DEPTH, BROWN.
GRAVEL (GM-GC), VERY SANDY, SLIGHTLY CLAYEY, SILTY, VERY DENSE, WET, GRAY.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE.
I DRTVE SAMPLE, 1 s/8-INCH l.D. SPLIT SPOON STANDARD PENETRATION TEST
.A/1' DRIVE SAMPLE BLOW COUNT. INDICATES THAT 24 BLOWS OF A 14o-POUND HAMMERL+/t tz FALLTNG Jo tNcHES WERE REeU|RED To DRtvE THE SAMPLER t2 lNcHES.
- DEPTH TO WATER LEVEL ENCOUNTERED AT THE TIME OF DRILLING.
---> DEPTH AT WHICH BORING CAVED.
I enncrrcaL AUGER REFUSAL.
NOTES
1 THE EXPLORATORY BORINGS WERE DRILLED ON DECEMBER 3, 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.
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 LEVELS SHOWN ON THE LOGS WERE MEASURED AT THE TIME AND UNDER
CONDITIONS INDICATED. FLUCTUATIONS IN THE WATER LEVEL MAY OCCUR WITH TIME.
7. LABORATORY TEST RESULTST
WC = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (PCI) (ISTU D2216);
-2ao= PERCENTAGE PASSING No. 200 SIEVE (ASTM Dl140).
20-7 -722 Kumar & Associates LEGEND AND NOTES Fig.3
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FROM:BoringlO10'
WC = 28.4 "Á, ÐD = 94 pcf
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SAMPLE OF: Sondy SÌlty Cloy
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WC = 15,0 %, DD = 111 pcf
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20-7 -722 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
I.* iiçlå'å"flfffii5'"'Êü'**TABLE 1SUMMARY OF LABORATORY TEST RESULTSSandy Silty ClaySandy Silty ClaySandy Silty ClaySandy Silty ClaySOIL TYPEATTERBERG LIMITSLIQUID LIMITUNCONFINEDCOMPRESSIVESTRENGTHPLASTICINDEX66PERCENTPASSING NO.200 stEVE67SAND(%)GRADATION(%)GRAVELr0794111104NATURALDRYDENSITY(pcflIololNATURALMOISTURECONTENTt9.228.415.02r.41050Itft)DEPTH5BORING12SAMPLE LOCATIONNo.20-7-722