HomeMy WebLinkAboutSubsoil Study¡(t Kumar&Associates, lnc. 5020 County Road 154
Geotechnical and Materials Eng¡neers Glenwood Springs, CO 81601
and Environmenrat sc¡entists pnonè: (970) g4s_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 S¿,IRONBRTDGE' PHASE IrI
TBD BLUE HERON DRIVE
GARFTELD COUNTY, COLORADO
PROJECT NO. 20-7-786
FEBRUARY 19,2021
PREPARED FOR:
scrB, LLC
ATTN: LUKE GOSDA
0115 BOOMERANG ROAD' SUITE 52018
ASPEN' COLORADO 81601
lu ke.gosda@sunriseco.com
TABLE OF'CONTENTS
PURPOSE AND SCOPE OF STUDY
PROPOSED CONSTRUCTION
SITE CONDITIONS....
DESIGN RECOMMENDATIONS
FOLTNDATIONS
FLOOR SLABS........
UNDERDRAIN SYSTEM ......
SURFACE DRAINAGE...............
LIMITATIONS
FIGURE 1 - LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 . LEGEND AND NOTES
FIGURES 4 AND 5 - S\MELL-CONSOLIDATION TEST RESULTS
TABLE I - SUMMARY OF LABORATORY TEST RESULTS
1
I
SUBSIDENCE POTENTIAL a
FIELD EXPLORATION....a
SUB SURFAC!; CONDI'I'ION S J
FOUNDATION BËARING CONDITIONS ......- 3 -
4
4
6
6
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Kumar & Associates, lnc.Project No,20.7.786
PURPOSE AND SCOPE OF STUDY
This report presents the results ofa subsoil study for a proposed residence to be located on
Lot 52,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 December 31,2020.
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 engineeringcharucteristics. 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 significantly 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 northeast with about 3
feet of elevation difference across the buildin g area. Vegetation consists of grass and weeds. The
lot was covered by about one foot of snow at the time of our fielcl exploration.
Kumar & Assoclates, lnc.Project No. 20-7-786
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SUBSIDENC]E POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the lronbridge development.
These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some
massive beds of gypsum ancl limestone. There is a possibility that massive gypsllm 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, several sinkholes were observed
scattered throughout the Ironbridge development. These sinkholes appear similar to others
associated with the Eagle Valley Evaporite in areas of the Roaring Fork Valley.
Sinkholcs wcre 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 52 throughout the service life of the proposed residence, in our
opiuion, is low aud sirtrilar to other lots in the area; however, the owner shoulcl 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 thc projcct was conductcd on January 12,202I. Two exploratory
borings were drilled at the locations shown on Figure 1 to evaluate the subsurface conditions.
'l'he borings were advanced with 4 inch diameter continuous flight augers powered by a truck-
mounted CME-45B 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 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 sanrples were taken arrd tlre penetlation resista:rrce vah¡es 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,Project No. 20-7-786
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SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. Below
about Yz foot of topsoil, the subsoils consist of very stiff, sandy silt and clay soils down to about
8 feet. Very stiff, sandy silty clay was then encountered down to a depth of 13 feet. This was
underlain by dense, silty sand and gravel with cobbles down to the drilled depths of 18 and
16 feet. Drilling in the dense granular soils with auger equipment was difficult due to the
cobbles and possible boulders and drilling refusal was encountered in Boring 1 in the deposit at
18 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 the silt and clay and sandy clay
soils, presented on Figures 4 and 5, indicate low to moderate compressibility under conditions of
loading and wetting with low collapse potential on the upper silt and clay soil when wetted. The
laboratory testing is summarizedin 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 silt and clay soils encountered at foundation level have a low bearing capacity and tend to
settle when they become wetted. A shallow foundation placed on the upper natural silt and clay
soils will have a risk of settlement and building distress. The amount of settlement will be
mainly related to the depth 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
report be followed. Due to the potential for post-construction settlement mitigation is
recommended to reduce the settlement potential. 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) a heavily reinforced structural slab
foundation.
The footing bearing level on Lot 52 could also be deepened below existing ground surface
around 6 to 8 feet, to limit the depth of silt and clay soils to around 6 feet below the bearing level
Kumar & Associates, lnc.Project No. 20-7-786
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as a foundation settlement mitigation measure. In sub-excavatecl areas, the on-site soils or road
basc could be replaced compacted to reestablish design bearing level. Ior typical shallow
footing depth of 3 feet, the depth of structural fill should be around 4 feet below footing bearing
level.
Recommendations for both a spread footing foundation and a drilled pier foundation are
presentetl below. If other foundation types are proposed, we should be contacted for additional
analysis and recommendations.
DESIGN RECOMMENDATIONS
FOI-INDATIONS
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 and silt or granular soils or compacted structural filIwith a settlement risk.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on up to about 6 feet of the undisturbed natural soils or on
compacted structural fill should be designed for an allowable bearing pressure of
1,500 psf. Based on experience, we expect initial settlement of footings designed
and constructed as discussed in this section will be about 1 inch or less.
Additional differential settlement up to about 1 inch could occur if the bearing
soils are wetted.
2) The footings should have a minimum u'idth of 20 inches for continuous rvalls and
) føøt fnr icnlaia¡l rrarlc
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
atea.
4)Continuous foundation walls should be heavily reinforced top and bottom to span
local attotttalies suclt as by assuming an unsupporlecl lenglh of at leasl 14 feet.
Foundation walls acting as retaining structures should also be designed to resist a
lateral earth pressure conesponcling to an equivalent fluid unit weight of at least
55 pcf for the onsite silt and clay soil as backfill.
Kumar & Associates, lnc.Project No. 20-7-786
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5)The topsoil, natural silt and clay soils and loose or disturbed soils should be
removed to at least 4 feet below bearing level in footing areas. The exposed soils
in footing areas at the sub-excavated level should then be moistened and
compacted. Structural fill (on-site silt and clay soils or road base) should extend
laterally beyond the footing edges a distance at least % the 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.
If the risk of settlement inherent in a spread footing foundation is not acceptable to the
homeowner, a drilled pier foundation that extends down into the underlying dense, gravel and
cobble soil could be used with low settlement potential. The design and construction criteria
presented below should be observed for a straight-shaft drilled pier foundation system.
1) The piers should be designed for an allowable end bearing pressure of 12,000 psf
and a skin friction of 1,200 psf for that portion of the pier embedded in gravel.
Pier penetration through the upper silt and clay soils should be neglected in the
skin friction calculations.
2) All piers should have a minimum total embedment length of 10 feet and a
minimum penetration into the gravel of 1 foot. The gravel and cobble soils will
tend to cave and penetration into the bearing soils should be limited to about
2 feet.
3) The pier holes should be properly cleaned prior to placement of concrete. The
natural silt and clay soils are stiff which indicates that casing of the holes should
not be required. Some caving and diffrcult drilling may be experienced in the
bearing soils due to cobbles and possible boulders. Placing concrete in the pier
hole the same day as drilling is recommended.
4) The pier drilling contractor should mobilize equipment of sufficient size to
achieve the design pier sizes and depths. We recommend a minimum pier
diameter of 12 inches.
5) Grade beams and pier caps should have a minimum depth of 3 feet for frost cover
and void form below them is not needed.
6)
Kumar & Associates, lnc.Project No, 20-7-786
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Free water was not encountered in the borings made at the site and clewatering
should not be needed.
A representative of the geotechnical engineer should observe pier drilling
operations on a full-time basis.
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 leeL of compacted structural fill similar to that placed below footings be placed below the floor
slab to help mitigate the settlement potential. 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. 'I'he requirements f'or 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 shoulcl consist of minus 2-inch
aggregate with at least 50% retained on the No. 4 sieve and less than T2%o passing the No. 200
sieve.
All fill materials for support of floor slabs should be compacted to at least 95Yo 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.
UI.{DEP.DR^.IN S Y S TEI\{
It is our understattding the ground hnished floor elevation of the residence is at or above the
surrounding grade. Therefore, a foundation drain system is not recommended. It has been our
cxpcricncc in thc arcathat local perched groundwater can develop during tirnes 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 basement
areas, if provided, be protected from wetting and hydrostatic pressure buildup by an underdrain
and wall drain system. An underdrain is not recommended around the crawlspace aÍea to help
limit the potential for wetting bclow thc shallow footings.
6)
t)
FLOOR SLABS
Kumar & Associates, lnc.Project No. 20.7.786
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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.
SURT'ACE 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 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 95Yo 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 l2 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 discharge well beyond the limits of all
backfill.
5) Landscaping which requires regular heavy irrigation should be located at least 10
feet from foundation walls. Consideration 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
principles and practices in this area al 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 atthe 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
Kumar & Associates, lnc.Project No. 20-7-786
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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 recoütmendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible fbr 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 verifu 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,
I(uruar & Associates. trnc.l#
David A. Noteboom, Staff Engineer
Reviewed by:
Steven L, Paw
SLP/kac I
15222
Kuma¡ & Associates, lne.Froject l{o, 20"7-786
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BENCHMARK:
SEWER MANHOLE RIM
ELEVATION 1 OO, ASSUMED
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LCT 51
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BORING 2 .
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BORING 1
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APPROXIMATE SCALE-FEET
20-7 -786 Kumar & Associates LOCATION OF TXPLORATORY BORINGS Fig. 1
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BORING
EL. 1 05
1 BORING 2
EL, 102,
0 0
17 /12
5
22/12
WC=4.4
DD= 1 06 5
14/12
WC=5.6
DD= 1 04
-200=78
20/ 12
WC=6.2
DD= l 07
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DD= 1 02
21/12
WC=9.9
DD= 1 02
15 1588/ 12 36/6, so/5.5
20 20
20-7 -786 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
1
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I
LEGEND
TOPSOIL; SILT AND CLAY, ROOTS AND ORGANICS, FIRM, MOIST, BROWN.
stLT AND CLAy (ML-CL); SANDY, StLTY, VERY STtFF, SLTGHTLY MOtST, LTGHT BROWN
CLAY (CL); SANDY, SILTY, VERY STIFF, SLIGHTLY MOIST, BROWN. LOW PLASTICITY
GRAVEL
BOULDE
(CV-OP); SANDY TO VERY SANDY, SILTY, WITH COBBLES AND PROBABLE SMALL
RS, VERY DENSE, SLIGHTLY MOIST, BROWN. ROUNDED ROCK.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
i DRTVE SAMPLE, 1 5/8-tNCH t.D. SPLIT SPOON STANDARD PENETRATION TEST
17/12 DRIVE SAMPLE BLOW COUNT. INDICATES THAT I7 BLOWS OF A 14o-POUND HAMMER
FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
I PRACTICAL AUGER DRILLING REFUSAL.
NOTES
1 . THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 12, 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 MEASURED BY INSTRUMENT LEVEL AND
REFER TO THE BENCHMARK ON FIG. 1.
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);
_2QO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D1140).
Fig. 320-7 -786 Kumar & Associates LEGEND AND NOTES
IJ
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SAMPLE 0F: Sondy Sllt ond Cloy
FROM:Boringl@^4'
WC = 4.4 %, DD = 106 pcf
full, w¡thout
1
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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APPLIED PRESSURE - KSF 10 100
20-7 -786 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
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SAMPLE OF: Sondy Cloy
FROM: Boring 1 @ 10'
WC = 7.4 %, DD = 102 pcl
EXPANSION UNDER CONSTANT
PRESSURE UPON WETTING
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1 1.0 APPLIED PRESSURE - KSF 10 100
SAMPLE OF: Sondy Cloy
FROM:Boring2@10'
WC = 9.9 %, DD = 102 pcf
Kumor
NO MOVEMENT UPON
WETTING
20-7 -786 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig.5
rcrf #,ffill[#:nruirniit'å *' "TABLE 1SUMMARY OF LABORATORY TEST RESULTSSOIL TYPESandy Silt and ClaySandy Silt and ClaySandy ClaySandy Silt and ClaySandy ClaylosflUNCONFINEDCOMPRESSIVESTREI,TGTHl%)PLASTICINDEXAÍTERBERG LIMITSOJLIQUID LIMITPERCENTPASSING NO.200 srEVE1064.47882SANDl:/"1GRADATIONP/,1GRAVEL(pcf)NATURALDRYDENSITY104102r07102I /0.NATURALÍIIOISTURECONTENT5.67.46.29.9tftlDEPTH4701501SAI'IPLE LOCATIONBORINGI2No.20-7-786
l(tÃiffiåffiffii#'i*"5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.corn
www.kumarusa.comAn Emdoycc ü,vttcd Cornpony
Offrce l¡cations: Denver {HQ), Parker, Colorado Spríngs, Fort Collins, Glenwood Springs, and Summit County, Colorado
Apr:l28,202l
Revised M.ay 5,2021
SCIB, LLC
Attn: Luke Gosda
0115 Boomerang Road, Suite 52018
Aspen, Colorado 81611
luke. sosda(@sunri seco. corn
Project No. 20-7-786
Subject: Structural Slab Addendum; Proposed Residence, Lot 52,Ironbridge, Phase 3,
Blue Heron Drive, Garfield County, Colorado
Gentlemen:
As requested by Mike Arbaney, we are providing this addendum for a structural slab alternative
to support the proposed residence at the subject site. We previously conducted a subsoil study
for design of foundations at the site and presented our findings in a report dated February 19,
2021, Project No. 20-7-786.
The residence is proposed to be a 2-story structure with slab-on-grade ground level floor.
Spread footings placed on at least 4 feet of structural filI were recommended for support in our
February 19,2021report. A structural slab foundation was also identified as a feasible method
to help mitigate differential foundation settlement and building distress. The slab will be
reinforced to spread out the loading and resist movements. Sub-excavation for 4 feet of
structural fill could also be used to limit differential settlement.
We understand that a structural slab foundation has been tentatively selected for the building
support which is consistent with our findings for suitable building foundation types. The
structural slab placed on the structural fill should be designed for an allowable bearing pressure
of 1,500 psf and capable of withstanding post-construction differential settlement of around 2
inches. Frost protected turn-down perimeter edges should be at least 24 inches deep and bearing
points should be at least 20 inches wide for continuous walls and 24 inches wide for point loads.
The structural slab should extend out to support the two front porch piers rather than have them
supported on isolated piers to limit differential settlement potential. Other recommendations
presented in our previous report which are applicable should also be observed.
If you have any questions or need further assistance, please call our office.
Sincerely,
Kumar & Associates,
Steven L. Pawlak, P
SLPlkac
cn 52221
cc: Colorado Structural -Arbaney ( )