HomeMy WebLinkAboutSubsoils Report for Foundation DesignI Cn fliffifi.ffi:#ni,'rsg**
An Employcc Grncd 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 (I{Q), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 833, FILTNG I, ASPEN GLEN
1.5 FOX PROWL
GARFIELD COUNTY, COLORADO
PROJECT NO. 22-7-106
SEPTEMBER7,2022
PREPARED FOR:
MICHELLE SIMMS
1042 ARATINA STREET
LOS ANGELES, CALTFORNTA 90042
ivobcm@gmail.com
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY
PR.OPOSED CONSTRUCTION ...
SITE CONDITIONS
SUBSIDENCE POTENTIAL
FIELD EXPLORATION
SUBSURFACE CONDITIONS .
FOUNDATION BEARING CONDITIONS
DESIGN RECOMMENDATIONS ..
FOUNDATIONS
FOUNDATION AND RETAINING WALLS
FLOOR SLABS
UNDERDRAIN S Y S'I'EM .......
SURFACE DRAINAGE
LIMTTATIONS...........
RtrFtrRDNCES : ...........
FIGURE 1 . LOCATION OF EXPLORATORY BORINGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 and 5 - SWELL-CONSOLIDATION TEST RESULTS
FIGURE 6 - GRADATION TEST RESULTS
TABLE I. SUMMARY OF LABORATORY TEST RtrSULTS
APPENDTX _ DEVELOPMENT IN SURFACE DEPRESSTON AREAS
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Kumar & Associates, lnc. @ Project No.22.7.106
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on
Lot E33, Filing 1, Aspen Glen, 15 Fox Prowl, Garfield County, Colorado. The project site is
shown on Figure l. The pu{pose 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 Michelle Simms dated January 6,2022. Chen-Northern previously
conducted a geotechnical study for the subdivision development and presented their findings in a
report dated December 20, 1991, Job No. 4 112 92.
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
Development plans for the lot were conceptual at the time of our study. In general, the proposed
residence will be a one and two-story wood-frame strucfure with attached garage. Ground floors
are assumed to 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 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 COITDITIONS
The lot was vacant and covered with about %foot of snow at the time of our field exploration.
The ground surface is relatively flat and gently sloping down to the north with around one foot of
elevation difference across the building area. Vegetation consists of grass and weeds.
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SUBSIDENCE POTENTIAL
Bedrock of the Pennsylvanian age Eagle Valley Evaporite underlies the Aspen Glen Subdivision.
These rocks are a sequence of gypsiferous shale, fine-grained sandstone and siltstone with some
massivc bcds 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, several sinkholes were observed
scattered throughout the Aspen Glen development (Chen-Northern, Inc., 1991). These sinkholes
appear similar to others associated with the Eagle Valley Evaporite in other areas of the lower
Roaring Fork River valley.
The site is mapped as lying within a broad depression area and sinkholes were mapped about
400 feet south and 400 fect northwest of the subjeet lot. The surface depression area is thought
to be associated with long-term ground subsidence. 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 E33 throughout the service life 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.
FTELD EXPLORATION
The field exploration for the project was conducted on January 21,2022. Three exploratory
borings were drilled at the approximate locations shown on Figure I to evaluate the subsurface
conditions. The borings were advanced with 4-inch diameter continuous flight augers porvered
by a truck-mounted CME-45B drill rig. The borings were logged by a representative of Kumar
& Associates.
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 14O-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 anindication of the relative density or consistency of the
Kumar & Aseociates, lnc. @ Project No.22-7-106
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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
Iaboratory for review by the project engineer and testing.
SUBST'RFACE CONDITIONS
Graphic logs ofthe subsurface conditions encountered at the site are shown on Figure 2. The
subsoils encountered, below about one foot of topsoil, consist of about 4Yzto7 feetof stiff to
very stiff, sandy silty clay overlying dense, silty sandy gravel and cobbles with probable
boulders. Drilling in the dense coarse granular soils with auger equipment was difficult due to
the cobbles and boulders and near practical drilling refusal was encountered in the borings at a
depthofaboutllfeet.
Laboratory testing performed on samples obtained from the borings included natural moisture
content and density and gradation analyses. Results of swell-consolidation testing performed on
relatively undisturbed drive samples of the upper clay soils, presented on Figures 4 and 5,
indicate low compressibility under light loading and'variable minor collapse or low expansion
potential when wetted. Results of gradation analyses performed on small diameter drive samples
(minus l%-inch fraction) of the coarse granular subsoils are shown on Figure 6. The laboratory
testing is summarizedin Table l.
No free water was encountered in the borings at the time of drilling and the subsoils were
slightly moist to moist with depth. ,
FOUI{DATION BEARING CONDITIONS
The silty clay soils encountered in the borings possess low bearing capacity and low to moderate
settlement potential, particularly when wetted. The underlying sandy gravel and cobble soils
possess moderate bearing capacity and typically low settlement potential. At assumed
excavation depths we expect the subgrade will expose either silty clay or gravel and cobble
subsoils. The residence can be supported on spread footings bearing on the silty clay soils with a
settlement risk or on the underlying gravel soils with a low settlement risk. Our experience
indicates the expansion potential measured on the silty clay soil sample from Boring 3 is
anomalous and can be ignored in the foundation design.
We have attached the Chen-Northern (1991) recommendations for building in the broad surface
depression areas of the subdivision. We believe these recommendations are conservative but
Kumar & Associates, lnc. @ Project No.22-7-106
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will reduce structural distress in the event of future ground movement and should be considered
in the building design.
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 below the topsoil with some risk of settlement. Placing footings down on the dense
gravel soils or on compacted structural fill placed on the natural granular soils can be done to
achieve a low settlement risk.
The design and construction criteria presented below should be observed for a spread footing
tbundation system.
l) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 1,500 psf. Footings placed entirely on the natural
dense coarse granular roit, ffi--"t"d structural fill should be designed for an
allowable bearing pressure otip@! Based on experience, we expect initial
settlement of footings designed and constructed as discussed in this section will
be about I inch or less. Additional differential sefflement could be around 1 inch
for footings placed on the silty clay soils under future wetted conditions.
2) The footings should have a minimum width of 18 inches for continuous walls and
2 feetfor 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
area. '-->
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 72 feet.
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.
5) The topsoil and any loose or disturbed soils should be removed and the footing
bearing level extended down to the firm natural soils. The exposed soils in
footing area should then be moistened and compacted. If structural fill is used to
re-establishdesignbearinggradesuchaswheresittyctffiTiffi,"d,the
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fill should consist of a relatively well graded granular material such as CDOT
Class 6 (%-inch) road base. Structural fill should be spread in thin horizontal lifts,
moisture conditioned to near optimum moisture content and compacted to at least
98 percent of maximum standard proctor density. The fill should extend laterally
beyond the footing edges a distance at least equal to one-half the depth of fill
below the footing.
6) A representative of the engineer should test any structural fill during
placement for compaction and observe excavations prior to concrete
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
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.
All foundation and retaining structures should be designed for appropriate hydrostatic and
surcharge pressures such as adjacent footings, traffic, 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 90Yo of the maximum
standard Proctor density at a moisture content near optimum. Backfill placed in pavement and
walkway areas should be compacted to at least 95% 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 ofthe
sliding resistance of the footing on the foundation materials and passive earth pressure against
Kumar & Associates, lnc. @ Project No. 22-7-106
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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 for clay soils and 0.50 for granular soils. 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. 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
-' compacted to at least 957o ofthe maximum standard Proctor density at a moisture content near
" optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded slab-on-grade
construction with a potential for settlement where underlain by silry clay soils. To reduce the
effects of some differential movement, floor slabs should be sepmated from all bearing walls and
^^1"-^..',i+L ^-^---:^- i^i-+-."1^:^L ^ll^.,,,,--^^+-^:-^A.,^*i^^l m^r,^s^€4 Dl^^- ^l-L ^^-+-^lwvlurrrrro vvrLrr v^P4rlJrvrr -lvrllrJ YYllrwl! clrtww urtlvDLt(llllvu Y9r Lrv4t rtt\rv9tItgttt. I It (rl Jll4t lelrllLrul
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 slab-on-grade construction for support. This material should
consist of minus 2-inch aggregate with at least 50olo retained on the No. 4 sieve and less than
l2olo passing the No. 200 sieve. The gravel layer below basement slab should be relatively free
draining with less than2%o passing the No.200 sieve.
All fill materials for support of floor slabs should be compacted to at least 957o 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 (plus 6-inch) rock.
UNDERDRNIN SYSTEM
Although frce water was not encr:unteted during our exploration, it has been our experienoe in
the area and where clay soils ae prescnt that loc,a-! perched gror:ndwater can develop during times
of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can also create a
perched condition. We recommend below-grade construction, such as retaining walls,
crawlspace and basement areas, be protected fiom wetting and hydroslal.io pressure buildup by
an underdrain system.
Kumar & Associates, lnc, @ Ptoject No.22.7.106
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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 minimum l%o to
a suitable gravity outlet or drywell based in the underlying gravel soils. Free-draining granular
material used in the underdrain system should contain less than 2Yopassingthe 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 and be covered by filter fabric such as Mirafi 140N.
SURFACE DRAINAGE
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 95o/o 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 ofthe 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 fine-grained
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, such as sod, and sprinkler
heads 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 atthis 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 l, 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. o Projec-t No.22-7-106
practice should be consulted. our nnaing,irlrr,rd" rrr,*olation 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 rc-evaluatio',f ftrc recommondations may bc'rade.
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 recommendations, and to verift that the recommendafions
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,
Kumar & Associates, fnc.
Steven L. Pawlak, P.E.
Reviewed by:
David A.P
slP/ljf
cc: JcffJohnsonArchiteots - JeffJohnsonjeff(4ljjarchitecturai'
REFERENCES:
Chen-Northe'r:r, Inc., ]?9^1, Preliminary Geotechnical Engineering Study, proposed Aspen Glen
levelopment, Garfield County, Coloratfui, prepared for Aspen Glen Comp-v, aut n December20,1991, Job No. 4ll29L.
Kuntar & Assoclales, lnc. e
Proiect No. 22-7-106
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APPROXIMATE SCALE-FEET
22-7 -106 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
BORING 1EL. 6020'BORING 2
EL. 6019.5'
BORINGEL. 602
30'
0 13/12
WC=7.6
DD=9 1
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WC=10.5
DD=89
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15 15
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*4=55
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22-7-tO6 Kumar & Associates LOGS OF EXPLORATORY BORINGS Fig. 2
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LEGEND
N
TOPSOIL; ORGANIC SANDY SILT AND CLAY, MOIST, BROWN, ROOTS.
CLAY
MOIST
(cr-);
WITH
SILTY, SLIGHTLY SANDY TO SANDY, STIFF TO VERY STIFF, SLIGHTLY MOIST TO
DEPTH, RED.
GRAVEL AND COBBLES (cM-cP); W|TH BOULDERS, SLTGHTLY S|LTY TO SILTY, SANDY, DENSE,
BROWN, ROUNDED ROCKS.
DRIVE SAMPLE, 2-INCH I.D. CALIFORNIA LINER SAMPLE
I DRTVE SAMPLE, 1 S/8-|NCH t.D. SPLTT SPOON STANDARD PENETRATION TEST.
,tq/l, DRTVE SAMPLE BLOW COUNT. |NDICATES THAT 15 BLOWS OF A 140-POUND HAMMER'-,.- FALLING 30 INCHES WERE REQUIRED TO DRIVE THE SAMPLER 12 INCHES.
NOTES
1. THE EXPLORATORY BORINGS WERE DRILLED ON JANUARY 21, ZO22 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 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 DRILLINU
7. LABORATORY TEST RESULTS:wc = WATER CONTENT (%) (ASTM D2216);DD = DRY DENSITY (PCT) (ASTU D2216);+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D6913);
-2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM Dl140).
22-7-106 Kumar & Associates LEGEND AND NOTES Fig. 5
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SAMPLE OF: Sondy Silty Cloy
FROM: Boring 2 @ 1'
WC = 7.6 %, DD = 91 pcf
H36.
ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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22-7 4A6 Kumar & Associates SVYELL-CONSOLIDATION TEST RESULTS Fig. 4
SAMPLE OF: Sondy Silty Cloy
FROM:BoringS@5'
WC = 16.1 ?(, DD = 107 pcf
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Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 522-7-106
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HYDROTIETER ANALYSIS SIEVE ANALYSIS
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I.425 127IDIAMETER OF PARTI IN
CLAY TO SILT COBBLES
GRAVEL 53 %
LIQUID LIMIT
SAMPLE OF: Silty Sondy Grovel
SAND 5/r %SILT AND E!.A]T 13 %
PLASTICITY INDEX
FROM: Boring 1 O 5' & l0' Combined
Th6o lcal rosulb opply only lo lhesomples rhlch wero tesl€d, Tholo!,tlng rsport 3holl nol bo roprcduc€d,oxcopl ln full, wllhoul th. wrlHonqpprcvol ot Kumqr & Asaoclolos, lnc.Slwe onolllls loltlng l! portomed lnoccordonco uilh ASTM 06915. ASTII D7924,ASTII C156 ond/or ASTM Dllito.
SAND GRAVEL
FINE MEDTUM lCOAnSe FINE COARSE
22-7-106 Kumar & Associates GRADATION TEST RESULTS Fig. 6
lcn l(unw & Associat€s, lnc.e
Geotechnical and Materials Engineers
and Environmental Scientists
TABLE 1
SUMMARY OF LABORATORY TEST RESULTS
SOIL TYPE
Slightly Silty Clay
Silty Sandy Gravel
Sandy Silty Clay
Sandy Silty Clay
(osfl
UNCONFINED
COMPRESSIVE
STRENGTH
lolol
PLASTIC
INDEX
ATTERBERG LIMITS
lolol
LIQUID LIMIT
PERCENT
PASSING NO,
200 stEVE
97
l3
t:hl
SAND
34
GRADATION
(%)
GRAVEL
53
19
r07
{ocfl
NATURAL
DRY
DENSITY
89
16.1
(%)
NATURAL
MOISTURE
CONTENT
10.3
t.2
7.6
5
tftl
DEPTH
2Yz
5&10
combined
1
SAMPLE LOCATION
BORING
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No.22-7-106
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practical due to the depttr of the sintholes. fire gmuting procedure should help roduce tle
settlcmentrisk but not totally eliminatc i[ Thercfore, we believe tlrat avoiding ttre sinldroles by
building setbeck is the lowcr risk and thc more appropriatc approaclr that slould bc n&ar.
Development in Surface Deptession Areas: Based on our fodings, develqment within the
ground surfacc depression areas (shown on Fig. 1) should be fasible provided appropriarc
mitigative designs are implemented for the residential buildings, utilities and madways as
described below. Tte appropriale lwel of ttre mitigative designs depend on thepotential ground
ddormation, thc building typc, location and configuration and levet of tolerable maintenrance
(mainly for rmdurays urd utilities). Building design considerations inelude use of a relatively
rigid foundation, (suclt as a stiffened slab or raft) urd a simply shaped building fooprint to
reduce poEntid darnage in ttrc event of differpntial movement. Tlcsc design concepts would
be included in the enginc€r€d foundations for residences locard in ttre depression areas.
Utilities should be dcsigncd and construcEd to bc rclatively flexible ud allow for diffcrential
movement without rupturing' \ilhere possible, rcttlement sensitive main utility Iines should be
routed outside of the ground surface de,pression areas. Roadways can bc conventionally designed
and constructed with provisions for maintenance if subsidence rehr€d disress is erperienced.
fitere are several gotechnical design conccpts which can be used to mitigate potential
subsidcncc damagc b residcntial buildings and underground utilities. Speciat mitigative designs
for a qner:ifin lnt chntrll ha r{anrolnn-,| lrrr rlra r...--!- ^-Lie-^. ^^ ) ,^ - ,:--r - -:i'-L---siL iw-i *i5iii.ii- us irsYEnir[nsii irt iiaE 9WiICf 5 iifctti-rEcE, aRC S-tRiCiUfAi engfneef Ud ShOUld
be based on the t)?c of building proposcd and the site specific foundation conditions. The
following design conce?ts are presented to assist in evaluating desrgn options prior to si*
Chen€Northern.Inc Conrfng €ngrr!€arsa.d &tdl&*s
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specific inrrestigations for an individual building site. The concept for underground utilities
should be incorporated into the utility design by the developer.
Building Configurations: Tte extent of damagc to a building subjectcd to the surface effece of
subsidence may be reduced by implernenting several architechral measuel in the building
design. fircse measures would include the following:
* Relatively flexible structural systems such as wood framc consfiuction, floxible
exterior siding, and dry wall interior partitions are preferable to less flexibte
nasnry structural systcm and erterior sidings.
* Intprior non-bearing.partitions resting on the floor slab should be prcvidcd with
slip joints so that slab movemcnts are not hansmified to tho uppcr structrfi€.
* The buitding should be a low sfrrtctue preferably limited to one or two stories.
* The buitding should have relatively small plan dimensions of @ feet or less. If
this is notpractical then the building should be divided into independent modules.
* Ih" building configuration should .be a simple rectarrgular configuration with
straight foundation walls and a minimum of side'projections from the main
building.
t' T'lre ground floor should be on a single level rather than using a split level design.
r Basements are particularly susceptible to subsidence dam4ge and are not
recommended unless rhe entire foundation is at basement level and dcsigncd for
Iateral earth loading.
Chen€Nortlrern,lnc Coflru[69 E €rn€*gaad ScraOUe
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Building Foundations: A raft foundation with a bearing level near the pxrerior grade appears
to bean appropriate foundation systrm for reducing the wlnerability of buildings to differential
subsidcnce damage. T1picat shallow spread footings would be a relativcly flexible systcm and
a rigid sysem is preferable for the larger magnitude ddormations.. Foundation syst€m
considerations arc outlined below:
I A raft foundation systcm is the prcferablc systen and should be designed
according to the sirc qpccific soil bearing conditions.
* The bosom surfrce of ttre raft should be smooth and ftee of vertical projections.
* Thc raft should be scparared from the bearing soils by placing the raft on a
minimum 4-inch thick compacted, clean sand. A polyethytene sheet should be
placcd benveen the rafr and the sand layer.
* Tltc usc of drcp foundation walls should bc minimized to the cxtent practical.
The soil plessure equal to at least tnrice the 'at rcstn earth prassure (on the order
of 80 to 100 pcf equivalent fluid unit wcigh$ should be assumed to act on aII
vertical burfaces in conact witlr the foundation soils.
* The bealing elevation of the raft should be placed bblow fr,ost depth or sufficient
soil cover should be provided for frost protection.
Underground Utilities: Underglound utilities are susceptible to the affecs of area subsidence.
As outlined below there are several mitigative design concepts which can.be used to reduce the
potenlial for darnage. In our opinion the mitigation measures should be ured where underground
utilities arc located in the ground surface depression areas shown on Fig l.
ChenQNorttrern,lnc Go|l'uttrng E Edllcrs anoscr3dEs
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Flexible joints should be used benveen adjacent pipe segments for both gravity
and pressure lines.
Positive restraints should bc provided in pressure lines to prevcnt pipe separation,
A florible joint should be provided as close as practical to any building, manhole,
or other rigid sfuctural connection.
A soil cushion in the immediate vicinity of the pipe should be provided by not
over-oompacting tlre bacldll soils close o the pipe.
Check valves should be.plaed at appopriale locations on all gas and uater mains
o permit intemrption of flow in case of subsidence disfress.
*
*
It
*
DEBRIS FLOW RISK AND MITIGATION
Haz4rd Evaluation: This study shows that the alluvial and debris ftlrs along the westem side
of the development are potential sitcs of water flooding and debris flows. The area erraluated
is shown on the atached Fig 1A. A summaly of the basins and fans evaluatcd is presented on
the atachd Table II. The calculated flow de,pths and volumes iue based on hydrological data
provided by Schmueser Gordon Meyer, Inc.
Potential water floods, with high sediment concentrations, should be considered for all
of the basins upslope of the fans. Appropriate surface water hydrologic methods should be used
to evaluate the flood haards on all fans. Futs I urd 2 in the southern part of the arca arc not
subject to debris flows, but debris flows should be considered on Fans 3 through 25 and the area
to the north (see Fig. lA).
Based on numerical debris flow rnodeling, we have designated thrce potential hazafi
Chen€l.lonhern.Inc Cdtst{Fg Engr.Eer3 rnd Scionurc.