HomeMy WebLinkAboutSubsoil Study for Foundation Design 01.17.2022rcrfiffirfåmmiy;--5020 County Road 154
Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email : kaglenwood@kumarusa.com
án Emplo'ycc Chrnccl Co'mpony www.kumarusa.com
Office Locations: Denver (FIQ), Parker, Colorado Springs, Fort Collinq Glenwood Springs, and Summit County, Colorado
SUBSOIL STUDY
FOR FOUNDATION DESIGN
PROPOSED RESIDENCE
LOT 4, SPRING RIDGE RESERVE
DRY PARK ROAI)
GARFIELD COUNTY, COLORADO
JOB NO. 2t-7-845
JANUARY 1t,2022
PREPARED FOR:
FUTURADO DEVELOPMENT
ATTN: DARRELL CORDOVA
P.O.B,O){2227
GLEN\ryOOD SPRTNGS, COLORADO 81602
(dcordova23 7@msn.com)
TABLE OF CONTENTS
PURPOSE AND SCOPE OF STUDY..
PROPOSED CONSTRUCTION
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1SITE CONDITIONS
GEOLOGIC HAZARDS REVIEW ....................- 2 -
SUBSURFACE CONDITIONS ...
DESIGN RECOMMENDATIONS ...............
FOLINDATIONS
FOLINDATION AND RETAINING WALLS
FLOOR SLABS
TINDERDRAIN SYSTEM ......
SURFACE DRAINAGE..........
LIMITATIONS......-7 -
FIGURE 1 - LOCATION OF EXPLORATORY BORTNGS
FIGURE 2 - LOGS OF EXPLORATORY BORINGS
FIGURE 3 - LEGEND AND NOTES
FIGURES 4 through I - SWELL-CONSOLIDATION TEST RESULTS
TABLE 1. SUMMARY OF LABORATORY TEST RESULTS
APPENDIX A _ HEPWORTH-PAWLAK GEOTECHNICAL, JLTNE 30,2004, ASSESSMENT
OF ROCKFALL RISK TO LOTS 1 _ 6, JOB NO. IOI 126.
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6
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Kumar & A¡sociate¡, lnc. €P'ojec{ No.21.7.845
PURPOSE AND SCOPE OF STUDY
This report presents the results of a subsoil study for a proposed residence to be located on Lot 4,
Spring Ridge Reserve, Dry Park Road, Garfield County, Colorado. The project site is shown on
Figure l. The purpose of the study was to develop recommendations for the foundation design.
The study was conducted in general accordance with our proposal for geotechnical engineering
services to Darrell Cordova dated October 25,2021. Hepworth-Pawlak Geotechnical previously
performed a preliminary geotechnical study for the subdivision development and presented the
findings in a report dated February 26,2001, Job No. I0l 126 and updated the study in a report
dated June 22,2004.
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 analyzed to 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 conclusionso design recommendations and other geotechnical
engineering considerations based on the proposed construction and the subsurface conditions
encountered.
PROPOSED CONSTRUCTION
Building plans were preliminary at the time of our study. In general, the proposed residence will
be a one and two-story wood-frame structure likely above a walkout basement level with an
attached garage and located within the building envelope as shown on Figure l. Ground floors
could be slab-on-grade or structural above crawlspace. Grading for the structure is assumed to
be relatively minor with cut depths between about 3 to 12 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 property was vacant at the time of our field exploration. The site is vegetated with grass,
weeds and sage brush with scrub oak and juniper trees within and above the building area. The
ground surface in the building area slopes moderately down to the northeast at about 20o/owith
about 15 feet of elevation difference across the building footprint. The grade steepens in the
Kumar & A¡oociates, lnc. @ Prcjecl No.21-7-845
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upper lot area to around 30o/o or more. Maroon Formation sandstone is exposed on the hillside to
the west of the lot.
GEOLOGIC HAZARDS REVIE\ry
Hepworth-Pawlak Geotechnical (now Kumar & Associates, Inc.) previously conducted an
assessment of rockfall risk to Lots I - 6, Spring Ridge Reserve and presented the findings in a
report dated June 30,2004, Job No. l0l 126 (attached).
lVe have reviewed the previous assessment and recommendations that the allowable building
area on the reviewed lots (including Lot 4) be limited to the less steep parts of the lots and at
least 50 feet downhill from the cunent upper building envelope line of Lots 1-5. Based on our
review of the previous report and our field observations on November 23,2021, we agree with
the previous assessment and recommend that the proposed building on Lot 4 be limited to at least
50 feet away (northeast) from the current upper building envelope line on Lot 4 to avoid the
potential rockfal I hazard.
FIELD EXPLORATION
The field exploration for the project was conducted on November 23,2021. Three 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 track-
mounted CME-45 drill rig. The borings were logged by a representative of Kumar & Associates.
Samples of the subsoils were taken with lyt inch and 2-inch I.D. spoon samplers. The samplers
were driven into the subsurface materials at various depths with blows from a 140 pound
hammer falling 30 inches. This test is similar to the standard penetration test described by
ASTM Method D-1586. The penetration resistance values arc 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 retumed to our laboratory for review by the project engineer and testing.
SUBSURFACE CONDITIONS
Graphic logs of the subsurface conditions encountered at the site are shown on Figure 2. The
subsoils encountered, below about I to l% feet of topsoil, consist of very stiff to hard, sandy silt
and clay with scattered gravel and cobbles to depths of about ll to 12 feet overlying medium
dense, silty sand with gravel and possible cobbles in Borings I and 3. Below the silt and clay
soil in Boring 2 and the silty sand soil at about 16 to l8 feet in Borings 1 and 3, relatively dense,
silty sandy gravel with cobbles and probable boulders was encountered to the boring depths of
Kumar & A¡sociater, lnc. @ PmJect l,lo. 21.7.845
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17 to 20 feet. Drillìng in the coarse granular subsoils was difficult due the cobbles and boulders
and practical drilling refusal was encountered in the deposit at Boring 2.
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 soils, presented on Figures 4
through 8, generally indicate low to moderate compressibility under light loading and a low
collapse potential (settlement under constant load) 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 upper silt and clay soils are of variable compressibility potential and tend to settle especially
when they become wetted. The top of the less compressible gravel soils appears to slope down to
the east generally with the ground surface slope and may be encountered in the deeper part of the
residence basement excavation and transition to sand, silt and clay in the remaining areas of the
excavation. A shallow foundation placed on the sand, silt and clay soils will have a risk of
settlement if the soils become wetted and care should be taken in the surface and subsurface
drainage around the building to keep the bearing soils dry. It will be critical to the long term
performance of the structure that the recommendations for surface grading and subsurface
drainage contained in this report be followed. Presented below are recommendations for shallow
spread footings with a risk of settlement. A lower settlement risk foundation support can be
achieved by extending the bearing down into the underlying relatively dense gravel soils such as
with piers or piles. If a deep foundation is desired, we should be contacted for additional
recommendations.
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 topsoil provided the owner accepts the risk of settlement and potential
building distress.
The design and construction criteria presented below should be observed for a spread footing
foundation system.
1) Footings placed on the undisturbed natural soils should be designed for an
allowable bearing pressure of 1,200 psf. Based on experience, we expect initial
Kumar & A¡sociates, lnc. o Poject ilo.2l-7-841i
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3)
settlement of footings designed and constructed as discussed in this section will
be about I inch or less. Additional differential settlement could be on the order of
I to 1% inches for a limited wetted depth of around 10 feet below the footings.
The footings should have a minimum width of 20 inches for continuous walls and
2 feet for 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
atea.
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
lateral earth pressures as discussed in the "Foundation and Retaining Walls"
section of this report.
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.
A representative of the geotechnical engineer should observe all footing
excavations prior to concrete placement to evaluate bearing conditions.
4)
5)
FOLINDATION 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 suffrciently to mobilize the full active earth pressure condition should
be designed for a lateral earth pressure computed on the basis of an equivalent fluid unit weight
of at least 45 pcf for backfill consisting of the on-site soils. Backfill should not contain organics
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, traffic, construction materials and equipment. The
pressures recommended above assume drained conditions behind the walls and a horizontal
backfïll 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.
2)
2)
6)
Kumar & A¡sociate¡, Inc. o Profect No.21.7.&15
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Backfill should be placed in uniform lifts and compacted to at least 90% of the maximum
standard Proctor density at near optimum moisture content. Backfïll placed in pavement and
walkway areas should be compacted to at least 95o/o of the maximum standard Proctor density.
Care should be taken not to overcompact the backfill or use large equipment near the wall, since
this could cause excessive lateral 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 foundæion 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 325 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 95o/o of the
maximum standard Proctor density at a moisture content near optimum.
FLOOR SLABS
The natural on-site soils, exclusive of topsoil, can be used to support lightly loaded slab-on-grade
construction. There could be differential settlement potential from wetting of the bearing soils
similar to that described above for footings. 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 slabs for supporl. This material should consist of minus 2-inch aggregate with
at least 50% retained on the No. 4 sieve and less than l2o/o passing the No. 200 sieve. The gravel
layer below the 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 95o/o of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the
onsite soils devoid of vegetation, topsoil and oversized (plus 6-inch) rock.
Kumar & A¡¡ociate¡, lnc. o Project No. 21-7-846
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UNDERDRAIN SYSTEM
Although free water was not encountered during our exploration, it has been our experience in
the area and where there are clay soils that local perched groundwater can develop during times
of heavy precipitation or seasonal runoff. Frozen ground during spring runoff can create a
perched condition. Wç recommend below-grade construction, such as retaining walls, be
protected from wetting and hydrostatic pressure buildup by an underdrain system.
\Mhere installed, the drains should consist of drainpipe placed in the boffom of the wall backfrll
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 lo/o to a suitable gravity outlet. Free-draining granular material used in the
underdrain system should contain less than 2% passing the No. 200 sieve, less than 50% passing
the No. 4 sieve and have a maximum size of 2 inches. The drain gravel backfill should be at
least lt/z feet deep. An impervious membrane such as 20 mil PVC should be placed beneath the
drain gravel in a trough shape and attached to the foundation wall with mastic to prevent wetting
of the bearing soils unless the bearing material is bedrock or non-moisture sensitive soil.
SURFACE DRAINAGE
Proper surface grading and drainage will be critical to limiting subsurface wetting below the
building. The following drainage precautions should be observed during construction and
maintained at all times after the residence has been completed:
l) lnundation 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 95Vo of the maximum standard Proctor density in pavement and slab areas
and to at least 90% of the maximum standard Proctor density in landscape areas.
3) The ground surface surounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of
3 inches in the first l0 feet in paved areas. Free-draining wall backfill should be
covered with filter fabric and capped with about 2 feet of the on-site 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 inigation should be located at least
l0 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 inigation.
Kumar & As¡ociater, lnc. o Prcject l{o. 21.7-846
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LIMITATIONS
This study has been conducted in acc,ordance 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 firture. If the client is concerned about MOBC, then a professional in this special field of
practice should be consulted. Our findings include interpolation and extrapolation of the
subsurface conditions idþntified at the exploratory borings and variations in the subsurface
conditions may not become evident until excavation is performed. If conditions encountered
during construction appear different from those described in this report, we should be notified so
that re-evaluation of the recommendations may be made.
This report has been prepared for the exclusive use by our client for design purposes. We are not
responsible for technical interpretations by others of our information. As the project evolves, we
should provide continued consultation and field services during construction to review and
monitor the implementation of our recommendations, and to veriff 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
ofexcavations and foundation bearing strata and testing ofstructural fi1l by a representative of
the geotechnical engineer.
Respectfu lly Submitted,
Kumar & Associates,
Steven L. Pawlak,
Reviewed by:
Robert L. Duran, P.E.
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21 -7 -845 Kumar & Associates LOCATION OF EXPLORATORY BORINGS Fig. 1
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BORING 1
EL. 6515'
BORING 2
EL. 6522'
BORING 3
EL. 6550'
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36/ 12
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WC=6.2
DD=94
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WC=7.5
DD=93
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-200=36
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20 50/6 50/ 1 20
21 -7 -845 Kumar & Associates LOGS OF TXPLORATORY BORINGS Fig. 2
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TOPSOIL; SILTY SAND WITH ORGANICS AND SCATTERED GRAVEL, FIRM, SLIGHTLY MOIST,
BROWN.
CLAY AND SrLr (CL-ML);
SLIGHTLY MOIST, BROWN,
SANDY, SCATTERED GRAVEL AND COBBLES, VERY STIFF TO HARD,
SLIGHTLY CALCAREOUS, SLIGHTLY POROUS.
SAND (SM); SILTY, SLIGHTIY GRAVELLY, POSSIBLE COBBLES, MEDIUM DENSE, SLIGHTLY MOIST,
RED_BROWN.
GRAVEL (0U); S¡HOy TO VERY SANDY, SILTY, COBBLES AND POSSIBLE BOULDERS, DENSE,
SLIGHTLY MOIST, RED AND BROWN.
DRIVE SAMPLE, 2.INCH I.D. CALIFORNIA LINER SAMPLE.
I DRTVE SAMPLE, 1 3/B-|NCH r.D. SPLIT SPOON STANDARD PENETRATTON TEST.
.,.Ê./ij DRIVE SAMPLE BLOW COUNT. INDICATES THAT 36 BLOWS OF A 140-POUND HAMMERVVl I. FALLING 50 INcHES WERE REQUIRED To DRIVE THE SAMPLER 12 INcHÊS.
I PRACTICAL AUGER REFUSAL.
NOTEg
1. THE EXPLORATORY BORINGS WERE DRILLED ON NOVEMBER 23, 2021 WITH A 4-INCH DIAMETER
CONTINUOUS-FLIGHT POWER AUGER.
2. THE LOCATIONS OF THE EXPLORATORY BORINGS WERE MEASURED APPROXIMATELY BY PACING
FROM FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY BORINGS WERE OBTAINED BY INTERPOLATION BETWEEN
CONTOURS ON THE SITE PLAN PROVIDED.
4. THE EXPLORATORY BORING LOCATIONS AND ELEVATIONS SHOULD BE CONSIDERED ACCURATE
ONLY TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THÊ EXPLORAÏORY BORING LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN ÏHE BORINGS AT THE TIME OF DRILLING
7, LABORAÏORY ÎEST RESULTSI
Wc = WATER CONTENT (%) (ASTM D2216);
DD = DRY DENSITY (PCi) (ASTU D2216);
_2OO= PERCENTAGE PASSING NO. 2OO SIEVE (ASTM D1140)
21 -7 -845 Kumar & Associates LIGTND AND NOTES Fig. 3
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SAMPLE 0F: Sondy Silt , ond Cloy
FROM:Boringl@4'
WC = 7.5 %, DD = 95 pcf
-2OQ = 74 %
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UNDER CONSTANT PRESSURE
DUE TO WETTING
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21 -7 -845 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fig. 4
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SAMPLE OF: Slightly Grovelly Silly Sond
FROM¡Boringl@14'
WC = 3.6 %, DD = 102 pcf
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
DUE TO WETTING
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21 -7 -845 Kumar & Associates SWELL-CONSOLIDATION ÏEST RTSULTS Fig. s
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SAMPLE OF; Very Sondy Silt ond Cloy
FROM:BoringZ@-4'
WC = 6.5 %, DD = 95 pcf
-2OO = 59 %
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ADDITIONAL COMPRESSION
UNDER CONSTANT PRESSURE
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21 -7 -845 Kumar & Associates SWELL-CONSOLIDATION TIST RESULTS Fig. 6
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SAMPLE OF: Sondy Silt ond Cloy
FROM: Boring 3 CD 2'
WC = 6.2 %, DÐ = 94 pcf
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UNDER CONSTANT PRESSURE
DUE TO WETTING
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21 -7 -845 Kumar & Associates SWELL-CONSOLIDATION TEST RESULTS Fis. 7
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SAMPLE OF: Sllghtly Grovelly Silty Sond
FROM;BoringS@14'
WC = 4.9 %, DD = 107 pcf
-2OQ = 36 %
EXPANSION UNDER CONSTANT
UPON WETTINGPRESSURE
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l(+llffiiffiffii$ü"*TABLE ISUÍIIMARY OF LABORATORY TEST RESULTSSandy Silt and ClaySlightly Gravelly SiltySandVery Sandy Silt and ClayVery Silty Sandy GravelSandy Silt and ClaySandy Silt and ClaySlightly Gravelly SiltySandSOILWPE{osflUI.¡CONFINEDCO¡IPRESSIVESTREI¡GTH{%ìPt-ASTlCIt¡DEX74RG UilITSATTERBT{%tLIQUID LffiTÍPERCENTPASSII{G ilO.200 slB/E5938&36(%)SAND(7.1GRAVEL{Dcl}NATURALDRYÐENSFY93rcz93959497r07tYo)NATURALrrorsT{REcot{TEt{T7.53.66.34.36.27.84.94t424l4ff0DEPÏH4t42aJBORING1No.2l-7-845
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I-leprvor rf i-Pau'ìak Ceotechnical, Inc.
5020 Cour,ty Road i54
Cler-ru,ood S¡rrings, Cololado B I 601
Phone: 970-945-7988
HEPWORTH. PAWLAK GEOTECHNICAL Fax: ct70.945-8454
ernaíl: hpgeo@hpgeotech.com
June 30,2004
The Greenwald Children's Trust
c/o Glenwood Brokers, Ltd.
Attn: Pat Fitzgerald
P.O. Box 1330
Glenwood Springs, Colorado 81602
Job No. 101 126
Subject:Assessment of Rockfall Risk to Lots 1-6, Proposed Springridge Place
Phase Il, County Road 125, Garfield County, Colorado
Dear Mr. Fitzgeralcl:
As requested, we have Çonducted a rockfall risk assessment to l,ots 1-6 of the proposed
Springridge Place Phase II subdivision. The analysis was performed as a supplement to
ow previous geotechnical study for the development (Hepworth-Pawlak Geotechnical,
2001) and to address General Conditions items 39 and 41 of the Garfield County
resolution for the subdivision approval dated February 3,2004. This report presents the
findings of our analysis and potential rockfall risk to the site. The analysis was performed
in accordance with our proposal for geotechnical engineering service to The Greenwald
Clrildren's Trust dated }./'ay 28,2004,
Background Information: Our previous reconnaissance iu 2001 identified possible
rockfall impacts to the area of Lots 1-6 on the u'estem side of the subdivision and on Lots
70-81 on the eastern side of the subdivision. Snow cover at the time of our previous
reconnaissance did not allow for thorougir inspection of the eastern lots. A reconnaissance
of tlre potential rockfalll areas was made on }i4.ay 24,2004. At this time, it is our opinion
that the risk of rockfall to Lots 63-8 i is low and rnitigation in that area is not warranted.
The focus of the eurent analysis is potential rockfall impacts to Lots 1-6. The
infonnation obtained fi'om our tecent field review and the Colorado Rockfall Simulation
Program (CRSP Version 4.0) was used in our current evaluation of potential rockfall risks
to Lots l-6.
Proposed Development: The development plans for the Phase II parl of the Springridge
Place developrnent consist of 81 single farnily residential lots having sizes ranging from
about 1 ame to greater than 4 acres. Lots i-6 will be located uphill of County Road 125
anrl the rest of the lots will be locatecl in the valley bottom mainly on the eastern valley
H
Parker 303-841-7119 ¡ ColoradoSprings 719-633-5567 o Silverthorne 970-468-1989
a-z-
side. A private roadway system will provide access to the east of County Road 125
Water and sewer services will be fiom central systems.
Rockfall Character and Potential: Sandstone outcrops near the top of the ridge on the
west sicle of the property are potential sources of rockfali that coulcl reach the proposed
building sites on I-ots 1-6. Sandstone blocks, many of which are the resuit of previous
rockfall events, are present near the proposed building envelopes on these lots. The
sandstone blocks on the hillside range ûp to about 2 to 3Y2 fèet in size. The rocks are
typically between 12 and 22 inches in size at the uphill side of building envelopes on Lots
1-5 and extend typically about 50 feet into the lots. l,ot 6 is located across a ravine and is
beyond the potential rockfall runout limit.
In our opinion, the upper part of the building envelopes on Lots 1-5 have a risk of rockfali
irnpact. The recurrence fi'equency for rockfall in these areas is likely long and rnay be
greater than 100 years. Although infrequent, if a rockfall were to reach the proposed
homes it could result in structural damage and could harm the occupants of the buildings.
if this risk is not acceptable, then rockfall mitigation to reduce the potential risk should be
considered.
Rockfall Mitigation Concepts: In the unlikely event that a rockfall were to reach the
building area, it should be feasible to reduce the risk by building lower on the lot,
providing a catching structure located upslope of the proposed home site or by direct
building protection. It should be possible to clesign the mitigation to provide an acceptable
level of protection to the building and its occupants, but some potential for property
damage is typically accepted by the owner.
Mitigation of the rockfall r:isk by a catching structure or direct protection does not appear
feasible during the subdivision developrnent because the building location aud type are
not known. The risk can be reduced by restricting building within about 50 feet of the
curent uphill building envelope line of Lots 1-5. In our opinion, buildings localed 50 feet
fi'om the upper envelope line and lower in the cunent building envelopes, and 50 feet
southeast of the nofihwest building envelope line o f Lot I , will have a low risk of rockfall
impact. This essentially places the allowable building area in the flatter alluvial apron
parts of Lots i, 2 and 5 anci the less steep parts of tlre colluvial deposits on Lots 3 and 4.
Potential impacts to the selected building sites on Lots 1-5 and the need for any additional
rnitigation to effectively avoid the potential rockfall hazards should be evaluated using the
Colorado Rockfall Simulation Program (CRSP version 4.0) at the tirne of the inclividual
lot developrnerrt.
Job No. 1Al 126
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Limitations: This study was conducted according to generally accepted engineering
geology principles and practices in this atea, atthis time. 'We make no waranty either
express or implied. The conclusions and recomrnendations subrnitted in this report are
based on our field observations, aerial photograph interpretations, interpretations of
previous geologic studies and mapping, and our experience in the area. This report has
been prepared exclusively for our client to evaluate the potential influence of the geology
on the proposed development. The infonnation is suitable for planning and prelirninary
design. W. ar" not responsible for teclinical interpretations by others of our information.
Geotechnical studies will be needed to provide project specific geologic hazard
mitigation, site grading, and design criteria.
if you have any questions or if we rnay be of further assistance, please let us know
Respectfully Submitted,
I-TEPWORTH - PAWLAK GEOTECFINICAL, INC
Steven L. Pawlak, P.E
Rev. by: DEH
SLPiksw
cc:Sopris Engineering- Attn: Yancy Nichol
REF'ERENCE
Hepworth-Pawlak Geotechnical, Prelin¡inaty Geoteclmical Study, Proposed Springr;idge
Place Phase II, County Road 125, Garfield County, Colorado, Job No. I0l 126,
report dated February 26,200I.
JobNo. 101 126
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