HomeMy WebLinkAboutSubsoil Study for Foundation Design 08.22.17H.PVKUMAR
Geotechnical Engineering I Engineering Geology
Materials Testing I Environmental
RECEIVED
JUN T g zOI$
GARFIELD COUNTY
COM M UNITY DEVELOPIIEI{T
5020 County Road 154
Glenwood Springs, CO 81601
Phone: (970) 945-7988
Far (970) 945-8454
Email: hpkglenwood@kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, Summit County, Colorado
August 22,2017
Integrated Mountain Development
Attn: Scott Dillard
21 County Road 126
Glenwood Springs, Colorado 81601
($çELtcl ill arclrealtor @ gmail.gau)
Project Na.L7-7-624
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot B-8, Aspen
Equestrian Estates, 156 Equestrian'Way, Garfield County, Colorado
Dear Mr. Dillard:
As requested, H-P/Kumar performed a subsoil study for design of foundations at the subject site.
The study was conducted in accordance with our agreement for geotechnical engineering
services to Integrated Mountain Development dated August 1,4,2017. The data obtained and our
recommendations based on the proposed construction and subsurface conditions encountered are
presented in this report. Hepworth-Pawlak Geotechnical, Inc, (now H-P/Kumar) previously
performed a preliminary geotechnical study for the proposed development (Preshana Farms) and
reported the findings August 31, 1998, Job No. 198 501.
Proposed Construction: The proposed residence will be one story wood frame construction
above a crawlspace and with an attached garage. The residence will be located in the building
envelope shown on Figure 1. Garage floor will be slab-on-grade. Cut depths are expected to
range between about 2 to 3 feet. Foundation loadings for this type of construction are assumed
to be relatively light and typical of the proposed type of construction.
If building conditions or foundation loadings are signifïcantly different from those described
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The property is vacant and vegetated with grass and weeds with scattered
cottonwoods to south of lot. There may have been minor grading during subdivision
a
development. The ground surface is relatively flat with a slight slope down to the west. There is
a drainage swale with willows along the western lot line and Blue Creek is located south of the
site.
Subsidence Potential: Bedrock of the Pennsylvanian Age Eagle Valley Evaporite underlies
Aspen Equestrian Estates. These rocks are a sequence of gypsiferious shale, fine-grained
sandstone/siltstone and limestone with some massive beds of gypsum. There is a possibility that
massive gypsum deposits associated with the Eagle Valley Evaporite underlie portions of the
property. 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 broad
subsidence areas and sinkholes have been observed in the lower Roaring Fork Valley,
No evidence of subsidence or sinkholes were observed on the property or encountered in the
subsurface materials, however, the exploratory pits were relatively shallow, for foundation
design only. Based on our present knowledge of the subsurface conditions at the site, it can not
be said for certain that sinkholes will not develop. The risk of future ground subsidence at the
site tfuoughout the service life of the structure, in our opinion is low, however the owner should
be 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.
Subsurface Conditions: The subsurface conditions at the site were evaluated by excavating two
exploratory pits at the approximate locations shown on Figure 1. The logs of the pits are
presented on Figure 2. The subsoils encountered, below about one foot of topsoil at Pit I and lVz
feet of fill and 6 inches of topsoil atPit2, consist of one foot of silty clayey sand overlying silty
sandy gravel with cobbles and small boulders. Results of a gradation analysis performed on a
sample of slightly silty sandy gravel with cobbles (minus 5 inch fraction) obtained from the site
are presented on Figure 3. Free water was observed in the Pit L at3Vz feet and Pit? at 6 feet at
the time of excavation. The upper soils were slightly moist to moist.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recofirmend spread footings
placed on the undisturbed natural granular soil designed for an allowable soil bearing pressure of
2,500 psf for support of the proposed residence. The fill, topsoil and clayey sand soils tend to
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Project No.17-7-624
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compress after wetting producing post-construction foundation settlement and should be
removed from beneath foundations and slabs. Footings should be a minimum width of 16 inches
for continuous walls and 2 feet for columns. Loose and disturbed soils and existing fill
encountered at the foundation bearing level within the excavation should be removed and the
footing bearing level extended down to the undisturbed natural gravel soils. Exterior footings
should be provided with adequate cover above their bearing elevations for frost protection.
Placement of footings at least 36 inches below the exterior grade is typically used in this area.
Continuous foundation walls should be reinforced top and bottom to span local anomalies such
as by assuming an unsupported length of at least l0 feet. Foundation walls acting as retaining
structures should be designed to resist a lateral earth pressure based on an equivalent fluid unit
weight of at least 50 pcf for the on-site soil as backf,ill.
Floor Slabs: The natural on-site granular soils, exclusive of topsoil, are suitable to sllpport
lightly loaded slab-on-grade construction. The existing fill and topsoil soils should be removed
from beneath slab areas. To reduce the effects of some differential movoment, 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 2 inch aggregate with less than SOVI passing the No. 4 sieve and
less than ZVo passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95Vo of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site granular soils or a suitable imported gravel devoid of vegetation, topsoil and oversized rock.
Underdrain System: Free water was encountered relatively shallow during our exploration and
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. We recommend below-grade construction, such as retaining walls and deep
crawlspace areas, be protected from wetting and hydrostatic pressure buildup by an underdrain
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system. A shallow crawlspace (less than 3 feet below exterior grade) and slab-on-grade Earage
should not require an underdrain system provided the exterior backfill is properly placed.
The drains (if needed) 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 I%a to a suitable gravity outlet. Free-draining granular material used in the
underdrain system should contain less than ZVo passingthe No. 200 sieve, less than 507o passing
the No. 4 sieve and have a maximum size of 2 inches, The drain gravel backfill should be at
least lVzfeet deep.
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 bacKill should be adjusted to near optimum moisture and compacted to
at least 957o of.the maximum standard Proctor density in pavement and slab areas
and to at least 90Vo of.the maximum standard Procto¡ density in landscape areas.
Free-draining wall backfill should be capped with about 2 feet of the on-site, finer
graded soils to reduce surface water infiltration.
3) The ground surface surounding the exterior of the building should be sloped to
drain away from the foundation in all directions. IVe recommend a minimum
slope of 6 inches in the first 10 feet in unpaved areas and a minimum slope of 3
inches in the first l0 feet in pavement and walkway areas.
4) Roof downspouts and drains should discharge well beyond thç limits of all
backfill.
Limitations: This study has been conducted in accordance with generally accepted geotechnical
engineering principles and practices in this area at this time. tiVe make no warranty either
express or implied. The conclusions and recommendations submitted in this report are based
upon the data obtained from the exploratory pits excavated at the locations indicated on Figure 1
and to the depths shown on Figure 2, the proposed type of construction, and our experience in
the area. Our services do not include determining the presence, prevention or possibility of mold
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or other biological contaminants (MOBC) developing in the future. 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 identified at the
exploratory pits 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 at once so re-evaluation of the
recommendations may be made.
This report has been prepared for the exciusive 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 verify 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 of structural fill by a representative of
the geotechnical engineer.
If you have any questions or if we may be of further assistance, please let us know
Respectfully Submitted,
H-P MAR
Louis E. Eller, Staff Engineer
'', :
'¿ ilReviewed by:
Daniel
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Attachments Figure 1 -Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Figure 3 - Gradation Test Results
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APPROXIMATE SCALE-FEET
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17 -7 -624 H-PryKUMAR LOCATION OF EXPLORATORY PITS Fig. 1
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LEGEND
TOPSOIL, ORGANIC CLAYEY SILTY SAND, SCATTERED CRAVEL, LOOSE, SLIGHTLY MOIST, DARK
BROWN.
FILL; SILTY SAND AND GRAVEL WITH COBBLES, LOOSE, SLIGHTLY MOIST, BROWN.
SAND (SC); CLAYEY, GRAVELLY, MEDIUM DENSE, MolsT, BRowN.
GRAVEL, COBBLES AND SMALL BOULDERS (GP-GM); SANDY, SLIGHTLY SILTY, DENSE, MOIST
TO WET WIÏH DEPTH, BROWN, ROUNÞED ROCK.
DISTURBED BULK SAMPLE.
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9 oEpru ro wATER LEVEL AT T¡ME oF EXcAVAINc.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A BACKHOE ON AUGUST 16,2017.
2, THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
FEATURES SHOWN ON THE SITE PLAN PROVIDED
3. THE ETEVATIONS OF THT EXPLORATORY PITS WERE MEASURED BY HAND LEVEL AND REFER TO
THE GROUND SURFACE AT PIT 1 AS ELEVATION lOO,O FEET.
4. THE EXPLORATORY PIT LOCATIONS AND ELEVATIONS SHOULD BT CONS|DERED ACCURATE ONLY
TO THE DEGREE IMPLIED BY THE METHOD USED.
5. THE LINES BETWEEN MATERIALS SHOWN ON THE EXPLORATORY PIT LOGS REPRESENT THE
APPROXIMATE BOUNDARIES BETWEEN MATERIAL TYPES AND THE TRA,NSITIONS MAY SE GRADUAL,
6. GROUNDWATER LEVELS SHOWN ON THE LOGS WERE MEASURED AT THE TIME ÄND UNDER
CONDITIONS INDICATED. FLUCTUATIONS IN THE WATER LEVEL MAY OCCUR WITH TIME,
7. LABORATORY TESÎ RESULTS:
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D 422);
-200 = PERCENTAGE PASSING No. 200 SIEVE (ASTM D 1140)
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H-PryKUMAR LOGS OF TXPLORATORY PITS Fig. 217 -7 -624
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CLAY TO SILT COBELES
GRAVEL 70 '( SAND 21 I
UQUID LIMIT PLASTICITY ¡NDEX
SAMPLE OF: Sllghlly Sllty Sondy Groval wlth Cobblcs
SILT AND CLAY 6 X
FROM:Pll 1O5'-¡l'
th.r. l.¡l ioulh opply only lo lh.romplil whloh w.n l.drd. Th.It.llng nporl rhqll trol b. r.producad.rxc.pl ln lull, vllhoul lhG wdll.n
opptovol ol Kumor & Attoolol.r, lñ0.
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HYOROMETER ANALYSIS SIEVE ANALYSIS
SAND GRAVEL
FINE MEDIUM COARSE FINE COARSE
17 -7 -624 H-PryKUMAR GRADATION TEST RESULTS Fis. 3