HomeMy WebLinkAboutSubsoil Study for Foundation Design 10.07.16H—P�~KUMAR
Geotechnical Engineering I Engineering Geology
Materials Testing I Environmental
October 7, 2016
Max Edelman
(maxedelman@gmail.com)
c/o Land and Shelter
Attn: Andi Korber
5020 County Road 154
Glenwood Springs, CO 81601
Prone: (970) 945-7988
Fax: (970) 945-8454
Email: hpkgienwood@kumarusa.com
Office Locations: Parker, Glenwood Springs, and Silverthome, Colorado
(andi@landandshelter.com)
Project No. 16-7-475
Subject: Subsoil Study for Foundation Design, Proposed Residence, Lot B7, Aspen
Equestrian Estates, 12 Equestrian Way, Garfield County, Colorado
Dear Mr. Edelman:
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 you dated September 16, 2016. 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 with an attached garage. The residence will be located on the site as shown
on Figure 1. Garage floor will be slab -on -grade. Cut depths are expected to range between
about 3 to 4 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 significantly different from those described
above, we should be notified to re-evaluate the recommendations presented in this report.
Site Conditions: The vacant property is vegetated with cottonwood trees and brush in the rear
portion of the lot and with grass and weeds in the building area. The site appears to have been
graded during subdivision development with shallow fill placed in the front part of the lot. The
ground surface is relatively flat with a slight slope down to the south. Blue Creek is located
south of the property.
Subsidence Potential: Bedrock of the Pennsylvanian Age Eagle Valley Evaporite underlies
Aspen Equestrian Estates. These rocks are a sequence of gypsiferious shale, fine-grained
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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 throughout 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, consist of silty
sandy gravel with cobbles. About one foot of man -placed clayey sand and gravel fill was
encountered at Pit 1 overlying the topsoil. Results of a gradation analysis performed on a sample
of sandy gravel with cobbles (minus 6 inch fraction) obtained from the site are presented on
Figure 4. No free water was observed in the pits at the time of excavation and the soils were
slightly moist to moist. It has been our experience in this area that groundwater can be
encountered at depths of 4 to 5 feet at some time during the year.
Foundation Recommendations: Considering the subsoil conditions encountered in the
exploratory pits and the nature of the proposed construction, we recommend 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 soils tend to compress after wetting and
there could be some post -construction foundation settlement. Footings should be a minimum
width of 16 inches for continuous walls and 2 feet for columns. Loose and disturbed soils,
topsoil 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 granular
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 10 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 45 pcf for the on-site soil as backfill.
Floor Slabs: The natural on-site soils, exclusive of topsoil, are suitable to support lightly loaded
slab -on -grade construction. To reduce the effects of some differential movement, floor slabs
should be separated from all bearing walls and columns with expansion joints which allow
unrestrained vertical movement. Floor slab control joints should be used to reduce damage due
to shrinkage cracking. The requirements for joint spacing and slab reinforcement should be
established by the designer based on experience and the intended slab use. A minimum 4 inch
layer of free -draining gravel should be placed beneath basement level slabs to facilitate drainage.
H -P z KUMAR
Project No. 16-7-475
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This material should consist of minus 2 inch aggregate with less than 50% passing the No. 4
sieve and less than 2% passing the No. 200 sieve.
All fill materials for support of floor slabs should be compacted to at least 95% of maximum
standard Proctor density at a moisture content near optimum. Required fill can consist of the on-
site granular soils devoid of vegetation, topsoil and oversized rock.
Underdrain System: Although free water was not encountered during our exploration, it has
been our experience in the area that local perched groundwater can develop during times of
heavy precipitation or seasonal runoff. Shallow groundwater is typical during the irrigation
season. Frozen ground during spring runoff can also create a perched condition. We
recommend below -grade construction, such as retaining walls and crawlspace areas, be protected
from wetting and hydrostatic pressure buildup by an underdrain system.
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 1 foot below lowest adjacent finish grade and sloped at a minimum I% to
a suitable gravity outlet or sump and pump. 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 1=/z feet 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 of the foundation excavations and underslab areas should be avoided
during construction.
2) Exterior backfill should be adjusted to near optimum moisture and compacted to
at least 95% of the maximum standard Proctor density in pavement and slab areas
and to at least 9017c: of the maximum standard Proctor 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 surrounding the exterior of the building should be sloped to
drain away from the foundation in all directions. We recommend a minimum
slope of 12 inches in the first 10 feet in unpaved areas and a minimum slope of 3
inches in the first 10 feet in pavement and walkway areas.
4) Roof downspouts and drains should discharge well beyond the 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. 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 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
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
H -P �t KUMAR Project No. 16-7-475
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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 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 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
of excavations 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-Pi� KU AR
Luis Eller
Reviewed by:
co
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Daniel E. Hardin, Pt.E,
LEElksw
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attachments Figure I - Locatt no of Exploratory Pits
Figure 2 - Logs of Exploratory Pits
Figure 3 - Legend and Notes
Figure 4 - Gradation Test Results
H -P t- KUMAR Project No. 16-7-475
PIT 1 PIT 2
0
0
0
a
o
w b
w o -� +4=76 o
w
z 5 -200=3s
R
w
CLW
10
10
LEGEND
® TOPSOIL; ORGANIC SILT AND SAND WITH GRAVEL, LOOSE, MOIST, DARK BROWN.
® FILL: CLAYEY SAND AND GRAVEL, LOOSE, SLIGHTLY MOIST.
I.AAGRAVEL (GM -GP), WITH COBBLES AND SMALL BOULDERS, SANDY, SILTY, DENSE, SLIGHTLY
p .
MOIST, LIGHT BROWN, ROUNDED ROCKS.
DISTURBED BULK SAMPLE.
T PRACTICAL DIGGING REFUSAL.
NOTES
1. THE EXPLORATORY PITS WERE EXCAVATED WITH A TRACKHOE ON SEPTEMBER 30, 2016.
2. THE LOCATIONS OF THE EXPLORATORY PITS WERE MEASURED APPROXIMATELY BY PACING FROM
FEATURES SHOWN ON THE SITE PLAN PROVIDED.
3. THE ELEVATIONS OF THE EXPLORATORY PITS WERE NOT MEASURED AND THE LOGS OF THE
EXPLORATORY PITS ARE PLOTTED TO DEPTH. PIT 2 IS ABOUT I FOOT LOWER THAN PIT 1.
4. THE EXPLORATORY PIT LOCATIONS SHOULD BE CONSIDERED 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 TRANSITIONS MAY BE GRADUAL.
6. GROUNDWATER WAS NOT ENCOUNTERED IN THE PITS AT THE TIME OF EXCAVATING.
7. LABORATORY TEST RESULTS:
+4 = PERCENTAGE RETAINED ON NO. 4 SIEVE (ASTM D 422);
-200 = PERCENTAGE PASSING NO. 200 SIEVE (ASTM D 1140).
16-7-475 1 H-P�KUMAR I LOGS OF EXPLORATORY PITS Fig. 2