HomeMy WebLinkAboutSubsoils Report for Foundation Design 05.25.22K+AKumar & Associates, for..
Geotechnical and Materials Engineers 5020 County Road 154
and Environmental Scientists Glenwood Springs, CO 81601
phone: (970) 945-7988
fax: (970) 945-8454
email: kaglenwood@kumarusa.com
An Employee Owned Compony www.kumarusa.com
Office Locations: Denver (HQ), Parker, Colorado Springs, Fort Collins, Glenwood Springs, and Sununit County, Colorado
May 25, 2022
Davis Construction
Attn: John Davis
P.O. Box 3770
Basalt, Colorado 81621
i olingdavi sc onstr_uction. campany
Project No. 21-7-756
Subject: Geotechnical Review for Building Envelope Adjustment, Proposed Residence,
Lot 12, Coryell Ranch, 541 Coryell Ranch Road, Garfield County, Colorado
Gentlemen:
As requested, we have reviewed the subsoil and geologic conditions information previously
identified at the site and presented in our subsoil study report dated October 27, 2021 for the
proposed adjustment of the current building envelope. The review was conducted supplemental
to and in accordance with our agreement for geotechnical engineering services to Davis
Construction dated September 15, 2021. A preliminary site plan of the adjusted building
envelope with the proposed building footprint was also provided for our review.
Current Building Envelope: CTL Thompson performed a geologic hazards investigation for
the overall Coryell Ranch subdivision and presented their findings in a report dated January 29,
1999, Job No. GS-2647 Part I. As part of this study, CTL Thompson mapped areas of high and
moderate subsidence risk. The northeast portion of Lot 12 near the Roaring Fork River is
mapped as a moderate subsidence risk apparently due to the irregular terrain and adjacent surface
depressions. The mapping provided as part of the report was probably used to set the current
building envelope for Lot 12 outside of the moderate subsidence risk area. We had
recommended if development into this higher risk area is desired, further subsurface
investigation including deep borings should be conducted to further evaluate the risk of
subsidence. Pits 1 and 2 of our previous study encountered relatively dense coarse granular soils
below the topsoil essentially the same as at Pits 3 and 4 located within the current building
envelope.
Proposed Building Envelope Adjustment: The eastern side of the current building envelope is
proposed to be moved approximately 60 feet to the east and placing the proposed building
footprint within the moderate subsidence risk area. A small depression is located next to Pit 4
(near the southwest proposed building corner) which appears relatively old and is backfilled with
cobble to small boulder size river rock. The surface of the rock is depressed 1 or 2 feet possibly
from ground subsidence and could be indicative of a sinkhole.
Conclusion and Recommendations: In our opinion, the risk of future ground subsidence
within the proposed building envelope on Lot 12 is probably higher than the current building
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envelope which is judged as low and similar to other lots in the area, but not totally risk free.
Provided the owner of the lot accepts the higher risk of future ground subsidence possibly
including sinkhole development, the building envelope can be adjusted with additional
consideration for possible future movement and distress of the residence and other site
improvements. The additional design and construction considerations are mitigation measures
to help limit the effects in the event of future subsidence. Similar considerations have been
recommended in some areas of Aspen Glen where a higher subsidence risk may be present.
Deep borings that extend well into bedrock could be drilled in an attempt to quantify the
presence or absence of subsurface voids and associated subsidence risk. The existing cobble
filled depression near Pit 4 should be excavated down to natural ground and backfilled in at least
the lower part with flow fill (lean concrete) during the initial grading of the building site. We
should observe the final excavation depth for indications of cause of the ground anomaly.
We have attached the Chen -Northern (1991) recommendations for building in a broad surface
depression area. We believe these recommendations are conservative but will reduce structural
distress in the event of future ground movement and should be considered in the building design.
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 Lot 12, the proposed construction,
and our experience in the area. 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. Significant design changes may require additional analysis or modifications to the
recommendations presented in this report.
If you have any questions or if we may be of farther assistance, please let us know.
Respectfully Submitted,
Kumar Associat
moo,.
Steven L. Pawla P V
Rev. by: DEH yti
SLP/kac
Attachment: Chen -No
Reference:
Surface Depression Areas
Chen -Northern, Inc., 1991, Preliminary Geotechnical Engineering Study, Proposed Aspen Glen
Development, Garfield County, Colorado, prepared for Aspen Glen Company, dated December
20, 1991, Job No. 4 112 92.
Kumar & Associates, Inc. �' Project No. 21-7-756
0
practical due to the depth of the sinkholes. The grouting procedure should help reduce the
settlement risk but not totally eliminate it. Therefore, we believe that avoiding the sinkholes by
building setback is the lower risk and the more appropriate approach that should be taken.
Development in Surface Depression Areas: Based on our findings, development within the
ground surface depression areas (shown on Fig. 1) should be feasible provided appropriate
mitigative designs are implemented for the residential buildings, utilities and roadways as
described below. The appropriate level of the mitigative designs depend on the potential ground
deformation, the building type, location and configuration and level of tolerable maintenance
(mainly for roadways and utilities). Building design considerations include use of a relatively
rigid foundation, (such as a stiffened slab or raft) and a simply shaped - building footprint to
reduce potential damage in the event of differential movement. These design concepts would
be included in the engineered foundations for residences located in the depression areas.
Utilities should be designed and constructed to be relatively flexible and allow for differential
movement without rupturing. Where possible, settlement sensitive main utility lines should be
routed outside of the ground surface depression areas. Roadways can be conventionally designed
and constructed with provisions for maintenance if subsidence related distress is experienced.
There are several geotechnical design concepts which can be used to mitigate potential
subsidence damage to residential buildings and underground utilities. Special mitigative designs
for a specific lot should be developed by the owner's architect and structural engineer and should
be based on the type of building proposed and the site specific foundation conditions. The
following design concepts are presented to assist in evaluating design options prior to site
Chen Northem Inc. codling Engineers and S
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specific investigations for an individual building site. The concept for underground utilities
should be incorporated into the utility design by the developer.
Building Configurations: The extent of damage to a building subjected to the surface effects of
subsidence may be reduced by implementing several architectural measures in the building
design. These measures would include the following:
* Relatively flexible structural systems such as wood frame construction, flexible
exterior siding, and dry wall interior partitions are preferable to less flexible
masonry structural system and exterior sidings.
* Interior non -bearing. partitions resting on the floor slab should be provided with
slip joints so that slab movements are not transmitted to the upper structure.
* The building should be a low structure preferably limited to one or two stories.
* The building should have relatively small plan dimensions of 60 feet or less. If
this is not practical then the building should be divided into independent modules.
* The building configuration should be a simple rectangular configuration with
straight foundation walls and a minimum of side - projections from the main
building.
* The ground floor should be on a single level rather than using a split level design.
* Basements are particularly susceptible to subsidence damage and are not
recommended unless the entire foundation is at basement level and designed for
lateral earth loading.
Chen @Northern, Inc. Consulting Engmeersand Scienusm
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Building Foundations: A raft foundation with a bearing level near the exterior grade appears
to be an appropriate foundation system for reducing the vulnerability of buildings to differential
subsidence damage. Typical shallow spread footings would be a relatively flexible system and
a rigid system is preferable for the larger magnitude deformations., Foundation system
considerations are outlined below:
* A raft foundation system is the preferable system and should be designed
according to the site specific soil bearing conditions.
* The bottom surface of the raft should be smooth and free of vertical projections.
The raft should be separated from the bearing soils by placing the raft on a
minimum 4-inch thick compacted, clean sand. A polyethylene sheet should be
placed between the raft and the sand layer.
* The use of deep foundation walls should be minimized to the extent practical.
The soil pressure equal to at least twice the "at rest" earth pressure (on the order
of 80 to 100 pcf equivalent fluid unit weight) should be assumed to act on all
vertical' surfaces in contact with the foundation soils.
* The bearing elevation of the raft should be placed below frost depth or sufficient
soil cover should be provided for frost protection.
Underground Utilities: Underground utilities are susceptible to the affects of area subsidence.
As outlined below there are several mitigative design concepts which can .be used to reduce the
potential for damage. In our opinion the mitigation measures should be used where underground
utilities are located in the ground surface depression areas shown on Fig 1.
Chen Northem, Inc. Corm,mng Engineers ana Svermss
* Flexible joints should be used between adjacent pipe segments for both gravity
and pressure lines.
* Positive restraints should be provided in pressure lines to prevent pipe separation.
* A flexible joint should be provided as close as practical to any building, manhole,
or other rigid structural connection.
* A soil cushion in the immediate vicinity of the pipe should be provided by not
over -compacting the backfill soils close to the pipe.
* Check valves should be placed at appropriate locations on all gas and water mains
to permit interruption of flow in case of subsidence distress.
DEBRIS FLOW RISK AND MITIGATION
Hazard Evaluation: This study shows that the alluvial and debris fans along the western side
of the development are potential sites of water flooding and debris flows. The area evaluated
is shown on the attached Fig IA. A summary of the basins and fans evaluated is presented on
the attached Table H. The calculated flow depths and volumes are 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 hazards on all fans. Fans 1 and 2 in the southern part of the area are not
subject to debris flows, but debris flows should be considered on Fans 3 through 25 and the area
to the north (see Fig. 1A).
Based on numerical debris flow modeling, we have designated three potential hazard
Chen -191Northern.Inc CansWupg Engineers ana Scierntas