HomeMy WebLinkAbout2021 Yearling Dr - Special Inspections/Engineering - 06/28/2013A.G. Wassenaar 2,$ radot80222 5710
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Geotechnical and Environmental Consultants
June 28, 2013
D R Horton
9555 South Kingston Court, Suite 200
Englewood, Colorado 80112-5943
Attention: Mr. Tim Karns
Subject: Soil and Foundation Study
Proposed Residential Structure
Lot 38, Block 12
Sidehill, Filing 2
Fort Collins, Colorado
Project Number 131357
Purpose
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As requested, we have performed a soil and foundation study at the subject site. The purpose of our
study was to observe subsurface conditions encountered and to recommend geotechnical design
criteria for the design and construction of the foundation for the proposed residence. Additionally,
we reviewed partial "Construction Observation and Testing" reports (Earth Engineering Consultants,
LLC, Project Number 1124098, dated December 28, 2012 and May 1, 2013) as a portion of our
analysis. This letter presents a summary of our findings and recommendations.
Subsurface Conditions
The field exploration included drilling a 4-inch diameter auger boring near the center of the lot to a
depth of approximately 24'/2 feet. The subsurface materials encountered consisted of:
0' to 6'/2' Fill, clay, very stiff, silty, sandy, scattered gravel, moist, mottled brown
6'/2, to 9'
9' to 13'
13' to 241/2'
Date of drilling:
Depth to water:
Laboratory Testing
Sand, dense, silty, clayey, gravelly, moist, brown
Sand and Gravel, very dense, silty, moist to wet, brown
Claystone (bedrock), very hard, silty, sandy, moist, rust to gray to olive
May 28, 2013
Dry at the time of drilling
12'/2 feet 6 days after drilling
Samples obtained during drilling were returned to the laboratory. They were visually classified and
testing was assigned to selected samples in an effort to evaluate the engineering properties -of the
subsurface materials encountered. Site specific laboratory swell/consolidation tests exhibited low
measured swell (0.1% at a depth of 2 feet and 1.1% at a depth of 14 feet) upon an increase in
Do 1305977
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D R Horton
Project Number 131357
June 28, 2013
Page 2
moisture content under a load of 1,000 pounds per square foot (psf). Based upon visual observation
of the subsurface conditions encountered and laboratory testing for this and/or adjacent lots, it is our
opinion that the subsurface materials generally exhibit high potential for expansion. Refer to the
Colorado Geological Survey Special Publication 43 for a description of expansive soils and their
impact on structure performance.
Foundation Recommendations
Based on our evaluation of the subsurface conditions, the proposed residence may be founded upon
straight shaft piers drilled at least 12 feet into competent bedrock. We recommend a minimum total
pier length of at least 24 feet in the lower level and 30 feet in the upper level. Pier lengths may only
be reduced after review by this office of site specific conditions encountered during drilling. The piers
should be designed for a maximum end bearing pressure of 30,000 psf, a maximum side shear of
3,000 psf for that portion of pier in competent bedrock, and a minimum dead load pressure of 30,000
psf. No side shear should be used within the upper 12 feet of each pier. In addition, no side shear
should be used for any portion of the pier in fill or natural soils. Where the minimum dead load
pressure cannot be obtained, bedrock penetration should be increased using the side shear value
given above.
Pier dimensions and structural foundation elements should be designed by a structural engineer.
The piers should be reinforced for their full length to resist the potential tension forces which may
develop as a result of the swelling of the subsurface materials in addition to other imposed tension
forces. The piers should be designed with sufficient reinforcing steel to resist an ultimate uplift
pressure of 4,800 psf applied over a 12-foot length of pier minus the dead load imposed on the top
of the pier by the structure. This steel should be coordinated with the other structural requirements
to resist shear, moment and/or structural uplift. Due to the stiffness of the supporting soil, the code
requirements for buckling of the piers may be waived for the portion of the pier below the ground
surface. A minimum 8-inch continuous void should be installed beneath the grade beams between
the piers for concentration of the dead load. Concrete in contact with the subsurface materials
should be designed for moderate (S1) sulfate exposure as defined by ACI 318.4.3. Concrete
should be placed as soon as practical after the drilling and cleaning of each pier hole. Pier
holes should not be left open overnight. Shallow ground water and or caving materials were
encountered during drilling. Therefore, casing will be required. Dewatering of the pier hole
or concrete placement using tremie or pumping methods will be necessary if more than 3
inches of water is present. Concrete for each pier should be placed in such a manner to maintain
a uniform diameter at the top of the pier.
The foundation walls backfilled with on -site materials should be designed for a lateral earth pressure
based upon an equivalent fluid density of 60 pounds per cubic foot (pcf) for the "at rest' condition or
45 pcf for the "active" condition. The "active" condition should only be used where wall movements
of at least 0.5% of the wall height are allowed. These values have been provided without
considerations for sloping backfill, surcharge loading or hydrostatic pressures. Construction of a
drain system and proper surface drainage as discussed later in this report may lower the potential
of developing hydrostatic pressure in the backfill materials. Minor cracking of concrete foundation
walls should be expected.
D R Horton
Project Number 131357
June 28, 2013
Page 3
Basement Floor Construction
A basement slab performance risk evaluation was conducted in general conformance with industry
guidelines for the local area. The risk assessment of a site for potential movement is not absolute;
rather, it represents a judgment based upon the data available and our experience in the area.
Movement of foundations and concrete flat work will occur with time in low to very high risk areas as
the soil moisture content increases. On low and moderate rated sites, slab movements of up to 3
inches or more across the slab, with slab cracking of up to %-inch or more in width and/or differential
are considered normal. The damage generally increases as the risk assessment increases and as
the depth of wetting increases. It must be understood, however, that assessing risk is an opinion.
There is currently no type of testing or correlation of factors that will definitively predict the amount
of heave that a floor slab will exhibit. Therefore, it may be possible that heave less than or in excess
of what is considered "normal" may be experienced.
For sites with a risk assessment of high or very high, we recommend an interior floor system
engineered for expansive soils be constructed. An alternative to the use of an engineered floor
system, such as soil modification to reduce the risk assessment, may also be considered. In
addition, an engineered interior floor system is recommended for all finished areas or any other areas
where floor movements cannot be tolerated.
Based upon our evaluation of the subsurface conditions at this site, it is our opinion that the
basement slab performance risk for this site is moderate. If this risk of movement is not acceptable,
engineered interior floors should be constructed or an alternative such as soil modification should be
considered.
If the Builder and/or Owner desires to construct a concrete slab -on -grade and accepts the risk of slab
movement, slabs supported by the expansive subsurface materials should be constructed using the
following criteria:
1. Slabs should be separated from exterior walls and interior bearing members with
a joint which allows free vertical movement of the slab.
2. Slab bearing partitions should be constructed with a minimum 2-inch void space.
Stairways bearing upon the slab should be constructed in such a way as to allow
at least 2 inches of slab heave. In the event of slab heave, the movement should
not be transmitted directly through the partitions to the remainder of the residence.
3. Plumbing and utilities should be isolated from the slab.
4. Where a forced -air heating system is used and the furnace is located on the slab,
we recommend provision for a collapsible connection between the furnace and the
duct work to allow for at least 3 inches of slab heave. Utility connections should
also be provided with flexible connections capable of accommodating the same
magnitude of movement as specified above.
5. Provide frequent control joints in the slab.
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