HomeMy WebLinkAbout2332 Spruce Creek Dr - Special Inspections/Engineering - 08/21/2014A.G. Wassenaar
S Cle-cok. Lr-
2180 South Ivanhoe Street, Suite 5
Denver, Colorado 80222-5710
Geotechnical and Environmental Consultants
Meritage Homes of Colorado, Inc.
6892 South Yosemite Court, Suite 1-201
Centennial, Colorado 80112
Attention: Mr. Michael Bird
Subject: Soil and Foundation Study
Proposed Residential Structure
Lot 11, Block 7
Mail Creek Crossing
Fort Collins, Colorado
Project Number 143634
Purpose
303-759-8100 Fax 303-756-2920
www.agwassenaar.com
August 21; -2014,. �. •�� ._ =7
1' All
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. 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, 1 foot
below the proposed overlot grade elevation, to a depth of approximately 20 feet. The subsurface
materials encountered consisted of:
0' to 5' Clay, stiff, silty, sandy, slightly calcareous, moist, brown to olive
5' to 17' Clay, medium stiff, silty, very sandy, with sand lenses, moist to very moist, brown
to reddish brown
17' to 20' Clay, stiff, silty, slightly sandy, moist, light brown to gray to olive
Date of drilling: July 16, 2014
Depth to water: Dry at the time of drilling
16 feet 2 days after drilling
Laboratory Testing
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. A site specific laboratory swell/consolidation test exhibited low
measured swell (0.1 % at a depth of 4 feet) upon an increase in 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 low potential for expansion or moderate potential for compression. Refer to the
Colorado Geological Survey Special Publication 43 for a description of expansive soils and their impact
on structure performance
Meritage Homes of Colorado, Inc.
Project Number 143634
August 21, 2014
Page 2
Site Preparation for Foundation Construction
It is possible that after excavation for the basement foundation system, soft, rutting, unstable soils will
be encountered which will not be suitable for support of the proposed construction.
If soft, rutting soils become a problem, stabilization with rockfill may be necessary. Site preparation for
the foundation system for the proposed structure may be accomplished by excavating the site to 1 or
2 feet below the proposed bottom of footing a distance of at least 2 feet beyond the edge of the footing.
Our office should be retained to observe a proof -roll of the foundation system subgrade to determine
if stabilization will be required. The soft, rutting soils should then be stabilized by placing 6-inch minus
crushed rock or an equivalent material, which should be evaluated by our office prior to use. Crushed
concrete should not be used. The material should be placed in approximately 8-inch loose lifts and
rolled into the underlying soft or loose soils. This procedure should be observed by a representative
of our firm and should continue until a front-end loader with loaded bucket, or equivalent, can be
supported on the rock fill with no significant deformation. Subsequent gravel layers should be placed
accordingly until foundation elevation has been obtained. A representative of our firm should verify
stabilization by observing a proof -roll prior to forming footings.
Foundation Recommendations
Based on our evaluation of the subsurface conditions, the proposed residence may be founded upon
spread or pad -type footings bearing below any existing fill on the natural undisturbed soils or on
properly placed and compacted fill. The footings should be designed for a maximum bearing pressure
of 1,200 psf. Footing dimensions and foundation structural elements should be determined by a
structural engineer. Concrete in contact with the subsurface materials may be designed for negligible
(SO) sulfate exposure as defined by ACI 318.4.3. Bearing materials loosened by machine excavation
should be removed prior to placing footing concrete. Occasionally, pockets of loose soils are
encountered in foundation excavations. If this condition occurs, the footings should extend to firm soils.
.Exterior footings should bear at least 3 feet below exterior grade for frost protection. The bearing
materials beneath footings should be protected from freezing during construction. All footing
excavations should be observed prior to placement of concrete to confirm the footings are bearing on
suitable materials as anticipated for design purposes.
The foundation walls backfilled with on -site materials should be designed for a lateral earth pressure
based upon an equivalent fluid density of 65 pounds per cubic foot (pcf) for the "at rest" condition or
50 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.
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.
Meritage Homes of Colorado, Inc.
Project Number 143634
August 21, 2014
Page 3
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 heaves 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 slab
performance risk for this site is low. 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.
Following these recommendations should reduce damage caused by movement of the floor slab;
however, the void spaces recommended are not intended to predict total slab movement. Care should
be taken to monitor and reestablish partition voids and flexible connections when necessary. We are
available to provide further consultation regarding basement slab performance risk assessments.
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