HomeMy WebLinkAbout2602 Palomino Ct - Special Inspections/Engineering - 07/25/2014A.G.
enaar
2180 South Ivanhoe Street, Suite 5
Denver, Colorado 80222-5710
Geotechnical and Environmental Consultants
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 1, Block 1
Bucking Horse, Filing 2
Fort Collins, Colorado
Project Number 131259
Purpose
July 25, 2014
303-759-8100 Fax 303-756-2920
® www.agwassenaar.com
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. Fill
placed during overlot grading was observed and tested by Earth Engineering Consultants,
Inc. (EEC) (EEC Project Number 1134107, dated June 16, 2014) on a part-time basis. A. G.
Wassenaar, Inc. also provided part-time observation of the fill placement and testing operations.
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 feet. The subsurface materials encountered consisted of:
0' to 6' Fill, clay, stiff to very stiff, sandy, with scattered gravel, moist, mottled brown
6' to 12'/z' Clay, stiff to very stiff, silty, sandy, moist to very moist, brown to dark brown
12'/2' to 24' Sand and Gravel, dense, silty to clean, wet, brown to gray
Date of drilling: June 9, 2014
Depth to water: 12 feet at the time of drilling
Caved at 13 feet 3 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. Site specific laboratory swell/consolidation tests exhibited
compression at depths of 2 and 7 feet upon an increase in moisture content under a load of
D R Horton
Project Number 131259
July 25, 2014
Page 2
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. Refer to the Colorado Geological Survey
Special Publication 43 for a description of expansive soils and their impact on structure
performance.
Existing Fill
Fill was encountered in our test borings. The construction observation reports of the fill placement
presented to this office indicated that all of the tests passed the compaction and moisture
specifications. Additionally, our periodic observations indicated the fill was being placed in a manner
that could likely be used to support foundations. Our sampling and testing of the fill encountered
indicated compaction was performed when placed. However, observations and testing were
performed on a part-time basis. The Client must understand that even with the engineering opinions
expressed within this report, the construction of foundations on the existing fill is a risk that they
must accept. We cannot and will not be liable for work conducted by others. An excavation
observation at the time of construction does not constitute our acceptance of the fill for structural
support.
Foundation Recommendations
Based on our evaluation of the subsurface conditions, the proposed residence may be founded
upon spread or pad -type footings bearing on the natural undisturbed soils/sandstone or on properly
placed and compacted fill. The footings should be designed for a maximum bearing pressure of
2,500 psf with a minimum dead load pressure of 800 psf. Four -inch void material should be
installed in areas where the minimum dead load cannot be attained. 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 priorto 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 30 inches 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.
D R Horton
Project Number 131259
July 25, 2014
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 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.
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