HomeMy WebLinkAbout3556 BEAR RIVER CT - SPECIAL INSPECTIONS - 6/26/201321 8penv-l~adot802 70A.G. Wassenaar e, (oo2251
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Geotechnical and Environmental Consultants
June 26, 2012
D R Horton
9555 South Kingston Court, Suite 200
Englewood, Colorado 80112-5943
Attention: Mr. Kyle S. Gunther
Subject: Soil and Foundation Study
Proposed Residential Structure
Lot 25, Block 2
Trail Head
Fort Collins, Colorado
Project Number 120911
Purpose
. 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. This letter
presents a summary of our findings and recommendations.
Subsurface Conditions
The field exploration included drilling two 4-inch diameter auger borings at the approximate diagonal
corners of the anticipated building footprint to depths of approximately 25 and 30 feet. The subsurface
materials encountered consisted of:
Boring 1 (northwest corner)
0' to 25' Clay, stiff to hard, silty, sandy, slightly moist to moist, brown to dark brown
Boring 2 (southeast corner)
0' to 25' Clay, stiff, silty, sandy, slightly moist to moist, brown to dark brown
25' to 30' Sand, silty, wet, brown to light brown
Date of drilling: May 15, 2012
Depth to water: Boring 1: Dry at the time of drilling
Dry 2 days after drilling
Boring 2: 25 feet at the time of drilling
Caved at 221/2 feet 2 days after drilling
D R Horton
Project Number 120911
June 26, 2012
Page 2
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 low
measured swell (0.8%) at a depth of 2 feet and compression at depths of 7 and 14 feet in Boring 1 and
compression at a depth of 9 feet in Boring 2 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. 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
spread or pad -type footings bearing on the natural undisturbed soils 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 should be
designed for very severe 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 55 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.
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 across the slab with slab cracking of up to'/< inch in width and/or differential are considered
normal. The damage generally increases as the risk assessment increases and as the depth of wetting
D R Horton
Project Number 120911
June 26, 2012
Page 3
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 interiorfloor 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 will reduce immediate 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.
D R Horton
Project Number 120911
June 26, 2012
Page 4
Crawl Space Construction
The crawl space ground surface should be sloped to the perimeter drain system. Trenching or dishing
out of the crawl space is not recommended unless a drain system is placed in these areas in such a
manner to facilitate drainage. The recommended clearance from the crawl space ground surface to the
engineered floor system should meet applicable codes as well as be increased by the recommended
foundation void height. In addition, all plumbing lines should be isolated from the ground surface or
foundation walls by at least the height of the previously recommended void thickness.
During construction, the crawl space area should be checked for standing water or very moist
conditions, construction debris, and other deleterious materials. If these conditions exist, the area
should be evaluated and mitigated, as necessary.
Crawl space areas should be constructed with consideration given to proper ventilation and moisture
management.. Provisions such as the installation of a vapor retarder should be utilized to reduce the
amount of moisture (humidity) in the crawl space air. The Client and any future Owner should be aware
that crawl space areas are subject to various air quality issues. A consultant specializing in ventilation
and air quality control should be contacted to provide any additional recommendations. Such
recommendations are beyond the geotechnical scope of this study. The environmental division of A.
G. Wassenaar, Inc. is capable of providing such services. Refer to "Homeowner's Guide To Moisture
Management" by Tri-County Health Department (Brochure Number S-323) for additional information.
Subsurface Drainage
As a minimum, we recommend providing a subsurface drainage system around the lowest below grade
area. The purpose of the drain is to collect water which may become trapped on the surface of the
excavation and enter the basement or crawl space areas. A drain should be constructed similar to the
attached drain detail (Figure 1) and should be uniformly sloped to a positive gravity discharge or sump.
If a sump pit is installed, it should be monitored forwater accumulation and proper operation. The water
level in the sump pit should not be allowed to rise above the foundation drain inlet pipe(s). If water rises
above the inlet pipe(s), a pump should be installed (if not originally equipped) or maintenance should
be performed on the existing pump.
Surface Drainage
The wetting of foundation soils and/or bedrock materials which causes heave may be reduced by
carefully planned and maintained surface drainage. The following recommendations should be
implemented during construction and maintained by the Homeowner after the residence is completed:
1. Excessive wetting or drying of the open foundation excavation should be avoided as
much as practical during construction.
D R Horton
Project Number 120911
June 26, 2012
Page 5
2. The ground surface surrounding the exterior of the foundation should be maintained
in such a manner as to provide for positive surface drainage away from the
foundation. At completion of construction, we recommend a minimum fall away from
the foundation of 6 inches in the first 5 feet. This slope should be continuous across
the backfill zone.
3. Backfill around the foundation should be moistened and compacted in such a manner
as to reduce future settlement. Areas which settle should be filled as soon as
possible in order to maintain positive drainage away from the foundation.
4. If lawn edging is used around the exterior of the foundation, it should be constructed
in a manner to prevent ponding of surface water in the vicinity of the backfill soils.
5. All drainage swales should be constructed and maintained a minimum of 5 feet away
from the foundation on side yards and 15 feet away from the foundation on back and
front yards. Drainage swales should maintain a slope of at least 2% off of the lot.
Swales must not be blocked by fences, landscaping, paths or other Homeowner
installed items.
6. Roof downspouts and drains should discharge well beyond the limits of foundation
backfill.
7. Watering adjacent to the foundation should be reduced as much as practical.
Landscaping which requires excessive watering should not be located within 5 feet
of foundation walls. Main sprinkler lines, zone control boxes and drains should be
located outside the limits of the foundation backfill. Sprinkler heads should be
positioned such that the spray does not fall within 5 feet of foundation walls.
8. Plastic membranes should not be used to cover the ground surface immediately
surrounding the foundation. These membranes tend to trap moisture and prevent
normal evaporation from occurring. We recommend the use of a weed suppressant
geotextile fabric.
Limitations
We believe the professional judgments expressed in this report are consistent with that degree of skill
and care ordinarily exercised by practicing design professionals performing similar design services in
the same locality, at the same time, at the same site and under the same or similar circumstances and
conditions. No other warranty, express or implied, is made. The location of the test boring drilled and
the laboratory testing performed for this study were designed to obtain a reasonably accurate picture
of subsurface conditions for design purposes. Variations in subsurface conditions not indicated by the
boring are possible and expected. Therefore, we should be retained to observe the foundation
excavation and construction in order to verify or revise our recommendations. If unexpected subsurface
conditions are observed by others during construction, we should be called to review our
recommendations.
D R Horton
Project Number 120911
June 26, 2012
Page 6
This report was prepared for the exclusive use of our Client for the sole purpose of providing
geotechnical design criteria for the subject structure based upon the existing site conditions as
encountered. The conclusions and recommendations contained in this report shall not be considered
valid for use by Others without written authorization from A. G. Wassenaar, Inc. In addition, the state
of practice in geotechnical engineering is constantly evolving. Therefore, findings presented in this
report should be reviewed and revised, if necessary, prior to actual construction.
If we can be of further service in discussing the contents of this letter or in analysis of the proposed
structure from the soil and foundation viewpoint, please call our office.
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Sincerely, 00 vo REG/'q i
A. G. WASSENAA/jR`O . Qc�' B9ytiF9m%
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Robert U. Brdfison,
Senior Engineer
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Reviewed by:
Keith D. Seaton, P.E
Senior Engineer
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Attachment: Figure 1
Statement of Services
A.G. Wassenaar
Geotechnical and Environmental Consultants ,I`y �°•
NOTES:
1. DRAIN MUST SLOPE TO A POSITIVE
GRAVITY OUTLET AND/OR TO SUMP
WHERE WATER CAN BE REMOVED
BY PUMPING
2. SLOPE BOTTOM OF TRENCH AND PIPE AT A
MINIMUM OF 1/8" PER FOOT (i.e. 1 %)
OR AS APPROVED BY THE GEOTECHNICAL ENGINEER
3. 4-INCH DIAMETER RIGID PERFORATED PVC
PIPE (ASTM D2729 MINIMUM SCHEDULE 20), OR
SUBSTITUTE APPROVED BY GEOTECHNICAL ENGINEER
4. GRAVEL SPECIFICATION: 2" MINUS WASHED
ROCK/GRAVEL, POORLY GRADED WITH NO MORE
THAN 30% PASSING THE 3/8" SIEVE AND NO
MORE THAN 10% PASSING THE #4 SIEVE, OR AS
APPROVED BY THE GEOTECHNICAL ENGINEER
CONCRETE SLAB ON GRADE
BACKFILL
INDATION MINIMUM 6 MIL POLYETHYLENE BARRIER OR
WALL EQUIVALENT APPROVED BY GEOTECHNICAL
ENGINEER. ATTACH TO WALL AFTER
DAMPPROOFING AND EXTEND AT LEAST 1
FOOT UP ON WALL AND BENEATH FLOW LINE
OF PIPE.
BACKFILL
NON -WOVEN GEOTEXTILE FILTER
FABRIC (MIRAFI 140N OR EQUIVALENT
APPROVED BY GEOTECHNICAL
ENGINEER) PLACED ACROSS ENTIRE
WIDTH OF DRAIN GRAVEL.
DRAIN GRAVEL (SEE NOTE #4) GRAVEL
SLIP
SHOULD FILL ENTIRE TRENCH AND EXTEND
DINT LATERALLY TO TOP OF FOOTING. GRAVEL
COVER ABOVE PIPE SHOULD BE AT LEAST 6"
DO NOT EXCAVATE WITHIN A 1:1 LINE —
EXTENDING DOWN AND AWAY FROM
EDGE OF FOOTING. MAINTAIN THIS ANGLE
OF EXCAVATION TO A DEPTH OF TWICE THE
FOOTING WIDTH.
...0. D ... _.0.
8"
MINIMUM
FOOTING FOUNDATION
D.R. HORTON-NORTH TYPICAL EXTERIOR DRAIN DETAIL
FfG 3 EXT-SOG FIGURE 1
SEPTEMBER 2007
EXTEND POLYETHYLENE TO OUTSIDE
EDGE OF BOTTOM OF TRENCH.
DRAIN PIPE - (SEE NOTES #2 & 3)
AT LOCATION OF HIGH POINT,
ESTABLISH BOTTOM OF DRAIN
PIPE AT OR BELOW
BOTTOM OF FOOTING OR PAD