HomeMy WebLinkAbout5721 Tilden St - Special Inspections/Engineering - 08/15/2013A.G. Wassenaar � � 218 panv 30378 00Fax
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Geotechnical and Environmental Consultants
August 15, 2013
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 3, Block 11
Kechter Crossing
Fort Collins, Colorado
Project Number 130409
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 a 4-inch diameter auger boring near the center of the lot to a
depth of approximately 20 feet. The subsurface materials encountered consisted of:
0' to 8' Fill, clay, stiff, silty, sandy, moist, brown
8' to 20' Clay, medium stiff to stiff, silty, sandy, slightly moist to wet, brown
Date of drilling: July 2, 2013
Depth to water: 18 feet at the time of drilling
Caved at 17'/ feet 6 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 low
to moderate measured swell (0.1% at a depth of 2 feet and 3.2% at a depth of 7 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 to moderate potential for
expansion. 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 130409
August 15, 2013
Page 2
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 2,000 psf. Footing dimensions and foundation structural elements should be determined
by a structural engineer. Concrete in contact with the subsurface materials should be designed for
moderate (S1) 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 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.
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 interiorfloor system is recommended for all finished areas or any other areas
where floor movements cannot be tolerated.
Meritage Homes of Colorado, Inc.
Project Number 130409
August 15, 2013
Page 3
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.
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. In addition, all plumbing lines
should be isolated from the ground surface or foundation walls by at least 2 inches.
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.
Meritage Homes of Colorado, Inc.
Project Number 130409
August 15, 2013
Page 4
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 for water 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.
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.
Meritage Homes of Colorado, Inc.
Project Number 130409
August 15, 2013
Page 5
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.
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.
Meritage Homes of Colorado, Inc.
Project Number 130409
August 15, 2013
Page 6
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.
Sincerely,
A. G. WASSENAAR, IN Q4g ..........
7e : d•�- by
1 / J rn
; ,%:Y_ 3�3 731 33:0
Michael R. Conner, P�tFo *.—LSJ�
Senior Engineer �p .
.r '•
Reviewed by:
L. r. � •®
MRC/DLT/lia
Attachment: Figure 1
Statement of Services
A.G. Wassenaar
Geotechnical and Environmental Consultants\
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
SLIP
CONCRETE SLAB ON GRADE
BACKFILL
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.
INDATION MINIMUM 10 MIL POLYETHYLENE BARRIER OR
WALL EQUIVALENT APPROVED BY GEOTECHNICAL
ENGINEER. ATTACH TO WALL AFTER
DAMPPROOFING AND EXTEND AT LEAST 1
FOOT UPON 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
SHOULD FILL ENTIRE TRENCH AND
ER ABOVE PIPE 3 SHOULD BFOOTING. GRAVEL
AT EXTEND COVER LEAST 4"
' EXTEND POLYETHYLENE TO OUTSIDE
EDGE OF BOTTOM OF TRENCH.
\ DRAIN PIPE - (SEE NOTES #2 & 3)
8" AT LOCATION OF HIGH POINT,
ESTABLISH BOTTOM OF DRAIN
MINIMUM PIPE AT LEAST 4" BELOW
BOTTOM OF FOOTING OR PAD
FOOTING FOUNDATION
FfG 3 EXT-SOG TYPICAL EXTERIOR DRAIN DETAIL
SEPTEMBER 2007 FIGURE 1