HomeMy WebLinkAbout868 CAMPFIRE DR - SPECIAL INSPECTIONS - 9/21/2012A.G.
Geotechnical and Environmental Consultants
D R Horton
9555 South Kingston Court, Suite 200
Englewood, Colorado 80112-5943
Attention: Mr. Kyle Gunther
Subject: Soil and Foundation Study
Proposed Residential Structure
Lot 20, Block 2
Trail Head
Fort Collins, Colorado
Project Number 121881
Purpose
2180 South Ivanhoe Street, Suites 5
Oanvor, Colorado 80222-5710
303-759-8100 Fax 303-756-2920
www.agwassenaar.com
September 21, 2012
i A Is
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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. 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 in
the upper level to a depth of approximately 20 feet. The subsurface materials encountered
consisted of:
0' to 51/2' Fill, clay, stiff, sandy, slightly moist to moist, brown
51/2' to 20' Clay, stiff to very stiff, silty, sandy, moist to very moist, brown
Date of drilling: August 17, 2012
Depth to water: Dry at the time of drilling
Dry 5 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
measured swell (1.9% at a depth of 4 feet and 0.9% at a depth of 9 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. Refer to the Colorado Geological Survey Special Publication 43 for a description of
expansive soils and their impact on structure performance
D R Horton
Project Number 121881
September 21, 2012
Page 2
Foundation Recommendations
Our sampling and testing of the fill encountered indicated it was likely placed in a manner that
would allow foundation support. However, construction observation reports of the fill placement
are not available. Therefore, we must be retained to observe the fill in the open excavation (prior
to foundation construction) to verify the quality of the fill. Additional recommendations will be made
at that time as necessary.
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 (determined as described previously). 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
(S3) 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 dry, hard
or very moist, soft fill may be encountered in the foundation elevation. If this condition occurs, the
footings should extend to properly moisture treated fill. 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 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
D R Horton
Project Number 121881
September 21, 2012
Page 3
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 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 121881
September 21, 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 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:
F1
D R Horton
Project Number 121881
September 21, 2012
Page 5
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.
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 dischargewell 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 otherwarranty, express or implied, is made. The location of the
D R Horton
Project Number 121881
September 21, 2012
Page 6
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.
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,
KIDS/lia
Attachment: Figure 1
Statement of Services
A.G. Wassenaar
Geoteoh,Ic I anC Erntronmental Consultants C•
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/W 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
ROCKIGRAVEL, 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
JOINT
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.
)NDATION MINIMUM 10 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
BACKFILL OF PIPE.
NON -WOVEN GEOTEXTILE FILTER
FABRIC (MIRAFI 140N OR EQUIVALENT
APPROVED BY GEOTECHNICAL
ENGINEER) PLACED ACROSS ENTIRE
WIDTH OF DRAIN GRAVEL.
LATERALLYDRAIN GRAVEL (SEE NOTE #4) GRAVEL
SHOULD FILL ENTIRE TRENCH AND
ER ABOVE PIPE SHOULD OLEAD BE AT EL
OST4"
P :.0
o •:
EXTEND POLYETHYLENE TO OUTSIDE
EDGE OF BOTTOM OF TRENCH.
\
DRAIN PIPE - (SEE NOTES #2 & 3)
8°
AT LOCATION OF HIGH POINT,
\
MINIMUM
ESTABLISH BOTTOM OF DRAIN
PIPE AT LEAST 4" BELOW
BOTTOM OF FOOTING OR PAD
FOOTING FOUNDATION
FTG 3 EXT-SOG TYPICAL EXTERIOR DRAIN DETAIL
SEPTEMBER 2007 FIGURE 1