HomeMy WebLinkAbout3557 Bear River Ct - Special Inspections/Engineering - 09/21/2012A.G
2180 South Ivanhoe Street, Suite 5
� Wassenaar I Denver, Colorado 5O222-5710
-W _ _ _. 303-759-8100 Fax 303-756-2920
Geotechnical and Environmental Consultants
September 21, 2012
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 31, Block 2
Trail Head
Fort Collins, Colorado
Project Number 121890
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 in the
upper level to a depth of approximately 25 feet. The subsurface materials encountered consisted
of:
0' to 13'
13' to 15'/2'
15'/2' to 25'
Date of drilling:
Depth to water:
Laboratory Testing
Fill, clay, very stiff, sandy, slightly moist, mottled brown
Fill, sand, medium dense to dense, gravelly, silty, clayey, with concrete and
brick rubble, slightly moist to moist, red brown
Clay, stiff, silty, slightly sandy, very moist, brown
August 17, 2012
Dry at the time of drilling
Dry 5 days after drilling
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 (3.4% at a depth of 2 feet and 1.3% 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
D R Horton
Project Number 121890
September 21, 2012
Page 2
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 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.
Foundation Recommendations
Our sampling and testing of the fill encountered indicated that there are soft muck or construction
debris beneath the fill. Construction observation reports of the fill placement are not available.
Additionally, overhead images (Google Earth) indicate that an irrigation canal likely ran beneath the
property prior to development. Therefore, we must be retained to observe the fill in the open _
excavation (prior to fill placement) to verify the suitability of the exposed materials to support
the new fill. Additional excavation may be required at that time. Additional recommendations
will be made at that time as necessary.
Based upon our evaluation of the subsurface conditions on this and adjacent lots, the proposed
residence may be founded upon spread or pad type footings bearing on moisture treated fill. The
moisture treated fill should be constructed by overexcavating the existing fill and natural soils to a
minimum depth of 10 feet below the bottom of the lowest foundation element (basement) over a
minimum area of 5 feet outside the structure footprint. Recommendations for construction of the
moisture treated fill and the foundation are presented below.
Overexcavation will be necessary prior to construction of the spread or pad type footing foundation.
The following recommendations should be followed during overexcavation operations in order to
enable the placement of a moisture treated fill that can be used for foundation support. These
recommendations may be modified during construction if soil conditions differing from those
anticipated are encountered.
The existing fill and natural soil should be excavated to a depth of at least 10 feet
below the bottom of the lowest foundation element on the lot to provide at least 10 feet
of moisture treated fill below the basement footings. The base of the excavation
should extend to a width of at least 5 feet beyond the potential foundation footprint
(including any counterforts). The excavation should be sloped following current OSHA
regulations. A licensed surveyor must verify the depth and width of the excavation
prior to any fill placement.
2. Once the excavation limits have been verified, fill placement may begin. The bottom
of the excavation should be scarified and moistened prior to fill placement. The
replacement fill should be placed in maximum 8-inch, loose lifts. If the replacement
fill consists of cohesive soils, the fill should be moistened and thoroughly mixed to
obtain a moisture content between 0% and +4% of optimum moisture content and then
compacted to a minimum of 95% of ASTM D 698 maximum dry density. If the
replacement fill will consist of granular soils, the fill should be moistened to a moisture
content of t2% of optimum moisture content and then compacted to a minimum of
95% of ASTM D 1557 maximum dry density. It is recommended that a grading
D R Horton
Project Number 121890
September 21, 2012
Page 3
compactor suited for such a fill be utilized to aid in compaction of the fill. Optimum
moisture content and maximum dry density should be obtained from appropriate
laboratory Proctor tests.
3. Observation and testing of fill placement must be performed by this firm on a full-time
basis. Testing should include in -place moisture content and dry density as well as
periodic swell -consolidation testing.
4. Placement and compaction of fill should continue to final overlot grade. We do not
recommend that placement of fill stop at foundation elevation as drying of the near
surface fill priorto foundation construction may allow sufficient heave after construction
to distress the residence. This may be waived if the foundation is to be constructed
within one month of completion of fill placement.
5. The subsurface conditions must be evaluated at the completion of fill placement by
conducting additional test borings, sampling, and testing. This requirement may be
waived if a representative of our firm observes and tests the fill on a full-time basis.
It must be understood that while these methods are used to reduce the likelihood of future heave,
it is not free of risk of foundation movement. While future heave is less likely, the possibility of
moisture induced settlement is increased. Therefore, the control and removal of surface water at the
site will continue to be very important.
Upon completion of the overexcavation process, 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 (S3) 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 dry, hard fill or very moist, soft fill may be encountered in the foundation
excavation. 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.
D R Horton
Project Number 121890
September 21, 2012
Page 4
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 will be low after the overexcavation process is completed. If this risk
of movement is not acceptable, engineered interior floors should be constructed.
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:
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.
D R Horton
Project Number 121890
September 21, 2012
Page 5
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 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.
D R Horton
Project Number 121890
September 21, 2012
Page 6
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.
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.
D R Horton
Project Number 121890
September 21, 2012
Page 7
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.
The recommendations provided in this report are based upon the specified extents of the
overexcavation for the original building footprint. The future Homeowner(s) should not construct any
additions to the residence utilizing the recommendations given in this report. Additional studies must
be provided if any additions are to be constructed.
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. WASSE
0
Keith D. Seaton, P.E �
,�:�/s
Senior EngineerO,n_..
KIDS/lia
Attachment: Figure 1.
Statement of Services
A.G. Wassenaar
Geotechnical and En*onmental 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 1/8" 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
JOINT
CONCRETE SLAB ON GRADE
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.
DRAIN GRAVEL (SEE NOTE #4) GRAVEL
SHOULD FILL ENTIRE TRENCH AND EXTEND
LATERLLY TO TOP RE ABOVE PIIPEOS OOUO GRAVEL
COVER BE AT LEAST 4"
FOOTING FOUNDATION
FfG 3 EXT-SOG TYPICAL EXTERIOR DRAIN DETAIL
SEPTEMBER 2007 FIGURE 1
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 LEAST 4" BELOW
BOTTOM OF FOOTING OR PAD