HomeMy WebLinkAboutSoils & Foundation Investigation 02.14.2008February 14, 2008
G. H. Daniels & Associates, Inc.
140 G. H. Daniels Boulevard
Gypsum, CO 81637
Attention: Mr. Jody Daniels
Subject: Soils and Foundation Investigation
Lots 1 through 7
Daniels/Hasenberg Subdivision
Garfield County, Colorado
Project No. GS05135-120
CTL ITHOMPSON
I N C O R P O R A T E D
CTL 1 Thompson, Inc. performed a geologic evaluation and preliminary
geotechnical investigation for the Daniels/Hasenberg Subdivision under this
project number, dated January 11, 2008. You asked if sufficient information was
obtained to provide design -level criteria for the planned residential construction.
In our opinion, we performed sufficient subsurface sampling to provide design -
level soils and foundation investigations for the seven (7) lots that comprise the
subdivision. However, we should be called to observe subsurface conditions
exposed in the completed foundation excavations for each lot to check that
conditions are as anticipated and appropriate for the foundation systems
recommended. This soils and foundation letter should be used in conjunction
with our geologic evaluation and preliminary geotechnical investigation.
Proposed Construction
Building plans for the residences were not developed at the time of our
investigation. If construction will differ significantly from the descriptions below,
we should be informed so that we can adjust our recommendations and design
criteria as necessary. We expect the proposed residences will be one or two-
story, wood -frame buildings with attached garages. A basement and/or crawl
space may be constructed below the buildings. Similar residences in the area are
typically constructed with slab -on -grade floors in basement and garage areas.
Maximum foundation excavation depths will likely be on the order of 7 to 9 feet if a
basement is constructed. Foundation loads are expected to vary between 1,000
and 3,000 pounds per lineal foot of foundation wall with maximum interior column
loads of 30 kips. Completed wall backfill depth may be slightly more than
excavation depth as final grades are adjusted for drainage.
Earthwork
We anticipate excavations for the building foundations and utilities can be
accomplished using conventional, heavy-duty excavation equipment. Excavation
sides will need to be sloped or braced to meet local, state and federal safety
234 Center Drive 1 Glenwood Springs, Colorado 81601
Telephone: 970-945-2809 Fax: 970-945-7411
regulations. We believe the soils at this site will generally classify as Type B and
Type C soils based on OSHA standards governing excavations. Temporary
slopes deeper than 5 feet should be no steeper than 1 to 1 (horizontal to vertical)
in Type B soils and 1.5 to 1 in Type C soils. Contractors should identify the soils
encountered in the excavations and refer to OSHA standards to determine
appropriate slopes.
We do not anticipate excavations for foundations or utilities will encounter
significant amounts of ground water. However, excavations should be sloped to a
gravity discharge or to a temporary sump where water can be removed by
pumping. The ground surrounding the excavations should be sloped as much as
practical to direct runoff away from the excavations.
Fill may be required to obtain subgrade elevations for the garage floor and
exterior concrete flatwork. Areas which will receive fill should be stripped of
vegetation, organic soils and debris. The on-site soils free of rocks larger than 3
inches in diameter, organic matter, and debris are suitable for use as fill below
new construction. Fill below structures should be placed in loose lifts of 10
inches thick or less, moisture conditioned to within 2 percent of optimum
moisture content, and compacted to at least 100 percent of standard Proctor
(ASTM D 698) maximum dry density. Moisture content and density of fill should
be checked by a representative of our firm during placement.
Properly placed backfill adjacent to foundation wall exteriors is important
to reduce infiltration of surface water and subsequent consolidation. Backfill
placed adjacent to foundation wall exteriors should be free of organic matter,
debris and rocks larger than 3 inches in diameter. Backfill should be moisture
conditioned to within 2 percent of optimum moisture content and compacted to at
least 95 percent of standard Proctor (ASTM D 698) maximum dry density.
Foundation
We recommend constructing the residences on footings constructed on
the undisturbed, natural sand or gravel soil. Criteria for construction of footings is
presented below.
1. Footing foundations should be supported entirely on the natural
sands or gravels. Soils loosened during the excavation or forming
process for the footings should be removed or the soils can be re -
compacted prior to placing concrete.
2. Footings on the natural sands or gravels should be designed for a
maximum soil bearing pressure of 4,000 psf.
3. Continuous wall footings should have a minimum width of at least
16 inches. Foundations for isolated columns should have minimum
dimensions of 24 inches by 24 inches Larger sizes may be
required, depending upon foundation loads.
G. H. DANIELS & ASSOCIATES, INC.
LOTS 1 THROUGH 7
DANIELS/HASENBERG SUBDIVISION
PROJECT NO. GS05135 120
S:\G505135.000112013. Letters \GS05135120 L1.doc
2
4. Grade beams and foundation walls should be well reinforced, top
and bottom. We recommend reinforcement sufficient to span an
unsupported distance of at least 10 feet. Reinforcement should be
designed by the structural engineer.
5. The soils under exterior footings should be protected from freezing.
We recommend the bottom of footings be constructed at a depth of
at least 36 inches below finished exterior grades for frost protection.
The Garfield County building department should be consulted to
verify the required depth.
Floor Systems and Slab -on -Grade
Slab -on -grade floors will likely be constructed in garage and some lower
level living areas of the residences. Based on our laboratory test data and our
experience, we judge slab -on -grade construction can be supported by the
undisturbed, natural sand or gravel or densely compacted structural fill with the
on-site gravel free of deleterious material and rock larger than 3 inches in
diameter or Class 6 aggregate base course with low risk of differential movement
and associated damage.
Slabs should be separated from exterior walls and interior bearing
members with slip joints which allow free vertical movement of the slabs.
Underslab plumbing should be pressure tested for leaks before the slabs are
constructed. Plumbing and utilities which pass through slabs should be isolated
from the slabs with sleeves and provided with flexible couplings to slab supported
appliances. Exterior patio and porch flatwork should be isolated from the
residence. These slabs should be well -reinforced to function as independent
units. Frequent control joints should be provided, in accordance with American
Concrete Institute (ACI) recommendations, to reduce problems associated with
shrinkage and curling.
The 2003 International Building Code (IBC) or 2003 International Residential
Code (IRC) may require a vapor retarder be placed between the base course or
subgrade soils and the concrete slab -on -grade floors. The merits of installation of
a vapor retarder below floor slabs and PT slabs depend on the sensitivity of floor
coverings and building to moisture. A properly installed vapor retarder (10 mil
minimum) is more beneficial below concrete slab -on -grade floors where floor
coverings, painted floor surfaces or products stored on the floor will be sensitive
to moisture. The vapor retarder is most effective when concrete is placed directly
on top of it. A sand or gravel leveling course should not be placed between the
vapor retarder and the floor slab. The placement of concrete on the vapor retarder
may increase the risk of shrinkage cracking and curling. Use of concrete with
reduced shrinkage characteristics including minimized water content, maximized
coarse aggregate content, and reasonably low slump will reduce the risk of
shrinkage cracking and curling. Considerations and recommendations for the
installation of vapor retarders below concrete slabs are outlined in Section 3.2.3 of
the 2003 report of American Concrete Institute (ACI) Committee 302, "Guide for
Concrete Floor and Slab Construction (ACI 302.R-96)".
G. H. DANIELS & ASSOCIATES, INC.
LOTS 1 THROUGH 7
DANIELS/HASENBERG SUBDIVISION
PROJECT NO. GS05135 120
5:IG505135.000112013. Letters\G505135120 L1.doc
3
Below -Grade Construction
Foundation walls which extend below -grade should be designed for lateral
earth pressures where backfill is not present to about the same extent on both
sides of the wall. Many factors affect the values of the design lateral earth
pressure. These factors include, but are not limited to, the type, compaction,
slope and drainage of the backfill, and the rigidity of the wall against rotation and
deflection. For a very rigid wall where negligible or very little deflection will occur,
an "at -rest" lateral earth pressure should be used in design. For walls which can
deflect or rotate 0.5 to 1 percent of wall height (depending upon the backfill types),
lower "active" lateral earth pressures are appropriate. Our experience indicates
that typical basement walls can deflect or rotate slightly under normal design
Toads, and that this deflection results in satisfactory wall performance. Thus, the
earth pressures on the walls will likely be between the "active" and "at -rest"
conditions.
If the on-site soils are used as backfill, we recommend design of below -
grade walls using an equivalent fluid density of at least 45 pcf for this site. This
equivalent density does not include allowances for sloping backfill, surcharges or
hydrostatic pressures. The recommended equivalent density assumes deflection;
some minor cracking of walls may occur. If very little wall deflection is desired, a
higher equivalent fluid density may be appropriate for design. Our recent
experience indicates most basement and below -grade walls designed with 45 pcf
to 50 pcf equivalent fluid density have performed satisfactorily. Backfill should be
placed and compacted in accordance with the recommendations outlined in the
Earthwork section
Subsurface Drainage
Water from rain, snow melt and surface irrigation of landscaping frequently
flows through relatively permeable backfill placed adjacent to a residence and
collects on the surface of relatively undisturbed soils at the bottom of the
excavation. This can cause wetting of foundation soils after construction. To
mitigate these concerns, we recommend provision of a foundation drain around
the below -grade areas of the residences. The drain should consist of a 4 -inch
diameter, slotted pipe encased in free draining gravel. The drain should lead to a
positive gravity outfall, or to a sump pit where water can be removed by pumping.
Typical foundation drain details are presented on Figures 1 and 2.
Surface Drainage
Surface drainage is critical to the performance of foundations, floor slabs
and concrete flatwork. We recommend the following precautions be observed
during construction and maintained at all times after the residence is completed:
1. The ground surface surrounding the exterior of the residence should
be sloped to drain away from the residence in all directions. We
G. H. DANIELS & ASSOCIATES, INC.
LOTS 1 THROUGH 7
DANIELS/HASENBERG SUBDIVISION
PROJECT NO. G505135 120
59G505135.000112013. Letters10505135120 1.1.doo
4
recommend providing a slope of at least 12 inches in the first 10 feet
around the residence, where possible. In no case should the slope
be less than 6 inches in the first 5 feet.
2. Backfill around the exterior of foundation walls should be placed in
maximum 10 inch thick loose lifts, moisture conditioned to within 2
percent of optimum moisture content and compacted to at least 95
percent of standard Proctor (ASTM D 698) maximum dry density.
3. The residence should be provided with roof gutters and
downspouts. Roof downspouts and drains should discharge well
beyond the limits of all backfill. Splash blocks and downspout
extensions should be provided at all discharge points.
4. Landscaping should be carefully designed to minimize irrigation.
Plants used near foundation walls should be limited to those with
low moisture requirements; irrigated grass should not be located
within 5 feet of the foundation. Sprinklers should not discharge
within 5 feet of the foundation and should be directed away from the
building.
5. Impervious plastic membranes should not be used to cover the
ground surface immediately surrounding the residence. These
membranes tend to trap moisture and prevent normal evaporation
from occurring. Geotextile fabrics can be used to control weed
growth and allow some evaporation to occur.
Limitations
This investigation was conducted in a manner consistent with that level of
care and skill ordinarily exercised by geotechnical engineers currently practicing
under similar conditions in the locality of this project. We should observe
condition in excavations to verify that soils are appropriate for support of the
foundation system as designed. A representative of our office should check the
moisture and density of fill soils. No warranty, express or implied, is made.
Very Truly Yours
CTL 1 THOMPSON, INC. Reviewed by:
Edward R. White, E.I. John Mechling, P.E.
Staff Engineer Branch Manager
ERW:JM:cd
(7 copies sent)
G. H. DANIELS & ASSOCIATES, INC.
LOTS 1 THROUGH 7
DANIELS/HASENBERG SUBDIVISION
PROJECT NO. 6505135120
S:1GS05135.000112013. Letters1G805135120 L1.doc
5
SLOPE
PER
OSHA
SLOPE
PER REPORT
BACKFILL
(COMPOSNION AND
COMPACTION PER REPORT)
BELOW GRADE WALL
NOTE:
DRAIN SHOULD BE AT LEAST 2 INCHES
BELOW BOTTOM OF FOOTING AT THE
HIGHEST POINT AND SLOPE DOWNWARD
TO A POSITNE GRAVITY OUTLET OR TO
A SUMP WHERE WATER CAN BE REMOVED
BY PUMPING.
ENCASE PIPE IN WASHED
CONCRETE AGGREGATE (ASTM
C33, NO. 57 OR NO. 67)
EXTEND GRAVEL TO AT LEAST
1/2 HEIGHT OF FOOTING.
COVER GRAVEL WITH
FILTER FABRIC.
REINFORCING STEEL
PER STRUCTURAL
DRAWINGS
PROVIDE POSITIVE SUP JOINT
BETWEEN SLAB AND WALL.
FLOOR SLAB
MINIMUM
8" MINIMUM
OR BEYOND 1:1
SLOPE FROM BOTTOM
OF FOOTING.
(WHICHEVER IS GREATER)
4—INCH DIAMETER PERFORATED DRAIN PIPE.
THE PIPE SHOULD BE LAID IN A TRENCH
WITH A MINIMUM SLOPE OF 0.5 PERCENT.
Project No. GS05135-120
FOOTING OR PAD
PROVIDE POLYETHYLENE
SHEETING GLUED TO
FOUNDATION WALL TO
REDUCE MOISTURE
PENETRATION
Exterior
Foundation
Wall Drain
Fig. 1
Ltr. 2/14/08
SLOPE
PER
OSHA
SLOPE
PER REPORT
K - BACKFILL
(COMPOSITION MID
COMPACTION PER REPORT)
BELOW GRADE WALL
PROVIDE POLYETHYLENE
SHEETING GLUED TO
FOUNDATION WALL TO
REDUCE MOISTURE
PENETRATION
COVER GRAVEL WITH
FILTER FABRIC
................
:rr}-
b—
NOTE:
DRAIN SHOULD BE AT LEAST 2 INCHES
BELOW BOTTOM OF FOOTING AT THE
HIGHEST POINT AND SLOPE DOWNWARD
TO A POSITIVE GRAVITY OUTLET OR TO
A SUMP WHERE WATER CAN BE
REMOVED BY PUMPING.
STRUCTURALLY SUPPORTEDJr
FLOOR SYSTEM
A - CRAWL SPACE
REINFORCING STEEL
PER STRUCTURAL
DRAWINGS.
FOOTING OR PAD
ENCASE PIPE IN WASHED -' "J orr 2" MINIMUM i
CONCRETE AGGREGATE (ASTM
C33, NO. 57 OR NO. 67).
EXTEND GRAVEL TO AT LEAST —4r MINIMUM — BOTTOM OF
1/2 HEIGHT OF FOOTING. OR BEYOND 1:1 EXCAVATION
SLOPE FROM BOTTOM
OF FOOTING.
(WHICHEVER IS GREATER)
4 -INCH DIAMETER PERFORATED
DRAIN PIPE. THE PIPE SHOULD
BE LAID IN A TRENCH WITH A
SLOPE RANGE BETWEEN 1/8
INCH AND 1/4 INCH DROP PER
FOOT OF DRAIN.
Exterior
Foundation
Wall Drain
Project No. GS05135-120
Fig. 2
Ltr. 2/14/08