HomeMy WebLinkAboutCT 99-10; Buerger Property; Geotechnical Investigation 06143-52-02; 1998-11-01GEOTECHNICAL INVESTIGATION
BUERGER PROPERTY
CARLSBAD, CALIFORNIA
PREPARED FOR
WESTERN PACIFIC HOUSING
CARLSBAD, CALIFORNIA
NOVEMBER 1998
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6. CONCLUSIONS AND RECOMMENDATIONS
6.1. General
6.1.1. No soil or geologic conditions were encountered which would preclude the development of
the property, provided the recommendations of this report are followed.
6.1.2 The surficial deposits (undocumented fill and topsoil) are not considered suitable for the
support of fill or structural loads in their present condition and will require remedial
grading.
6.2. Soil and Excavation Characteristics
6.2.1. The soil conditions are anticipated to vary from very low to low expansive, silty sand
derived from Lindavista Formation, to low to medium expansive, silty and clayey topsoil.
6.2.2. Excavating within the Lindavista Formation will generally vary in difficulty with the depth
of excavation. Oversized rock generated during excavation should be placed in accordance
with Recommended Grading Specifications (Appendix C) and the requirements of the City
of Carlsbad. Since proposed fill areas and depths are relatively limited, most oversize rock
will require breakage to smaller than 6 inches or exportation from the property.
6.3. Corrosive Potential
• 6.3.1. Potential of Hydrogen (pH) and resistivity tests were performed to evaluate the corrosion
™ potential to subsurface metal structures. The tests were performed in accordance with
_ California Test Method No. 643, and, indicate a "mildly corrosive" condition may exist on
^ the site. The results are presented in Appendix B and should be considered for the design
of underground metal structures. This information should be provided to the project civil
• and corrosion engineers in preparation of improvement and plumbing plans.
6.3.2. Laboratory tests were performed to determine the percentage of soluble sulfate and
chloride. Results from the laboratory soluble sulfate and chloride tests are presented in
Appendix B, Tables V and VI, and indicate that the on-site materials possess "moderate"
and "negligible" sulfate and chloride exposure, respectively, to steel reinforced concrete
structures as defined by UBC Table 19-A-4. Recommendations for concrete mix design
when exposed to "moderate" sulfate conditions include using Type II cement, limiting the
water-cement ratio to a maximum of 0.50, and designing with a minimum concrete
compressive strength of 4000 psi.
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6.3.3. Laboratory tests were performed to determine the percentage of TKN or organic nitrogen
which is potentially corrosive to buried copper pipes. The laboratory test results presented
in Appendix B, Table VII indicate a high potential of exposure for copper pipes. Geocon
Incorporated does not practice in the field of corrosion engineering. Therefore, due to the
potentially corrosive environment for copper pipes it is recommended that further
evaluation by a corrosion engineer be performed to incorporate the necessary precautions
to avoid premature corrosion of underground copper pipes in direct contact with the on-site
soils. This will require corrosion protection design during plumbing plan preparation.
6.3.4. Geocon Incorporated does not practice in the field of corrosion engineering. Therefore,
due to the potentially corrosive environment, it is recommended that further evaluation by a
corrosion engineer be performed to incorporate the necessary precautions to avoid
premature corrosion on underground pipes and buried metal in direct contact with
the soils.
*
6.4. Grading
6.4,1. All grading should be performed in accordance with the attached Recommended Grading
Specifications (Appendix C). Where the recommendations of this section conflict with
Appendix C, the recommendations of this section take precedence. All earthwork should
be observed and all fills tested for proper compaction by Geocon Incorporated.
6.4.2. Prior to commencing grading, a preconstruction conference should be held at the site with
• the owner or developer, grading contractor, civil engineer, and geotechnical engineer in
attendance. Special soil handling and/or the grading plans can be discussed at that time.
6.4.3. Site preparation should begin with the removal of all deleterious material and vegetation.
The depth of removal should be such that material exposed in cut areas or soil to be used as
fill is relatively free of organic matter. Material generated during stripping and/or site
demolition should be exported from the site.
6.4.4. All potentially compressible surficial soils within areas of planned grading should be
removed to firm natural ground and properly compacted prior to placing additional fill
and/or structural loads. Overly wet, surficial materials, if encountered, will require drying
and/or mixing with drier soils to facilitate proper compaction.
6.4.5. The site should then be brought to final subgrade elevations with structural fill compacted
in layers. In general, soils native to the site are suitable for re-use as fill if free from
vegetation, debris and other deleterious material. Layers of fill should be no thicker than
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will allow for adequate bonding and compaction. All fill, including backfill and scarified
ground surfaces, should be compacted to at least 90 percent of maximum dry density near
optimum moisture content, as determined in accordance with ASTM Test Procedure
D 1557-91.
6.4.6. To reduce the potential for differential settlement, it is recommended that the cut portion of
cut-fill transition building pads be undercut at least 3 feet and replaced with properly
compacted "very low" to "low" expansive fill soils.
6.4.7. Where practical, the upper 3 feet of all building pads (cut or fill) and 12 inches in pavement
areas should be composed of properly compacted or undisturbed formational "very low" to
"low" expansive soils. Medium expansive soils, if encountered, should be placed in the
deeper fill areas greater than 3 feet and properly compacted. "Very low" to "low"
expansive soils are defined as those soils that have an Expansion Index of 50 or less in
accordance with UBC Table 18-I-B. Rock greater than 6 inches in maximum dimension
are typically not placed within 10 feet of finish grade or 3 feet from the deepest utility.
Rock greater than 6 inches in maximum dimension should not be placed within 3 feet of
finish grade in building pad areas.
6.5. Slope Stability
6.5.1. The results of the slope stability analysis, using soil strength parameters based on
laboratory tests and experience with similar soil materials in nearby areas indicate that the
proposed cut and fill slopes will have calculated factors-of-safety in excess of 1.5 under
static conditions of both deep-seated and shallow sloughing conditions. The stability of the
slopes was analyzed for cut and fill slopes constructed at 2:1. Slope stability, calculations
for surficial stability and deep seated stability are presented on Figures 3 through 5.
6.5.2. It is recommended that all cut slope excavations be observed during grading by an
engineering geologist to verify that soil and geologic conditions do not differ significantly
from those anticipated.
6.5.3. The fill slopes should either be over built a minimum of 3 feet and cut back to final grade
or, as a minimum, backrolled with a sheepsfoot compactor at maximum 4-foot high
intervals and track-walked upon completion so that a dozer track covers the entire slope at
least twice to achieve the required compaction.
6.5.4. The use of terrace drains on cut or fill slopes proposed for the project is not considered
necessary to maintain the gross stability of the slope.
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6.5.5.
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6.6.
6.6.1.
All slopes should be planted, drained and properly maintained to reduce erosion. Slope
planting should generally consist of drought tolerant plants having a variable root depth.
Slope watering should be kept to a minimum to just support the plant growth.
Foundations
The following preliminary foundation recommendations apply to one and two story
structures and are separated into categories dependent on depth and geometry of fill soils
for a particular pad, as well as the expansive characteristics of the soil within
approximately three feet of the finished pad grade. Due to the abundance of low expansion
material on site it should be practical to cap all building pads with low or very low
expansive soil.
TABLE 7.6.1.
FOUNDATION RECOMMENDATIONS BY CATEGORY
Foundation
Category
I
II
III
Minimum
Footing Depth
(inches)
12
18
24
Continuous Footing
Reinforcement
One No. 4 bar top and bottom
Two No. 4 bars top and bottom
Two No. 5 bars top and bottom
Interior Slab
Reinforcement
6 x 6 - 10/10 welded wire
mesh at slab mid-point
No. 3 bars at 24 inches on
center, both directions
No. 3 bars at 18 inches on
center, both directions
CATEGORY CRITERIA
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Category I Maximum fill thickness is less than 20 feet and Expansion Index is less manor
equal to 50.
Category II Maximum fill thickness is less than 50 feet and Expansion Index is less than or
equal to 90, or variation in fill thickness is between 10 feet and 20 feet.
Category III: Fill thickness exceeds 50 feet, or variation in fill thickness exceeds 20 feet, or
Expansion Index exceeds 90, but is less than 130.
Notes:
I . All footings should have a minimum width of 1 2 inches.
2. Footing depth measured from lowest adjacent subgrade.
3. AH interior living area concrete slabs should be at least four inches thick for Categories I and II
and 5 inches thick for Category III.
4. All interior concrete slabs should be underlain by at least 4 inches (3 inches for Category III) of
clean sand or crushed rock.
5. All slabs expected to receive moisture sensitive floor coverings or used to store moisture
sensitive materials should be underlain by a vapor barrier covered with at least 2 inches of the
clean sand recommended in No. 4 above.
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6.6.2. Foundations for either Category I, II, or III as described on the following page, may be
designed for an allowable soil bearing pressure of 2,000 pounds per square foot (psf) (dead
plus live load). This bearing pressure may be increased by one-third for transient loads
such as wind or seismic forces.
6.6.3. No special subgrade preparation is deemed necessary prior to placing concrete, however,
the exposed foundation and slab subgrade soils should be sprinkled, as necessary, to
maintain a moist soil condition as would be expected in any such concrete placement.
6.6.4. Where buildings or other improvements are planned near the top of a slope steeper than 3
to 1, special foundations and/or design considerations are recommended due to the
tendency for lateral soil movement to occur.
• For fill slopes less than 20 feet high, building footings should be deepened such
that the bottom outside edge of the footing is at least 7 feet horizontally from the
face of the slope.
• Where the height of the fill slope exceeds 20 feet, the minimum horizontal distance
should be increased to H/3 (where H equals the vertical distance from the top of
the slope to the toe) but need not exceed 40 feet. For composite (fill over cut)
slopes, H equals the vertical distance from the top of the slope to the bottom of the
fill portion of the slope. An acceptable alternative to deepening the footings would
be the use of a post-tensioned slab and foundation system or increased footing and
slab reinforcement. Specific design parameters or recommendations for either of
these alternatives can be provided once the building location and fill slope
geometry have been determined.
• For cut slopes in dense formational materials, or fill slopes inclined at 3 to 1 or
flatter, the bottom outside edge of building footings should be at least 7 feet
horizontally from the face of the slope, regardless of slope height.
• Swimming pools located within 7 feet of the top of cut or fill slopes are not
recommended. Where such a condition cannot be avoided, it is recommended that
the portion of the swimming pool wall within 7 feet of the slope face be designed
assuming that the adjacent soil provides no lateral support. This recommendation
applies to fill slopes up to 30 feet in height, and cut slopes regardless of height.
For swimming pools located near the top of fill slopes greater than 30 feet in
height, additional recommendations may be required and Geocon Incorporated
should be contacted for a review of specific site conditions. It should be noted that
difficult excavation with very heavy effort should be anticipated during swimming
pool construction operations that encounter formational material.
• Although other improvements which are relatively rigid or brittle, such as concrete
flatwork or masonry walls may experience some distress if located near the top of a
slope, it is generally not economical to mitigate this potential. It may be possible,
however, to incorporate design measures which would permit some lateral soil
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6.6.5.
6.6.6.
movement without causing extensive distress. Geocon Incorporated should be
consulted for specific recommendations.
Although the previously discussed foundation recommendations are considered adequate,
consideration should be given to the use of post-tensioned concrete slab and foundation
systems for the support of the proposed structures. The post-tensioned systems should be
designed by a structural engineer experienced in post-tensioned slab design and design
criteria of the Post-Tensioning Institute (UBC Standard No. 29-4, Part H). Although this
procedure was developed for expansive soils, it is understood that it can also be used to
reduce the potential for foundation distress due to differential fill settlement. The post-
tensioned design should incorporate the geotechnical parameters presented on the
following table entitled Post-Tensioned Foundation System Design Parameters for the
particular Foundation Category designated,
TABLE 7.6.2.
POST-TENSIONED FOUNDATION SYSTEM DESIGN PARAMETERS
Post-Tensioning Institute (PTI)
Design Parameters
1 . Thornthwaite Index
2. Clay Type - Montmorillonite
3, Clay Portion (Maximum)
4. Depth to Constant Soil Suction
5. Soil Suction
6. Moisture Velocity
7. Edge Lift Moisture Variation Distance
8. Edge Lift
9. Center Lift Moisture Variation Distance
10. Center Lift
Foundation Category
I
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Yes
30%
7.0ft.
3.6ft.
0.7 in./mo.
2.6ft.
0.41 in.
5.3ft.
2. 12 in.
II
-20
Yes
50%
7.0ft.
3.6ft.
0.7 in./mo.
2.6ft.
0.78 in.
5.3ft.
3.21 in.
Ill
-20
Yes
70%
7.0ft.
3.6ft.
0.7 in/mo.
2.6ft.
1.15 in.
5.3ft.
4.74 in.
UBC Standard No. 29-4 Part II uses interior stifTener beams in its structural design
procedures. If the structural engineer proposes a post-tensioned foundation design method
other than UBC Standard No. 29-4, Part II, it is recommended that interior stiffener beams
be used for Foundation Categories II and III. The depth of the perimeter foundation
should be at least 12 inches for Foundation Category I'. Where the Expansion Index for a
particular building pad exceeds 50 but is less than 91, the perimeter footing depth should be
at least 18 inches; and where it exceeds 90 but is less than 130, the perimeter footing depth
should be at least 24 inches. Geocon Incorporated should be consulted to provide
additional design parameters as required by the structural engineer.
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6.7. Retaining Walls and Lateral Loads
6.7.1. The following recommendations for retaining walls are provided based on our experience
with similar site and soil conditions. Modifications may be required depending on actual
site conditions.
6.7.2. Retaining walls not restrained at the top and having a level backfill surface should be
designed for an active soil pressure equivalent to the pressure exerted by a fluid density
of 30 pounds per cubic foot (pcf). Where the backfill will be inclined at no steeper than 2.0
to 1.0, an active soil pressure of 40 pcf is recommended. These soil pressures assume that
the backfill materials within an area bounded by the wall and a 1:1 plane extending upward
from the base of the wall possess an Expansion Index of less than 50. For those pads where
backfill materials do not conform to the above criteria, Geocon Incorporated should be
consulted for additional recommendations.
6.7.3. Unrestrained walls are those that are allowed to rotate more than 0.001H at the top of the
wall. Where walls are restrained from movement at the top, an additional uniform pressure
of 7H psf (where H equals the height of the retaining portion of the wall in feet) should be
added to the above active soil pressure.
6.7.4. All retaining walls should be provided with a drainage system adequate to prevent the
I buildup of hydrostatic forces and should be waterproofed if required by the project
architect. The use of drainage openings through the base of the wall (weep holes, etc.) is
• not recommended where the seepage could be a nuisance or otherwise adversely impact the
^ property adjacent to the base of the wall. The above recommendations assume a properly
— compacted granular (Expansion Index less than 50) backfill material with no hydrostatic
| forces or imposed surcharge load. If conditions different than those described are
anticipated, or if specific drainage details are desired, Geocon Incorporated should be
(contacted for additional recommendations.
6.7.5. In general, wall foundations having a minimum depth and width of one foot may be
designed for an allowable soil bearing pressure of 2,000 psf, provided the soil within 3 feet
below the base of the wall has an Expansion Index of less than 90. The proximity of the
foundation to the top of a slope steeper than 3:1 could impact the allowable soil bearing
pressure. Therefore, Geocon Incorporated should be consulted where such a condition is
anticipated.
6.7.6. For resistance to lateral loads, an allowable passive earth pressure equivalent to a fluid
density of 300 pcf is recommended for footings or shear keys poured neat against properly
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compacted granular fill soils or undisturbed natural soils. The allowable passive pressure
assumes a horizontal surface extending at least 5 feet or three times the surface generating
the passive pressure, whichever is greater. The upper 12 inches of material not protected
by floor slabs or pavement should not be included in the design for lateral resistance. An
allowable friction coefficient of 0.4 may be used for resistance to sliding between soil and
concrete. This friction coefficient may be combined with the allowable passive earth
pressure when determining resistance to lateral loads.
6.7.7. The recommendations presented above are generally applicable to the design of rigid
concrete or masonry retaining walls having a maximum height of 8 feet. In the event that
walls higher than 8 feet or other types of walls are planned, such as crib-type walls, Geocon
Incorporated should be consulted for additional recommendations.
6.8. Drainage and Maintenance
6.8.1. Establishing proper drainage is imperative to reduce the potential for differential soil
movement, erosion and subsurface seepage. Positive measures should be taken to properly
finish grade the building pads after structures and other improvements are in place, so that
drainage water from the building pads and adjacent properties is directed away from
foundations and tops of slopes to controlled drainage devices. Under no circumstances
should water be allowed to pond adjacent to footings. Experience has shown that even
with these provisions, a shallow groundwater or subsurface condition can and may develop
in areas where no such condition existed prior to site development. This is particularly true
where a substantial increase in surface water infiltration results from an increase in
landscape irrigation.
6.9. Grading and Foundation Plan Review
6.9.1. It is recommended that Geocon Incorporated review proposed grading and/or foundation
plans prior to finalizing. The need for additional analysis or comments can be determined
at that time.
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