HomeMy WebLinkAboutCT 05-14; LA COSTA OAKS NORTH NEIGH 3.1; GEOTECHNICAL REVIEW OF PRECISE GRADING; 2010-12-14L C
LGC Geotechnical Inc
Geotechnical, Inc.
December 14,2010 Project No 10072-01
I
Mr. Peter Hemphill
I Standard Pacific Homes
26 Technology
Irvine, CA 92612
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Subject Geotechiucal Review of the Precise Grading Plan for Villages of La Costa, The Oaks North
I Neighborhood 3.1, CT 05-14, City of Carlsbad, California
I In accordance with your request, LGC Geotechnical, Inc. has prepared this report to present the results of our
geotechnical review of the precise grading plan for the Villages of La Costa, The Oaks North Neighborhood Z 3.1, Carlsbad Tract No. 05-14, located in Carlsbad, California. This report provides a summary of our 0 I conclusions and recommendations relative to the development of the subject tract
0
I Should you have any questions regarding this report, please do not hesitate to contact our office. We
appreciate this opportunity to be of service. z
I Respectfully,
LGC Geotechnical, Inc. 0 so
140.220
CER10ED E2 0 No. 277
or Kevin B. Colson, CEG 2210 GIE01OG'S' De is Borat nec, GE 2770 7.31-12 Vice President Vice Pres7id-t Or C"
Clk
Distribution (3) Addressee (includes two wet-signed copies)
1 co
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I >-
1319 Calle Avanzado • San Clemente • CA 92673-6351 • (949) 369-6141 • Fax (949) 369-6142
TABLE OF CONTENTS
Section Page
1.0 INTRODUCTION ..................................................................................................................... 1
1.1 Precise Grading Plan Review........................................................................................1
1.2 As-Graded Condition .................................................................................................... 1
2.0 GEOTECHNICAL CONCLUSIONS.......................................................................................3
3.0 GEOTECHNICAL RECOMMENDATIONS ..........................................................................4
3.1 Site Earthwork ............................................................................................................... 4
3.1.1 Site Preparation.................................................................................................4
3.1.2 Removal and Recompaction..............................................................................5
3.1.3 Fill Placement and Compaction.........................................................................5
3.1.4 Trench Backfill and Compaction......................................................................5
3.1.5 Slope Stability ..................................................................................................6
3.2 Provisional Foundation Recommendations ..................................................................6
3.2.1 Post-Tensioned Foundation Subgrade Preparation and Maintenance..............6
3.2.2 Soil Bearing ........................................................................................................ 7
3.2.3 Vapor Retarder and Sand Below Slabs ............................................................8
3.2.4 Foundation Setback from Top of Slope and Bottom of Slope .........................8
3.3.5 Potential Fill Settlement...................................................................................9
3.3 Slope Creep...................................................................................................................9
3.4 Lot Stretching .............................................................................................................. 10
3.5 Corrosivity to Concrete and Metal..............................................................................10
3.6 Nonstructural Concrete Flatwork................................................................................10
3.7 Retaining Walls...........................................................................................................11
3.8 Fences and Freestanding Walls...................................................................................15
3.8.1 Freestanding Walls.........................................................................................15
3.8.2 Fences at the Top-of-Slopes ........................................................................... 15
3.9 Surface Drainage and Yard Landscaping ...................................................................16
3.9.1 General ...........................................................................................................16
3.9.2 Precise Grading ..............................................................................................16
3.10 Swimming Pools and Spas..........................................................................................16
3.11 Seismic Design Criteria..............................................................................................16
3.12 Geotechnical Observation/Testing During Construction............................................17
4.0 LIMITATIONS .......................................................................................................................19
Project No. 10072-01 Page i December 14, 2010
TABLE OF CONTENTS (Cont'd)
LIST OF TABLES, ILLUSTRATIONS, AND APPENDICES
Tables
Table 1 - Provisional Geotecimical Parameters for POst-Tensioned Foundation Slab Design (Page 7)
Table 2 - Nonstructural Concrete Flatwork for Very Low Expansion Potential (Page 11)
Table 3 - Equivalent Fluid Pressures (Page 12)
Table 4A - Seismic Design Parameters (Page 17)
Table 4B - Seismic Design Parameters Modified for Site Class D (Page 17)
Figures
Figure 1 - Site Location Map (Page 2)
Figure 2 - Typical Retaining Wall Backfill and Drainage Detail (Page 14)
Appendices
Appendix A - References
Appendix B - Laboratory Testing Procedures and Test Results
Project No. 10072-01 Page ii December 14, 2010
1.0 INTRODUCTION
LGC Geotechnical has completed a geotechnical review of the precise grading plan for the Villages of La
Costa, The Oaks North Neighborhood 3.1, Carlsbad Tract No. 05-14, located in Carlsbad, California
(Hunsaker, 2010). This report summarizes our conclusions and recommendations with respect to the precise
grading plan provided for the subject tract.
The Oaks North Neighborhood 3.1 development currently consists of finish graded pads graded under the
observation and testing services of others (Geocon, 2007a and b). LGC Geotechnical has taken over as
consultant of record for the project. This precise grading plan review report has been prepared to address the
geotechnical conditions of the tract. This report supersedes all previous reports unless specified otherwise.
This report should be referenced on the subject precise grading plan as the project geotechnical report.
1.1 Precise Gradin.g Plan Review
Based on our review of the subject precise grading plan (Hunsaker, 2010), the elevations of the
building pads are similar to those shown on the previous rough grading plan. As a result, no
substantial grading has been indicated. However, minor additional grading, in the form of
reprocessing the near surface materials on the pads as well as repair of minor erosion are anticipated
to reestablish the building pad grades and appropriate pad drainage. We understand a post-tensioned
foundation slab system will be used to support the proposed residential structures.
1.2 As-Graded Condition
Rough grading of the subject tract was completed as part of grading for the overall development
under the observation and testing of Geocon, Inc. (Geocon, 2007a and b). Based on the as-graded
reports, the depths of fill beneath the subject lots within the tract range from approximately 4 feet to
50 feet (as reported by Geocon, 2007a). Compacted fill was primarily derived from Escondido Creek
Granodiorite bedrock.
Project No. 10072-01 Page 1 December 14, 2010
2.0 GEOTECHNICAL CONCLUSIONS
Based on our review of the previous geotechnical reports for the project (Geocon, 2007a and b), the results
of our in-progress update evaluation for the overall project, and our review of the subject precise grading
plan, the precise grading plans are geotechnically acceptable and the site is considered suitable for its
intended development, provided that the recommendations contained in this report are appropriately
implemented. Continuous erosion control, rodent control, and maintenance are essential to the long-term
stability of all slopes within and adjacent to the subject site.
Project No. 10072-01 Page 3 December 14, 2010
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I 3.0 GEOTECHNICAL RECOMMENDATIONS.
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The following recommendations should be considered minimal from a geotechnical viewpoint, as there may
be more restrictive requirements from the architect, structural engineer, building codes, governing agencies,
or the City of Carlsbad.
It should be noted that the following geotechnical recommendations are intended to provide sufficient
information to develop the site in general accordance with the 2007 C.B.C. requirements. With regard to the
potential occurrence of potentially catastrophic geotechnical hazards such as fault rupture, earthquake-
induced landslides, liquefaction, etc. the following geotechnical recommendations should provide adequate
protection for the proposed development to the extent required to reduce seismic risk to an "acceptable
level". The "acceptable level" of risk is defined by the California Code of Regulations as "that level that
provides reasonable protection of the public safety, though it does not necessarily ensure continued structural
integrity and functionality of the project" [Section 3721(a)]. Therefore, repair and remedial work of the
proposed improvement may be required after a significant seismic event. With regards to the potential for
less significant geologic hazards to the proposed development, the recommendations contained herein are
intended as a reasonable protection against the potential damaging effects of geotechnical phenomena such
as expansive soils, fill settlement, ground-water seepage, etc. It should be understood, however, that our
recommendations are intended to maintain the structural integrity of the proposed development and
structures given the site geotechnical conditions, but cannot preclude the potential for some cosmetic distress
or nuisance issues to develop as a result of the site geotechnical conditions.
3.1 Site Earthwork
We anticipate that earthwork at the site will consist of reprocessing of the near surface materials on the
building pads followed by utility construction and foundation construction. We recommend that
earthwork onsite be performed in accordance with the following recommendations and the City of
Carlsbad Grading Requirements. The following recommendations should be considered preliminary
and may be revised based on the actual as-graded conditions of the site once grading is completed. If
necessary, revisions will be provided in our recertification letter report for the lots following the
completion of re-grading. .
- 3.1.1 Site Preparation ..
I Prior to grading of areas to receive structural fill or engineered structures, the areas should be
cleared of surface obstructions and potentially compressible material (such as desiccated fill
material and vegetation). Vegetation and debris should be removed and properly disposed of
I offsite. Holes 'resulting from the removal of buried obstructions, which extend below proposed
design grades, should be replaced with suitable compacted fill material. .
3.1.2 Removal and Recompaction
We anticipate removals on the site will be minor and should typically include ripping and
heavily watering the upper 10 inches, followed by recompaction of the existing fill materials in
place; this should be done for building footprints plus 5 feet. Localized, deeper removals should
be anticipated where deemed necessary by the geotechnical consultant based on observations
during grading.
Removal bottoms and/or reprocessing should be observed and accepted by the geotechnical
consultant prior to fill placement. From a geotechnical perspective, material that is removed
may be placed as fill, provided the material is relatively free of organic material and/or
deleterious debris, is moisture-conditioned or dried (as needed) to obtain above-optimum
moisture content, and then recompacted prior to additional fill placement or construction. Areas
to receive fill and/or other surface improvements should be scarified, moisture conditioned, and
recompacted to at least 90 percent relative compaction (based on American Society for Testing
and Materials [ASTM] Test Method D1557).
3.1.3 Fill Placement and Compaction
From a geotechnical perspective, the onsite soils are generally suitable for use as compacted fill,
provided they are screened of organic materials and construction debris. Areas prepared to
receive structural fill and/or other surface improvements should be scarified, brought to at least
optimum-moisture content, and recompacted to at least 90 percent relative compaction (based
on ASTM Test Method Dl 557). The optimum lift thickness to produce a uniformly compacted
fill, will depend on the type and size of compaction equipment used. In general, granular fill
should be placed in uniform lifts not exceeding 8 inches in compacted thickness. Generally,
placement and compaction of fill should be performed in accordance with local grading
ordinances under the observation and testing of the geotechnical consultant.
Import soils (if necessary) should consist of granular soils of very low-to-low expansion
potential (expansion index 50 or less based on ASTM D 4829). Source samples should be
provided to the geotechnical consultant for laboratory testing a minimum of 48 hours prior to
any planned importation.
3.1.4 Trench Backfill and Compaction
The onsite materials may generally be suitable as-, trench backfill provided the soils are screened
of rocks and other material greater than 6 inches in diameter and organic matter. If trenches are
shallow or the use of conventional equipment may result in damage to the utilities, clean sand
having a SE> 30 should be used to bed and shade the pipes. Sand backfill may be densified by
jetting or flooding and then tamping to ensure adequate compaction. Otherwise, trench backfill
should be compacted in uniform lifts (generally not exceeding 12 inches in compacted
thickness) by mechanical means to at least 90 percent relative compaction (per ASTM Test
Method D1557). A representative from LGC Geotechnical should observe and test the
backfill to verify compliance with the project specifications
Project No. 10072-01 Page 5 December 14, 2010
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3.1.5 Slope Stability
The existing site slopes with a slope ratio of 2:1 (horizontal to vertical), are anticipated to be
both grossly and surficially stable as built. The cut slopes should be protected with properly
designed vegetative covers. If trees are planned on cut slopes, tree wells should be
considered.
3.2 Provisional Foundation Recommendations
It is our understanding that post-tensioned slab-on-grade foundation systems will be utilized for the
proposed structures. We recommend that the project slab design engineer design the proposed
foundations using the following PT parameters.
In utilizing these parameters, the foundation engineer should design the foundation system in
accordance with the allowable deflection criteria of applicable codes and the requirements of the
structural engineer/architect. Other types of stiff slabs may be used in place of the C.B.C. post-
tensioned slab design provided that, in the opinion of the structural engineer, the alternative type of
slab is at least as stiff and strong as that designed by the C.B.C. method.
3.2.1 Post-Tensioned Foundation Subrirade Preparation and Maintenance '
Presoaking of the subgrade soils is recommended prior to trenching the foundation. Presoak
recommendations specific to the anticipated site soil conditions are presented in Table 1. The
subgrade moisture condition of the building pad soils should be maintained at near optimum
moisture content up to the time of concrete placement. This moisture content should be
maintained around the immediate perimeter of the slab during construction and up to
occupancy of the structures.
The geotechnical parameters provided in Table 1 assume that if the areas adjacent to the
foundation are planted and irrigated, these areas will be designed with proper drainage and
adequately maintained so that ponding, which causes significant moisture changes below the
foundation, does not occur. Our recommendations do not account for excessive irrigation
and/or incorrect landscape design. Sunken planters placed adjacent to the foundation should
either be designed with an efficient drainage system or liners to prevent moisture infiltration
below the foundation. Some lifting of the perimeter foundation beam should be expected
even with properly constructed planters.
In addition to the factors mentioned above, future owners (and owner's landscape architect)
should be made aware of the potential negative influences of trees and/or other large
vegetation. Roots that extend near the vicinity of foundations can cause distress to
foundations. Future owners (and the owners' landscape architect) should not plant trees/large
shrubs closer to foundations than a distance equal to half the mature height of the tree or 20
feet, whichever is more conservative, unless specifically provided with root barriers to
prevent root growth below the house foundation.
Project No. 10072-01 Page 6 December 14, 2010
It is the owner's responsibility to perform periodic maintenance during hot and dry periods to
insure that adequate watering has been provided to keep soil from separating or pulling back
from the foundation. Future owners should be informed and educated regarding the
importance of maintaining a constant level of soil-moisture. The owners should be made
aware of the potential negative consequences of both excessive watering, as well as allowing
potentially expansive soils to become too dry. Expansive soils can undergo shrinkage during
drying, and swelling during the rainy winter .season, or when irrigation is resumed. This can•
result in distress to building structures and hardscape improvements. The builder should
provide these recommendations to future owners and their landscape architect.
TABLE 1
Provisional Geotechnical Parameters for Post-Tensioned Foundation Slab Design
Parameter PT Slab with
Perimeter Footing
Center Lift
Edge moisture variation distance, em 9.0 feet
Center lift, Ym 0.3 inches
Edge Lift
Edge moisture variation distance, em 5.3 feet
Edge lift, ym 0.61 inch
Minimum Perimeter footing/thickened edge
embedment below finish grade 10 inches
Minimum slab thickness 5 inches'
1. Recommendations for foundation reinforcement and slab thickness are
ultimately the purview of the foundation engineer/structure engineer based upon
the geotechnical criteria and structural engineering considerations.
2: Presoak to optimum moisture content to a depth of 12 inches prior to trenching.
3.2.2 Soil Bearinji
An allowable soil bearing pressure of 2,000 pounds per square foot (psf) may be used for the
design of footings having a minimum width of 12 inches and minimum embedment of 10
inches below lowest adjacent ground surface. This value may be increased by 300 psf for each
additional foot of embedment or 100 psf for each additional foot of foundation width to a
maximum value of 2,500 psf. These allowable bearing pressures are applicable for level
(ground slope equal to or flatter than 5H: 1V) conditions only.
Resistance to lateral loads can be provided by friction acting at the base of foundations and by
passive earth pressure. A coefficient of friction of 0.35 may be assumed with dead-load forces.
An ultimate passive lateral earth pressure of 300 psf per foot of depth to a maximum of 3,000
psf may be used for the sides of footings poured against properly compacted fill. This passive
pressure is applicable for level (ground slope equal to or flatter than 5H: 1V) conditions only.
Passive resistance for sloping conditions (2 horizontal to 1 vertical or flatter) may be taken as
150 pcf (ignoring the upper 1-foot) to maximum 2,250 psf.
Project No. 10072-01 Page 7 December-14, 2010
Bearing values indicated above are for total dead loads and frequently applied live loads. The
above vertical bearing may be increased by one-third for short durations of loading which will
include the effect of wind or seismic forces. The passive pressure may be increased by one-third
due to wind or seismic forces
3.2.3 Vapor Retarder and Sand Below Slabs
Interior floor slabs with moisture sensitive floor coverings should be underlain by a minimum
10-mil thick polyolefin (or equivalent) moisture/vapor retarder to help reduce the upward
migration of moisture from the underlying subgrade soils. The moisture/vapor retarder
product used should meet the performance standards of an ASTM E 1745 Class A material,
and be properly installed in accordance with ACT publication 302. It is the responsibility of
the contractor to ensure that the moisture/vapor retarder systems are properly placed in
accordance with the project plans and specifications, and that the moisture/vapor retarder
materials are free of tears and punctures prior to concrete placement. Additional moisture
reduction and/or prevention measures may be needed, depending on the performance
requirements of future interior floor coverings.
Sand layers placed below slabs and above/below vapor retarders for the purpose of protecting
the retarder and to assist in concrete curing are the purview of the foundation
engineer/structural engineer, and should be provided in accordance with ACT Publication 302
"Guide for Concrete Floor and Slab Construction". We have provided recommendations in
Table 1 for moisture retardation that we consider to be suitable from a geoteclmical
perspective. These recommendations must be confirmed (and/or altered) by the foundation
engineer, based upon the performance expectations of the foundation. Ultimately, the design
of the moisture retarder system and recommendations for concrete placement and curing are
the purview of the foundation engineer, in consideration of the project requirements provided
by the architect and developer.
3.2.4 Foundation Setback from Top of Slope and Bottom of Slope
Per the C.B.C., the minimum setback is H/3 with a maximum setback of 40 feet. Top of slope
setbacks should be measured perpendicular to the top of slope. Ultimately, the setback
criteria shall be reviewed and determined by the geotechnical consultant.
A review of the plans, depicting the currently proposed building layouts for each lot indicates
that all the proposed layouts for the Model Lots and Phases 1 & 2 will meet minimum
setback requirements. Finalized building layouts for the emaining lots were not available at
the time of this review and therefore LGC Geoteclmical must review setbacks for each phase
as they are incorporated into the final plan-set.
Should future plan changes result in building footprint moving any closer to the slope, LGC
Geotechnical must again review plans for setback compliance.
Project No. 10072-01 Page 8 December 14, 2010
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I Future homeowners should be advised not to plan movement sensitive structures within close
vicinity of slopes. If balconies are to be constructed, we recommend that these isolated
footings be connected to the building foundation system with grade beams.
- 3.2.5 Potential Fill Settlement
- As mentioned earlier in this report, the subject site is underlain by fill. Due to the self-weight
consolidation of the fill, the subject lots will experience some amount of fill settlement during
the project design life. In areas where fill thickness is constant and fill settlement occurs
uniformly, there will be little or no significant influence on the structures. However,
differential settlement if not considered, could have an adverse impact on structures.
- Considering the thicknesses of fill below the subject lots and the granular nature of the fill
soils, self-weight consolidation of the fill is expected to be mininial.
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3.3 Slope Creep
As with most natural and manmade slopes and pad areas, some degree of slope creep should be
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expected for this site. The amount of slope creep is usually influenced by such factors as the slope
geometry, slope exposure, aspect, height, composition, as well as plant type, precipitation, irrigation
and landscaping programs. Since the depth of the creep zone is somewhat unknown and analytically
is in its infancy, our estimates of the extent and magnitude of slope creep are, therefore, based on our
I observations at previous sites with similar soil conditions. In general, the effects of slope creep are
most prevalent in the outer 20 feet of the slope. In general, more slope creep occurs as the slope
- height increases, expansion potential increases, and changes in the moisture content of the soil occur.
Slope creep is not expected to significantly influence the proposed buildings, which are constructed
with a post-tensioned slab and meet or exceed the setback recommendations.
- Although rear yard top-of-slope improvements are generally not considered structural, we
recommend that decorative walkways, patios, pools and spas, and other landscaping features be
I constructed with flexibility to accommodate the effects of slope creep. Typical remediation methods
include construction joints, separation joints, flexible payers, flexible structures, deep foundation
system, or additional reinforcement to reduce cracking (see Section 3.6, Nonstructural Concrete
I Flatwork). The exact amount of movement due to slope creep cannot be determined at this time; it is
dependent to some extent upon irrigation practices of homeowners and homeowner associations.
Future homeowners should be made aware of these conditions so they can design their improvements
I appropriately. See Section 3.8 for more specific geoteclmical recommendations for freestanding
walls and fences close to the top-of-slopes.
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Project No. 10072-01 Page 9 December 14, 2010
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3.4 Lot Stretchinj'
Lot stretching is a term used to describe the predominately lateral deformation or extension of lots,
which are located near the top of slopes generally containing expansive soils. Based on our previous
experience, the effects of lot stretching generally extend further back from the top of slope than slope
creep and have been observed up to 100 feet from the top of slope. In general, the effects of lot
stretching manifest themselves in the form of distortion of improvements and/or separation of
flatwork from adjacent improvements. It has been our experience, that the effects of lot stretching
generally do not significantly influence the performance of post-tension foundations. Although the
effects of lot stretching have been observed for many years, it is still not completely understood.
Based on limited theoretical models, lot stretching is believed to occur as a result of the wetting front
gradually penetrating through expansive soils.
We anticipate lot stretching could result in lateral movement on the order of 6 inches; however, it is
also possible that no significant lot stretching will occur due to the granular nature of the onsite soils
and relatively small slope heights (-20 feet). The amount of lot stretching is dependant in part on
homeowner activity and is not fully understood. Therefore it is not possible to accurately determine
the amount of movement that may occur. Based on the estimated depth of fill and proposed usage of
post-tensioned foundation system, the effects of lot stretching are not expected to significantly affect
the proposed residential foundations. However, separation of the flatwork from adjacent
improvements and/or distortion of other landscaping improvements may occur. Please see previous
section for design recommendations to help reduce the effects of lot stretching.
3.5 Corrosivitp to Concrete and Metal
Based upon previous laboratory testing performed at the site, sulfate exposure to concrete structures
in contact with onsite soils should be considered 'negligible' and therefore concrete in contact with
these soils should consist of cement that meets the minimum compressive strength requirements of
the locally governing agency. Onsite soils should also be considered corrosive to buried metallic
objects/structures. If necessary, we recommend you contact a corrosion engineer for specific
mitigation techniques to help prevent corrosion of buried structures.
3.6 Nonstructural Concrete Flatwork
Concrete flatwork (such as walkways, bicycle trails, patio slabs, etc.) has a high potential for
cracking due to changes in soil volume related to soil-moisture fluctuations. To reduce the potential
for excessive cracking and lifting, concrete should be designed in accordance with the minimum
guidelines outlined in Table 2. These guidelines will reduce the potential for irregular cracking and
promote cracking along construction joints, but will not eliminate all cracking or lifting. Thickening
the concrete and/or adding additional reinforcement will further reduce cosmetic distress.
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Project No. 10072-01 Page 10 December 14, 2010
TABLE 2
Nonstructural Concrete Flatwork for Low Expansion Potential
Homeowner Private Drives Patios/Entryways City Sidewalk Curb
Sidewalks and Gutters
Minimum 4 (nominal) 4 (full) 4 (full) City/Agency
Thickness (in.) Standard
Moisture Wet down prior Wet down prior Wet down prior City/Agency
Conditioning to placing to placing to placing Standard
Reinforcement -
No. 3 at 24 No. 3 at 24 City/Agency
inches on centers inches on centers Standard
Thickened Edge
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City/Agency
(in.) 8 x 8 Standard
Saw cut or deep Saw cut or deep Saw cut or deep
Crack Control open tool joint open tool joint to open tool joint to City/Agency
Joints to a minimum of a minimum of a minimum of Standard 1/3 the concrete 1/ the concrete 1/ the concrete
thickness thickness thickness
Maximum Joint 10 feet or quarter City/Agency
Spacing 5 feet cut whichever is 6 feet Standard closer
Aggregate Base City/Agency
Thickness (in.) - - Standard
To reduce the potential for driveways to separate from the garage slab, the builder may elect to install
dowels to tie these two elements together. Similarly, future homeowners should consider the use of
dowels to connect flatwork to the foundation.
1 3.7 Retaining Walls
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The following parameters may be utilized for preliminary design of retaining walls. These
recommendations are for walls not at the top of slopes. Should walls be designed at these locations,
the geotechnical consultant should be contacted to provide specific recommendations.
I Onsite soils having low to very low expansion potential and a SE > 20 may be used as backfill
behind retaining walls. Alternatively, retaining wall backfill should comprise of select clean sand or
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gravel with SE > 30.
In addition to the nature of the backfill soils, the magnitude of the lateral soil pressures on a retaining
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wall also depends on the amount of deformation that the wall can yield under load. If the wall can
yield enough to mobilize the full shear strength of the soil, it can be designed for "active" pressures.
Walls designed for "active" conditions should contain construction joints. These construction joints
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should be based on the recommendations of the civil engineer, but at a minimum they should be
placed at every direction change. If the wall cannot yield under the applied load, the shear strength of
the soil cannot be fully mobilized and the earth pressure will be higher. Such walls should be
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Project No. 10072-01 Page 11 December 14, 2010
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designed for the "at-rest" conditions. If a structure tends to move toward the soils, the resulting
resistance developed by the soil is the "passive" resistance.
For design purposes, the recommended equivalent fluid pressures for level and sloping backfill are
provided in Table 3. In utilizing these geotechnical parameters, the civil engineer should clearly
indicate within the construction plans, that the backfill is approved onsite (SE > 20) or select clean sand
or gravel (SE >30).
TABLE -
Equivalent Fluid Pressures
Condition Equivalent Fluid Unit Weight (psf/ft.)
Level Ground I 2H: IV Slope
Backfill Approved Onsite (SE> 20) or Clean Sand or Gravel (SE> 30)
Active
At-Rest
35
55
55
75
Onsite Materials
Passive 300 -
Please note that the above values do not take into account surcharge, due to structures in the vicinity of
the retaining walls, or compaction-induced wall pressures. If the retaining walls are influenced by such
surcharge, additional load due to the same should be considered when designing the subject walls.
Heavy construction equipment should maintain a distance of 3 feet away from the walls while
backfilling the wall to avoid overstressing the wall.
Soil resistance developed against lateral structural movement can be obtained from the passive
pressure value provided above. Since it is likely that the sides of the foundation will be bearing against
onsite material, the passive resistance of this material has been included within Table 3. The lateral
passive resistance should be taken into account only if it is ensured that the soil against embedded
structures will remain intact with time. Further, for sliding resistance, a friction coefficient of 0.35 may
be used at the concrete and soil interface. These lateral and frictional resistance values represent
ultimate values, so appropriate safety factors should be applied by the structural engineer during wall
design.
Retaining wall backfills should be compacted to a minimum of 90 percent relative compaction (based
on ASTM Test Method D1557). The wall backfihls should be keyed in to meet the existing fill at the
site. During construction of retaining walls, backcuts should be made in accordance with the
requirements of Cal/OSHA Construction Safety Orders. It is the contractor's responsibility to ensure
these requirements are met.
Retaining wall footings should have a minimum embedment of 18 inches below the adjacent lowest
grade. The project geotechnical consultant should review all retaining wall plans, particularly where
walls are adjacent to structures.
Project No. 10072-01 Page 12 December 14, 2010
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I Retaining structures should be provided with a subdrain system as shown on Figure 2. If drainage
cannot be provided over the full length of the wall, an additional lateral force due to water accumulation ' behind the wall should be taken into consideration for that portion retaining the undrained zone. It
should be noted that these recommended backdrain features do not provide protection against seepage
through the face of the wall. If such seepage is undesirable, retaining walls should be waterproofed to
I limit seepage. Please note that the retaining wall designer should include a backfill detail in accordance
with the diagram on the following page on the design plans to ensure proper construction by the
contractor. Wall subdrains should be placed as low as possible during construction to prevent ponding
of water within the backfill materials.
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Project No. 10072-01 Page 13 December 14, 2010
EXTENT OF APPROVED ON-SITE BACKFILL
(El <20, FREE DRAINING)
OR IMPORT BACKFILL (El <30, FREE
DRAINING)MATERIAL MINIMUM HEEL WIDTH
OR H/2,WHICH EVER IS GREATER -
NATIVE SOIL CAP
12-INCH MINIMUM
18-INCH MAXIMUM
'
WATER PROOFING PER I I CIVIL ENGINEER AND / OR ARCHITECT
RETAINING WALL BACKFILL
COMPACTED TO MINIMUM 90% IA A
RELATIVE COMPACTION PER ASTM D 1557
Lu
i BACKCUT PER OSHA F I
MINIMUM I CUBIC FOOT GRAVEL / CRUSHED
ROCK (3/4 INCH) PER LINEAR FOOT OF
PERFORATED PIPE WRAPPED IN FILTER FABRIC \. - .•
(MIRAFI 140N OR APPROVED EQUIVALENT) \
4-INCH DIAMETER, SCHEDULE 40 PERFORATED
PVC PIPE MAINTAINED AT BASE OF WALL TO FLOW . . A. TO DRAINAGE OUTLET I I . .
..
-I lj. I F
•.
A . . H H I- -
.' Ill FI_I1 Iii
FOOTINJGM/ALL DESIGN PER : I • .
STRUCTURAL ENGINEER F iI
NOTE:
PLACEMENT OF SUBDRAIN AT BASE
OF WALL IS INTENDED TO DRAIN THE
RETAINING WALL BACKFILL AND WILL
NOT PREVENT SATURATION OF SOILS
BELOW AND / OR IN FRONT OF WALL
Version 1012009
FIGURE 2 PROJECT NAME La Costa 3.1
______ Typical Retaining Wall PROJECT NO. 1007201
Backfill and Drainage Detail ENG. I GEOL. DJB / TJL
Approved On-Site/Import SCALE Not to Scale Material Backfill DATE December 2010
3.8 Fences and Freestandinr Walls
As their name indicates, freestanding walls are walls that are not designed to retain soil and/or water.
I These walls are generally located at the rear of the lot, or along the side yard or between lots.
I
As a result of the many factors that influence the rate and magnitude of slope creep, it is not possible
to accurately determine at the present time the extent or amount of slope creep. The amount of
distress that occurs to these improvements as a result of slope creep depends to a certain extent on
I
how much movement occurs and how flexible the improvement is. For the purpose of this report,
freestanding walls generally considered to consist of masonry or concrete blocks, while flexible
fences generally consist of wood or tube steel.
The following recommendations have been developed by experience generated in working in similar
geotechnical conditions rather than a calculated solution. These recommendations will not eliminate
all movement of freestanding walls at the site, but should limit movement to within tolerable limits of
the structures, thereby maintaining their functionability and reducing cosmetic distress. The
following recommendations also assume proper homeowner/homeowner association maintenance,
landscaping, and irrigation practices. Should future owners not properly maintain the subject slopes,
then additional distress may be observed.
3.8.1 Freestandinj' Walls
In recognition that the subject walls will be subject to slope creep, we recommend
freestanding walls located parallel to the top-of-slope should have a minimum embedment of
24 inches below the adjacent lowest grade. At this depth, an allowable bearing pressure of
1,800 psf may be utilized in design.
Freestanding walls located perpendicular to the top-of-slope should have a minimum
embedment of 18 inches below the adjacent lowest grade. At this depth, an allowable bearing
pressure of 1,650 psf may be utilized in the design.
The bearing values above may be increased by one-third for wind or seismic loading. To
I reduce the potential for unsightly cracks due to the onsite expansive soils, we recommend the
inclusion of construction joints at a maximum of 15-foot on center. This spacing may be
altered by the structural engineer based upon the wall reinforcement. If the soil-moisture
I content below the wall foundation varies significantly, some wall movement should be
expected; however, this movement is unlikely to cause more than cosmetic distress.
3.8.2 Fences at the Top-of-Slopes
I The fence designer should engineer the construction of the fence to be as flexible as possible
while still maintaining integrity. Typical design features should include such items as slip
couplings at the junction of top of slope fences running and fences/walls perpendicular to the
I slope face.
I
Project No. 10072-01 Page 15 December 14, 2010
I
3.9 Surface Drainage and Yard Landscaping
3.9.1 General
Surface drainage should be carefully taken into consideration during precise grading,
building construction, future landscaping and throughout the design life of the residential
structure. Positive drainage should be provided to direct surface water away from
improvements and towards either the street or other suitable drainage devices. Ponding of
water adjacent to any structural improvement foundation must be avoided. The performance
of structural foundations is dependant upon maintaining adequate surface drainage away
from them, thereby reducing excessive moisture fluctuations. From a geotechnical
perspective, roof gutters, area drains, drainage swales, and finished grade soils should be
aligned so as to transport surface water to a minimum distance of 5 feet away from the
proposed foundations.
3.9.2 Precise Grading
From a geotechnical perspective, we recommend that compacted finished grade soils adjacent
to proposed residences, be sloped at a minimum of 2 percent away from the proposed
residence and towards an approved drainage device or unobstructed swale. Drainage swales,
wherever feasible, should not be constructed within 5 feet of buildings. Where lot and
building geometry necessitates that the side yard drainage swales be routed closer than 5 feet
to structural foundations, we recommend the use of area drains together with drainage
swales, particularly in areas of minimal positive drainage. Drainage swales used in
conjunction with area drains should be designed by the project civil engineer so that a
properly constructed and maintained system will prevent ponding within 5 feet of the
foundation.
3.10 Swimming Pools and Spas
LGC Geotechnical should be contacted for specific design recommendations for proposed pools
located at the top of slopes. Recommendations are typically provided on a case-by-case basis.
3.11 Seismic Design Criteria
The site seismic characteristics were evaluated per the guidelines set forth in Chapter 16, Section
1613 of the 2007 C.B.C. Site coordinates of latitude 33.101712 degrees north and longitude
-117.223935 degrees west, which are representative of the site, were utilized in our analyses. The
initial results of our analyses for the maximum considered earthquake (MCE) spectral response
accelerations (Ss and S1 ) are presented on Table 4A.
Project No. 10072-01 Page 16 December 14, 2010
TABLE 4A
Seismic Design Parameters
Selected Parameters from the 2007 C.B.C.
Section _1613_-_ Earthquake _Loads Seismic Design Values
Site Class per Table 1613.5.2 D
Spectral Acceleration for Short Periods (Ss)* 1.083 g
Spectral Accelerations for 1-Second Periods (5)* 0.408 g
Site Coefficient Fa per Table 1613.5.3(1) 1.067
Site Coefficient F per Table 1613.5.3(2) 1.592
* Calculated from the USGS computer program "Seismic Hazard Curves, Response
Parameters and Design Parameters" v5.0.9a (10/21/09)
The spectral response accelerations (SMS and SMI) and design spectral response acceleration
parameters (SDS and SDI), adjusted for Site Class D, were evaluated for the site in general accordance
with section 1613 of the 2007 C.B.C. These site class adjusted parameters are listed on Table 413.
TABLE 4B
Seismic Design Parameters Modified for Site Class D
Selected Parameters from the 2007 C.B.C. Seismic Design Values
Section 1613 - Earthquake Loads Modified for Site Class D
Site Modified Spectral Acceleration for Short
Periods (SMs) for Site Class D 1.155 g
[Note: SMS = FaS]
Site Modified Spectral Acceleration for 1-Second
Periods (SM1) for Site Class D 0.650 g
[Note: SMI = FS1]
Design Spectral Acceleration for Short Periods
(SDS) for Site Class D 0.770 g
[Note: SDS = (2/3)SMs]
Design Spectral Acceleration for 1-Second Periods
(SDI) for Site Class D 0.433 g
[Note: SDI = (2/3)SMI]
In accordance with Table 1613.5.6 (1, 2), the seismic design category for the subject site is Category
D, where SDS > 0.5 and SDI > 0.2.
Section 1802.2.7 of the 2007 C.B.C. states that the PGA for a site may be defined as SDs/2.5. The SDS
for the subject site has been calculated as 0.770 g.
Therefore, PGA = 0.770 /2.5 = 0.31 g.
3.12 Geotechnical Observation/Testing During Construction
Project No. 10072-01 Page 17 December 14, 2010
The project geotechnical consultant should perform observation and/or testing at the following
stages:
During precise grading or pad recertification process;
After building footing and retaining wall excavation and prior to placing concrete and/or
reinforcing;
During installation of retaining wall drainage and placing backfill (where applicable);
During backfill of interior plumbing trenches and dry/wet utility trenches and house connections;
During preparation of subgrade and placing of aggregate base; and
When any unusual soil conditions are encountered during any construction operation subsequent
to issuance of this report.
Project No. 10072-01 Page 18 December 14, 2010
4.0 LIMITATIONS
U
Our services were performed using the degree of care and skill ordinarily exercised, under similar
circumstances, by reputable soils engineers and geologists practicing in this or similar localities. No other
warranty, expressed or implied, is made as to the conclusions and professional advice included in this report.
This report is based on data obtained from limited observations of the site, which have been extrapolated to
characterize the site. While the scope of services performed is considered suitable to adequately characterize
the site geotechnical conditions relative to the proposed development, no practical investigation can
completely eliminate uncertainty regarding the anticipated geotechnical conditions in connection with a
subject site. Variations may exist and conditions not observed or described in this report may be
encountered during construction.
This report is issued with the understanding that it is the responsibility of the owner, or of his/her
representative, to ensure that the information and recommendations contained herein are brought to the
attention of the other consultants and incorporated into the plans. The contractor should properly implement
the recommendations during construction and notify the owner if they consider any of the recommendations
presented herein to be unsafe, or unsuitable.
The findings of this report are valid as of the present date. However, changes in the conditions of a site can
and do occur with the passage of time, whether they be due to natural processes or the works of man on this
or adjacent properties. The findings, conclusions, and recommendations presented in this report can be relied
upon only if LGC Geotechnical has the opportunity to observe the subsurface conditions during grading and
construction of the project, in order to confirm that our preliminary findings are representative for the site.
This report is intended exclusively for use by the client, any use of or reliance on this report by a third party
shall be at such party's sole risk.
In addition, changes in, applicable or appropriate standards may occur, whether they result from legislation or
the broadening of knowledge. Accordingly, the findings of this report may be invalidated wholly or partially
by changes outside our control. Therefore, this report is subject to review and modification.
Project No. 10072-01 Page 19 December 14, 2010
Appendix A
References
I
I APPENDJX A
References
GEOCON Inc., 2007a, Final Report of Testing and Observation Services Performed During Site Grading,
Villages of La Costa - The Oaks North Neighborhood 3.1, Lots 1 through 80, Carlsbad, California,
I Project No. 06105-52-20, dated August 7, 2007.
2007b, Final Report of Testing and Observation Services Performed During Master Developer
I Improvements, Villages of La Costa - The Oaks North Neighborhoods 3.1 through 3.5, 3.7, and
Avenida Soledad Stations 11+50 through 15+68, Carlsbad, California, Project No. 06105-52-21, dated
December 26, 2007.
Geocon Inc., 2010, Update Geotechnical Report, Villages of La Costa - The Oaks North Neighborhood 3.1,
Lots 1 through 80, Carlsbad, California, Project No. 06105-52-28, dated January 20, 2010.
Hunsaker & Associates, 2010, Precise Grading, Villages of La Costa - The Oaks North Neighborhood 3.1, 3
Sheets, dated December 14, 2010.
LGC Geotechnical Inc., 2010a, Recommended Pavement Sections for Interior Streets within La Costa 3.1,
Carlsbad, California, Project No. 10072-01, dated October 27, 2010.
2010b, Foundation Recommendations, La Costa Oaks, Tract 10815, City of Carlsbad, California,
Project No. 10072-01, Dated November 1,2010.
Project No. 10072-01 A-i December 14, 2010
I'
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I
Appendix B
Laboratory Testing Procedures & Test Results
APPENDIX B
Laboratory Testing Procedures and Test Results
Expansion Index Tests: The expansion potential of selected materials was evaluated by the Expansion Index
Test, U.B.C. Standard No. 18-I-B. Specimens are molded under a given compactive energy to approximately
the optimum moisture content and approximately 50 percent saturation or approximately 90 percent relative
compaction. The prepared 1-inch thick by 4-inch diameter specimens are loaded to an equivalent 144 psf
surcharge and are inundated with tap water until volumetric equilibrium is reached. The results of these tests are
presented in the table below.
Lot Sample Description Expansion
Index Expansion Potential Source
1 through 3 El-C 0 Very Low Geocon, 2007a
4 and 5 EI-D 0 Very Low Geocon, 2007a
6 and 7 EI-W 30 Low Geocon, 2007a
8 through 12 EI-G 20 Very Low Geocon, 2007a
13 through 16 El-F 5 Very Low Geocon, 2007a
17 through 20 EI-X 7 Very Low Geocon, 2007a
21 through 24 EI-E 6 Very Low Geocon, 2007a
25 through 30 EI-Y 1 Very Low Geocon, 2007a
31 through 35 EI-Z 0 Very Low Geocon, 2007a
36 through 39 El-AF 9 Very Low Geocon, 2007a
40 through'43 EI-AE 2 Very Low Geocon, 2007a
44 through 47 El-AD 0 Very Low Geocon, 2007a
48 through 50 El-AC 8 Very Low Geocon, 2007a
51 through 54 EI-AB 0 Very Low Geocon, 2007a
55 through 58 EI-AA 0 Very Low Geocon, 2007a
59 through 62 El-AK 0 Very Low Geocon, 2007a
63 through 66 EI-AJ 1 Very Low Geocon, 2007a
67 through 70 El-Al 0 Very Low Geocon, 2007a
71 through 75 El-AH 1 Very Low Geocon, 2007a
76 through 80 El-AG 0 Very Low Geocon, 2007a
Project No. 10072-01 B-i December 14, 2010