HomeMy WebLinkAboutMS 14-07; PRELIMINARY FOUNDATION DESIGN; 2015-05-15LGC Valley, Inc.
Geotechnical Consulting
April 15, 2015
Mr. Tae Dixon
Shape/i Properties, Inc.
8383 Wilshire Boulevard, Suite 700
Beverly Hills, California 90211
I ECE•D
SEP28 2015
1 LAND DEVEl-0-sT
EG!F—ERIIk
Project No. 133023-07
Subject: Preliminary Foundation Design for the Proposed Multi-Family Residential Development,
Robertson Ranch Planning Areas PA-7 and PA-8, 4980 El Cam/no Real, Carlsbad,
California
Introduction
In accordance with your request, LGC Valley, Inc., (LGC) has prepared this letter to provide preliminary
foundation design and seismic design parameters for the future development of Planning Areas PA-7, and
PA-8 of the Robertson Ranch project located in the City of Carlsbad, California. The purpose of this letter is
to provide preliminary foundation design and seismic parameters to be in accordance with the latest adopted
building code (i.e. 2013 California Building Code, which is based on the 2012 International Building Code).
The recommendations contained herein should be verified by LGC at the completion of grading to confirm the
interpretations used herein.
Planning Areas PA-7 and PA-8 are located in the south-central portion of the Robertson Ranch project and are
approximately 6.5 and 14.5 acres in size, respectively. PA-8 is divided into two parcels: Parcel A is 4.8 acres in
size and is planned for senior housing; while Parcel B is 9.7 acres in size, and along with PA-7, will contain a
multi-family apartment complex. The site (PA-7 and PA-8) is bounded by El Camino Real to the south;
proposed single-family communities (PA-3 and PA-6) on the west, northwest, and northeast; and a retail
development (PA-i 1), Gage Road and West Ranch Street on the southeast. Robertson Road separates PA-7 and
PA-8 Parcel B. A future community recreation center (PA-4) is located at the southwest corner of PA-8 on the
north side of Robertson Road but is not part of the multi-family development.
It is our understanding that the proposed multi-family development will consist of 24 one- to three-story
residential buildings with 263 units and 376 parking spaces. The project will include a leasing/recreation
building, maintenance building, pool, landscape areas, appurtenant structures, and driveways. We also
understand that the senior housing project will include the construction of one large building with 96 units and
143 parking spaces with associated driveways, landscape areas, and appurtenant structures.
Based on review of the latest site grading plans, PA-7 will require approximately 7,700 cubic yards of fill while
PA-8 will require 18,000 cubic yards. The precise or fine grading of the site will consist of minor cuts and fills
of less than two to three feet to achieve the site design finish grades. Fills of up to six to eight feet will be
necessary to backfill the existing storm drain retention basins. Once fine grading is completed, finish grade
expansion and corrosion testing should be performed to confirm the final as-graded pad properties.
2420 Grand Avenue, Suite F2 • Vista • CA 92081 '(760) 599-7000. Fax (760) 599-7007
Current As-Graded Geolechnical Conditions
Rough/mass grading of the Robertson Ranch started September 3, 2014 and is still on-going as of this date. The
rough/mass grading operations within Planning Areas PA-7 and PA-8 included: removal of potentially
compressible soils such as alluvium, colluvium, topsoil, and weathered formational material; installation of
canyon subdrains; excavation of cut materials; overexcavation of cut/fill transition conditions within the
proposed building pad locations; and placement of fill. As recommended in the project geotechnical report, fill
soils deeper than 40 feet below proposed grade were placed and compacted to a minimum 93 percent relative
compaction. Fills soils placed within 40 feet of finished grade were compacted to a minimum 90 percent
relative compaction.
Based on the as graded conditions, the building pads of Buildings 8, 9, 12, and 17 consist of cut while the
remaining building pads consist of fill. Buildings 1, 2, 5, 6, 10, and 13 are cut/fill transition pads that were
overexcavated a minimum of 5 feet to a distance of at least 10 feet outside the proposed building limits and
replaced with compacted fill. During the rough/mass grading operations, remedial grading was performed so
that the fill differential beneath the proposed building pads were less than a 3:1 (maximum fill thickness to the
minimum fill thickness).
Building pads having more than 40 feet of fill include Buildings 3, 4, 5, 16, and 20 through 25. Settlement
monument monitoring was recommended in the project geotechnical report in these areas. A total of five
settlement monuments have been installed and are currently being monitored. Buildings 3, 4, 5, 16, and 20
through 25 should not be constructed until the monitoring indicates the primary and secondary settlement of the
fill is essentially complete.
Based on our professional experience on the site, we anticipate that the finish grade soils within Planning Areas
PA-7 and PA-8 will have expansion potentials ranging from very low to medium (i.e. an expansion index of 0
to 90). The actual expansion potentials of the finish grade soils of the building pads should be determined after
the site is fine-graded.
In addition, based on limited soil testing prior to the grading of the site, the on-site soils are classified as
having a negligible sulfate exposure condition in accordance with AC! 318R-08 Table 4.3.1. Concrete in
contact with on-site soils should be designed in accordance with AC! 318R-08 Table 4.3.1 for the negligible
category. It is also our opinion that on-site soils should be considered moderately to severely corrosive to
buried metals. The actual corrosivity of the finish grade soils should be verified with confirmatory sampling
and testing upon completion of the site rough and fine grading.
Preliminary Foundation L)esi2n
The following preliminary foundation designs are provided for your review and use and are based on the site
conditions as currently understood and interpreted by LGC, and are in accordance with the latest building code
requirements (2013 California Building Code).
Project No. 133023-07 Page 2 April 15, 2015
General Foundation Recommendation
Preliminary recommendations for foundation design and foundation construction are presented herein. When
the structural loads for the proposed structures are known they should be provided to our office to verify the
recommendations presented herein.
The following three foundation recommendations are provided. The foundations recommended for the
proposed structures are: (1) Conventional foundation for very low expansion potential and shallow fills; (2)
Post-Tension foundations; or (3) Mat Slabs.
Based on the site geotechnical conditions after mass/rough grading, the site is considered suitable for the
support of the anticipated structures using a conventional, post-tensioned, or mat slab-on-grade foundation
system for very low to medium expansion potential (0-90 Expansion Index).
The information and recommendations presented in this section are not meant to supersede design by the
project structural engineer or civil engineer specializing in the structural design nor impede those
recommendations by a corrosion consultant. Should conflict arise, modifications to the foundation design
provided herein can be provided.
Bearini' Cavaci
Shallow foundations may be designed for a maximum allowable bearing capacity of 2,000 lb/ft2 (gross), for
continuous footings a minimum of 12 inches wide and 18 inches deep, and spread footings 24 inches wide
and 18 inches deep, into certified compacted fill or bedrock. A factor of safety greater than 3 was used in
evaluating the above bearing capacity value. This value maybe increased by 250 psf for each additional foot
in depth and 100 psf for each additional foot of width to a maximum value of 3,000 psf.
Lateral forces on footings may be resisted by passive earth resistance and friction at the bottom of the
footing. Foundations may be designed for a coefficient of friction of 0.35, and a passive earth pressure of
250 lb/ft2/ft. The passive earth pressure incorporates a factor of safety of greater than 1.5.
All footing excavations should be cut square and level as much as possible, and should be free of sloughed
materials including sand, rocks and gravel, and trash debris. Subgrade soils should be pre-moistened for the
assumed vey low to medium expansion potential (to be confirmed at the end of grading). These allowable
bearing pressures are applicable for level (ground slope equal to or flatter than 5H: IV) conditions only.
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.
Project No. 133023-07 Page 3 April 15, 2015
Conventional Foundations
Conventional foundations may be used to support proposed structures underlain by very low expansive soils
(i.e. Expansion Index less that 20 and Plasticity Index less than 15) and with less than 30 feet of fills.
Continuous footings should have minimum widths of 12 inches, 15 inches or 18 inches for one-story, two-
story or three-story structures, respectively. Individual column footings should have a minimum width of 24
inches.
Footings for proposed structure should have minimum depths (below lowest adjacent finish grade) of 18
inches and 12 inches for exterior and interior footings, respectively for a very low expansion potential (0-20
Expansion Index). The subgrade should be moisture-conditioned and proof-rolled just prior to construction to
provide a firm, relatively unyielding surface, especially if the surface has been loosened by the passage of
construction traffic.
The underslab vapor/moisture retarder (i.e. an equivalent capillary break method) may consist of a minimum
15-mil thick vapor barrier in conformance with ASTM E 1745 Class A material, placed in general
conformance with ASTM E1643, underlain by a minimum 1-inch of sand. The sand layer requirements
above the vapor barrier are the purview of the foundation engineer/structural engineer, and should be
provided in accordance with ACI Publication 302 "Guide for Concrete Floor and Slab Construction". 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 concrete mix design, which will address bleeding, shrinkage,
and curling are the purview of the foundation engineer, in consideration of the project requirements provided
by the architect and developer. The underslab vapor/moisture retarder described above is considered a
suitable alternative in accordance with the Capillary Break Section 4.505.2.1 of the CALGreen code.
Subgrade soils should be pre-saturated to optimum moisture content to a depth of 12 inches for a very low
expansion potential. Expansion index testing should be performed at the end of grading for confirmation.
The minimum thickness of the floor slabs should be at least 4.5 inches, and joints should be provided per usual
practice.
Post-Tension Foundations
Based on the site geotechnical conditions and provided the previous remedial recommendations have been
implemented during site grading, the site may be considered suitable for the support of the anticipated
structures using a post-tensioned slab-on-grade foundation system, for the anticipated very low to medium
expansive soils. The following section summaries our recommendations for the foundation system.
The following table contains the geotechnical recommendations for the construction of PT slab on grade
foundations. The structural engineer should design the foundation system based on these parameters
including the foundation settlement as indicated in the following section to the allowable deflection criteria
determined by the structural engineer/architect.
Project No. 133023-07 Page 4 April 15, 2015
Preliminary Geotechnical Parameters for Post-Tensioned Foundation Design
Parameter Value
Expansion Classification (Assumed to be Very Low to Low and for Medium Expansion
confirmed at the completion of grading):
Thomthwaite Moisture Index (From -20
Figure 3.3):
Constant Soil Suction (From Figure 3.4): PF 3.6
Center Lift Very Low to Low Medium Edge moisture variation distance
(from Figure 3.6), em: 9.0 feet 9.0 feet
Center lift, Ym 0.3 inches 0.5 inches
Edge Lift Very Low to Low Medium Edge moisture variation distance
(from Figure 3.6), em: 5.2 feet 5.0 feet
Edge lift, ym: 0.7 inches 1.1 inches
Soluble Sulfate Content for Design of
Concrete Mix in Contact with Site Soils in Negligible Exposure
Accordance with American Concrete (Based on preliminary testing - needs to be confirmed at the
Institute standard 318, Section 4.3: completion of grading)
Corrosivity of Earth Materials to Ferrous
Metals: Moderately to Severely Corrosive
Modulus of Subgrade Reaction, k
(assuming presaturation as indicated 100 pci (very low to low)
below): 85 pci (medium)
Additional Recommendations:
Pre saturate slab subgrade to at least optimum-moisture content, or to 1.2 times optimum moisture to
minimum depths of 12 and 18 inches below ground surface, respectively for very low to low, and
medium expansion potentials.
Install a 15-mil moisture/vapor barrier in direct contact with the concrete (unless superseded by the
Structural/Post-tension engineer*) with minimum 1 inches of sand below the moisture/vapor barrier.
Minimum perimeter foundation embedment below finish grade for moisture cut off should be 12 and
18 inches, respectively for very low to low and medium expansion potentials.
Minimum slab thickness should be 5 inches.
Project No. 133023-07 Page 5 April 15, 2015
The above sand and moisture/vapor barrier recommendations are traditionally included with geotechnical
foundation recommendations although they are generally not a major factor influencing the geotechnical
performance of the foundation. The sand and moisture/vapor barrier requirements are the purview of the
foundation engineer/corrosion engineer (in accordance with ACI Publication 302 "Guide for Concrete Floor
and Slab Construction") and the homebuilder to ensure that the concrete cures more evenly than it would
otherwise, is protected from corrosive environments, and moisture penetration of through the floor is
acceptable to future homeowners. Therefore, the recommendations provided herein may be superseded by
the requirements of the previously mentioned parties.
The underslab vapor/moisture retarder (i.e. an equivalent capillary break method) may consist of a minimum
15-mil thick moisture/vapor barrier in conformance with ASTM E 1745 Class A material, placed in general
conformance with ASTM E1643, underlain by a minimum 1-inch of sand, as needed. The sand layer
requirements above the vapor barrier are the purview of the foundation engineer/structural engineer, and
should be provided in accordance with AC! Publication 302 "Guide for Concrete Floor and Slab
Construction". 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 concrete mix design, which will address bleeding,
shrinkage, and curling are the purview of the foundation engineer, in consideration of the project
requirements provided by the architect and developer. The underslab vapor/moisture retarder described
above is considered a suitable alternative in accordance with the Capillary Break Section 4.505.2.1 of the
CALGreen code.
Mat Foundations
A mat foundation can be used for support of proposed residential buildings. An allowable soil bearing
pressure of 1,000 psf may be used for the design of the mat at the surface under the slab area. The allowable
bearing value is for total dead loads and frequently applied live loads and may be increased by one-third for
short durations of loading which will include the effect of wind or seismic forces. A coefficient of vertical
subgrade reaction, k, of 85 pounds per cubic inch (pci) may be used to evaluate the pressure distribution
beneath the mat foundation. The magnitude of total and differential settlements of the mat foundation will be
a function of the structural design and stiffness of the mat.
Resistance to lateral loads can be provided by friction acting at the base of foundations and by passive earth
pressure. Foundations may be designed for a coefficient of friction of 0.35. Minimum perimeter footing
embedment provided in the previous sections maybe reduced for the mat slab design. The underslab
vapor/moisture retarder should be installed as recommended above in the post-tension foundation section.
Coordination with the structural engineer will be required in order to ensure structural loads are adequately
distributed throughout the mat foundation to avoid localized stress concentrations resulting in potential
settlement. The foundation plan should be reviewed by LGC to confirm preliminary estimated total and
differential static settlements.
Project No. 133023-07 Page 6 April 15, 2015
Foundation Settlement
Based on the site design relative to native grades and the site remedial removals currently being performed
during site rough grading to remove all unsuitable potentially compressible soils underlying the site, fill at
the site will range from approximately 5 to a maximum depth of 75 feet (southeast corner of PA-7) in
thickness with the majority of the fill thicknesses less than 40 feet in depth. The deepest fills located in the
southeast corner of PA-7 along the north-south trending canyon. It is anticipated that most of the
consolidation will be complete by the time final design grades are achieved due to the sandy nature of site soils.
Settlement monuments have been installed in deeper fill areas (fills greater than 40 feet in thickness) at the
completion of the rough/mass grading activities, to monitor the primary and secondary consolidation of deep
fills. Deeper fill lots will be released when the primary and secondary consolidation is within acceptable
limits.
Based on the as-graded condition and our preliminary review of site grading plans, remedial grading was
performed during rough/mass grading activities so that the fill differential beneath the proposed building pads
were less than a 3:1 (maximum fill thickness to the minimum fill thickness); therefore, no major fill
differentials are anticipated across the current building pad areas.
Based on our evaluation, the static post-construction settlements for the lots with less than 40 feet of fill, and
deeper fill lots after they are released for construction is estimated to be up to a maximum differential
settlement of approximately 3/4-inch in 30 feet.
Foundation Setback -Top of Slope
Building foundations located close to the top of descending slopes should have a minimum setback per
Figure 1808.7.1 of the 2013 CBC. Figure 1808.7.1 of the 2013 CBC shows that the setback distance from the
bottom edge of the building foundation to the slope surface is equal to the height of slope divided by 3 up to
a maximum setback of 40 feet. The setback distances should be measured from competent materials on the
outer slope face, excluding any weathered and loose materials.
Toe of Slope Condition - Building Clearance
Section 1808.7.1 of the 2013 California Building Code (CBC) provides recommendations/discussion with
regards to the building clearance from ascending slopes. The intent of this section of the code is that the
proposed building structure below slopes shall be set a sufficient distance from the slope to provide
protection from slope drainage, erosion, and shallow failures. The code clearance for building foundations
below slopes is equal to the smaller of the height of slope divided by 2 or 15 feet. CBC Section 1808.7.5
permits an alternate clearance subject to the approval of the building official provided a geotechnical
evaluation is performed to demonstrate that the intent of Section 1808.7 would be satisfied. LGC can review
the site plans once available to evaluate the proposed building clearances.
Project No. 133023-07 Page 7 April 15, 2015
Seismic Desi2n Criteria
The site seismic characteristics were evaluated per the guidelines set forth in Chapter 16, Section 1613 of the
2013 California Building Code (CBC). The maximum considered earthquake (MCE) spectral response
accelerations (SMS and SMI) and adjusted design spectral response acceleration parameters (SDS and SDI) for
Site Class D are provided in the following table.
Seismic Design Parameters
Selected Parameters from 2013 CBC, Section 1613 - Earthquake Loads Seismic
Design Values
Site Class per Chapter 20 of ASCE 7 D
Risk-Targeted Spectral Acceleration for Short Periods (Ss)* 1.083g
Risk-Targeted Spectral Accelerations for 1-Second Periods (Si)* 0.417g
Site Coefficient Fa per Table 1613.3.3 (1) 1.067
Site Coefficient F per Table 1613.3 .3(2) 1.583
Site Modified Spectral Acceleration for Short Periods (SMs) for Site Class 0
[Note: SMS = FaSs] 1155 g
Site Modified Spectral Acceleration for 1-Second Periods (SM1) for Site Class D
[Note: SMI=FVS1] 0.66g g
Design Spectral Acceleration for Short Periods (SDs) for Site Class D
[Note: SDS (2/3)SMs] 0.77 g
Design Spectral Acceleration for 1-Second Periods (SDI) for Site Class D
[Note: SDI = (2/3)SMI] 0.44g g
Mapped Risk Coefficient at 0.2 sec Spectral Response Period, CRS (per ASCE 7) 0.968
Mapped Risk Coefficient at 1 sec Spectral Response Period, CR1 (per ASCE 7) 1.02
* From USGS, 2013
Section 1803.5.12 of the 2013 CBC (per Section 11.8.3 of ASCE 7) states that the maximum considered
earthquake geometric mean (MCEG) Peak Ground Acceleration (PGA) should be used for geotechnical
evaluations. The PGAM for the site is equal to 0.453g (USGS, 2013).
A deaggregation of the PGA based on a 2,475-year average return period indicates that an earthquake
magnitude of 6.79 at a distance of approximately 11.4 km (7.1 mile) from the site would contribute the most
to this ground motion (USGS, 2008b).
Project No. 133023-07 Page 8 April 15, 2015
Corrosivity to Concrete and Metal
The National Association of Corrosion Engineers (NACE) defines corrosion as "a deterioration of a
substance or its properties because of a reaction with its environment." From a geotechnical viewpoint, the
"environment" is the prevailing foundation soils and the "substances" are the reinforced concrete foundations
or various buried metallic elements such as rebar, piles, pipes, etc., which are in direct contact with or within
close vicinity of the foundation soil.
In general, soil environments that are detrimental to concrete have high concentrations of soluble sulfates
and/or pH values of less than 5.5. AC! Criteria (ACI 318R-08 Table 4.3.1), provides specific guidelines for
the concrete mix design when the soluble sulfate content of the soils exceeds 0.1 percent by weight or 1,000
ppm. The minimum amount of chloride ions in the soil environment that are corrosive to steel, either in the
form of reinforcement protected by concrete cover, or plain steel substructures such as steel pipes or piles, is
500 ppm per California Test 532.
Based on limited soil testing prior to the grading of the site, the onsite soils are classified as having a
negligible sulfate exposure condition in accordance with ACT 318R-08 Table 4.3.1. Concrete in contact with
onsite soils should be designed in accordance with AC! 318R-08 Table 4.3.1 for the negligible category. It is
also our opinion that onsite soils should be considered moderately to severely corrosive to buried metals.
Control of Surface Water and DrainaRe Control
Positive drainage of surface water away from structures is very important. No water should be allowed to pond
adjacent to buildings. Positive drainage may be accomplished by providing drainage away from buildings at a
gradient of at least 2 percent for a distance of at least 5 feet, and further maintained by a swale or drainage path
at a gradient of at least 1 percent. Where necessary, drainage paths may be shortened by use of area drains and
collector pipes.
Planters with open bottoms adjacent to buildings should be avoided. Planters should not be designed adjacent
to buildings unless provisions for drainage, such as catch basins, liners, and/or area drains, are made.
Overwatering must be avoided.
Limitations
Our services were performed using the degree of care and skill ordinarily exercised, under similar
circumstances, by reputable 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.
Changes in conditions must be evaluated by the project soils engineer and geologist and design(s) adjusted as
required or alternate design(s) recommended.
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 architect and/or project engineer and incorporated into the plans, and the necessary steps are
taken to see that the contractor and/or subcontractor properly implements the recommendations in the field.
The contractor and/or subcontractor should notify the owner if they consider any of the recommendations
presented herein to be unsafe.
Project No. 133023-07 Page 9 April 15, 2015
Closure
This letter 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 structural/foundation engineer and the necessary steps are taken to see that the information is
implement in the structural/foundation design, as necessary.
If you should have any questions, please do not hesitate to contact us. The undersigned can be reached at
(661) 702-8474.
Respectfully submitted,
LOC Valley, Inc.
( #Z-*-
Basil Hattar, GE 2.734
Principal Engineer
RKW/BIH
Attachments: References
Distribution: (1) Addressee
Project No. 133023-07 Page 10 April 15, 2015
References
American Society of Civil Engineers (ASCE), 2013, Minimum design loads for buildings and other structures,
ASCE/SEI 7-10, Third Printing, 2013.
California Building Standards Commission (CBSC), 2013a, California Building Code, California Code of
Regulations, Title 24, Part 2, Volume 1 and 2 of 2 (based on the 2012 International Building Code).
CBSC, 2013b, California Residential Building Code, California Code of Regulations, Title 24, Part 2.5,(based on
the 2012 International Residential Code).
CBSC, 2013c, California Green Building Standards Code, California Code of Regulations, Title 24, Part 11.
LGC Valley, Inc., LGC Valley, Inc., 2014, Geotechnical and environmental recommendations for Robertson
Ranch West, Carlsbad Tract No. 13-03, Carlsbad, California, Project Number 133023-03, dated 4/29/14.
O'Day Consultants, 2013, Grading plans for Rancho Costera, Robertson Ranch West Village, Carlsbad Tract No.
13-03, Drawing No. 480-3A, 26 sheets, dated November 25.
O'Day Consultants, 2014, Vesting tentative map for Carlsbad Tract No. 13-03-2, 23 sheets, dated 1/16/2014.
Post-Tensioning Institute, 2006, Design of post tensioned slabs-on-ground, Third Addition, Addendum I dated
May 2007, and Addendum 2 dated May 2008, with errata February 4, 2010.
United States Geological Survey (USGS), 2008a, "2008 National Seismic Hazard Maps - Fault Parameters"
retrieved from: http://geohazards.usgs. gov/cfusionlhazfaults search/hf search main.cfm
USGS, 2008b, "2008 interactive Deaggregations (Beta)," retrieved from:
hqps:Hgeohazards.usgs.gov/deaggint/2008/
USGS, 2013, U.S. Seismic Design Maps, retrieved from:
http://geohazards.usgs.gov/designmaps/us/batch.php#csv
Project No. 133023-07 Page 11 April 15, 2015