HomeMy WebLinkAboutPD 2018-0008; CHESTNUT AVENUE RESIDENCE; PRELIMINARY GEOTECHNICAL INVESTIGATION; 2018-02-08Geotechnical Exploration, Inc.
SOIL AND FOUNDATION ENGINEERING • GROUNDWATER • ENGINEERING GEOLOGY
08 February 2018
Mr. Tom Scott
3320 Highland Drive
Carlsbad, CA 92008
lob No. 17-11714
Subject: Preliminary Geotechnical Investigation
Scott Residential Lot
1161 Chestnut Avenue
Carlsbad, California
Dear Mr. Scott:
In accordance with your request, a representative of Geotechnical Exploration,
Inc. has visited the subject site and performed an evaluation of the soil conditions
in the area of the proposed new residence. It is our understanding that the lot is
being developed to receive a new two-story, single-family residence and associated
improvements which will utilize continuous footings and slab on grade. As part of
our investigation, we observed and evaluated the shallow soil conditions at three
locations within the proposed new building area.
In addition, we reviewed an architectural site plan and the as-built grading plan by
RCE, Inc., dated November 15, 2005. We should review the final plans to confirm
they have been prepared in accordance with the recommendations included in this
report.
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LAN_~ Dt-VELOPMENT
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7420 TRADE STREIT• SAN DIEGO, CA. 92121 • (858) 549-7222 • FAX: (858) 549-1604 • EMAIL: geotech@gei-sd.com
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Scott Residential Lot
Carlsbad, California
Job No. 17-11714
Page 2
The field work, conducted on November 21, 2017, consisted of logging three hand-
excavated test pits/hand auger borings in the location of the proposed new
residence. The excavations revealed that the building site is underlain by
approximately 3 to 6 feet of medium dense, silty sand fill soil over medium dense
to dense, silty sand formational materials. The on-site soils are considered to have
a very low expansion potential with an Expansion Index of less than 20.
Based upon our observation, probing of the on-site soils, it is our opinion that the
new foundations for the residence can be founded in the existing medium dense fill
soil. Any loose fill soils in the proposed building pad area should be removed and
properly compacted as part of site preparation for the new foundation and slab
areas. All fill should be compacted to at least 90 percent of Maximum Dry Density.
The Maximum Dry Density of the soil has been determined per ASTM D1557-12.
1. It is our opinion that the medium dense fill soil will provide adequate bearing
strength for the proposed new foundations. New footings placed in the
existing medium dense fill or dense formational soils can be designed for an
allowable soil bearing capacity of 1,500 pounds per square foot (psf). We do
recommend that the proposed footings and slabs contain at least a nominal
amount of reinforcing steel to reduce the separation of cracks should they
occur. The allowable soil bearing capacity may be increased one-third for
structural design including seismic or wind loads.
2. The proposed footings should have a minimum depth of 18 inches and a
width of at least 15 inches, founded in the medium dense fill soil. A
minimum of steel for continuous footings should include at least four No. 5
bars continuous, with two bars 3 inches from the bottom of the footing.
Scott Residential Lot
Carlsbad, California
Job No. 17-11714
Page 3
3. Site-specific seismic design criteria to calculate the base shear needed for the
design of the residential addition are presented in the following table. The
design criteria was obtained from the California Building Code (CBC) 2016
edition, and is based on the distance to the closest active fault and soil
profile classification.
4. The proposed structure should be designed in accordance with Section 1613
of the 2016 CBC, which incorporates by reference the ASCE 7-10 for seismic
design and the following parameters should be utilized. We have determined
the mapped spectral acceleration values for the site based on a latitude of
33.1583 degrees and longitude of 117.3364 degrees, utilizing a program
titled "Design Maps and Tools," provided by the USGS, which provides a
solution for ASCE 7-10 (Section 1613 of the 2016 CBC) utilizing digitized files
for the Spectral Acceleration maps.
In addition, we have assigned a Site Classification of D. The response
parameters for design are presented in the following table. The design
spectrum acceleration vs. Period Tis attached.
TABLE I
Mapped Spectral Acceleration Values and Design Parameters
1.136 1.045 1.564 1.188 0.792
5. The liquefaction of saturated sands during earthquakes can be a major cause
of damage to buildings. Liquefaction is the process by which soils are
transformed into a viscous fluid that will flow as a liquid when unconfined. It
occurs primarily in loose, saturated sands and silts when they are sufficiently
shaken by an earthquake.
Scott Residential Lot
Carlsbad, California
Job No. 17-11714
Page 4
On this site, the risk of liquefaction of foundation materials due to seismic
shaking is considered to be remote due to the relatively shallow, medium
dense to dense nature of the natural-ground material and the lack of a
shallow static groundwater surface under the site. No soil liquefaction or soil
strength loss is anticipated to occur due to a seismic event.
6. New concrete slabs on-grade (on existing fill soils) should be a minimum of 5
inches actual thickness and be reinforced with at least No. 4 steel bars on
18-inch centers, in both directions, placed at mid-height in the slab. The
interior slab should be underlain by a 15-mil vapor barrier (15-mil
StegoWrap) placed directly on properly compacted subgrade. The sand base
may be waived.
We recommend that isolation joints and sawcuts be incorporated to at least
one-fourth the thickness of the slab in any slab designs. The joints and cuts,
if properly placed, should reduce the potential for and help control floor slab
cracking. In no case, however, should control joints be spaced farther than
20 feet apart, or the width of the slab. Control joints should be placed within
12 hours after concrete placement as soon as concrete sets and no raveling
of aggregate occurs.
7. If any retaining walls are planned, the active earth pressure (to be utilized in
the design of cantilever, non-restrained walls) should be based on an
Equivalent Fluid Weight of 38 pounds per cubic foot (for level backfill only) if
on-site soils are used. Additional loads applied within the potential failure
block should be added to the active soil earth pressure by multiplying the
vertical surcharge load by a 0.31 lateral earth pressure coefficient.
Scott Residential Lot
Carlsbad, California
Job No. 17-11714
Page 5
For restrained wall conditions, we recommend an equivalent fluid weight of
56 pcf. Surcharge loads may be converted to lateral pressures by
multiplying by a factor of 0.47. Should seismic soil increment be required,
the unrestrained walls with level backfill should be designed for an additional
pressure of 14 pcf, in addition to the regular static loading, with zero
pressure at the top and the maximum pressure at the bottom of the wall.
8. The passive earth pressure of the encountered fill soil to be used for design
of shallow foundations and footings to resist the lateral forces, should be
based on an Equivalent Fluid Weight of 200 pcf. This passive earth pressure
is valid for design only if the ground adjacent to the foundation structure is
essentially level for a distance of at least three times the total depth of the
foundation and is properly compacted or dense natural soil. An allowable
Coefficient of Friction of 0.35 times the dead load may be used between the
bearing soils and concrete foundations, walls or floor slabs.
9. Adequate measures should be taken to properly finish-grade the site a~er
the new structure and other improvements are in place. Drainage waters
from this site and adjacent properties should be directed away from
perimeter foundations, floor slabs, footings and slope tops, and onto the
natural drainage direction for this area or into properly designed and
approved drainage facilities. Proper subsurface and surface drainage will
help minimize the potential for waters to seek the level of the bearing soils
under the foundations, footings, and floor slabs. Failure to observe this
recommendation could result in undermining, differential settlement of the
building foundation or other improvements on the site, or moisture-related
problems.
Scott Residential Lot
Carlsbad, California
Job No. 17-11714
Page 6
It is not within the scope of our services to provide quality control oversight
for surface or subsurface drainage construction or retaining wall sealing and
base of wall drain construction. It is the responsibility of the contractor
and/or their retained construction inspection service provider to provide
proper surface and subsurface drainage.
10. Due to the possible build-up of groundwater (derived primarily from rainfall
and irrigation), excess moisture is a common problem in below-grade
structures or behind retaining walls that may be planned. These problems
are generally in the form of water seepage through walls, mineral staining,
mildew growth and high humidity. In order to minimize the potential for
moisture-related problems to develop, proper cross ventilation and water-
proofing must be provided for below-ground areas, in crawl spaces, and the
backfill side of all structure retaining walls must be adequately waterproofed
and drained.
Proper subdrains and free-draining backwall material (such as gravel or
geocomposite drains such as Miradrain 6000 or equivalent) should be
installed behind all retaining walls on the subject project in addition to wall
waterproofing. Geotechnical Exploration, Inc. will assume no liability for
damage to structures that is attributable to poor drainage.
11. Planter areas and planter boxes should be sloped to drain away from the
foundations, footings, and floor slabs. Planter boxes should be constructed
with a closed bottom and a subsurface drain, installed in gravel, with the
direction of subsurface and surface flow away from the foundations, footings,
and floor slabs, to an adequate drainage facility. The finish grade around the
Scott Residential Lot
Carlsbad, California
Job No. 17-11714
Page 7
addition should drain away from the perimeter walls to help reduce or
prevent water accumulation.
Exterior slabs or rigid improvements should also be built on properly
compacted soils and be provided with concrete shrinkage reinforcement and
adequately spaced joints.
Geotechnical Exploration, Inc. recommends that we be asked to verify the
actual soil conditions revealed in footing excavations prior to form and steel
reinforcement placement. In addition, any new fills or loose soils should be
properly compacted under the observations and testing of our firm.
Should you have any questions regarding this matter, please contact our office.
Reference to our lob No. 17-11714 will help to expedite a response to your
inquiries.
Respectfully submitted,
ECHNICAL EXPLORATION, INC.
,..,._._,~. Heiser
Senior Project Geologist R.C.E. 34422/G.E. 2007
Senior Geotechnical Engineer
lilJSGS Design Maps Summary Report
User-Specified Input
Report Title 1161 Chestnut Avenue, Carlsbad, CA
Thu February 8, 2018 18:09:57 UTC
Building Code Reference Document ASCE 7-10 Standard
(which utilizes USGS hazard data available in 2008)
Site Coordinates 33.1583°N, 117.3364°W
Site Soil Classification Site Class D -"Stiff Soil"
Risk Category I/II/III
USGS-Provided Output
S5 = 1.136 g
s1 = 0.436 9
Oceansidi
SMS = 1.188 g
SM1 = 0.682 g
'II
S05 = 0.792 g
S01 = 0.455 g
For information on how the SS and 51 values above have been calculated from probabilistic (risk-targeted) and
deterministic ground motions in the direction of maximum horizontal response, please return to the application and
select the "2009 NEHRP" building code reference document.
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For PGAM, Tv CR5, and CR1 va lues, please view the detailed reP.ort.
Although this information Is a product of the U.S. Geological Survey, we provide no warranty, expressed or implied, as to the
accuracy of the data contained therein. This tool is not a substitute for technical subject-matter knowledge.
EIJSGS Design Maps Detailed Report
ASCE 7-10 Standard (33.1583°N, 117.3364°W)
Site Class D -"Stiff Soil", Risk Category I/II/III
Section 11.4.1 -Mapped Acceleration Parameters
Note: Ground motion values provided below are for the direction of maximum horizontal
spectral response acceleration. They have been converted from corresponding geometric
mean ground motions computed by the USGS by applying factors of 1.1 (to obtain S5) and
1.3 (to obtain S1). Maps in the 2010 ASCE-7 Standard are provided for Site Class B.
Adjustments for other Site Classes are made, as needed, in Section 11.4.3.
From Figure 22-1 r11 S5 = 1.136 g
From Figure 22-2 r21 S 1 = 0.436 g
Section 11.4.2 -Site Class
The authority having jurisdiction (not the USGS), site-specific geotechnical data, and/or
the default has classified the site as Site Class D, based on the site soil properties in
accordance with Chapter 20.
Table 20.3-1 Site Classification
Site Class
A. Hard Rock
B. Rock
C. Very dense soil and soft rock
D. Stiff Soil
E. Soft clay soil
F. Soils requiring site response
analysis in accordance with Section
21.1
-Nor Nch -Vs Su
>5,000 ft/s N/A N/A
2,500 to 5,000 ft/s N/A N/A
1,200 to 2,500 ft/s >SO >2,000 psf
600 to 1,200 ft/s 15 to 50 1,000 to 2,000 psf
<600 ft/s <15 < 1,000 psf
Any profile with more than 10 ft of soil having the
characteristics:
• Plasticity index PI > 20,
• Moisture content w ;:::: 40%, and
• Undrained shear strength su < 500 psf
See Section 20.3.1
For SI: 1ft/s = 0.3048 m/s llb/ft2 "" 0.0479 kN/m2
Section 11.4.3 -Site Coefficients and Risk-Targeted Maximum Considered Earthquake (.M~.~)
Spectral Response Acceleration Parameters
Table 11.4-1: Site Coefficient F.
Site Class Mapped MCE R Spectral Response Acceleration Parameter at Short Period
S5 ~ 0.25 S5 = 0.50 S5 = 0.75 S5 = 1.00 S5 2:: 1.25
A 0.8 0.8 0.8 0.8 0.8
B 1.0 1.0 1.0 1.0 1.0
C 1.2 1.2 1.1 1.0 1.0
D 1.6 1.4 1.2 1.1 1.0
E 2.5 1.7 1.2 0.9 0.9
F See Section 11.4.7 of ASCE 7
Note: Use straight-line interpolation for intermediate values of S5
For Site Class = D and S5 = 1.136 g, F. = 1.045
Table 11.4-2: Site Coefficient Fv
Site Class Mapped MCE R Spectral Response Acceleration Parameter at 1-s Period
S1 ~ 0.10 S1 = 0.20 51 = 0.30 S1 = 0.40 51 2:: 0.50
A 0.8 0.8 0.8 0.8 0.8
B 1.0 1.0 1.0 1.0 1.0
C 1.7 1.6 1.5 1.4 1.3
D 2.4 2.0 1.8 1.6 1.5
E 3.5 3.2 2.8 2.4 2.4
F See Section 11.4.7 of ASCE 7
Note: Use straight-line interpolation for intermediate values of S1
For Site Class = D and s1 = 0.436 g, F v = 1.564
Equation (11.4-1): SMS = Fas s = 1.045 x 1.136 = 1.188 g
Equation (11.4-2):
Section 11.4.4 -Design Spectral Acceleration Parameters
Equation {11.4-3): S05 = ½ S Ms = ½ X 1.188 = 0.792 g
Equation (11.4-4): S01 = ½ SMl = ½ x 0.682 = 0.455 g
Section 11.4.5 -Design Response Spectrum
From Figure 22-12 131 TL = 8 seconds
-------
Figure 11.4-1: Design Response Spectrum
T<T0 : s. = S[lfl ( 0.4 + 0.6T /10 }
T~STST5 :S1 =Sos
T.s <l STL: S1 = S01 /T
-• I • •
, Section 11.4.6 -Risk-Targeted Maximum Considered Earthquake (MCER) Response Spectrum
The MCER Response Spectrum is determined by multiplying the design response spectrum above by
1.5.
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• Section 11.8.3 -Additional Geotechnical Investigation Report Requirements for Seismic Design
Categories D through F
From Figure 22-7 r41 PGA = 0.448
Equation (11.8-1): PGAM = FPGAPGA = 1.052 x 0.448 = 0.471 g
Table 11.8-1: Site Coefficient FPGA
Site Mapped MCE Geometric Mean Peak Ground Acceleration, PGA
Class
PGA::; PGA = PGA = PGA = PGA;?;
0.10 0.20 0.30 0.40 0.50
A 0.8 0.8 0.8 0.8 0.8
B 1.0 1.0 1.0 1.0 1.0
C 1.2 1.2 1.1 1.0 1.0
D 1.6 1.4 1.2 1.1 1.0
E 2.5 1.7 1.2 0.9 0.9
F See Section 11.4. 7 of ASCE 7
Note: Use straight-line interpolation for intermediate values of PGA
For Site Class = D and PGA = 0.448 g, FPGA = 1.052
Section 21.2.1.1 -Method 1 (from Chapter 21 -Site-Specific Ground Motion Procedures for
Seismic Design)
From Figure 22-17 cs1 CRs = 0.947
From Figure 22-18 c51 CRl = 0.999
,# Section 11.6 -Seismic Design Category
Table 11.6-1 Seismic Design Category Based on Short Period Response Acceleration Parameter
RISK CATEGORY
VALUE OF S05 I or II III IV
S05 < 0,167g A A A
0.167g S Sos < 0.33g B 8 C
0.33g .:S S 0 s < O.SOg C C D
0,50g S S05 D D D
For Risk Category= I and S0s = 0.792 g, Seismic Design Category= D
Table 11.6-2 Seismic Design Category Based on 1-S Period Response Acceleration Parameter
RISK CATEGORY
VALUE OF S 01 I or II III IV
S01 < 0,067g A A A
0.067g s S01 < 0.133g B B C
0.133g S S 01 < 0.20g C C D
0.20g .:S 501 D D D
For Risk Category = I and S01 = 0.455 g, Seismic Design Category = D
Note: When S1 is greater than or equal to 0.75g, the Seismic Design Category is E for
buildings in Risk Categories I, II, and III, and F for those in Risk Category IV, irrespective
of the above.
Seismic Design Category = "the more severe design category in accordance with
Table 11.6-1 or 11.6-2" = D
Note: See Section 11.6 for alternative approaches to calculating Seismic Design Category.
References
1. Figure 22-1: https://earthquake.usgs.gov/hazards/designmaps/downloads/pdfs/2010_ASCE-7 _Figure_22-1.pdf
2. Figure 22-2: https ://earthquake .usgs.gov/hazards/designmaps/downloads/pdfs/20 lO_ASCE-7 _Figure_22-2. pdf
3. Figure 22-12: https ://earthquake .usgs.gov/hazards/designmaps/downloads/pdfs/2010_ASCE-7 _Figure_22-12. pdf
4. Figure 22-7: https ://earthquake.usgs.gov/hazards/designmaps/downloads/pdfs/20 lO_ASCE-7 _Figure_22-7 .pdf
5. Figure 22-17: https ://earthquake.usgs.gov/hazards/designmaps/downloads/pdfs/2010_ASCE-7 _Figure_22-17. pdf
6. Figure 22-18: https ://earthquake.usgs.gov/hazards/designmaps/downloads/pdfs/201 O_ASCE-7 _Figure_22-18.pdf