14050161Geotechnical Investigation
Vallco Town Center
Cupertino, California
27 March 2018
770633101
Page 7
• West of N. Wolfe Road: Ground surface elevations range from Elevation 178.1 feet at
the northern portion of the parcel to 198.4 feet at the southwest corner of the parcel,-
East
arcel;
East of N. Wolfe Road: Ground surface elevations range from Elevation 176.4 feet at
the northwest corner of the parcel to Elevation 197.5 at the eastern portion of the
parcel.
4.2 Subsurface Conditions
Where asphalt pavement was encountered, the section consists of 1'/2 to 6 inches of asphalt
concrete (AC) over 3 to 10 inches of aggregate base (AB). In general, the project site is
underlain by alluvial deposits consisting of stiff to hard clays and sandy clays and medium
dense to very dense sand and gravel. TRC (as Lowney Associates) encountered 11/2 and
4'/2 feet of clay fill in borings LB -6 and LB -8, respectively. The surficial clayey soil has moderate
to high expansion potential'; where tested, the upper clay layers have plasticity indices of
25 and 39. Where tested, laboratory test results of the undrained shear strength of relatively
undisturbed samples of the alluvium ranges from 1,220 to 4,750 pounds per square foot (psf).
An undrained shear strength of 640 psf was recorded during testing of a disturbed sample
collected from boring B-1 at a depth of 75'/2 feet bgs. In addition, the consolidation laboratory
test results indicate the alluvium is overconsolidated3 and has compression ratios ranging from
0.1 to 0.12.
Idealized subsurface profiles, Figures 4 and 5, illustrate the general subsurface conditions at the
site
Based on our review of published maps (California Division of Mines and Geology, 2002),
historic high groundwater in the project vicinity is deeper than 50 feet bgs. Based on previous
geotechnical investigation at or nearby the project site, (Langan Treadwell Rollo, 2014 and TRC,
2015), groundwater was encountered at depths of approximately 65 to 75 feet bgs. During our
current investigation, the groundwater levels were measured at depths of approximately 48 and
96 bgs (corresponding to Elevations 146 to 86 feet) at Borings B-1 and B-4, respectively.
However, this depth was measured during drilling and may not represent a stabilized ground
water level. Groundwater levels may fluctuate due to seasonal rainfall.
2 Highly expansive soil undergoes large volume changes with changes in moisture content.
3 An overconsolidated clay has experienced a pressure greater than its current load.
L A NGA N
Geotechnical Investigation
Vallco Town Center
Cupertino, California
27 March 2018
770633101
Page 8
Pore -pressure dissipation tests' (PPDTs) were attempted at CPT -1 through CPT -5 at depths of
approximately 62 feet to 75 feet bgs; groundwater was not encountered at those depths.
Groundwater depth and elevation data from the current and prior investigations are summarized
in Table 1.
TABLE 1
Summary of Groundwater Depth and Elevation Data
Consultant
Location
Year of
Exploration
Ground
Surface
Elevation
(ft)
Exploration
Depth
(ft)
Groundwater
Depth
(ft)
Groundwater
Elevation
(ft)
B-1
2016
194.2
141
48
146.2
B-2
2016
197.6
101.5
-
B-3
2016
196.1
50
-
B-4
2016
182.4
100
96
86.4
B-5
2016
179.8
50
-
Langan
CPT -1
2016
195.4
75.3
-
CPT -2
2016
194.2
75.3
-
CPT -3
2016
194.0
75.5
-
CPT -4
2016
176.4
75.3
-
CPT -5
2016
189.2
75.5
-
TRC (as
Lowney
Associates)
EB -9
1 2004
1 184.2
1 84.5
1 68
1 116.2
Notes:
1. Groundwater level obscured by drilling method in Boring B-2.
2. Groundwater not encountered in Borings B-3, B-5, and CPT -1 to CPT -5.
3. TRC (as Lowney Associates or Lowney Kaldveer Associates) borings that did not encounter
groundwater are not included.
Downhole suspension logging was performed in Boring B-1. Shear wave velocities ranged
from about 790 to 2,498 feet per second in the alluvial deposits. A plot of shear wave velocity
with depth is presented in Appendix C.
5.0 REGIONAL SEISMICITY
The major active faults in the area are the San Andreas, Monte Vista -Shannon, Hayward, and
Calaveras faults. These and other faults of the region are shown on Figure 6. For each of the
' PPDTs are conducted at various depths to measure hydrostatic water pressures and to determine the
approximate depth of the groundwater level. The variation of pore pressure with time is measured behind the
tip of the cone and recorded.
LANGAN
Geotechnical Investigation
Vallco Town Center
Cupertino, California
27 March 2018
770633101
Page 9
active faults within approximately 100 km from the site, the distance from the site and
estimated mean characteristic Moment magnitude' [2007 Working Group on California
Earthquake Probabilities (WGCEP) (2008) and Cao et al. (2003)] are summarized in Table 2.
TABLE 2
Regional Faults and Seismicity
Approx.
Distance from
Fault Segment fault (km)
Direction
from Site
Mean
Characteristic
Moment
Magnitude
Monte Vista -Shannon 4.8
Southwest
6.50
N. San Andreas - Peninsula 10.6
Southwest
7.23
N. San Andreas (1906 event) 10.6
Southwest
8.05
N. San Andreas - Santa Cruz 17
South
7.12
Total Hayward 20
Northeast
7.00
Total Hayward -Rodgers Creek 20
Northeast
7.33
Total Calaveras 22
Northeast
7.03
Zayante-Vergeles 27
South
7.00
San Gregorio Connected 33
West
7.50
Monterey Bay-Tularcitos 46
South
7.30
Greenville Connected 46
East
7.00
Mount Diablo Thrust 48
Northeast
6.70
Great Valley 7 63
Northeast
6.90
Green Valley Connected 64
North
6.80
Ortigalita 65
East
7.10
N. San Andreas - North Coast 71
Northwest
7.51
Quien Sabe 73
Southeast
6.60
SAF - creeping segment (jl0.sa-creep,
modified) 75
Southeast
6.70
Rinconada 76
Southeast
7.50
Great Valley 8 77
East
6.80
Great Valley 5, Pittsburg Kirby Hills 78
North
6.70
Rodgers Creek 92
Northwest
7.07
Great Valley 9 94
East
6.80
West Napa 95
North
6.70
Point Reyes 100
Northwest
6.90
Figure 6 also shows the earthquake epicenters for events with magnitude greater than 5.0 from
January 1800 through December 2000. Since 1800, four major earthquakes have been
recorded on the San Andreas Fault. In 1836 an earthquake with an estimated maximum
' Moment magnitude is an energy -based scale and provides a physically meaningful measure of the size of a
faulting event. Moment magnitude is directly related to average slip and fault rupture area.
LANGAN