Jamison Steidl Page 1 4/6/01 Institute for Crustal Studies University of California at Santa Barbara Borehole Observations of Pore Pressure and Ground Motions from the 1999 Hector Mines Earthquake at the Garner Valley Engineering Seismology Test Site. Ground motions and pore pressure are monitored at Garner Valley via vertical array instrumentation in the near-surface alluvium, the weathered rock, and down at 0.5 km depth in the crystalline Granite bedrock (Figure 1). Shallow Pore pressure Observations from the Liquefaction Array While the largest motions recorded at GVDA from the Hector Mine earthquake were on the order of only 10% g, still considered in the linear range for dynamic soil behavior, evidence of the onset of nonlinear soil response is seen in the acceleration and pore pressure records of the Garner Valley vertical array in the upper 10-20 meters (Figures 2&3). Dynamic pore pressure oscillations in the near surface alluvium and a steady increase in pore pressure with time correlate with a breakdown in the linear behavior of the stress-strain time histories (Figure 3). At UC Santa Barbara, we have been developing modeling techniques for dynamic soil behavior at large strain or “strong-motion” levels. Nonlinear modeling including the effects of pore pressure, are used to simulate the ground motions and pore pressure observations in the shallow alluvium. A critical element to the development of any nonlinear model of soil behavior is having control experiments for different soil types, where the input motion below the soil, and the surface motions are recorded. Even better, if we can have a series of observations at different levels within the soil layers, similar to the experiment at the Garner Valley test site. Permanently instrumented field sites to monitor soil behavior at different strain levels and at multiple sites with different soil types are critical to any validation and calibration of state-of-the-art modeling techniques. We are involved in initiatives to try and increase the number of these types of arrays in the United States. In addition, structural arrays at some of these sites to investigate soil structure interaction and building response would also be extremely useful to the engineering community to improve design methods and public safety.
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Jamison Steidl Page 1 4/6/01Institute for Crustal StudiesUniversity of California at Santa Barbara
Borehole Observations of Pore Pressure and Ground Motions from the 1999Hector Mines Earthquake at the Garner Valley Engineering Seismology TestSite.
Ground motions and pore pressure are monitored at Garner Valley via verticalarray instrumentation in the near-surface alluvium, the weathered rock, and downat 0.5 km depth in the crystalline Granite bedrock (Figure 1).
Shallow Pore pressure Observations from the Liquefaction Array
While the largest motions recorded at GVDA from the Hector Mine earthquakewere on the order of only 10% g, still considered in the linear range for dynamicsoil behavior, evidence of the onset of nonlinear soil response is seen in theacceleration and pore pressure records of the Garner Valley vertical array in theupper 10-20 meters (Figures 2&3). Dynamic pore pressure oscillations in the nearsurface alluvium and a steady increase in pore pressure with time correlate with abreakdown in the linear behavior of the stress-strain time histories (Figure 3).
At UC Santa Barbara, we have been developing modeling techniques for dynamicsoil behavior at large strain or “strong-motion” levels. Nonlinear modelingincluding the effects of pore pressure, are used to simulate the ground motions andpore pressure observations in the shallow alluvium. A critical element to thedevelopment of any nonlinear model of soil behavior is having control experimentsfor different soil types, where the input motion below the soil, and the surfacemotions are recorded. Even better, if we can have a series of observations atdifferent levels within the soil layers, similar to the experiment at the GarnerValley test site. Permanently instrumented field sites to monitor soil behavior atdifferent strain levels and at multiple sites with different soil types are critical toany validation and calibration of state-of-the-art modeling techniques.
We are involved in initiatives to try and increase the number of these types ofarrays in the United States. In addition, structural arrays at some of these sites toinvestigate soil structure interaction and building response would also be extremelyuseful to the engineering community to improve design methods and public safety.
Dep
th (
m)
GL-0GL-6
GL-15GL-22
GL-220
GL-500
0
25
50
75
100
125
150
175
200
225
500
Dual-gain Three-component Accelerometer
Alluvium
Weathered Granite
GL-50
Granite
Garner Valley Engineering Seismology Test SiteVertical Accelerometer Array
Figure 1.
2
SANDS
SANDS
CLAYEY SILT & SILT
CLAY & SILTY CLAY
SANDY SILTS&
SILTY SANDS
0
3
6
9
12
15
Dep
th (
met
ers)
Dual-gain Three-component Accelerometer
GL-0
Near-surface Sensors in the Garner Valley Alluvium
GL-22
GL-6
GL-15
Druck / KPSI / Paroscientific Pore Pressure Transducer
Figure 2.
Acc
eler
atio
n (m
/s2 )
Acc
eler
atio
n (m
/s2 )
Time (Seconds)
GL-0 meter
GL-22 meter
GL-12 meter
Por
e P
ress
ure
(PSI
)
E/W
Str
ain
(%)
Time (seconds)
E/W
She
ar S
tres
s (k
Pa)
Time (seconds)
Time = 0-15 s Time = 55-68 s
Time = 15-28 s Time = 28-40 s Time = 40-55 s
E/W
She
ar S
tres
s (k
Pa)
E/W Strain (%) E/W Strain (%)
E/W Strain (%)E/W Strain (%) E/W Strain (%)
E/W
She
ar S
tres
s (k
Pa)
Pore Pressure in the Near-surface Alluvium Observed on the Liquefaction Array
Pore Pressure Increases
Material Strength Decreases
Five minute pore pressure record
Time (seconds)
Por
e P
ress
ure
(PSI
)
30.4
30.0
29.6
29.2
Pore Pressure Build-up and Decay
0 100 200 300
Hector MinesGL-12 m (LQ6)
Garner Valley Test Site, California
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