Lessons learned and need for instrumented liquefaction sites

doi:10.1016/j.soildyn.2004.06.006

Soil Dynamics and Earthquake Engineering

Volume 24, Issues 9–10, October 2004, Pages 639-646

https://doi.org/10.1016/j.soildyn.2004.06.006 Get rights and content

Abstract

Instrumented sites provide essential information for understanding and modeling of ground response and ground deformation. For example, significant new lessons were learned from responses at the Wildlife Liquefaction Array (WLA) including: (1) soil softening led to lengthening of period of transmitted ground motions; (2) soil softening also led to attenuation of short-period spectral accelerations (<0.7 s); (3) amplification of long period motions (>0.7 s) occurred due to liquefaction-induced ground oscillation; and (4) ground oscillation led to a continued rise of pore water pressures after strong ground shaking ceased. A new and expanded instrumented site is being developed 70 m downstream from the old WLA site as part of the NSF Network for Earthquake Engineering Simulation (NEES). The new site has more accelerometers, piezometers and ground deformation measurement devices and the data will be streamed to the NEES-grid in near real time.

Introduction

Liquefaction of subsurface sediment may lead to two possible hazards: unacceptably large ground deformations or ground failure and modification of seismic waves propagating through the liquefying layer. The latter modifications could affect spectral accelerations required for design of bridges, buildings, pipelines and other constructed works. To better understand and model these liquefaction-induced hazards, records from instrumented sites are needed to provide a database of actual pore pressure and ground response and ground deformation. In this paper, we evaluate and show the benefit of instrumental records from the Wildlife Liquefaction Array (WLA) that was installed by the lead author and his colleagues at the US Geological Survey in 1982 [1]. Those instruments recorded responses during two significant earthquakes in 1987: the November 23 Elmore Ranch event (M_w=6.2; a_max=0.16g) which produced no significant rise of pore water pressure; and the November 24 Superstition Hills event (M_w=6.6; a_max=0.21g) which generated widespread liquefaction at the site.

Because of the wealth of data gained and lessons learned from these responses, WLA is being re-instrumented at a locality about 70 m downstream from the 1982 site as part of the NSF Network for Earthquake Engineering Simulation (NEES). The new locality was chosen because of disturbances that occurred at the old site due to post earthquake investigations and because of increased potential for ground deformation at the new site. The new site is located adjacent to a steep bank of the Alamo River that flows through the area. The amount of instrumentation at the site is being enhanced with more surface and downhole accelerometers, electrically transduced piezometers, and flexible casings for measurement of ground deformation.

Section snippets

Lessons learned from the wildlife liquefaction array (WLA)

The primary purpose for instrumenting WLA in 1982 was to record ground motions above and below a liquefiable layer and pore water pressures within the liquefiable layer during earthquake shaking. That goal was achieved during the 1987 Elmore Ranch and Superstition Hills earthquakes. Fig. 1 shows the general location of WLA and epicenters and magnitudes of the three important earthquakes that shook the site prior to and after development of the site. The magnitude 5.9 event is the 1981

Ground motions and pore pressure response

Acceleration and pore pressure records from the 1987 Superstition Hills earthquake are reproduced in Fig. 3. These records show ground accelerations recorded above and below the liquefiable layer and pore pressures generated within that layer. Some notable aspects of the records are: (1) a sharp rise in pore pressures began with the arrival of the peak acceleration pulse (a_max=0.21g) that propagated through the site 13.6 s after instrumental triggering. (2) Discordance developed between

Spectral response

To analyze the influence of liquefaction on ground motions transmitted through liquefying or liquefied layers, we compared ground motions and response spectra determined from motions recorded at ground surface (termed actual motions and spectra) with motions and spectra predicted from acceleration recorded by instruments installed immediately below the liquefiable layer (termed predicted motions and spectra). To generate the predicted motions and spectra, the motions recorded beneath the

Reinstrumentation of WLA

The pore pressure piezometers at WLA ceased to function sometime after the 1987 records were obtained and the site was greatly disturbed during investigations subsequent to the 1987 earthquakes. Also through NEES, an opportunity arose to move and upgrade the site, with more extensive arrays of downhole and surface accelerometers, piezometers, and ground deformation measuring devices. The new site is located 70 m downstream from the old site and near a steep bank of the Alamo River. The steep

Conclusions

1.
Significant new lessons were learned from the recorded responses at the WLA including: (1) soil softening led to lengthening of period of transmitted ground motions; (2) induced ground oscillation led to continued increase of pore water pressures after strong ground shaking ceased; (3) soil softening led to attenuation of short-period spectral accelerations (<0.7 s); and (4) the induced ground oscillations led to amplification of long period motions (>0.7 s).
2.
A new WLA site is being redeveloped

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Geotechnical investigation of liquefaction sites, Imperial Valley, California
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Piezometer performance at the Wildlife liquefaction site
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In-place calibration of USGS transducers at Wildlife liquefaction site, California, USA
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In situ measurement of pore pressure build-up during liquefaction
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Prediction of pore water pressure buildup and liquefaction of sands during earthquakes by the cyclic strain methods
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Lessons learned and need for instrumented liquefaction sites

Abstract

Introduction

Section snippets

Lessons learned from the wildlife liquefaction array (WLA)

Ground motions and pore pressure response

Spectral response

Reinstrumentation of WLA

Conclusions

Geotechnical investigation of liquefaction sites, Imperial Valley, California

Piezometer performance at the Wildlife liquefaction site

J Geotech Eng, ASCE

In-place calibration of USGS transducers at Wildlife liquefaction site, California, USA

In situ measurement of pore pressure build-up during liquefaction

Compaction of sands by repeated shear straining

J Soil Mech Found Div, ASCE

Prediction of pore water pressure buildup and liquefaction of sands during earthquakes by the cyclic strain methods