Geotecnical Aspects of Liquefaction

General Definition

Liquefaction is a phenomenon that occurs when a sudden shock, or cyclic loading causes soil pore pressures to temporarily increase until the effective pressure is zero. Residual pore pressure increases on successive stress cycles. Pore pressure also increases with increasing strain. This causes the soil particles to "float", leaving the soil with a very low shear strength and allowing the unconfined soil to flow laterally. Another explanation is that when saturated sand is subjected to vibrations, it tends to compact and decrease in volume. If the sand is not allowed to drain, the pore water pressure will increase. If the pore water pressure builds up to equal the overburden pressure, the effective stress becomes zero. The sand loses strength and becomes liquefied.

Loading Conditions

The loading conditions necessary to cause liquefaction exist most commonly seen during pile driving operations, and under earthquake conditions. Soils most susceptible to liquefaction are saturated, fine to medium-fine grained, loose sands, however, non-saturated clays may also be subject to liquefaction.

Building Failure Mechanism

The safety concern with liquefaction is related to buildings, or other structures bearing on these soils. When soils become liquefied, these structures lose support and could potentially collapse. The entire mass of soil under a structure does not have to be in a state of liquefaction for failure of the foundation to occur. Liquefaction at a single point may cause loads to transfer and adjacent soils to become overstressed, resulting in settlement and tilting of the structure.

Photos/Case Studies

Northern Iran, June 1990. Photos showing building failures due to liquefaction.

Iran earthquake

 

Hyogoken-Nanbu earthquake, 1995. Photos showing damage in and around Kobe.

Kobe Eqrthquake Report

 

 Hyogoken-Nanbu earthquake. Case study.

http://www.ce.berkeley.edu/Programs/Geotech/Kobe/KobeReport/ch3/ch3-7.html

 

General information on liquefaction.

http://Soil.princeton.edu/~igor/background.html

 

Prepared by Lori K. Thomas, Dec. 1997