Sub-module 2.1 Lectures on seismic hazard- Liquefaction
copied from "Earthquake Basics Brief No. 1".
"Purpose
This pamphlet, the first in a series, has been written by members of the Earthquake Engineering Research Institute (EERI) to explain the phenomenon of liquefaction and what can be done about it. This brief is intended for other EERI members, and local officials, public policymakers, and property owners who are faced with decisions regarding the hazard of liquefaction. It is not intended to replace evaluations conducted by a geotechnical expert to assess the hazard at any particular site. The discussion on the liquefaction process and its effects on the built environment is primarily meant for the design engineer. The discussion on options for mitigation, preparedness, response, and recovery is primarily aimed at policymakers. However, the effort was made to explain both the process and its public policy implications to all readers.

This brief is meant to be used with two slide sets that illustrate
(1) the phenomenon of liquefaction and
(2) mitigation options for liquefaction.
The authors hope that the information presented here conveys, to policymakers in particular, that better understanding of the risk from liquefaction at a particular site or area leads to better decisions regarding mitigation options, response planning, and preparedness strategies. With good liquefaction opportunity and susceptibility maps as a starting point, public officials and private property owners can make informed decisions about how to concentrate limited resources to manage and reduce the risk.

Liquefaction Process
Liquefaction is a process by which sediments below the water table temporarily lose strength and behave as a viscous liquid rather than a solid. The types of sediments most susceptible are clay-free deposits of sand and silts; occasionally, gravel liquefies.
The actions in the soil which produce liquefaction are as follows: seismic waves, primarily shear waves, passing through saturated granular layers, distort the granular structure, and cause loosely packed groups of particles to collapse
(Fig.1 below)
These collapses increase the pore-water pressure between the grains if drainage cannot occur. If the pore-water pressure rises to a level approaching the weight of the overlying soil, the granular layer temporarily behaves as a viscous liquid rather than a solid. Liquefaction has occurred.

Sketch of a packet of water-saturated sand grains illustrating the process of liquefaction. Shear deformations (indicated by large arrows) induced by earthquake shaking distort the granular structure causing loosely packed groups to collapse as indicated by the curved arrow (Youd, 1992).

Figure 1 [after EERI]

In the liquefied condition, soil may deform with little shear resistance; deformations large enough to cause damage to buildings and other structures are called ground failures. The ease with which a soil can be liquefied depends primarily on the looseness of the soil, the amount of cementing or clay between particles, and the amount of drainage restriction. The amount of soil deformation following liquefaction depends on the looseness of the material, the depth, thickness, and areal extent of the liquefied layer, the ground slope, and the distribution of loads applied by buildings and other structures.
Liquefaction does not occur at random, but is restricted to certain geologic and hydrologic environments, primarily recently deposited sands and silts in areas with high ground water levels. Generally, the younger and looser the sediment, and the higher the water table, the more susceptible the soil is to liquefaction. Sediments most susceptible to liquefaction include Holocene (less than 10,000-year-old) delta, river channel, flood plain, and aeolian deposits, and poorly compacted fills. Liquefaction has been most abundant in areas where ground water lies within 10 m of the ground surface; few instances of liquefaction have occurred in areas with ground water deeper than 20 m. Dense soils, including well-compacted fills, have low susceptibility to liquefaction.