LIQUEFACTION - Geotechnical Earthquake Engineering - page 161

TABLE 6.1 Estimated Susceptibility of Sedimentary Deposits to Liquefaction during Strong
Seismic Shaking Based on Geologic Age and Depositional Environment

General

distribution of Likelihood that cohesionless sediments, when saturated,

cohesionless would be susceptible to liquefaction (by age of deposit)

Type of sediments in

deposit deposits 500 years Holocene Pleistocene Pre-Pleistocene

( a ) Continental deposits

Alluvial fan and plain Widespread Moderate Low Low Very low

Delta and fan-delta Widespread High Moderate Low Very low

Dunes Widespread High Moderate Low Very low

Marine terrace/plain Widespread Unknown Low Very low Very low

Talus Widespread Low Low Very low Very low

Tephra Widespread High High Unknown Unknown

Colluvium Variable High Moderate Low Very low

Glacial till Variable Low Low Very low Very low

Lacustrine and playa Variable High Moderate Low Very low

Loess Variable High High High Unknown

Floodplain Locally variable High Moderate Low Very low

River channel Locally variable Very high High Low Very low

Sebka Locally variable High Moderate Low Very low

Residual soils Rare Low Low Very low Very low

Tuff Rare Low Low Very low Very low

( b ) Coastal zone

Beach—large waves Widespread Moderate Low Very low Very low

Beach—small waves Widespread High Moderate Low Very low

Delta Widespread Very high High Low Very low

Estuarine Locally variable High Moderate Low Very low

Foreshore Locally variable High Moderate Low Very low

Lagoonal Locally variable High Moderate Low Very low

( c ) Artificial

Compacted fill Variable Low Unknown Unknown Unknown

Uncompacted fill Variable Very high Unknown Unknown Unknown

Source: Data from Youd and Hoose (1978), reproduced from R. B. Seed (1991).

6.4.2 Cyclic Stress Ratio Caused by the Earthquake

If it is determined that the soil has the ability to liquefy during an earthquake and the soil
is below or will be below the groundwater table, then the liquefaction analysis is performed.
The first step in the simplified procedure is to calculate the cyclic stress ratio, also com-
monly referred to as the seismic stress ratio ( SSR ), that is caused by the earthquake.
To develop the CSR earthquake equation, it is assumed that there is a level ground sur-
face and a soil column of unit width and length, and that the soil column will move hori-
zontally as a rigid body in response to the maximum horizontal acceleration a max exerted by
the earthquake at ground surface. Figure 6.4 shows a diagram of these assumed conditions.
Given these assumptions, the weight W of the soil column is equal to t z, where t total