DESIGN SPECIFICATIONS FOR HIGHWAY BRIDGES - IISEE

Chapter 8 Effects of Seismically Unstable Ground
8.1 General
(1) In verification of seismic performance of a bridge on a ground changing into
unstable state during an earthquake, the effects of the unstable ground shall be
taken into account. In the above, an unstable ground is defined as an extremely soft
soil layer in seismic design, or a sandy layer susceptible to liquefaction and affect
the bridge due to the liquefaction and the liquefaction-induced ground flow.
(2) In verifying seismic performance of a bridge with conditions shown in the above (1),
a case assuming a stable ground shall also be considered, in order to ensure the
seismic performance of the bridge for both the stable and unstable grounds.
8.2 Geotechnical Parameters of Extremely Soft Layer and Sandy Layer
Prone to Liquefaction
8.2.1 General
For an extremely soft soil layer in seismic design specified in Section 8.2.2 or a sandy
layer in Section 8.2.3 which may affect the bridge due to liquefaction, geotechnical
parameters used in the seismic design shall be reduced in accordance with the
provisions in Section 8.2.4.
8.2.2 Assessment of Extremely Soft Soil Layer in Seismic Design
For a clayey layer or a silt layer located up to three meters below the ground surface,
and having compressive strength of 20kN/m 2 or less obtained from an unconfined
compression test or an in- situ test, the layer shall be regarded as an extremely soft
layer in the seismic design.
8.2.3 Assessment of Soil Liquefaction
(1) Sandy layer requiring liquefaction assessment
For an alluvial sandy layer having all of the following three conditions, liquefaction
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