Basic Geotechnical Earthquake Phần 8 docx

102 Basic Geotechnical Earthquake Engineering

RETAINING WALL ANALYSES

FOR EARTHQUAKES

10

CHAPTER

102

10.1 INTRODUCTION

Retaining wall is a structure whose primary purpose is to provide lateral support for soil

or rock. It may also support vertical loads. They could be of gravity, cantilever, counterfort

and crib wall type. Basement walls and bridge abutments are typical examples. Performance

of retaining wall during earthquake is very complex. Due to seismic forces, walls can move

by translation and/or rotation depending on wall design. Magnitude and distribution of dynamic

wall pressure is influenced by mode of wall movement. Maximum soil thrust acts on wall

when the wall translates or rotates towards the backfill. It is minimum when the wall translates

or rotates away from the backfill. The shape of earthquake pressure distribution and the

point of application of resultant changes as the wall moves. Dynamic wall pressure and

permanent wall displacement increase significantly, near the natural frequency of wall backfill

system under earthquake loading. Increased residual pressure may remain on wall after episode

of strong shaking has ended.

It has been stated that the allowable bearing pressure and allowable passive pressure

should be increased by a factor of one-third while performing seismic analysis. This increase

is appropriate if retaining wall bearing material and soil in front of wall consists of massive

crystalline bedrock and sedimentary rock that remains static during earthquake, soils which

dilate due to earthquake, soils having little reduction in shear strength with strain, clay with

low sensitivity and soils located above water table. However the increase is not recommended

if the soil consists of foliated rock that fractures in earthquake, loose soil located below water

table, sensitive clays and soft clays. Former group of soils do not loose shear strength during

seismic shaking while later group of soils loose shear strength during seismic shaking.

This chapter deals with methods of retaining wall analysis under earthquakes.