Soil Retaining Structures

9.2.1 Introduction

As the non-seismic design of soil-retaining structures is well discussed elsewhere, little other than seismic considerations are dealt with here. The principal types of structure covered in this section are retaining walls and basement walls.

The magnitude of the seismic soil pressures acting on a soil-retaining structure in part depends upon the relative stiffness of the structure and the associated soil mass. Two main categories of soil-structure interaction are usually defined:

(1) flexible structures which move away from the soil sufficiently to minimize the soil pressures, such as slender free-standing retaining walls;

(2) rigid structures, such as basement walls or tied-back retaining walls.

In case (1), active pressures will occur, and the amount of movement required to produce the active state is of the order indicated in Table 9.3. The amount of wall movement which will occur during earthquakes depends mainly upon the foundation fixity and the wall flexibility. Unless a more exact analysis is made, the following soil pressure states may be used:

Table 9.3 Movement of retaining wall required to produce the active state


Wall movement/height

Cohesionless, dense


Cohesionless, loose


Firm clay


Loose clay


(1) Flexible: walls founded on non-rock materials or cantilever walls higher than 5 m; assume active soil state.

(2) Intermediate: cantilever walls less than 5 m high founded on rock.

(3) Rigid: counterfort or gravity wall founded on rock or piles; at-rest soil state.

In general, wall stiffness will lie somewhere between the extremes of flexible and rigid, and interpolation between the forces generated in these states may be appropriate.

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