5.1 Introduction

Lateral wall deformations and ground surface settlements represent the performance of excavation support systems.These are closely related to the stiffness of the supporting system，the soil and groundwater conditions，the earth and water pressures，and the construction procedures.The decision to set particular wall deflection and ground movement limits can be of significant economic importance.The setting of appropriate limits should be considered carefully and clearly communicated.

Figure 5.1 shows the general deflection behavior of the wall in response to the excavation presented by Clough and O’Rourke（1990）.Figure 5.1（a）shows that at early phases of the excavation，when the first level of lateral support has to be installed，the wall will deform as a cantilever.Settlements during this phase may be represented by a triangular distribution having the maximum value very near to the wall.As the excavation activities advance to deeper elevations，horizontal supports are installed restraining upper wall movements.At this phase，deep inward movements of the wall occur［Figure 5.1（b）］.The combination of cantilever and deep inward movements results in the cumulative wall and ground surface displacements shown in Figure 5.1（c）.Clough and O’Rourke（1990）stated that if deep inward movements are the predominant form of wall deformation，the settlements tend to be bounded by a trapezoidal displacement profile as in the case with deep excavations in soft to medium clay；and if cantilever movements predominate，as can occur for excavations in sands and stiff to very hard clay，then settlements tend to follow a triangular pattern.Similar findings were presented by Ou et al.（1993）and Hsieh and Ou（1998）who，based on observed movements of case histories in clay，proposed the spandrel and concave settlement profiles.It has to be noted that Figure 5.1 only describes the general wall deflection behavior in response to the excavation and neglects important factors such as soil conditions，wall installation methods，and excavation support system stiffness，which have been shown to influence the magnitude and shape of both lateral wall movements and ground settlements.

Figure 5.1 Typical profiles of movement for braced and tieback walls（After Clough and O’Rourke，1990）