4.4 Design of Retaining Walls
The penetration depth of a retaining wall directly affects the stability of the excavation.Concerning the design of the penetration depth of a retaining wall，please refer to the contents of stability analysis.This section focuses on designs for sections and dimensions of different types of retaining walls.Table 4.2 lists the comparative values of nominal stiffness for different retaining walls for the references in preliminary design.
To design the sections and dimensions of a retaining wall，we should first carry out the stress analysis.Three methods can be adopted for the stress analysis of a retaining wall：the assumed support method，the beam on elastic foundation method，and the finite element method，which have been introduced in Section 4.3.The actual construction procedures should be simulated in a stress analysis.Because the maximum bending stress occurring at each construction stage does not take place at the same depth，the design of the wall sections should adopt the envelope of the maximum bending stresses of all the construction stages.Figure 4.18 illustrates the typical diagrams of bending moment and shear at each stage for the stress analysis of a retaining wall.According to the mechanics of materials，the bending moment and shear of a retaining wall are the products of EI and the quadratic and cubic differential values of the deformation curve，respectively.If any two computed wall deformation curves are close，their computed bending moment and shear should be close too.To analyze the stresses of a retaining wall by using the beam on elastic foundation method or the finite element method，it is necessary to ensure that the analytical pattern of the wall deformation be close to the observed or empirical values.
Figure 4.18 Typical bending moment and shear diagrams of a retaining wall by stress analysis
Basically，the deformation curves computed with the finite element method and the beam on elastic foundation method are also related to the selected parameters.The correct choosing of the parameters is a question of experience and is still to be studied.As a result，to discuss the differences of the results with the beam on elastic foundation method and the finite element method respectively is meaningless.The assumed support method and its determination of the assumed support are too rough.Besides，the method cannot simulate the construction process completely.Thus，the analytic results by using the assumed support method and those by using the finite element method or beam on elastic foundation method may be different.The results of the assumed support method may be only applicable to small scaled excavations.