Section 3 Design philosophy, design method and earth pressures3.1 Design philosophy3.1.1 GeneralThe design of earth retaining structures requires consideration of the interaction between the ground and the structure. It requires the performance of two sets of calculations: 1）a set of equilibrium calculations to determine the overall proportions and the geometry of the structure necessary to achieve equilibrium under the relevant earth pressures and forces; 2）structural design calculations to determine the size and properties of thestructural sections necessary to resist the bending moments and shear forces determined from the equilibrium calculations.Both sets of calculations are carried out for specific design situations (see 3.2.2) in accordance with the principles of limit state design. The selected design situations should be sufficientlySevere and varied so as to encompass all reasonable conditions which can be foreseen during the period of construction and the life of the retaining wall.3.1.2 Limit state designThis code of practice adopts the philosophy of limit state design. This philosophy does not impose upon the designer any special requirements as to the manner in which the safety and stability of the retaining wall may be achieved, whether by overall factors of safety, or partial factors of safety, or by other measures. Limit states (see 1.3.13) are classified into: a) ultimate limit states (see 3.1.3); b) serviceability limit states (see 3.1.4).Typical ultimate limit states are depicted in figure 3. Rupture states which are reached before collapse occurs are, for simplicity, also classified and treated as ultimate limit states. Ultimate limit states include: a) instability of the structure or any hart of it, including supports and foundations, considered as a rigid body; b) failure by rupture of the structure or any part of it, including supports and foundations.3.1.3 Ultimate limit states3.1.3.1 GeneralThe following ultimate limit states should be considered. Failure of a retaining wall as a result of: a) instability of the earth mass, e.g. a slip failure, overturning or a rotational failure where the disturbing moments on the structure exceed the restoring moments, a translational failure where the disturbing forces (see 1.3.8) exceed the restoring forces and a bearing failure. Instability of the earth mass aim-involving a slip failure ，may occur where:1) the wall is built on sloping ground which itself is close to limiting equilibrium; or2) the structure is underlain by a significant depth of clay whose undrained strength increases only gradually with depth; or3) the structure is founded on a relatively strong stratum underlain by weaker strata; or4) the structure is underlain by strata within which high pore water pressures may develop from natural or artificial sources. b) failure of structural members including the wall itself in bending or shear; c) excessive deformation of the wall or ground such that adjacent structures or services reach their ultimate limit state.3.1.3.2 analysis methodWhere the mode of failure involves a slip failure the methods of analysis, for stability of slopes, are described in BS 6031 and in BS 8081. Where the mode of failure involves a bearing capacity failure, the calculations should establish an effective width of foundation. The bearing pressures as determined from 4.2.2 should not exceed the ultimate bearing capacity in accordance with BS 8004.Where the mode of failure is by translational movement, with passive resistance excluded, stable equilibrium should be achieved using the design shear strength of the soil in contact with the base of the earth retaining structure.Where the mode of failure involves a rotational or translational movement, the stable equilibrium of the earth retaining structure depends on the mobilization of shear stresses within the soil. The full mobilization of the soil shear strength gives rise to limiting active and passive thrusts. These limiting thrusts act in concert on the structure only at the point of collapse, i.e. ultimate limit state.3.1.4 Serviceability limit statesThe following serviceability limit states should be considered: a) substantial deformation of the structure; b) substantial movement of the ground.The soil deformations, which accompany the full mobilization of shear strength in the surrounding soil, are large in comparison with the normally acceptable strains in service. Accordingly, for most earth retaining structures the serviceability limit state of displacement will be the governing criterion for a satisfactory equilibrium and not the ultimate limit state of overall stability. However, although it is generally impossible or impractical to calculate displacements directly, serviceability can be sufficiently assured by limiting the proportion of available strength actually mobilized in service; by the method given in 3.2.4 a