【正文】 ose material and the earth pressures consequently tend to increase once more to active values based on critical state strength.The initial presumption should be that the design earth pressure will correspond to that arising from the design soil strength, see . But the mobilized earth pressure in service, for some walls, will exceed these values. This enhanced earth pressure will control the design, for example. a) Where clays may swell in the retained soil zone, or be subject to the effects of paction in layers, larger earth pressures may occur in that zone, causing corresponding resistance from the ground, propping forces, or anchor tensions to increase so as maintain overall equilibrium. b) Where clays may have lateral earth pressures in excess of the assessed values taking account of earth pressures prior to construction and the effects of wall installation and soil excavation or filling, the earth pressure in retained soil zones will be increased to maintain overall equilibrium. c) Where both the wall and backfill are placed on pressible soils, differential settlement due to consolidation may lead to rotation of the wall into the backfill. This increases the earth pressures in the retained zone. d) Where the structure is particularly stiff, for example fully piled boxshaped Bridge abutments, higher earth pressures, caused, for example by paction, may be preserved, notwithstanding that the degree of wall displacement or flexibility required to reduce retained earth pressures to their fully active values in cohesionless materials is only of the order of a rotation of 103 radians.In each of these cases, mobilized soil strengths will increase as deformations continue, so the unfavorable earth pressure conditions dill not persist as collapse approaches.The design earth pressures are derived from design soil strengths using the usual methods of plastic analysis, with earth pressure coefficients (see ) given in this code of practice being based on Keriselamp。 b) the independence or interdependence of the various parameters involved in the calculation。 b) substantial movement of the ground.The soil deformations, which acpany the full mobilization of shear strength in the surrounding soil, are large in parison 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。 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。 or2) the structure is underlain by a significant depth of clay whose undrained strength increases only gradually with depth。 b) serviceability limit states (see ).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。Section 3 Design philosophy, design method and earth pressures Design philosophy 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 ) in accordance with the principles of limit state design. The selected design situations should be sufficientlySevere and varied so as to enpass all reasonable conditions which can be foreseen during the period of construction and the life of the retaining wall. 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 ) are classified into: a) ultimate limit states (see )。 b) failure by rupture of the structure or any part of it, including supports and foundations. Ultimate limit states GeneralThe following ultimate limit states should be considered. Failure of a retaining wall as a result of: a) instability of the earth mass, . 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 ) exceed the restoring forces and a bearing failure. Instability of the earth mass aiminvolving a slip failure ，may occur where:1) the wall is built on sloping ground which itself is close to limiting equilibrium。 or3) the structure is founded on a relatively strong stratum underlain by weaker strata。 c) excessive deformation of the wall or ground such that adjacent structures or services reach their ultimate limit state. 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 involve