【正文】 ssures is approached, with sufficient deformation of the structure, the active earth pressure (see ) in the retained soil reduces to the fully active pressure and the passive resistance (see ) tends to increase to the full available passive resistance (see ).The patibility of deformation of the structure and the corresponding earth pressures is important where the form of structure, for example a propped cantilever wall, prevents the occurrence of fully active pressure at the prop. It is alsoparticularly important where the structure behaves as a brittle material and loses strength as deformation increases, such as an unreinforced mass gravity structure or where the soil is liable to strain softening as deformation increases. Design values of parametersThese are applicable at the specified limit states in the specified design situations. All elements of safety and uncertainty should be incorporated into the design values.The selection of design values for soil parametersshould take account of: a) the possibility of unfavorable variations in the values of the parameters。 b) the independence or interdependence of the various parameters involved in the calculation。 c) the quality of workmanship and level of control specified for the construction. Applied loadsThe design value for the density of fill materials, should be a pessimistic or unfavorable assessment of actual density.For surcharges and live loadings different values may be appropriate for the differing conditions of serviceability and ultimate limit states and for different load binations. The intention of this code of practice is to determine those earthpressures which will not be exceeded in a limit state, if external loads are correctly predicted. External loads, such as structural dead loads or vehicle surcharge loads may be specified in other codes as nominal or characteristic values. Some of the structural codes, with which this code interfaces, specify different load factors to be applied for serviceability or ultimate limit state the checks and for different load binations,See .Design values of loads, derived by factoring or otherwise, are intended, here, to behere most pessimistic or unfavorable loads which should he used in the calculations for the structure. Similarly, when external loads act on the active or retained side of the wall these same external loads should be derived in the same way. The soil is then treated as forming part of the whole structural system. Design soil strength (see )Assessment of the design values depends on the required or anticipated life of the structure, but account should be taken also of the shortterm conditions which apply during and immediately following the period of construction. Single design values of soil strength should be obtained from a consideration of the representative values for peak and ultimate strength. The value so selected will satisfy, simultaneously, the considerations of ultimate and serviceability limit states. The design value should be the lower of: a) that value of soil strength, on the stressstrain relation leading to peak strength,which is mobilized at soil strains acceptable for serviceability. This can be expressed as the peak strength reduced by a mobilization factor M as given in or 。 or b) that value which would be mobilized at collapse, after significant ground movements. This can general be taken be the critical state strength.Design values selected in this way should be checked to ensure that they conform to . Design values should not exceed representative values of the fully softened critical state soil strength. Design earth pressuresThe design values of lateral earth pressure are intended to give an overestimate of the earth pressure on the active or retained side and an underestimate of the earth resistance on the passive side for small deformations of the structure as a whole, in the working state. Earth pressures reduce as fully active conditions are mobilized atpeak soil strength in the retained soil, under deformations larger than can be tolerated for serviceability. As collapse threatens, the retained soil approaches a critical state, in which its strength reduces to that of loose 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 pr