【正文】 ent design situations.The structural geometry of the retaining wall and the equilibrium calculations should be determined from the design earth pressures derived from the design soil strength using the appropriate earth pressure coefficients.Design earth pressures will lead to active and passive pressure diagrams of the type shown in figure 4. The earth pressure distribution should be checked for global equilibrium of the structure. Horizontal forces equilibrium and momentequilibrium will give the prop force in figure 4a and the location of the point of reversed stress conditions near the toe in figure 4b. Vertical forces equilibrium should also be checked. Design situations GeneralThe specification of design situations should include the disposition and classification of the various zones of soil and rock and the elements of construction which could be involved in a limit state event. The specification of design situations should follow a consideration of all uncertainties and the risk factors involved, including thefollowing: a) the loads and their binations, . surcharge and%or external loads on the active or retained side of the wall。 c) the material characteristics of the structure, . following corrosion。scour, erosion and excavation, leading to changes in the geometry of the ground surface。 weathering。 the presence of gases emerging from the ground。 e) earthquakes。 g) the tolerance of the structure to deformations。 i) for structures resting on or near rock, the consideration of: interbedded hard and soft strata。 solution cavities such as swallow holes or fissures, filled with soft material, and continuing solution processes. Minimum surcharge and minimum unplanned excavationIn checking the stable equilibrium and soil deformation all walls should be designed for a minimum design surcharge loading of 10 kN/m2 and a minimum depth of excavation in front of the wall, which should be: a)not less than m。unplanned39。 or b) 75% of the design shear strength to be mobilized in the soil itself, that is using: (5) (6) Since for the soil mass: (7) this is equivalent to: (8) similarly, in total stress analysis: (9)The friction or adhesion, which can be mobilized in practice, is generally less than the value deduced on the basis of soil sliding against the relevant surface. It is unlikely for example, that a cantilever wall will remain at constant elevation while the active soil zone subsides creating full downward wall friction on the retained side, and the passive zone heaves creating full upward wall friction on the excavated side. It is more likely that the wall would move vertically with one or other soil zone,reducing friction on that side, and thereby attaining vertical force equilibrium. The 25% reduction in the design shear strength in b) above makes an allowance for this possibility. Further reductions, and even the elimination of wall friction or its reversal, may be necessary when soil structure interaction is taken into account. Wall friction on the retained or active side should be excluded when the wall is capable of penetrating deeper, due to the vertical thrust imparted by inclined anchors on an embedded wall, by structural loads on a basement wall, or where a clay soil may heave due to swelling during outward movement of the wall. Wall friction on the passive side should be excluded when the wall is prevented from sinking but the adjacent soil may fail to heave, due for example to settlement of loose granular soils induced by cyclic loads, or when the wall is free to move upwards with the passive soil zone, as may happen with buried anchor blocks. Design to structural codesThe earth pressures to be used in structural design calculations are the most severe earth pressures determined for serviceability limit state, see . These are the most severe that can credibly occur under the design situations, see . Accordingly the application of partial load factors to the bending moments and internal forces derived from these earth pressures, is not normally required. Hacking determined the earth pressures using design the structure increases it should be assumed that loads and design soil strengths, the structural load affects (bending moments, and shears) can be calculated using equilibrium principles in the usual way without applying any further factors. Finally, the material properties and sections should be derived from the load effects according to the structural codes. Reference should be made to the documentary source for the loadings, such as BS 5400:Part 4 for guidance on the respective design values.Structural design calculations based upon ultimate limit state assume that the moments and forces applicable at ultimate larger than limit state are significantly at serviceability limit state. BS8110: Part 1 and Part。 no water pressure。 uniformly distributed surcharge only.In these restricted circumstances, the active pressure at depth z is given by: (11)where the earth pressure coefficient Ka is based on design values of soil parameters.The total active thrust normal to the between ground level and depth z is then: (12)If there is static ground water beneath a water table at depth for zzw. (13)Where (14)Then (15)This equation is general。s formula may be used: (16)The design value of the angle of wall friction t,