【導(dǎo)讀】pressuresandforces;foundations.

　　

【正文】 e . 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。 BS 5400:Part 4 and BS 5950:Part 1 and Part 5 make this assumption. At ultimate limit state, the earth active or retained side are not pressures on the a maximum. Because the structural forces and bending moments due to earth pressures reduce as deformation of the most severe earth pressures, which are usually determined for the serviceability limit state, also apply to the ultimate limit state structural design calculations. The design at serviceability limit state for flexible structures such as steel or reinforced and prestressed undertaken in a like concrete may be manner to the analysis in to of BS 8110:Part 2:1985. For gravity mass walls such as masonry structures, which are relatively rigid, the earth pressures on the retained or active side are likely to be higher than the fully active values in the working state. The earth pressures at serviceability and ultimate limit states will be similar, because the displacement criteria will be similar. Disturbing forces General The disturbing forces to be taken into account in the equilibrium calculations are the earth pressures on the active or retained side of the wall, together with loads due to the paction of the fill (if any) behind the wall, surcharge loads, external loads and last, but by no means least, the water pressure. Atrest earth pressures The earth pressures which act on retaining walls, or parts of retaining walls, below existing ground, depend on the initial or atrest state of stress in the ground. For an undisturbed soil at a state of rest, the ratio of the horizontal to vertical stress depends on the type of soil, its geological origin, the temporary loads which may have acted on the surface of the soil and the topography. Soil suction and empirical correlations with in situ tests including static cone and dilatometer. The value of Ki depends on the type of soil, its geological history, the loads which may have topography, the temporary acted on the ground surface and changes in ground strain or ground water regime due to natural or artificial causes. Where there has been no lateral strain within the ground, Ki can be determinable from equated with K0 the coefficient onedimensional consolidation and swelling tests conducted in a stresspath triaxial test using appropriate stress cycles. For normally consolidated soils, both granular and cohesive: ???? sin 10K (10) For overconsolidated soils, K0 is larger and may approach the passive value at shallow depths in a heavily overconsolidated clay, (see for example Lambe and Whitman, quoting Hendron and Wroth 1975). Ki is not used directly in earth retaining structure design because the construction process always modifies this initial value. The value of Ki is however, important in assessing the degree of deformation which will be induced as the earth pressure tends towards active or passive states. In normally consolidated soil the ground deformation necessary to mobilize the active condition will be small in relation to that required to mobilize the full passive resistance, while in heavily overconsolidated soil the required ground deformation will be of similar magnitude. Additional ground deformation is necessary for the structure to approach a failure condition with the earth pressures moving further towards their limiting active and passive values. Where a stressed support system is employed ( anchorage) then the partial mobilization the active state on the retained side is reversed during installation of the system and,in the zone of support, the effective stress ratio in the soil may pass through the original toward the value of K0 ， and tend toward the value of Kp. Active earth pressures General Active earth pressures are generally assumed to increase linearly with increasing depth. However there may be variations from a linear relationship as a consequence, for example, of wall flexure. This can result in reduced bending moments in the structure, where the structure is deformations of the retaining structure are caused by transient loads, as encountered in highway structures, lockedin moments may remain after the load has been removed. These lockedin stresses will accumulate under repeated loading. This effect will limit the application of reduced bending moments in such structures. The design soil strength, derived in accordance with should be used in evaluating the active earth pressure. Cohesionless soil The basic formula for active pressure is applicable in the following simple situation: uniform cohesionless soil。 no water pressure。 mode of deformation such that earth pressure increases linearly with depth。 uniformly distributed surcharge only. In these restricted circumstances, the active pressure at depth z is given by: )( qzKaan ?? ??