本科毕业设计外文 翻译

Section 3 Design philosophy, design method and earth pressures 3.1 Design philosophy 3.1.1 General The 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 sufficiently Severe 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 design This 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 states 3.1.3.1 General The 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: the wall is built on sloping ground which itself is close to limiting equilibrium; or 2) the structure is underlain by a significant depth of clay whose undrained strength increases only gradually with depth; or 3) the structure is founded on a relatively strong stratum underlain by weaker strata; or 4) 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 method Where 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 states The 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 and 3.2.5. The design earth pressures used for serviceability limit state calculations will differ from those used for ultimate limit state calculations only where structures are to be subjected to differing design values of external loads (generally surcharge and live loads) for the ultimate limit state and for the serviceability limit state. 3.1.5 Limit states and compatibility of deformations The deformation of an earth retaining structure is important because it has a direct effect upon the forces on the structure, the forces from the retained soil and the forces which result when the structure moves against the soil. The structural forces and bending moments due to earth pressures reduce as deformation of the structure increases. The maximum earth pressures on a retaining structure occur during working conditions and the necessary equilibrium calculations (see 3.2.1) are based on the assumption that earth pressures greater than fully active pressure (see 1.3.11) and less than fully passive will act on the retaining structure during service. As ultimate limit state with respect to soil pressures is approached, with sufficient deformation of the structure, the active earth pressure (see 1.3.1) in the retained soil reduces to the fully active pressure and the passive resistance (see 1.3.15) tends to increase to the full available passive resistance (see 1.3.12). The compatibility 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 also particularly 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. 3.1.6 Design values of parameters These 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 parameters should 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. 3.1.7 Applied loads The 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 combinations. The intention of this code of practice is to determine those earth pressures 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 combinations, See 3.2.7 .Design values of loads, derived by factoring or otherwise, are intended, here, to be here 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. 3.1.8 Design soil strength (see 1.3.4) Assessment of the design values depends on the required or anticipated life of the structure, but account should be taken also of the short-term 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 stress-strain 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 3.2.4 or 3.2.5; or b) that value which would be mobilized at collapse, after significant ground movements. This can general be taken t.o be the critical state strength. Design values selected in this way should be checked to ensure that they conform to 3.1.6. Design values should not exceed representative values of the fully softened critical state soil strength. 3.1.9 Design earth pressures The 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 3.1.8. 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 compaction 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 t.o 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 compressible 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 box-shaped Bridge abutments, higher earth pressures, caused, for example by compaction, 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 10-3 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 1.3.9) given in this code of practice being based on Kerisel&Absi(1990). The same design earth pressures are used in the default condition for the design of structural. sections, see 3.2.7. 3.2 Design method 3.2.1 Equilibrium calculations In order to determine the geometry of the retaining wall, for exampal the depth of penetration of an embedded wall (see 1.3.10), equilibrium calculations should be carried out for care formulated design situations. The design fully calculations relate to a free-body diagram of forces and stresses for the whole retaining wall. The design calculations should demonstrate that there is global equilibrium of vertical and horizontal forces, and of moments. Separate calculations should be made for different 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 moment equilibrium 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. 3.2.2 Design situations 3.2.2.1 General The 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 the following: a) the loads and their combinations, e.g. surcharge and%or external loads on the active or retained side of the wall; b) the geometry of the structure, and the neighbouring soil bodies, representing the worst credible conditions, for example over-excavation during or after construction; c) the material characteristics of the structure, e.g. following corrosion; d) effects due to the environment within which the design is set, such as: -ground water levels, including their variations due to the effects of dewatering possible flooding or failure of any drainage system; -scour, erosion and excavation, leading to changes in the geometry of the ground surface; -chemical corrosion; -weathering; -freezing; -the presence of gases emerging from the ground; -other effects of time and environment on the strength and other properties of materials; e) earthquakes; f) subsidence due to mining or other causes; g) the tolerance of the structure to deformations; h) the effect of the new structure on existing structures or services and the effect of existing structures or services on the new structure; i) for structures resting on or near rock, the consideration of: -interbedded hard and soft strata; -faults, joints and fissures; -solution cavities such as swallow holes or fissures, filled with soft material, and continuing solution processes. 3.2.2.2 Minimum surcharge and minimum unplanned excavation In 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 0.5 m; and b)not less than10% of the total height retained for cantilever walls, or the height retained lowest support level for propped or anchored walls. These minimum values should be reviewed for each design and more adverse values adopted in particularly critical or uncertain circumstances. The requirement for an additional or unplanned excavation as a design criterion is to provide for unforeseen and accidental events. Foreseeable excavations suet as service or drainage trenches in front of a retaining wall, which may be required at some stage in the life of the structure, should be treated as a planned excavation. Actual excavation beyond the planned depth is outside the design considerations of this code. 3.2.2.3 Water pressure regime The water pressure regime used in the design should be the most onerous that is considered to be reasonably possible. 3.2.3 Calculations based on total and effective stress parameters The changes in loading associated with the construction of a retaining wall may result in changes in the strength of the ground in the vicinity of the wall. if"here the mass permeability of the ground is low these changes of strength take place over some time and therefore the design should consider conditions in both the short- and long-term. Which condition will be critical depends on whether the changes in load applied to the soil mass cause an increase or decrease in soil strength. The long-term condition is likely to be critical where the soil mass undergoes a net reduction in load as a result of excavation, such as adjacent to a cantilever wall. Conversely where the soil mass is subject to a net increase in loading, such as beneath the foundation of a gravity or reinforced stem wall at ground level, the short-term condition is likely to be critical for stability. When considering long-term earth pressures and equilibrium, allowance should be made for changes in ground water conditions and pore water pressure regime which may result from the construction of the works or from other agencies. Calculations for long-term conditions require shear strength parameters to be in terms of effective stress and should take account of a range of water pressures based on considerations of possible seepage flow conditions within the earth mass. Effective stress methods can also be used to assess the short-term conditions provided the pore water pressures developed during construction are known. A total stress method of analysis may be used to assess the short-term conditions in clays and soils of low permeability, but an inherent assumption of this method is that there will be no change in the soil strength as a result of the changes in load caused by the construction. For granular materials and soils of high permeability all excess pore water pressure will dissipate rapidly so that the relevant strength is always the drained strength and the earth pressures and equilibrium calculations are always in terms of effective stresses. 3.2.4 Design using total stress parameters The retaining wall should be designed to be in equilibrium design clay when based on a mobilized undrained strength (design cu) which does not exceed the representative divided by a mobilization undrained strength factor M. The value of M should not be less than 1.5 if wall displacements are required to be less than 0.5 % of wall height. The value of M should be larger than 1.5 for clays which require large strains to mobilize their peak strength. 3.2.5 Design using effective stress parameters The retaining wall should be designed to be in equilibrium mobilizing a soil strength the lesser or: a) the representative peak strength of the soil divided by a factor M=1.2: that is: （3） （4） or b) the representative critical state strength of the soil. This will ensure that for soils which are medium dense or firm the wall displacements in service will be limited to 0.5 % of the wall height. The mobilization factor of 1.2 should be used in conjunction with the front of the wall, the 'unplanned' excavation inminimum surcharge loading and the water pressure regime, see 3.2.2.2 and 3.2.2.3. A more detailed analysis of displacement should be are to be applied or for soft or loose soils. The criteria a) and b), taken together, should provide a sufficient reserve of safety against small unforeseen loads and adverse conditions. In stiff clays subject to cycles of strain, such as through seasonal variation of pore water pressure, the long-term peak strength may deteriorate to the critical state strength. The requirements of a) and b) above are sufficiently cautious to accommodate this possibility. 3.2.6 Design values of wall friction, base friction and undrained wall adhesion These should be derived from the representative strength determined in accordance with 2.2.8,using the same mobilization actors as for the adjacent soil. The design value of the friction or adhesion mobilized at an interface with the structure be the lesser of: a) the representative value determined by described in 2.2.8 if such test results are available; 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. 3.2.7 Design to structural codes The earth pressures to be used in structural design calculations are the most severe earth pressures determined for serviceability limit state, see 3.1.9. These are the most severe that can credibly occur under the design situations, see 3.2.2. 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 3.1 to 3.4 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. 3.3 Disturbing forces 3.3.1 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 compaction of the fill (if any) behind the wall, surcharge loads, external loads and last, but by no means least, the water pressure. 3.3.2 At-rest earth pressures The earth pressures which act on retaining walls, or parts of retaining walls, below existing ground, depend on the initial or at-rest 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 one-dimensional consolidation and swelling tests conducted in a stress-path triaxial test using appropriate stress cycles. For normally consolidated soils, both granular and cohesive: (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 (e.g.ground 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. 3.3.3 Active earth pressures 3.3.3.1 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 flexible.Where deformations of the retaining structure are caused by transient loads, as encountered in highway structures, locked-in moments may remain after the load has been removed. These locked-in 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 3.1.8 should be used in evaluating the active earth pressure. 3.3.3.2 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: (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 zw.then for z>zw. (13) Where (14) Then (15) This equation is general; it is not limited to uniform soils or hydrostatic water pressures or to modes of deformation such that earth pressure increases linearly with depth. More than one surcharge can be accommodated, but each must be uniformly distributed. Using the design soil strength, the value of Ka should be determined from the graphs in annex A. Ka in these graphs is the horizontal component. In the special case of a smooth vertical wall and horizontally retained soil surface(β=0，α=900，δ=0), Rankine's formula may be used: (16) The design value of the angle of wall friction t,o be used in the graphs in annex A should be determined in accordance with 3.2.5. Where the ground surface is irregular the active thrust may be determined by the graphical procedure shown in figure 5. A slip plane is chosen and the thrust on the wall is determined from the triangle of forces. The procedure is repeated withother slip planes until sufficient values have been obtained to enable the maximum thrust to be found by graphical interpolation. Not less than three planes should be used, but it is not usually necessary to have more than five. The position of the centre of pressure on the back of the wall may be taken as the point of intersection with the back of the wall of a line drawn through the centre of gravity of the wedge parallel to the slip plane of the wedge. An alternative approach is to consider the additional soil mass above a horizontal retained surface as a surcharge load, see 3.3.4. Where there is a superimposed line load for a considerable distance along and parallel to the wall, the weight per unit length of this load may be included in the force W in the diagram. If there are several different strata of cohesionless soils behind the wall, the foregoing procedure can be used for the uppermost stratum in contact with the wall and, unless the wall is appreciably inclined from the vertical, the active pressures exerted by the lower strata can be calculated from equation 15 using an assumed average ground surface level for the estimation of the effective overburden pressure. 第3章 理念、方法与土压力 3.1 设计理念 3.1.1 综述 挡土墙的设计要求考虑地面结构之间的相互作用，要求两部分计算的效应： 1）一部分平衡（稳定性）计算来确保结构体的比例和几何形状，且在相关的土压力和荷载下达到必要的平衡； 2）结构设计结算来确保结构截面的尺寸和性能，须能够抵抗由平衡计算确定的弯矩和剪切应力。 对于特殊情形的设计（见3.2.2）运用，两部分的计算都与极限状态设计法一致。选定的设计情形应当足够的严谨和广泛，以能够包涵所有在结构施工期和挡土墙使用期，可以被预见的可能性因素。 3.1.2 极限状态设计 现行规范采取极限状态法设计理念。这种理念没有给设计者提出任何具体的要求，所以在这种方法中，无论通过全部安全因素，或者是部分的安全因素，还是其他的，挡土墙都会具有有稳定性和安全性。极限状态（见1.3.13）可以分为： a) 承载能力极限状态（见3.1.3）； b) 正常使用极限状态（见3.1.4）。 典型的承载能力极限状态如图3所述。结构在倒塌发生前达到破裂状态，破裂状态可以简易地划分和视为承载能力极限状态。承载能力极限状态包括： 结构或者其任何部分的不稳定，包括支撑和基础，均视为一个牢固的整体； 由结构体或者其任何部分，包括支撑和基础的破裂而导致的破坏。 3.1.3 承载能力极限状态 3.1.3.1 综述 承载能力极限状态应当考虑以下，导致连续墙破坏的原因为： 土体失稳，例如在滑移破坏，倾覆破坏和转动破坏中，作用在结构体上的扰动力矩超过了它自身的恢复力矩，在移动破坏中，扰动力（见1.3.8）超过了恢复力和承载破坏。涉及滑移破坏的地面大面积失稳可能发生在以下地方： 1）墙体建造在倾斜的地面上，而地面本身接近极限平衡； 2）或者结构体下伏相当深度的粘土层，且其地下水压力随深度的增加而逐渐增大； 3）或者结构体建造在有薄弱下伏层的相对牢固地层上； 4）或者，结构体下伏的地层中有较高的天然或人为的孔隙水压力。 a）包括墙本身的结构体部件承受弯矩和剪力； b) 墙或地面的过度变形导致邻近结构或辅件达到自身的基本极限状态。 图3 挡土构筑物的极限状态 图3 挡土构筑物的极限状态（续） 图3 挡土构筑物的极限状态（续） 3.1.3.2 方法 由滑移引起的破坏模式中，分析的方法，如斜坡稳定性，在《BS 6031》和《BS 8081》中有描述。由承载能力引起的破坏模式，计算应建立一个有效宽度的基础。根据《BS 8004》，由4.2.2确定的承载力不宜超出基本极限承载力。 由移动引起的破坏模式，除去被动阻力外，应用与地面连续结构物底面相接触的土层的抗剪能力来获得稳定平衡性。 由转动或移动引起的破坏模式，地面连续结构物的稳定平衡性依靠土体中剪切压力的变化情况。土体剪切能力的足够变化，会引起有限的主动或被动推力的增加。这些有限的推力只有在结构体发生倒塌的时刻共同起作用，也就是基本极限状态。 3.1.4 正常使用极限状态 正常使用极限状态法应考虑以下情况： 结构的大量变形； 地面的大幅度运动。 伴随着周围土体中剪切能力的共同作用，相对于使用中可承受的主应变，土体的变形很大。因此，对于大多数的挡土墙，位移的正常使用极限状态法将是符合要求的平衡能力的控制标准，而不是承载能力极限状态法的整体稳定性。然而，尽管直接计算位移时不大可能和不太实际的，但利用在3.2.4和3.2.5中给出的方法，通过限制在使用中起作用的有效承载力，可以充分地确定正常使用性。 适用于正常使用极限状态法的挡土墙设计，和适用于承载能力极限状态法的挡土墙设计，两种方法区别之处，只在于结构体受外部荷载不同设计值。 3.1.5 极限状态和位移的协调性 挡土墙的位移很重要，因为其在结构体的作用力之上产生一个直接作用，这种作用力来自固有土体，并且在结构题背离土体移动时产生。因土压力而产生的结构内力和弯矩随结构位移的增大而减小。 作用于挡土墙上的最大土压力在使用环境中（见3.2.1）产生，且必要的平衡计算的依据是一个假设：土压力大于全主动压力（见1.3.11）并且小于全被动压力，土压力会在挡土墙使用时对其起作用。ＡＳ承载能力极限状态法考虑了土压力（见1.3.1）是可取的，随着结构的大量变形，固有土体中的主动土压力迫使全主动压力和被动抗力（见1.3.15）趋向接近到可用被动抗力（见1.3.12）。 在结构体的构造类型处，结构体的变形和相应土压力的容许值是很关键的，例如支撑悬臂墙，能够防止在支撑上产生全主动压力。在脆性材料结构以及随变形增大而强度损失的结构中尤为关键，比如在无筋重力结构或随变形增大而应力易于软化的土体中。 3.1.6 参数值设计 参数值适用于规定的极限状态法在特定的设计情况。所有的安全与不确定因素都应当是设计值的一部分。 土工参数的设计值选择应考虑以下方面： 参数值不利变化的可能性； 计算中各种参数的独立性与相互依赖性； 3.1.7 实用荷载 实心材料的密度设计值应当是实际密度的保守值或者不利估计值。 对于不同环境下的使用性和承载能力极限状态法，以及不同的荷载组合，超载和活载的不同参数值需合理适当。现行规范的目的是，在假设外荷载正确估算的情况下，确保土压力在一种极限状态中不会超限。外荷载，如结构恒载或者车辆活载，可能在其他规范中作为名义值或者特定值已经规定。与本规范相继的一些规范，规定了不同的荷载因素，用于结构的使用性或者承载能力极限状态检查，以及不同的荷载组合（见3.2.7）。荷载的设计值由一些因素推导，亦或此时设计成最保守和最不利荷载，这些荷载应利用到结构的计算中。同样，当外荷载作用在墙身的主动测或者作用侧，这些相同的外荷载应用相同方法推导。然后把土体视为组成整个结构体系的一部分。 3.1.8 土体强度设计 设计参数值得估算取决于规范要求，或者结构使用寿命，但也应考虑到造价以及施工期间和紧随其后的短期环境要求。土体强度的单一设计值的取值，应考虑最大承载力和极限承载力的代值。这样选择的设计值，同时满足承载能力极限状态法和正常使用极限状态法的考虑。设计值在以下情况中应减小： 在应力-应变关系中推导峰值强度值，土体应变符合使用性的土体强度设计值。这可以表示成峰值强度因因子M的作用而减小，3.2.4或3.2.5中已给出。 可能致使在崩塌的值在很大的地面位移后，这通常看作是临界强度。 通过这种方法选择的设计值应得以检查，来确保其符合3.1.6。设计值不宜超出全软化临界状态的土体强度的代表值。 3.1.9 土压力设计 对于工作状态下整体结构的小位移，侧向土压力设计值适用于给出的主动侧或开挖前后土体一个高估土压力，和一个低估被动侧的土体抗力。土压力因土体中全主动条件达到成土强度峰值。作为倒塌危险，土体接近临界状态，临界状态下，土体强度减小到相当于松散材料的强度，且结果，土压力趋向于再次增加至基于临界状态强度主动值。 初始推测宜使设计土压力与设计土体强度一致，见3.1.8。但对于某些墙，在使用期土压力会超出这些参数值。这些强化的土压力会控制设计，例如： 在粘土会膨胀的区域，或遭遇土层挤密作用的地方，会出现较大的土压力，导致来自地面、支撑了力、锚固张力的相应抗力增大，以保持整体平衡。 在结构建造和墙体安装，以及土体开挖或填筑影响之前，粘土可能有侧向土压力超出考虑了土压力的估计值的地方，原有土体范围内的土压力会增大，以保持整体平衡。 在墙体和回填土都在可压缩土层的地方，固结产生的不均匀沉降可能会导致墙身转动进入回填土中。 结构稳定性非常好，例如箱形截面桥全桩桥墩，例如通过挤压导致的较高的土压力，可能会传递，尽管无粘性材料墙要求，减小固有土压力达到它的全主动值，它的位移或弹性程度是约为10-3弧度的转角。 在以上的每一种情况下，土体强度将随着位移的增加而增大，因此，不利土压力的条件不再随倒塌的接近而持续。 设计土压力根据设计土体强度推导而来，运用塑性分析的方法，利用基于Kerisel & Absi（1990）的现行规范给出土压力系数（见1.3.9）。相同的设计土压力用于结构设计章节的缺省条件中，见3.2.7。 3.2 设计方法 3.2.1 平衡计算 为了确定挡土墙的几何形状，例如嵌入式挡土墙的渗透深度（见1.3.10），平衡计算应依设计情形的详细计算。对于整个挡土墙，设计计算涉及到力和应变的隔离体受力图。设计计算应论证竖向力、水平力和力矩的整体平衡。单独计算宜当用于不同的设计情形。 挡土墙的几何形状和平衡计算应由设计土压力确定，而设计土压力根据设计土体强度，用合适的土压力系数推导得出。 设计土压力会引出如图4所示的主动和被动土压力图。设计土压力的分布宜根据结构整体平衡来检查。水平力平衡和力矩平衡，会在图4a中得出支撑力，在图4b中得出墙趾附近反向应力位置点的条件。竖向力平衡也能得到检查。 图4 压力图 3.2.2 设计情形 3.2.2.1 综述 设计情形规范应包括各区域土层和岩石的特征、分类和可用在一个极限状态事件中的元素组成。 设计情形规范应遵循综合考虑全部不确定因素和危险因素的原则，包括以下： 荷载和荷载组合，例如作用在墙主动侧或作用侧的超载和（或）外荷载； 结构的几何形状，和代表最可信条件的邻近土体，例如在施工期间或施工后的过度挖掘； 结构材料特征，例如材料在腐蚀后； 设计时规定的环境的影响，如： —地下水位，包括由于抽水、可能的洪灾或排水系统的破坏而引起的水位变化； —冲刷、腐蚀和挖掘而引起的地面几何形状的改变； —化学腐蚀； —气象条件； —冰冻作用； —土壤气体的存在； —力作用时间与环境，以及材料性能的其他影响； 地震作用； 采矿或其他原因引起的下沉； 结构对位移的承受能力； 新结构对现有结构或辅件的影响，现有结构或辅件对新结构的影响； 若结构建于岩石之上或靠近岩石，考虑以下情况： ①硬夹层或软土； ②断层、接头和裂缝； ③被软物质填充的溶洞（如溶沟或裂缝），并溶解在继续发育。 3.2.2.2 最小超载和计划外开挖 在检查稳定平衡和土体位移时，所有墙的设计，宜按10KN/m2加载的最小设计超载来设计，以及规定墙前额外计划外开挖最小深度： 不能小于0.5m； 不能小于悬臂墙原有总高度的10﹪，或不能小于支撑墙或锚定墙原有高度低于最低支撑水平的10﹪。 对于采用在特别的临界或不确定情况中的各种设计值和更不利值，应检验这些最小值。作为设计标准，对附加或计划外的开挖的要求，是提供给不可预测和意外事件的。可预测开挖，如挡土墙前的辅助沟渠或排水沟渠，可能在结构寿命的某些阶段，要求宜视为计划内开挖。本规范不考虑实际开挖超过计划深度。 3.2.2.3 水压力状况 用于设计的水压力状况，应是视为合理可能性的极为繁琐的情况。 3.2.3 总应力参数和有效应力参数的计算 挡土墙施工期荷载的变化，可能会导致挡土墙附近土体强度的变化。在地面土的渗透性差的地方，有时候这些强度变化会出现超标，因此设计时应都考虑短期及长期条件。每种条件会不会达到临界状态，在于作用在土体上荷载的变化是否会导致土体强度的增大或减小。长期条件在开挖后土体经历加载呈网状减小的位置，可能达到临界状态，例如与悬臂墙相毗连的位置。相反地，在土体经历加载呈网状增大的位置，例如在低于重力式或加筋挡土墙地面基础的位置，短期条件可能达到稳定临界状态。当考虑了长期条件土压力和平衡，限值应根据地下水条件和孔隙水压体制的变化制定，限值可能来自本工程的建造或其他机构。 长期条件的计算要求剪力参数按照有效应力，并且应考虑一些与土体中可能渗流流动条件相关的水压。有效压力方法也可以用来评价施工明确时短期条件给出的孔隙水压的变化情况。总体应力分析方法可能用于评价短期条件粘土和土体的低渗透性，但这种方法的固有假设是，施工导致的荷载变化没有引起土体强度的变化。对于强渗透性的颗粒材料和土层，所有超孔隙水压会迅速消散，因此有意义的强度通常是排水强度与土压力，而且有效应力通常是平衡计算的依据。 3.2.4 用总应力参数进行设计 挡土墙宜在平衡状态下设计，当基于不排水粘土强度设计（cu设计），未超过由因子M控制的代表不排水强度。如果墙的位移要求小于墙高的0.5﹪，M的值不宜小于1.5。对于要求大应变的粘土，M的值应大于1.5，来达到他们的峰值强度。 3.2.5 用有效参数应力进行设计 挡土墙设计应处于平衡状态使土体强度去下面较小者： 土体的代表峰值强度除以因子M=1.2： 即： （3） （4） 或者土体临界状态代表强度。 这会确保对于中等密实或坚固的土体，墙在使用期的位移将被限制在墙高的0.5﹪。调节因子1.2宜与墙前计划外开挖相结合使用，最小超载加载和水压力体制见3.2.2.2和3.2.2.3。 对于标准更严格的软弱或松散的土体，应当用更详细的位移分析方法。强调准则a）和b），应提供充分的安全储备来抵抗小的不可预测荷载和不利因素。 在承受循环应力的坚硬土层，例如通过孔隙水压季节交替，长期峰值可能承受临界状态强度值。为适应这种可能性，要十分注意a）和b）上文的要求。 3.2.6 挡土墙的摩擦力、基底摩擦力和无筋墙的粘聚力设计 此部分应根据2.2.8决定的标准强度进行设计，用相同的因子，就邻近土体而言。 和结构产生在某一接触面的摩擦力或粘聚力的设计值去下面较小者： 如2.2.8中所述，如果试验结果可用的话，标准值由试验得到； 或者，75﹪的设计剪切强度在土体自身中发挥作用，用： （5） （6） 因对土体： （7） 等价于： （8） 同样，总应力分析： （9） 实践中可调节的摩擦力或粘聚力，通常小于由沿相关表面滑移土体基础上的推导值。例如，当主动区土体沉降而在作用侧产生全部向下的墙体摩擦力，以及被动区土体上升而在开挖侧产生全部向上的墙体摩擦力时，悬臂墙将保持持续的升高的可能性不大。而更有可能的是，挡土墙会随某一区域或其他区域土体竖向移动，在该侧产生摩擦力，并且在另一侧达到竖向力平衡。上面b）中设计剪切强度减小的25﹪，是这种可能性限值。当考虑了挡土墙和土体之间的相互作用时，可能有必要进一步减小、甚至消除墙体摩擦力或其反向力。挡土墙作用侧或主动侧的摩擦力宜消除，当挡土墙具有更强渗透能力时，导致通过倾斜的锚固件在嵌入式挡土墙上，通过结构荷载在地下室墙上提供竖向推力，或者在挡土墙向外移动时，因粘土体隆起而引起土体的上升的位置。当墙体阻止下沉但邻近土体可能未上升时，挡土墙被动侧的摩擦力应消除，例如由循环荷载引起的松散颗粒土的沉降，或当墙体随主动区土体自由移动时，比如可能随地埋锚块而产生。 3.2.7 结构设计规范 用于结构设计计算的土压力是最严格的土压力，为正常使用极限状态法而确定的，见3.1.9。那些可以可靠的发生在设计情形之下的情况也是最为严格的，见3.2.2。因此，局部荷载要素在由土压力推导的弯矩和内部荷载上应用，是没有正常规定的。用结构设计增大土压力，应假设：荷载、设计土体强度和结构荷载效应（弯矩和剪切力）可用没有更多要素的平常方法，通过平衡原则来计算。最后，材料特性和截面应根据结构规范，由荷载效应推导而定。加载应参考文献，比如《BS 5400-4》对各个设计参数的指导。 根据承载能力极限状态法的结构设计计算，假设适用于承载能力极限状态法的力矩和力，明显大于正常使用极限状态中的。《BS 8110-1/4》和《BS 5400-4》，以及《BS 5950-1/5》做了这个假设。在承载能力极限状态中，作用在挡土墙主动侧或作用侧的土压力不是最大值。因为结构荷载和弯矩，导致了土压力随着最严格土压力的变形而减小，结构荷载和弯矩通常由正常使用极限状态规定，但也用于承载能力极限状态的设计计算。正常使用极限状态中的设计，对于相对有弹性的结构体，比如钢结构或者预应力钢筋混凝土结构，可能同样的与《BS 8110-2 1985》中的分析类似。 对于重力式挡土墙，如相对脆性的石砌结构，作用在主动侧或作用侧的土压力，可能高于使用态中的全主动值。正常使用极限状态和承载能力极限状态中的土压力相似，因为位移标准相似。 3.3 扰动荷载 3.3.1 综述 考虑在平衡计算中的扰动荷载，是作用在挡土墙主动侧或者作用侧的土压力，以及墙后土体的挤密作用引起的荷载，超载，外荷载，最后甚至是水压力。 3.3.2 静止土压力 作用在挡土墙或挡土墙局部的土压力，在现有地面以下，依靠土中最初或者静态应力。对于处于静态的未扰动土，水平应力和竖直应力比取决于土的类型，它的地质起源，作用在土体表面和地表的临时荷载。 对于处于静态的土体，水平和竖直有效应力比（ ）可以通过一系列方法来估计，包括自钻式旁压试验，实验室测定土壤吸力以及原位测试经验关系式，包括静态锥和膨胀计。 值取决于土体类型，地质过程，地形，以及可能作用在地面上的临时荷载，自然或人工原因导致的地面应变和水文情况变化。 在没有侧向应变的土中， 等同于 ，一维固结和膨胀试验的系数可测，进行用合适的应力圆的应力路径三轴试验。对于正常固结土，颗粒状和粘性土都有： （10） 对于超固结土， 更大且可能在严重超固结土浅深度处接近被动值（例如，见Lambe 和 Whitman，引证Hendronb 和 Wroth 1975）。 并不直接用于挡土墙结构设计中，因为施工过程中经常会修改其原始值，即使 的值在评价变形程度是非常重要，土压力趋向主动或被动态将会诱导变形。在正常的固结土中，必要的地面变形使得主动条件会小一些，要求主动条件发挥全被动抗力，然而在严重固结土中，要求地面变形的程度更小。对于结构随着土压力进一步向极限主动或被动值靠近而接近破坏条件时，附加地面变形是必要的。 在采用应力支撑体系（如地锚）的地方，那么，在作用侧上主动态的部分作用在体系安装过程中是相反的，并且在支撑区，土中有效应力比超过 原始值，且趋向 。 3.3.3 主动土压力 3.3.3.1 综述 主动土压力通常假定随深度增大而直线增大。然而这种直线关系可能会有一些变化，例如墙的挠曲结果的影响，将会导致结构挠曲位置的弯矩减小。在暂时荷载导致的挡土结构变形的位置，内弯矩可能在荷载去除之后还会持续，如公路中的结构。内应力在循环荷载作用下会累积。这个影响会限制这类结构的弯矩应用性。 由3.1.8推导土体设计强度，应用于主动土压力的求解。 3.3.3.2 无粘性土 主动土压力基本公式用于以下简单情形： —均匀无粘性土； —无水压； —变形方式使得土压力随深度而直线增大； —只有均布超荷载。 在这些约束环境中，深度z处的主动土压力为： （11） 土压力系数 是根据土体参数设计值计算的。 垂直于墙的地面标高和深度z 之间的总主动推力为： （12） 当在地下水位以下深度 处有静止地下水，那么对于 ： （13） 若 （14） 于是 （15） 这个公式是通用的，不受限于均匀土体或是静止水压，也或是使土压力随深度而直线增大的变形方式。多个超荷载可以重叠，但每一个必须均布。 用设计土体强度， 应从附A表中取值。图表中的 水平构成。 光滑竖直挡土墙和水平土体表面（β=0，α=900，δ=0）的特定情况，用朗肯公式： （16） 用于附A表中的设计摩擦角应根据3.2.5确定。不规则地面位置，图5中的图解过程可推导主动推力。选择A滑动面并由力三角形推导挡土墙上的推力。随其它滑动面重复图解过程，直到得到充分值，通过图表内插能够得到最小推力为止。使用滑动面不能少于三个，但一般没必要多于五个。墙背上的压力中心位置，可以取墙后画直线通过楔形重心到楔形滑动面的交叉点。 还以一个方法是把水平面上的附加土体视为超荷载，见3.3.4。在只有叠加线荷载，且距墙一定距离及平行与墙的地方，此荷载每单位长度的值包括图表中的力W。 如果墙后有若层无粘性土，前述过程可用于与墙接触的最上层土，并且，除了墙明显地从竖向倾斜，上层主动土压力可以通过低层依式（15）计算，用一个假设的平均地面标高，来估算上覆土层压力。 图5 粘性土主动土压力的变形图解 重力式挡土墙在有限深度软弱土层上的承受能力破坏 重力式挡土墙沿BC倾斜上AB面上的剪力键破坏 _1234567905.unknown _1234567913.unknown _1234567921.unknown 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