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1、Section 3 Design philosophy, design method and earth pressures3.1 Design philosophy3.1.1 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 cal
2、culations 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
3、 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 sufficientlySevere and varied so as to encompass all reaso
4、nable conditions which can be foreseen during the period of construction and the life of the retaining wall.3.1.2 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
5、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 l
6、imit 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
7、 a rigid body; b) failure by rupture of the structure or any part of it, including supports and foundations.3.1.3 Ultimate limit states3.1.3.1 GeneralThe 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 fa
8、ilure, 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 oc
9、cur where:1) the wall is built on sloping ground which itself is close to limiting equilibrium; or2) the structure is underlain by a significant depth of clay whose undrained strength increases only gradually with depth; or3) the structure is founded on a relatively strong stratum underlain by weake
10、r strata; 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; c) excessive deformation of the wall or ground such that adjacent structures or s
11、ervices reach their ultimate limit state.3.1.3.2 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 involves a bearing capacity failure, the calculations should establis
12、h 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 desi
13、gn 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 mobilizat
14、ion 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 statesThe following serviceability limit states should be considered: a) substa
15、ntial 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
16、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 ass
17、ured 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 su
18、bjected 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 deformationsThe deformation of an earth retaining structure is important because it has a direct effec
19、t 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 st
20、ructure 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
21、 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
22、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 alsoparticularly important where the structure behaves as a brittle
23、 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 parametersThese are applicable at the specified limit states in the specified design situations. All e
24、lements 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 inv
25、olved in the calculation; c) the quality of workmanship and level of control specified for the construction.3.1.7 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
26、 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 earthpressures which will not be exceeded in a limit state, if external loads are correctly predicted. External l
27、oads, 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 fo
28、r different load combinations,See 3.2.7 .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
29、 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 a
30、lso 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
31、 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 re
32、duced 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
33、.6. Design values should not exceed representative values of the fully softened critical state soil strength.3.1.9 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 e
34、arth 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 thr
35、eatens, 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 corre
36、spond 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 o
37、f 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 acc
38、ount 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 settlemen
39、t 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 pre
40、served, 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 deformat
41、ions 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 o
42、n 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 method3.2.1 Equilibrium calculationsIn order to determine the geometry of the retaining wall, for exampal the depth of penetration of an embedded wall
43、(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
44、 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
45、 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
46、 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 situations3.2.2.1 GeneralThe specification of design situations should include the disposition and classification of the various zones
47、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 combinations, e.g. surcharge and%o
48、r 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 corr
49、osion; 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 g
