As 4678

AS 4678—2002

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J10.2 Design surcharge loads

The distribution of stresses on a retaining structure due to surcharge loads should be

carefully assessed in design. The design load factors are 1.5 for strength and stability limit

states, and 0.7 for the serviceability limit state. See Table J1 for examples of load

components for each limit state.

J10.3 Assessment of the effect of surcharge for conventional retaining structures

Each pressure induced by surcharge loads will depend on the load spreading properties of

the retained earth and the stiffness of the wall. Two approaches may be used to assess the

magnitude and distribution of lateral pressures induced by surcharge loading, as follows:

(a) Rankine active earth pressure theory—the trial wedge method and associated force

polygon methods may be employed (see Figure J6).

(b) Elasticity theory supported by experimental measurements—the earth pressure design

charts given by Terzaghi (Ref. 4) given in Figure J7 may be used to estimate lateral

pressure due to vertical line loads, point loads and horizontal line loads. These have

been modified from the Boussinesq (Ref. 5) solution for distribution of stresses in an

isotropic semi-infinite elastic medium based on experimental evidence.

Numerical modelling approaches (finite element analysis) may be used to assess the

magnitude and distribution of earth pressures for complex loaded retaining structures.

J10.4 Assessment of the effect of applied loadings on reinforced soil structures

For reinforced soil-retaining structures, the internal stresses are derived from two separate

load conditions, resulting from—

(a) the supporting function, and

(b) the retaining function.

These stresses should be superimposed for the determination of the total stresses due to

applied loadings and for the calculation of reinforcement stresses.

The supporting function requires the determination of the additional vertical and horizontal

stresses resulting from the diffusion of the applied loadings.

For vertical loadings, the distribution of vertical strip loadings through the fill may be

determined by a lateral dispersion defined by a slope of 2 vertically to 1 horizontally (see

Figure J8), or by calculating an appropriate stress distribution such as defined by

Boussinesq (Ref. 5). Vertical load dispersion has to take into account the effect of the face

on the available dispersion area.

For horizontal loadings, the distribution of horizontal strip loadings through the fill may be

determined by a vertical dispersion over the outside surface or facing of the structure

defined by a slope from the rear of the contact surface. The horizontal pressure distribution

on the outside surface or facing has to vary linearly from a maximum at the level of the

contact surface to zero at its lowest point (see Figure J8).

The retaining function takes into account the overall retaining structure loadings such as

self weight, superstructure and general surcharge loadings. These should then be taken in

combination with the overturning moments generated by the applied loads on the structure.

The diffusion of stresses imposed by applied loadings depends on the stiffness

characteristics of the structure. Further guidance on the selection and use of appropriate

design methods is provided in BS 8006.