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-29 -The boundary between the two materials should be constructed so thatthere is no inherent loss of strength on the surface. Benching the insitumaterial ensures that the failure surface is almost entirely through insitu orwell compacted material.3. JO PRESSURES PROPUCE1? EV COMPACTIONProper compaction of- backfill to a retaining wall is necessary inorder to increase the backfill shearing strength and to prevent its excessivesettlement later. Care should be taken to ensure that the compaction processdoes not cause damage to the wall, as pressures produced by compaction canvary considerably in magnitude and distribution and can be much larger thanthose predicted using classical earth pressure theories,Aggour & Brown (1974) give guidance on the formulation of numericalsolutions to compaction problems and include in their paper graphical solutionswhich indicate the influence of some factors affecting residual pressures , e.g.backfill geometry, wall flexibility , end wall restraint.Broms^ (1971) has presented a method for the determination of lateralearth pressures due to compaction against unyielding structures and proposesthe earth pressure distribution shown in Figure 14 (i) for use in design. Theassociated data relating to the figure are for a limited range of compactionplant.Ingold (1979) has presented a simple analytical method which can beused to give a working approximation of compaction induced pressures forroutine designs. The method is based on the following assumptions :(a)(b)An idealised stress path is followed in the compaction processBelow a critical depth Z c there is no reduction in horizontalstress after removal of compactive force. Ingold shows thatapproximate values of Z c may be obtained from the followingexpressions :(7)(c)The increase in vertical effective stress, A0 r v* at a depthZ due to a dead weight of vibratory roller applying a unitweight p/unit length may be obtained from the expression :