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-55-The factor of safety with respect to shear failure is given by :where the terms are defined in Figure 25. Where F s is less than 2 substantialdeformations may occur with consequent loss of ground, and the probability offailure exists. Where soft clay extends to considerable depth below theexcavation 9 the effect of increased sheeting stiffness,or depth* is minimal,However driving the sheeting into a hard stratum before commencing theexcavation can appreciably reduce the deformations.Control of the groundwater may be necessary to prevent piping orheave associated with groundwater flow. Methods to achieve this are discussedin Section 5.5.7.3 ANCHOREV FLEXIBLE WALLS7.3.1 WaJUU> Anckosi&d mast the. TopThe deformation of an anchored sheet pile depends on the relativestiffness of the pile/soil system. For a relatively rigid system, such as aheavy pile section in a loose sand, the earth pressure distribution correspondsclosely to the triangular active and passive conditions. The toe of the pileis assumed pinned, and the Free Earth Support design method as outlined byTeng (1962) is appropriate.As the stiffness of the system decreases the pressure distributionalters in such a way as to reduce the bending moment in the pile. As aconsequence, the sheet pile section used may be reduced as compared with aninfinitely stiff wall. Rowe's Theory of Moment Reduction (1952, 1955, 1957)takes this effect into account; it is summarised by Teng (1962) and in CIRIAReport No. 54 (1974).When calculating the toe penetration, it is recommended that nofactor of safety should be applied to the active pressures. The passiveresistance may be factored by 2.0, or, as recommended in the CIRIA report, thefollowing factored values of 0 ? and