(c) Bentonite displaced by concrete

(d) Soil excavated in front of wall

Fig. 7.4 The construction stages of a diaphragm wall.

7.3.3 Contiguous and secant bored pile walls Contiguous bored pile walls

This type of wall is constructed from a single or double row of piles placed beside each other. Alternate piles are cast first and the intermediate piles are then installed. The construction technique allows gaps to be left between piles which can permit inflow of water in granular conditions. The secant bored pile wall offers a watertight alternative.

Secant bored pile walls

The construction technique is similar to that of the contiguous bored pile wall except that the alternate piles are drilled at a closer spacing. Then, while the concrete is still green, the intermediate holes are drilled along a slightly offset line so that the holes cut into the first piles. These holes are then concreted to create a watertight continuous wall.

7.4 Design of earth retaining structures

The traditional approach for the design of earth retaining structures involved establishing the ratio of the restoring moment (or force) to the disturbing moment (or force) and declaring this ratio as a factor of safety. This factor had to be high enough to allow for any uncertainties in the soil parameters used in the analysis, and the approach was generally referred to as the factor of safety approach. (Guidance on this method was previously given in the Code of Practice CP2 Earth retaining structures (1951) which has now been revised as BS8002: 1994.) An alternative approach now becoming widely adopted is the limit state design approach. This method is advocated in both BS 8002 and the Eurocode Programme.

The Eurocode Programme was initiated to establish a set of harmonised technical rules for the design of building and civil engineering works across Europe. The rules are known collectively as the Structural Eurocodes which comprise a series of nine design documents. Work is continuing on developing the series of Eurocodes including Eurocode 7 Geotechnical design. This code is at a draft stage and it is recommended that the reader refers to the latest draft to gain a full understanding of the procedures and the most recent published values of partial factors.

In the limit state design method described in Eurocode 7, partial factors are applied to characteristic values of actions and ground properties to yield the design values of each. This approach allows for the effects of uncertainties in the magnitudes of the characteristic values.

• Actions include soil weight, stresses in the ground, surcharges, pore water pressures and seepage forces and are categorised as either permanent (e.g. dead loads) or variable (e.g. imposed loads). Further, actions are considered as having either unfavourable or favourable effects with respect to limit states. The characteristic value of the action is multiplied by the partial factor appropriate for the nature of the action, to give the design value.

• Ground properties are cu, c' and tan <p. The characteristic value of the property is divided by the appropriate partial factor, to give the design value.

In BS8002: 1994 the design values of loads are intended to be the most pessimistic and unfavourable, whether derived by factoring or otherwise. To satisfy the requirements of both the ultimate and serviceability limit states, the design soil strength values are obtained from consideration of the representative values for peak and ultimate strength. The design values are taken as the lower of:

(a) the soil strength mobilised at a strain acceptable for serviceability: this can be expressed as the peak strength reduced by a mobilisation factor, M;

(b) the soil strength which would be mobilised at collapse, following significant ground movements: this can generally be taken as the critical state strength.

The value of the mobilisation factor, M, depends on whether the design is concerned with undrained or drained conditions. For undrained conditions, the design clay strength (design cu) is taken as the representative undrained strength divided by a value of M not less than 1.5, if the wall displacement is not to exceed 0.5 per cent of the wall height.

For drained conditions, the lesser of two values of soil strength should be used:

(a) the representative peak strength divided by a value of M = 1.2, i.e.

Representative tan 0'max

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