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*ULS = ultimate; SLS = serviceability limit state tDesign Approach B is not suitable for SLS calculations $A minimum surcharge of 10kPa should be applied to walls with retained height of 3m or more

If worst credible parameters are chosen for retaining wall design (C580's Design Approach B), then these could be regarded as design values for the purposes of Eurocode 7.

12.3.2 Unplanned excavation

EN 1997-1 requires an allowance to be made for the possibility of an unplanned excavation reducing formation level on the restraining side of a retaining wall (see Figure 12.3).

When normal levels of site control are employed, verification of ultimate limit states should assume an increase in retained height AH given by:

where H is the retained height of a cantilever wall (left hand side of Figure 12.3) or the height below the lowest support for a propped wall (right hand side of Figure 12.3). [en 1997-1 §9.3.2.2(2)]

Eurocode 7 also warns that, where the surface level is particularly uncertain, larger values of AH should be used. However, it also allows smaller values (including AH = 0) to be assumed when measures are put in place throughout the execution period to control the formation reliably.

The rules for unplanned excavations apply to ultimate limit states only and not to serviceability limit states. Anticipated excavations in front of the wall should be considered specifically - they are not, by definition, unplanned. Planned excavations include French drains, pipe trenches, buried close-circuit television cables, etc.

These rules provide the designer with considerable flexibility in dealing with the risk of over-digging. A more economical design may be obtained by adopting AH = 0, but the risk involved must be controlled during construction - leading to a supervision requirement that must be specified in the Geotechnical Design Report (see Chapter 16). A designer who wants to minimize the need for supervision must guard against the effects of over-digging by adopting AH = 10%H.

The rules for unplanned excavations in BS 80029 were changed in 2001 to match those of (the then draft) Eurocode 7 Part 1.

12.3.3 Selection of water levels

Selection of appropriate water levels is particularly important in retaining wall design and Eurocode 7 makes specific demands in this regard.

When the wall retains medium or low permeability soils (mainly fine soils), then the wall must be designed for a water level above formation level. If a reliable drainage system is provided, then the water level may be assumed to occur below the top of the wall. Without reliable drainage, the water must be taken at the surface of the retained material. [en 1997-1 §9.6(3)P]

It would be unusual to install a drainage system behind an embedded retaining wall (weep holes provided through a sheet pile wall should not be considered a 'reliable drainage system'). Eurocode 7's requirements appear far more onerous than would normally be adopted in current practice. It is rare for the natural water table to rise to ground surface and so, when it can be demonstrated to come below formation level, it seems excessive to design for such large sustained water pressures behind the wall.

As discussed in Chapter 11, current British practice10 recommends taking groundwater level, when drainage is absent, at a depth dw given by:

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