Basis of design

Eurocode 7 requires gravity walls to be designed so that ultimate limit states of bearing and sliding ('foundation failures') will not occur. Verification of these limit states must follow the Principles of §6 of EN 1997-1, 'Spread foundations', which are discussed in Chapter 10. [en 1997-1 §9.7.3(1)P]

Additionally, gravity walls must not fail due to overall instability of the ground in which they are installed. [EN 1997-1 §9.7.2(1)P]

Regrettably, Eurocode 7 gives little detailed guidance on the design of gravity retaining walls. However, it does indicate the level of reliability that is required whichever design method is adopted. Reliability is ensured through a combination of partial factors on parameters (see Chapter 6), tolerances on dimensions (see Section 11.3.1), and selection of suitable water levels (see Section 11.3.2).

This book does not attempt to provide complete guidance on the design of gravity walls, for which the reader should refer to any well-established text on the subject.1

11.3.1 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 11.4).

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

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 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.

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 80022 were changed in 2001 to match those of (the then draft) Eurocode 7 Part 1.

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