9.5.3 Limiting values of earth pressure

(1)P Limiting values of earth pressures shall be determined taking account of the relative movement of the soil and the wall at failure and the corresponding shape of the failure surface.

(2) Limiting values of earth pressure assuming straight failure surfaces can significantly deviate from the values assuming curved failure surfaces for high angles of internal friction and wall-ground interface parameters δ, and so lead to unsafe results.

NOTE Annex C provides some data of relative movements that cause limiting values of earth pressures.

(3) In cases where struts, anchorages or similar elements impose restraints on movement of the retaining structure, it should be considered that the limiting active and passive values of earth pressure, and their distributions, may not be the most adverse ones.

9.5.4 Intermediate values of earth pressure

(1)P Intermediate values of earth pressure occur if the wall movements are insufficient to mobilise the limiting values. The determination of the intermediate values of earth pressure shall take account of the amount of wall movement and its direction relative to the ground.

NOTE Annex C, figure C.3, gives a diagram, which may be used for the determination of the mobilized passive earth pressure

(2) The intermediate values of earth pressures may be calculated using, for example, various empirical rules, spring constant methods or finite element methods.

9.5.5 Compaction effects

(1)P The determination of earth pressures acting behind the wall shall take account of the additional pressures generated by any placing of backfill and the procedures adopted for its compaction.

NOTE Measurements indicate that the additional pressures depend on the applied compactive energy, the thickness of the compacted layers and the travel pattern of the compaction plant. Horizontal pressure normal to the wall in a layer may reduce when the next layer is placed and compacted. When backfilling is complete, the additional pressure normally acts only on the upper part of the wall.

(2)P Appropriate compaction procedures shall be specified with the aim of avoiding excessive additional earth pressures, which may lead to unacceptable movements.

9.6 Water pressures

(1)P Determination of characteristic and design water pressures shall take account of water levels both above and in the ground.

(2)P When checking the ultimate and serviceability limit states, water pressures shall be accounted for in the combinations of actions in accordance with 2.4.5.3 and 2.4.6.1, considering the possible risks indicated in 9.4.1 (5).

(3)P For structures retaining earth of medium or low permeability (silts and clays), water pressures shall be assumed to act behind the wall. Unless a reliable drainage system is installed (9.4.2 (1)P), or infiltration is prevented, the values of water pressures shall correspond to a water table at the surface of the retained material.

(4)P Where sudden changes in a free water level may occur, both the non-steady condition occurring immediately after the change and the steady condition shall be examined.

(5)P Where no special drainage or flow prevention measures are taken, the possible effects of water-filled tension or shrinkage cracks shall be considered.