9.8 Serviceability limit state design
(1)P The design of retaining structures shall be checked at the serviceability limit state using the appropriate design situations as specified in 9.3.3.
(2)P Design values of earth pressures for the serviceability limit state shall be derived using characteristic values of all soil parameters.
(3)P Permanent surcharge loads behind the retaining wall shall be taken at their characteristic values.
(4) The assessment of design values of earth pressures should take account of the initial stress, stiffness and strength of the ground and the stiffness of the structural elements.
(5) The design values of earth pressures should be derived taking account of the allowable deformation of the structure at its serviceability limit state. These pressures may not necessarily be limiting values.
(1)P Limiting values for the allowable displacements of walls and the ground adjacent to them shall be established in accordance with 2.4.8, taking into account the tolerance to displacements of supported structures and services.
(2)P A cautious estimate of the distortion and displacement of retaining walls, and the effects on supported structures and services, shall always be made on the basis of comparable experience. This estimate shall include the effects of construction of the wall. The design may be justified by checking that the estimated displacements do not exceed the limiting values.
(3)P If the initial cautious estimate of displacement exceeds the limiting values, the design shall be justified by a more detailed investigation including displacement calculations.
(4)P It shall be considered to what extent variable actions, such as vibrations caused by traffic loads behind the retaining wall, contribute to the wall displacement.
(5)P A more detailed investigation, including displacement calculations, shall be undertaken in the following situations:
- where nearby structures and services are unusually sensitive to displacement;
- where comparable experience is not well established.
(6) Displacement calculations should also be considered in the following cases:
- where the wall retains more than 6 m of cohesive soil of low plasticity,
- where the wall retains more than 3 m of soils of high plasticity;
- where the wall is supported by soft clay within its height or beneath its base.
(7)P Displacement calculations shall take account of the stiffness of the ground and structural elements and the sequence of construction.
(8) The behaviour of materials assumed in displacement calculations should be calibrated by comparable experience with the same calculation model. If linear behaviour is assumed, the stiffnesses adopted for the ground and structural materials should be appropriate for the degree of deformation computed. Alternatively, complete stress-strain models of the materials may be adopted.
(9)P The effect of vibrations on displacements shall be considered with regard to 6.6.4.