9.4 Design and construction considerations
(1)P Both ultimate and serviceability limit states shall be considered using the procedures described in 2.4.7 and 2.4.8.
(2)P It shall be demonstrated that vertical equilibrium can be achieved for the assumed pressure distributions and actions on the wall.
(3) The verification of vertical equilibrium may be achieved by reducing the wall friction parameters.
(4) As far as possible, retaining walls should be designed in such a way that there are visible signs of the approach of an ultimate limit state. The design should guard against the occurrence of brittle failure, e.g. sudden collapse without conspicuous preliminary deformations.
(5) For many earth retaining structures, a critical limit state should be considered to occur if the wall has displaced enough to cause damage to nearby structures or services. Although collapse of the wall may not be imminent, the degree of damage may considerably exceed a serviceability limit state in the supported structure.
(6) The design methods and partial factor values recommended by this standard are usually sufficient to prevent the occurrence of ultimate limit states in nearby structures, provided that the soils involved are of at least medium density or firm consistency and that adequate construction methods and sequences are adopted. Special care should be taken, however, with some highly over-consolidated clay deposits in which large at rest horizontal stresses may induce substantial movements in a wide area around excavations.
(7) The complexity of the interaction between the ground and the retaining structure sometimes makes it difficult to design a retaining structure in detail before the actual execution begins. In this case use of the observational method for the design (see 2.7) should be considered.
(8)P The design of retaining structures shall take account of the following items, where appropriate:
- the effects of constructing the wall, including:
- the provision of temporary support to the sides of excavations;
- the changes of in situ stresses and resulting ground movements caused both by the wall excavation and its construction;
- disturbance of the ground due to driving or boring operations;
- provision of access for construction;
- the required degree of water tightness of the finished wall;
- the practicability of constructing the wall to reach a stratum of low permeability, so forming a water cut-off. The resulting equilibrium ground-water flow problem shall be assessed;
- the practicability of forming ground anchorages in adjacent ground;
- the practicability of excavating between any propping of retaining walls;
- the ability of the wall to carry vertical load;
- the ductility of structural components;
- access for maintenance of the wall and any associated drainage measures;
- the appearance and durability of the wall and any anchorages;
- for sheet piling, the need for a section stiff enough to be driven to the design penetration without loss of interlock;
- the stability of borings or slurry trench panels while they are open;
- for fill, the nature of materials available and the means used to compact them adjacent to the wall, in accordance with 5.3.
9.4.2 Drainage systems
(1)P If the safety and serviceability of the designed structure depend on the successful performance of a drainage system, the consequences of its failure shall be considered, having regard for both safety and cost of repair. One of the following conditions (or a combination of them) shall apply:
- a maintenance programme for the drainage system shall be specified and the design shall allow access for this purpose;
- it shall be demonstrated both by comparable experience and by assessment of any water discharge, that the drainage system will operate adequately without maintenance.
(2) The quantities, pressures and eventual chemical content of any water discharge should be taken into account.