Hong Kong Geoguide 1: Guide to Retaining Wall Design (2020 Edition)

4 Verification of Safety and Serviceability

4.1 General Principles

The general approach which should be used to verify the safety and serviceability of a proposed retaining wall is described in this Chapter. Essentially, it consists of the following steps :

(a) Firstly, a list of required performance criteria should be compiled.

(b) Secondly, the relevant limit states at which the various performance criteria would be infringed should be determined.

(c) Thirdly, it must be demonstrated that the limit states are sufficiently unlikely to occur.

In compiling the list of required performance criteria and relevant limit states, the designer should consider the various design situations which can be foreseen during the construction and use of the proposed retaining wall. In the last step, the limit states must be shown to be sufficiently unlikely in each design situation. This is normally achieved by the use of calculation methods (see Section 4.3). In some cases, prescriptive measures (Section 4.4) or the observational method (Section 4.5) are applied. Sometimes it may be appropriate to use these methods in combination. Guidance is given in Chapters 9 to 11 on the methods to be used for checking limit states for the design of various types of retaining walls.

The guidance given in this Geoguide cannot possibly cover all situations. The designer should select performance criteria, limit states, loadings and design parameters appropriate to particular conditions.

4.2 Design Situations

Situations considered in design should be sufficiently severe and varied so as to cover all reasonable conditions that may occur during construction and throughout the design life of the proposed retaining wall.

The following information is required in order to specify design situations :

(a) the nature of the environment within which the retaining wall is to be constructed over its design life, for example : (i) the geological profile and variations in soil and rock properties, (ii) changes in surface water and groundwater levels, and changes in pore water pressures, (iii) effects of chemical attack and electrolytic corrosion, (iv) excavation in front of and surcharge behind the retaining wall, (v) effects of planned development in the vicinity, (vi) effects of earthquakes,

(b) aspects of construction that are pertinent to the design, e.g. compaction behind the retaining wall, and

(c) users' functional requirements.

Each design situation should be clearly defined, together with the relevant types of loading.

Conceivable accidents should be considered in design and the retaining wall should be designed in such a way that it will not be damaged disproportionately or lose its stability under such accidental conditions. However, the effects of accidental loadings, e.g. pressures due to bursting of water mains, are often difficult to quantify, and consideration of combinations of accidental loadings usually results in costly designs. Hence, accidental loadings are usually best dealt with by prescriptive measures, e.g. re-routing or ducting of water services.