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

4 Verification of Safety and Serviceability

4.3 Calculation Methods

4.3.4 Factors of Safety

(1) General. The reliability of a geotechnical design depends not only on the factor of safety, but also on the methods of analysis, the calculation models, the way in which factors of safety are defined, the reliability of the geological model, the assessment of appropriate geotechnical parameters and the quality achieved in construction. Therefore, the minimum factors of safety recommended in this Geoguide should not be used out of context. Where the designer wishes to adopt different minimum factors of safety, the following factors should be considered :

(a) the consequences of the limit states being reached, (b) possible slight increases in load beyond those covered by the design, (c) reliability of the geological model, (d) inaccurate assessments of loading and unforeseen stress redistribution within the retaining wall, (e) the uncertainties in the method of analysis and the applicability of the design calculation model, (f) possible differences between the strengths of the materials in the actual ground and structure and the strengths derived from test specimens, (g) the level of supervision to be provided and the likely quality of workmanship, e.g. variations in dimensional accuracy, target compaction, etc., and (h) the design life of the wall.

It should be noted that factors of safety cannot cover gross errors and non-compliance with specifications.

(2) Partial Safety Factors. The partial safety factor approach, allowing different safety factors for loading and material properties, commensurate with different reliabilities and consequences, is adopted in this Geoguide. Factored values of loading and geotechnical parameters, as defined below, should be used directly in the design calculations :

The recommended minimum partial factors of safety for ultimate limit state design of retaining walls are given in Tables 6 and 7. The use of the partial safety factor approach for calculating 'ultimate' values of earth pressures and sliding resistance is illustrated below.

The first step involves applying the partial material factor γm to the selected values (see Section 5.3 for guidance on its determination) of c' and tan ϕ' to obtain the factored shear strength parameters c_f' and tan ϕ_f' :

The angle of skin friction δ_s and wall friction δ are then assessed using ϕ_f' (see Section 5.11). Finally, the ultimate earth pressures should be obtained using the factored c_f' and ϕ_f' values and the δ_s or δ values assessed earlier.

The ultimate sliding resistance of a gravity or reinforced concrete retaining wall is assessed using the factored angle of base shearing resistance δ_bf. δ_bf should be calculated from tan^-1[(tan δ_b)/γ_m] using the selected δ_b value, which should be based on the unfactored selected value of the relevant soil or interface shear strength parameters (see Section 5.12).

For the design of slopes, reference should be made to the Geotechnical Manual for Slopes (GCO, 1984), which gives the relevant recommended minimum factors of safety.

For serviceability limit state checks, all values of γ_f and γ_m should be set to unity. An exception is that the value of γ_f should be set to zero for those surcharge loads which produce a favourable effect, e.g. surcharge in the area between the wall stem of a R.C. L-shaped retaining wall and its virtual back.

Appropriate partial factors should be applied to the loadings and selected values of material parameters used in the various calculation models given in this Geoguide, depending on whether the calculation is to check against an ultimate or a serviceability limit state.

4.3.5 Structural Design

Structural design of retaining walls should be carried out in accordance with the requirements of relevant structural codes and standards.

In structural design, the loadings on retaining walls due to earth and water pressures, and the effects of surcharge and seismic load, should be evaluated in accordance with the principles given in this Geoguide. Generally, unfactored parameters should be used in the calculations, but this would depend on the requirements of the structural code adopted. In the commonly-used limit state structural design codes, the ultimate limit state loads are evaluated based on unfactored parameters. These are then multiplied by appropriate partial load factors in the checking of limit states.

4.4 Prescriptive Measures

For certain limit states, e.g. failure due to chemical attack, calculation models are either not available or unnecessary. Instead, the limit state can be avoided by adopting 'prescriptive measures'. These may involve conventional or generally conservative details in the design, and attention to specification and control of materials, workmanship, protection and maintenance procedures.