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

4 Verification of Safety and Serviceability

4.3 Calculation Methods

4.3.1 Available Approaches to Design Calculations

Limit states relevant to a design situation are usually checked by calculation methods. Two approaches, viz. the direct approach and the indirect approach, may be followed in design calculations. Chapters 9 to 11 indicate where each of the following approaches is applicable.

In the direct approach, each limit state is checked by direct calculations which must show that the limit state would not be reached. Limit states involving the formation of failure mechanisms in the ground are often checked using this approach. For limit states involving considerations of deformations, the deformations may either be calculated or otherwise assessed if this approach is used.

In some cases, especially for routine design, rigorous analysis may not be justified and a simple indirect approach may be more appropriate. The indirect approach is particularly useful for serviceability checks, e.g. where the deformation criteria may be deemed to be satisfied by limiting the eccentricity of the reaction at wall base. An example of this approach is the application of the middle-third rule in the design of gravity retaining walls (see Section 9.2.4).

4.3.2 Design Calculation Models

A design calculation model generally consists of two elements : (a) a method of analysis, often based on a theoretical approach including simplifications, and (b) if necessary, a modification to the results of the analysis to ensure that the results of the calculation are either accurate or err on the side of safety.

Where an unconventional theory is to be used, or where a conventional theory is to be applied in an untried situation, the uncertainty in the applicability of the design calculation model should be assessed. Whenever possible, the method of analysis should be calibrated against observations of field performance of previous similar designs, load tests or more reliable analysis.

Design calculation models are given in Chapters 9 to 11 for the checking of limit states for various types of retaining walls.

4.3.3 Loadings, Geometric and Geotechnical Parameters

(1) General. Some general guidance is given in the following Sections. Specific guidance on the selection of values of geotechnical parameters for design, and on the evaluation of various types of loadings, viz. earth pressures, surcharge (and seismic) and water loads, is given in Chapters 5, 6, 7 and 8 respectively. Where appropriate, parametric studies of the effect of a variable loading or parameter should be carried out.

(2) Loadings and Load Combinations. For each design situation, concentrated or distributed loads which may result in forces acting on the retaining wall should be evaluated. The common types of direct loads are : (a) weights of soil, rock and water, (b) earth pressures, (c) free water and groundwater pressures, (d) seepage forces, and (e) surcharge and seismic loads.

Indirect loadings that need to be tolerated by the retaining wall should also be considered and, where appropriate, their magnitudes should be determined. These include loadings arising from : (a) geological factors, e.g. creep of slope masses, solution of soluble rock and collapse of sinkholes in cavernous rock, (b) conceivable man-made activities, e.g. dewatering, excavation, tunnelling and blasting, and (c) temperature effects.

No guidance is given on how to evaluate the magnitudes of these indirect loadings and major dynamic loads (e.g. from machine foundations), nor their effects on retaining walls. The designer should refer to specialist literature on these subjects.

In selecting values of loadings for design, the duration of the loadings should be considered with reference to possible changes in soil properties with time, especially the drainage properties and compressibility of fine-grained soils. It may be helpful to distinguish between very short-term transient loads (e.g. wind loads) during which soils are likely to display enhanced strength and stiffness, short-term loads (e.g. construction loads) during which soil drainage is likely to be negligible, and long-term loads.

It is sometimes necessary to check several load combinations in a design situation and to design for the most critical condition.

(3) Geometric Parameters. The most important geometric parameters in geotechnical design are usually the level and slope of the retained ground surface, the geometric characteristics of the geological model, and the levels of excavations. While small variations in such parameters are generally covered by the factors of safety applied, gross changes in levels should be allowed for in the design directly. For limit states with severe consequences, geometric parameters should represent the most unfavourable values which could occur in practice. In some cases, it may be economical to design for a range of anticipated levels, e.g. founding levels in the design of rock-socketed caissons, and to incorporate in the contract permissible variations in design subject to the actual conditions encountered during construction.

(4) Geotechnical Parameters. The parameters relevant to retaining wall design include unit weight, shear strength, permeability, insitu stresses and deformation parameters of soil and rock. Detailed guidance on the selection of such parameters for design is given in Chapter 5.