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

2 Design Considerations

2.1 General

The purpose of a retaining wall is to withstand the forces exerted by the retained ground, and to transmit these forces safely to the foundation. Excessive deformation or movement of the retaining wall and of the adjoining ground is also to be avoided.

The cost of constructing a retaining wall is usually high compared with the cost of forming a new slope. Therefore, the need for a retaining wall should be assessed carefully during design and an effort should be made to keep the retained height as low as possible.

Special considerations are often necessary for retaining walls to be constructed close to land boundaries, particularly in urban areas. Land-take requirements often place limitations on the use of certain forms of earth retention. The permission of the adjacent land owner will need to be sought for excavation limits or structural elements to be extended into the adjacent land.

2.2 Performance Criteria, Limit States and Ductility

2.2.1 Performance Criteria

A retaining wall, and each part of it, is required to fulfil fundamental requirements of stability, stiffness, durability, etc., during construction and throughout its design life.

The performance criteria which need to be considered in the design are indicated for each type of retaining wall in Chapters 9 to 11. Guidance on design calculation models and prescriptive measures which can normally be used to ensure that the performance criteria will be met are also given in these chapters. However, alternative design approaches can also be used where they can be justified (see Chapter 4). Such alternatives will usually necessitate additional geotechnical and soil-structure interaction analyses and calculations, or additional supervision and monitoring of site works. Wherever possible, the design should be critically examined in the light of local experience.

2.2.2 Limit State Method

When a retaining wall, or part of it, fails to satisfy any of its performance criteria, the wall is deemed to have reached a 'limit state'. This Geoguide adopts the 'limit state method' in which various limit states are considered separately in the design and their occurrence is either eliminated or is shown to be sufficiently unlikely. The limit states discussed cover, as far as possible, the full range of possible limit states for each type of retaining wall. However, the designer must judge whether additional limit states should be taken into account based on the particular site conditions or other requirements not covered herein.

The following two main classes of limit state should be considered :

(a) Ultimate limit state - a state at which a failure mechanismcan form in the ground or in the retaining wall, or severe structural damage (e.g. yielding or rupture) occurs in principal structural elements.

(b) Serviceability limit state - a state at which specified serviceability criteria are no longer met.

For simplicity in design, states prior to collapse are often considered in place of the collapse itself, e.g. forward sliding of a gravity retaining wall. These states are also classified and treated as ultimate limit states.

Serviceability limit states include strains or movements in a retaining wall which would render the wall unsightly, result in unforeseen maintenance or shorten its expected life. They also include deformations in the ground which are of concern, or which affect the serviceability of any adjacent structures or services.

2.2.3 Ductility

As far as possible, retaining walls should be designed in such way that adequate warning of danger (i.e. approaching an ultimate limit state) is given by visible signs. The design should guard against the occurrence of brittle failure, e.g. sudden collapse without conspicuous preliminary deformations. Retaining walls designed in accordance with this Geoguide should exhibit sufficient 'ductility' in approaching geotechnical limit states to give visible warning and no additional provision should be necessary.

2.3 Site Investigation and the Geological Model

Guidance on the planning of site investigations and the conduct of desk study for retaining walls, and on the execution of ground investigations, is given in Geoguide 2: Guide to Site Investigation (GEO, 2017d).

A site investigation for retaining wall construction will normally proceed in stages, as follows : desk study; site reconnaissance; collection of field data for design, including ground investigation, topographic and hydrographic survey; and if necessary, follow-up investigations during construction.

The ground investigation should aim to establish the suitability of the site for the type of retaining wall being considered in the design, including the adequacy of the overall stability of the site with respect to the proposed works, the suitability of the foundation, and the suitability of materials on the site for fill. Preliminary design of the proposed works is often helpful in identifying parameters that need to be obtained from the ground investigation.

The investigation should identify specific groundwater and surface drainage conditions in the vicinity of the site, and their likely response to, for example, heavy rain or tidal action. In some cases, chemical testing of groundwater or surface water may be warranted (e.g. for salinity, sulphate ion content and pH). Further guidance on the chemical testing of groundwater is given in Geoguide 2 (GEO, 2017d).

To avoid confusion and misinterpretation in the communication of design information, it is essential that rocks and soils are adequately described. Guidance on this is given in Geoguide 3 : Guide to Rock and Soil Descriptions (GEO, 2017a).

One of the end-products of a ground investigation is one or more geological models. It is important to state clearly in the geotechnical design report (see Section 4.6) the geological models considered in the design. In areas of insitu weathered rock in Hong Kong, a stratified model consisting of distinct layers may not be realistic.