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

Foreword

This Geoguide presents a recommended standard of good practice for the design of retaining walls in Hong Kong. The first edition of the Geoguide was published in July 1982 and preparation for a second edition begun in 1985 with a consultation exercise to obtain the views of practitioners on the first edition. Work on the revision began in 1987.

The Geoguide covers the types of retaining walls which are commonplace in Hong Kong, including conventional reinforced concrete walls, gravity walls such as crib walls, gabion walls and mass concrete walls, and cantilevered retaining walls. It does not aim to deal comprehensively with maritime structures nor structures used to support the sides of deep excavations, such as sheet pile walls. The recommendations given herein are also not intended for retaining wall design which is outsite existing experience derived from practive in Hong Kong. In giving recommendations on design this Geoguide adopts the limit state method with partial factors of safety.

Reference has been made in preparing this Geoguide to the results of research on retaining walls and to various national codes and guidance ducuments. These include the UK draft revised CP2, the Canadian Foundation Engineering Manual, the Danish Code of Practive for Foundation Engineering, the US Navy Design Manual 7.1 and 7.2, the then New Zealand Ministry of Works and Development's Retaining Wall Design Notes and the draft Eurocode No. 7.

The Geoguide was prepared by a team in the Special Projects Division of the Office led by Dr P.L.R. Pang under the overall supervision initially of Dr R.P. Martin and later Mr Y.C. Chan and including at various times Mr W.T. Chan, Mr W.K. Pun and Dr C.Y. Yung. The membership of the Steering Committee at various times over the past five years is givin on the opposite page.

To ensure that the Geoguide would be considered a consensus document by interested parties in Hong Kong, a draft version was circulated locally and abroad for comments in early 1992. Those consulted included consulting engineers, contractors, academics, professional bodies and Government departments. Many individuals and organisations made very useful comments, which have been taken into account in finalising the Geoguide, and their contributions are gratefully acknowledged.

As with other Geoguides, this document gives guidance on good engineering practive, and its recommendations are not intended to be mandatory. It is recognised that experienced practitioners may wish to use alternative mathods to those recommended herein. Practitioners are encouraged to comment at any time to the Geotechnical Engineering Office on the contents of this Geoguide, so that improvements can be made to future editions.

by A.W. Malone, Principal Government Grotechnical Engineer, October 1993.

1 Introduction

1.1 Purpose and Scope

The purpose of this Geoguide is to recommend a standard of good practice for the design of retaining walls in Hong Kong. The document is aimed at qualified civil, geotechnical and structural engineers who are conversant with the relevant engineering principles and procedures.

The geotechnical standards set out in this Geoguide are for new permanent earth retaining walls on land. Design of maritime structures and assessment of existing retaining walls are beyond its scope. Temporary structures which are used to support the sides of deep excavations (e.g. strutted and anchored walls formed with sheet piles, soldier piles, grouted piles or diaphragm wall panels) are also not covered. A review of the design methods for such structures is given in GEO Publication No. 1/2023: Deep Excavation Design and Construction.

General considerations in the design of retaining walls are outlined in Chapters 2, 3 and 4. The limit state approach incorporating partial safety factors has been adopted in this Geoguide. Appropriate partial safety factors for different types of loading and material parameters are given in Chapter 4.

In carrying out the structural design for a retaining wall, the loadings due to soil, water and surcharge should be calculated using the principles and procedures givin herein. If a limit state structural design code is adopetd, appropriate partial safety factors on loading and structural material parameters as recommended in the code should be used. The loadings calculated in accordance with this Geoguide may also be used in conjunction with structural codes adopting the permissible stress approach to design.

Guidance on the evaluation of geotechnical parameters, earth pressures, and the effects of surcharge, seismic load and water is given in Chapters 5, 6, 7 and 8.

Many types of retaining walls are in use in Hong Kong. Those covered by this Geoguide are:

(a) gravity retaining walls, which include mass concrete retaining walls, crib walls, gabion walls and reinforced fill structures,

(b) reinforced concrete (R.C.) retianing walls, which include R.C. L-shapred and inverted T-shaped cantilever, counterfort and bettressed retaining wall, and

(c) cantilvered retaining walls, e.g. bored pile walls.

Guidance on the choice of the type of wall to be used is given in Section 2.4. Selected design and construction aspects of the various types of retaining walls listed above are dealt with in Chapters 9, 10 and 11 respectively.

Masonry retaining wall are not covered in this Geoguide.

As with other forms of construction, adequate supervision should be provided during construction of a retaining wall. Where necessary, the retaining wall should be monitored during and after construction. Retaining walls should be regularly maintained. Guidance on these subjects is given in Chapters 12 and 13.

There are a few terms used with specific meanings in this Geoguide or in Hong Kong. Their meanings are given in the Glossary of Ters at the end of this document.

1.2 General Guidance

1.2.1 General

Recommendations on different aspects of design are given throughout this document, and frequent reference is made to relevant codes, textbooks and other published information. The reader should consult these original publications for more detailed coverage of particular aspects of the subject matter.

Engineering judgement should always be exercised in applying the theories and design methods presented. In particular, the practitioner should be aware of the limitations of the basic assumptions employed in a particular theoretical or computational method.

1.2.2 Personnel

The various stages of planning, investigation, design and construction of a retaining wall demand a range of skills. It is essential that personnel involved in the design have geotechnical knowledge appropriate to the project in hand. In particular, personnel involved in ground investigation should have appropriate specialized knowledge and experience, see Geoguide 2 (GEO, 2017d).

It is essential that the relevant information and design assumptions are communicated clearly at all stages to safeguard against misunderstanding and the misuse of information. This is particularly important where the geotechnical and structural design are not carried out by the same person. The personnel responsible for supervision of site work must be aware of the critical design assumptions and check their validity on site.

1.2.3 Local Experience

In geotechnical designs, previous experience of the construction and performance of similar structures in similar conditions is often quoted. The term ''local experience'' refers to documented or other clearly-established information related to the ground conditions and type of structure being considered in design, involving the same geological materials and for which similar geotechnical behaviour is expected. Knowledge of local experience can be invaluable in the design of retaining walls.

1.2.4 Geology

The geology of Hong Kong is very briefly summarised in Figure 1. Much of the accumulated local experience in geology has been derived within a fairly restricted area of the territory, broadly the older developed areas of urban Hong Kong Island, Kowloon Peninsula and Tsuen Wan. With spread of development into the Northwest New Territories and Lantau Island, areas with less familiar geology are being encountered. Due care must be exercised in carrying out geotechnical design in areas with less familiar geology and ground conditions.