Civil Engineering Association

The purpose of this document is to give guidance for the design and construction of foundations in Hong Kong. It is aimed at professionals and supervisory personnel involved in the design and construction of foundations. The document has been prepared on the assumption that the reader has some general knowledge of foundations. Foundations can be classified as shallow and deep foundations, depending on the depth of load-transfer from the structure to the ground. The definition of shallow foundations varies in different publications. BS 8004 (BSI, 1986) adopts an arbitrary embedment depth of 3 m as a way to define shallow foundations. In the context of this document, a shallow foundation is taken as one in which the depth to the bottom of the foundation is less than or equal to its least dimension (Terzaghi et al, 1996). Deep foundations usually refer to piles installed at depths and are :
(a) pre-manufactured and inserted into the ground by driving, jacking or other methods, or
(b) cast-in-place in a shaft formed in the ground by boring or excavation.
Traditional foundation design practice in Hong Kong relies, in part, on the British Code of Practice for Foundations (BSI, 1954), together with empirical rules formulated some 40 years ago from local experience with foundations in weathered rocks. Foundation design and construction for projects that require the approval of the Building Authority shall comply with the Buildings Ordinance and related regulations. The Code of Practice for Foundations (BD, 2004a) consolidates the practice commonly used in Hong Kong. Designs in accordance with the code are 'deemed-to-satisfy' the Buildings Ordinance and related regulations. Rational design approaches based on accepted engineering principles are recognised practice and are also allowed in the Code of Practice for Foundations. This publication is intended as a technical reference document that presents modern methods in the design of foundation. Rational design approaches require a greater geotechnical input including properly planned site investigations, field and laboratory testing, together with consideration of the method of construction. The use of rational methods to back-analyse results of loading tests on instrumented foundations or the monitored behaviour of prototype structures has led to a better understanding of foundation behaviour and enables more reliable and economical design to be employed. This should be continued to further enhance the knowledge such that improvements to foundation design can be made in future projects. A thorough understanding of the ground conditions is a pre-requisite to the success of a foundation project. An outline of geological conditions in Hong Kong is given in Chapter 2, along with guidance on the scope of site investigations required for the design of foundations. Shallow foundations are usually the most economical foundation option. The feasibility of using shallow foundations should be assessed. Chapter 3 provides guidance on some key design aspects and clarifying the intent of the methods. In Hong Kong, tall buildings in excess of 30 storeys are commonplace both on reclamations and on hillsides. Steel and concrete piles are generally used as building foundations. Timber piles, which were used extensively in the past to support low-rise buildings and for wharves and jetties, are not covered in this document. Guidance on the types of foundations commonly used in Hong Kong is given in Chapter 4. Factors to be considered in choosing the most appropriate pile type and the issue of design responsibility are given in Chapter 5, along with guidance on assessing the suitability of reusing existing piles. Guidance on methods of designing single piles and methods of assessing pile movement are given in Chapter 6. The design of pile groups and their movement are covered in Chapter 7. Given the nature of the geology of the urban areas of Hong Kong where granular soils predominate, emphasis has been placed on the design of piles in granular soil and weathered rock, although pile design in clay has also been outlined for use in areas underlain by argillaceous rock. Consideration of the practicalities of pile installation and the range of construction control measures form an integral part of pile design, since the method of construction can have a profound influence on the ground and hence on pile performance. A summary of pile construction techniques commonly used in Hong Kong and a discussion on a variety of issues to be addressed during construction, together with possible precautionary measures that may be adopted, are given in Chapter 8. In view of the many uncertainties inherent in the design of piles, it is difficult to predict with accuracy the behaviour of a pile, even with the use of sophisticated analyses. The actual performance of single piles is best verified by a loading test, and foundation performance by building settlement monitoring. Chapter 9 describes the types of, and procedures for, static and dynamic loading tests commonly used in Hong Kong.