ABSTRACT:

Reinforced concrete codes of practice in the UK and US consider limited definitions of the ductility of reinforced concrete sections involving minimum quantities of steel for flexural sections. This paper discusses an alternative method of defining a ductile section that is unique to underground structures. This paper describes the interaction between the ground and shotcrete tunnel linings for tunnels driven in materials that deform and are displaced as a result of excavation, such as soft ground and weak rock in shallow civil engineering scenarios. The relative stiffness of shotcrete linings is discussed and definitions for soft and stiff linings are given. The limitations of codes of practice and standards for design of structural concrete when applied to tunnel lining design are discussed. The suggested method presented here considers the relative stiffness of tunnel lining and the surrounding ground under which conditions a tunnel lining can be considered to deform in a ductile manner regardless of the structural reinforcement content.

DEFINITION OF DUCTILITY USED IN CODES OF PRACTICE

The use of codes of practice in design of tunnel linings poses a series of interesting questions and problems of applicability, as there is no designated code of practice for tunnelling or tunnels design. British and U.S. practice generally use the codes for design of reinforced concrete structures with occasional reference to highway or bridge design codes for definition of load cases. There has been a large amount of research and empirical study of the incompatibilities of these codes of practice. exceeded by the code of practice minimum steel requirements. In addition, the minimum steel for section ductility may require even more steel to satisfy serviceability and thermal cracking requirements. Standard design practice leads to over conservative factors of safety against maximum conceivable earth pressure in many tunnels.

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