CHARACTERISTICS OF LINING BEHAVIOUR

A list of common characteristics that pervade all lining systems are (afterKuesel and King 1996):

The processes of ground pre-treatment (e.g. grouting), excavation andground stabilization (e.g. rock bolting) alter the pre-existing state of stressin the ground, before the lining comes into contact with the ground.

A tunnel lining is not an independent structure acted upon by well-definedloads. The loads acting on a tunnel are not well defined, and itsbehaviour is governed by the properties of the surrounding ground.Design of a tunnel lining is not a structural problem, but a ground-structure interaction problem, with the emphasis on the ground. Definingthe loads on the tunnel lining is one of the most challenging aspects fora civil engineer on a tunnel project.

Tunnel lining is a four-dimensional problem. During construction, theground conditions at the tunnel heading involve both transverse andlongitudinal arching, or cantilevering from the excavated face. All theground properties are time-dependent, particularly in the short-term,which leads to the commonly observed phenomenon of stand-up time,without which most practical tunnel construction methods would beimpossible. The timing of the lining installation is an important variable.In addition, some tunnel linings such as sprayed concrete lining (shot -crete) can itself have time-dependent characteristics. In the case ofsprayed concrete lining the stiffness is extremely time-dependent andthe effects of early creep have to be taken into account before the liningreaches its full strength.

The most serious structural problems encountered with actual liningbehaviour are related to the absence of support rather than to theintensity and distribution of the load, for example inadvertent voidsleft behind the lining. However, exceptions exist and poor groundconditions could result in additional, unexpected loading on the lining.

In most cases in hard rock, the bending strength and stiffness ofstructural linings are small compared with those of the surroundingground. The properties of the ground therefore control the deformationof the lining, and changing the properties of the lining will notsignificantly change this deformation. It is important that the lininghas adequate ductility to conform to the imposed deformations, andadequate strength to resist bending stresses is therefore secondary. Thelining therefore forms a flexible ring confined by the ground.

4.3.2 Sprayed concrete (shotcrete)

Sprayed concrete is concrete which is conveyed under high pressure througha pneumatic hose and projected into place at high velocity, with simul-taneous compaction (DIN 2005). Sprayed concrete can also be calledshotcrete and both terms are used in this book.

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