General Principles for the Design of the Cross-section - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 17

depends on the ground pressure. The better the load-bearing capacity of the rock mass be-
ing passed through, the less ground pressure has to be assumed in the design of the support,
particularly pressure from the side. The higher the lateral pressure is in relationship to the
vertical pressure, the more a near-circular cross-section will be suitable. The shape of the
cross-section thus depends on the external and internal forces acting on the perimeter of the
cavity (Fig. 1-14). In competent rock, which does not tend to be weathered, the excavated
profile will stand up without any structural contribution from the support, and a thin layer of
shotcrete can resist any effects of weathering. Ways of optimising the tunnel cross-section to
an ideal shape are offered by the laws of structural geotechnical engineering.
Figure 1-14 Basic cross-
section shapes [135].
L. Müller [160] stated the following considerations:
“In competent rock, design is relatively unrestricted: cross-sections with vertical sides or flat
vaults, even horizontal crowns are possible in stable rock as long as there are not too many
joints, such as in conglomerates, some compact limestones and above all in undisturbed
granite. Such tunnels are then mostly not lined or only provided with a weak lining. Fully
inappropriate are linings, in this case actually only a facing, which are only installed to pre-
sent a smooth face; underground cavities have their own aesthetics and should be designed
according to the character of the rock; there is no reason to coyly hide it.
In fairly competent but jointed rock, profiles with weak support are used or those where
systematic rock bolting provides the actual support. Also in this case we no are longer
ashamed to specify an uneven lining that follows the structure of the rock, like for example
is created by spraying shotcrete, and we leave the heads of the rock bolts visible even in
important structures. This saves support work and concrete, which would only serve to
create a geometrically precise outline by filling the hollows formed by overbreak without
any actual structural purpose. Why should we not, where the surface structure imposes
such an excavation shape, sometimes specify the required clearance gauge as a pointed
arch or similar irregular profile, when otherwise rock bolting and concrete would have to
be provided to avoid such profiles but without any actual structural purpose?
Rock that is competent but tends to subsequent loosening is often only permanently support-
ed in the top heading, sometimes only with mesh and rock bolts. Formerly, when masonry