Structural design verifications, structural analysis of tunnels - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 95

3 Structural design verifications, structural analysis

of tunnels

3.1 General

The terms “load determination”, “structural analysis”, “calculation”, “dimensioning”, and
“safety coefficient” cannot be transferred from structural engineering to tunnelling with-
out comment. The usual practice until now of using the existing standards and guidelines
for reinforced concrete, steel construction and others, which were normally intended for
structures above ground, for tunnelling should also be reconsidered. The exploitation of
interpretations of or even deviations from standards when local conditions are not relevant
for use in tunnels is not a way to perpetuate existing experience for innovative new de-
velopments. For many years, structural verifications have been worked out from scratch,
working with new engineers responsible for checking the calculations. By the end of the
project, a more or less good and perhaps also economic compromise will have been found.
But compromises are often far from the optimal solution. Collected experience is only
transferred sparingly, because the next project will be based again from the existing stand-
ards and guidelines and start once more from the beginning. The extent of the efforts made
by the specialist world of tunnelling to fit tunnel engineering into the existing environment
of verification calculations is demonstrated by the chapter heading “Attempts at structural
calculation” in the topic “Rock construction” volume 3, by L. Müller [160].

In order to justify the special situation of tunnelling, the following differences compared
to conventional structural engineering can be stated:
Rock mass. The rock mass is simultaneously a loading and load-bearing element. The
loads are not normally acting on a load-bearing member from the outside. The designer
only has information at isolated locations from the geological survey. The available in-
formation is, however detailed, not adequate to perform a conclusive structural verifica-
tion. Measurements made as the tunnel progresses demonstrate the real behaviour of the
unsupported and supported tunnel as a full-scale model, presuming they are appropriately
performed and evaluated. Unexpected changes of ground conditions can however still lead
to collapses, even when great caution is exercised.
Construction process. The size and shape of the cross-section influence the ground pres-
sure and the loading. The excavation process influences the degree of loosening of the
rock mass. The time taken for support to become effective influences the distribution of
the development of ground pressure.
Support materials. Support materials can support at points (steel profiles) or as a bonded
composite (shotcrete) and they influence the loosening of the rock mass and its contribu-
tion to structural action. The strength development of the construction materials with a