General Principles for the Design of the Cross-section - Tunnel Engineering: Basics and Additional Services for Design and Construction - page 18

lining was used, the lining was often restricted to certain less stable areas and these were
supported with masonry bands and corners. This practice died out with the introduction of
formed concrete but should be reintroduced today with the availability of shotcrete, which
can be applied as required. This would however only become established practice if saving
money was regarded as a virtue and the supervisory engineers felt responsible not only for
the avoidance of damage but for the economical use of construction materials.
In competent rock, which is fractured into large to very large blocks by jointing, and if the
diameter is less than twice the average joint spacing, support can be designed on the same
basis as in competent rock without large joints if the anchoring is installed according to
the structure of the rock mass. If the diameter is larger, a type of punching pressure on the
support has to be expected, so the lining has to be designed to resist this shear or the joints
have to be dowelled.
Brittle rock with medium strength, which requires transverse support to assist the forma-
tion of a protective zone, demands well rounded profiles that are approximately circular
including the invert and support to all sides, possibly making use of permanent rock bolts
behind a thin layer of concrete.
In squeezing and strongly squeezing rock, irrespective whether the ground pressure derives
from high primary stress or low rock strength, the cross-section should always be nearly
circular, although the invert is normally given a flatter profile (arched profile). The sup-
port in squeezing rock is normally made stronger without, however, having to completely
abandon the advantages of a relatively slender support layer, as recent knowledge shows
that the formation of a supporting ring in the rock mass, normally achieved or reinforced
with systematic rock bolting, should always be the intention. The circular structure of the
tunnel support must always be closed in squeezing rock, including the invert, and the cur-
vature of the invert arch must be more pronounced the higher the ground pressure is. The
closure of the invert is also very important in rock susceptible to softening.
Ground pressure acting on only one side (a), as for example is characteristic of tunnels
beneath a slope, particularly slopes susceptible to creep pressure, require an asymmetric
lining (Fig. 1.15), in which the abutment on the valley side, that is away from the pressure,
is strengthened and this always requires a widening of the foundation.
Figure 1-15 Tunnel profile for a tunnel below a
slope with strong downhill thrust, designed using a
classic line of thrust calculation [160].