Civil Engineering Reference
In-Depth Information
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Figure 8.29
Crack pattern occurring in the walls of the side drifts
bending moment around points 10 and 11 (tensile stress at the extrados),
and a positive bending moment/elongation around points 12 and 13. The
sprayed concrete was, at this time, some weeks old and already hardened.
Stress redistribution inside the sprayed concrete, e.g. due to creeping effects,
was negligible. The movements therefore immediately caused an increase
in stress inside the sprayed concrete, which was made visible by the cracks.
This was confirmed by the stress-intensity-index (see section 7.3.4.6) as
shown in Figure 8.28 for the inner wall of the right-hand side wall drift
at Chainage 60 (see marking in Figure 8.28 between points 13-11-15).
With the face of Kern1 passing the monitoring cross section at Chainage
60, the stress changes from a compressive stress (negative sign) to a tensile
stress (positive sign) and leads to the cracks. The stress-intensity-index
shows a tensile stress of only about 20% after the development of the
cracks. The safety factor against failure (
= 100%) was still around five.
This gave the certainty that the tunnel was in a very stable situation. With
the knowledge of the stress-intensity-index another conclusion could be
made: Due to the cracks and the low stress level in the inner wall, most
of the load had been redistributed into the outer walls of the side wall
drifts. This effect was considered desirable with respect to the later demoli-
tion of the inner walls. Demolition was safer and easier with unloaded
inner walls. Overall, the development of cracks in the sprayed concrete
lining was beneficial in this case.
