Shotcrete support with special reference to Mexico City drainage tunnels

The use of shotcrete is described as a structural support in tunnels in soft, waterbearing, semi-consolidated soil-rocks with an unconfined compressive strength ranging about 27 psi. The tunnels were 20′–25′ span at springline, widening to 56′ in a transition section, 20′–30′ in height. Continuous water inflows of 25 litres/sec with surges to 50 litres/sec were experienced. No interior steel reinforcement was used in the transition section, though 3 m grouted bolts were used for rock reinforcement of the arch. Shear and tensile stresses slowly developed in the walls about the transition section, which were then bolted. In the face of waterinduced wall cavitations occurring in subsequent (20′) tunnelling steel wire mesh was introduced in the shotcrete below springline. Abandoned for 12 months and left under water, floor heave of 20–30 cm developed, without damage to the shotcrete arch and walls. The inverts had been left open without lining.