A cross passage is a short tunnel connecting two larger tunnels. The main function is to provide emergency access from one tunnel to another. Cross passages are often spaced approximately every 100 meters. Construction of cross passages is often difficult. Unlike the larger tunnels they connect, tunnel boring machines cannot be used. In soil conditions with very high water pressures, ground freezing is used.By freezing the zone between the two larger tunnels, the contractor can mine through the hard, water-tight ground without collapse or groundwater infiltration.
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