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Abstract Collapse is one of the main risks during the construction of tunnels through soft ground in urban areas. Understanding the characteristics of tunnel collapsing helps the prevention of the problem in tunneling through the soft ground. Model tests were performed to study the collapsing characteristics of a deep tunnel and a shallow tunnel under the same soft ground condition respectively. The results showed that collapses of the tunnels in soft ground under different covering depths had two similar characteristics，the progressive failure of surrounding rock and the sudden occurrence of tunnel collapse. However，the characteristics of collapses in the deep tunnel were different from ones in the shallow tunnel in the following aspects：(1) The failures of surrounding rock around the deep tunnel did not spread to the ground surface. Collapses occurred several times and there was a short-time and stable collapsing arch during each collapse. The collapses did not stop until the formation of a stable collapsing arch. All the boundaries of collapsing arches were in the shapes of typical quadratic parabolas. (2) The failures of surrounding rock around the shallow tunnel spread to the ground surface，and eventually caused the ground surface to collapse. A short-time and stable collapsing arch was also observed to exist during the failures of surrounding rock，and its boundary was also in the shape of a typical quadratic parabola. (3) The failures first occurred on the two sides of the tunnel and then at the tunnel crown in the case of the deep tunnel，while the ruptures of the ground surface occurred earlier than the surrounding rock around the tunnel during the construction of the shallow tunnel. Therefore，the key measures of preventing the collapses should be taken in the tunnel for deep tunnels，and both in the tunnel and on the ground surface for shallow tunnels.
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Received: 21 October 2013
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2014, Vol. 33 
