Abstract
Ground composed of an accretionary prism was found in prior investigation to have a seismic velocity of over 4.0km/s and a compressive strength of over 20MN/m2. However, the falling of rocks from the tunnel cutting face is a frequent occurrence and inside, the displacement of the tunnel was large during the excavation. In terms of typical mechanical properties of the ground, composed of an accretionary geological system in the excavation of the tunnel, ground behaviour was described as follows:
1. In the excavation of the tunnel, seismic velocity decreased from 4.0km/s to under 2.0km/s around the tunnel.
2. Modulus of elasticity after the excavation dropped 20%; in other words, the competence factor reduced from 30 down to less than 4.
To account for these analyses, it is thought that the particular micro structure based on pressure-solution cleavage in Thrust Fault complex was revealed because of the stress release in the excavation of the tunnel.
In conclusion, according to past investigation methods, it is too hard to predict the collapse in excavation due to stress dependency of an accretionary geological system.
