Abstract
The earth pressure working on the portal of the tunnel are quite complicated. The tunnel receives a widely changing earth pressure because of topography and geology as well as of difference of excavation method.
Loosening of ground after excavation progressed in general as time elapse and pressure increase accordingly.
It is especially so where the tunnel has a little earth covering at the top part and the topography has a very steep gradient or where the discontinuty of the layer are inclining against the cross section of the tunnel.
Past experience, also, teach us that many of the landslide, rockfall and falling accident of the tunnel during the construction were caused by the loosening of the ground.
