Stabilizing the face is fundamental for tunnelling through unstable grounds, especially so far tunnelling through a sand zone with high water pressure.
The Urasa Tunnel, one of the Joetsu Shinkansen railway (which is under construction from Tokyo to Niigata) tunnels, had encountered face collapses as many as ten times in such a short distance of 20m in advancing the side headings through high water pressured zone of 9 kgf/cm2. The tunnel 6,025m in total length is situated in the southern part of Niigata Prefecture. The area through which the tunnel was driven is very mountainous Comprising mainly tuff of the Tertiary Period which is widely distributed around Niigata Prefecture. The prior geological investigation had indicated that a silt and a sand lay between tuff and mudstone layers (Fig. 1).
All the methods conventionally used in Japan such as deep well, LW grouting (sodium silicate and cement), dewatering boring and pilot tunnel were tried, but Only the grouting using sleeve pipe of retarded-setting hard silica gel (sodium silicate and triacetin) and shield tunneling method were found successful.
This paper deals with application of various methods carried out to stabilize the tunnel face, and the grouting using sleeve pipe of retarded-setting hard silica gel (RSG-3 grouting) and shield tunnelling method which enabled the successful completion of the project, and the observation method to ensure the statisfactory treatment effect by checking density-and moisture loggings by use of radioisotope.
(Figure in full paper)
The first method to stabilize the tunnel face was dewatering boring. The most troublesome problem was how to drill. Muddy water including silt came out whenever the drill penetrated into the ground. So it was difficult to drill the hole of 100m which had been planned in the stage of designing, and many short borings (20- 30m) were repeated instead of digging a few long designed holes (Fig. 2).
(Figure in full paper)
Since the rock around the tunnel was impermeable (10–4cm/sec), it was almost impossible for drilling to lower the ground water table in a short time. The amount of inflow water decreased depending on result of the jamming on a pipe; in case of remarkable jamming the inflow water stopped immediately; in a moderate jamming it stopped in a few days.
Since the dewatering borings had been found ineffective to stabilize the tunnel face, the grouting was adopted. In the designing stage, the grouting was thought to fill up the collapsed area to reinforce the loosening area and to mobilize arch action around the tunnel. Therefore, in selecting the grouting material, the following factors were considered:
The permeation into the ground.
The strength of the injected rock.
Watertightness.
In the case of injection into the sand, the material of low viscosity and long gel time (permeable grouting material) have been usually used to enable a low pressured injection. But in this tunnel, it was predicted that it was not effective to use the permeable grouting materials due to the high water pressure.
