Introduction

The Hokuetsu North Line, which connects Muikamachi on the Joetsu Line with Saigata on the shinetsu Line is the new planned line of the 59.4km long railway system of the Japanese National Railway. The Nabetachiyama tunnel, 9,117m long single truck tunnel, located at nearly middle part of the route of the Hokuetsu North Line is being constructed under the supervision of the Japan Railway Construction Public Corporation, and is divided into three work sections.

In west work section the geological structure is very complicated such that Gimyo anticline is a recumbent fold with disturbed zone, and the geological condition is generally so bad, especially around Gimyo anticline The rock mainly consists of soft tertiary mudstone. The overburden is 150m to 300m deep and the uniaxial compressive strength of rock is 10 to 40kg/cm2, and near anticlinal axis, fractured rock and clay caused by fold exist with about 1 to 10kg/ strength.

The ratio between the uniaxial compressive strength and the overburden pressure σgd/Pv) is below 0.2. Besides the overburden pressure, there is potentially tectonic pressure. Therefore the heavy squeezing pressure is working on this tunnel. So NATM was adopted in this 124m long tunnel.

then this paper, authors discuss the design, the execution, and results of measurement in the heavy squeezing zone, and particularly refer to the tunnelling that utilized ground arch, which is characteristic of NATM as well as practical proof on the formation of the ground arch by the in-situ measurement.

Geology

The geology at the Nabetachiyama tunnel situated on the Niigata oil field is composed of submarine sedimentary rocks in late Miocene age, called Shiiya formation and Teradomari formation. They are alternation of mainly tuffaceous mudstone, sandstone and so on.

The geological conditions are generally so bad, especially around Gimyo anticline associated with reverse fault and disturbed or sheared zone. And the geological structure shows a steep dipping around Gimyo anticlinal axis.

Furthermore, ground around the tunnel contains crude oil and inflammable gas (mainly methane gas). The physical properties of rocks around this part are as follows:

  1. Though it was hardly possible to pick up adequate samples for testing, the uniaxial compressive strength σgd is very low (3.7~7.0kg/cm2 or less, and σgd/Pv calculated by the above value is 0.1~0.2.

  2. The content of montmorillonite is high and rocks dipped into water are fragile.

    (Figure in full paper)

    Rock is easily changed into mudyclay by increase of water content due to inflow water etc.

  3. The velocity of seismic wave in the initial state is about O.9km/s,-J 2.0km/s. Behavior of heavy squeezing is indicated in the rock in the tunnel.

Design and execution

In west work section, the conventional method was adopted to excavate 3310m long tunnel from portal. As the face approched to the Gimyo anticlinal axis, the squeezing phenomenon began to occur.

So the construction method was changed to the NATM from the location 35k380m. The driving rate per round was 1.0m, and the standard support pattern consisted of shotcrete of 20cm

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