Heretofore, the measurement of ground behavior has not generally been a factor in conventional methods of tunnel construction. with the recent introduction of the New Austrian Tunneling Method (NATM), however, increasing attention is being given to the observation of ground creep tendencies for the purposes of tunnel construction control.
Nevertheless, most such observations are generally restricted to measurement of stress conditions and deformation occuring in the plane perpendicular to the direction of the tunnel axis, with the displacement behavior of tunnel faces (in other words deformation in the direction of the tunnel axis) being rarely subject to consideration.
It is the intent of this paper to demonstrate that monitoring of tunnel face displacement behavior offers useful information for Control of tunnel construction by providing an index for accurately evaluating the total effect of the earth stress and the material properties of the ground structure.
The analysis of tunnel face creep as discussed hereunder has been effected through in-situ measurements by micro-creep meter in the Shin-Usami Tunnel currently under construction for the Japanese National Railways, and through computer simulation of ground displacement behavior utilizing the finite element method.
The tunnel site is located along the east coast of the Izu Peninsula. The ground structure consists primarily of Yugashima formation and Ajiro basaltic rock of the Neogene Period. Measurement of tunnel face displacement was carried out at points located in the Yugashima formation composed chiefly of alterated andesite, some green tuff and tuff breccia. Some portions of these materials have been arg1llizated as the result of hydrothermal alteration. Table 1 gives the mineral composition of the alterated andesite, as determined from the mineral distribution ratio elicited through a micrograph and x-ray diffraction peak study of sample material. As shown in Table 1, the alterated andesite tested was 31% montmorillonite, indicating rock that is heavily argillizated.
(Table in full paper)
The Shin-Usami Tunnel is a 3km long, single track tunnel under construction for the Itoh Line of the Japanese National Railways. The tunnel section is circular, with a diameter of 7m.
Since the ground rock is rather soft the NATM was adopted for construction.
In order to minimize excess excavation, to prevent excessive ground loosening, and to permit early closure, excavation is being carried out with a short bench. (length 3.2 4.8m), utilizing independent road header (MRli-S45) for upper and lower half sections Following excavation, yield able supports (MU-29) are installed with O.8 l.2mpitch depending on rock characteristics and the overburden pressure, after which shotcrete is applied in a l0~l5cm thick layer to the tunnel wall.
SN anchors(¢24mm)are used as rockbolts with a 1~1.2m2/bolt design criteria.
Samples of alterated andesite were recovered from the tunnel face and subjected to various laboratory tests.
Their material properties as elicited through testing are indicated in Table 2.
(Table in full paper)
Certain samples were. exposed to the unconfined compression test conducted according to the strain control method.