Plateaus of pyroclastic flow deposit not exceeding 300 m on the elevation from the sea, widely distributed in southern Kyushu, Japan, consists of volcanic glassy soil called Shirasu, fallen pumice and tuff. There are densely distributed many houses On Such plateaus not more than 40 m on the elevation. A short-cut tunnel has been Planned through a low plateau in order to introduce some quantity of flow water from a river which has frequently caused floods, and there has been carrying out such investigations as borings and an in-situ tunnel excavation test for the purpose of establishing the design and execution method of the actual tunnel with the dimensions o1f about 10 m diameter and about 1600 m long. However the engineers engaged in the Project were faced with many difficult problems regarding the excavation method against the underground water actions, especially from the fallen pumice layer laid in the Shirasu plateau.
Shirasu has been counted to be an unusual Soil material for its extreme erosivity and fluidity when it is subjected to the water flow or the seepage (Tsubaki et al, 1975) even if the mechanism has not been Made clear being known its facts only. meantime the mechanical characteristics have fully been clarified and it is known that the material has a reasonably large angle of shear resistance and cohesion due to a kind of metamorphism welding so that the material can be said to have the properties ranging from granular soil to weak rock (Horie et al, 1975).
This paper describes a series of the studies carried out experimentally and analytically for obtaining the useful information to the project involving the problem of tunnel route location. Firstly, the Shirasu samples, sorted into a few kinds of ranges on the diameter, were tested with regard to the minimum fluidization velocity as well as the undisturbed Shirasu samples and fallen pumice samples were investigated on the failure resistance against various hydraulic gradients. Secondly, a model experiment and some finite element analyses were performed aiming at investigating the seepage actions to the tunnel stability in relation to the relative site of the fallen pumice layer. Thirdly, six typical sections, supposed to be encountered in the tunnel works, were elastoplastically analysed using the finite element method and adopting the seepage force as a body force.
Values of the critical hydraulic gradient were measured by an experiment on the Shirasu and fallen pumice specimens in both compacted and undisturbed states.
The experiment was conducted using an apparatus which can provide the head difference up to 3 m by means of a constant head permeability test leading seepage water upward from the specimen bottom. The compacted specimens were tested to investigate the effects of welding and magnitude of void ratio.
Figure 1 shows the relationship between hydraulic gradient and intrinsic seepage velocity obtained on the compacted samples.