Excavating a weak ground for a large cutting or tunnelling, if we can catch a sign of collapse at an early time, then, we shall be able to prevent its occurrence by setting a suitable protection. When the ground preserving a equilibrium has been excavated, some changes of stress field around the digging area should occur so that a new stable state may be established. This paper suggests a new method which can easily detect the change of stress within ground. Using this new measuring meter, we have carried out several tests in laboratory and in situ, thus investigated the aspects of ‘stress-rearrangement’ around a cavity.
Fig. 1 shows the new meter, which is able to measure the degree of stress-change within ground. This meter has been devised by us and manufactured by DOBOKU-SOKKI CENTER.
This meter, named as VALIMUE CELL, consists in four parts:
(Part A) is a rubber-cell contained an unfrozen liquid. This cell is burried in a drilled hole of ground and is able to deflate or expand according to the change of ground pressure;
(Part B) is a flexible pressure tube leading the unfrozen liquid in a cell to a liquid chamber in part C;
(Part C) is a metal cylinder having a movable diaphragm supported by spring, so that the latter can press the liquid;
(Part D) is a detector attached to part C.
It gives us an amount of diaphragm's displacement, that is, an elastic deformation of spring.
We can convert a deformation to a stress by using the spring constant. As a base stress, we shall now adopt one when the cell was set up initially, then, a stress reduced from it shows a shrinkage of the cell volume, that means a increment of ground pressure
(Figure in full paper)
(We take it positive). On the contrary, a stress increased from it means a decrement of ground pressure (negative). Setting up the cell with a suitable pressure in a bored hole, therefore, a variation of the spring stress of Part D may show a change of ground pressure. In this paper, a spring-stress is called a ‘cell stress’.
To explore the basic behaviour of VALIMUE CELL, we have carried out the uniaxial compression tests on the 30 cm cubic cementmortal blocks in which have two cells installed parallel to each other.
These cubic specimens made of ‘SIRASU (a white volcanic ash, cohesionless)’ instead of sand have the various strengths between 10 to 300 kg/cm2 according to the cement-contents. Inside of them have been prepared two holes which have a 40 mm diameter, 250 mm length and parallel to each other with 100 mm interval.
(Figure in full paper)
Corresponding to the increments of uniaxial static load on this specimen, we have measured these cell-stresses and correlated them to the cracking or fracturing of specimen.