When tunnelling we sometimes meet squeezing ground which leads to awful delay of construction. But even now the mechanism of squeezing is not known clearly. To make this mechanism clear the authors have done three dimensional model tests on soft ground tunnels and discussed the mechanism of squeezing and groud behavior around a tunnle.
So called competence Factorl)2) (the ratio of the uniaxial compressive strength of the ground to the net pressure of overburden) is considered to be one of the most useful indexes to estimate squeezing. But the authors have thought that Competence Factor is misleading without paying attention to the internal friction angle of earth. So the authors have done two model tests of different kind of earth. The two different kind of earth have the same (rather small) uniaxial compressive strength but different internal friction angles, that is, the one is Cohesive the other is sandy. It has been studied how the difference of friction angle influences the behavior of ground in case of tunnelling under weak rock and large pressure of Overburden.
When some similarity exists between models and prototypes, model tests can be efficient to solve problems of prototype. To pursue this similarity it is necessary to find non-dimensional numbers which must be the same value in both prototypes and models. These non-dimensional numbers are often refered as II numbers. Dimensional analysis is useful to find necessary n numbers.
Initial factors of dimensional analysis must be the ones which have great influence on the ground behavior when tunnelling. The authors have chosen seven factors, H,D,d,r,C,Φ,T (refer to Table 1 for notations). F(force)-L(length) system has been chosen as the dimensions. According to the Buckingham's theorem the number of necessary n numbers is 7–2=5.
Table 2 shows these five n numbers which have been calculated by dimensional analysis.
(Table in full paper)
The way of the experiments is as follows. First the authors put earth into a tank of 2×2×2m and consolidate it (Fig 1). Then the following two steps are repeated by turns until the whole model tunnel is accomplished. In the first step a semicircular hole is excavated for 10cm. In the second step a semicircular Steel linig is pushed into the hole also 10 cm.(Fig 2). Durig tunnelling various measurements are carried out One experiment ends when the tunnel gets 2 m long. The authors have done two such experiments. The characteristics of model earth have been changed.
The initial stress of earth including loading pressure and the stress caused by the weight of overburden earth is 0.8 kgf/cm2(78.4kPa) at the level of the tunnel. The uniaxial strength of compression of the model earth is 0.2 kgf/cm2(19.6kPa). In other words Competence Factor is 0.25. This condition is common in both two experiments.
In a word the way of model tunnelling looks like pipe jacking method. The hole is excavated manually with picks and shovels. One excavation length is 1Ocm.