The 'Kuroko' is an unique complex ore consiting of Cu, Pb and Zn sulphide containing gold and silver. The surrounding rocks of Kuroko deposits are generally clayey, soft, and sticky and the underground rock pressure is 'quick and heavy'. Therefore, in the Kuroko mines the 'underhand cut & fill mining method with artifitial cemented roof' is usually adopted to attain the complete are extraction and to prevent the surface subsidence.

Recently, new technique called the 'preburied cap timber technique' (Ohtsuki,1975) as been incorporated into the abovementioned mining method. This method is illustrated in Fig. 1, and outlined as follows. After a slice has been mined-out, wooden timbers are placed' about 1 meter apart parallel to a crosscut, 15% cement mortar is poured onto a floor about 60 cm thickness, and the rest of the mined space is filled by 3% cement mortar. When the underneath slice is mined, these pre-buried timbers are supported by post timbers and act as cap timbers. This method has successfully strengthened the artifitial roof and resulted in the cost reduction and labor saving.

(Figure in full paper)

On the other hand, miners working underground experience (Chonan,1973), that the damage of the supports is most severe during mining the top slice but it reduces considerably as the mining down to the lower slices.

The purpose of this research investigation is to explain the various rock pressure phenomena and to clarify the effect of the filling in this method by an elasto-plastic stress analysis.


Uni-axial compression, confined compression, and Brazilian tests were preformed on the clayey ore, artifitial roof material (consisting of tailing sand, volcanic ash, and 15% portland cement), and filling material (consisting of tailing sand, volcanic ash, and 3% portland cement). During the sample preparation and laboratory tests, sufficient care was taken to keep the water content of the samples as same as the field condition. The test results are tabulated in Table 1.

(Table in full paper)


In Fig. 2 the stress-strain curves of the clayey ore, artifitial roof material, and filling material obtained by uni-axial compression tests are shown. As is obvious from this figure, non-linear behaviour accompanied with 'strain softening' is remarkble; therefore, it is considered necessary to develop an elasto-plastic stress analysis method in order to explain the various phenomena in these soft and weak rock mass.

For this purpose, firstly, the constitutive equation suitable to these soft and weak rock materials has been derived from the mathematical theory of plasticity based on the 'extended von Mises' yield criterion' and the 'associated flow rule'. Secondly, a new computational procedure using FEM has been developed. An iterative method ('stress transfer' method) has been incorporated for solving the material non-linear problems such as the elasto-plasticity including the strain softening characteristics (Yamatomi, 1979).

In this FEM program, 'tensile fracturing' is also considered. If the maximum principal stress (σ1) exceeds the tensile strength of an element,

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