Up to now, studies of the bearing capacity are not enough to be related to the property of soft rock itself, because these-strain softening, non-linear failure envelope, etc are not all clear.

This paper presents the load bearing characteristic of soft rock foundation, which is based on recent studies and the property on soft rock. The main part of study is following.

  1. To investigate the bearing capacity and failure mechanism of the foundation With and without a virtical joint from two dimensional model bearing tests and in-site bearing tests.

  2. To determine the numerical models of mechanical properties of soft rock and joint for numerical analysis.

  3. To analyze the results of above experiments with the models obtained from (2) and finite element method, and to consider the applicability of this method to practical problems.

Experimental method

A series of the bearing tests was performed with use of a rigid and rectangular footing with 5 cm width and 15 cm length on the foundation. The size of arranged rock as it was 15cm width, 50cm length and 30cm hight. This was put in rigid frame to satisfy the test condition which was plane strain and grease was used to reduce the side friction. Joint was introduced with a diamond saw, it a direction was vertical and parallel to footing axis. Joint was plain, no filling and closed.

Loading rate was about 2 kgf/cm2/min. considered to porous nature of rock material for drained condition test. The position of joint was varied from (X/B)=O to 2.5, where B is footing width and X is the distance between the joint and the centerline of footing.

Soft rock used in tests was tuff commonly called Oya-lshi, saturated state under the experiment and used rocks were two series which were A and B but B was mainly used in tests. Physical properties of the rock are shown in Table.1.

Experiments results

Fig. 1 shows load-settlement curves obtained from a series of bearing model tests and Fig. 2 shows failure state at the side face of the foundation under footing, which was investigated after experiment operations. Load-settlement relations present loading force(P)-settlement(S) of footing curves and P-SD curves, where SD is settlement of the foundation at distance 5mm from the footing edge.

In case of the foundation without a joint shown in Fig. 1(a), P-S relations show that settlements are increasing remarkable after yielding in comparison with initial elasticity and these relations are very similar to S-SD curve. This is relation between P and SD is linearity, so bearing capacity is closely related to ability of stress transfer to the sides and depends on the growth of fissure. The loads at inflection points of both curves are nearly equal. Then a wedge shown in Fig. 2 was always produced under the footing and at same time tensile fissure was progressed downward it.

The difference of the mechanical properties of foundation rock for the bearing capacity appears sensitively.

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