There has been a great improvement in mechanical and measuring devices about the in-situ testing technique for rock mass in the last two decades. Above all the borehole jack test is easily influenced by geological and geometrical conditions of test point, for instance rock substances, discontinuities, roughness of borehole wall and so on. One should take advantage of the instrument to disclose the distribution of rock deformational properties in a large rock mass through suitable preliminary investigations Global and local geological surveys, testing procedures, bases of data interpretation etc. are included in the investigations.
Goodman et al. (1968) presented a useful study for determining the in-situ rock elastic modulus from the borehole jack test data. They solved the problem using the complex stress functions in the plane elasticity. They adopted a boundary condition that a uniform unidirectional pressure acts on two diametrically opposed sectors of the borehole wall. De la Cruz (1978) presented a modified Goodman's test method with a more accurate interpretation of determining the rock elastic properties which is derived by using the Airy stress function.
This paper describes a finite element procedure which yields rock deformation moduli — elastic modulus, creep compliance and relaxation modulus— from the borehole jack test data. The borehole jack has rigid bearing plates to transmit hydraulic pressure to the borehole wall. In this paper the bearing plates are regarded as rigid compared with the soft rock and contact of curved surfaces of the plates with the borehole wall as complete. This problem should be, therefore, analyzed as a contact problem of rigid to deformable solids.
The borehole jack used in this investigation is one of KKT jacks developed in Kawasaki Geoscience Survey Ltd. which is named high pressure type. The outline of the borehole jack is shown in Fig. 1 and its geometry is summarized in Table 1.
(Table in full paper)
The applied pressure and the diametral displacement of the bearing plate are measured by a pressure transducer directly connected to the plate.
Figure 2 (a) shows diagramatically a part of a trans- verse cross section of the deformed borehole under the unidirectional jack pressure 2P through the bearing plate. When a geological condition around the test point is homogeneous and length of the plate is enough longer than its width, the rock at the central region of the jack may be considered to be under plane strain state. A numerical solution for such a contact problem can be readily obtained, although it is very difficult to derive an analytical solution. The finite element method is one of the powerful tools to get the numerical solution.
Figure 2 (b) shows a mesh pattern for the region near the bearing plate. At nodes 1 to j on the wall loads and displacements are unknown, but the following conditions can be assigned.
(Figure in full paper)
where a and Q are summations of elements of matrices [M] and [L] respectively.
