The availability of a large amount of data from borehole load testing has made the determination of in situ mechanical characteristics of rock a comparitively easy matter.
However, because much of the data that ecxists is based on a complex set of enviromental factors, there are many unclear aspects.
In this paper we will introduce a method for evaluation of strength and deformation characteristics of rock by using results obtained from borehole load testing considering the special features of loading in the method used.
The borehole load tester used is shown in Fig. 1 and summarized in Table-1. Fluid under pressure is supplied to the sonde by means of a pump on the surface. Pressure and deformation are measured by a pressure guage and a deformation measuring system built in to the sonde, called a CONTACT BALANCER. The measured data is recorded by a X-Y recorder on the surface. The contact balancer contains a bellows that responds to changes in cubic volume. A sencer maintains contact with the inside of a rubber tube that moves in response to displacement of the borehole wall. The movement is transmitted by the sencer to a differencial transformer that in turn moves the bellows.
If a round hole with radius a is opened in a plate of infinite area as shown in Fig. 2, and pressure p is exerted within this hole, stress over the distance r from the center of the hole may be expressed as follows:
(Figure in full paper)
Because both in apressure p and amount of borehole wall dispracement ∆r are determined by borehole load testing, the deformation modulus can be determined using integral r from eq.2 by the following formula:
as shown in eq.1, stress in the direction of the radius is compression, while circumferential stress, i.e., tention, is of the same amount. consequently, radial fissures in the borehole wall are produced because of this tensil stress mentioned above that occureds in the direction of circumference.
Acording to M. Rocha et al. (1967), displacement in the borehole wall in such a case can be seen as displacement of wedge shaped deformation of rock surrounded by fissures. As shown in Fig. 3. wedge shaped deformation of rock surrounded by fissures may be expressed as follows:
where St is tensil strength of rock. This formular is based on the theory that radial stress in a model wedge is reduced in inverse proportion to r by stress dipersion while no stress is produced in the intersecting direction. In this case, stress is described as uniaxial compressive stress.
(Figure in full paper)
As pointed out above, the borehole lateral load test is distinguished by stress and deformation that it produces. When this test is conducted, The effects of geostress corresponding to boring depth may be considered to be substantial.
