Three aspects of rock mechanics, namely, in-situ stress estimation by acoustic emission (AE) method, strength of rock
mass and role of chemicals to reduce the strength are covered. It is possible to detect the previously applied maximum
stress by stressing a rock specimen to the point where there is a substantial increase in AE activity. This is known as Kaiser
effect. From the AE signatures in the second and subsequent loadings, AE take-off point was identified more easily than in
the first loading. In determining the compressive strength of rock mass, two factors have to be considered, namely, the size
effect on the compressive strength of intact rock and the effect of discontinuities on the compressive strength of rock mass.
Although a modified Bieniawski criterion gives best agreement with the triaxial test data, modifications have been
suggested to Hoek-Brown criterion due to its popularity. It is possible to reduce the tensile strength of sandstone by
saturating it with weak chemical solutions made with dodecyltrimethyl ammonium bromide, polyethylene oxide and
aluminium chloride by up to 30%. In the case of compressive strength, there is no appreciable effect. The possible
explanation is that the chemical solutions produce an effect on the strength of sandstone only when the failure mechanism
is dominated by tensile mode.