Three aspects of rock mechanics, namely, in-situ stress estimation by acoustic emission (AB) method, strength of rock mass and role of chemicals to reduce the strength are covered. It i:s possible to detect the previously applied maximum stress by stressing a rock specimen to the point where there is a substantial increase in AB activity .This is known as Kaiser effect. From the AB signatures in the second and subsequent loading.s, AB 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 dis,continuities on the compressive strength of rock mass. Although a modified Bieniawski criterion gives best agreement '".ith the triaxial test data, modifications have been suggested to Hoek-Brown criterion due to its popularity. It is po~;sible 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.