Laboratory experiments of volume changes of Jinan gabbro and Chang-ping granite in various stress paths are performed.Firstly,we loaded the rock sample to a given stress state,and the further loadings were ap-plied according to the following three modes:Case A,the maximum prin-cipal stress was increased until the rock sample failed;Case B,the minimum principal stress was decreased until the sample failed;Case C,the minimum principal stress was increased,but no rupture occurred.In experiments we measured axial and lateral strains by means of strain gages so that the volumetric strains could be calculated.It has been found that the dilatancy effects of the same rock are quite different in various stress paths.The experimental results suggest that there exist the super-dense state in case B,and the ultra-dilatant state in case C.In case B,just before rupture the state of rocks changes,from the super-dense to the ul-tra-dilatant.The studies of other authors on acoustic emission,wave velo-city and microcraeks have confirmed the existence of the super-dense state and the ultra-dilatant state.The magnitude of the super-dense and ultra-dilatant effects of gabbro was calculated.As we have known,the rock under cyclic hydrostatic pressure has hysteresis effects,and in fact,both the super-dense state and the ultra-dilatant state are also hysteresis effe-cts.Because the microfracturing occurred in these states,these hysteresis in magnitude are about one order greater than that in hydrostatic pres-sure experiments and influence greatly on the properties of rocks.
The conditions of the super-dense appeared in case B has been stu-died.The super-dense state exists only in those experiments of case B,in whi-ch the minimum stress is decreased from the stress state about 80% of rock strength.The results of more complex stress paths are reported.Finally,the possible application of the experimental results to earthquake predic-tion has been discussed.