Changes of Micro-Structures of a Soft Rock on the Creep Loading Process. Studies on the creep mechanism under compressive load of rocks (Part-1).

Abstract

In this study, drained creep tests under uni-axial compressive load were carried out on Kimachi-sandstone to clarify the creep process and mechanism of the soft sedimentary rock.During these experiments, axial strain, lateral strain and the Acoustic Emission (AE) activity were observed in the loading time intervals of several minutes to one month.Furthermore, thin sections for microscope were made of the tested specimens and micro-photographs on these sections were taken by a microscope. On these micro-photographs, using an image processing technique of a micro-computer, numerical data for the changes of pores and micro-cracks at controlled creep time or at prescribed creep region were acquired.The main results obtained from these tests are as follows; 1) In loading process of the creep load on a rock specimen, firstly the axial strain rate is shown to decrease gradually to a minimum value, after it increases gradually and finally it increases rapidly, and then the specimen is going to fail, 2) In the process of decreasing the axial strain rate, closure of pores in the rock occurs and the compacting of the rock takes place. But, the compressive strength of the specimen, which is unloaded in this loading process, is not affected by the loading time and is almost the same as that of the intact rock, 3) In the process of increasing the axial strain rate, that is after passing through the minimum point of the axial strain rate, cracks in the specimen develop toward the direction parallel to the loading axis. And also, the compressive strength of the specimen, from which load is removed in this process, decreases as compared with that of intact rock.