Characterization of a mudstone in a deep underground cavern - comparison of full-scale behaviour and field and laboratory tests

Abstract

In Sagamihara city of Japan, a 50m-deep vertical shaft and large diameter cavern were excavated for research purpose in a sedimentary soft rock deposit (mudstone). In this paper the behavior of mudstone were studied in three different scales, in which the discussions are based on results of full-scale field behaviors and laboratory and field tests. The stiffness of the soft rock was back-calculated from field monitoring data. The results of various shear tests such as field shear wave velocity measurements, pressuremeter tests and plate loading tests, as well as detailed laboratory tests such as unconfined compression tests, conventional triaxial tests and ultrasonic wave velocity measurements were analyzed to study the scale effect and obtaining the properties of mudstone in different measuring methods. Also to measure the shear strength and deformability of rock mass in field by a single test, new down-hole triaxial tests were conducted and its relevant results were compared with those by traditional field and laboratory testing methods. The undertaken analysis and comparison show that, because of loosening and disturbing and some inherent errors in measuring the deformation of rock mass, the stiffness calculated by pressuremeter tests and plate loading tests are underestimated. Moreover, sampling disturbance reduced the stiffness and strength of rock material measured in the laboratory. It is also concluded that the down-hole triaxial testing is a reliable measurement method to estimate the stiffness and strength of rock mass.