Two kinds of rock shear tests are performed by two ways in Japan. One is rock shear test (RST) shearing rock mass directly. Another is block shear test (BST) shearing boundary between rock mass and concrete block. There are few studies, however, being conducted to compare the strength properties obtained by RST and BST. Therefore, the purpose of this study is to obtain the relationships between the cohesion and the friction angles obtained by RST and those by BST. The results demonstrated that, for the hard rock masses [H], the strength properties by BSTs are lower than those by BSTs by about 11–26%. On the other hand, for the soft rock masses [S], the strength parameters by RSTs are higher than those by BSTs by about 10–252%.
Japanese Geotechnical Society published a database of various field tests on rock masses¹). This database included as many as 301 data of rock shear tests and 862 data of block shear tests conducted in Japan. Attached to the database was a report where these test results were analyzed as relationships between the strength properties, cohesions and frictional angles, evaluated from the test results and the relevant engineering classification of rock mass by the Japanese Geotechnical Standard for rock mass classification, JGS: 3811–2004. However, it is should be noted that the results of rock shear tests (RST) and those of block shear tests (BST) were not separately treated in the analyses. Fig. 1 shows the difference between RST and BST. Although these two test methods are designed to rock masses strengths, the obtained results may represent different strength. The RSTs can ideally evaluate the shear strengths of rock mass directly, whereas the BSTs may evaluate the shear strengths on the boundary between rock mass and concrete block if the failure plane develops along the boundary. Therefore, the purpose of this study is to compare the RST's results and the BST's results.
The values of cohesion, c, and frictional angle, f, were obtained for individual site. Then, the representative values of strength properties for the relevant engineering classification were determined as the average values. The analyses were conducted for the first-step, second-step and third-step classifications specified in the Japanese Geotechnical Standard for rock mass classification, JGS: 3811–2004. In this report, the relationship between strength properties obtained by RST's and those by BST's is compared by two methods. In Analysis 1, the average values of strength properties were calculated for the individual sub-classification of JGS's rock mass classification system. In Analysis 2, the relevant strength properties were directly compared for both RST's and BST's at the same site.
The relationships between the strength properties by RST's and those by BST's are compared by the average values for the first-step, second-step, and third-step classifications.
Fig. 2 compares the cohesion (cRST, cBST) and the friction angle (fRST, fBST).