The Inference of Geo-Mechanical Properties of Soft Rocks and their Degradation from Needle Penetration Tests

Needle penetration tests (NPTs) are used for inferring the uniaxial compressive strength of soft rocks, particularly in tunneling through squeezing rocks and foundations on weathered rocks in Japan. The device measures the applied load and the penetration depth of its needle. The ratio of applied load to penetration depth was originally called the needle penetration index (NPI). In this study, this device has been used to infer the geo-mechanical properties of soft rocks from Japan, Turkey, Indonesia, and Egypt. Various equations are presented to infer the geo-mechanical properties in terms of the NPI and compared with the experimental results. The possibility of evaluating the anisotropy of geo-mechanical properties is shown. Furthermore, the characterization of geo-mechanical properties of fault/fracture and slip (shear) surfaces is explained. Some additional equations are given to consider the degradation of geo-mechanical properties as a function of water content, weathering state, and number of cycles of freezing–thawing. Furthermore, the possibility of evaluating the time-dependency characteristics of soft rocks by needle penetration testing is discussed through experiments. It is shown that the effects of water content, weathering state, and number of cycles of freezing–thawing and time-dependency can be evaluated using the NPT technique.