How to get the uniaxial compressive strength value of soft rocks is always a problem in engineering because of their low strength

loose structure and difficult processing. Needle penetrometer invented by Japanese

can be used to calculate compressive strength of soft rocks indirectly by testing its needle penetration index (NPI). The paper introduces experimental instrument

operating procedures

announcements and data processing methods of needle penetration test(NPT). Then the strengths of different soft rocks are tested by needle penetrometer in situ and laboratory. According to statistical analysis of test results

the discreteness of sampled value is closely related to rock uniformity and structural discontinuous surface. Finally

contrast methods of NPT and point load test on a set of soft rock samples. It turns out that the results based on these two methods are similar. Therefore

NPT is reasonable and feasible in soft rock strength test and it can be popularized and applied to the soft rock engineering in China.