Abstract: The most of construction coal mines in South of Sichuan province have been discovered with the difficult problems of much soft surrounding rock, easily weathering、developmental joint and fracture、large deformation、difficult support、multiple repairmen , which have influenced on the process of coal exploration and production in recent years. The soft style and severity should be conformed in order to choose the appropriate support technology. We investigated the engineering geology and choose the thick-bedded dark grey mud rock mainly surrounding the roadway. The fresh rocks with irregular shape have been tested by the point load testing in tunnel. After the screening testing data with the method of Grubbs criterion, the rock strength has been calculated by the formula defined by the traditional method、International Society for Rock Mechanics (ISRM) standard rate and Chinese engineering standard specification. The random and system error about the result of calculation with different formulas is tested by the F-test and t-test. The results show that the average uniaxial compressive strength obtained by the traditional method and ISRM standard rate is more accuracy than the Chinese engineering standard specification. The strength is appropriate to the soft rock evaluation. The change factor of the mudstone strength is between 11%-30%, which is approach to the general uniaxial compressive strength. The point load testing is the credibility, fast, convenience and cheap way to obtain the strength of soft rock with difficult coring, which could be used to evaluate the difficulty of the soft rock engineering.

Key words: point load test, rock strength, reliability, standard deviation, feasibility, regularities of distribution