Abstract:

As a difficult point in rock mechanics，end effect has been widely concerned by scholars.The phenomenon of end effect is caused by the friction between the indenter of testing machine and rock specimen in the process of compression，which usually causes the measured compressive strength of rock increased.However，the more intuitive performance of end effect is that the radial strain of rock is larger in the middle and smaller at both ends.According to Saint-Venant’s principle，when the height diameter ratio is greater than 2.0，the influence of the end effect can be ignored，but only the influence of the end effect on the measured uniaxial compressive strength can be ignored，which doesn’t form uniform stress in the specimen.Therefore，the method of weakening the end effect should be further studied.By summarizing the research of domestic and foreign scholars，it is found that there are two main methods to reduce the end effect，they are reducing the end friction and controlling the end deformation.The first method is to add friction reducing agent between the indenter of the testing machine and the end of the rock specimen，but it can’t completely eliminate the end effect.The second method is to use the metal specimen with the same mechanical parameters as the rock specimen as the cushion block，but it is too difficult to find this kind of metal material.Based on the second idea，to reduce the influence of end effect in the measurement of uniaxial compressive strength，a new uniaxial compression test method was proposed.In this method，uniaxial compression test was carried out using the rocks with the same material as the specimen as the cushion block.Two combination forms of （25+50+25）mm and （20+60+20）mm are set for the experiment，and the experimental results are compared with those specimens with the height of 50 mm and 60 mm.The results show that the new experimental method can reduce the influence of end effect on the measurement of uniaxial compressive strength of rock and obtain more uniform radial strain.Compared with the specimens with the height of 50 mm and 60 mm，the uniaxial compressive strength of the specimens with the height of （25+50+25）mm and （20+60+20）mm is reduced by 38.41% and 39.69% respectively，ad-ditionally，the strength is also decreased comparing with the standard specimen.Numerical simulation results show that the uniaxial compressive strength of the combined specimen with or without end friction is close to the uniaxial compressive strength of the standard specimen in ideal state.All of the specimens have uniform radial strain without end friction.The radial strain of the combined specimen is relatively uniform when there is end effect.Numerical simulation results prove that the new experimental method reduces the end effect，but it can’t completely eliminate the end effect.

Key words: rock mechanics, end effect, uniaxial compression, numerical simulation, sandstone, cushion block, strain distribution