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Gold Science and Technology ›› 2021, Vol. 29 ›› Issue (3): 382-391.doi: 10.11872/j.issn.1005-2518.2021.03.003

• Mining Technology and Mine Management • Previous Articles    

Study on a New Method of Weakening End Effect in Uniaxial Compression Test

Jingkai JIA1,2(),Gun HUANG1,2(),Long WANG1,2,Qiang CHENG1,2,Libing ZHEN1,2   

  1. 1.State Key Laboratory of Coal Mine Disaster Dynamics and Control，Chongqing University，Chongqing 400044，China
    2.School of Resources and Safety Engineering，Chongqing University，Chongqing 400044，China
  • Received:2020-12-23 Revised:2021-03-04 Online:2021-06-30 Published:2021-07-14
  • Contact: Gun HUANG E-mail:cqujjk@163.com;hg023@cqu.edu.cn

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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

CLC Number: 

  • TD315

Fig.1

Combined specimen form"

Table 1

Test scheme"

试验编号上垫块高度/mm待测试件高度/mm下垫块高度/mm
10500
20600
3206020
4255025
501000

Table 2

Uniaxial compressive strength of rock specimens"

试件类型试件高度/mm单轴抗压强度（均值）/MPa单轴抗压强度降低幅度/%
单一试件5060.7938.41
组合试件25+50+2537.44
单一试件6067.6239.69
组合试件20+60+2040.78
标准试件10056.73

Fig.2

Radial strain at different positions of specimens"

Fig.3

Failure pattern of rock in each test"

Fig.4

Numerical model"

Table 3

Parameters of numerical model"

模型体积模量 /GPa剪切模量 /GPa弹性模量 /GPa泊松比黏聚力 /MPa内摩擦角 /（°）
岩石3.32.050.251245
压头111.183.32000.20--

Table 4

Contact surface parameters of different schemes"

试验方案有无摩擦接触面 编号接触面参数
黏聚力/MPa内摩擦角/（°）
1无接触面-100
接触面-2830
2有接触面-1415
接触面-2830

Table 5

Uniaxial compressive strength of all tests"

模型有无摩擦单轴抗压强度/MPa
50无58.32
有86.67
25+50+25无58.09
有58.60
100有59.27
无58.04

Fig.5

Section location"

Fig.6

Radial strain nephogram without end friction"

Fig.7

Radial strain at different positions without end friction"

Fig.8

Radial strain nephogram with end friction"

Fig.9

Radial strain at different positions with end friction"

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