Study On The Size Effect Of Rock Mechanical Parameters Of Multiple Lithologie

How to obtain the strength of rock accurately and quickly is of great significance to the structural design of rock mass engineering.Due to the heterogeneity and anisotropy of the rock itself,it has obvious size effect characteristics,and the rocks used in the project are usually orders of magnitude larger than the laboratory samples,so it is necessary to carry out laboratory tests to study the law of rock mechanical parameters change with size.Moreover,the rock samples of different lithologies are affected by geological processes,so the study of the rock sample size effect of different lithologies is of great significance for engineering construction and industrial development.In this paper,based on previous studies,uniaxial compression tests were conducted on rock samples of different lithologies to comprehensively analyze the effects of lithology and dimensional changes on the mechanical properties of rock samples,to explore the trend of axial strain-radial strain curves of different rock samples,and to analyze the causes of dimensional effects.The main research contents and conclusions are as follows:(1)Uniaxial compression tests were carried out on sedimentary rock(siltstone and bluestone),magmatic rock(two kinds of granite),metamorphic rock(two kinds of marble)and rock-like gypsum samples respectively to analyze the influence of lithology and size changes on rock mechanical properties,and combined with previous scholars’ research results,The variation trend of uniaxial compressive strength of different rock samples at rising stage and failure forms of rock samples at different sizes are analyzed.The results show that when the ratio of height to diameter is 2:1,the size effects of strength,elastic modulus and deformation modulus of different rock samples are different with the increase of size,and the variation trend of strength rising area can be divided into linear,logarithmic and exponential forms.The strength of sedimentary rock and magmatic rock samples increases linearly and logarithmically with the increase of size,the strength of metamorphic rock samples increases logarithmically with the increase of size,and the strength of rock-like materials increases exponentially with the increase of size.Poisson’s ratio has no definite change law.The failure of siltstone,celestone and two kinds of marble develops from single inclined plane shear failure to X conjugate inclined plane shear failure.The failure of two kinds of granite is mostly X conjugate inclined plane shear failure,and the failure of gypsum sample is mostly compound failure.(2)The static strain measurement system is used to measure the rock deformation,analyze the axial strain-radial strain curves of different rock samples,and discuss the irregularity of Poisson’s ratio data.The results show that the axial and radial strain curves of siltstone,bluestone and white marble samples are concave,and the axial and radial strain curves of granite,off-white marble and gypsum samples are linear from the elastic stage to the stable development stage of micro-fracture.The non-definite size effect of Poisson’s ratio is mainly caused by the difference between the location of strain gauge sticking and the location of rock failure main crack.(3)Combined with the previous research results and the relationship between the failure form and strength of the rock samples in this test,the influence of rock heterogeneity and the end face friction effect of the indenter and the end of the sample on the rock size effect was discussed,and the main causes of the rock size effect were analyzed.The results show that the main reason for the size effect of rocks is their own heterogeneity,followed by the end effect.The test results can provide data support for the uniaxial test instrument in the selfdeveloped portable "backpack laboratory" in-situ testing equipment(can test the rock sample size of φ20 mm×40 mm,and then obtain the strength of a rock sample size according to the size effect of uniaxial compressive strength),and can also provide reference and basic data for the study of size effect of small size rock sample.