Experimental Study On Expansion Characteristics And Mechanical Characteristics For Limestone And Sandstone On The Coal Roof Under High Temperature

The wall rock will experience certain high temperature effect in these fields, such as underground gasification of coal, coal and gas explosion under the coal mine, exploitation and utilization of geotherm resources, and so on. The mechanical characterics of rock and the mechanical behavior laws of rock play the decisive role to geotechnical design, numerical analysis and long-time safety prediction. Considering the fact that the wall rock of underground gasification furnace of coal has one or two free surfaces and is under high temperature during the gasification, and selecting the limestone and sandstone on the coal roof as study objects, the longitudinal wave velocity, micro-structure, expansion stress, mechanical characterics of limestone and sandstone after high temperature are studied on such three conditions as uni-directional restraint, bi-directional restraints and non-restraint. The statistical constitutive model for rock damage considering temperature effect is established. The study can provide rock mechanical basis under high temperature for such these engineerings as the underground gasification of coal and so on. The research results is valuable to the whole course analysis of other engineering structure under high temperature and some fire damage structures and to improve rock safety. The main research results and conclusions are listed as follows:(1) By the ultrasonic wave experiment, energy spectrum analysis experiment and scanning electronic microscope experiment of limestone and sandstone after high temperature, the reason of longitudinal wave velocity of rock decreasing, the elemental composition and micro-structure characteristics are studied. The variation process of elemental composition and development laws of fractures in the rock are analysed.(2) The expansion stress laws of limestone and sandstone on the condition of high temperature and uni-directional restraint are studied by the experiments and the reasons are analysed. The expansion stresses of limestone and sandstone gradually increase with the temperature increasing during the temperature rising. The relationship curve of expansion stress and temperature is piecewise linear, but the whole curve is close to a second degree parabola. The slope of curve between expansion stress of sandstone and temperature is bigger than that of limestone, which indicates the sensibility of sandstone to temperature is more obvious than that of limestone. The expansion stresses of limestone and sandstone all increase with the temperature-keeping time increasing during keeping temperature before 600℃. The limestone and sandstone reach to the expansion limit, but the detailed process is different. The expansion stress laws of rock under high temperature are caused by the mineral expanding and contracting, crystal transition, chemical decomposition and some gaps and fractures variation in the rock. (3) The heat expansion stress laws of limestone and sandstone on the condition of high temperature and bi-directional restraints are studied by the experiments and the anisotropy reasons of expansion stress are analysed. The expansion stresses of limestone in two restraint directions gradually increase with the temperature increasing during the temperature rising before 600℃, and the expansion stresses decrease after 600℃. The expansion stresses of sanstone in two restraint directions gradually increase with the temperature increasing during the temperature rising before 700℃, but the increasing speed decreases after 600℃. It indicates the temperature intervals which the expansion stresses of limestone and sandstone on the condition of bi-directional restraints reach to the limit in are all in 600℃～700℃. The expansion stresses of limestone and sandstone tend to steady values at last with temperature-keeping time increasing. The research shows the expansion stresses of limestone and sandstone on the condition of bi-directional restraints present anisotropy. The expansion stress perpendicular to bedding plane is bigger than that parallel to bedding plane. The expansion stresses of limestone and sandstone experience three periods during high temperature:the expansion stress increasing period during temperature rising, the expansion stress increasing period during keeping temperature and the expansion stress steady period.(4) A series of unaxial compressive tests for limestone and sandstone specimens after different temperatures have been carried out on the RMT-150C type rigid test machine system for rock. The influence law of temperature to the rock mechanical parameters and failure mode are analysed and summarized. The rock mechanical parameters laws are compared on three kinds of temperament conditions. The stress-strain curves during unaxial compress experience four periods:consolidation, elasticity, yield and failure. The temperature is higher, the stress-strain curve moves towards the right more evidently, and the slpoe of curve becomes less. The limestone and sandstone present brittle failure after reaching to the peak-strength. The peak-stress, elastic modulus, and deformation modulus of limestone and sandstone all decrease with temperature increasing. The peak-strain increases with temperature increasing. High temperature weakens the strength of rock to a certain extent.(5) Based on the strain strength theory and the random statistical distribution hypothesis of rock strength, a statistical constitutive model for rock damage considering temperature effect is established by adopting the damage mechanics theory. Accordingly, the stress-strain curves of rock under different temperature (before 600℃) can be calculated by the stress-strain curve of rock under normal temperature.