| The rock mass of deep underground engineering is broken due to excavation and unloading,and the broken surrounding rock still has a certain load force.Together with the supporting structure,it maintains the stability of the engineering structure,and continues to suffer from engineering disturbance,resulting in the repeated failure state of the surrounding rock after breaking and then loading.In view of this characteristic,a series of triaxial loading and unloading tests were carried out with ZTCR-2000 rock triaxial test system,taking fractured sandstone as the research object,to study the mechanical properties of fractured rock under single loading and cyclic loading and unloading,and to analyze the stress-strain curve,strength and deformation characteristics,residual strain and failure forms of fractured rock.The energy distribution and evolution law of fractured rock in the process of cyclic loading and unloading are discussed,and the following main research results are obtained:(1)Under the triaxial condition,the peak stress and strain values of intact sandstone increase with the increase of confining pressure,but the elastic modulus remains unchanged.When the cracked sandstone prepared by loading under the confining pressure of 5MPa is loaded again under different confining pressures(2MPa,4MPa,6MPa and 8MPa),the post-peak curve of the cracked sandstone shows sudden drop,softening and similar hardening with the increase of confining pressure.(2)In the cyclic loading and unloading test,the axial hysteresis rings of complete sandstone samples all move to the right,but the cyclic hysteresis rings move to the right only when the upper limit of the cycle is 0.4 times the peak stress;Under three stress levels,the movement directions of the ring to the hysteresis ring of the fractured sandstone are all to the right.(3)The peak strength of intact sandstone decreases with the increase of stress ratio after the constant amplitude cycle,but the peak strength of fractured rock samples increases with the increase of stress ratio after the cycle.In addition,under the constant amplitude cycle condition of 0.8 times the peak stress,the secondary peak strength is greater than the first peak strength under each confining pressure.(4)The loading and unloading deformation moduli of complete rock samples in the cycle process are basically equal,and the number of rings required increases with the increase of stress level;However,the loading and unloading deformation moduli of the broken rock samples are close to each other and equal quickly after the 2nd ～ 3rd cycle under all stress levels.(5)The development stages of residual strain in the whole cycle are divided into acceleration and stability stages.The specific gravity occupied by the acceleration stage of the intact rock sample increases gradually with the increase of the stress level,while the specific gravity occupied by the stable stage of the broken rock sample occupies the majority of the cycle under each stress level.(6)By introducing the elastic strain-energy ratio and dissipated energy ratio coefficient,the relationship between the elastic strain-energy ratio and dissipated energy ratio and stress level in 30 cycles is analyzed.It shows that the stress ratio can not only understand the transformation law of dissipated energy and strain energy,but also predict the instability of intact and fractured rock samples in advance.(7)Shear failure is the main failure mode of intact and broken rock samples under the conditions of monotone loading and cyclic loading and unloading,indicating that the loading condition and state of rock samples(whether it is broken or not)will not have a great influence on the final failure mode of rock samples.Figure [38] table [8] reference [37]... |
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