The Damage Effect Of Pore Pressure On Rock

With long-term exploitation and utilization of mineral resources,deep resources is crucial to develop due to shallow resources of the earth have been nearly exhausted.The deep rock mass is in the complicated mechanical environment with three “high” and one “disturbance”-high ground stress,high earth temperature,high karst water pressure,and mining disturbance,which makes it difficult to mine the deep resources safely and efficiently.The damage effect of pore pressure on rock,as part of the complicated mechanical environment,has been concerned by the researchers of underground engineering,but the damage effect of initial uniform pore pressure on rock is rarely studied.Consequently,physical experiments,theoretical analysis and other methods are used to study the effect of pore pressure on rock strength weakening and structural transformation in detail in this paper.The standard gritstone samples were selected to test the mechanical parameters.And then the pore pressure experiment is carried out by using the conventional triaxial fluid solid coupling experimental system.After the experiment,the mechanical properties of the samples are tested again,and the acoustic emission signal is monitored at the same time of the uniaxial compression experiment.The stress-strain curve,acoustic parameter curve of the rock before and after the pore pressure were obtained with the uniaxial compression test during the acoustic emission monitoring process.The experimental results show that: pore pressure weakens the strength of rock,and its average uniaxial compressive strength is reduced by 25.5%,and its average elastic modulus is reduced by 20.2%.In the same time,the acoustic emission count and energy of the sample were also significantly reduced by 13.8% and 25.5% respectively at the moment of fracturing after pore pressure.Scanning electron microscope test and high-resolution three-dimensional X-ray micrograph test were carried out on the gritstone samples before and after pore pressure.The pore pressure had a structural transformation effect on the rock during the pore pressure process after analysing test results processed by visual software.Before pore pressure,the mineral particles on the surface of the sample,with good integrity,were tightly cemented with each other and the development of pores and fractures was not obvious.Most of the pores had small pore size,low porosity and poor connectivity.After pore pressure,the cementation between the particles on the surface of the samples was destroyed,which created interlaced micro cracks.And many obvious developed pores and cracks in the internal of sample connected pore distribution areas,causing good connectivity between the pores and the porosity increased by 0.49%.Through drawing the histogram of equivalent diameter distribution,it was found that a large number of micro pores have expanded in the rock sample after infiltration and the degree of pore development was mainly controlled by macro pores.By using the control variable method,the effect of pore pressure on the weakening of rock strength was studied when the main factors such as pore pressure,time of pore pressure and confining pressure were changed.The test results showed that the failure mode of rock in the process of uniaxial compression gradually transited from brittle failure to plastic failure with the increasing of pore pressure and time of pore pressure.The uniaxial compressive strength and elastic modulus of rock showed a decreasing trend.The acoustic emission count and energy at the moment of rock fracturing also gradually decreased,which showed that the increase of pore pressure and time of pore pressure can enhance the weakening effect of pore pressure on rock strength,while the confining pressure was the opposite.On the basis of previous studies,based on the fluid-solid coupling effect of pore pressure on rock damage,a fluid-solid coupling mechanical model of single-phase fluid based on elastic damage theory was established.Based on the experimental results in the previous chapters,the rock damage model under pore pressure was obtained.There are 36 figures,9 tables and 81 references in this thesis.