Effect of mass loss on pore permeability in gravel aquifers

During the occurrence of water and sand inrush, the loss of sand grains inside the gravel aquifer is an important cause of its permeability change.In order to study the permeability evolution law of the gravel aquifer during mass loss, the axial compression displacement control method was used to study the sand loss law of different initial porosity specimens under different initial water pressure gradients, and the sensitivity of the evolution of initial porosity to permeability during variable mass permeation was analyzed.The results show that ① the water pressure gradient and flow rate in the sand loss process first start to change slowly from the initial value.Then the infiltration state changes abruptly, the water pressure gradient drops abruptly and the flow rate increases dramatically.Eventually, both stabilize at a certain value, and the increase of the initial porosity and initial water pressure gradient will advance the moment of abrupt change.② The loss of sand during infiltration is influenced by the combination of water pressure gradient, porosity, sand particle size and other factors.Under the same conditions, fine sand is more likely to be lost, and an expression for the change in porosity E is given based on the effect of initial porosity φ₀ and initial water pressure gradient G_p0 on sand loss, which can give a quantitative description of the change of pore structure during infiltration.③ After the permeability state undergoes a sudden change, the permeability of each specimen increases from 10^-13 to 10^-11 m² magnitude.The range of values of the porosity sensitivity coefficient keeps decreasing with the increase of the initial water pressure gradient.The peak of the porosity sensitivity coefficient curve of φ₀=0.404 decreases from 550 to about 200 at one time, and the peak moment of each curve advances with the increase of φ₀ and G_p0, all of them are before the abrupt change stage.It indicates that the effect of initial porosity on permeability is more obvious before the abrupt change of permeability state.With the strengthening of the initial water pressure gradient, the influence of initial porosity on the whole permeability process is decreasing.Based on the above results, the sand loss law is in good agreement with the permeability change of the aquifer.Combining the porosity change E and the porosity sensitivity coefficient C can predict the evolution trend of the permeability of the aquifer during the water inrush and sand collapse.The study of its influence law can be used as an important reference basis for design, implementation and effect evaluation in aquifer grouting reconstruction projects.