Abstract
In order to investigate the water inrush mechanism in completely weathered granite, a large-scale triaxial testing system is designed and manufactured, which can induce the mass transfer and monitor the flow properties. Using this system, the effects of water pressure and initial porosity on the mass transfer and flow properties were determined, and the relative critical conditions for water inrush were proposed. The results indicate that (1) the particle transfer could cause an increase in porosity, permeability, and water inflow, which is the essential reason for water inrush in completely weathered granite. (2) Due to the effect of particle transfer, the flow properties may change from a Darcy to a non-Darcy flow, which is a key signal for water inrush. (3) With the increase of water pressure, the mass transfer, permeability, and water inflow increased gradually, and a critical value (p=0.6 MPa) that caused the water inrush was obtained. Furthermore, with the decrease of initial porosity, the mass transfer and flow properties were suppressed rapidly, and a critical porosity (0.23) to anti-inrush was observed. The results obtained can provide an important reference for understanding the mechanism, forecasting the risk, and taking the effective control measures for water inrush.
Highlights
In recent years, a large number of tunnels have been constructed in unfavorable geology such as completely weathered granite, karst, and fault [1,2,3,4], causing various challenges to the geotechnical engineers, for example, collapse [5], land subsidence [6], groundwater inflow [7, 8], and total environmental hazards [9]
In order to investigate the impact of water pressure and porosity on seepage-erosion properties of completely weathered granite and to propose the respective critical conditions for water inrush, a series of tests consider the last factors as water pressure and initial porosity were conducted, and the mass transfer, porosity, water inflow, permeability, and the flow pattern through the tests were monitored
The flow pattern may be changed with the mass transfer; at the beginning, the flow is a Darcy flow; with the continuous particle transfer, the flow may be transferred into a nonlinear flow, leading a rapidly increase of permeability and water inflow, which is a significant signal for water inrush
Summary
A large number of tunnels have been constructed in unfavorable geology such as completely weathered granite, karst, and fault [1,2,3,4], causing various challenges to the geotechnical engineers, for example, collapse [5], land subsidence [6], groundwater inflow [7, 8], and total environmental hazards [9]. The completely weathered granite, one of the typical unfavorable geologies, is widely distributed in subtropical, tropical, and humid temperature zones [10,11,12]. This material has the properties of weak water stability, low strength, and strong disintegration [13]. In order to analyze and forecast the water inrush, many scholars proposed various simplified methods, such as water inrush coefficient [17, 18], underlying belt theory [19], and key strata model [20] These methods oversimplify the geological conditions and cannot accurately predict the probability of water inrush. Considering the flow properties, some scholars proposed the flow stress coupling models which the flow equation adopted a Darcy or modified Darcy’s equation [21,22,23] and many other analytical and semianalytical solution to predict the water inflow and water inrush ([7, 18, 24]); these models do not consider the erosion property and nonlinear flow, which cannot reflect
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