Solid grain migration on hydraulic properties of fault rocks in underground mining tunnel

Fault fracture zones and undesirable geological conditions are common in mining tunnel construction and excavation. This is often attributed to deposit construction and complex hydrological conditions. Due to the high permeability, low strength and easy deformability of the surrounding rocks, the fracture faults can create a conducting pathway for water ingress, resulting in serious causalities and economic losses. This can be exacerbated by high-mineralized water emissions and high temperatures. While it is important to repair crushing tunnels after excavation, the vicious cycle may lead to weak excavations, waste resources, and unsafe working environments.

The grain migration effects on seepage has been substantially studied using different approaches. Although radical seepage mode is more common in fault fill, most studies assumed that water inflow flow in a single direction, where the confined water moves from the periphery to the inside wall of the tunnel. Additionally, most studies on grain migration have focused on numerical simulation with a limited experimental study on the effects of radial seepage on ground migration. This has been mainly attributed to the complicated nature of grain migration in the rock mass and poor understanding of the hydrological characteristics and configuration of the existing faults.

Herein, Professor Dan Ma, Dr. Hongyu Duan and Professor Jixiong Zhang from China University of Mining & Technology performed radical seepage experiments to study the effects of solid grain migration on hydraulic properties of fault rocks in underground mining tunnels. In their approach, the authors commenced by performing the one-dimensional (1D) radial sandstone erosion seepage experiments. The experiments were carried out under variable water pressure and grain size distribution. Different hydraulic properties, such as permeability and porosity, were investigated under the 1D radial grain migration and their corresponding effects on the grain migration were also analyzed. In addition, three models for predicting permeability were verified by comparing the experimental and theoretical results. The work is currently published in the journal, Tunnelling and Underground Space Technology.

The research team showed that the porosity and permeability increased with time, and the water inflow process was grouped into four distinct stages: rapid growth, decelerated increase, slow climbing and stable period. The initial two stages were characterized by a significant increase in the porosity and were considered the main phases of fine grain migration. In the other two stages, a gradual decrease in the removable fine grains and its eventual approach to zero was observed. After the seepage test, sample porosity increased from the top to the bottom, attaining maximum at the lowest part, suggesting non-uniform spatial distribution due to radial grain migration.

An increase in the water pressure and decrease in grain size distribution resulted in the fluctuation of the migration ratio and severe fine grain migration associated with significant changes in the hydraulic properties. Among the three verified prediction models, Carman-Kozoeny model produced the most accurate prediction results. Samples with smaller grain migration capacity exhibited higher prediction accuracy for similar predictive models. Furthermore, closeness to water outlet corresponded to higher permeability and the predicted permeability and porosity values had similar spatial distribution.

In summary, the groundwater inflow mechanism induced by mining and fine grain migration was experimentally investigated. Several strategies were presented for controlling and preventing the inflow of water into the faults. First, it was necessary to take the measurements in the initial water flow stages quickly. Second, the rocks surrounding tunnels needed reinforcement to prevent solid grain migration around the region. Lastly, conducting grouting and drainage were shown to suppress the effects of grain migration by improving the grain size distribution and reducing the water pressure of the fault rocks. In a statement to Advances in Engineering, Professor Dan Ma, corresponding author said their findings provided useful insights for advancing underground mining.