Water Seepage Induced Deformation and Stability Control of Weakly Cemented Soft-Rock Roadway

Deep beneath the earth's surface, where coal miners carve out long tunnels to reach valuable seams, the ground is not a solid, unyielding block. In many places, the rock surrounding these tunnels is soft, crumbly, and held together by weak natural glue. This is known as weakly cemented soft rock. When water seeps through the ground and touches this rock, a slow but destructive process begins. The water acts like a solvent, dissolving the weak bonds and causing the minerals within the rock to swell and soften, much like a sponge absorbing water. This transformation turns the rock from a structural support into a weak, muddy substance that struggles to hold its shape. For engineers tasked with keeping these underground tunnels open and safe, this interaction between water and rock is one of the most difficult challenges they face. If the rock fails, the tunnel can collapse, trapping equipment and workers, or simply becoming too narrow to use.

Researchers at the CCTEG Coal Mining Research Institute and Hunan University of Science and Technology set out to understand exactly how this failure happens in a specific coal mine in Ningxia, China. They focused on a tunnel known as the 110803 transport roadway, where the surrounding rock is made of argillaceous soft rock, a type of clay-rich material that is particularly sensitive to water. The team combined on-site inspections, laboratory tests, and computer simulations to watch how the tunnel behaved over time. They discovered that the tunnel's problems were not just about the rock being weak, but about a specific chain reaction triggered by water. The rock contains high amounts of clay minerals that expand when wet, creating internal pressure. At the same time, the water weakens the grip of the metal bolts and cables used to hold the rock in place. The original support system, which relied on these bolts, was simply not strong enough to fight against the swelling rock and the loss of grip caused by the water.

The researchers observed that the tunnel did not fail all at once. Instead, it went through three distinct stages of movement. First, there was a period of rapid change where the roof and floor began to move toward each other quickly. This was followed by a phase of accelerated deformation, where the movement sped up even more as the rock continued to soften and the support system began to lose its hold. Finally, the movement slowed down, but it never truly stopped; the tunnel continued to shift and settle for months, indicating a state of long-term instability. In some sections of the tunnel, the floor heaved upward by more than 700 millimeters, while the roof sank by about 300 millimeters. The sides of the tunnel also squeezed inward by over 600 millimeters. This massive deformation was driven by a combination of factors the team called "one water and three weaknesses." The "one water" was the constant seepage that softened the rock. The "three weaknesses" were the poor foundation where the bolts were anchored, the weak layer that transferred the tightening force of the bolts to the rock, and the overall low strength of the rock itself.

To solve this problem, the team proposed a new strategy they described as "one isolation and three strengthening." The first step, isolation, involved stopping the water from touching the soft rock. They achieved this by injecting grout, a fluid cement-like material, deep into the rock before and after digging the tunnel to seal off the water sources. They also sprayed a layer of concrete over the exposed rock surfaces to create a waterproof barrier. The second part of the strategy involved strengthening the support system in three specific ways. They improved the anchoring foundation by using special waterproof materials that could grip the rock firmly even in wet conditions. They strengthened the layer that transfers the force of the bolts by using larger, stiffer plates and a thicker layer of concrete. Finally, they increased the overall strength of the rock mass by using high-strength cables and injecting more grout to bind the rock together. This created a fully enclosed support system that held the roof, the sides, and the floor in place.

When the team applied this new method to the tunnel, the results were immediate and dramatic. In the sections where the old support system was in place, the tunnel continued to deform significantly, with the roof and floor moving nearly half a meter over a few months. In contrast, the sections reinforced with the new "one isolation and three strengthening" method showed almost no movement. The roof and floor moved less than 50 millimeters over an 80-day period, and the sides moved by only about 30 millimeters. The rock separation, which is the gap that forms between layers of rock as they pull apart, was reduced to a tiny fraction of what it was before. The bolts and cables held steady, and the tunnel maintained its shape. The study demonstrated that by understanding the specific way water and soft rock interact, and by designing a support system that addresses both the water and the structural weaknesses, engineers can keep these deep underground tunnels stable and safe for the long term. This approach offers a clear path forward for mining operations in areas where water and soft rock threaten to undermine the stability of the earth.