The core issues in tunnel construction are essentially concentrated in the excavation and support processes, with other tasks serving as means to address and solve these core problems. As an active support method, anchor technology is applied in tunnel engineering to enhance the mechanical characteristics of surrounding rock, increase its load-bearing capacity, and demonstrate numerous advantages, thus becoming the primary form of tunnel rock support.
Anchor rods serve as a technique for internally reinforcing the rock mass, enhancing the anti-cracking strength of fractured rock, improving the physical and mechanical properties of the rock, and connecting discontinuous rock masses. This effectively leverages the self-bearing capacity of the rock mass. Among various anchoring methods, the use of self-drilling anchor systems is increasingly prevalent in tunnel support projects.
1. Introduction to Self-Drilling Anchor System
The self-drilling anchor system is an anchoring tool that integrates drilling, grouting, and anchoring functions into a single system. It ensures effective anchoring in complex geological conditions (such as fault zones, fractured zones, soft rock, and soil layers where conventional drilling is challenging). It outperforms other conventional anchor rods in terms of tensile strength and yield. Additionally, it can be easily cut, connected, pre-stressed, and load-relieved. Self drilling anchor system is characterized by its reliability, efficiency, and simplicity.
2. Performance and Characteristics of Self-Drilling Anchor System
2.1. The self-drilling anchor bolt is fabricated from high-quality thick-walled seamless steel pipe material, combined with self-drilling anchor drill bits and other self-drilling anchor accessories, achieving a unified functionality of drilling, grouting, and anchoring. The self-drilling anchor bolt serves as both a drilling rod and an anchor rod. In weathered rock, fragmented rock layers, backfill zones, sand, clay, gravel layers, etc., it can form anchor holes without the need for casing protection, ensuring effective anchoring and grouting results.
2.2. The self-drilling anchor system offers efficient grouting, filling cracks, consolidating rock and soil layers, with a good grout diffusion radius and reliable anchoring quality.
2.3. Equipped with a high-penetration drill bit, the self-drilling anchor system can easily penetrate various types of rock under the action of typical rock drilling machinery.
2.4. The self-drilling anchor bolt doesn’t need to be removed after drilling. Its hollow structure serves as a grouting channel, allowing grout to be injected from inside to outside. Grout enters from the rod’s end, eventually overflowing from the top of the drill bit and returning to the back end for grouting. This ensures complete filling of gaps around the anchor bolt, solidifies the rock mass, and guarantees grouting and anchoring quality. The grout plug ensures compact grouting and can reach pressures of 1.5MPa to 2.0MPa.
2.5. The self-drilling anchor system is equipped with washers and nuts, allowing pre-stressing of the contact layer.
2.6. The continuous thread design enables the self-drilling anchor bolt to be cut and connected as needed, making it suitable for construction in narrow workspaces. Simultaneously, the continuous thread design gives the self-drilling anchor bolt stronger bonding assistance compared to smooth steel pipes.
3. Key Points of Self-Drilling Anchor Installation
Before excavating the heading face, excavate along the designed contour line of the tunnel roof, drill a row or multiple rows of longitudinal self-drilling anchor rods into the rock mass, and then perform cement grout injection to form a consolidated support structure at the top. Subsequently, excavation is carried out under the protection of this structure. After excavating a certain distance, repeat the above steps and continue in a cyclic manner.
4. Construction Process of Self-Drilling Anchor Support
4.1 Installation of Self-Drilling Anchor
4.1.1 Mark the anchor bolt positions along the designed excavation contour line at the specified spacing.
4.1.2 Anchor assembly. ①Inspect the water holes of the self-drilling anchor bolt and drill bit for any obstructions and clear them if necessary; ②Connect the drill bit to the anchor bolt; ③Connect the anchor bolt to the rock drill.
4.1.3 Align the anchor bolt with the designed drill hole position and initiate the drilling process. When drilling in soft rock, the water holes of the drill bit are prone to clogging, so drilling speed should be reduced.
4.1.4 Once the designed drill hole depth is reached, use water or high-pressure air to clean the hole, ensure its clearance, then detach the rock drill, leaving 10-15cm of the anchor rod exposed at the hole entrance.
4.1.5 During the grouting process, ensure continuous grouting operation.
5. Construction Precautions
5.1 Ensure a certain distance between drilling points during construction; otherwise, excessive water injection into the rock may lead to collapse of the tunnel face.
5.2 During the drilling process, it is crucial to maintain clear water holes in both the anchor bolt and drill bit. Pay attention to the water flow from the holes. If water hole blockage occurs, retract the anchor bolt by approximately 50cm, repeatedly clean the holes to ensure water hole clearance, and then proceed with drilling slowly until reaching the design depth.
5.3 The grout should be mixed strictly according to the specified proportions and used immediately to prevent premature coagulation during the grouting process.
5.4 If bod blockage occurs during the grouting process, promptly clean the anchor bolt, grouting hose, and pump.
In Conclusion
The self-drilling anchor system integrates drilling, grouting, and support functions, avoiding potential hole collapse issues associated with traditional anchor systems that could lead to rework, thus effectively saving manpower and resources. Additionally, the self-drilling anchor system can be applied in complex rock formations, overcoming challenges such as hole collapse and insufficient grouting often encountered with conventional systems, ensuring overall stability of the surrounding rock.
