Advance Support of Pipe Umbrella System in Loose and Broken Strata in Tunnel

In recent years, pipe umbrella system support has been widely used in the construction of tunnel engineering and mining engineering, and has become one of the effective advanced support methods when passing through loose and broken strata in underground engineering construction. The application of this technology not only simplifies the tunnel construction process, and accelerates the construction speed, but also greatly improves the construction safety and economic benefits.

However, in the current tunnel construction, it is also found that there is still the unreasonable and inappropriate application of pipe umbrella system support, which not only affects the effect of pipe umbrella system support, but also often buries quality and safety hazards to tunnel construction, and even causes accidents.

Application Conditions of Tunnel Advanced Pipe Umbrella Support

According to the principle of rock mechanics, whether the surrounding rock can be stable after tunnel excavation depends on the stress state of the surrounding rock and the strength of the surrounding rock. After the tunnel is excavated, the surrounding rock will be damaged, and even the phenomenon of caving or caving will appear. Therefore, it needs to be carried out safely under the protection of advanced support, otherwise, the working face will be unstable and may be damaged at any time.

Stress Variation of Pipe Umbrella System in Advance Support

In the design and construction of the advanced support of the pipe umbrella system, great attention should be paid to the forced change of the pipe umbrella system at each construction stage to ensure that the pipe umbrella system is stable at each stage without obvious deformation and displacement.

As the tunnel construction progresses, the force of the advanced support pipe umbrella system varies and is divided into four stages.

In the first stage, the pipe umbrella system is driven into the stratum and the steel arch is erected. At this time, the pipe umbrella system is ahead of the excavation surface and is not stressed;

In the second stage, under the protection of the pipe umbrella system, the tunnel was driven forward by 1 cycle footage. At this time, the load of the loose rock above the tunnel acts on the tube shed, and the tube shed can be regarded as a beam bearing a uniform load, mainly bearing the bending moment;

The third stage is to first drive bolts above the tunnel, hang steel mesh, spray concrete, and complete temporary support; then drive into the next circulation pipe umbrella system. At this time, the stress of the pipe umbrella system that was driven in the first is basically the same as that in the second stage, and the role of the anchor rod has not yet been exerted (except for the prestressed anchor rod);

In the fourth stage, the tunnel excavation face is pushed forward by another cycle footage. At this time, due to the influence of rock excavation, the rock at the front end of the pipe umbrella system that is driven first loses its supporting force and tends to sink, so that the bolt is stressed. The displacement of the tube shed is limited, and the tube shed becomes a multi-span continuous beam bearing uniform loads. At this time, the pipe umbrella system and the bolting net together constitute the primary support of the tunnel.

Rock Bolt Length and Lap Length

In the construction of tunnels in loose and broken rock layers, the method of excavating in steps to reduce the excavation height is beneficial to timely temporary support, shorten the exposure time of surrounding rock, and ensure construction safety. It is beneficial to shorten the lap length between tube sheds, reduce material consumption and reduce construction costs.

Rock Bolt Spacing

The advanced support of the pipe umbrella system is mostly used in the loose and broken rock formations. If the spacing is too large, it will not only easily lead to large pieces of rock falling, but also the load on the pipe umbrella system is too large, which is not conducive to the stability and safety of the pipe umbrella system support system. The use of large-diameter shed pipes to improve the system's bearing capacity increases the difficulty of construction; however, if the spacing is too small, it will inevitably increase the workload of pipe umbrella system installation and affect the construction progress. The spacing between the tube sheds can generally be selected between 400 and 800 mm; when the rock is broken and easy to flow, the smaller value is used. It should be noted that the pipe umbrella system groups should be staggered and overlapped to facilitate construction.

Rock Bolt Inclination

In actual construction, the pipe umbrella system is inclined outward at a certain angle. The camber angle of the tube shed should be 3° to 8°. The larger the cycling footage, the smaller the camber angle of the tube shed. In general, the center of the borehole hole in the pipe umbrella system should fall on the outline of the tunnel excavation, and the bottom of the hole should fall outside the outline.

Summarize

In tunnel construction, when encountering strata that are not strong enough to maintain their own stability, such as loose and broken, advance support is required. The pipe umbrella system is a commonly used form of advanced support. Its function is to maintain the stability of the surrounding rock of the tunnel, ensure the safety of construction, and at the same time limit the deformation and displacement of the surrounding rock, control the subsidence of the tunnel roof, and create good conditions for the later secondary support construction.