What is Tunnel Pipe Roof Forepoling?

What is Tunnel Pipe Roof Forepoling?

Forepoling Method for Underground Projects

forepoling technology refers to the technical measures to form an arch-shaped continuum in the tunnel cross-section by injecting slurry, freezing, drilling steel pipes, steel plates, and anchors into the stratum in front of the palm face before the tunnel excavation, so that it can strengthen the stratum in front of the hole while using its support force to keep the stability of the soil in front and reduce the amount of surface settlement.

The study shows that grouting reinforcement can improve the strength and deformation modulus of surrounding rock, and fundamentally improve the deformation law of surrounding stone. The data show that the strength of sandstone after grouting can increase by 50 % -70 %, and the power of siltstone and argillaceous rock can increase by 2-4 times. The increase in rock strength can reduce the support load by 2/3-4/5.

The practice has shown that forepoling systems can effectively limit ground settlement and keep the natural ground in a stable state for tunneling. Research shows that 30%-40% of the entire settlement of the ground and 40%-50% of the entire settlement of the underground strata occurs before the general support starts to take effect, and the forepoling has a 30-35% inhibiting effect on the ground settlement and a 40% inhibiting effect on the settlement of the strata on top of the tunnel (vault), so it is very important to reinforce the strata in front of the palm face to inhibit ground settlement. Therefore, reinforcing the ground in front of the palm face is very important to suppress ground settlement. As an auxiliary construction method to reinforce the stratum, stabilize the vault and palm face, and reduce the ground settlement, forepoling technology has been widely used in underground engineering construction.

According to the influence of reinforcement measures on the surrounding strata characteristics and stress distribution, the forepoling method can be divided into the strata improvement method and the pre-support method. The stratum improvement method is to improve the characteristics of soil around the excavation face. This method includes grouting, soil reinforcement, drainage, and stratum freezing. The forepoling method is to reinforce the surrounding rock in advance before the tunnel excavation to increase the self-stability of the surrounding rock and minimize the stress interference around the excavation face. Forepoling methods mainly include pipe shed method, mechanical pre-cutting method, pre-lining method, horizontal jet grouting method, advanced small pipe method, advanced bolt method, freezing method, and so on. Among them, the pipe shed method, horizontal rotary jet grouting method, small pipe method, and other supporting methods also improved and strengthened the stratum.

Pipe Shed Grouting Support

It is a set of steel pipes drilled into the stratum along the excavation outline and combined with a steel arch to form a strong scaffold pre-support reinforcement system, which supports the load from the upper part of the pipe shed and injects slurry into the stratum by pressurizing the plum-shaped grouting holes of the steel pipes to strengthen the weak and broken stratum and improve the self-stabilizing ability of the stratum. Pipe shed grouting is a long-distance overrun support method with long overrun distance and large stiffness, which is suitable for the stratum that cannot be self-stabilized and water-bearing at the palm face, and has a better effect on controlling surface settlement and preventing seepage and stopping water, and has higher construction technology requirements. Such the combination of pipe shed grouting and small conduit supplemental grouting method, in addition to the characteristics of a large pipe shed, can prevent the collapse of the triangular soil below the pipe shed, this combination of long and short pre-supporting effect is more ideal.

Main Functions and Advantages of Pipe Shed Support

(1) Beam arch effect: The pipe shed applied first, with the palm face and rear support as the fulcrum, forms a beam structure, both of which form a shell-like structure around the tunnel outline, which can effectively inhibit the surrounding rock from loosening and collapsing.

(2) Reinforcement effect: The slurry injection is pressed into the fissures of surrounding rock through the pipe wall holes, which makes the loose rock body gel and solidify, thus improving the physical and mechanical properties of the weak surrounding rock, enhancing the self-supporting capacity of the surrounding rock, and achieving the purpose of reinforcing the weak surrounding rock around the steel pipe.

(3) Ring groove effect: the propagation of the blast shock wave generated by the palm face blast and the expansion of the blast gas is reflected, absorbed, or bypassed after encountering the dense ring hole groove of the pipe shed, which greatly reduces the extent of damage and disturbance to the surrounding rock caused by the reverse stretching wave.

(4) To ensure construction safety: pipe shed support stiffness, construction, such as collapse, the collapse of ballast is also falling on the upper part of the pipe shed rock ballast, play a buffering role, even if the pipe shed destabilization, the damage is also slow.