# Numerical Analysis of Friction Reduction of Grouting in Long-Distance Pipe Jacking
## 1 Introduction
This process results in an increase in density and the development of internal forces within the slurry. These forces can influence the stability, pressure distribution, and overall performance of the slurry in its application. [PERSON] et al. [10] conducted detailed research and discussion on the slurry scheme for large-diameter, long-distance pipe jacking based on the construction of the Hong Kong-Zhuhai-Macau Bridge tunnel at the Gongbei site. Through the research, the engineers significantly reduced the friction resistance in practical engineering and solved the problem of excessive jacking force. The grouting process is as follows. At the beginning of the grouting, we need to pressurize first, then jack. During the process, we need to pressurize and jack simultaneously and regrout timely. An appropriate amount of friction-reducing slurry is injected into the outer wall of the pipe section through the grouting annular pipe. By using a multi-point symmetrical grouting method, the slurry can be evenly distributed in the gap between the outer wall of the pipe section and the surrounding soil, thereby reducing the friction resistance between the pipe section and the soil and lowering the jacking force. The main technical parameters of grouting are as follows:  
1. Grouting Volume: This parameter quantifies the relationship between the pipe jacking length and the thickness of the grout layer. By dynamically monitoring the grouting volume, jacking length, and grout layer thickness, the actual friction-reducing effect of the grouting can be evaluated. 2. Grouting Pressure: During the grouting process, it is essential to strictly control the grouting pressure. On the one hand, if the pressure is too low, the grouting effect may not be achieved. On the other hand, if the pressure is too high, it may fail to form an effective grout layer and could cause grout leakage (overflow). 3. Grouting Speed: The grouting speed is influenced and controlled by several factors, including the arrangement of the grouting holes, the formation of the slurry sleeve, and the jacking speed. There are generally two methods for grouting in long-distance pipe jacking: 1. Double-layer Grouting Method: The double-layer grouting method effectively reduces friction resistance by forming a protective layer between the slurry and the surrounding soil. This method first injects a consolidation grout behind the jacking head. As the jacking head and pipe advance, the consolidation grout gradually hardens to form a solid shell. Then, an expansion device compresses the consolidation grout, forming a strong protective layer. Finally, grout is injected into the gap between this protective layer and the pipe to ensure the integrity of the grout sheath. This method requires two types of slurry to be stored and injected separately, with consolidation grout primarily used for support and lubrication and supplementary grout used for further lubrication [11]. 2. Pipe Membrane Pushing Method: This method installs a pipe with a plastic membrane inside at the rear of the tunneling machine. During the jacking process, the plastic membrane is continuously drawn out from the pipe, and grout is injected between the membrane and the pipe to facilitate pipe sliding, avoiding uneven grouting and grout leakage. Regardless of the grouting material or method used, the similar grouting-friction-reduction processes work as follows: after the grout comes into contact with the soil, the grouting pressure causes the free water in the grout to permeate into the cracks of the surrounding soil, leading to water loss in the grout [12]. As the permeation process continues, the grout gradually transforms into a gel-like substance, filling the voids in the soil. During this permeation process, a dense permeation layer forms between the grout and the soil, causing the solid particles in the grout to adhere to this layer, forming a low-permeability slurry sheath. This reduces water loss from the grout.