## 2 Project Overview  
The Beijing South-to-North Water Diversion Auxiliary Project, which aims to partially solve city water supply problems, involves a ring of tunnels surrounding the city areas of Beijing, the capital of China. The target tunnel is the final part of the ring, and its completion means that this water supply project can be put into operation. The job site of the tunnel lies in the northwestern area of the city, and the ground surface is already well planned. As shown in Figure 1, there are many houses, scenic sites, and public facilities in the vicinity. The tunnel, 1711 m in length, was excavated by a slurry shield machine, and a launch shaft and a reception shaft were necessary to accommodate the machine. Usually, a separation plant is arranged near the launch shaft to optimally adapt to the requirements of the tunneling process. Due to the limitation of the ground surface, the separation plant was finally set up within close proximity of an auxiliary shaft, which was connected to the launch shaft by an existing tunnel approximately 842 m away from the launch shaft, as shown in Figure 1. The extended slurry feed pipelines and discharge lines were simply laid on the floor of the existing tunnel, which undoubtedly worsened the problem of pipeline damage and wear over the period of slurry shield driving. As shown in Figure 2, after being launched, the slurry shield machine drove downslope at a slope of 2.1% until reaching a tunnel overburden depth of approximately 36 m and then drove almost horizontally to the end. Before tunnel excavation, a series of boreholes were drilled to examine the geological conditions. In this project, there were 10 boreholes in the cobble section, 17 boreholes in the bedrock section, and 1 borehole in the clay section. Soil and rock samples were tested to determine their getotechnical parameters. Finally, the excavated ground was divided into three sections, namely, the pebble layer, rock ground, and clay with gravel, which have horizontal lengths of 848 m, 830 m, and 33 m, respectively. Locally, sandy clay with gravel, siltstone, and mudstone were encountered. The pebble layer, with a pebble content of approximately 55-65% and grain size of 4-6 cm, is well graded in general and mainly backfilled by medium sand, and a small number of cobbles and boulders are found in the layer. The discovered pebble and cobble particles are rounded or subrounded but very hard, and the uniaxial compressive strength of some samples exceeds 200 MPa. The rock ground can be subdivided into two parts: a fault fracture zone and a zone of sandstone and quartz sandstone. The fault fracture zone mainly consists of weathered sandstone and quartz sandstone, and their strength, in the range of 5-101 MPa, is closely related to the degree of weathering. For the fresh sandstone and  
Figure 1: Plan view of the project. quartz sandstone encountered, a strength of over 100 MPa was measured in some test samples. Fragmented rock particles due to rock cutting by disc cutters of the slurry shield machine when tunneling in the rock ground exhibited sharp angularities. The pipes used for the slurry circuit, 300 mm in outer diameter and 250 mm in inner diameter, were made of Chinese standard Q235B mild steel. The key chemical composition of the steel is listed in Table 1. The slurry circulation system was composed of pipes (straight and curved), elbows/bends, pumps, pipe joints, and gate valves. As shown in Figure 3, a grooved joint was first employed due to its easy assembly and therefore lower labor cost, but was later changed to a flanged joint because of frequent leks at the grooved joints. Because the slurry discharged had a high content of large solids, a knife gate valve was used to completely shut off the fluid flow when necessary. The valve owes its ability to handle these fluids to a knife-edged disc.