In shield tunnel construction, shield segment bolts play a key role in connecting segments and maintaining the stability of the tunnel structure. Different geological conditions will cause shield segment bolts to present different stress characteristics, which profoundly affect the safety and durability of the tunnel.
First, in soft soil layers, such as silty clay or silty clay. This type of stratum is relatively soft, with high compressibility and low strength. During the shield advancement process, the pressure of the surrounding soil on the segments is relatively uniform, but due to the fluidity of the soil, the segments are prone to relative displacement. At this time, the shield segment bolts are mainly subjected to the combined effects of shear force and tension. The shear force comes from the dislocation tendency between the segments, while the tension is to prevent the segments from separating. For example, in shield tunnel projects in some soft soil layers in Shanghai, the design of segment bolts needs to focus on their shear and tensile resistance. Generally, high-strength alloy steel is used, and the bolt diameter and preload are appropriately increased to cope with the deformation and displacement of the segments that may be caused by soft soil layers.
Secondly, hard rock formations have a unique impact on the stress characteristics of shield segment bolts. Hard rock has high hardness and low deformability. When the shield is excavated, the segments are mainly subjected to huge pressure from the rock, while the bolts are mainly subjected to the tension generated by the pressure transmission. In addition, due to the unevenness of the rock and the existence of joints and fissures, the stress on the segments may be locally concentrated, which makes the stress on the bolts also present unevenness. In the construction of hard rock shield tunnels in mountainous areas such as Chongqing, in order to ensure the reliability of the bolts under high tension and uneven stress, in addition to selecting high-quality steel, the arrangement of the bolts will also be optimized, such as increasing the number of bolts in the parts with greater stress or using a special fastening system to disperse the tension and prevent the bolts from failing due to local overload.
Furthermore, in sandy soil formations, the friction between sand particles is large and the stability of the soil is relatively good, but during the shield crossing process, the sand is prone to loosening and collapse. In such formations, in addition to bearing the conventional tension and shear force between the segments, shield segment bolts also need to deal with the instantaneous impact force caused by the flow and impact of sand. For example, in shield projects in desert areas or coastal sandy soil strata, the connection structure of the segment bolts needs to have good buffering and energy absorption characteristics. Elastic washers can be set at the bolt connection or special connection structures can be used to reduce the damage to the bolts caused by instantaneous impact forces and ensure the long-term stability of the segment connection.
Finally, complex geological conditions, such as fault fracture zones or soft and hard uneven strata, are most unfavorable to the stress of shield segment bolts. In these strata, the geological conditions around the segments change frequently, and the bolts may be subjected to multiple complex forces at the same time, including the alternating effects of tension, pressure, shear and impact. This requires that before the design and construction of the shield tunnel, geological exploration should be fully carried out, and the stress state of the segment bolts should be accurately calculated and simulated according to the specific geological changes. Adaptable bolt design and installation schemes, such as bolt systems with adjustable preload or bolts made of composite materials, should be adopted to cope with various challenges under complex geological conditions and ensure the safe and stable operation of the shield tunnel.
