1. Consideration based on segment thickness
The length and diameter of shield segment bolts should first consider the thickness of the segment. Segment thickness is one of the key factors affecting bolt length. Generally speaking, the length of the bolt needs to be long enough to pass through the bolt hole on the segment and to ensure sufficient thread engagement length after the nut is installed to ensure the reliability of the connection. Usually, the bolt length is slightly longer than the segment thickness, and the extra length is used for the installation space of the nut and washer. For example, for a segment with a thickness of 30 cm, the bolt length may be around 32-35 cm, of which 2-5 cm is used for the installation of nuts and washers. The bolt diameter also has a certain relationship with the thickness of the segment. Thicker segments often require bolts with larger diameters to provide sufficient connection strength, because thicker segments will generate greater shear and tension on the bolts when the tunnel is stressed.
2. Calculation based on tunnel diameter and pressure
The diameter of the tunnel and the pressure it bears are of great significance to the size determination of shield segment bolts. As the diameter of the tunnel increases, the external pressure (such as earth pressure, water pressure, etc.) on the segment will also increase. In this case, bolts with larger diameters and appropriate lengths are needed to resist these pressures. Bolts with larger diameters can withstand greater axial and shear forces. For example, in shield projects with large diameter tunnels (such as those with a diameter of more than 10 meters), bolts with larger diameters (such as from 20 mm to 30 mm) and longer lengths (increased accordingly according to the thickness of the segment) may be used compared to those with small diameter tunnels (such as those with a diameter of 3-5 meters). During the calculation process, the forces that the bolts need to withstand are determined by structural mechanics calculations based on the geological conditions of the tunnel, such as the type of soil (soft soil, rock, etc.), the pressure of groundwater, etc., and then the appropriate diameter and length are determined.
3. Consider the influence of the segment connection method
The segment connection method also affects the bolt size. Different connection methods (such as straight bolt connection or bent bolt connection) have different stress conditions on the bolts. When connected with straight bolts, the bolts mainly bear axial forces and certain shear forces; while in bent bolt connections, due to their special connection paths, the bolts may bear additional bending stresses. For segments connected with bent bolts, bolts with larger diameters and appropriate lengths may be required to resist this complex stress state. For example, the diameter of the bent bolt may be 5-10 mm larger than that of the straight bolt to ensure that it can still provide sufficient connection strength while bearing bending stress and ensure the stability of the segment connection.
IV. Reference to engineering experience and standard specifications
When determining the length and diameter of the shield segment bolts, engineering experience and standard specifications are also indispensable bases. In the long-term practice of shield engineering, a lot of experience has been accumulated on the use of bolt sizes under different geological conditions, tunnel diameters and segment types. These empirical data provide important references for new projects. At the same time, each country has corresponding engineering standards and specifications, which stipulate the size, performance and other aspects of shield segment bolts. For example, some countries stipulate that under specific geological conditions and tunnel scales, the diameter of the bolt should be within a certain range, and there are also clear requirements for the proportional relationship between the length and the thickness of the segment.
