# Optimization Study on Longitudinal Joints in Quasi-Rectangular Shield Tunnels
## 2 Test Setup  
### Test Specimens  
Given the fact that when the joints of curved or linear lining segments sustain the same axial force and bending moment combinations, the mechanics in the joints can be considered equal [28], and the specimens in the tests were designed as planes rather than curved in order to achieve more precise bending moment values. In addition to this, all the structural details of the joints were retained as they are used in a real tunnel, such as the grooves for waterproofing belts. Each specimen was composed of two segments, and the size of each segment was 1250 \(\times\) 1200 \(\times\) 450 mm. Two DIJPs were preinstalled in each segment through five connecting reinforcements, as shown in Figure 3. The DIJPs in adjacent segments were connected by two short straight bolts (diameter of 33 mm, yield stress of 480 MPa, tensile stress of 600 MPa). The adopted concrete class was C50 (characteristic cube compressive strength of 50 MPa). The geometric details at the joint section are shown in Figure 4.  
Figure 3: Tested segments: (**a**) ductile iron joint panel (D|JP); (**b**) connecting reinforcements; (**c**) segment after curing.  
Figure 2: Comparison of a quasi-rectangular double-track shield tunnel and two circular single-track shield tunnels.  
In a tunnel, the joints can be subjected to two types of bending moments, which we refer to as positive and negative bending moments. A positive bending moment creates tension at the inner side of the segment and compression at the outer side, while a negative bending moment creates the reverse stress situation. In the test specimens, the original position of the bolt was 200 mm from the inner segment surface, and we define this joint as a Type-A joint. Differently from circular tunnels, the horizontal axis of the quasi-rectangular tunnels has to be strictly controlled to be parallel to the sea level. As such, most of the longitudinal joints are subjected to either positive or negative moments only depending on their positions in the cross-section. This feature makes an optimization of the joint geometry possible based on the occurring bending moment type. It has to be pointed out that the \"joint optimization\" or \"optimizing method\" in this research does not mean that it is proven to be the best possible joint configuration; rather, it reflects a possible improved configuration compared to the original joint design. In order to increase the distance from the bolts to the compression zone so as to resist a higher bending moment, the bolts were moved 50 mm to the inner side for the joints in the case of a positive bending moment (Type-BPos joint) and 50 mm to the outer side for the joints in the case of a negative bending moment (Type-BNeg joint). The three configurations are shown in Figure 5. Each segment had two DIJPs, but only one DIJP of each segment is presented in Figure 5 for brevity. Figure 6 shows the cross-sections of the DIJPs that are indicated in Figure 5. These three configurations are used to compare the effects of the bolt position improvements.  
Figure 4: Geometric details of the joint section (unit: mm).  
Figure 5: Front view of the DIJP configurations: (**a**) Type-BPos; (**b**) Type-A; (**c**) Type-BNeg.