Mechanical Properties of the Shield Tail Brush and Its Pressure Distribution Acting upon the Segment  
[PERSON]  
1 Department of Geotechnical Engineering, College of Civil Engineering, Tongji University,  
Shanghai 200092, China; [EMAIL_ADDRESS] (W.D. ); [EMAIL_ADDRESS] (Y.Q. ); [EMAIL_ADDRESS] (Y.Q. );  
[PERSON]  
1 Department of Geotechnical Engineering, College of Civil Engineering, Tongji University,  
Shanghai 200092, China; dingruq@tongji. According to extensive research, the force upon the tunnel segment induced by tail brushes is considerable. According to the mechanical test and field measurement, the pressure upon the segment would reach the maximum value while the shield tail brush passes through, which is about twice the theoretical earth pressure, and even up to four times more than the theoretical earth pressure in some projects, resulting in a significant effect on the distribution of the segment internal force [11; 12; 13; 14]. The more unfavorable situation is that when the shield adjusts its posture, the shield tail clearance changes, leading to the tail brushes behind the segment compressing the segment unevenly and to a great risk of dislocation, damage, and cracking of the segment [15; 16; 17; 18; 19; 20] (Figure 2), in turn causing great risk to engineering safety. Therefore, the shield tail brush pressure has gradually attracted more attention from researchers. [PERSON] et al. [15] and [PERSON] et al. [21] took the tail brush in the shield shell into consideration in 3D numerical models to estimate the effect of tail brush extrusion on segments under different shield postures. The results suggest that shield tail brush extrusion would cause extremely uneven pressure on the segment, resulting in a large internal force and displacement of the segment, while the dislocation of the key block usually exceeds the safe limits, and tends to cause cracks and fracturing of the block. Figure 1: Shield tail brushes in an actual project [10]. Figure 2: Common defects in the segment during shield tunnel construction [22]. The aforementioned efforts have considerably improved many aspects of our understanding of shield tail brush pressure. However, the existing studies focus on the effects of the tail brush extrusion on segments, but few studies have looked at the mechanical properties of the tail brushes and the method to calculate the magnitude of the tail brush extrusion pressure, which is the key factor in reasonably predicting the extrusion effect. In addition, the relevant research on the properties of the shield tail brushes is mostly limited to the structural design [23], wear resistance [24] and replacement schemes [25; 26]. As a remedy for this deficiency, [PERSON] et al. [27], [PERSON] [28], [PERSON] et al. [29], and [PERSON] et al. [30] conducted mechanical tests on the shield tail brush. [PERSON] et al. [27] conducted a compression test on the tail brush and in situ monitoring of the pressure acting upon a large shield tunnel, and obtained the characteristics of the load-compression curve, which shows that the tail brush pressure increases linearly with the compression deformation at first, then increases exponentially. In addition, they studied its effect on the segment internal force and cracking by numerical simulation and in situ monitoring, pointing out that the uneven shield tail brush pressure may be the main factor leading to the cracking of a segment. [PERSON] et al. [28] focused on the sealing performance of the tail brush and studied the deformation of the tail brush under different shield tail clearances and transverse grease pressures. [PERSON] et al. [29] and [PERSON] et al. [30], while also focusing on the sealing performance, carried out cyclic loading compression experiments on the tail brush. The results show that the pressure on the tail brush increases slowly at first and then rapidly with the decrease in the tail clearance. Additionally, the cyclic loading will lead to a gradual accumulation of plastic deformation of the tail brush.