Abstract: In high-water-pressure shield tunneling, contact failure zones between the tail brush and tunnel segments arise from factors such as wear, plastic deformation, and the caking and hardening of the tail brush, and further compromise the shield-tail sealing system. To address this issue, rheological testing and computational fluid dynamics (CFD) simulations are used to quantitatively investigate how the rheological properties of the seal grease and the dimensions of the contact failure zones affect sealing performance. Two custom-formulated greases (high-flow, low-viscosity and low-flow, high-viscosity) as well as a commercially available brand seal grease (Grease A) are taken as test subjects. Steadystate shear, dynamic oscillatory stress, and temperature scanning tests are conducted using a rheometer at 25 °C to measure the viscosity-shear rate relationships, yield stress, and viscoelastic transition characteristics. A two-dimensional CFD model is developed, with the tail brush simplified as a porous medium, to simulate the breakdown process under eight different sizes of contact failure areas ranging from 0 to 15 mm. Variations in the flow field, pressure transmission, and failure time are analyzed. The results reveal the following: (1) All three greases exhibit pseudoplastic behavior, with the high-flow-low-viscosity type, low-flow-high-viscosity type, and Grease A exhibiting yield stresses of 5.65, 11.41, and 22.90 Pa, respectively. (2) When the failure zone exceeds 3 mm, system breakdown times significantly decrease, leading to a sharp decline in sealing performance. The low-flow, high-viscosity grease exhibits the longest breakdown time across all failure zone sizes, followed by Grease A, whereas the high-flow, low-viscosity grease has the shortest breakdown time. (3) Contact failure between the tail brush and the tunnel segment critically affects the sealing system’s performance. The low-flow, high-viscosity grease demonstrates the best resistance to breakdown, making it ideal for conditions with significant contact failure zones. By contrast, the high-flow, low-viscosity grease has good fluidity, making it suitable for rapid filling of smaller failure zones. Grease A offers a balanced performance while exhibiting enhanced characteristics of the low-flow, high-viscosity type. (4) In practical applications, the choice of grease should be tailored to the condition of the tail brush; specifically, when the failure zone exceeds 3 mm, management of the shield-tail gap is essential.

Key words: shield-tail seal grease, rheological properties, contact failure, numerical simulation, failure mechanism