With the rapid development of the tunnels constructed under the rivers and seas, the research on face stability of shield tunnel in water-rich sand has important theoretical value and engineering application significance. In addition to the loads exerted by overlaying strata, the tunnels constructed in water-rich strata are usually subjected to high hydrostatic pressure or seepage forces, which are apt to cause the ground collapse of the shield tunnel face. The distribution of hydraulic head field around the tunnel face is critical to assess the impacts of the seepage forces on the tunnel face stability. This paper investigates the axisymmetric problem of the face stability of the shield tunnel under a seepage condition within the framework of limit equilibrium analysis. First, numerical simulations are carried out in this paper to analyze the distribution rules of total hydraulic head and pore water pressure near the tunnel face of the shield tunnel under the condition of stable seepage with different cover depths. Then, based on the distribution rules of total hydraulic head, new formulas for predicting the total hydraulic head along the horizontal and vertical directions are proposed and compared with the numerical simulations in this paper and existing approximate analytical solutions. Second, the classical axisymmetric limit equilibrium model is revised by incorporating the new approximate analytical solutions of hydraulic head field to determine the failure modes and the limit support pressures with a numerical optimization procedure. Lastly, the comparisons of the results obtained from the theoretical analysis model in this paper and the existing approaches are conducted, which shows that the failure mechanism proposed in this paper could provide relatively satisfactory results for the limit support pressures applied to the tunnel face.

Shape is an important physical property of aggregates. However, the large amount of work required to measure with precision the size of coarse aggregate and the lack of relevant tests obscure how the shape of the coarse aggregate affects the strength of concrete. In this work, granite coarse aggregate was first divided into flaky stone and massive stone by manual screening. Next, the coarse aggregate was selected at random for size and volume measurements to obtain and analyze the aggregate shape parameters such as sphericity and flatness. Based on the selected aggregate, three aggregate combinations were designed according to the content of flaky stone. Finally, we measured the compressive strength and splitting tensile strength of hydraulic concrete (7, 28, and 60 days) with water‐binder ratios of 0.33, 0.41, and 0.5. The results show that the three aggregates clearly differ in shape, and the measured compressive strengths and splitting tensile strengths of the concrete with water‐binder ratios of 0.41 and 0.5 are similar. However, the compressive strength of the concrete with water‐binder ratio of 0.33 is significantly correlated with the three selected aggregate combinations.

The measured temperature of a concrete pouring block depends strongly on the position of the buried thermometer. Only when the temperature measured by the thermometer accurately reflects the actual temperature of the concrete pouring block do reasonable temperature-control measures become possible. However, little research has been done on how to determine the proper position of thermometers buried in a concrete pouring block embedded with cooling pipes. To address this situation, we develop herein a method to determine the position of thermometers buried in a concrete pouring block. First, we assume that the design temperature-control process line characterizes the average-temperature history of the concrete pouring block. Under this assumption, we calculate the average-temperature history of the concrete pouring block by using the water-pipe-cooling FEM, following which the temperature history of an arbitrary point in the concrete pouring block is obtained by interpolating the shape function. Based on the average-temperature history of the concrete pouring block and the temperature history of the arbitrary point, we build a mathematical model to optimize the buried position of the thermometer and use the optimization algorithm to determine this position. By using this method, we establish finite-element models of concrete prisms with four typical water-pipe spacing cases for concrete-dam engineering and obtain the geometric position of the thermometers by using the optimization algorithm. By burying thermometers at these positions, the measured temperature should better characterize the average-temperature history of the concrete pouring block, which can provide useful information for regulating the temperature of concrete pouring blocks.

In this paper, numerical simulations of face stability of shallow tunnels are carried out by using the particle discrete element codes PFC2D from the microscopic view. Progressive instability of the tunnel face is achieved through the withdrawal of the baffle used to simulate the tunnel face. Under different retreating displacement of the tunnel face, the evolution laws of support pressure on the tunnel face, the ground deformation, ground surface settlement and contact force chain in front of the tunnel face are studied. The results show that with the withdrawal of the tunnel face, the support pressure on the tunnel face can be divided into four stages, namely, the rapid decline stage, the minimum stage, the slow rising stage, and the horizontal stability stage. Moreover, based on the simulation results of the particle contact force chain, discriminated methods of failure zones are proposed. The research results obtained from this paper will provide theoretical support for the reasonable value of support pressure of a tunnel face in practical engineering.

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