Upper-bound solutions for face stability of circular tunnel in purely cohesive soil using continuous rotational velocity field

This paper presents a 2D analysis model based on limit analysis and slip-line theories for the face stability of a circular tunnel in purely cohesive soil. The analysis model depends on the ratio of cover depth C to tunnel diameter D. When C/D is equal to 0.5, the mechanism consists of three blocks, namely, Zone I, Zone II, and Zone III. When C/D is greater than 0.5, the mechanism consists of four blocks, namely, Zone I, Zone II, Zone III, and a possible Zone IV. Zone II is a transition zone satisfying the normality conditions. The possible Zone IV is a Rankine zone that is subjected to the influence of the vertical soil arching effect appearing at the top of Zone III. The criterion for the collapse thickness limit of the tunnel is proposed based on Terzaghi’s theory of relative soil pressure. The results show that significant improvements have been made to the existing solutions using the proposed failure mechanism for the face stability of circular tunnels in purely cohesive soil.     

Experimental investigation of tornado-like vortex dynamics with swirl ratio: The mean and turbulent flow fields

An experimental simulation of tornado-like vortices is conducted in a small tornado vortex simulator in order to study the effect of swirl ratio on flow characteristics. Particle Image Velocimetry (PIV) method is employed to quantitatively determine the tornado-vortex velocity field for swirl ratios ranging from 0.08 to 1. The radial and tangential components of velocity as well as the core radius of the tornado increase with increase in swirl ratio. The location of the maximum radial and tangential velocities is adjacent to the ground where the tornado vortex interacts with the surface. The values of normal and shear turbulent stresses indicate the existence of a laminar core for small swirl. As expected the shear stresses increase with swirl ratio as the vortex becomes turbulent. The highest turbulent production corresponds to the critical case of vortex touchdown.     

Effect of seepage force on tunnel face stability reinforced with multi-step pipe grouting

Tunneling in difficult geological conditions is often inevitable especially in urban areas. Ground improvement and reinforcement techniques are required to guarantee safe tunnel excavations and/or to prevent damage to adjacent structures. The steel pipe-reinforced multi-step grouting method has been recently applied to tunnel sites as an auxiliary technique in Korea for impermeabilization in underwater tunnels as well as for reinforcement. However, this technique has been usually employed empirically without much understanding with regard to its effect on the tunnel safety. In this study, the face stability with steel pipe-reinforced multi-step grouting in underwater tunnels was evaluated by simultaneously considering two factors: one is the effective stress acting on the tunnel face calculated by limit theorem and limit equilibrium method; the other is the seepage force obtained by means of numerical analysis. This study revealed that the influence of the steel pipe-reinforced multi-step grouting on the support pressure required for the stability of the tunnel face in dry condition is not significant while there is relatively a significant reduction in seepage forces by adopting the technique in the underwater tunnel. The effect of permeability anisotropy on the seepage force acting on the tunnel face was also assessed by conducting a coupled analysis.     

改进数字图像相关法测量混凝土柱弯转角

由于混凝土是一种各向异性材料,其破坏特性比较难以预测。对其力学性能(尤其新开发产品)进行实验研究往往需要耗费大量的人力、物力,高精度、操作方便的测试手段对于提高实验数据的可靠性和减轻实验人员的劳动强度是非常必要的。数字图像相关法(Digital Image Correlation,DIC)是一种基于光电成像和计算机处理的高精度、非接触的全场测量手段,相比于其他测量技术,该方法原理简单,对系统和环境的要求也比较低。但是,目前有关该法在土木工程研究中应用的报道还比较少,还需要根据混凝土构件特性对测量中的一些问题进行探索。本文将结合在混凝土构件实验中遇到的问题,首先通过数值模拟和实测对比分析对有关问题进行探讨验证,并在此基础上给出改进方法。标定验证实验结果验证了该方法用于混凝土构件变形测试的可行性,是非常有前途的测量方法,其中改进策略可作为类似工程应用的有益参考。