الگوي دگرشكليهاي خاك بر اثر احداث تونل دايره اي در آن

The Pattern of Ground Deformations due to Circular Tunneling

This paper presents the results of a recent study about the following aspects relevant to tunneling in soft grounds: The domain of deformations due to tunneling in soft ground can be specified within a boundary of a parabolic shape. This boundary is defined by a parabolic formula as a function of a central angle (i) which depends on the soil type; i.e., either cohesive or cohesionless. This parabolic shape can also be verified by a finite element computation. A finite element program has been applied to investingate the deformation characteristics around and above circular tunnels and to find the settlement ratio (a.) as a function of known variables such as, depth ratio, modulus of elasticity, and the thickness of soil layer beneath the tunneL The finite element computations were carried out by assuming a given distribution of displacements around the tunnel perimeter, for which reason the method may be called "compulsory displacements". It was found that although all the variables mentioned affect both the settlement ratio and the type of soil deformations, changing the values of modulus of elasticity affects only on the amount of deformation components, but not the settlement ratio. The results of finite element computations for the settlement ratio have been compared to other analytical curves and empirical data from some available case studies; from which excellent agreements were found Also the contours of Equal deformation components from the finite element program and from the simple formulae proposed by the author were found to be quite similar and in acceptable agreement.Because the results obtained from the proposed formulae for the distribution of settlement at the ground surface are in excellent agreement with the relationships recently proposed by Loganathan & Poulos and the empirical data available, it is concluded that the simple analysis proposed here and the finite element computations based on the elasticity assumption can both be used to predict the deformation pattern around excavations in soft ground.