Model investigations of the performance of a tunnel excavation in anisotropic (orthotropic) rocks at a shallow depth

Abstract

The majority of rocks through which hydraulic tunnels are driven are not isotropic and their elastic properties vary with direction. This is due either to the inhomogeneous nature of fissuring of the rock mass in various directions or to its bed structure, with different dips of the beds. The plastic characteristics of the mass in the direction of stratification and in the direction perpendicular to the beds may differ several times over.On account of the anisotropy of the rocks, a circular tunnel excavation is subjected to nonuniform deformations around its circumference, under the action of nonuniform internal pressure, resulting in additional bending stresses in the lining. Even in homogeneous isotropic rocks because of the shallow location irregular deformations of the excavation contour remit from the uniform internal pressure.A theoretical solution has been formulated by S. G. Gutman [1] for a circular tunnel excavation in a homogeneous isotropic elastic medium (E = const), which can be used for determining the deformation both of the excavation contour and of the free surface. S. G. Gutman has also provided a solution for a particular case of anisotropy, for an orthotropic medium under conditions where its elastic characteristics are governed by the relationship =2G(l+ ~, the adoption of which provides a strict solution to the problem, on the basis of experiments on an isotropie model [2]. There is no analogous solution for the general case of anisotropy, and it is probably very difficult to obtain one.Experimental investigations have been carried out in the laboratory of the V. V. Kuibyshev Moscow Civil Engineering Institute, Faculty of Hydraulic Engineering Works, for investigating tunnel excavation and lining of a tunnel at a shallow depth, employing models in an orthotropic medium with horizontal bedding of the rocks. This paper examines the results of the investigations.The experiments were carried out on models of coarse-grained material with a gypsum binder, since it is relatively simple to set up models of an anisotropic rock mass using gypsum stone. Prior to setting up the model, work was carried out for determining the physical-mechanical properties of the model material for the purpose of selecting the mortar composition. For this purpose, use was made of the recommendations by G. M. Koganov, drawn up by him at the above-mentioned laboratory.Gypsum with a water-gypsum ratio 1:1 was taken as the basic material. Tests on prismatic specimens 7 X 7 X 21 cm yielded mean values, from 15 specimens, of Young's modulus E=23.8 X 103 kg/cm z, Poisson's ratio 0.19-0.20, and critical compressive strength 24.5 kg/cm z. As the result of the investigations, a procedure was formulated for preparing the mortar, by which the scatter of the properties of the specimens did not exceed 20-25%, which can be considered as satisfactory for this type of investigation. The investigations were carried out on a frame rig (Fig. 1) made up from No. 14 100 x 100-cm channel iron.The orthotropic model of the rock mass was set up ...

Effect of the structure of a rock mass and properties of rocks on the stress state of hydraulic pressure tunnels

“…The ranges of the relationships of the deformation characteristics of materials of the structures and foundation rocks for which a refinement of their values does not lead to the need to change the parameters of the structures were investigated [3]. However, the stress state and, consequently, the design parameters of a lining of a tunnel, particularly a pressure tunnel, are determined not only by the deformation characteristics of the rock in the immediate vicinity of the structure but alsoby the structure of the mass, particularly its anisotropy [4], surface relief, and depth [5], and also by the technical level of performing the tunneling and concreting work [6]. In the case of shield or mole tunneling overbreaks are absent and the stress state of the structure is more favorable [7].…”