2. An improved rotational tunnel face failure mechanism for non-linear materials
2.1. Principles of the collapse mechanism of Mollon et al.
The analytical collapse mechanism developed by Mollonetal. [22], in the framework of the kinematical theorem of limit analysis applied to MC soils, relies on two main assumptions: (i) the collapse involves the rotational motion of a single rigid block around an axis (Ox) (x being the horizontal direction perpendicular to the tunnel axis, see Fig.1a), and(ii) the collapsing block intersects the whole circular surface of the tunnel face. These kinematic assumptions were made after observations of numerical
[12] and experimental [8] simulations of face collapses. Besides the kinematic aspects, the normality condition related to the assumptions of the kinematical upper bound theorem of limit analysis has to be fulfilled. In the case of a frictional soil, with or without cohesion,this condition states that the normal vector pointing outward of the slip surface should make an angle p/2þj with the discontinuity velocity vector in any point of the discontinuity surface, with j being the internal friction angle of the material. Even in a homogeneous MC soil, there is no simple surface which is able to satisfy both the kinematic and the normality conditions. It was thus necessary to use a complex discretization scheme to generate the external surface of the moving block from close to close, using a collection of triangular facets respecting locally the normality condition. The method and equations used for this generation are described in detail in Ref. [22] and will be briefly recalled here to make understandable the changes needed to adapt this formulation to the HB criterion, as described in Section 2.2.