International Journal of Rock Mechanics and Mining Sciences
Effect of soil–infilled joints on the stability of rock wedges formed in a tunnel roof
Introduction
One of the most challenging tasks in the design of geo-structures (tunnels, dams, foundations, etc.) in a rock mass is to simulate the mechanical behaviour of the rock mass, which usually consists of an interlocking assembly of discrete blocks. The shear and normal displacements of rock discontinuities and their shear and normal stiffnesses control the distribution of stresses and displacements within the discontinuous rock mass. In conditions where an equivalent continuum-based approach is not applicable, the joint material model should be able to describe important mechanisms, such as the asperity sliding, deformation and shearing, post-peak behaviour, and the effect of a possible soft infilling.
Bandis [1] discussed a range of studies of the shear behaviour of rock joints as being theoretical or empirical. The theoretical approaches usually utilise numerical methods coupled with advanced constitutive laws to model the behaviour of the joint interface. Although advanced mathematically, these constitutive laws are still empirically inspired from the results of laboratory or field tests and do not explicitly model the kinematics of shear development at the interface [2]. As a result, many practical solutions are still based on empirical approaches which involve the analysis of test data to obtain a correlation between the joint and material characteristics, and the observed behaviour. Consequently, many analyses may only be valid for limited rock formations (individual projects), but not for a universal application as is the JRC–JCS model developed by Barton [3], [4] and Barton and Bandis [5], [6].
The complex and irregular geometries of rock joints can only be estimated statistically, which means that the simultaneous interacting processes of sliding, shearing, dilation, and elastic-plastic deformation are also of a complex nature. In addition, rock joints may present different conditions of weathering and contact surfaces, varying from clean, fresh surfaces to clay covered and slickensided surfaces. Amongst these contact surfaces, soil–infilled rock joints are likely to be the weakest planes because of the low frictional properties of the infill [7], [8]. Because of lack of models, it has been common practice to assume that their shear strength is equal to that of the infill material alone, usually described by a Coulomb slip model. This assumption (i.e. joint strength equal to that of the infill) may lead to unsafe conditions, where the rock–infill contact has lower strength [29] or uneconomical designs, where the overall strength of the infilled joint is not fully accounted for.
The most pronounced effect of an infill is to reduce the friction between the discontinuity boundaries or joint walls below that for the rock-to-rock contact condition. The shear behaviour is influenced by the nature of the infill material and the characteristics of the wall–infill interfaces. In addition, parameters such as the boundary condition, roughness of the joint, thickness of the infill material, pore-pressure, and the over-consolidation ratio may influence the shear strength of an infilled joint [8], [9].
Because of the lack of a generalised model that correctly simulates the behaviour of soil–infilled joints, limited attempts to capture the numerous factors affecting infilled joints have been made in recent years [9], [10], [11], [12], [13]. For this purpose, it is important to understand the governing mechanisms occurring during the shearing of infilled rock joints.
Section snippets
Shearing mechanisms in soil–infilled joints
Apart from the properties of the constituent materials, the infill thickness is perhaps the most important parameter controlling the strength of the joint. In non-planar joints, as the thickness of the infill to asperity amplitude ratio (t/a) increases, the overall shear strength of the joint decreases as illustrated in Fig. 1. Once a critical t/a ratio is attained, the joint walls no longer affect the overall behaviour and the joint shear strength may be represented by that of the infill alone.
The soil–infilled joint model
Based on the shearing mechanisms described in the previous section, Indraratna et al. [14] proposed a semi-empirical shear–shear displacement criterion for soil–infilled joints. Their model is based on a homogenised Coulomb slip model, in which the effect of infill squeezing during shearing is accounted for. Oliveira and Indraratna [15] noted that the model did not describe well the post-peak behaviour of clean joints in cases of pronounced asperity degradation. They modified the original
Constitutive equations for numerical codes
Numerical analyses are often essential components of the design of urban tunnels in order to evaluate the interaction of the geo-structure with other structures in the vicinity of the excavation (buildings, public services, driveways, etc.), as well as for the design of the support and to establish alert limits during construction. Therefore, for practical applications, it is important to implement the soil–infilled model in numerical codes.
The general stress–strain relation for an
Stability of a rock wedge formed in the roof of a tunnel
The mechanical behaviour of a rock block formed in the roof of an underground excavation is governed by its geometry, the mechanical characteristics of the joints forming the block, the deformability of the block and that of the surrounding rock mass, and the stresses within the rock. In this scenario, it becomes apparent that soil–infilled rock joints will have a significant role controlling the stability of the block due to its reduced friction. Therefore, the effect of soil–infilled joints
Back-analysis based on a cavern collapse
The Yellow Line of the São Paulo (Brazil) Metro is 12.5 km long and links the city centre to the western suburbs with four interchange stations. The stations are under construction by either cut and cover or NATM methods. The Pinheiros Station was being built by sequential excavation method (NATM) and included a large-diameter shaft (40 m diameter×36 m in depth), two platform tunnels (18.6 m wide×14.2 m high×46 m long) and two access tunnels. The station has side-platforms with a central double-track
Conclusions
The soil–infilled joint model proposed by [15] applied here for tunnel wedge stability describes accurately the occurrence of dilation and compression with lateral displacements of soil–infilled joints. Consequently, it better represents some of the shearing mechanisms associated with infill squeezing and asperity interference that cannot be readily captured by the existing joint models originally developed for clean discontinuities (e.g. Barton–Bandis, continuously yielding) or conventional
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2020, Engineering Fracture MechanicsCitation Excerpt :Natural rock masses are always heterogeneous due to the presence of different kinds of discontinuities. These discontinuities are normally filled with weak materials such as sheared-off or broken rock fragments and clay [1,2], which are key issues for the stability of tunnels, dam foundations and landslides [3–7]. Several authors have investigated the mechanical properties of rock joints filled with soft materials such as clay, sand and gypsum [1,5,8–15].