13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Boundary Fractures and Indentation Tests A.P.S. Selvadurai1 1 Department of Civil Engineering and Applied Mechanics, McGill University, Montréal, QC, Canada H3A 0C3 Abstract The paper presents an evaluation of the factors influencing fracture initiation at the boundary of a rigid test plate that are used to estimate the in-situ deformability characteristics of a geologic medium. The paper outlines the techniques that are used to perform in situ plate load tests and focuses on the problem of boundary fracture generation at the edges of the geologic medium. If the mechanical behaviour of the rock mass can be assumed to display brittle elastic behaviour, computational methods based on boundary element techniques can be used to examine the mode of crack extension within the elastic geomaterial. The process of fracture generation can influence the extent of the region being evaluated and, more importantly, this can adversely affect the theoretical relationships for the interpretation of test plate data. In most instances the boundary crack may not be visible; this is especially true if the plate load test is conducted with some nominal embedment. This paper discusses issues associated with the interpretation of plate load tests conducted as a validation of experimental data determined from plate load tests. The methodology for the correct interpretation of plate load tests conducted on brittle elastic materials requires knowledge of additional parameters governing the mechanical behaviour of the rock; this involves laboratory evaluation of fracture toughness data. The paper presents results concerning the influence of axisymmetric boundary fractures on the estimated deformability characteristics of the rock mass. Keywords plate load tests, brittle edge fracture, boundary elements, interpretation of fields tests 1. Introduction The evaluation of the effective geomechanical characteristics of complex and heterogeneous geological materials is best accomplished through static load tests that are conducted in-situ. A technique that has been used extensively in this connection is the plate loading test where a plate of known dimensions and flexural rigidity is maintained in contact with the surface of the geological medium under examination and is then subjected to an axial loading [1,2]. As the elastic stiffness of the geomaterial increases large loads are required to attain measurable test plate deflections. When plate load tests are conducted in galleries and adits, the loads needed to indent the test plate can be achieved through reaction against the walls of the gallery or enclosure. When plate load tests are performed on large open surfaces this facility is not available and recourse must be made to provide the test loads through a self stressing reaction system. The method of cable jacking introduces the reactive loads through an anchor region located in the medium that is being tested. The method was first proposed by Zienkiewicz and Stagg [3] and presents a simpler test configuration than that involving anchor piles and a bracing frame to accommodate the remoteness of the anchoring loads from the plate location. The influence of the anchor load on the resulting net settlement of the test plate was first examined by Selvadurai [4], who examined the problem of the interaction between a smoothly indenting plate and a Mindlin force [5] located at a finite depth from test plate. The analysis was subsequently extended to cover distributed anchor loads [6], transverse isotropy of the rock mass [7], flexibility of the test plate [8-11] and creep effects of the geologic medium [12]. In this paper we examine the problem of crack extension in a brittle elastic geologic medium during the indentation of the brittle elastic half-space by a cylindrical punch with a smooth flat contact surface. The paper discusses a procedure for locating the point of nucleation of the crack within the brittle elastic solid and employs a boundary element technique to locate the progress of crack evolution as the force on the loading device is increased [13]. The numerical results illustrate how the extent of crack development influences the load vs. displacement relationship for the rigid test plate. The development of boundary fracture is characteristic of any indentation problem involving brittle elastic materials and sharp-edged indenters. Results are developed for geomechanical investigations that are carried out both at the surface of a geomaterial and at depth. The work can also be extended to include flexibility of the plate that is applying the indentation loads.
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