Rock anchoring for stabilization of infrastructures with focus on the arching effect and rock-grout bond

High-capacity rock anchors are used to stabilize rock slopes, retaining walls and foundations of large-scale infrastructures. Failure of the rock anchors would jeopardize the infrastructures and lead to serious economic and social consequences. Rock anchors are composed of a single or multi strands of cables which have a bonded length that is fixed deeply in the rock mass and a free length that is attached with fixture units on the ground surface. The current rock anchor dimensioning has some uncertainties because of the limited knowledge in the failure of the rock mass and the rock-grout bond strength. New knowledge in rock anchoring is required to improve the rock anchor design and to enhance relevant engineering education. ROCARC aims to develop an updated method for the dimensioning of rock anchors. The investigations include, among others, the load transferring between the anchor and the rock mass, the failure mode of the rock mass, and the bond strength between the grout and the rock. The research has been carried out through laboratory model tests, field trials and numerical modelling. The laboratory tests aim to study how the rock mass responds to the anchor load and how the failure is initiated and propagates in the model materials. A two-dimensional test rig has been constructed for model tests, where the deformation and load transferring in the model materials are monitored. The field trials were carried out in a limestone quarry to investigate how the anchor load was transferred to the rock mass and how the rock mass failed under the anchor load. Pullout tests of full-scale rock anchors were carried out in the same quarry to study the bond failure of the anchor. Numerical modelling has been also conducted for parameter studies. In the end of the project, a methodology will be established for rock anchor design. The field tests and the laboratory model tests that have been conducted so far indicate that load-transferring arches are built up in the rock mass; the load capacity of the rock mass is significantly higher than the empirical value used in the current designs; and the bond of the anchor fails in different modes depending on the configurations. The findings can be used to modify the current anchor design and to improve the rock anchoring technology.

Rock anchors have been widely used in stabilization of large-scale of infrastructures like windmills, concrete dams and bridge towers. The basic principle for dimensioning of rock anchors was developed in the 1970’s and has remained largely unchanged since then. Engineering practisers believe that the current design practise is too conservative and urge to formulate a better and more realistic dimensioning approach. The current principle of dimensioning was formulated largely based on the consideration of the following failure modes: 1) along a cone surface in the rock mass, 2) in the rock-grout bond, 3) in the grout-strand bond, and 4) in the strand. The design for failure Modes 1 and 2 are most uncertain because of the lack of knowledge in the failure pattern of the rock mass under the anchor load and in the determination of the rock-grout bond strength. The project aims to create new knowledge in the uplifting behaviour and failure mode of the rock mass and the characteristics of the rock-grout bond failure, which helps to formulate an updated dimensioning approach for rock anchoring. The project is executed through laboratory tests, field tests and numerical modelling. Laboratory models are constructed with blocks of rock / rock-like materials. The arching effect, displacements and formulation of the failure planes in the blocks are experimentally studied. Field tests are carried out to study the uplifting mechanism and the cone failure in full-scale rock masses through measurements of the rock deformation, stress change, and propagation of the cone fracture with help of extensometers, load cells and a microseismic monitoring system. The dependence of the rock?grout bond strength on the bonded length and borehole diameter will be also studied in the field tests. Numerical modelling is conducted to simulate both the laboratory and field tests and finally a numerical modelling methodology for dimensioning of rock anchors is formulated.