Environmentally Controlled, Multi Scale, Dynamic Behavior of Rock Masses

EXECUTIVE SUMMARY This research focuses on studying the propagation of dynamic perturbations through a discontinuous rock mass from seismic and environmental sources, and the role of dynamic friction along sliding interfaces, in the context of seismic response of discontinuous rock masses.

The numerical Discontinuous Deformation Analysis (DDA) method is used in this research. The two dimensional DDA code is used to model wave propagation through a discontinuous rock mass and the recently developed three dimensional version is verified and validated using analytical solutions and shaking table experiments for the dynamic response of a three dimensional wedge resting on two planes..

Preliminary attempts to model wave propagation phenomena through a discontinuous rock mass using 2D-DDA revealed a shortcoming in the ability of the existing 2D-DDA line generation (DL) and block cutting (DC) algorithms to simulate mechanical layering effects, so common in sedimentary rock masses which comprise the bulk of the exposed outcrops in Israel.

To overcome this limitation we developed a 2D-DDA preprocessor based on FracMan®, which allows us to assign different statistical distributions to the different structural parameters and to impose realistic geological constraints of mechanical layering on the generated discrete fracture network (DFN). The ability of the 2D-DDA code to model dynamic perturbations through a discontinuous rock mass on large scale is still being studied with the more advanced discrete element mesh generator developed by us.

We propose an original analytical solution for dynamic, single and double face sliding in 3D. Analytical and 3D-DDA numerical solutions for dynamic sliding of a tetrahedral wedge are compared with results of shaking table experiments performed on a concrete model, the interface friction of which is determined by constant velocity and velocity stepping, direct shear tests.

Results of constant and variable velocity direct shear tests reveal that the tested concrete interface exhibits a velocity weakening behavior. This is confirmed by shaking table experiments where friction degradation upon multiple cycles of shaking culminated in wedge run-out. The measured shaking table results are fitted with our 3D analytical solution to obtain a remarkable linear logarithmic relationship between friction coefficient and sliding velocity which remains valid for five orders of magnitude of sliding velocity.

We conclude that velocity-dependent friction across rock discontinuities should be integrated into dynamic rock slope analysis, either analytical or numerical, to obtain more realistic results when strong ground motions of relatively long duration are considered.

Towards the last year of the research program we began monitoring climatic effects on block motion in discontinuous rock slopes. Preliminary analysis of data indicates an inverse relationship between joint opening and temperature, suggesting that the rock block material contracts and expands as a response to environmental cooling and warming, respectively. A similar behavior was monitored in the East slope of the mountain along the snake path cliff.

Comparison of displacement results indicates that the amplitude of displacement fluctuation is proportional to block width, suggesting a thermo-elastic mechanics. A theoretical weather induced wedging failure mechanism is currently being developed to explain the field monitoring results.