Slope Stability in Jointed Rock Masses

1. Slope Stability in Jointed Rock Masses Maurice B. Dusseault Porous Media Research Institute University of Waterloo, Waterloo ON Canada

2. Uncertainty in Slope Design • What are joint patterns at slip surfaces? • What are the joint properties? • What will the loads be over time? • Pore pressures can be seasonal, or a storm • Earthquake loads are hard to predict • Will the rock strength deteriorate over time? • Geochemical changes in shale • Dissolution of limestones • Did blasting affect properties? • A highly indeterminate design problem

3. Solutions to Uncertainty • Deterministic slope stability models have now reached their useful limits • We need probabilistic models • Fabric from oriented core + stochastics • Likely property variations (e.g. c, ) • Return period probabilities on pore pressure • Same for earthquake loads • Cohesion loss with time in shales • Safety factor method is not adequate • Monitoring vital in critical cases

4. Some Factors Depth of tension cracks Earthquake loads Joint properties Cohesion deterioration in regions of high stresses Seasonal pore pressure variations

5. Fabric Uncertainty How to extract fabric data from boreholes? How do we extrapolate jointing data from the surface to depth? ??

6. Geometry and Pore Pressure Joint intensity decreases with depth? Are there sub-parallel stress joints? Do joint properties vary with depth? Which is the most critical slip plane? What is the p response to rainfall?

7. Whither Slope Mechanics? • Since the advent of 3-D block models, there is nothing new in slope stability models • Also, the continued pursuit of better deterministic models is a futile effort. • The random “noise” because of uncertainty exceeds the precision of models • Therefore, the only realistic developments are in the area of stochastic (probabilistic) approaches to stability, combined with monitoring

8. Mining and Civil Approaches • Civil slopes are designed conservatively, using Factor of Safety approaches • Minimize risk to people and structures • Long service life required • Minimal long term deformations • Mining slopes are almost always empirically built to be close to failure at all times • As steep as possible to minimize excavation • Monitored carefully for deformations • Generally experience continued slow movement

9. Factor of Safety FS = 1.6 FS is not necessarily same as Pf FS = 1.9 Pf = 0.05 Pf = 0.01 Safety factor

10. Monitoring Methods • Displacement gauges, alarm tapes • Piezometric measurements • EDM 3-D measurements, resolved to give a map of movement vectors with time • Satellite InSAR interferometry • Direct visual measurements by mine personnel (face raveling…)

11. Protecting People • Rock catching berms, ditches • Deflecting devices and diversion structures • Wire mesh blankets to minimize acceleration of individual blocks • Surface block scaling (removal) • Horizontal drains into slip planes • Shotcrete or concrete coat the entire surface • Controlled deliberate inducing of failures • Massive cable anchors, etc…