As a consultant you can only advise. You cannot force. You can only provide your professional opinion. You cannot make the client do the right thing. You can set out facts and possible consequences. You cannot make the outcome be what you desire or believe it should be.
As a consultant you are providing a service for money. The client is paying and makes the decisions as to how to proceed. They are the ultimate recipient of pain or pleasure.
You may have superior knowledge, experience, and insight about what will happen if a particular course of action is implemented or not. But this is of no consequence if the client has other motives—even venial, selfish motives.
This week I experienced the full force of these facts. The story is thus: as part of a pre-feasibility study, the major, respected consultant (one of those large, international outfits) concluded the pit slope will be stable to a great depth and at a steep angle. They noted that it will be necessary to control water pressures in the slope: install cut-off walls, grout curtains, and dewatering wells, the full panoply.
There is a deep lake but a few tens of meters from the pit perimeter. The lake depth is about a third of the depth of the final pit depth. They concluded the rock is so tight and strong that even if the dewater facilities they recommend are not installed, the slope of the open pit will remain stable.
Good enough as it goes.
As a blogger and cynic, I asked this simple question: “If stress relief occurs as the pit is developed, will there be a change in the rock permeability, and hence water pressures, and could that affect slope stability?”
Afterall this is a topic I dealt with doing my master thesis more than forty years ago. This is hardly a new or revolutionary issue.
The response exceeded wild anticipation. The meeting, as one, attacked me as a heretic. How dare I cast doubt on conclusions? How dare I question fundamentals? Was I impugning the experts who had preceded me?
Not being directly involved in the outcome, I beat a hasty retreat. I had done my duty: raise the issue. If they choose to ignore it, so be it. The outcome is their responsibility. In that the project is but a pre-feasibility, nobody riding the gravy train wants the project killed thus early.
I retreated to Google to check out the current state of the opinion on the issue of rock stress relief, hydraulic conductivity increase, and decrease of slope stability. I found a number of papers. Email me if you want them—I downloaded them and they support my opinions.
Here is a link to the best of them. It is a forty-one page paper by T.D. Sullivan who is billed as a consultant and university professor thus: Pells Sullivan Meynink Pty Ltd and The University of New South Wales, Australia. Here is the abstract of the paper:
This paper presents a review of the general state of the art in the understanding of the interaction between water and pit slope deformations. The topics covered include; the theory of hydromechanical coupling, the interaction between stress and pore fluid responses for rock structure and rock masses and the impacts of these on open pit slopes.
The hydrogeological cycle for an open pit is explained and the importance of transient and partial pore pressures are highlighted. The paper includes an explanation of pit slope deformations and failure movements. A new system for classifying pit slope movements is presented together with critical movement thresholds.
The various methods for slope depressurisation are addressed and the accuracy and methodology for assessing pore pressure input to slope design at all slope scales are reviewed.
Examples of hydromechanical coupling, pore pressure responses in structure and rock masses, statistics on the interaction between rainfall and pit slope failures and the role of undrained loading for the liquefaction potential for pit slopes are presented.
It is concluded that the effective management of pit slopes is only possible by the integration of pit slope deformations and movements with the hydraulic properties and pore pressure responses of the rock mass.
We address these issue in the EduMine courses I have written alone and with others. See these links:
Maybe this is a study for feasibility study and detailed design attention!