concept of the three-legged stool

Salt and potash rock mechanics

tHe IMPortaNt CoNCePt of tHe tHree-leGGeD stool

rock mechanics involve developing an understanding of how rock responds to changes in equilibrium and then applying that understanding to addressing scenarios that range from localized ground-control issues to mine-wide issues related to accessibility and stability. a rock-mechanics program amasses information from three interdependent elements to develop effective solutions that work within the confines of the local geology. the elements that comprise the three “legs” of the rock-mechanics stool are mine measurements and field observation; laboratory testing; and numerical modeling.

In the mine

We can informally determine the mine behaviour by observing what is good behaviour or bad behaviour, but some indications of change or instability are subtle and may not be visually apparent. Mine measurements provide data to quantify the mine behaviour by evaluating trends in the recorded data. these data, when taken throughout the mine on a regular basis, enhance worker safety by identifying areas where ground-control problems are developing. a recorded behaviour against which numerical models can be validated is also provided by this data.

In the laboratory

tests are the fundamental method for measuring rock behaviour under simple and well-controlled conditions. tests provide data to interpolate and extrapolate the expected rock behaviour under various conditions; however, bad data are often worse than no data. salt and potash are difficult to fail under moderate confining stresses; consequently, unconfined compressive strength (UCs) tests do not adequately represent strength, and poorly designed creep tests may not properly characterize longterm deformation. Good data improve our understanding of the fundamental rock behaviour, and interpretation of the data helps us to develop site-specific behaviour models for numerical modeling.

On the computer

Numerical modeling has become a prominent tool to forecast and interpret mine behaviour. an effective modeling approach should be grounded in reality, and arbitrarily changing modeling parameters under the guise of “engineering judgement” can grossly mischaracterize mine behaviour, especially in potash. this approach has resulted in the adage of “garbage in, garbage out”, which has led some to believe that modeling is worthless, when in reality, the modeling approach was poorly implemented. an approach based on accurate field and laboratory data can be used to forecast the mine behaviour in real-world terms, such as the susceptibility for roof falls, load-bearing pillar capacity, and long-term response of the mined excavation. the results can also be used to interpret the mine behaviour by comparing the measured and predicted behaviours. Good agreement suggests that the mine is behaving as expected. the extent or severity at which the actual behaviour deviates from the expected behaviour can indicate the potential impacts on the global stability of the mine.

this three-legged approach strives to reduce the inherent risk in mining by developing a better understanding of mine behaviour. each leg is equally important, and heavily relying on a single component can lead to illogical ends, such as an overly conservative design or unsafe conditions. this consideration is especially important in salt and potash mines, where the mine behaviour is unlike that of many other commodities. over the past 50-plus years, resPeC has tested, modeled, and observed salt and potash behaviour from nearly every actively mined evaporite deposit in the world. our approach has successfully guided decisions regarding mine design optimizations and provided a comprehensive assessment of the utility of existing workings and long-term stability, as well as suitable management of the resource. s