Leveraging mineral composition to tackle convergence and creep
By Cody Vining, RESPEC
Understanding each deposit’s unique characteristics in potash mining is crucial to maintaining efficient, productive, and safe operations. A significant but sometimes overlooked factor affecting underground potash mines, particularly in regions like Saskatchewan, is how the mineral composition of potash influences creep rate (i.e., the gradual movement and deformation of the rock mass).
By studying and planning for these natural behaviours, operators can better predict and mitigate issues that slow production, increase costs, and pose hazards to mine personnel and installed infrastructure.
Mineral composition and creep rate matter
The potash extracted from Saskatchewan’s rich deposits is often a mixture of minerals, such as halite (a common salt), sylvite (potassium chloride), and carnallite (a magnesium chloride compound), each of which behaves differently under stress.
For example, carnallite creeps significantly faster than halite and sylvite. Thus, areas with higher carnallite content close more quickly, leading to what geotechnical experts call total convergence. Total convergence affects the clearance around mine infrastructure (e.g., conveyor belts), creating the potential need for unplanned maintenance and temporary shutdowns; total convergence and the manifestation of ground-control hazards are typically correlated.
Convergence rates directly impact how long a mined room can remain accessible and safe. An unmonitored or misunderstood mineral composition can lead to the unexpected closure of access routes when the surrounding rock creeps faster than anticipated. These changes can result in costly rework and downtime while operators halt production, remove equipment, and re-cut access areas to continue operations safely.
The mineralogy of the rock is critical for rock mechanics modeling and understanding how the rock will creep. For decades, our RESPEC geologists have worked alongside engineers to log potash and salt cores to support these calculations extensively. We focus on critical core aspects, including mineralogy, crystal size, insoluble content, and structural features. These observations are logged into a database to build geological models and resource estimates. The rock mechanics team then creates geomechanical models.
RESPEC’s data-driven approach
RESPEC teams can predict cavern closure centres by linking mineral composition to creep behaviour, thus empowering mining operations to forecast convergence and plan accordingly. Our team of geomechanics specialists has developed a mineralogy-dependent creep law, first introduced in a landmark 2015 study [Vining and Nopola, 2015][1]. This case study provided an exclusive opportunity to validate a constitutive model’s stress, temperature, and mineralogy dependence by comparing convergence rates predicted during numerical simulations to convergence rates measured underground.
Using data from this study and ongoing laboratory and field tests, RESPEC helps mining operators measure and anticipate specific closure rates while factoring in variables such as stress, temperature, and the mineral makeup of each mined section.
Operators achieve improved efficiency by:
- Planning maintenance downtime: Understanding the specific creep rates for areas high in carnallite or other fast-moving minerals helps mines more accurately schedule downtime for equipment adjustments and infrastructure repairs, minimizing interruptions in production.
- Adjusting design layouts: Operators can proactively adjust room design and equipment placement when mineral composition suggests accelerated convergence, maintaining safety and efficiency over time.
These insights allow mine operators to focus on operational flow and long-term productivity. By knowing the expected timeline for room accessibility, they can prepare for the necessary adjustments before these aspects become critical.
Potash mining’s proven partnership
With growing potash demand, proactive planning is essential to keep mines operational and safe. RESPEC has worked in salt and potash geomechanics for over 50 years, bringing our clients academic insight and hands-on experience. By combining sophisticated modeling techniques with our extensive knowledge of evaporite behaviour, we provide potash producers with the strategic foresight to optimize mining schedules, improve safety, and ultimately enhance profitability.
[1] Vining C. A. and J. R. Nopola, 2014. “Validation of a Mineralogy-Dependent Constitutive Model for Potash,” Proceedings, Mechanical Behavior of Salt VIII, South Dakota School of Mines and Technology, Rapid City, SD, United States, May 26–28, L. Roberts, K. Mellegard, and F. Hansen (eds.), Taylor & Francis Group, London, UK, 2015, pp. 311–317.
