Estimation of pillar strength is a vital part of assessing the stability of pillars used in mines. In this paper, numerical modelling using strain softening constitutive law have been employed to estimate the strength of 32 stable St. Peter Sandstone pillar cases. The model results show a satisfactory agreement with the general Obert and Duvall empirical equation. A major limitation of using Obert and Duvall empirical equation was revealed and a methodology was proposed to address this shortcoming. In conclusion, an empirical pillar strength equation has been proposed to evaluate the stability of St. Peter Sandstone pillars. The findings are expected to improve mine planning and design of room and pillar mine in St. Peter Sandstone formation in Iowa.


The St. Peter Sandstone formation has been a primary source of hydraulically fracture sand (frac sand) in the United States. Hydraulically fractured sand is used to keep cracks open so that natural gas can flow freely into wells in a process called hydraulic fracturing. Hydraulic fracturing technology has made extraction of unconventional oil and gas resources feasible in the United States. Advances in the use of this technology has contributed to the United States, being the world’s largest producer of natural gas in recent years.

The surge of hydraulically fractured sand has led to increased production capacities of mining industries exploiting St. Peter Sandstone deposit. Currently, surface mining techniques are mostly used to extract this flat lying deposit. However, room and pillar underground mining technology has been employed under circumstances where the cost of removing the overburden cover becomes too expensive. In recent times, the use of room and pillar underground mining method is considered more sustainable over open pit surface mining method due to little environmental concerns.

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