: Time-dependent ground behavior is recognized as an important consideration for designing stable excavations in trona measure rocks of the Green River Basin in southwestern Wyoming, USA. Although a considerable degree of experience has been acquired with time-dependent ground response for single-seam mining, little is known locally of the short- and long-term effects of thin-interburden two-seam mining. Future production at OCI Wyoming, L.P. ’s (OCI), Big Island Mine includes plans for superimposed room-and-pillar mining in two flat-lying trona seams separated by as little as 9 m of interburden. As part of the mine planning effort, three-dimensional finite difference modeling was conducted to predict two-seam mining stress conditions while considering the crucial creep behavior of the host strata. Barrier columnization, time lag between bed mining, panel extraction ratio, and barrier width were determined to be the most significant issues influencing global stability. Implications for two-seam mine design are discussed in the paper.
1 INTRODUCTION
The OCI Big Island Mine is one of four active producers of soda ash in the Green River Basin of southwest Wyoming (Fig. 1). OCI''s underground mine supplies trona, a mixture of hydrous sodium carbonate and sodium bicarbonate, to the refinery on surface for conversion to soda ash (Na2CO3). Current operations at OCI utilize drum-type continuous mining machines to extract nearly 3.2 million metric tons of trona per year (1999 production) by single-seam room-and-pillar mining methods. The mineable resource occurs in two flat-lying seams (Beds 24 and 25), each ranging in thickness from 3.0 to 3.5 m. Present workings extend over an area of approximately 24 sq km and vary in depth between 240 and 335 m (Fig. 2).
