American Society of Civil Engineers

Influence of Softening on Mine Floor-Bearing Capacity: Case History

by Gennaro G. Marino, P.E., M.ASCE, (President, Marino Engineering Associates, 1101 E. Colorado Ave., Urbana, IL 61801 E-mail: and Abdolreza Osouli, (corresponding author), M.ASCE, (Geotechnical Engineer, Marino Engineering Associates, 1101 E. Colorado Ave., Urbana, IL 61801 E-mail:

Journal of Geotechnical and Geoenvironmental Engineering, Vol. 138, No. 10, October 2012, pp. 1284-1297, (doi:

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Document type: Case Studies
Abstract: This paper introduces a new approach that considers the effect of softening to more accurately calculate floor-bearing capacities where difficult mine conditions are present. Because of softening and changes in confining pressures, the geotechnical properties of immediate fine-grained rock vary in the mine floor. Therefore, when floor softening is present, the conventional equations used to determine the floor-bearing capacity are not very accurate. In this study, a methodology is presented that considers the floor softening and existence of a durable layer in the mine floor. The proposed method is based on analysis of a case study located in central Illinois utilizing finite-element method (FEM) and rock mechanics laboratory data. For this case study, extensive geological mapping and laboratory tests, including rock classification, rock swell properties, and triaxial compression tests, were conducted on samples of fine-grained rocks that predominantly consisted of mudstone. The results of laboratory tests are presented and discussed in detail. Aerial and cross-sectional analyses of the floor lithology and stratigraphy were performed to evaluate the important bearing conditions across the project site. From the analyses, the immediate floor thickness and type of the nondurable and the underlying durable rock across the site were determined. DuroIndex is presented and used to determine and rate the durability of mine floor material. Considering certain pillar-to-room width ratios, two-dimensional FEM analyses were performed to evaluate mine floor capacity with both softened and unsoftened floor conditions. Consequently, the softening correction factor, or the correction for the softening effect, was determined. Finally, a procedure that takes into account the effect of softening and the existence of a durable layer was developed to determine allowable floor-bearing capacity.

ASCE Subject Headings:
Case studies
Load bearing capacity

Author Keywords:
Rock softening
Mine floor properties
Bearing strength
Floor stability analysis