Date of Graduation


Document Type


Degree Type



Statler College of Engineering and Mineral Resources


Mining Engineering

Committee Chair

Syd S. Peng.


The research described in this dissertation is a direct consequence of the author's quest to find solutions to several problems he faced while designing optimum pillar layouts for some Illinois Basin coal mines over weak immediate floor strata. When using the existing methods of floor stability analysis, conclusions were sometimes drawn that were either not supported by real-world mining experience or in some cases even physically impossible results were obtained. Therefore, it was necessary to delve deeper into the existing methods and understand their limitations to arrive at alternatives that were consistent with practical observations. In improving upon the existing methods, however, it was not necessary to radically alter the way floor stability was being currently investigated in the Illinois Basin. Rather, it was necessary to gather a large amount of additional laboratory and in-situ data to develop improved design equations. It was also necessary to adopt advanced numerical modeling tools to investigate the accuracy of bearing capacity equations used currently.;For this research, the largest database of laboratory and in-situ properties was put together from all three states of the Illinois Basin. Analysis of the laboratory data showed that the engineering characteristics of the weak floor in the Basin differed between the Eastern and Western shelves. Therefore, the past practice of using single design approach for the entire Basin must be replaced by the two-shelf methodology for a satisfactory floor stability analysis. Guidelines and the necessary design equations for this purpose were developed in this dissertation. The in-situ plate test data revealed the conservative nature of the Speck and SIUC floor strength equations used currently. It was also shown that for some moisture content values, such as those determined at a few Western shelf mines, both the Speck and SIUC equations predicted physically meaningless floor strength.;The laboratory and in-situ data showed that the simple and reliable moisture content test could be used to estimate all the necessary design parameters for a preliminary floor stability analysis. Empirical correlations between the moisture content and several engineering properties of the floor were developed in this research. It was shown that critical properties of the floor like cohesion, peak and residual angle of internal friction, modulus of deformation and permeability could be approximately estimated as a function of its natural moisture content.;Back analysis of several stable and unstable floor case histories showed that while the current practice of using the Vesic's non-homogeneous soil bearing capacity solution for routine stability analysis could be continued, the Speck's equation to estimate floor strength need to be replaced by the author's relations. Finally, it has been shown that several important aspects of the coal mine bearing capacity problem that have significant impact on the floor strength could be considered in the design using the numerical modeling methodology. With the modeling technique, it was also possible to investigate floor stability under very complex pillars such as those created during perimeter mining' as practiced at some Illinois Basin mines.