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The paper presents laboratory, and field characterization studies of the weak floor strata associated with the Illinois No. 6 and No. 5 coal seams. The research included:

  1. Laboratory measurement of engineering index properties; clay mineral composition, and strength-deformation properties;

  2. Field measurement of shear strength properties using a Rock Borehole Shear Tester (RBST);

  3. Field measurement of Ultimate Bearing Capacity (UBC) and deformability using plate loading tests; and

  4. Correlation between laboratory and field determined properties. Results show that:

    1. Inherent moisture content (MC), Atterberg Limits (AL), indirect tensile strength, and lithologic description of weak floor strata are most important to describe weak floor strata behavior in the field;

    2. Most claystones associated with the immediate floor strata may be characterized as clays of low to medium plasticity;

    3. A minimum square plate size of 20-cm is recommended for conducting plate loading tests; and

    4. Plate UBC and deformation modulus in the field can be estimated from the MC and AL of weak floor strata down to a depth of 30- cm below the coal seam.

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