In the design of entries, pillars, and longwall powered supports for underground mining floor bearing capacity must be evaluated, especially in the weak floor conditions encountered. Experimental studies have been performed by many researchers to delineate relevant bearing capacity of mine floor. Floor bearing capacity test is usually conducted using a series of small bearing plates much smaller than pillars or base plates of powered support. The results sometimes do not properly represent design values.

This paper describes an investigation of floor bearing capacity in an underground coal mine in Alabama. From the experimental results and theoretical analyses of two typical floor conditions, it was found that the bearing plate sizes obviously affected on floor bearing capacity measurement. It was also found that different floor conditions have different floor failure modes, stress-deformation relationships, and loading conditions which are parameters for determining floor bearing capacity. A method is also developed to determine floor bearing capacity for various floor conditions and plate sizes.


Because of lower bearing capacity of the weak floor, floor heaves have been frequently observed at a mine in the Warrior Coal Field in Alabama. These problems forced cancellation of mining activities at a section. Two different floor conditions as shown in Fig. 1 exist at the mine. Type 1 is the floor that consists of two layers: the upper layer is composed of dark shale which is harder than the layer beneath, composed of fireclay. Type 2 is the floor only consisted of fireclay, having lower cohesion, internal friction angle and compressive strength. In order to determine floor bearing capacities, a series of floor bearing capacity tests were performed on two typical floor conditions at the mine. This paper describes the results and findings of the investigation. The different floor characteristics are discussed and compared.


The study involved laboratory rock property testing and field bearing capacity tests. The test results were analyzed and compared according to the different floor conditions.

2.1 Laboratory Test and Rock Properties

Floor rocks, dark shale and fireclay, collected from the typical sites in the mine were tested in the laboratory. The testing program included slake durability test, water content test, and uniaxial compression test under various water contents. Slake Durability Test. Slake durability tests were conducted to determine weathering properties of rock. Normally, rock specimens of 40 to 60 g in weight each giving a total weight of 450 to 550 g are selected. The Samples were in natural condition or soaked in water for different periods of time before testing. Figure 2 shows test results of the slake durability indices for two different floor rocks with various time durations of soaking in water. It was found that the slake durability indices of fireclay were much smaller than those of dark shale, and it decrease rapidly after the samples were soaked in water. This indicates that fireclay contains more clay, resulting in more reduction in its strength as moisture content increases.

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