# Field Test Study on Vertical Bearing Characteristics of Coal Rock
## 1 Introduction  
For the foundation design of buildings sitting on rock, the basic design parameters such as the uniaxial compressive strength of rock are usually determined through laboratory tests. Combining these parameters with the characteristics of joints and fissures of the rock mass, the bearing capacity of the shallow foundation or the deep foundation can be calculated according to the theoretical formulas or the empirical formulas [1, 2]. Coal rock is a special type of rock that is composed of, or contains, coal. Its composition is complex, its properties vary greatly and its mechanical properties are different from other types of rocks [3]. At present, the research on coal rock mostly focuses on the damage, the fracture and the seepage characteristics from the perspective of mining [4, 5, 6]. It is rare for coal rock to be used for supporting buildings. At present, there is still a lack of basic physical and mechanical parameters of coal rock, and there is no corresponding theoretical or empirical formula verified to calculate the bearing capacity of coal rock. Therefore, in the field of engineering applications, the bearing capacity of coal rock can only be obtained through in situ testing [7]. Plate load test is a common in situ testing method. By applying a vertical load on a rigid plate, the pressure-deformation curve of the ground can be obtained, and the bearing capacity can be determined [8, 9]. The shallow plate load test corresponds to the Boussinesq solution, i.e., applying a vertical load on the surface of the elastic semi-infinite space. The deep plate load test corresponds to the Mindlin solution, i.e., applying a vertical load at a certain depth below the surface. According to the Boussinesq solution or the Mindlin solution, the stress and the deformation in the ground can be calculated [10, 11, 12, 13, 14]. Thebearing capacity of rock is generally high, and it is difficult to apply large tonnage test loads, so small-diameter rigid plates are usually used; when the testing conditions are available, a larger size bearing plate or even a full-scale bearing plate can be used to obtain the bearing capacity parameters [9; 15]. The bearing capacity of the pile foundation is often tested by the static load tests. Traditional testing methods include the kentledge method and the anchor pile method, etc. The tip resistance and the side resistance of the pile can be tested directly by using transducers embedded in the pile or interpreted from the load-settlement curve through certain methods [16; 17]. The bearing capacity of the rock-socketed pile is usually very high, and it is difficult for the traditional test methods to provide sufficient reaction force, so the bi-directional load test is often used. For the bi-directional load test, the tip resistance and the side resistance of the pile are tested separately, so the corresponding bearing capacity and the load transfer function are obtained, respectively, and then the equivalent pile top load-settlement curve can be obtained [18; 19; 20; 21]. The bearing capacity of the pile foundation depends on the load transfer functions of the tip resistance and the side resistance. In general, the displacement required to fully mobilize the tip resistance is significantly larger than that of the side resistance, but the load transfer functions of both resistances are usually hyperbolas. For rock-socketed piles, it is difficult to load the ground to failure during the test, and this will result in a small amount of displacement; the load transfer function of the tip resistance is mostly a straight line, a double line, etc. ; the load transfer function of the side resistance depends on the shear characteristics of the concrete-rock interface, and it is associated with the development of the tip resistance [22; 23; 24; 25]. In this paper, three plate load tests were carried out for coal rock, and the bearing capacity and the deformation parameters of the shallow foundation were obtained.