Anchoring mechanism and application of hydraulic expansion bolts(2)

3.Application of hydraulic expansion bolts used in soft rock roadway floor heave control

3.1 General condition of test roadway

The conveyance roadway of the N207 working face in Liaoyuan Group Jinbaotun Coal Mine is buried at 800 m depth, which belongs to the deep mining roadway. Both the roof and floor rocks are mudstone as shown in Table 1, whose mechanical strengths are low as shown in Table 2. Thus the rock belongs to the typical soft rock. In the initial excavation stage of the roadway, the section shape has a semicircle arch shape, and the support form is single-layer I steel beam shed with 500 mm distance between two sheds. But the deformation of the roadway is very large especially the floor heave at the distance of about 100 m and driving face of 400 mm. At the moment, the roadway is enlarged with 2250 mm round shape and supported by the double layer 40 U-type sheds with 330 mm distance between the two sheds. But the deformation is still very large. This seriously influences the driving speed and use effect of the roadway.

3.2 Mechanism in soft rock roadway floor heave
Because the thickness of the immediate floor is larger than 1/3 the width of the roadway, and the immediate floor is sheared into some wedge blocks when the disturbance stress is larger than the strength of the immediate floor rock. The corresponding mechanical model is showed in Fig. 6 [17]. The pressure arch JKL is generated in the roof rock of the roadway, whose arch height is H. The load over the pressure arch is transferred to the deep surrounding rock via the arch foot J and K. The load c(h + H) inside the pressure arch applies on the BE plane. According to Rankine theory, the rock in the BDE area is in active plastic pressure status, and the rock in the ABD area is in passive plastic pressure status. Based on the fact that the active pressure on the right of the D point equals to the passive pressure on the left of the D point, then the depth of the D point can be calculated by Eq. (9):

3.3 Support parameters of hydraulic expansion bolts

Through the analysis concerning the anchoring mechanism of the hydraulic expansion bolt and the mechanism of floor heave in the mining roadway in the N207 working face, we found that the hydraulic expansion bolts can control floor heave in the soft rock. The control mechanism is revealed in the following two aspects [19,20].

(1) Preventing development of plastic area in floor rock

When the disturbance stress of the floor rock is more than its yielding strength, the plastic area is generated. If the hydraulic expansion bolts are installed in the floor, the yielding strength of the floor will be increased. Correspondingly, this will lead to prevention of the plastic area development. And the bolts should be installed as early as possible, which can make the floor rock more integral and more rigid.

(2) Preventing the flow of plastic area in the floor rock

The axial direction of the hydraulic expansion bolt can be vertical or oblique to the moving direction of the plastic active area and passive area in the floor. The rock must overcome the circumfluence resistance of the bolts if it moves. If the value of the circumfluence resistance is large enough, the rock in the active Rankine area will be balanced, and then the floor heave is controlled. Generally, the vertical relationship between the axial direction of the bolt and the moving direction of the floor rock can provide the biggest circumfluence resistance. So the installation angle of the bolt should be cautiously selected according to the circumfluence resistance and the construction feasibility.Through the comparison of the once-forming method and the multiple-forming method for stability control and excavation speed of the roadway, the mining roadway is excavated by the once-forming method, then the resin bolts, the cables, the hydraulic expansion bolts and the double layer 40 U-type sheds are installed. The support parameters of the roadway are shown in Fig. 7. Among them, the diameter of the hydraulic expansion bolt is 30 mm and the length is 2400 mm. After the hydraulic expansion bolt is fully expanded, the diameter reaches 48 mm. The main technological parameters of the hydraulic expansion bolt are shown in Table.

(The content of the article comes from ScienceDirect)