Further discussion on the quantitative structure of “combined short cantilever-articulated rock beam” in roof with large mining height stope

Quantitative research on the structure of “combined short cantilever-articulated rock beam” in roof with large mining height stope, the increase of mining height will inevitably lead to the increase of the activity range of the overlying strata in the horizontal and vertical directions. According to the characteristics of bottom-up movement and caving expansion of the overlying strata in the stope, the caving space in the goaf will be close to zero at a certain time, it is proposed that the roof strata that product stress on the support are within a certain range, that is, there is a boundary strata in the roof, and the concept and judgment method of the boundary strata are given. With the continuous advancement of the stope, the falling gangue will continue to creep and compress again under the action of the overlying strata, which will cause the overlying strata to sink many times, and cause surface subsidence until it finally stabilizes. Therefore, the overlying strata from the stope to the surface are divided into near-field strata and far-field strata from the perspective of the influence on the ground pressure behavior of the stope, the near-field rock strata refer to the composite rock strata that from the support to the boundary strata, and the far-field rock strata refers to the composite rock strata that from the boundary strata to the surface. The movement law of rock strata in the near-field is the focus of studying the ground pressure of the stope, and the movement law of rock strata in the far-field is the focus of studying the surface subsidence of the stope. Based on the characteristics that the lower composite roof of the large mining height stope cannot touch the gangue before it collapses and presents a “combined short cantilever beam” structure, and the upper composite roof rock strata present an “articulated rock beam” structure in the horizontal direction of the stope under the support of coal wall, the combined support body of support and direct roof, and caving gangue in the goaf, the new concept of immediate roof and main roof in the near-field of large mining height stope is proposed and the quantitative judgment method of immediate roof and main roof is obtained. Therefore, the quantitative structure of “composite short cantilever beam-articulated rock beam” of roof in large mining height stope is formed. The main roof strata that form the “articulated rock beam” structure generally have one or more layers, depending on the mining height and the thickness, strength, and position of the roof strata. The basic conditions for the formation of the articulated structure are:① the possible subsidence of the main roof is less than its limit subsidence (\begin{document}$ {\Delta }_{\mathrm{m}} < {\Delta }_{\mathrm{j}} $\end{document}), ② the stress in the upper part of the middle section of the structural plane is greater than its tensile strength (\begin{document}$ {\sigma }_{\mathrm{t}}＞[{R}_{\mathrm{t}} $\end{document}]), the stress in the lower part of the middle section is less than its compressive strength (\begin{document}$ {\sigma }_{\mathrm{c}} < \left[{R}_{\mathrm{c}}\right] $\end{document}), ③ The fracture distance of the rock strata is greater than 2 times of its thickness. Taking the 10 m super large mining height longwall face of Caojiatan Coal Mine as the engineering background, the objectivity of the existence of boundary strata is obtained by the field measurement and analysis, as the application of the quantitative structure theory of “composite short cantilever beam-articulated rock beam” of roof in large mining height stope, the support strength of the special stacking support for the longwall face retracement with the 10 m large mining height was determined.