## 4 Double-Layer Flexible Support Technique and its Key Parameters
### Double-Layer Flexible Support Technique  
Currently, a rigid anchor bolt made from non-longitudinal reinforced threaded steel has been extensively used in most roadway supports [22; 23]. The ultimate length of this type of anchor bolt is constrained by the following three factors: (1) the anchor bolt's inability to bend; (2) the design height of the roadway limits the roof bolt length; and (3) a portion of the gauge in the excavation face is not transported out in a timely manner, but instead serves as a working platform for roof support (otherwise, the workers are unable to access the roof if the roadway elevation is too high). Consequently, the length of the roof anchor bolt is limited. Evidently, the support length of the rigid anchor rod is limited and fails to meet the demands of thick-layer anchorage.  
The steel strands possess flexible properties and are capable of being bent. Therefore, the length of the steel strand is not limited by the height of the roadway or the available working space. The tensile strength of the steel strand is much higher than that of the threaded steel anchor bolt. Table 2 presents the mechanical parameters of the most commonly used anchor bolts and steel strands in coal mine roadways. The tensile strength and yield strength of the steel strands are 2.78 times and 3.2 times that of the rebar, respectively; therefore, the steel strands have a better bearing capacity. The cable support is adapted to variable load conditions [24; 25], especially shear stresses, which may occur during mining disturbances. The utilization of steel strands for supporting the roadway roof enables not only effective thick anchorage, but also the application of high pre-tightening force. This approach significantly reduces the likelihood of bolt detachment, thus achieving robust anti-disturbance support.  
As shown in Figure 9, the double-layer flexible support technique adopts two lengths of steel strands. The first layer of flexible support is used to construct the thick fundamental anchorage layer, and the second layer of flexible support is used to realize the thicker reinforced anchorage layer. The advantages of this technique include the following:  
Figure 8: The relationship between the thickness of the anchored rock beam and the maximum subsidence/the maximum tensile stress of the roof.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Materials** & **Tensile Strength/MPa** & **Yield Strength/MPa** & **Elastic Modulus/GPa** & **Elongation Rate/\%** \\ \hline Bolts & 670 & 500 & 210 & 20.0 \\ Anchor cables & 1860 & 1600 & 202 & 5.3 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Mechanical parameters of bolt and cable most commonly used in coal mine roadway.  
(1) fully utilizing the high load-bearing capacity of the steel strands; (2) by using the thick fundamental anchorage layer, a certain thickness of rock layers on the roof is consolidated into one unit, and anti-disturbance support is achieved; and (3) by using the thicker reinforced anchorage layer, the stability of the deeper rock masses can be fully mobilized in order to constrain the deformation of shallow, unstable rock masses, thereby further improving the anti-disturbance performance of the support structure.