A combined supporting system based on foamed concrete and U-shaped steel for underground coal mine roadways undergoing large deformations

In order to develop a satisfied support scheme for roadways excavated in soft rock and subjected to large deformation in coal mines, a combined supporting system is proposed, which includes steel mesh, bolt, anchoring cable, shotcrete, compressible U-shaped steel, foamed concrete damping layer, and fractured rock cushion layer. The interpretation of the behavior of the supporting system is carried out by numerical analysis and field monitoring. The results from the numerical analysis reveal that: due to the fact that the foamed concrete absorbed most of the deformations of the surrounding rock, the shrinkage of the U-shaped steel was significantly reduced, and the relationship between the average axial force of the U-shaped steel and the foamed concrete thickness is a second order equation. It is further noticed that, in engineering practice, the damping effect was no longer evident when the thickness of foamed concrete exceeded 30 cm. The field monitoring results show that both the force sustained by the U-shaped steel and the displacement of the surrounding rock exhibited a trend of increase in the first 20 days after the support system was formed but the increasing trend reduced after that, which means the proposed combined supporting system is effective to control the large deformation of roadways excavated in soft rock.