# Stressing State Analysis of CFST Arch Supports in Deep Roadway Based on NSF Method
## 1 Introduction  
With the depletion of shallow coal resources, the mining of coal resources gradually turns to the deep, and the depth of many mines even reaches more than 1000 m, which brings about serious influence of high ground stress, nonlinear rock mechanics behavior, complex geological conditions, etc. [1; 2; 3]. However, due to deficient control capability of rock mass stability, low ultimate bearing capability, etc., conventional support methods, such as rock bolting, U-steel support, reinforced concrete arc and so on, would not be suitable for withstanding the complicated high stress field environment [4; 5; 6]. Nowadays, the concrete-filled steel tube (CFST) are gaining more and more popularity in industrial factory building and bridge construction because of its high bearing capacity, light self-weight and convenient construction [7; 8]. As for CFST structures, the steel tube can provide the confinement of internal concrete resulting in the significant increase of compressive performance and ductility, meanwhile the internal concrete can improve the local buckling of the steel tube as well [9; 10; 11]. Hence, these advantages prompt CFST supports as a new support technology to apply in deep mine roadways. However, the load mode and stressing state of CSFT supports are different from those of the common CFST structures in ground, so it is necessary for researchers to further study the structural working behavior characteristics of CFST supports under the radical uniformly distributed load, in order to provide a theoretical basis for engineering practice [12; 13; 14]. As early as around 2000, the CFST supports were industrial tested and researched to verify the feasibility of the technology, and then the supports were successfully applied to the China's Qianjiaying coal mine in 2009 [15; 16]. Since then, the practical engineering cases and researches based on the CFST supports became more and more, which confirmed the control ability of the to the supports surrounding rocks [17; 18]. For example, [PERSON] et al. tested axial compressive strength, lateral flexural performance and hardening rate of core concrete of the CFST supports in soft rock roadway and then built the compressive strength-maturity relationship through limit equilibrium method, fixed-end arch theory and the results of laboratory tests [19]. [PERSON] et al. presented the numerical study on the mechanical performance of CFST supports and proposed an interface based model with friction coefficient of 0.4. The results showed that higher concrete strength and steel ratio could bring about higher mechanical performances [20]. [PERSON] et al. carried out full-size mechanical tests of CFST arch supports by using a number of hydraulic cylinders along the arch profile to simulate the surrounding rock pressure. The finding determined the failure mode as elastic-plastic instability with an extreme point and the reliable and an accurate theoretical calculation method was also proposed [21]. [PERSON] et al. developed an innovative CFST support for a 1000 m-deep roadway in a high situ stress field verified the outstanding advantages of CFST supports in terms of a higher bearing capacity and structural stability through laboratory tests and long-term deformation observation [22]. [PERSON] et al. compared the mechanical performances of U-steel support and CFST support through theoretical calculation and numerical simulation and discovered that the CFST support could provide greater supporting force to the roof-floor and two sides of roadway [23; 24]. In general, the existing researches can greatly promote the application of CFST supports indeed, while three problems in the working behavior of them have been puzzling researchers and limiting their development to some extent:  
1. As a new support in deep mine roadway, many an experiment of CFST supports is still required to investigate its working behavior and failure mechanism, while the high experimental cost results in insufficient experimental data.