## 1 Introduction  
To reduce the difficulty in excavation and improve production efficiency, mining roadways with a short service life are generally arranged in low-strength coal strata and mostly adopt rectangular sections [1]. Under the conventional anchorage support, tensile stress distribution areas appear on the surface of roadways with rectangular sections as a result of the shape of roadway sections, the morphological characteristics of anchorage body and the secondary stress distribution during excavation [2]. Meanwhile, the anchorage system has a limited effect on the shoulder angle of the roadway. The roadway roof and the -anchorage bodies at the two sides are independent of each other, failing to achieve roof-side synergy [3]. In addition, affected by the geological condition of mining, the two sides and roof of the extremely soft and fragile fully mechanized caving face roadway (ESFFMCFR) are composed of extremely soft and fragile coal bodies. An increasing mining depth leads to a high ground stress. Under such a special geological condition, it is difficult to maintain a rectangular section, and the conventional anchor support systems for roadways often fail. Thus, steel sections is required for maintenance and frequent renovation. Roof accidents occur at a rising frequency [4]. The support of the ESFMCFR requires much effort, which seriously affects the recovery efficiency in fully mechanized caving faces. [PERSON] et al. [5] studied the failure mechanism of ESFMCFR with high ground stress; they thought that an \"inverted trapezoid\"-shaped plastic zone was formed in its roof coal, and stressed the importance of a cable-stayed anchor cable support. [PERSON] et al. [6] believed that the root causes of the serious deformation of the ESFMCFR lay in the weak roof supporting structure, weak synergistic control of the roof and sides as well as high tensile stress in the middle of the roof. [PERSON] et al. [7] studied the stress and deformation characteristics of surrounding rock (SR) in a fully mechanized caving face roadway (FMCFR) under different influencing factors, obtained the relationships between SR stress distribution and influencing factors, and considered that the preload of anchor bolts and cables should form a compressive stress area where they are connected and superimposed with each other. Through similar simulation tests, [PERSON] et al. [8] found that the support of an arch-shaped integral anchor structure in roadways could effectively control the serious deformation of roadway SR. By means of discontinuous deformation analysis, [PERSON] et al. [9] analyzed the deformation and failure characteristics of straight-wall semi-circular arch roadways in large-dip-angle soft and broken thick coal seams under different support conditions. By analyzing the mechanism of bolt support, [PERSON] et al. [10] put forward a one-time permanent support for a high pre-stressed and strong support system to control serious SR deformation in complex and difficult roadways of the coal mine. In summary, studies on roadway support in soft and broken thick coal seams have mainly focused on the control technologies for roadway SR, such as anchor bolts and cables, bolt mesh shotcrete and U-shaped steel shed support. However, for the ESFMCFR, the above support measures of anchor bolts (cables) and U-shaped steel sheds with an increased density or strength not only fail to effectively support roadways, but also waste a lot of materials. To sum up, current conventional roadway support technologies have certain limitations, failing to effectively control the deformation and failure of SR in ESFMCFRs. The support technology for ESFMCFRs requires further research [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. This study took an ESFMCFR as the research object. First, the hydraulic characteristics of extremely soft and fragile coal were studied by laboratory tests. With the aid of mechanical tests of coal and rock mass and on-site monitoring results, the failure mechanism and support measures for SR of the ESFMCFR were explored.