# Investigation of Mechanical Properties for Group Anchors
## 1 Introduction  
Anchor support is the most common, fastest, and cheapest support method in mining roadway [1; 2; 3]. In recent years, with the degradation of mining environment, the adaptability of existing anchor support technology has become weaker and weaker. The strong time-dependent properties of deep rock masses have led to frequent occurrences of high-energy and large-scale deep engineering disasters, and the disaster mechanism is unclear and difficult to predict and effectively control [4; 5; 6; 7; 8]. The main reason is that the bearing capacity of a single anchor structure is limited and cannot provide sufficient support resistance. The nature of surrounding rock support is mechanical balance [7]; if the anchors cannot provide sufficient support resistance, the surrounding rock cannot maintain stability. Prestressed anchor support is the most direct way to provide support resistance. From the perspective of mechanical balance alone, a substantial increase in support resistance will help the long-term stability of the surrounding rock. The existing anchor bolt support theory and technology are not perfect, and there are still many problems in field application [9; 10; 11]. As shown in Figure 1, the roof sinks violently and the try deforms greatly in the roadway of anchor support. Although the design value of the anchor prestress in the coal mine has reached 200 kN; however, the 200 kN anchor still cannot meet the support requirements of deep high-stress, strong mining, and soft rock roadways. There is an urgent need to develop new anchor support technologies to meet theneeds of safe and efficient mining of coal resources in China for a long period of time in the future. In the fields of rock soil and slopes, the support technology of the 1000 kN-level prestressed anchor and cable has been mature [12; 13; 14]. However, this technology is restricted by time and space and cannot be directly used in coal mines. Researching and designing new structures is very necessary for the realization of super pre-stress of coal mine anchors. Compared with other fields, coal mine anchors are characterized by short length, fast installation speed, and strong anti-disturbance ability. These three characteristics hinder the realization of super anchors for mines. Fortunately, the short service cycle of coal mine anchors is a positive feature. There are two reasons for the short length of coal mine anchors. On the one hand, it is to save support costs, on the other hand, it is restricted by the construction space and construction speed. The narrow construction space of coal mine roadways is not conducive to the construction of ultra-long anchors. The construction speed of long anchors is slower than that of short anchors, which is not conducive to the rapid excavation of roadways. The installation speed of the anchor is not only restricted by the length of the anchor, but also by the solidification time of the anchoring agent. In the field of geotechnical anchorage, the solidification time of grouting material can be as long as several hours to several weeks, while for mineral anchorage, it is generally required to reach the solidification strength and complete the installation of anchor rod within a few minutes. Coal mine anchors need strong anti-interference ability because they are frequently affected by engineering disturbances, which accelerate the weakening process of anchors. All these factors restrict the application of super prestress in coal mine support [15; 16; 17]. This paper proposes a group structure to realize higher prestressed anchor support and solve the problem of deep roadway support. At present, the group anchor structure has not been used in coal mine roadway support, which is a new topic. For the new topic, the design method and basis of the group anchor support are lacking.