What is the first time the old top comes to pressure, what are the characteristics?

The first time to press: the top of the working face caused by the first instability of the old top, called the first time of the old top.
The first step of the pressure step: the distance from the opening of the eye to the initial pressure of the working face is called the initial pressure step of the old top.

2. The characteristics of the old top when it comes to the first time

The roof is heavily sunk and the strut load is increased.

2) supporting the wall increases the pressure within the coal, the coal and spalling severe deformation of the walls. Pressurized zone, decompression zone, and voltage regulator zone.

3) Direct top crushing and slag dropping.

4) The first time the pressure is relatively sudden, it is easy to cause a serious accident.

The size of the initial pressure step is related to the mechanical properties of the old top rock layer, the thickness, and the degree of mutual bite between the broken rock blocks. At the same time, it is also related to geological structures.

Generally 20~35m, individual 50~70m or even larger.

According to statistics, in the existing production mines in China, the initial pressure step is:

10~30m 54%

30~55m 37.5%

The rest is greater than 55m, the maximum is about 160m, such as Datong.