# Identification for Abutment Stress by Drilling Cuttings
## 3 In Situ Investigation
### Stage #3
#### 3.4.2 Discussions  
The variation of drilling cuttings has good correspondence with roof movement and weighting. The drilling cuttings amount reflects the different layers of roof strata. In this investigation, when the lower roof is active, the concentration range of drilling cuttings is relatively narrow, usually about 15-45 m. When the upper roof is active, the lower roof will be forced to break off and move together; its corresponding drilling cuttings concentration range is relatively large, up to 45-75 m. Therefore, through detecting the variation of the concentrated area of drilling cuttings, the pattern of variation can be obtained, which can be used to predict the weighting and its influence range.  
The results of drilling cuttings detection showed that, with the change of coalface advances, stress concentration zones can be identified in general. Especially, the heterogeneity and fluctuation of stress concentration can be mapped. As matter of fact, the coal seam is uneven, and the regional characteristics are obvious, which is due to the uneven distribution of joints and fractures in different scales in the coal seam. As the coalface advances, in addition, the change of abutment pressure leads to the uneven damage and destruction of the area, which causes the obvious non-uniformity of stress concentration and the \"multi-region\" appearance. This investigation breaks the previous understanding of homogenizing the mining-induced stress concentration area. Thus, it is very important to obtain the stress concentration area based on the inhomogeneity of the defects in the coal seam, in order to meet the need of accurate pressure relief in rock burst prevention.  
At present, vibration wave detection and online stress monitoring methods are the regular methods for rock burst early warning in some coal mines in China. However, a lot of trials showed vibration wave detection accuracy is limited by penetration distance. Online stress monitoring accuracy is limited by the low number of sensors installed. Generally, there are two sensors installed in one station: the near sensor is only 5 m deep and the far sensor is only 10 m deep, along the horizontal direction of the coal seam. In addition, the stiffness matching between sensor and fractured rock or coal is another factor. Thus, drilling cutting is an indispensable method for rock burst early warning. The description revised as \"the detection accuracy of abutment pressure and its changes by using vibration wave detection and online stress monitoring\" should be improved  
How much influence does the fracture and fault structure of different scales have on the concentration and distribution of stress due to extraction of the coal seam? How can you get close to the actual result through the detection method? These are important issues that need to be addressed. At present, the results of numerical simulation are mainly used as the basis of cognition. However, from the drilling cutting results, the characteristics of stress concentration aroused by extraction are far beyond the previous understanding. Drilling cutting may be an effective approach to obtain the influence of different scale fractures or faults on stress concentration patterns.  
It should be noted that the large spacing and short length of boreholes affected the accuracy of results in this investigation. For the detection of pressure concentration and the  
Figure 13: Detection results of stage #3 upper roof weighting.  
prediction of roof weighting, the length and spacing of boreholes have a great influence, and the borehole arrangement should satisfy the engineering needs.