During the tunnel excavation process, due to stress concentration and pressure relief, the surrounding rock near the excavation face will produce a new rupture zone, which is similar in shape to a circular or elliptical circle, generally called the surrounding rock loose circle. In order to be able to detect the thickness of the loose ring at different positions of the surrounding rock in each section of the roadway, a detection point is selected between the two gangs and the top of the roadway around the circumference of the roadway to detect the looseness of the surrounding rock looseness of the roadway. The order is right gang → vault → left gang → bottom plate. According to experience, in order to effectively suppress the cross-section fluctuation and the interference caused by the metal mesh in the roadway, low-pass filtering data processing means is adopted in the loose circle detection, so that the image processing precision can be effectively improved. The electromagnetic wave velocity is 0.11m/ns, and the relevant formula can be used to determine the roadway looseness range. Due to space limitations, the detection results of two typical sections, mudstone section and limestone section, typical section 1 and typical section 2 are selected (see Figure 1). (2) Limestone section. The radar profile of the surrounding rock and limestone detection of the roadway in the typical section is shown in Fig. 4. In the figure, U-shaped steel interference is present at the abscissas of 1.1, 2, and 3.2 m. The range of the roadway from the left to the bottom of the arch to the right side is not much different, ranging from 1.3 to 2.3 m, but the maximum at the vault. Intermittent reflections can be seen at 4m in the vault, indicating that the vault has looseness in the rock mass at 4.0m due to the development effect. According to the geological radar detection image, the development pattern of the limestone loose circle is shown in Fig. 4. Article source: Mining Technology; 2017.17(3) Author: Zhou Kaijun, Pengguang Qing; Zhengzhou Coal Industry (Group) Co., Ltd. Pei coal mine, Xinmi City 452 382 Copyright: Outdoor Binoculars,Lightweight Binoculars Optics For Hunting,Outdoor Binoculars For Hunting,Sport Hunting Binoculars Guangzhou Miaozhun Jie Trade Co.,Ltd. , https://www.focuhunter.store
Its formation is mainly related to the geological conditions and construction methods of excavated rock mass [1]. The shape and variation range of the surrounding rock loose ring have an important guiding role in the choice of roadway support mode. It is usually necessary to carry out the loose circle measurement in a typical section. There are many methods for detecting loose rock in surrounding rock of roadway, each with advantages and disadvantages [2]. Commonly used methods include geological radar method [3-6], acoustic wave method [7-10], eye drilling method [11], damage theory method [12] and borehole imaging method [13]. Consider GPR apparatus having portability, easy to operate quickly, it is possible site real-time imaging and data image processing more convenient, etc. [2], this study selected GPR method for deep new secret coal fields Peigou coal extension 42 District down The typical section is used to detect the loose rock of the surrounding rock.
1Geological radar test basic principle of surrounding rock loose circle
Geological radar is a non-destructive continuous detection with high resolution and high precision. The main purpose is to use the launching device to emit high-frequency electromagnetic waves into the surrounding rock of the roadway. After being reflected by the interface between the intact rock mass and the fractured rock mass, it is received by another device, and the strength and the arrival time of the electromagnetic reflected wave signal are recorded to determine the circumference. The size of the rock loose circle.
Due to the excavation disturbance, macroscopic cracks are generated inside the surrounding rock, and the electromagnetic waves emitted by the geological radar are scanned along the roadway section. When encountering the broken rock mass, the waveform shows a disordered state; when passing through the crack and the complete interface, it will cause strong The reflection, so that the surrounding rock loose circle of the roadway can be determined according to the image features formed by the reflected wave.
A large number of measured results show that [3-6], the electromagnetic wave propagation velocity in dry coal is 0.13 ~ 0.15m / ns, and the propagation velocity in dry sandstone and limestone is 0.11 ~ 0.13m / ns. The surrounding rock of the underground roadway in this construction test is wet. The water content of the surrounding rock is observed on the site, and even some water sees out in the borehole. The comprehensive analysis of the detected wave velocity is 0.11m. /ns.
In general, the antenna frequency of a geological radar is inversely proportional to the depth of detection and is proportional to the accuracy of the detection [5]. In order to reconcile the contradiction between detection accuracy and depth, it is equipped with Ramac's complete radar antenna system, including 10MHz unshielded antenna, 100MHz unshielded antenna, 250MHz shielded antenna, 500MHz shielded antenna and 1000MHz shielded antenna. Meet the various tasks of geological radar. Considering the 42 District Dip Gateway inside thereof and a metal of electromagnetic noise, this GPR 250MHz shielded antenna selection test for loose rock zone of the tunnel probe.
2 detection section overview
The 42 mining area of ​​Yanghe Coal Industry is an important mining area for the replacement of mine production and has a long service life. The 3 downhill slopes of the 42 mining area are deeply buried and affected by the sliding structure, which is a typical soft rock roadway. The deformation of the three downhill roadways that have been developed is very serious. The deformation of the track downhill roadway seriously affects the normal transportation of the 42 mining area and the operation of the overhead passenger device. In order to ensure the safe production and transportation convenience of the 42 mining area, special research on the pressure relief support technology for the severely deformed section of the 42-track downhill slope is carried out to ensure the normal use of the 42-track downhill in its service life, in which the surrounding rock loose circle Testing is a major part of the research.
The elevation of the top plate of the 42-track downhill test section is between -279.7~-360.3m. Considering the surface elevation, the actual buried depth is about 530~600m. The plane position of the specific test section is shown in Figure 1, and there are two rails 27 to 34. Between the measuring points. If the average gravity of the overlying rock layer is considered to be 0.025 MN/m3, the vertical stress of the original rock is about 15.0 MPa (<20 MPa) according to the maximum depth of the roadway. According to the measured geological section drawn during the roadway excavation of the 42-track downhill test section, the lithology of the test section is mainly composed of mudstone and limestone. The lithology of the roadway is measured from the downhill direction of the self-measuring point 27 down to the point of the track at 31:18. Mudstone is the main one, the Platts coefficient is 2~4, the hardness is soft, so it is easy to deform after support; the lithology changes after the F7 fault is exposed near the 31-point track, and the surrounding rock of the roadway is basically L7-8. Limestone with a Platts coefficient of 8 and a hard lithology. There are two faults in F7 and F8 in the roadway of the test section. The joint cracks in the affected area are extremely developed, easily broken, and the surrounding rock is poor. Due to the influence of fault structure, the stress level of the surrounding rock of the roadway is greatly increased. If the stress concentration factor of the structure is considered according to 2 to 3, the stress level of the surrounding rock of the roadway is as high as 30 MPa or more. 
3 detection results and analysis
(1) Mudstone section. The radar detection profile of the surrounding rock and mudstone of a typical section is shown in Figure 2. The abscissa is 0, 0.4, 1, 2, 3, in the figure. There are cables, water pipes and U-shaped steel interlocking obstacles at 2m. Excluding the above interference, we can see from Fig. 2 that there is a dense intermittent intermittent reflection wave at 85 ns, the corresponding part is the top of the roadway, and the surrounding rock loose damage range is about 3.6 m; there is continuous reflection at the floor of the roadway, indicating The surrounding rock of the bottom plate has good integrity, and the breaking depth is within 2m; the breaking depth of the two gangs is relatively large, but the loosening range is not much different, between 2.7 and 3.2m. According to the geological radar detection image, the development pattern of the roadway loose circle is shown in Figure 3.
4 conclusion
(1) On the basis of simple analysis of the basic principle of detecting the surrounding rock loose circle of the roadway by geological radar method, the field measurement of the typical section of the 42nd mining track in the deep extension of the Weigou coal mine was carried out, and the two main sections along the line were obtained. The main quantitative parameters.
(2) The mudstone of the two loose zone is 2.7~3.2m, the limestone is about 2.1~2.3m; the top loose mudstone is about 3.6m, the limestone is about 4.0m; the bottom is loose. The circle is the smallest, the mudstone is about 2.0m, and the limestone is about 1.3m.
(3) Comparative analysis found that the surrounding rock loose circle of the mudstone and limestone sections in the detection section has great differences. The maximum position of the loose circle is located at the top of the roadway, and the limestone is larger than the mudstone, but the two gangs and the bottom and the mudstone are relatively large.
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