Research on Surrounding Rock Stability Test of Tunneling Roadway under Complex Geological Conditions

Different geological conditions are often encountered in the excavation of coal mine roadways, with fault-fracture zone being the most commonly seen complex geological conditions. Fault-fracture zone is characterized by complex lithologic property and joint development and can easily cause safety accidents when excavation burrows through the fault. Therefore, grouting reinforcement of fault-fracture zone is often implemented to ensure coal mine safety production. Based on the tunnel excavation case of −530 −650 m belt conveyor inclined roadway at Huainan Pan’er Coal Mine, borehole optical fiber and electrical testing technologies were applied to monitor and analyze the dynamics of the surrounding rock stability when roadway excavation passed through the F1 fault, and evaluate the effect of grouting reinforcement on fault-fracture zone. According to the results of optical fiber and electrical methods, the distributional characteristics and evolution patterns of strain and electric resistivity were analyzed. The research pointed out the distinct difference in variation characteristics of strain and electrical fields between grouted reinforced fault-fracture zone and normal rock strata sections. This indicates that the grouting reinforcement effectively improve physical properties of rock strata in the fractured section, the stability of the rock strata at the fault-fracture zone was effectively increased, the degree of fault activation and deformation was relatively small, and roadway surrounding rock basically retained its original properties, pointing to high stability.


Introduction
A variety of geological problems are often encountered when underground roadway excavation is carried out in coal mine. These geological problems are usually attributable to changes in geological conditions, such as fault zone, collapse column and seam thinning zone. These geological conditions are complex for normal sedimentary strata and can cause accidents like water inrush and collapse of roof rock, thus affecting coal mine safe production. Specifically, fault-fracture zone is one of the most commonly seen complex geological conditions. Subject to the effect of mining activities in the coal-bearing strata, the fault-fracture zone is often activated, causing geological disasters like water inrush, pressure bump and coal and gas outbursts [1]. Therefore, researching the testing technology of surrounding-rock stability in roadway excavation under complex geological conditions is of great significance for guiding safe production of coal enterprises.
Presently, extensive scholarly research and analyses on stability control technology in roadway excavation at faults and fractured rock mass have been conducted with good outcomes. Zhang Jinggong proposed that applying combined forepoling technology in fault construction can effectively control surrounding rock deformation, increase safety and reliability of construction and provide approaches and referenceable experience for handling problems like roof caving [2].
Through an analysis of fault occurrence and throw, Shi Zhiyin proposed measures like horizon control and laying metal mesh on fault-fracture zone to ensure safet production on the fully mechanized caving face [3]. Utilizing grouting reinforcement measures such as Gutelong (GN-4 model), Zhou Qinglin et al. effectively solved the geological degree of fracture development and increased the stability of surrounding rock [4]. Based on an analysis of the mechanism of grouting of fractured rock mass caving, Zhang Yong proposed the advanced deep-borehole grouting reinforcement technology to prevent roof caving of fault-fracture zone, which yielded good results in practice [5].
As can be seen from above, present scholarly research has focused on controlling stability of fault-fracture zone through integrated measures like advanced borehole grouting and pre-support, however, few studies have addressed stability analysis of the rock mass of fault-fracture zone after grouting reinforcement [6] [7] [8] [9]. More scholarly attention should be focused on what methods should be adopted to evaluate the stability characteristics of rock mass in faultfracture zone after grouting treatment. On that basis, this paper presents a study of the F1 fault zone passed through the −530 -−650 m belt conveyor inclined roadway at Huainan Pan'er Coal Mine. Integrated measures encompassing the borehole distributed optical fiber sensing and electrical method were mainly applied to conduct full-cycle dynamic monitoring, which derived stability conditions of the fracture zone after receiving grouting reinforcement treatment, followed by an evaluation of the stability of the zone. Through this study, the reliability of the relevant methods is tested, and the stability characteristics of F1 fault are obtained, which can effectively guide the safe production of the mine.

Project Overview
The F1 fault zone is abounding fault between Panbei and Pan'er mining areas, with a fault throw of 10 -300 m. The Panbei and Pan'er coal mining areas were required to be merged for purpose of resource integration. The merging was fulfilled by an inclined roadway connecting the two coal mining areas using the −530 -−650 m belt conveyor inclined roadway, which would pass through the F1 fault zone during excavation process, as shown in Figure 1. An investigation reveals that the F1 fault zone involved massive rock core fracture, obvious mudstone kneading and a large number of cracks with regional cracks being filled with calcite vein. The fault-fracture zone was characterized by a great thickness, developing fault zone and subjection to great compressional stress. Rock in the fractured zone had a moderate degree of cementation, and on both sides of the fault were largely soft rock. Given the complex geological conditions of the F1 fault zone such as large fault throw and high fracture degree in the fractured zone, as well as redistribution of stress in surrounding rock masses caused by roadway excavation, considerably large safety hazard was expected to be encountered when the −530 -−650 m belt conveyor inclined roadway was excavated. As such, grouting reinforcement of the F1 fault zone had been conducted from ground surface in the initial stage.
To evaluate the effect of ground grouting treatment of the F1 fault zone passed through the −530 -−650 m belt conveyor inclined roadway and analyze the stability of surrounding rock of the roadway section, distributed optical fiber sensing technology and parallel electrical method were adopted in the integrated monitoring project of the F1 fault zone.

Optical Fiber Sensing
The Brillouin optical time domain reflectometry (BOTDR)-based distributed optical fiber sensing technology was adopted in monitoring. By principle, after the pulsed light emitted by the demodulator is injected into the optical fiber, the photons and phonons in the optical fiber experience elastic and inelastic collisions and Brillouin scattering will be generated along the opposite direction to the transmission of the pulsed light. Brillouin scattering light is sensitive to both strain and temperature in the testing environment. When change in strain or temperature of a certain point occurs, the Brillouin frequency shift of the point will occur. Changes in strain and temperature can be obtained based on the change in Brillouin frequency shift. The relationship between Brillouin frequency shift and the two factors can be expressed by Equation (1).

Electrical Resistivity Testing
With respect to the electrical resistivity method in boreholes, the parallel electrical testing system was applied. Based on the working principle of high-density resistivity method, the testing system adopts the "distributed parallel intelligent electrode potential difference signal acquisition method and system" to collect and interpret data. The biggest advantage of the testing technology lies in the fact that when any electrode deployed in the system is supplied with electricity, potential measurement can be simultaneously conducted at all of the rest electrodes, allowing for a clear reflection of changes in spontaneous potential in detected area and in potential of the primary field potential of electricity supply and significantly higher data collection efficiency compared with traditional high density resistivity method [10]. Depending on the field source of electricity supply point, data collection of the parallel electrical testing system can be divided into the AM and ABM methods. A large body of electrical data can be obtained by automatic sequential switch of electrodes implemented by the AM and ABM-based devices, which not only fulfill data inversion of all existing DC-high density resistivity methods (e.g., Zener diode, triode and tetrode) and high-resolution inversion by resistivity method [11]- [16]. In AM method, wenner tripole method is used for electrical data acquisition, and in ABM method, Wenner quadrupole method is used for electrical data acquisition.

Deployment of Monitoring Borehole
Based on the investigation tasks and actual conditions, a borehole monitoring system encompassing devices based on the BOTDR and electrical methods was deployed in −530 -−650 m belt conveyor inclined roadway. The monitored cross section was located near the 38 m ahead of the PD77 point in the inclined roadway excavated by the belt conveyor, and the testing systems were respectively installed on both sides of the roadway from the drilling field. The boreholes are shown in Figure 1. When testing was conducted, monitoring sites were deployed on the cross section to research the effect of inclined roadway excavated by belt conveyor on the rock strata of the F1 fault zone. As shown in Figure 1(b), the 4 boreholes on the two cross sections were deployed at a certain depth of the F1 fault-fracture zone. As such, the boreholes at 4 different directions formed up a 3-dimensional monitoring space capable of robustly evaluating the structural stability of the F1 fault-fracture zone near the same section of inclined roadway excavated by the belt conveyor.

Data Collection and Analysis
On-site coal mine data collection started on March 26, 2020. Depending on the speed of roadway excavation and schedule of the mine owner, the frequency of data collection slightly varied. In the early stage, data collection frequency was once every 3-4 days; the construction of inclined roadway by the belt conveyor

Analysis of Strain Field Data
Given limited length of this paper, data of F3-3# borehole was focused despite a large body of monitoring data collected from the test site. From April 17, 2020 to August 21, 2021, a total of 95 groups of strain deformation data by the BOTDR method were collected. Based on the analysis of the results of the BOTDR tests at different depths of the monitoring borehole, the observational data of surrounding rock deformation within the control range of the borehole in the monitoring cycle was obtained. On that basis, the stability of rock in F1 fault within the control range of the borehole was analyzed, followed by an evaluation of the grouting reinforcement from ground surface and monitoring of the stability of surrounding rock in the F1 fault during roadway construction and post-construction utilization stage. Figure 2 shows the strain distribution based on the BOTDR method in the F3-3# borehole during the monitoring period. As can be seen, BOTDR-measured strain exhibited different strain characteristics, including both tensile and compressive strain, for different locations of borehole during the monitoring periods. The compressive strain was dominant and had relatively small amplitudes with the largest strain not exceeding 600 με, indicating a small rock strata deformation within the control range of the borehole and relative stability in fault-fracture zone.
To better analyze deformation and damage of rock strata within the control range of borehole, the strain curve and geological profile were combined to comprehensively analyze rock strata deformation, as shown in Figure 3. The

Analysis of Geoelectrical Field Data
The data generated by the underground electrical method was collected between March 26, 2020 to August 21, 2021; a total of 298 groups of data were collected.