The Establishment Of Microseismic Monitoring System In Zijing Tunnel And The Preliminary Analysis Of Microseismic Characteristics Of Surrounding Rock

With the significant investment for highway construction, more and more highway tunnel was built, especially in western China which developed at an unprecedented speed. At the same time, there have been many rock fracture problems caused by surrounding rock deformation during tunnel excavation, producing a series of fissures or cracks, and leading to instability and failure of tunnel surrounding rock under certain conditions, which have a great impact on production equipment and construction duration, and threaten the safety of construction workers, which has become one of the major technical problems in China’s underground construction. Therefore, the tunnel surrounding rock stability analysis has a very important significance.Microseismic monitoring system is an effective three-dimensional space rock fracture orientation and intensity of monitoring approach which has been rapidly developed, and has been widely used in underground construction projects. Microseismic monitoring technology can use the waveform to analyze time, location and magnitude of the source sabotage events(also known as the three elements of strong spatial and temporal), provide technical support for the research surrounding rock stability, and monitor the magnitude smaller rock microfracture events in the same time.In this paper, the research background is about the construction of Zijing tunnel in Highway 303. Established microseismic monitoring system basing on drill and blast in highway tunnel, through the Zijing tunnel monitoring data analysis, the monitor paragraph is K13 + 670 ~ K13 + 770. Conducts geological sketch and field research, based on the microseismic monitoring results contrast with the scene investigation records and rock tunnel excavation stability analysis. By microseismic monitoring data analysis and summary, draw the following conclusions:(1)Combining with the collection of field geological investigation, ground stress test, rock mass investigation and related data, did the engineering geological evaluation for Zijin tunnel in highway 303 line. In the ground stress test, the maximum principal stress is 29.2MPa, the intermediate principal stress is 16.5 MPa, and the minimum principal stress is 14.6 MPa. The maximum main stress direction ofσ is 135°, the axis direction of the tunnel is about 231°, while the maximum principal stress direction angle is 84 ° to the tunnel axis, with a large Angle intersection, and adverse to the surrounding rock stability of ground stress direction. At the same time, the ground stress measurement points of three to the principal stress angle is 3° and 87°and 0 °respectively. Indicates that the maximum main stress direction is given priority to the horizontal, and the principal plane is slightly tilted.(2)According to the engineering environment and geological features of the Zijing tunnel in highway 303, initially realized the tunnel 24 hours real-time monitoring and analysis. Through the deep analysis and research on the geology, geophysical exploration and construction materials in the Zijing tunnel in highway 303, under meeting the microseismic event location accuracy and sensitivity requirements, select the axis acceleration sensor, and optimized sensor layout program. Final layout plan is to select 6 axis acceleration sensor, which is arranged on 3 sections. In each section, there are 2 array sensors and the distance between the first section and the section from the tunnel face is 30 m. Meanwhile, in order to prevent the close distance between secondary lining construction machinery and the tunnel face which may influences the installation distance between each sensor, selected another sensor arrangement as an alternative scheme. In the alternative scheme, arrange two sections with 4 sensors inside and the distance between the first section and the section from each tunnel face is always 35 m.(3)Through the analysis and study of the engineering construction features of Drilling and Blasting method in Zijing tunnel, degug and set the parameters affecting microseismic monitoring system performance. Determine the overall velocity model equivalent Bauhinia tunnel test methods for artificially fixed by blasting and obtained that the minimum positioning error is 18.45 m in the microseismic monitoring system when P wave velocity is 3900m/s. Through comparative analysis, select Extract frequency filtering and Butterworth filtering as the noise reduction measures for their absolute Positioning error is small which is 31.7m and 30.2m respectively. Meanwhile, choose Moving window with noise rejection for the P-wave arrival time, because it eliminates the noise to some extent and the positioning error is reduced to 21.9m. Based on the above testing and optimization of various parameters, determine the context of monitoring microseismic event source location error by blasting test, the positioning error of less than 9m and 10 m in the horizontal direction(X and Y directions), it is less than 13 m in the vertical direction(Z direction), and the absolute Positioning error is less than 15 m.(4)Summarizes the predecessor of deep buried tunnel in different time, different mechanism and different excavation method of different types of rock burst characteristics of seismic information research, for the subsequent of the tunnel microseismic monitoring work, and the analysis of seismic information. in the study of the Zijin tunnel in different vibration signal combined with the field exploration and construction of environmental information, the integrated use of time-frequency analysis, study all kinds of methods of shock wave amplitude frequency characteristics. Through the observation and research in construction process of the Zijing tunnel in highway 303, the events are divided into three categories: excavation blasting, rock burst events, as well as the mechanical vibration and noise.(5)Through the analysis of the events of microseismic monitoring data density and density contours, found that due to stress redistribution around the working face, caused frequent microfracture incidents, increases of event density and rocks falling phenomenon in the tunnel vault, especially in the period of half an hour to several hours after excavation blasting. At the same time, it can be seen from the effect map produced by the microseismic events that microfracture events generated mainly at the top and bottom of the tunnel and only a few micro-fracture event Produced on both sides of the tunnel.(6) Monitor paragraph K13 + 670 ~ K13 + 770 is divided into three segments according to the surrounding rock. ParagraphⅠ is K13 + 670 ~ K13 + 705, Ⅱ is K13 + 705 ~ K13 + 740, and Ⅲ is K13 + 740 ~ K13 + 770. While Analyze and summary the seismic data with the surrounding rock characteristics of each relationship respectively. It is concluded that although there are microseismic in the monitoring surrounding rock mass, there is no obvious macroscopic fracture the surrounding rock, the microseismic in this rock segment is the result of normal rock stress adjustments after excavation which has no significant effect on surrounding rock stability and the surrounding rock is in a stable state. Otherwise, in Ⅲ level surrounding rock, microseismic information can also reflect differences in geological characteristics of surrounding rock. Generally, less effective microseismic event, smaller events energy release and smaller intensity of events represent the surrounding rocks joints, the developed fracture, the broken rock, fissure block-cracked structure as well as the diabase development and groundwater exposure. If the microseismic event is more effective, intensity is higher and some events can release larger energy, represent strong stress adjustment in the sections of the tunnel after excavation, good rock integrity, blocky-mosaic structure, and no groundwater exposure.