Keywords

1 Introduction

With the rapid development of underground engineering, various deep underground projects are planned, are under construction, and have been built. Microseismic (MS) monitoring technology has been widely used in deep tunnel engineering for safety monitoring as a geophysical and seismic-based method, and has achieved a series of helpful results. The MS wave signal associated with rockmass fracturing can be captured by MS sensors in spatial. Then, the MS wave signal can be systematic analyzed and information such as the time, location and energy of the fracture can be obtained. Therewith, the stability and its development trend of rockmass can be judged based on the information [1]. MS monitoring technology originated in the US rock explosion research of deep mines in the early 20th century. However, because the technology was not mature enough at that time, the research did not achieve satisfying results. The technology was first put into use officially by South Africa in the 1960s. South Africa successfully used the technology to monitor the microseismicity in deep gold mining, and the stability of rockmass was judged based on the microseismicity monitored. Later, the MS monitoring technology was widely used as an emerging rockmass stability monitoring method in the fields such as tunnels, slopes and mines in developed countries. In 2004, China introduced the ISS MS monitoring system from South Africa, and a MS monitoring system in Dongguashan copper mine was established [2]. In 2010, Chen et al. [3] introduced the MS monitoring technology to the deep TBM tunnel in hydropower engineering for rockburst monitoring in China. In 2015, A MS method for quantitative warning of rockburst development processes in deep tunnel was first proposed by Feng et al. [4]. And the MS method has been successfully applied to rockburst warning in deep tunnels at the Jinping II hydropower project.

In recent years, with the development of science and technology, MS monitoring technology realized informatization, automation and intelligence, which has been widely used in deep tunnels in China with a series of helpful results [5,6,7,8,9,10,11,12,13,14,15,16,17,18,19]. In order to understand the developments of MS monitoring technology in deep tunnels in China, four aspects, i.e. distribution of projects, environment and system characteristic, purpose, and effect of MS monitoring in deep tunnels in China were analyzed and summarized. In addition, the future development direction of MS technology in deep tunnel engineering is prospected. The results will be helpful for a rapid development of MS monitoring technology in deep tunnel in China.

2 Distribution Characteristic of Tunnel Projects with MS Monitoring

In order to understand the distribution of tunnel projects with MS monitoring, the literature in the past decade related to tunnel projects with MS monitoring was investigated, and then, the related information, i.e. project name, project region, application time and project type, were counted. Distribution of several tunnel projects with MS monitoring is shown in Table 1.

According to the statistics in Table 1, we can see that MS monitoring technology has been widely used in deep tunnel projects in China in the past decade, and the projects were mainly distributed in western China. The area was mainly concentrated in Sichuan Province and there were several projects in Tibet and Xinjiang. This is mainly because the terrain characteristics of China. The western region is with a lot of mountains, high altitude, and rich water conservancy resources. As an effective and mature mean of rockmass stability monitoring technology, MS monitoring technology was mainly applied in water conservancy and hydropower project, traffic project, and laboratory. The application types of projects were comprehensive.

Table 1. Distribution of several tunnel projects with MS monitoring
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3 Environment and System Characteristic of MS Monitoring

Furthermore, based on the analysis of the literature related to tunnel project with MS monitoring in the past decade, the maximum buried depth, lithology, construction method and monitoring system of the tunnel projects with MS monitoring were counted. And the environment and system characteristic of MS monitoring were obtained. The statistical results are shown in Table 2. The projects corresponding to the serial numbers in Table 2 are consistent with those described in Table 1.

Table 2. Environment and system characteristic of MS monitoring
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According to the statistics of Table 2, the MS monitoring system applied in deep tunnel projects in China at present mainly includes ISS, IMS, ESG and SSS. The IMS is a new generation product of ISS. With the development of deep tunnel projects, the maximum buried depth with MS monitoring reached 2525 m. And most of the buried depth was deeper than 2000 m (Fig. 1). MS monitoring technology can be applied to a variety of lithologies, including basalt, marble, diorite, schist, gneiss, mudstone and granite, in which marble and granite were the most ones. The tunnel construction methods were TBM and D&B methods, among which the D&B method was in the majority.

Fig. 1.
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Maximum buried depth for tunnel with MS monitoring

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4 Purpose of MS Monitoring in Deep Tunnel

Furthermore, statistical analysis was made on the purposes of MS monitoring applied in the above tunnel projects. The statistical results are shown in Table 3. The projects corresponding to the serial numbers in Table 3 are consistent with those described in Table 1. As can be seen from Table 3, the purpose of MS monitoring technology applied in the deep tunnel projects in China was mainly for disaster warning, as well as the disaster mechanism understanding.

Table 3. Main purposes of MS monitoring in deep tunnel
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5 Effect of MS Monitoring in Deep Tunnel: Case Study

5.1 Case 1: The Deep Tunnels in Jinping II Hydropower Project

Jinping II hydropower project was located on the Yalong River, Sichuan Province in southwestern China. The project and MS monitoring information have been described in detail by the references [4, 5] and [10]. A microseismicity-based method of rockburst quantitative warning in the deep tunnels was proposed to warn of the rockburst risk. The method was shown in Fig. 2. During the continuous MS monitoring activity, there were no serious rockburst casualties. The safety and construction schedule of the tunnels were ensured.

Fig. 2.
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Flowchart for quantitative warning of rockburst in deep tunnel using microseismicity [4]

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5.2 Case 2: The Deep Tunnels in Baihetan Hydropower Station

The dam area of Baihetan hydropower station was a mountain canyon landform, and the rockmass along the diversion tunnels in this project was monoclinal rock with medium to high geostress. The columnar jointed basalt developed in the project was generally dark gray or gray, and the rock blocks were mostly columnar mosaic structure. MS monitoring technology was utilized during the excavation of columnar jointed basalt in the deep tunnels. The monitoring results provides a helpful understanding and suggestion for the application of MS monitoring technology, reasonable excavation and supporting design for the tunnels in the similar rockmass with columnar joint sets. Some results are shown in Fig. 3 [13].

Fig. 3.
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Statistical distribution of MS events in spatial [13]

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6 Conclusions

MS monitoring technology has achieved unprecedented development with a series of helpful results in deep tunnel projects in China. The technology was mainly applied in the western region of China, and the project distribution was mainly concentrated in Sichuan Province, and partly in Tibet and Xinjiang. It has been used in a wide range of project types, covering many fields such as water resources and hydropower engineering, transportation engineering and laboratory. The maximum buried depth of the projects reached a maximum of 2525 m and most of the buried depth was deeper than 2000 m. There were many types of MS monitoring systems used, i.e. IMS, ESG, ISS and SSS. The construction methods related were TBM and D&B methods, among which the D&B method was in the majority. And the lithologies of the projects were mainly marble and granite. The monitoring purposes were mainlyfor disaster warning and mechanism understanding. And the technology has been successfully used in a lot of deep tunnels in China.

The tunnel project is becoming deeper and deeper and with a more complex geological environment. Therewith, the type of disasters under high stress in deep tunnel and their mechanism are becoming complex. Therefore, a higher requirement for the performance and analysis method of the MS monitoring technology is required. The improvement of the performance of MS monitoring system, the optimization of the monitoring scheme, and the precision and efficiency of MS data analysis will be several key research directions in the future. The other one key research direction is the MS based disaster analysis technology, such as big data analysis on the large number of MS monitoring data, deeper understanding of disaster development process and establishment of a more accurate MS based early warning method for disaster.