An overview of ahead geological prospecting in tunneling
Introduction
Underground infrastructure is becoming a regular activity with a variety of objectives: transportation, water supply, hydropower etc. All of them involve the excavation of tunnels. The fast increase in the world’s population and economics, increasing number of tunnels are being constructed to provide better transportation, water resources and power energy. The geological structures encountered in tunnel construction is sometimes complicated. Maximum safety should always be ensured in tunneling. Moreover, improving the reliability and the speed of tunneling is mandatory.
Many abnormal geological structures such as fracture zones, faults, aquifers and karst (e.g., karst-caves and underground rivers) may exist at the front of the tunnel face. These are challenges to overcome for a safe and economic tunneling. If adverse geological bodies ahead of the tunnel face could not be detected clearly, geological hazards may occur due to the excavation perturbation. These geological hazards, including water and/or mud inflow, tunnel collapse and large deformation, could result in the delay of construction schedule, instrument damages, economic loss, and tremendous casualties. For example, during the construction of Maluqing Tunnel of Yiwan Railway in China, 19 times of water-inrush hazards have occurred, resulting in 15 lives losses. Another example, the auxiliary tunnels of the Jinping hydropower station in Sichuan Province (China) faces a maximum buried depth exceeds 2000 m and a length of over 20 km, the water pressure reaches 10 MPa, and the highest rate of water-inrush flow reaches 7 m3/s, which is an obstacle for construction safety. Obviously, from the viewpoint of safety and efficiency of the tunnel construction, it is highly advantageous to know the geology before tunneling.
Surface measurements such as geological field observation, borehole and surface geophysical surveys must be executed before the tunnel excavation, aiming to provide general geological information in the tunnel project region. However, because of complicated geologies, terrains and surface vegetation coverage, extrapolating the surface geology in the depth axis with an acceptable resolution is always difficult. Results of the surface geological and geophysical investigation along the planned tunnel trace don’t feature enough resolution and not always reliable. They usually deviate from the actual geological structures revealed by the excavation. Besides, the resolution of investigating data is not always sufficient to identify geological bodies, lithological boundaries or other geological features. Although regular surface borehole drilling has high vertical resolution, it only provides information on the vertical axis at particular points along the axis of the planned tunnel (it is not a 2D or 3D method). Particularly, in some tunnels with steep terrain and great depth, surface borehole drilling is difficult to be carried out. Failing of detecting and recognizing the geological structures ahead of the tunnel face may make the tunnel construction a challenging activity. In order to obtain more reliable and accurate geology along the planned tunnel, additional investigations are required during tunnel excavation. Therefore, using ahead prospecting technologies is necessary and important in order to predict lithological and structural heterogeneities ahead of the tunnel face.
Preliminary geological investigation is considered as the basic step of ahead geological prediction. Various geophysical prospecting techniques (e.g., seismic reflection methods, electromagnetic methods, electrical methods, infrared detection methods) are used as dominant methods to detect the geological structures and identify the adverse geological bodies. In addition, other methods such as horizontal pilot boring, geological sketch and mapping, geological analysis, geochemical exploration are also used in geological prediction.
In general, geophysical prospecting techniques are used to reveal geological structures and physical conditions ahead of the tunnel face within a certain range and, to identify the spatial location, shape and size of adverse geological bodies. The quantitative and qualitative information obtained by advanced prospecting can provide reliable basis for managing the tunneling operation, informative construction and disaster prevention. Therefore, advanced prospecting has become an essential routine procedure in tunneling. In addition, a requirement of ahead prospecting is that it must be a rapid process from data acquisition to analysis of results. Ideally, data acquisition should not interrupt tunneling operations, and prospecting reports should be provided within hours.
Specifically, tasks of the tunnel geological forecast are as follows:
- (1)
Detect adverse geological bodies ahead of the tunnel face and identify their spatial location, shape, size and altitude; identify the nature of fillings in adverse geological bodies, e.g., harmful gas, groundwater, deposit or mud; and estimate the volume of the groundwater.
- (2)
Detect the interface between different lithological structures and provide location and geometry.
- (3)
Predict the surrounding rock quality ahead of the tunnel face, and analyze the type, risk and scale of geo-hazards that may encountered during tunneling.
In order to effectively acquire accurate geological structures ahead of the tunnel face, researchers have focused on theories and technologies of tunnel advanced prospecting since 70s in the 20th century. During the last 50 years, ahead prospecting has been evolved from drilling and geological analysis to non-destructive geophysical prospecting and integrated interpretation. The integrated interpretation is defined as a combination of several geophysical methods based on the employment of different physical properties (magnetization, density, resistivity, polarizability, elastic, thermal and radioactive properties), along with an intra-method integration involving different modifications of a single geophysical method (Khesin et al., 1996). In general terms, the development of advanced prospecting could be divided into three stages: (1) drilling and geological analysis; (2) non-destructive geophysical prospecting; and (3) integrated prospecting and interpretation. The geological prospecting used in drilling and blasting tunnels have achieved a great progress. Predicting the geology in TBM activities needs still to be improved to increase speed and safety of tunneling.
To summarize the developing progress, achievements, problems and future trends of tunnel ahead prospecting technologies, a technical overview is given in this paper. And the principles, technical levels, applicability, trends, key problems and applications of tunnel ahead geological prospecting are also introduced. As the major part of this paper, new advance and progress of drilling and geological analysis, non-destructive geophysical ahead prospecting and integrated prospecting/interpretation are highlighted. Furthermore, key problems of tunnel advanced technologies, especially in TBM drilling, are discussed, and then the corresponding solutions and the trends are suggested.
Section snippets
Conventional surface geology and analysis in tunnel drilling activities
Surface geology mapping and analysis, taking into account concepts on engineering geology, hydrogeology and karst geology, is the basis of non-destructive geophysical advanced prospecting. These provide a guideline in the interpretation of geophysical data. Usually, surface geology mapping and analysis use methods such as surface investigation, advance drilling, borehole logging, and sketch. Usually, surface investigation is used to offer a general knowledge of the geology in the construction
Non-destructive geophysical ahead prospecting in drilling and blasting tunnel
Non-destructive geophysical methods are efficient techniques to investigate and predict lithology, structural heterogeneities and adverse geologies. And its investigation distances can reach from tens of meters to several hundred meters ahead of tunnel face. Geophysical non-destructive evaluations include electric methods, seismic methods and electromagnetic methods.
Non-destructive ahead prospecting in TBM tunnel
In addition to the drilling-and-blasting technique, the tunnel excavation can be performed using TBM. It has many advantages over the drilling-and-blasting technique: (1) it has a much higher rate of excavation; (2) it reduces rock damage and the production of excavated materials; (3) it produces a smoother internal surface and makes the construction safer; and (4) it can reduce the labor cost. One obvious disadvantage of this technology is the limited flexibility and suitability responding to
Traditional integrated ahead geological prospecting
Similar to the surface geophysical prospecting, results of ahead prospecting in tunnels are usually non-unique, unreliable, and even inaccurate. Especially for a very complex geology, a single ahead prospecting technique cannot always provide the required information for tunneling. One of the most used approaches is to apply several suitable ahead prospecting techniques and make integrated interpretation of data and results from all employed techniques for reducing the ambiguity. Integrated
Discussion
An overview of tunnel ahead geological prospecting is given in this paper. The principles, technical levels, highlights, applications and key problems of common ahead prospecting technologies are analyzed. This is of great use to be aware of the international research status and focus. Also, to give a solution to the key problems and conclude the trend of future research are another aim of this overview. Although the techniques of tunnel ahead geological prospecting have achieved progress in
Conclusion
Ahead geological prospecting has indeed become a routine in tunneling. We have given an overview on the principles, technical levels, the trend and applications of tunnel ahead prospecting.
Geological investigation and horizontal drilling are the basic work for ahead prospecting. Integrated logging and measuring while drilling should be also considered as they can play a very important role to obtain a priori knowledge of the geology ahead of the tunnel face (geology structure and physical
Acknowledgments
This paper is supported by the National Program on Key Basic Research Project of China (973 Program) (Nos. 2013CB036002, 2015CB058101, 2014CB046901), National Key Scientific Instrument and Equipment Development Project (No. 51327802), Consulting research project of Chinese Academy of Engineering (No. 2015-05-ZD-002). The above supports are greatly acknowledged.
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