Global warming and related environmental problems are some of the most talked about issues today. In fact, at current warming trends, major global cities like New York, Shanghai, and Mumbai are at risk of being partially submerged by the year 2100 due to rising sea level. The scientific community at-large agrees that one major human activity that contributes to global warming is the emission of CO2 and other greenhouse gasses into our atmosphere from factories and vehicles. Surprisingly, the tunneling industry presents a unique opportunity to combat a warming atmosphere by taking traffic underground. Building roadway tunnels in major cities like Los Angeles will significantly reduce the carbon footprint of gas-powered vehicles through directed ventilation systems that precipitate gaseous carbon. This would be a step in the right direction for all of humanity in mitigating risks associated with global warming.
As global urbanization accelerates, roadway tunneling will become increasingly in demand and useful to major metropolitan areas not only for air quality issues, but also in regard to concerns of limited land availability and rampant noise pollution. Many cities such as Boston have already invested in taking entire freeways underground. Furthermore, with recent years’ advances in high speed locomotive travel, it’s quite possible that the twenty-first century will see tunneling as the fastest growing and most important sector of local and international infrastructure.
Undoubtedly, governments and civil and geotechnical engineering firms all over the world will push to develop cheaper and more effective methods in tunneling. Fortunately, geophysical applications to the tunneling industry present many cost-effective and non-destructive methods in tunnel investigation, planning, and construction. This paper will touch specifically on seismic and electric methods used in geophysical surveys for the tunneling industry.
Before the use of geophysics in tunneling, tunnels were constructed based on engineering geology methods including surface mapping of faults and topography and vertical borehole drilling and subsequent compositional and mechanical analysis of the recovered cores. Drilling methods are severely limited by their relative high-cost nature. As a result, many tunnel projects only drill cores every one-hundred meters or more. For deep tunnels and tunnels under bodies of water the distance between cores can easily reach one-thousand meters. The distance in between cores limits knowledge of the subsurface and involves a serious amount of guesswork as to what lies in between each core.
Seismic methods, on the other hand, interpret information from artificially induced seismic (elastic) waves to model subsurface structures. They are necessary for making detailed layer profiles of the subsurface in question and for understanding specific objects such as faults and aquifers, which dictate how and where to construct the tunnel. Explosions, seismic vibrators, and airguns are three examples of tools used to induce seismic waves. Geophones are used to record the seismic waves through converting ground motion into an electric potential. The geophone data is then compiled and migrated in a computer model to create a 2D or 3D rendering of the subsurface. Seismic geophysical methods, in combination with surface surveys and vertical boreholes effectively fill in the missing information between drilling sites.
Nevertheless, vertical boreholes and seismic methods in tunneling do not provide all the information necessary for a project. Based on the subsurface model created with borehole and geophysical data, tunnel boring machines (TBM’s) are programmed to operate a certain way along the intended path of the tunnel excavation. Essentially, the forward velocity of the machine and the torque and rotational velocity of the cutterhead are programmed to operate and adjust most efficiently given what is known about the subsurface material. Often times during construction the tunnel boring machine encounters unexpected subsurface environments. This can lead to boring inefficiencies and potential human and equipment hazards from flooded or partially collapsed tunnels, for example. Electrical geophysical technology is a promising and current area of innovation used to effectively ‘look ahead’ into unexcavated material as the boring machine moves along. Electrical geophysical methods, commonly known as geoelectrics, can assess composition, saturation, temperature, porosity, salinity, and more up to about 30m ahead of the tunnel face, depending on the conditions.
Geoelectric applications induce and retrieve an electric current (either direct or alternating current) and measuring electric field. Then, based on the electrical resistivity they predict the nature of the ahead subsurface. Resistivity is primarily controlled by the nature of the pore space rather than the solid bedrock itself, although the presence of conductive minerals such as metal ore also exert a strong impact. Temperature, presence of fluids and fluid type, porosity, and clay content within pore space all impact resistivity. On the cutterhead of a TBM are various cutting and fracturing tools used to dig the tunnel. The TBM has a metal construction and therefore low resistivity whereas the resistivity of the earth is variable but almost always higher than the TBM itself. Another region of variable resistivity lies in between the cutterhead and the tunnel face and the TBM shield and adjacent tunnel walls. This is due to the fact that, especially in hard ground conditions, the TBM creates large debris that limit the quality of contact with the tunnel face. Geolectrics in TBM’s are equipped with an injection electrode located on the cutterhead, a retrieval electrode some distance behing the TBM, a central processing unit, and isolated electrodes measuring voltage. When a TBM is boring through hard ground conditions and, as stated, loses quality contact with the tunnel face, tunnel engineers should be sure to place the injecting electrode on one of the cutting tools of the TBM, rather than on the face of the cutterhead. Some complications can occur in regard to injection electrode isolation from the cutterhead, as the electric current is prone to flow back through the metal TBM rather than propogating outward through the subsurface earth. Furthermore, the retrieval electrode is not connected to the TBM and must be moved along periodically as the TBM moves forward.
Figure 6 shows the transition of a TBM from clay to sand and a subsequent increase in the change of potential between the isolated electrodes measuring voltage. Given assumed background noise at the level of ten millivolts, the change in potential can be noticed until seven meters away from the boundary change. This so called ‘Look ahead distance varies based on the level of background electric signal, the quality of the devices used, and the transition the TBM is about to encounter. Compositional, temperature, and water level transitions all display varying changes in potential as the TBM approaches. A current active field of research aims to better predict what changing electric signal indicates about the type of solid material the pore space characteristics ahead of the TBM.
In conclusion, geoelectrics in tunneling present cost-effective solutions to increase the boring efficiency and hazard reduction in tunneling operations and are becoming more important as boring machine technology allow us to create larger and longer tunnels through more complex subsurface than ever before.