Development and implementation of design proce-dures, equipment and technologies to improve the safety levels in drilled and blasted rock tunnels is emphasized in the research project "Tunnel stability; documentation and verification". Fundamental aspects of the methods of verifying tunnel stability is studied, both during construction and during the operation of the tunnel.
The focus is on specific technologies of interest to the industry, and where findings and results can be incorporated into a wider industry programme with direct commercial applications and benefits.
The Norwegian method of tunnelling has been developed for road systems with moderate traffic density levels, where the requirement for quick and cost effective excavation of tunnels is paramount.
The rock is excavated by drill and blast methods. The rock type is in most cases hard, igneous, metamorphic or sedimentary rock which is generally capable of supporting itself with only the use of sprayed concrete in thickness between 6 and 25 cm and rock bolts to retain blocks of rock that could be released by discontinuities such as fractures and joints. In very poor rock mass quality like weathered rock, fault zones and other weakness zones reinforced ribs of sprayed concrete or cast concrete lining is applied as permanent support.
The effective use of sprayed concrete and rockbolts requires that the amount of support is related to the rock mass conditions, particularly the presence of discontinuities and weathering. There are several rock classification systems that are used to characterise the rock mass qualities to determine the required support. In Norway the Q-system is most commonly applied. The Q-system requires an experienced rock engineer and/or engineering geologist to characterise the rock quality and relate this to a table of support requirements. However in order to reduce construction time and costs it has become practice for the support to be standardised as much as possible. Decisions on temporary support are taken at the face by the tunnel workers who can call out specialists when they are uncertain about the ground conditions. The rock is com¬monly covered by sprayed concrete immediately after excavation which in practice makes it difficult to map and characterise the rock conditions in the detail required to plan adequate and sufficient support afterwards.
Desk studies and site investigations prior to tunnelling give information suitable for general plan¬ning purposes for the excavation but rarely provide accurate or detailed information on the conditions at the tunnel face and cannot be used to determine the most cost effective support required throughout the tunnel.
The required support is determined by observing the rock conditions immediately after excavation. This is sometimes supplemented, especially when difficult or unforeseen rock conditions are encountered, by experienced professional geologists or engineers mapping and recording the rock conditions. The installed support is documented by recording the number and type of rock bolts installed and the volume of shotcrete and reinforced ribs of shotcrete applied after each blast round.
After break through the workmen or engineering geologists map the temporary support throughout the tunnel, and decide the additional permanent support. Normally the additional support consist of more sprayed concrete and rock bolts, and some reinforced ribs of sprayed concrete or cast concrete lining in weakness zones.
After the final support is installed the tunnel is often lined with a non-structural liner to manage water ingress combined with frost protection and partly for aesthetic purposes. There is generally no easy or practical way of monitoring the condition of the support or rock behind the non-structural lining. Regular maintenance to remove loose rocks and check the condition of the sprayed concrete and other support is required in unlined tunnels.
Internationally, the rock conditions and construction techniques are often very different from those encountered in Norway. Tunnels are frequently only excavated where there are significant traffic levels, and ground conditions are often much weaker rocks that can deform plastically. In addition the management systems are often less flexible and the support must be designed and approved in advance. Under these conditions a cast concrete lining is often preferred. Although a cast concrete lining is a very safe option for tunnel support it is also very expensive and would rarely be eco-nomically justifiable under Norwegian conditions. There are however many countries where the Norwegian method of tunnelling could be applied. Improvements in tunnelling equipment, tech-nologies and methodologies would help in exporting Norwegian competence.
The proposed research will make use of all relevant results of the recent (2000-2003) industry research program "Miljø- og samfunnstjenelige tunneler" ("Environmentally sustainable tunnels") supported by The Research Council of Norway and headed by the Directorate of Public Roads (Veg¬direktoratet). This earlier project focused on limiting groundwater leakage into rock tunnels. The new research in this proposal deals with the topic of stability of tunnels in rock, in particular the development of new methods. Some of the results of "Samfunnstjenelige tunneler" are relevant, for instance newer geophysical techniques for mapping of rock quality and by use of borehole mapping techniques.
This project will to a far higher degree than in the past focus on different methods of documentation of the rock mass quality during excavation.
There have been a number of failures resulting in partial collapses in tunnels in recent years, the best known being in the Hanekleiv road tunnel near Sande, in the Oslofjord subsea road tunnel which collapsed when they were in service, the "T-bane ring" subway tunnel, the Lærdal and Svartdal road tunnels which had cave in during excavation. These failures highlight the need for research into tunnel stability and improvement of how tunnel support is designed in practice. The consequence of such unexpected failures with respect to risk to life and the trust of the public in tunnelling technology are so important that the profession needs to improve its methods and rethink its practice.
The Hanekleiv tunnel collapse, 25 December 2006, on E18 between Drammen and Holmestrand.
The main challenge in tunnelling under Norwegian conditions lies in gathering of necessary infor-mation to design an effective support system in the short time between excavation and instal¬lation of the support. The information currently used for design normally comes from surface mapping and boreholes which are able to provide some general information on the expected quality of the rock and which may determine faults zones or weak rock conditions where special support may be necessary. However it is not practical to investigate the conditions in sufficient detail to plan effective support until the rock is partly excavated and investigated during tunnel construction. The main basis for deciding on the temporary support is the observed conditions at the tunnel face shortly after blasting. Information on rock conditions and the presence of water ahead of the face is available from the drilling of the blast holes, special probe holes and when applicable, drilling of holes for grouting ahead of the face is required.
Data from drilling of such holes (e.g. drilling rates, water flow, drill bit pressure etc.) can be used to give an indication of the rock mass quality, but the information is generally not obtained in a useful form, and has to be interpreted by a specialist. The most reliable infor¬mation on rock mass quality comes from detailed mapping during scaling after blasting and muck-out and before appli¬cation of sprayed concrete. This mapping is currently done by the drilling crew and also often supplemented by a site engineering geologist. But there is often very little time to undertake such mapping and it is perceived by the industry as expensive to have a professional on site at all times. Information could also be obtained after application of shotcrete, but would require the development of geophysical methods to "see through" the sprayed concrete.
It is also difficult to document the applied support, such as thickness of sprayed concrete applied and location of rock bolts. At present a nominal thickness of shotcrete is applied, but this can vary widely over the area of application. Generally only the number of rock bolts installed following a blasting round is recorded, together with a nominal pattern spacing where this is applicable. The lack of documentation on the rock conditions and installed support means that it is not possible to verify that the designed support has been installed, or to independently verify the design.
Upon completion of the tunnel it is generally lined with a non-structural lining as described earlier. This makes it very difficult and expensive to inspect the rock behind the lining to determine whether the support is withstanding the applied stresses or to identify incipient failure.
The solution to these problems lies in finding methods of gaining the required information at the right time and in a cost effective manner. The proposed research will therefore address and develop the following key determinant elements for tunnel safety:
- Methods for improved pre-investigations for tunnel support design
- The use of drilling parameters (MWD) from blast holes and/or longer sounding holes drilled from the tunnel face to assess rock mass quality and rock support needs and development of software for data interpretation
- Methods to verify rock mass quality immediately after blasting by means of fully automated proce¬dures. Such procedures could be measurement of the compressive strength, photographic scanning, infrared photography and various geophysical techniques. The result should be compared with the results from drilling parameters
- An approach for automated documentation of implemented sprayed concrete and rock bolting by scanning and data collection from drilling jumbo, and visualisation by DAK systems
- Instrumentation and warning systems based on acoustic emission, microseismics and new sys-tems for continuous deformation monitoring based on fibre optic sensor cables fixed continuously to the crown through tunnels in service, including verification via a pilot test.
The project will concentrate on the fundamentals for the required approaches, documentation and verification. It is believed that the new procedures and technologies developed as a result of these new studies can form a good basis for development of new guidelines for practical design and verification of tunnel support systems for drilled and blasted rock tunnels.
It is the intention of the consortium together with other relevant partners in the Norwegian tunnelling industry to ensure implementation of the methods and technologies developed in practice, through a future user-oriented R&D program which will also incorporate more practical aspects like organisation of tunnelling projects and definition and clarification of responsibilities of the different parties, e.g. owner, consultant, and contractor, in defining, implementing and controlling rock support.
In light of the tunnel failures that have occurred recently, there is an absolute requirement in this way to involve the entire sector of the tunnelling industry in the improvement of the Norwegian tunnelling practice.
Several of these topics are excellent subjects for doctoral and master's studies and two MSc students will be associated with the project. The universities will be involved as partners in the research, both because of their competence to bring in new ideas, and to direct their graduate students towards this important study topic.
The research approaches and methods that are considered within the proposed project to be most relevant to improve on tunnel stability are as follows:
- Review existing methods of pre-investigations and evaluate the applicability of new remote sens¬ing techniques. Make use of results on mapping techniques developed and explored in the recent (2000-2003) industry program "Miljø- og samfunnstjenelige tunneler" ("Environmentally acceptable tunnels") which among other things included testing of various geophysical and borehole logging techniques for defining zones of weakness or high hydraulic conductivity.
- Evaluate the data available from drilling of blast holes and probe holes and in collaboration with manufacturers of drilling jumbos and drill parameter collection systems develop ways of inter-preting rock mass quality and presenting such data in a form that can be used in determining support requirements.
- Develop and test out technologies and equipment for mapping rock mass quality in the tunnel crown and walls after blasting and before shotcrete is applied. Compare against manual mapping. Develop methods to record, analyse and present such data in a form that could be used in deter¬mining support requirements.
- Consider new and assess existing technologies and equipment for measuring and documenting installed support, such as thickness of sprayed concrete and location and orientation of rock bolts. Develop methods of presenting such data in a form that could be used as as-built docu-mentation and verification that the installed support is in accordance with the design require-ments.
- Develop and test out technologies and equipment available to determine the satisfactory per-formance of the tunnel and installed support during the operational life of a tunnel. Develop instru¬mentation systems in collaboration with existing suppliers of relevant equipment to monitor the condition of the tunnel and give warning about potential instabilities. Technologies that will be considered are use of deformation measurements in the rock mass and stress measurements in installed support as well as direct measurements of deformations by means of fibre optic cable that can measure deformations along an entire tunnel on a continuous basis.
The results of the research will be published in both refereed scientific and technical journals and trade journals, and at conferences. Both dissemination channels will highlight the problem areas under investigation, discuss and propose solutions and spread knowledge of the tech¬nologies and equipment being developed.