< Previous28 자연,터널 그리고 지하공간 기술기사 2 Application of Norwegian Method of Tunnelling (NMT) Principles to bypass landslides in mountainous terrain 3. Rock Mass Characterization and Design of Rock Support In NMT great emphasis is placed on a thorough description of geological and geotechnical aspects of the project (Barton et al., 1992). The Q-system is an empirical method providing a quantitative evaluation of the rock mass based on the structure of the rock mass, its roughness & frictional characteristics and active stress conditions. The rock tunnelling quality Q is considered a function of three parameters which are crude measures of: 1) Block size(RQD/Jn) 2) Inter block shear strength(Jr/Ja) 3) Active stress(Jw/SRF) The Q-value is expressed by The numerical value of Q ranges from 0.001 for exceptionally poor quality squeezing ground up to 1000 for exceptionally good quality rock, which is practically unjointed (Barton et al., 1974). Based on the Q-value and dimensions of the opening, the rock support in a tunnel can be selected using the Q-system support chart as shown in Figure 1. One of the key feature of the rock support chart is the recommendation of reinforced ribs of shotcrete (RRS) which has practically replaced relatively expensive rock support techniques such as lattice girders, steel ribs and cast concrete arches. Figure 2 shows a section of RRS and its execution in a tunnel in Oslo. The arrangement of the 16 mm diameter reinforcing bars on a preliminary and smoothing shotcrete is shown in Figure 3 before being covered by another layer of <Fig. 1> Q-system support chart (Barton and Grimstad, 2014 & NGI, 2015) <Fig. 2> Section showing RRS and the execution of RRS in a tunnel in Oslo (Grimstad et al., 2003 and NPRA, 2010)Vol. 22, No. 1 29 shotcrete, which may be 30 cm thick. This rock support structure is quite flexible. The RRS is bolted approximately at 1 m interval around the arch. The spacing of RRS along the tunnel length is dependent on the value of Q and the support chart shown in Figure 1 provides guidelines for its spacing. The execution of RRS is described by Grimstad et al., 2003 and in NPRA Technology Report No. 2538 (2010). NGI has instrumented several sections of RRS in tunnels and has numerically modelled RRS to verify and calibrate the load and rock support requirements in tunnels (Bhasin et al., 1999). Figure 4 shows the RRS along the tunnel length prior to shotcrete spraying. Figure 5 shows the finished RRS product. 4. Site Investigations In the past few years, NGI has been utilizing airborne electromagnetic (AEM) surveys for carrying out site investigations for aligning the tunnel corridor. Advanced airborne electromagnetic AEM surveys are performed along the tunnel corridor to provide information on the rock mass quality along the potential tunnel alignment and for visualizing the existing sub-surface geological conditions (Figure 6). Specifically, high resistivity areas i.e. competent bedrock can be distinguished from low resistivity areas i.e. incompetent or weathered rock (Figure 7). The Airborne Electromagnetic (AEM) method is based on the physical effect of electromagnetic induction where an electrical current is induced in the ground and thus a secondary magnetic field is created. <Fig. 5> Finshed product showing tunnel support with RRS in a tunnel in Oslo (NPRA, 2010) <Fig. 3> Set-up of RRS comprising of six 16 mm diameter- reinforcing bars <Fig. 4> RRS along the tunnel length prior to shotcrete spraying30 자연,터널 그리고 지하공간 기술기사 2 Application of Norwegian Method of Tunnelling (NMT) Principles to bypass landslides in mountainous terrain This secondary magnetic field is governed by the electrical resistivity of the ground. AEM systems measure the EM time decay or frequency response and the related resistivity distribution is subsequently obtained by inverse modelling. Time-domain systems (TEM) measure an EM step response decaying with time. They are generally well suited for deeper investigations due to the higher transmitter moment. Some TEM systems can provide highly accurate and well-calibrated data. AEM data provides a powerful tool for geotechnical projects due to coverage and survey speed. Significant cost reductions can be achieved by planning geotechnical drillings based on the preliminary geological model derived from AEM. Integrated with AEM, limited drilling sites can be linked and combined to a model covering the complete area of interest. 5. Numerical Verification of Rock Support Numerical verification of rock support is increasingly carried out to optimize the rock support selected using the Q-support chart in Figure 1. The use of numerical codes for predicting rock reinforcement requirements in underground excavations in both static and dynamic conditions have proved to highly useful, especially in mountainous terrain (see e.g. Bhasin and Pabst 2013 and Bhasin et al., 2017). Complimentary analysis using both finite and distinct element techniques are usually performed to understand the rock mass deformation and verification of rock support selected using the Q-system. Dynamic and pseudo-static analysis may also be conducted to assess the behavior of the tunnel subjected to dynamic loads (earthquake). Figure 8 shows an example of the displacements around the periphery of the tunnel using the distinct element code UDEC. The numerical modelling results shown in this Figure are for very poor to extremely poor rock qualities (Q-values from 0.4 - 0.04) for a planned road tunnel in the Himalayas. In the above example, the failure of rock bolts and tunnel lining was modelled for providing a better insight into the behavior of the tunnel support with overburden. Thus, one can become better prepared to tackle the situation through extra reinforcements in the tunnel. <Fig. 6> Performing geophysical surveys (AEM) along the tunnel alignment in hilly terrain in Bhutan <Fig. 7> Results from AEM survey showing resistivity in a vertical section along a tunnel alignment, red is conductive (incompetent rock) while blue is resistive (competent rock)Vol. 22, No. 1 31 Figure 9 shows the bolts failure (in %) for various Q-values and overburden, under static and dynamic (earthquake or EQ) loadings. It can be clearly seen in this Figure that with low Q-values and increased overburden there is an increased risk of bolt failure requiring a strengthening of the bolt-system. Similarly, the tunnel lining (RRS, etc.) can also be modelled for verifying different tunnel supports. 6. Norwegian Examples of Road Tunnels Bypassing Unstable Sloping Areas In areas of difficult topography in Norway, hundreds of kilometres of tunnels have been constructed to help shorten road routes and permit development without disturbing the existing landscape. The great majority of the road tunnels constructed in Norway have been intended to improve transport conditions in rural district. Before embarking on a tunnel project, there is always a discussion and debate in Norway on various mitigation measures for keeping the road safe and open throughout the year. A simple cost benefit analysis is performed taking into consideration the long-term benefits to the society as a whole. Very often, it is concluded that a tunnel is the best long-term solution that provides a good communication link to overcome the rough Norwegian topography with fjords and mountains where existing slope instability hazards exist (Grimstad, 1986). Some recent examples of tunnels constructed in rugged Norwegian topography are presented below. 6.1 South Kjostunnel project This is a recent tunnel project, completed in 2018, to bypass an area exposed to unstable slopes with frequent rock and snow avalanches. Figure 10 shows a map of the area in the north of Norway beyond the Arctic Circle where the road tunnel has been constructed. Figure 11 <Fig. 8> Example of results obtained with UDEC models showing displacements around the tunnel (Bhasin et al., 2017) <Fig. 9> Bolts failure (in %) for various Q-values and over- burden, under static and dynamic (earthquake or EQ) loadings (Bhasin et al., 2017)32 자연,터널 그리고 지하공간 기술기사 2 Application of Norwegian Method of Tunnelling (NMT) Principles to bypass landslides in mountainous terrain shows the portal of the tunnel. The principles of NMT were utilized for constructing the 4.5 km long tunnel where the average cost of the tunnel per meter was estimated to be about 9,700 Euros. Another example in Norway is the European highway number 6 (E6) Nordnes-Skardal tunnel to avoid rock fall along the coastal road. This tunnel is close to the city of Tromsø, which is also beyond the Arctic Circle. Figure 12 shows a map of the area where frequent rock falls have occurred on the road. The constructed tunnel, which opened in November 2019, is 5.8 km long and has reduced the distance of the European highway E6 by 8 km. 6.2 Laerdal tunnel The Laerdal tunnel is the World’s longest road tunnel with a length of 24.5 km. It was built to have an all year connectivity between the two largest cities Oslo and Bergen through the European highway E16. The tunnel avoids difficult mountain crossings, which are open only about 5 months annually, (see Figure 13) and make a ferry free connection between Norway’s two largest cities. There was no connection without a long ferry link that took approximately 1 hour. Another improvement is that the inner part of the County Sogn and Fjordane has got a new and safe link to Bergen, the capital of west Norway. The tunnel has a maximum overburden of 1450 m, which corresponds to a vertical stress of approximately 39 MPa. During the excavation, spalling and rock burst were observed in large parts of the tunnel. In areas with intensive spalling and rock burst, cracks were developed in the sprayed concrete during <Fig. 10> Map of the area showing the existing road and the new tunnel alignment <Fig. 11> Portal of the South Kjos tunnel <Fig. 12> A 5.8 km long road tunnel has reduced the distance along the European Highway E6 by 8 km and has increased the safety along the roadVol. 22, No. 1 33 construction, even when proper rock bolting was carried out. The cost of the tunnel, which was completed in the year 2000, was about 1 billion NOK. Figure 14 shows the entrance and one of the safety caverns allowing a U-turn for long vehicles inside the road tunnel. The Q-system was used to classify the rock and the Norwegian Method of Tunnelling (NMT) principles were used for the construction of the tunnel (Grimstad and Kvale, 1999). Typical recorded rock mass qualities from the tunnel were: SRF J J J J RQD Qw a r n 7.06.0 200 1 1 4 3 10090 In the above case, very massive rock is affected by heavy spalling and rock burst immediately after blasting. SRF J J J J RQD Qw a r n 250.2 51 1 21 5.1 6 10080 <Fig. 13> Photos showing the summer road above the Laerdal tunnel (commons.wikimedia.org/wiki) <Fig. 14> Entrance to Laerdal tunnel (left) and a safety cavern inside World’s longest road tunnel (right)34 자연,터널 그리고 지하공간 기술기사 2 Application of Norwegian Method of Tunnelling (NMT) Principles to bypass landslides in mountainous terrain In this case, no sign of stress could be observed at the face but deformations may occur and therefore the SRF value is up to five. The tunnel, which is one of many that lies along the European Route E16, allows uninhibited flow of traffic while preserving the alpine environment of the region. As mentioned earlier, many places in Norway, where natural hazards such as landslides and rock fall exist, have been linked by tunnels. 7. Conclusions This paper has provided some examples of tunnelling to bypass major landslide areas using the Norwegian Method of Tunnelling. It is experienced that tunnelling is a long-term environmentally friendly solution to combat major landslides in mountainous areas with rugged terrain. Several hundreds of kilometres of road and rail tunnels have been built in Norway to combat major landslide and rock fall areas. The benefits of constructing tunnels in landslide areas include savings in time and increased safety. More than 5000 kilometres of tunnels have been constructed in Norway over the past few decades using Norwegian tunnelling techniques. The application of updated rock support techniques including reinforced ribs of shotcrete (RRS) has replaced the use of passive steel sets in underground support in Norway. The use of single shell rock support technique in Norwegian tunnelling is considered fast, safe and cost effective. This technology has a good potential to be used for underground excavations in Vietnam especially along some parts of the Ho Chi Minh Highway, which is prone to frequent landslides. References 1. Barton, N., R. Lien and J. Lunde, 1974, "Engineering classification of rock masses for the design of tunnel support", Rock Mechanics, Vol. 6, No. 4, pp. 189-236. 2. Barton, N., Grimstad, E., Aas, G., Opsahl, O.A., Bakken, A., Pedersen, L., and Johansen, E.D., 1992. Norwegian Method of Tunneling, WT Focus on Norway, World Tunneling, June/August 1992. 3. Barton, N. and Grimstad, E. 2014. Tunnel and cavern support selection in Norway, based on rock mass classification with the Q-system. Norwegian Tunneling Technology Publication No. 23, Norwegian Tunneling Society. 4. Berggren, A., Nermoen, B., Kveen, A., Jakobsen, P.D. and Neby, A. 2014. Excavation and support methods, Norwegian Tunneling Technology, Publication No. 23, Norwegian Tunneling Society. 5. Bhasin, R., Grimstad, E., Aarset, A. and Malik, S. (2019). Overcoming rock engineering challenges for construction of transport tunnels in the Himalayas. Proceedings American Rock Mechanics Association, New York, 2019, Paper ARMA 19-1616. 6. Bhasin, R. and Løset, F. and Barton, N., (1999). Rock Support Performance of a Sub-Sea Tunnel in Western Norway. Proc. third International Symposium on Sprayed Concrete Gol, Norway, September 1999, pp.58-69. 7. Bhasin, R., Grimstad, E., Larsen, J.O., Dhawan, A.K., Singh, R. and Verma, S.K. 2002. Landslide Hazards and Mitigation Measures at Gangtok, Sikkim Himalaya. Engineering Geology, Vol. 64, pp. 351-368. 8. Bhasin, R., Tshering, T. and Olsson, R. 2012. The Effect of Earthquake on Rock Support in Tunnels. World Tunnel Congress 2012, Bangkok, Thailand, 20-23 May 2012, Tunnelling and underground Space for a Global Society. 9. Bhasin, R., Pabst, T. and Aarset, A. (2017) Detailed engineering geological investigations and numerical modelling for a planned road tunnel in Bhutan Himalaya. Proc. World Tunnel Congress 2017 - Surface challenges - Underground solutions. Bergen, Norway. 10. Grimstad, E., Barton, N. 1993. Updating the Q-system for NMT. International Symposium on Sprayed Concrete. Fagernes, September 1993. Proceedings, pp. 46-66. Norsk Betongforening/NIF, Oslo 1993. 11. Grimstad, E. 1986. Rock-Burst problems in road tunnels. Norwegian Road Tunneling, Publication no. 4, Norwegian Soil and Rock Engineering Association, Tapir Publishers, Vol. 22, No. 1 35 N-7034 Trondheim. 12. Grimstad, E. and KvKle, J. 1999. The influence of Rock Stress and Support on the Depth of the Disturbed Zone in the Lærdal Tunnel. A key to Differentiate the Rock Support. Proceedings pp. 341-346. ITA Conference, Oslo June 1999. 13. Grimstad, E., Bhasin, R., Hagen, A.W., Kaynia, A. and Kankes, K., (2003) Q-system advance for sprayed lining. Tunnels and Tunneling International (pp. 44-47), January 2003. 14. NGI, 2015. Using the Q-system, Rock mass classification and support design. Handbook, published at ww.ngi.no, 2015. 15. NPRA Norwegian Public Roads Administration. 2010. Technical Report No. 2538. Works ahead of the tunnel face and rock support in road tunnels, NPRA, Oslo (Norwegian). [This article is composed entitely based on the authors’ opinion and does not have any relation to do with the Korean tunnelling and underground space association’s official stance]36 자연,터널 그리고 지하공간 기술기사 3 1. 일반사항 2019년 유럽, 유라시아 및 지중해 지역의 전략적 대규모 인프라 프로젝트 100가지를 순서대로 나열한 보고서가 있다 (CG/LA Intrastructure, 2019). 그 중 상위 20개 프로젝트는 다음 표 1과 같으며, 여기에 노르웨이 프로젝트가 2개를 차지하고 있다. 그 중 가장 전략적 대규모 프로젝트로 주목을 받고 있는 프로젝트가 E39 피오르드 크로싱 프로젝트이다. 최근 국내에서도 관심이 많은 노르웨이 E39 피오르드 크로싱 프로젝트의 궁극적인 목적은 Kristiansand와 Trondheim 사이의 8개의 구간을 페리를 타고 건너는 대신 부유식 터널이나 부유식 또는 고정식 교량을 건설하여 피오르드를 건너고자 하는 것이다. 노르웨이 공공 도로 관리국 서부 지부(Western Region of the Norwegian Public Roads Administration) 는 매우 깊고 넓은 피오르드 지역에 터널이나 교량 건설의 타당성 검토를 수행하기 위해 일련의 연구 개발을 수행하는 과정에서, 송내 피오르드(Sognefjord) 현장을 타당성 검토 구역으로 선정하였다. 이 현장의 수심은 1300미터, 거리는 3700미터이다. 이는 매우 도전적인 현장을 선택하여 프로젝트를 진행하고 있는 것이며, 노르웨이 해상 유전 개발 기술과 경험을 바탕 으로 터널 또는 교량을 이용하여 피요르드를 크로싱하는데 많은 문제가 해결되기를 바라고 있다. 이 연구의 최종 목표는 피요르드를 크로싱 하기 위한 방법으로 부유식 교량 또는 부유식 터널에 대한 개략적인 컨셉을 확보하는 것이다. 이 타당성 조사의 첫 번째 과제로 핵심 기관들이 참여한 가운데 두 차례 “Think Tank” 세미나를 개최하였다. 이 세미 나에서는 부유식 터널, 부유식 교량, 현수교 및 이러한 대안의 조합들에 대해서 논의 되었고, 이 작업은 2010년 여름에 부유식 교량 또는 부유식 터널 어떻게 넓고 깊은 피오르드를 건널 것인가? 신윤섭 Norwegian Geotechnical Institute yus@ngi.noVol. 22, No. 1 37 완료되었다. 세미나를 통하여 제안된 대안과 다른 새로운 아이디어에 대해 프로젝트 그룹이 구성되었고, 설계 세부 사항 및 시공 조건에 대한 논의도 현재 진행되고 있다. 본 기술기사는 타당성 조사에서 수행했던 내용을 요약하였으며, 향후 프로젝트가 진행되는데 있어서 필요한 향후 연 구사항 및 권고사항 등을 정리하였다. <표 1> 유럽, 유라시아 및 지중해 지역 전략적 탑 20 인프라 프로젝트(CG/LA Intrastructure, 2019) 2. 부유식 교량 2.1 컨셉 스터디 1250미터 이상의 깊은 수심을 가진 송내 피오르드(Sognefjord)를 건너기 위한 고정식 기초구조물은 타당성이 없는 것 으로 결정되었다. 3700미터 폭의 송내 피오르드(Sognefjord)를 가로지르는 교량의 경우 파도, 해류 및 바람으로부터의 오는 외부 환경하중에 충분히 견딜 수 있는 수평 강도와 강성을 보장할 수 있어야 한다. 송내 피오르드 크로싱을 위한 가 장 유망한 두 가지 대안은 다음과 같다.Next >