INTERNATIONAL SOCIETY FOR SOIL MECHANICS AND GEOTECHNICAL ENGINEERING

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Le Projet du Tunnel d’Eurasie: les leçons apprises de fouilles profondes de terre difficiles

Alexis Rose, Adrian Dolecki Ground Engineering, WSP | Parsons Brinckerhoff, UK, [email protected]

Tolga Toğan Geotechnical Engineering, WSP | Parsons Brinckerhoff, USA

Mark Glendinning Civil and Structural, WSP | Parsons Brinckerhoff, Singapore

ABSTRACT: The Eurasia Tunnel project is a 14.6km major roadway, which includes 3.4km of TBM tunnelling beneath the Bosphorus to link the Asian and European sides of . The 13.7m diameter TBM was deep underground when Istanbul’s latest mega-project “The Grand Istanbul Tunnel” was announced in early 2015. It will have a tunnel diameter of 16.8m and will be the first three-level tunnel in the world, consisting of both a metro line and twin-deck motorway, which is planned to open in 2020. It is important that lessons learned from previous project experiences are used to better direct and influence similar impending projects. This paper focuses on the ground conditions and the inherent difficulties of construction in highly faulted, folded and fractured rock where groundwater is also a significant problem. Such conditions were encountered during excavation of the Asian Transition Box; the launch pit for the Eurasia Tunnel TBM. As a consequence, it proved necessary to redesign the support of excavation system following unexpected ground movements that occurred once excavation had begun. A greater importance must be placed on a good quality ground investigation and understanding of the complexities in data interpretation whilst formulating design parameters.

RÉSUMÉ: Le projet du Tunnel d’Eurasie est une route principale de 14.6km qui comprend 3.4km de percement de tunnel TBM au- dessous du Bosphorus, pour joindre les côtés asiatique et européen d’Istanbul. Le TBM, à diamètre de 13.7m, était déjà sous-terre lorsqu’on a annoncé au commencement de 2015, le dernier grand projet d’Istanbul, “Le Grand Tunnel d’Istanbul”. Ce tunnel aura un diamètre de 16.8m et sera le premier tunnel à trois étages du monde, ayant une ligne de métro et une autoroute à deux étages. L’ouverture est prévue pour 2020. Il est absolument essentiel que les projets d’avenir profitent des leçons apprises de nos expériences. Ce papier concentre sur les conditions de terre et les difficultés propres à la construction dans une roche très imparfaite, pliée et fracturée, où l’eau de terre est aussi un problème significatif. On a découvert ces conditions pendant les fouilles du “Asian Transition Box”; la fosse de lancement pour le Tunnel d’Eurasie TBM. Par conséquent, il a fallu reconcevoir le soutien du système d’excavation à la suite de quelques mouvements de terre inattendus, après le commencement de fouilles. Il faut attribuer beaucoup plus d’importance sur la bonne qualité de l’investigation de terre et la compréhension des complexités de données d’interprétation en formulant des desseins des paramètres. KEYWORDS: observational method, risk management, deep excavations, parameter selection

The TBM tunnel is a twin-deck arrangement with traffic on 1 INTRODUCTION the upper deck travelling towards Asia and traffic on the lower deck travelling towards Europe. The route officially opened on 20th December 2016. The Grand Istanbul Tunnel is located 1.1 Project background about 7km northeast of the Eurasia Tunnel and the Tunnel (a metro tunnel that opened in October 2013) is located The Istanbul Strait Road Tube Crossing Project, also known as about 1.5km to the north. The locations of all three are the ‘Eurasia Tunnel project’ connects the Asian and European shown by dashed lines in Figure 1. sides of Istanbul. The project has been constructed along a This paper focuses on a specific aspect of the project, the 14.6km route, comprising 5.4km of tunnels and 9.2km of Asian Transition Box (ATB), which is an excavation of 173m approach roads. The principal aim of the project was to relieve in length and up to 38m deep, comprising a two-tier piled wall Istanbul’s substantial transcontinental traffic pressure, but the with pre-stressed ground anchor tiebacks. It is where the 13.7m project was also driven by economic and environmental diameter TBM was launched from and also where the tunnel benefits. The Design-Build-Operate-Transfer contract for the form transitions from TBM to mined NATM tunnel. Parsons project was awarded to the Turkish-Korean JV, ATAŞ, formed Brinckerhoff were the Designers and IGT Muhendislik, based through the partnership of Yapı Merkezi and SK Engineering in Istanbul, were their Sub-designers. and Construction. The approximate lengths of the route components are as follows: 5.4km of European approach roads, 3.8km of Asian 2 GROUND CONDITIONS approach roads, 3.4km of tunnel boring machine (mixed shield slurry), 1.0km of NATM twin tunnels (utilising initial shotcrete and rockbolt support) and 1.0km of cut and cover tunnel. The 2.1 Geology maximum depth of the TBM tunnel is 106.4m below the Bosphorus surface water level, meaning that the TBM was The geological setting is within a large scale fault region called exposed to pressures in excess of 11bar, the highest ever for the North Anatolian Fault Zone, which is one of the most TBM construction. seismically active areas in the world. The Marmara Fault

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System is the part of this fault zone that runs beneath Istanbul The selection of parameters employed data from the site and many strong earthquakes occur along these fault lines. specific ground investigation, but also considered data from The geology within the region comprises Carboniferous and other excavation sites in Istanbul. Local experts were called Devonian sandstones, claystones, limestones and greywacke. upon to assist and offer their advice and experience of the The greywacke is the principal rock type at the location of the ground conditions in Istanbul from previous ground ATB and is locally known as the Trakya Formation. investigations and construction projects. There were numerous The Trakya is weathered argillaceous sandstone and iterations of the geotechnical parameter table put forward for deposition of these sediments together occurs from turbidity design; a clear indication of the challenges in confidently currents or submarine avalanches. The Trakya has also been assigning parameters to the materials and quantifying their intruded by igneous dykes of dolerite (a subvolcanic mafic performance in a rock mass situation. rock). Close to the Bosphorus and adjacent to these intrusions, The final version of the parameter table showed the rock the rock is more intensely folded, faulted and fractured and as a divided into four zones; fair, poor, very poor and extremely result it has highly variable strength and stability properties. poor, with Mohr-Coulomb strength values as follows: effective Three primary joint sets were identified during the ground friction angle (’) ranging between 20o and 35o and the investigation, one is approximately horizontal, and the other effective cohesion (c’) ranging between 60kPa and 160kPa. The two are sub-vertical and are oriented orthogonally. ground model adopted for the design was largely based on the sequencing of the geological units from the closest borehole to the wall section under consideration. An increased emphasis was placed on the monitoring and instrumentation programme, which was planned as part of the construction phase.

3 DESIGN AND CONSTRUCTION

3.1 Design

Various options were considered for the excavation support design, although most centred on the use of piled walls with pre-stressed ground anchor tiebacks, the preferred construction method of local contractors. The design considered that the excavation would be open for more than two years and many Figure 1. Location of Istanbul’s transcontinental tunnels; Marmaray, elements of the design adopted permanent works assumptions Eurasia and the planned ‘Grand Istanbul Tunnel’. See detail for ATB. over temporary. The option taken forward to final design involved an upper secant piled wall and a lower contiguous 2.2 Ground investigation piled wall, both with ground anchor tiebacks. The secant piles were up to 21m in length and the contiguous piles were up to Three main phases of ground investigation were completed for 25m in length, all with a diameter of 800mm and reinforced the project between 2010 and 2012. A total of 12 boreholes over their full depth (see Figure 2). were drilled for the ATB, eight deep holes and four shallow, with an average depth of 36m. The majority of rock core was logged as sandstone, with thick zones of mudstone and thin zones of fault breccia, as well as the dolerite intrusions. Limited thicknesses of made ground and superficial deposits were also recorded. Eurocode 7 suggests boreholes may be spaced at 50m intervals for such a structure. The perimeter of approximately 400m would indicate that the 12 boreholes was acceptable; however, this is open to interpretation and is dependent on numerous other factors. Standard Penetration Tests (SPTs) and undisturbed samples were taken where appropriate, and core samples were taken in the rock. A range of soil and rock laboratory testing was subsequently undertaken. The groundwater level is at a depth of approximately 4m below ground level at the western end and approximately 8m below ground level at the higher eastern end. The western end of the ATB is only about 70m from the Bosphorus.

2.3 Ground model and parameters Figure 2. Cross section showing an example of the piled wall layout Rock mass classification was evaluated using both Rock Mass Rating (RMR) and Geological Strength Index (GSI) and both The ground anchors were between 24 and 48m in length systems showed the rock to have very poor to fair quality. (see Figure 3). They were initially intended to be double However, reflecting this variation in derived geotechnical corrosion protection (DCP) anchors however issues with site parameters is particularly difficult. When testing core samples trials and low interface bond resulted in the outer encapsulation from a poor quality rock mass, it is important to evaluate the being abandoned. All were pre-stressed to either 45 or 60 results knowing that there can be a tendency to undertake the tonnes with a working bond length of 8m. tests on better quality samples of rock material and therefore It was preferred to construct the wall in a two-phase may not fully reflect the characteristics of the whole rock mass. approach because there were concerns over buildability and the Allowance must be made for this. installation of ~50m long piles into rock. This preferred

- 1926 - Technical Committee 206 / Comité technique 206 solution would provide a stiff ground control system and allow 4 REDESIGN flexibility in adapting the design via the observational approach. The secant piled wall was required for the top half of the excavation, where ground conditions and water seepage was 4.1 Back-analysis considered more likely to be a problem. The design included inclined drain holes within the lower part of the secant piled Site observations showed that in places the material had wall to reduce the water pressures over the upper section. A properties more akin to a soil, and that the sandstone properties contiguous piled wall was considered appropriate for the lower did not govern the behaviour of the material mass. The ground level, where the ground conditions were believed to be more model and parameters were revisited and amended accordingly, favourable and water ingress anticipated to be lower. with the most fundamental change being the introduction of a The design was undertaken using Eurocode Design cohesionless material. Approach 2 and Design Approach 3. Finite Element analysis The load cell data indicated no loss of load during the was used to design the support of excavation system, to allow accelerated wall movements, which would have suggested scrutiny of the ground stresses and deflections and the forces in anchor creep or yielding in the bond zone. Horizontal the piles and anchors. Limit Equilibrium analysis was used to extensometer data reported the expected degree of elongation of check the global stability. the anchors free length, consistent with the theory that the fixed The contract requirements set out that the deflection length of the anchors was satisfactorily resisting the loads. tolerances should follow the guidance from Clough & From review of the data and the events that had preceded, it O’Rourke. Accordingly, horizontal deflections were limited to was theorised that the wall movements had occurred due to 0.1-0.2% (approx.) of the excavation depth. There were no poorer ground conditions in the Trakya formation along an specific assets in the immediate vicinity that needed protecting. interface between the highly weathered rock and the more The instrumentation included 14 inclinometers spaced competent rock. The inclinometer data in the area with the most around the perimeter walls. Horizontal extensometers and load significant wall deflections suggested that there was an cells were used to monitor the anchors, and piezometers and 3D interface at approximately 20m below ground level where a survey points were also utilised. notable change in the wall deflection profile was observed. The back analysis was undertaken using Finite Element analysis programme Plaxis 2D and used a simple ground profile, with a limited thickness of made ground on top of completely weathered rock underlain by more competent rock. The back analysis aimed to replicate the observed deflections with reasoned adjustment of parameters. The task was repeated at discrete intervals around the ATB perimeter. The previously used Mohr-Coulomb soil model was unable to reproduce the observed deflections and it was proposed to use the Hardening Soil model. The Hardening Soil model allows stress dependency of soil stiffness, where different stiffnesses can be applied to primary and unloading moduli. It was not originally considered appropriate to use this soil model for the initial design because the added degree of complexity Figure 3. Plan layout of the ATB showing size and extent of anchors could not be supported by the available data. However, with the addition of the monitoring data to allow model calibration, this approach was now regarded as sufficiently reliable. The 3.2 Excavation analysis revealed a failure surface directly behind the wall within the free anchor length zone. The analyses also found that Excavation began in April 2013. The material at some locations the excavation support system was highly sensitive to changes revealed itself to be extremely weak and clay-like and could be in excavation depth as the material approach failure, which bulk excavated with minimal effort. It was apparent at this stage matched the recorded observations of wall movement. that some of the assumptions regarding the structure and nature of the material were not as originally anticipated. This also initiated a review of the adopted geotechnical design parameters 4.2 Ground model and parameters given the observations from the on-going site works. Additionally, groundwater ingress was greater than some of the The redesign considered the live monitoring data in parallel advice that local experience had indicated; and was observed with revisiting the original geotechnical parameter selection and flowing through many of the anchor head locations and ponding inspecting rock core obtained during the ground investigation. in the base of the excavation. Ground conditions were clearly more complex than first In the first week of June 2013, monitoring data was thought, with a frequent variation in properties both vertically beginning to point towards a potential problem with the support and laterally over the height and length of the ATB. As a of excavation. Unexpected lateral displacements that rapidly consequence of this, a much simpler conceptual ground model increased from 13 to 26mm were measured from the was adopted with fewer strata, which overall were inclinometers in the western section of the south wall, where representative of the combined material mass. What was excavation at the time was at a depth of just 12m. apparent was that the decision to adopt the stratigraphy from the Corresponding increases in anchor loads were also recorded and borehole logs resulted in an overly precise but unrealistic an amber alert level (80% of yield load) was reached at one ground model, because the material was so variable between the location. The amber alert, coupled with the acceleration of wall borehole locations as the controlling structural discontinuities movement for small additional depth of excavation, triggered were inclined as opposed to horizontal. One limitation with the temporary suspension of excavation operations. The understanding the material was the low number of boreholes excavation was partially re-filled to restore passive resistance in available to provide the subsurface information necessary to front of the wall while the cause of the movement could be develop the conceptual ground model given the variability in better understood and quantified. the material observed.

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Closer inspection of the rock core and the borehole logs 5 CONCLUSIONS revealed that the strength of the rock mass was overestimated. For example, a description of “moderately strong, slightly Ground conditions encountered during construction can often weathered rock” was noted in one case; the sections of rock that differ to those anticipated from ground investigations or were recovered were indeed moderately strong, but, were existing knowledge. The experiences of the ATB excavation accompanied by a TCR (total core recovery) of 25% and an illustrate the real consequences of this problem. RQD (rock quality designation) of 7% (see Figure 4). The overall strength of the ground is much more dependent on properties of the unrecovered material, likely to be a heavily fractured fault breccia material or a soft clayey fault gouge material, both of which would probably have been washed out with the borehole flush fluid.

Figure 5. Excavation complete

Figure 4. Core box showing typical rock core samples (example taken from eastern end of ATB at approximately 40-45m depth) A far greater importance must be placed on undertaking a ground investigation suitable for the site conditions and

complexity of the structure being built, particularly when Since the ATB was very close to the Bosphorus and variability in the material is known to exist or is discovered potentially closer to fault zones, the conditions were more during the investigative works. Meticulous core logging is adverse than those typically experienced in Istanbul. The most required and rock mass classifications must be used with notable difference was that the conditions within the ATB site caution and with a full understanding of their limitations. required the introduction of a completely weathered rock Similarly, a clear understanding and appreciation of material classification into the ground model. Much of the complexities in data interpretation is required, combined with observed material being excavated revealed that the structure the knowledge that a simpler, more rationalised conceptual had been completely destroyed and was visibly more soil-like. ground model may provide a more appropriate solution in These redefined parameters were an extension to traditional locations that are spatially highly complex. Also of note is that guidance for the locally well-known ground conditions. designs based on limited information in complex ground and without holistic consideration of the surrounding conditions can 4.3 Redesign lead to a confidence in the ground model that cannot be justified and this approach is discouraged. Communication between With the revised and rationalised conceptual ground model and engineering geologists and geotechnical designers is essential accompanying parameters the redesign could be undertaken whilst formulating ground models and geotechnical parameters. using a similar method and approach used for the initial design. Effective implementation and management of the For those sections already constructed this involved installing observational approach, along with proactive use of monitoring additional rows of anchors between those already in place. For data to reappraise ground conditions and allow dynamic the lower sections not yet built the spacing could be optimised. redesign, was key to the success of the Asian Transition Box. It The contiguous wall piling had not commenced and so the is important that lessons are learned from these experiences to opportunity was taken to increase the reinforcement into the better direct and influence similar impending projects. pile up to the maximum 4% permitted by the codes to provide some additional redundancy. As a consequence of the redesign, the zone of influence of 6 ACKNOWLEDGEMENTS the anchors overlapped where additional ones were installed. The Authors would like to thank Ray Castelli (WSP USA) and This necessitated those anchors being installed past the bond Anıl Kurban (IGT Muhendislik) for their assistance and input zone of the existing anchors to avoid interference between bond into the successful completion of the Asian Transition Box. zones. Design was completed in sections and on completion of each section construction drawings were issued progressively. 7 REFERENCES

4.4 Excavation completion Castelli, R. J., Richards, D. P. and Clark, G. T (2015). Design and Construction of a Deep Excavation in Extremely Poor Rock Mass. Following the successful reanalysis and the revised design of US Rock Mechanics Symposium. the ATB, the excavation recommenced. Work was only Clark, G et al. (2015). Elements of the Istanbul Strait Highway Tunnel. suspended for two weeks before the new design started to be Rapid Excavation and Tunnelling Conference. implemented on site. Final excavation depth was reached in Clough, G. W. and O’Rourke, T. D. (1990). Construction induced January 2014 after minimal delay to the construction movements of insitu walls. ASCE Special Publication 25. programme (see Figure 5). The 13.7m diameter, 120m long Keskin, H. B. (2008). Analysis of the performance of retaining systems TBM was launched successfully in April 2014 and broke in deep excavations in greywackes. PhD Thesis, Bo aziçi through on the European side of Istanbul in August 2015. ğ University.

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