1.1 Project Presentation
The first underground road link between the European and Anatolian parts of Istanbul, The Eurasia Tunnel (Avrasya Tüp Tunnel) was opened on December 20, 2016.This tunnel was something that the population of the Turkish metropolis had been keenly waiting for. After all, the city had grown massively on both sides of the Bosporus in recent decades. This had also resulted in an enormous increase in commuter traffic between the European and Anatolian sides. For years, the existing bridges spanning the river have been unable to cope with individual transportation so that the city saw itself faced with incessant tailbacks (Fig. 2).
Prior to the opening of the Eurasia Tunnel, it took some one and a half to two hours to cross the Bosporus, now it should only take around 15 minutes. Capacity has been rated at up to 130 000 vehicles per day.
The complete tunnel structure was commissioned as a B.O.T. (Build Operate Transfer) scheme. In other words, the responsible operating company “Avrasya Tüneli İşletme İnşaat ve Yatırım A.Ş.“ (ATAŞ), comprising Avrasya Tüneli, Yapı Merkezi and SK E&C will ultimately hand over this major project to the client after 29 years. During these 29 years, the ATAȘ will cooperate to cover the planning and construction of the project. During the remaining period of time, from the opening until transference to the client, the ATAȘ acts as concession holder of the Europe-Asia link project. During this time, it will levy toll charges and bear the responsibility for repair and maintenance jobs.
Consequently, it is of enormous importance for the contractor when B.O.T. projects are awarded to ensure their completion, and receive the green light to become operational as rapidly as possible. The overall construction time lasted 55 months. This subsequently results in an operating concession of 24 years and 5 months. The total contractual sum for the B.O.T. project amounted to around 1.245 billion US dollars.
A special consortium “Yapi Merkezi and SK E&C Joint Venture (YMSK JV) was established to build this complex tunnel project under the guidance of ATAS. Both JV partners also belong to the operating company. The JV took over the construction project in the form of an EPC (Engineering Procurement Construction) contract. The assigned construction costs amounted to around 814 million US dollars.
2 Boundary Conditions
In spite of the relatively moderate length of the tunnel (the bored tunnel is 5.4 km) this tunnelling project is classified as something superlative. The actual crossing below the Bosporus is only 3.4 km long and at its deepest point is some 106.4 m BSL. There, the tunnel shell is located only 25 m below the existing bedrock (Fig. 3). From the two portals the angle of inclination remains consistently at 5 % until the deepest point is reached. The intermittent geological layers represented one of the difficulties to be mastered. On the one hand, there were loose sands and on the other, hard rock to be penetrated. The water pressure of almost 11 bar and the prevailing geology posed demanding tasks for the tunnellers. The commissioned TBM manufacturer Herrenknecht designed a special machine for the purpose, based on Mixshield technology, previously unheard of in this form.
2.2 Technical Data
The Eurasia Tunnel project was driven with a single tube. The tunnel cross-section’s useful internal diameter of 12 m catered for a twin-deck structure so that two traffic routes can flow directionally – separated from each other, with two lanes and an additional hard shoulder. The lowest part of the tunnel serves as a supply channel. Every 200 m, there is a simple protective chamber and every 800 m a more complex one for emergencies.
The tunnel is designed to last for 100 years. In addition to the difficult starting position for producing the structure, the engineers were confronted with a further problem. The tunnel is located close to the contact zone of two tectonic plates. The distance to the edge of the northern plate is just some 16 km.
In order to master all these requirements (geology, water pressure, potential seismic activities), segment elements were installed with a thickness of 600 mm. The strength of the applied concrete amounts to roughly 50 N/mm². The entire structure was tested intensively in advance, including its resistance to fire. The section subjected to fire was exposed to a temperature of 1200 °C for a period of 55 minutes. It was cooled down to 20 °C within 110 minutes.
2.3 Seismic Joints
On account of potential seismic activities, caused by the nearby Marmara Graben, the responsible engineers were compelled to contend with possible displacements of the rock around the tunnel. Studies revealed that such displacements could occur at two particular places in the tunnel longitudinal section. These two critical points are located on the incline at the Asian side, in the transition zone between gravel/hard rock and sand-loam layers. The steel segmental rings installed there as seismic joints permit expansion of up to 75 mm, compression of up to 75 mm and shear of up to 50 mm.
3.1 Starting Position for Sealing
It goes without saying that leaks occur when producing a segmental tunnel. There are many causes – as for instance, displaced or torn segment elements, cracks, voids behind the tunnel shell resulting from washed out cement suspension, etc. In order to ensure that the required fire protection can be installed properly and in a sustainable manner, these leaks first had to be appropriately sealed. It was only possible to start work on fire protection after these waterproofing operations had been successfully concluded. Solutions promising rapid and reliable progress were required to ensure that this time-related ambitious measure was not held up.
There were two decisive forms of leaks in the Eurasia Tunnel. First of all, a large number of hairline cracks occurred in the segments (Fig. 4); furthermore, problems arose in the seismic joint zone. Initial attempts to prevent the leaks using PU foams were unsatisfactory. It is no secret that PU foams can only be used for temporary waterproofing or for pre-injections Notwithstanding, these products are frequently used in Turkey as standard solutions. The reason: pump technology for two-component injections is seldom available. In addition, the knowhow of the unskilled workforce often does not suffice to enable the pumps to be operated properly. Regardless of this fact, prior tests revealed that the necessary long-term degree of tightness could only be attained by means of a two-component reaction resin. In conjunction with a local specialised sealing company and with technical support from the material supplier, it was possible to execute two-component injections successfully and efficiently.
However, a further obstacle had to be surmounted, for the ingressing water also possessed an excessively high salt content. Whereas the salt content of the surface water possesses a relatively low salt concentration (salinity is roughly 18; corresponding to 18 g of salt per 1000 g of water), the proportion of salt is higher in the lower layer. The reason for this phenomenon is due to the flow conditions in the Bosporus. The current is not unidirectional but runs in two directions at the same time. The water flowing from the Black Sea possesses a lower salt content and thus occupies the surface layer on account of its correspondingly lower density. The water from the Sea of Marmara, which contains roughly twice as much salt (salinity 38) flows in the lower layer, at a depth of some 40 m, towards the Black Sea.
3.2 Preliminary Tests with Two-Component Injection Resins
Before the actual injections could start, various types and brands of resin were tested to discover how they performed against hairline cracks as well as their general resistance to the surrounding salty water exerting 11 bar of pressure (Fig. 5). Two-component polyurethanes remain essentially unaffected when reacting in a salty environment. The injectability in the case of hairline cracks did not produce the desired results. As a result, acrylate gels were lent consideration. In order to check the reaction behaviour during grouting, 1:1 injections were executed and analysed in the lab by means of core drilling. The minimum stipulation was that the area covered by the filling agent in the core had to amount to at least 80 %. The long-term resistance had to be verified by means of lab tests.
3.3 Waterproofing Injections for the Segment Hairline Cracks
The hairline cracks in the segments mainly occurred at the Asian side of the tunnel. Predominantly, over a length of some 2 km in the section between the access shaft and the first seismic joint. The bulk of the cracks had occurred in the roof and wall area. The tunnel was excavated from that side. The local team first had to become accustomed to the optimal means of driving and develop a proper feeling for it. The hairline cracks that subsequently occurred did not represent a static problem, but of course, carried water owing to the high water pressure.
In keeping with ZTV-Ing, the hairline cracks were drilled at a 45° angle (distance to crack = component thickness/2) and with a reciprocal packer gap of some 35 cm (rule of thumb: drill hole depth = gap). As these segments had been calculated to sustain the most extreme loads, a correspondingly large amount of reinforcing steel and a concrete with high compressive strengths were applied. In order to minimise the number of drilling errors, the local engineers marked the exact positioning of the drill holes by means of an ultrasonic appliance. This approach turned out to be highly effective.
Following the results of the preliminary tests with hairline cracks, the client approved an acrylate with the best ratings (behaviour during injection phase, long-term resistance to chemical influences).
Basically, the first injections were carried out most satisfactorily with regard to quality (Fig. 6). However, on account of the findings which had already been obtained, an extrapolation indicated that it was impossible to adhere to the proposed maximum timeframe if this procedure was to be maintained. Thus, a speedier and more reliable solution was sought and decided on in conjunction with the sealing specialists. The answer was to seal a zone of hairline cracks by backfilling them with gel.
Here too, the procedure was monitored most carefully. In this conjunction, the drilling time and path, grouting period and material consumption were assessed and compared with the classical crack injection. In the process, it was discovered that although material consumption grew enormously, simultaneously the working time could be substantially reduced. The added costs for material were on the one hand compensated for by the working costs; furthermore, it was now possible to stick to the constricted time frames set for the project so that further potential costs could be saved. The application of the low-viscosity acrylate had furthermore the advantage that it enabled material escaping from the crack to be observed. Here too, the minimum filling level of the crack amounting to 80 % was verified through core samples.
In addition, the risk of an error during drilling dropped by applying this grouting method as the drill holes could be created vertically at the drilling point. By adding an accelerating agent to the basic components, it was also possible to optimise material consumption as the material’s open time could be set as low as required. In this way, the grouting agent could not escape in an uncontrolled manner.
3.4 Frictional Injection Sealing for broken Segment Elements
Apart from the classical follow-up injections for sealing cracks and butt joints, occasionally there were defective segment elements. Depending on the defect (usually parts that had broken off or were otherwise damaged) the responsible engineers opted for a frictional injection to enhance the load-bearing capacity (Fig. 7). In this case, a specially formulated polyurethane was selected, which also attained the appropriate tensile strength given the wet-moist environment.
3.5 Injections for seismic Joint Elements
As already mentioned, there were two seismic joint transition zones in the Eurasia Tunnel. In each case, they were installed with two joint connections to the classical segmental ring (Fig. 8). The initial injection phase was applied point-for-point to cope with larger leaks along the radial joint between the steel and concrete segments (Fig. 9). For this purpose, a viscoplastic two-component PU injection resin with an ultimate strength of roughly 60 N/mm² was applied. It could be assumed that the larger flows of water at these points were favoured among other things by voids resulting from washed out beds of mortar. Owing to the high water pressure and the high flow speed, the PU injection resin was preset on-site by adding accelerator to achieve a reaction time of only a few seconds. In this way, the loss of material due to the injection agent being washed out could be substantially reduced during grouting (liquid phase of the injection resin).
During the second planned injection phase, all four radial joints were systematically grouted. Towards this end, drill holes were produced in a circular pattern along the joint at gaps of 50 to 75 cm. Drilling was undertaken at a slanted angle on the side of the concrete segment so that the end of the drill hole was positioned in the apex area of butt joint and rock. Here too, the engineers made use of an ultrasonic appliance to locate the reinforcement in the segment. As a result, the number of abortive drill holes was marginalised.
During the planning phase the client and the sealing specialist had already decided on a permanent ductile two-component PU injection resin with low viscosity. The aim was on the one hand, to fill the remaining voids completely and seal any mini-cracks located nearby. On the other hand, it had to be ensured that light deformations caused by an earthquake did not result in further leaks.
Once again, this tunnel project clearly revealed that it is essential to make the right choice with regard to the applied injection products. Providing they are adjusted to the prevailing situation and the corresponding method, an optimal result is possible. The search for a so-called “multi-purpose weapon” when choosing the products is seldom crowned with success. The alleged lost time, spent on practical trials, was retrieved later on many times over. The entire sealing operations described here were completed within a year. Towards this end, at its peak, up to five teams each involving three injection specialists were busy during the day and night shifts. The subsequent lining activities were carried out according to schedule. This enabled the opening ceremony to be held on December 20, 2018.