The winTer olympics in Sochi, Russia, that took place from 7-23 February 2014 were marked by major investments aimed at overcoming the city’s lack of infrastructure and to improve its transport network. One of these projects is the new Sochi motorway, also known as “The alternative to the Kurortnyi road”, which runs parallel to the Black Sea and makes it possible to reach the Olympic sites and the Adler airport without having to cross the city. Construction of the new artery, of decisive and strategic importance, required the construction of eight double-bore natural tunnels, as well as open-cut sections, embankments and bridges for a total length of 16km. Due to the difficulties caused by the nature of the ground and the short amount of time available for construction, Russia decided to maximise productivity and minimise risk by adopting the ADECO-RS approach for the design and construction of the T8 and T8A tunnels (these two being the longest and most complex of the motorway).

This approach was judged to have been reliable during the construction of the new Bologna-Florence high-speed railway. This called for more than 100km of tunnelling in similar ground to that in Sochi. The T8 and T8A tunnels are therefore of interest given that other tunnels required for the same motorway by-pass (facing similar ground conditions, although shorter and perhaps less difficult overall) have been excavated by NATM, which is well-established in the Russian tunnelling culture. For the first time therefore, the author is in position to work out a comparison between the two methods based on real data.

General characteristics of the project

The new Sochi bypass presents two separate carriageways; each with two lanes in each direction, designed for speeds of 120km/h. Construction required the excavation of a series of tunnels (Figure 2). The T8 and T8A tunnels, excavated by using ADECO-RS, in addition of being longer than the others, are of a more significant cross sectional area, ranging from 120 to 220m2 (Figure 3). It is noted that the 220m2 cross-section is in the proximity of the North Portal (in a landslide zone) where a junction of the new with the old road network is located. Another point of relevance is the lower cover of the tunnels, which were excavated in heavily inhabited areas as shown in Figure 4.

Geology and Geotechnical Aspects

Sochi is located at the western slope of the Caucasus Mountains, a chain that stretches for around 1,100 – 1,200km between the Black sea and the Caspian Sea. Like the Italian Alps, the mountain system formed during the Cenozoic era – around 25 million years ago – following the collision between the Arabian plate and Eurasian plate. In this context, the Sochi motorway by-pass is located along the shore of the Black Sea, which is characteristic for the hilly and heavily vegetated countryside.

As illustrated in the longitudinal geological profile of Figure 5, the T8 and T8A tunnels cross the Sochi Formation, composed of clay-stones and silty-clays, and the Mamai Formation, with marly-flysch with a scaly structure. The two formations are intersected by faults. Also to be highlighted is the presence of colluvial-eluvial sedimentary covers met near the tunnel portals and in low overburden zones, where also present are active landslides. A very dreaded ‘Voronzoff overthrust zone’ is located near the northern portal.

Design aspects of The T8 and T8a Tunnels

In line with the predictions based on ADECO-RS, the T8 and T8A tunnels were excavated along the entire length instable conditions in the short term or unstable core-face conditions (type “B” and “C” stress-strain behaviour categories). As shown in Figure 6, excavation was full face using the necessary protective and/or reinforcing measures of the “core-face” and by keeping the kickers and the invert of the final tunnel lining in close proximity of the advancing tunnel face (less than one diameter).

It is noted that in all cases, for the cross sections where the core-face reinforcement could be avoided, this last stabilising measure has always been adopted due to the presence of a significant in situ horizontal stress component. At the landsliding north portal, the C2W section type with an invert strut was adopted in order to limit deformation to the highest level. As regards to types in section C, alongside reinforcement of the core-face by means of fibre-glass structures, the figure highlights the treatment along its outline which was carried out by operating injections through fibre-glass structures equipped with manchette valves. Figure 7 gives a view of the tunnel face with the stabilization measures completed. Tunnels T8 and T8a: productions

Productions and construction times were affected by:

¦ The necessary time (at least six months) to acquire experience and generally become familiar with the ADECO-RS approach (which had never been used before in Russia);

¦ Logistical difficulties due to transportation of construction material, machinery and necessary spare parts to the site. The enormous investments allocated towards construction in Sochi led to an incredibly high concentration and volume of demand for raw materials.

The transportation and rail systems were not initially capable of supporting such a massive movement of material; furthermore, the winter weather conditions made it difficult for ships to dock, thus invalidating sea delivery as well.

This, obviously, created significant difficulties towards the efficient application of a highly industrial system such as the ADECO-RS. Despite this, the T8 and T8A tunnels were excavated with a rate of excavation of 40 to 90m per month, except when under-passing the landslide zone at the North Portal, where the same rate became equal to 20m per month, also due to the large tunnel cross section adopted (220m2). It is noteworthy that, independent of the difficulties of various kinds met in order to getting well organized and to adopt the novel and, for the Country, yet unknown, technologies which characterize the ADECO-RS approach, the productivity of the excavation process soon became as expected, also due to the gradually gained experience in the application of the approach by the Contractor.

Comparison between aDeCo-rs anD natm Production

It is of interest to draw attention to Figure 8 where the “volume of rock excavated/month/face” and “metre of completed tunnel/ month/face” ratios are shown for the T8/T8A (ADECO-RS) tunnels and the T7, T7A,…,T3, T3A (NATM) tunnels (see Figure 2). The superior performance of ADECO-RS with respect to NATM in terms of these two parameters is clearly seen. The “volume of rock excavated/month/face” with the first approach is 2.4 times that attained with the second approach; similarly, the “metre of completed tunnel/month/ face” is 40 per cent greater for ADECO-RS with respect to NATM.

An additional interesting comparison between the two tunnelling approaches is possible by plotting, as shown in Figure 9, the time (days) between the breakthrough in a tunnel and the time elapsed before the final lining was installed, i.e. tunnel stability and safety were reached. It is clear that the ADECO-RS, for its own nature, is characterised by the very short time (three weeks approximately) of the final lining installation with respect to the NATM (from 10 weeks minimum to 43 weeks maximum).

Costs

It would seem reasonable to suppose that the full-face excavation of a tunnel with concomitant stabilisation measures applied to the core, typical for the ADECO-RS tunnels excavated in difficult ground, is more costly. The experience gained in Sochi shows that it is not so: NATM was generally more expensive because of the higher cost of workforce and machinery (NATM employs on average one and a half times more manpower than ADECO-RS and a large number of small machines against to a few, large and powerful machines used by ADECO-RS).

Furthermore, taking into consideration the production data shown in Figure 8 the highest values of ADECO-RS comparing to NATM are more than evident. The difference of production in terms of average percentage is in the order of 30 per cent with a minimum guaranteed value of 5 per cent. This is a direct consequence of the job site organization and this means a sharp reduction in terms of construction times and, as a consequence, costs (site active for fewer days). If you add to this the high industrialization of the tunnel construction guaranteed, ADECO-RS appear without doubt more advantageous also with reference to the costs.

Safety

Regarding safety, the objective data collected during excavation of the tunnels for the new motorway by-pass of Sochi has shown the following: although the Russian company and designers were experienced in NATM rather than ADECO-RS, the full-face excavation of tunnels using ADECO-RS proceeded straightforwardly and without accidents. Unfortunately, the same was not true for those carried out in partitioned face using NATM criteria. Indeed, in these cases there were many accidents, some of which quite serious and involving workers. This fact is clearly due to the large workspace made available at the face by ADECO-RS thanks to full-face advancement, which allows complete mechanization of excavation: the presence of unprotected workers in the face area is practically excluded. Another reason is that little time incurs from the moment of excavation to the set of first-phase and final linings, thus stopping stress from evolving to the point of developing serious manifestations of instability. Indeed, continuous monitoring the stress-strain state of the advance core was of great importance, anticipating and blocking any risky situation before it occurred.

Conclusions

The recent excavation of Sochi motorway tunnels was of particular interest given that two of them (T8 and T8A) have been excavated using the ADECO-RS approach, whereas the remaining ones (T7, T7a,…,T3, T3a) have been completed by adopting the NATM method. This has allowed the author to make a first straightforward comparison, under quite similar conditions, between the NATM and ADECO-RS design and construction methods.

It is the author’s opinion that it has been shown that the ADECO-RS approach, above all when applied to tunnels in difficult ground conditions, as in the case of Sochi, when full face excavation with concomitant stabilization of the core-face takes place, offers significant advantages in terms of production rates that can be achieved, the shorter time needed for tunnel completion, the reduced man power needed at the face, the high industrialisation of the tunnel work, and the greater safety conditions obtained. Regarding safety, it is important to underline that during the construction of tunnels T8 and T8A not a single accident has occurred, in contrast to the other tunnels excavated using NATM where several injures have been counted. The tunnels were completed before the agreed deadline and – which is very important – without incidents. This latter fact has had great response in Russia and has been recognised and decorated by the Ministry of Transport of the RF. Introducing the ADECO-RS approach in Russia will certainly take time and will necessarily entail passing through a series of changes including amending building regulations.