2
Geology of Turkey and Istanbul, expected problems, some cuttability characteristics of the rocks

2.1 Introduction

Recent studies have revealed that tunnels were excavated under hundreds of Neolithic settlements all over Europe, and the fact that so many tunnels have survived 12,000 years indicates that the original networks must have been huge, from Scotland to Turkey. Some experts believe that the network was a way of protecting man from predators, while others believe that some of the linked tunnels were used like motorways are today, for people to travel safely regardless of wars or violence or even weather above ground [1]. There are several underground cities from Roman Imperial Times, and even older in Cappadocia, Nevsehir in Turkey, the historical underground cities are linked with a network of tunnels. Tunneling activities in Turkey have being carried out for centuries since then.

There are currently 1,700 hydropower projects, with more than 800 tunnels being excavated. Turkey is a mountainous country necessitating continuous road tunneling activities, all over the country. The national target up to 2023 is 330 km of road and highway tunnels and 78 km of railway tunnels. The need for metro tunnels in big cities is increasing, reflecting the growing population; the length of metro lines in Istanbul was 141 km in 2013 and it is expected that this will reach to 400 km by 2019. The length of utility tunnels to be built in the near future in Istanbul is expected to be around 85 km. Altogether, it is predicted that Turkey will invest more than 35 billion USD in tunneling projects in the near future, and the majority of tunnels will be driven using tunnel boring machines (TBM). The anticipated problems are directly related to the complexity of the geology of Turkey, including frequent face collapses, squeezing of TBM, high water inflow and excessive cutter wear. In this book a brief summary of the geology of Turkey, and Istanbul in particular, will first be given, describing the main geological formations with physical, mechanical and cuttability characteristics. It is hoped that the information given in this chapter will help to make rational decisions in designing and executing the tunneling projects.

2.2 Geology of Turkey

Turkey's varied landscapes are the product of a wide variety of tectonic processes that have shaped Anatolia over millions of years and which continue today, as evidenced by frequent earthquakes and occasional volcanic eruptions. Turkey's terrain is structurally complex. Nearly 85% of the land is at an elevation of more than 450 m, the median altitude of the country being 1,128 m. It is hard to explain the complexity of the geology of Turkey within the limited length of this book, but the following paragraph taken from a well-known paper published by Okay [2] explains the complexity of the geology.

"Geologically Turkey consists of a mosaic of several terranes, which were amalgamated during the Alpide orogeny. The relics of the oceans, which once separated these terranes, are widespread through the Anatolia; they are represented by ophiolite and accretionary complexes. The three terranes, which make up the Pontides, namely the Strandja, Istanbul and Sakarya terranes, have Laurasian affinities. These Pontic terranes bear evidence for Variscan and Cimmeride orogenies. Their Palaeozoic and Mesozoic evolutions are quite different from the Anatolide-Taurides. The Pontides and the Anatolide-Taurides evolved independently during the Phanerozoic and they were first brought together in the Tertiary. In contrast to the Pontic terranes, the Anatolide-Tauride terrane has not been affected by the Variscan and Cimmeride deformation and metamorphism but was strongly shaped by the Alpide orogeny. It was part of the Arabian Platform and hence Gondwana until the Triassic and was reassembled with the Arabian Platform in the Miocene. The Anatolide-Tauride terrane is subdivided into several zones mainly on the basis of type and age of Alpide metamorphism. The southeast Anatolia forms the northernmost extension of the Arabian Platform and shares many common stratigraphic features with the Anatolide-Tauride terrane. The final amalgamation of the terranes in the Oligo-Miocene ushered a new tectonic era characterized by continental sedimentation, calc-alkaline magmatism, extension and strike-slip faulting. Most of the present active structures, such as the North Anatolian Fault, and most of the present landscape are a result of this neotectonic phase."

As Bozkurt and Mittwede [3] stated, Anatolia forms a superb laboratory for the study of subduction, ophiolite obduction, continent-continent collision, metamorphism, the relationship between lithospheric deformation and magmatism, fold and thrust belts, suture zones, active strike-slip faulting, active normal faulting and associated basin formation.

All the complexity of the geology as explained above makes it really difficult to excavate tunnels in some parts of Turkey and this book is aimed at summarizing the possible difficulties that may occur during TBM excavation, with possible solutions given based on past experience.

2.3 Geology of Istanbul

Palaeozoic, Mesozoic and Cenozoic formations are recognized in Istanbul as seen in Figure 2.1 [4].

Figure 2.1 The geology of Istanbul [4].

The oldest rocks are Palaeozoic rocks consisting of quartz, quartz arenite and arcose. From the Ordovician to the middle of the Carboniferous there is a concordant rock sequence outcropping several thousand meters thick. This sequence mainly consists of variable facies of clastic and carbonate rocks. Some of the main characteristics of the Palaeozoic sequence are horizontal and vertical transitions, alternations of different rocks and lenticular structures. Palaeozoic rocks cover large areas of Istanbul. Rocks of Ordovician, Silurian and Devonian age outcrop mostly on the Asian side, and Carboniferous rocks are located mostly on the European side. Andesitic and diabasic dykes are also present. Triassic formations are represented by conglomerate, sandstone, dolomite, dolomitic limestone and clayey limestone, and Cretaceous units by sandstone, shale, and limestone interbedded with lavas and pyroclastic rocks. The Mesozoic rocks, in turn, are covered by Tertiary units. These units are generally fossiliferous limestone, clayey limestone or marl, and uncemented or loosely cemented sand, silt and clay. Halic and Bosphorus sediments, old and new alluvium and artificial fill cover the other units in Istanbul. Halic and Bosphorus sediments generally consist of black clay, sand and silt. The thickness of these materials is 5-13 m on the Marmara Sea coasts, and 60-70 m on the Halic (Golden Horn) coasts. Old and new alluvium can be seen in the Baltalimani, Istinye and Bebek Valleys on the European side, and close to the Kurbagali, Maltepe-Cevizli and Buyuk Rivers on the Asian side. They consist of uncemented gravel, sand and clay which originate from other units in the area. The thickness of the alluvium is 8-20 m, and artificial fill 2-40 m thick can be seen on the Historical Peninsula (Eminonu). This consists of gravel, sand, silt, clay, rubble, brick and tile fragments [5].

2.4 TBM performance in different projects in Istanbul

Some typical examples of TBM performance in different projects are given in Tables 2.1, 2.2 and 2.3. As seen from these tables main daily advance rates vary in different geologic formations. The main expected problems are frequent changing of the strata, dykes, ancient water wells etc. Sometimes in earth pressure balance (EPB) TBM applications, excessive ground deformations may cause damage to the surrounding buildings as experienced in the Otogar-Esenler metro tunnels, which caused an extra cost of 35.6 million USD of the project [4].

For an efficient tunneling operation in Istanbul it is essential to understand the behavior of TBMs in different geological formations. That is why detailed information of each stratum will be given within this chapter.

Table 2.1 Some of the completed metro tunnels [4].

Table 2.2 Some of recent TBM applications in sewerage projects in Istanbul [4].