Geotechnical investigations and preliminary support design for the Geçilmez tunnel: A case study along the Black Sea coastal highway, Giresun, northern Turkey

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Highlights

  • Performing a geotechnical investigation along the Geçilmez tunnel.

  • Performing kinematic and limit equilibrium analyses for portal slope stability.

  • Determining empirical preliminary tunnel support requirements.

  • Determining preliminary tunnel support for discontinuous and fractured rock masses.

  • Comparing the analytical, numerical and thrust–moment interaction analyses results.

Abstract

This study encompasses geotechnical investigations, stability assessments and design of the preliminary support systems for the Geçilmez tunnel which is constructed in Giresun for the improvement of the highway along the Black Sea coast. During the study, a detailed geological map of the study area was prepared and the geotechnical characteristics of the rock masses were determined. The rock mass classification of the tunnel grounds was performed by utilizing the RMR method, Q system, NATM and the Geological Strength Index (GSI) classification which was followed by performing a geotechnical investigation along the tunnel grounds in order to obtain the geotechnical parameters for the stability analyses of the portals and of the tunnel. Lugeon (water pressure) tests were performed in order to determine the permeability of the rock mass along the tunnel. The appropriate geotechnical parameters were utilized in order to perform rock slope stability kinematic and limit equilibrium analyses at the portals of the tunnel. Empirical preliminary tunnel support systems according to the RMR method, Q-system and NATM were determined. The structurally controlled instabilities within the tunnel sections were identified and the required preliminary tunnel support systems were determined to overcome these instabilities. Regarding the structurally controlled rock failures along the probable weak zones and lineaments (i.e., inactive probable faults or shear zones) during tunneling, wedge stability analysis was utilized to determine the potential wedge failures that could possibly occur during tunneling and to apply the necessary support systems for stabilizing any wedge failure in the tunnel. The induced stress distributions and deformations in the rock mass surrounding the tunnel grounds was investigated and the interaction of the support systems with the rock mass was analyzed by using numerical (finite element) modeling. In the finite element analyses it was assumed that the rock mass behaved as a fractured rock mass since the tunnel grounds were moderate to highly jointed. The objective of the numerical modeling was to check the validity of the empirical preliminary tunnel support requirements and also to compare the results with those obtained through assuming structurally controlled failures during tunneling. The performance of the preliminary tunnel support was also validated on the basis of thrust–moment interaction analyses. The results of the structurally controlled failure analyses, numerical analyses and thrust–moment analyses were compared in an attempt to determine the preliminary tunnel support systems to stabilize the Geçilmez tunnel.

Introduction

The Turkish General Directorate of Highways (KGM) adjudicated a highway project for the improvement of the government highways along the Black Sea coast. The project included construction of highways, viaducts and tunnels for the improvement of the highway between Giresun–Espiye (KM: 0 + 000–24 + 576.14) along the Black Sea coast. As a result of the preliminary geotechnical studies carried out in the region, four tunnels were constructed, which are the Aksu tunnel, Geçilmez tunnel, Uluburun tunnel and Arıdurak tunnel, respectively (Fig. 1). This study consists of geotechnically assessing the portal slope stability and tunnel stability of the Geçilmez tunnel which is constructed as a single tube horseshoe shaped tunnel between KM: 12 + 247.50–12 + 555.00 of the Giresun–Espiye highway project with a total length of 307.5 m.

The construction of the Geçilmez tunnel is part of a project that involves the improvement of the Black Sea coast government highway between Giresun–Espiye (KM: 0 + 000–24 + 576.14) and the Geçilmez tunnel is constructed approximately at the midway of this project which is situated about 500 m after the Yolağzı township. It is accessible through all year by the existing government Black Sea coast highway. Fig. 1 gives a location map of the study area. The best times to visit the area is between April and August, as there is heavy rain in the region during the other months. Fig. 2 gives a view of the location of the entrance portal of the Geçilmez tunnel prior to excavation.

Section snippets

General geology

In general, the Eastern Pontides (Eastern Black Sea) has formed by the intercalation of Upper Cretaceous basic and acidic volcanic rocks and their pyroclastics (agglomerate and tuff), which are the products of subduction zone in island arc volcanism related with the tectonism of the region. The volcanosedimentary and pelagic units have formed during dormant periods of volcanism in the region. The oldest volcanic rocks in the region are the lower basic series and the youngest volcanic rock is

General

A geological map with a scale of 1/1000 and a section along the tunnel axis was prepared (Fig. 4). The length of the tunnel is 307.5 m and the entire tunnel cuts through basalt. A general side view of the tunnel route is given by Fig. 5. The rocks that are exposed in the study area are agglomerate, biotite bearing dacite and basalt which are described below. Colluvium is generally observed along the coastline and at the mouths of major rivers. It is generally composed of sand, gravel with a

General

The geotechnical properties of the rock mass constituting the tunnel grounds were determined through dividing the tunnel grounds into seven geotechnical sectors. The boundaries of these seven geotechnical sectors were delineated by the three inactive probable faults (shear zones) situated at KM: 12 + 306–12 + 316 (F2) and at KM: 12 + 448–12 + 461 (F3 and F4; Fig. 4). Since the inactive probable fault (shear zone) F1 is located about 9 m west of the entrance portal and lies outside of the tunnel space,

Stability analyses of the portals

In order to decide the most suitable slope geometry and location for portal construction, several closely spaced geological sections were drawn at the west (entrance) and east (exit) portals of the tunnel. As a result of the evaluation of the sections, it was decided to construct the entrance portal between KM: 12 + 235–12 + 247.5 and the exit portal between KM: 12 + 555–12 + 570 of the highway. Cut portal side slopes (i.e., cut slopes along the sides of the portal locations) and portal face slopes

Summary and conclusions

In this study, the stability assessment and the design of the Geçilmez tunnel which is constructed in Giresun, along the Black Sea coastline, Turkey was investigated. The study began with 1/1000 scale geological mapping of the study area and the determination of the geotechnical characteristics of the rock masses in the area. Then, rock mass classification of the tunnel grounds was completed and the geotechnical parameters that were necessary for the stability analysis and for the design of the

Acknowledgements

Thanks are due to Mr. Hasan Özaslan of Yüksel Proje International A.Ş. for his kind assistance throughout the project and for supplying Fig. 2. The authors are thankful to Dr. Kıvanç Okalp, Coordinator of Projects, Beray Engineering Consultancy Ltd., for his assistance in re-drafting Fig. 4 and to Dr. Evrim Sopacı, Senior Geological Engineer, Temelsu International Engineering Services Inc., for his valuable suggestions in some of the numerical modeling steps.

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