Engineering geological investigations along the Ilıksu Tunnels, Alanya, southern Turkey

doi:10.1016/S0013-7952(02)00204-1

Engineering Geology

Volume 68, Issues 3–4, March 2003, Pages 141-158

https://doi.org/10.1016/S0013-7952(02)00204-1 Get rights and content

Abstract

The main objective of this study is to assess the engineering geological characteristics of the rock mass and to suggest appropriate support recommendations along the two autoroad tunnel projects, named as Ilıksu 1 and Ilıksu 2, which are located along the Antalya–Alanya Highway. The study area consists of pelitic schist and calc schist overlain by a thick sequence of recrystallized limestone and an intercalation of pelitic schist, calc schist and graphite schist. The tunnel ground support types and categories were determined according to the Q-system, Rock Mass Rating (RMR) method and New Austrian Tunneling Method (NATM). Slope stability analyses were performed at the portal and cut slope sections. Kinematic and limit equilibrium analyses incorporating the effects of water pressure were performed for the failed rock slopes. Slope stability analyses of irregularly jointed, highly foliated lithologies were performed using soil slope stability software. The interactions of the tunnel support systems with the rock mass were analyzed through finite element analysis.

Introduction

The purpose of this study is to investigate the engineering geological characteristics of the rock material and rock mass and to suggest appropriate stabilization techniques along the two autoroad tunnel projects located along the fourth division route of the Antalya–Alanya autoroad. These tunnels are named as Ilıksu 1 (km: 127+545−128+074) and Ilıksu 2 (km: 128+155−128+317), respectively (Fig. 1).

Units that belong to the Yumrudağ Nappe, which is the structurally highest unit of the Alanya Massif, were observed in the project area. These units comprise the uppermost level of the Cebirreis Formation that consists of pelitic schist, calc schist, metadolomites overlain by a thick sequence of recrystallized limestone and the complete part of the Asmaca Formation that consists of a gradational intercalation of pelitic schist, calc schist and graphite schist (Fig. 2).

A detailed geological and geotechnical study (for example, scan-line survey, discontinuity measurements, kinematic analyses, etc.) was carried out in the project area to determine the engineering geological characteristics of the rock masses. The rock mass was classified according to the Q-system, RMR method and NATM. The support systems were selected according to each of the three different classification systems. The shear strength parameters (angle of internal friction φ′ and cohesion c′) of the rock mass were obtained by the RMR method (Bieniawski, 1989).

In order to decide on the most suitable geometry and to determine the stability of the portal and cut slope sections, slope stability analyses were performed. Kinematic analyses followed by limit equilibrium analyses were performed for the kinematically failed rock slopes incorporating the effect of water pressure. Stability analyses of irregularly jointed, highly foliated lithologies were performed through the use of the Slope/W software. Following the slope stability analyses, recommendations were made regarding the required support systems to prevent the most probable failure mechanisms at the tunnel sections and to stabilize the portals.

By using the appropriate geotechnical parameters (i.e., shear strength, deformation modulus, Poisson's ratio, etc.), deformations and stress concentrations around the tunnels were investigated and the interaction of the support systems with the excavated rock masses were analyzed by the finite element method.

The study area is included in the 1/25 000 scale topographic map of the General Directorate of Highways in the Alanya Section. It is located 7-km west of Alanya on the Antalya–Alanya D-400 highway, which is situated along the coast of the Mediterranean Sea, and in the close proximity of the General Directorate of Highways Recreation Park, which lies in between the Avsallar and Değirmendere villages (Fig. 1).

Section snippets

Geology of the region

The study area is overlain by the Alanya Massif, which is the name given to a large area of metamorphic rocks situated towards the east of Antalya Bay in the Eastern Mediterranean region (Özgül, 1984). The Mesozoic continental margin type lithologies of the Antalya unit crop out beneath the Alanya Nappes in a large tectonic window. In the east of the Antalya Bay between Alanya and Anamur, rocks of the Antalya unit are in turn tectonically overlain by the metamorphic rocks of the Alanya Massif,

Engineering geological assessment of the rock masses in the project area

A detailed engineering geological investigation was carried out in the project area. A geological map with a scale of 1/2000 showing the borehole locations, and a geological cross-section along the Ilıksu 1 and 2 tunnel axes with a 1/2000 horizontal scale and a 1/500 vertical scale are presented in Fig. 3. A detailed discontinuity survey was carried out at the portals of the Ilıksu tunnels. Approximately 598 discontinuity data (joint data and bedding plane data) were taken in order to perform

Rock mass classification and empirical underground support design for the Ilıksu tunnels

The geotechnical properties of the rock units comprising the project area were assessed by using three empirical rock mass classification systems, namely the NGI method (Q-system) and CSIR method (Rock Mass Rating) as well as NATM (New Austrian Tunneling Method). Assessment with the NATM was made by using correlations with the RMR and Q-systems according to the procedure given by the Turkish General Directorate of Highways (1997). Each classification system has been applied to classify the rock

Theoretical background

The most important parameter in the stability analysis of slopes is to determine safe slope geometries with minimum support, excavation, benching, deformation and/or modification of the natural topography. In order to decide on the most suitable geometry and determine the stability of the portal and cut slope sections, slope stability analyses were performed. This section of the paper consists of a detailed geotechnical evaluation of the entrance and exit portal structures of the Ilıksu 1 and

General

This part of the paper deals with the stability assessment of the Ilıksu tunnels and with the necessary reinforcement measures for those unstable tunnel sections where large stress changes and deformations are induced when the underground openings are excavated. The finite element software package Phase² (Rocscience, 2001) was used to determine the induced stresses and deformations developed around the Ilıksu tunnels and to investigate the interaction of the proposed support systems with the

Summary, conclusions and recommendations

This paper assesses the engineering geological characteristics of the rock material and rock mass and suggests appropriate support recommendation and stabilization techniques along the two highway tunnel projects. A detailed engineering geological study (scan-line survey, discontinuity measurements, kinematic analyses, etc.) was carried out in the project area. Rock mass classification systems, namely, the Q-system and the Rock Mass Rating systems have been applied. The tunnel grounds were

Acknowledgements

The authors would like to express their gratitude to Mr. Hasan Özaslan of Yüksel Proje International for his kind assistance throughout the project.

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