GroundIGC 2009 Response, Guntur, and INDIA Support Measures for Pir Panjal Tunnel in the Himalayas GROUND RESPONSE AND SUPPORT MEASURES FOR PIR PANJAL TUNNEL IN THE HIMALAYAS K.S. Rao Professor, Department of Civil Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi–110 016, India. E-mail: [email protected] ABSTRACT: The Pir Panjal tunnel linking between Banihal and Qazigund stations is the important tunnel in the railway line from Udhampur to Baramula in the Himalayas. The Pir Panjal ranges are having complex geological set up with major folds and faults. More than six major lithological units are traled along the 11 km length of the tunnel with very high overburden at many sections. The phenomena of squeezing is studied using the limit equilibrium and FLAC methods for this tunnel. A detailed stress and displacement assessment has been attempted in this study, in order to stabilise the tunnel sections with suitable support measures. 1. INTRODUCTION Sequence of rock mass behaviour leading to regional failure is explained schematically by Szwedzicki (2003) as shown in A large number of power and transport tunnel projects are being Figure 1. Accordingly, there will be several indicators and constructed in the tectonically active and young Himalayan precursors which will lead to local damage and subsequently mountains. The main areas of concern regarding tunnel stability regional failure. An indicator is defined as a sign, a state or a are the existence of weak, highly deformable and anisotropic contributing factor that points out or suggest that the rock rock mass and high degree of weathering and fracturing. Tunnel mass may be prone to damage or failure. In general potential squeezing is common in the Himalayas in weak rock such as failure is indicated by geotechnical and operational factors. A shale, slate, phyllite, schist and in weakness/fault zones and geotechnical precursor is a state or behaviour that suggests represents one of the major areas of concern regarding stability. that the structure of the rock mass has been damaged prior to Also in some areas due to very high overburden and brittle rock possible failure. Precursors, including results from mass, explosive conditions develop resulting in rock bursts. instrumentation, warn of the development of excess ground Rock burst is the explosive failure in rock which occurs when deformations or high stress. Local damage is manifested by the very high stress concentrations are induced around underground following precursors e.g. spalling, squeezing, bursting, roof openings. Though ravelling, swelling, running and flowing sagging, local falls, slabbing, joint dilation, creep, floor are occasional but rock squeezing is common in the Himalayas, heaving, support damage etc. leading to tunnel collapses. Several tunnels and bridges are being constructed by Ircon Int. Ltd and Konkan Rly. Corp. Ltd for ambitious railway link between Katra to Qazigund in the Himalayas for the Northern Railways. The Pir Panjal rail tunnel is a part of the new railway line from Udhampur to Baramulla. The tunnel crosses the different formation of Pir Panjal range and runs around 11 km length. In this study an attempt is made to evaluate ground response of Pir Panjal tunnel through limit equilibrium and Finite element approaches. Especially efforts were made to assess the squeezing and rock bursting conditions through out the length of the tunnel and stability measures are suggested for the affected sections. 1.1 Rockmass Response and Collapses Changes in stress around tunnel excavations can result in the behaviour of the rock mass which in turn may lead to damage, failure and consequent collapse of the rock mass. Fig. 1: Sequence of Rock Mass Behaviour 920 Ground Response and Support Measures for Pir Panjal Tunnel in the Himalayas 1.2 Squeezing excavation method is NATM with drill-blast. The tunnel layout is shown in Figure 3. Time dependent large displacement occurring around tunnels and other openings essentially associated with creeping is known as “squeezing rock conditions”. Several authors tried to explain the phenomena in the past (Dube et al. 1986, Ayden et al. 1993, Singh et al. 1999, Goel et al. 1995 and Panthi & Nilsen 2007). Tunnels in weak rocks such as schists, shales, slates and phyllites when subjected to high ground stresses, experience squeezing conditions. This behaviour implies that yielding will occur around the tunnel resulting in convergence and face displacements. Singh et al. (1992) differentiated squeezing and non-squeezing overburden pressure and Q-ratting. Similarly several authors defined Fig. 3: 2D View of Pir Panjal Tunnel squeezing based on other criteria as well. Hoek & Marinos (2000) defined squeezing based on strain % 2.1 Geological Setup (which is tunnel closure/tunnel diameter × 100) and ratio of rock mass strength and in situ stress. Based on this they The tunnel alignment traverses through steeply slopping proposed a classification for squeezing level as shown in highly undulating hill slopes of the Pir Panjal range which is Figure 2. When scm/po is low, the strain is very high (10%) part of the young lower Himalayas. Formation levels at tunnel indicating extreme squeezing conditions where as when the portals are at elevation 1713.63 (South portal) and 1956.70 stress ratio is high (0–6), the strain % is < 1% implying few (North portal). The highly folded and faulted mountain ranges support problems. The above classification is used in this have a strike of bedding is NW-SE. Distinct folding is visible study to define the squeezing problems. in the central regions. Bedding of the southern slopes dip with 60–90° towards NE while on northern slopes dip with 36–45 towards SW. Contact between rock units are often faulted. The lithological units are Zewan beds, Gangamopteris bed, Panjal traps, conglomerate bed, agglomerate slates, fenestella shale and syringothyeis limestone. The main rock units are limestones, quartzites, shales, sandstone, conglomerate and fluvoglacial materials. Table 1 details the anticipated rock units at different chainage of the tunnel. The table also presents maximum and minimum overburden at different sections of the tunnel. 2.2 Geotechnical Parameters Extensive geotechnical investigations were carried out through drilling number of boreholes as well as several shafts and drifts. Because of high overburden, there was a Fig. 2: Classification of Squeezing limitation of drilling depth. However, based on surface mapping, drilling and mapping in the drifts and adits, the 2. THE PIR PANJAL TUNNEL rock type classes were determined. Extensive laboratory tests were carried out for obtaining bulk density, cohesion, c, The proposed Pir Panjal tunnel is part of the railway line friction f, uniaxial compressive strength, s c, tensile strength, from Udhampur to Baramula in the Himalayas. The tunnel s t, Modulus, Et and Poisson’s ratios for all rock varieties. across Pir Panjal range is located between the future railway Rock mass properties and joint parameters were also stations Banihal in the south and Qazigund in the north. The established through relevant field and laboratory tests. total length of the horse shoe shape with flat floor tunnel will Adopted geotechnical parameters for the ground response be 11 km (10960 m) length with 8.0 m height and 8.94 m analysis are presented in Table 2. width. It is completely straight and runs almost parallel to North-South direction. The overburden at both portals is 3. GROUND RESPONSE ANALYSIS: ANALYTICAL about 10m above tunnel crown, while the maximum APPROACHES overburden is approximately 1150 m. About 4 km of the tunnel length has an overburden of more than 500 m and Predictions of stresses and displacements around a circular about 650 m of the length has more than 1000 m. The opening in rock mass at great depth is an important problem 921 Ground Response and Support Measures for Pir Panjal Tunnel in the Himalayas in geotechnical, petroleum and mining engineering. The feature discretisation only along interior or exterior main analytical approaches adopted are: boundaries. Ground response in the study was obtained using 1. Limit Equilibrium Method the powerful FLAC method. The Fast Langrangian Analysis 2. Numerical Method of Continua (FLAC) is a two dimensional explicit finite difference program. In order to setup a model to run a simulation with FLAC, in the fundamental components of Table 1: Anticipated Rock Types the problem shall be specified: a finite difference grid, constitutive behaviour and material properties and boundary and initial conditions. The general solution procedure as indicated in FLAC manual version 5.0 is adopted for the study. The Mohr-Coulomb model which is convenient is used in the study. Table 2: Geotechnical Design Parameters The descretised model of the Pir Panjal tunnel obtained by FLAC is shown in Figure 4. Because of axisymmetric half of the tunnel is descretised into 15876 square and rectangular zones. The boundary conditions are applied in terms of both The closed form solutions are based on simplified stresses and displacements. The bottom boundary is fixed in assumptions e.g. shape of the opening is regular (mostly Y-direction where as the left vertical boundary is restrained circular, elliptical, or spherical), the media is homogeneous in X- direction. A vertical stress Syy and the horizontal and isotropic. They are easy and provide insight into how the stress, Sxx are applied on top boundary and vertical right mechanical variables influence the deformation behaviour side boundary. Stresses and deformations before and after (Hoek & Brown 1994). To identify the magnitude of stresses support system are calculated using both the methods. and deformations in Pir Panjal tunnel, calculations were carried out based on closed form solution for circular shape of equivalent opening in elasto-plastic medium with primary stress field of Ko = 1. Numerical methods include such techniques as finite element, finite difference and boundary element. Depending upon geological media two approaches to numerical modelling is identified.