Ground Response and Support Measures for Pir Panjal Tunnel in the Himalayas

Ground Response and 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.

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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.

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Ground Response and Support Measures for Pir Panjal Tunnel in the Himalayas

1. INTRODUCTION

A large number of power and transport tunnel projects are being constructed in the tectonically active and young Himalayan mountains. The main areas of concern regarding tunnel stability are the existence of weak, highly deformable and anisotropic rock mass and high degree of weathering and fracturing. Tunnel squeezing is common in the Himalayas in weak rock such as shale, slate, phyllite, schist and in weakness/fault zones and represents one of the major areas of concern regarding stability. Also in some areas due to very high overburden and brittle rock mass, explosive conditions develop resulting in rock bursts. Rock burst is the explosive failure in rock which occurs when very high stress concentrations are induced around underground openings. Though ravelling, swelling, running and flowing are occasional but rock squeezing is common in the Himalayas, 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. Sequence of rock mass behaviour leading to regional failure is explained schematically by Szwedzicki (2003) as shown in Figure 1. Accordingly, there will be several indicators and precursors which will lead to local damage and subsequently regional failure. An indicator is defined as a sign, a state or a contributing factor that points out or suggest that the rock mass may be prone to damage or failure. In general potential failure is indicated by geotechnical and operational factors. A geotechnical precursor is a state or behaviour that suggests that the structure of the rock mass has been damaged prior to possible failure. Precursors, including results from instrumentation, warn of the development of excess ground deformations or high stress. Local damage is manifested by the following precursors e.g. spalling, squeezing, bursting, roof sagging, local falls, slabbing, joint dilation, creep, floor heaving, support damage etc.

Fig. 1: Sequence of Rock Mass Behaviour

1.2 Squeezing

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 squeezing based on other criteria as well.

Hoek Marinos (2000) defined squeezing based on strain % (which is tunnel closure/tunnel diameter × 100) and ratio of rock mass strength and in situ stress. Based on this they proposed a classification for squeezing level as shown in Figure 2. When σcm/po is low, the strain is very high (10%) indicating extreme squeezing conditions where as when the stress ratio is high (0–6), the strain % is < 1% implying few support problems. The above classification is used in this study to define the squeezing problems.

Fig. 2: Classification of Squeezing

2. THE PIR PANJAL TUNNEL

The proposed Pir Panjal tunnel is part of the railway line from Udhampur to Baramula in the Himalayas. The tunnel across Pir Panjal range is located between the future railway stations Banihal in the south and Qazigund in the north. The total length of the horse shoe shape with flat floor tunnel will be 11 km (10960 m) length with 8.0 m height and 8.94 m width. It is completely straight and runs almost parallel to North-South direction. The overburden at both portals is about 10m above tunnel crown, while the maximum overburden is approximately 1150 m. About 4 km of the tunnel length has an overburden of more than 500 m and about 650 m of the length has more than 1000 m. The excavation method is NATM with drill-blast. The tunnel layout is shown in Figure 3.

Fig. 3: 2D View of Pir Panjal Tunnel

2.1 Geological Setup

The tunnel alignment traverses through steeply slopping highly undulating hill slopes of the Pir Panjal range which is part of the young lower Himalayas. Formation levels at tunnel portals are at elevation 1713.63 (South portal) and 1956.70 (North portal). The highly folded and faulted mountain ranges have a strike of bedding is NW-SE. Distinct folding is visible in the central regions. Bedding of the southern slopes dip with 60–90⁰ 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 limitation of drilling depth. However, based on surface mapping, drilling and mapping in the drifts and adits, the rock type classes were determined. Extensive laboratory tests were carried out for obtaining bulk density, cohesion, c, friction φ, uniaxial compressive strength, σc, tensile strength, σt, Modulus, Et and Poisson’s ratios for all rock varieties. Rock mass properties and joint parameters were also established through relevant field and laboratory tests. Adopted geotechnical parameters for the ground response analysis are presented in Table 2.

3. GROUND RESPONSE ANALYSIS: ANALYTICAL
APPROACHES

Predictions of stresses and displacements around a circular opening in rock mass at great depth is an important problem in geotechnical, petroleum and mining engineering. The main analytical approaches adopted are:

1. Limit Equilibrium Method

2. Numerical Method

Table 1: Anticipated Rock Types

The closed form solutions are based on simplified assumptions e.g. shape of the opening is regular (mostly circular, elliptical, or spherical), the media is homogeneous and isotropic. They are easy and provide insight into how the mechanical variables influence the deformation behaviour (Hoek Brown 1994). To identify the magnitude of stresses 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. A continuum approach treats the rock mass as continuum intersected by a number of discontinuities, while a discontinuum approach views the rock mass as an assemblage of independent blocks or particles (Goodman John 1977). Further, continuum models are of two types: differential and integral. Differential models characterise the entire region of interest and include the finite difference and the finite element methods. Where as integral or boundary element models feature discretisation only along interior or exterior boundaries. Ground response in the study was obtained using the powerful FLAC method. The Fast Langrangian Analysis 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 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 stresses and displacements. The bottom boundary is fixed in Y-direction where as the left vertical boundary is restrained in X- direction. A vertical stress Syy and the horizontal stress, Sxx are applied on top boundary and vertical right side boundary. Stresses and deformations before and after support system are calculated using both the methods.

Fig. 4: Descretize Model of Tunnel

4. RESULTS AND DISCUSSION

Analysis based on closed form solutions for the six varieties of rock masses available along the Pir Panjal tunnel has been carried out and stresses and displacement are obtained. Variation of radial and tangential stresses with distance from the centre of the tunnel and also variation of displacements (radial) with distance are plotted for all rock types. Typical such variations for shale are shown in Figure 5(a) and (b). As per the figures, it is clear that the sq = 19.56 MPa occurs at 10.5 m from the centre of tunnel, where as radial stress is zero at the boundary. Maximum deformation of 59.6 m was observed at the tunnel boundary and also radius of plastic zone is 10.5 m. Results for the rocks at different sections are summarised in Table. 3.

Table 3: Comparison of Analysis

For prediction of squeezing behaviour, strains were calculated from the displacements and presented in Table 3 for all six types of rocks. As shown in table agglomeratic shale and shale show 1.03 and 1.33% of strain respectively indicating moderate squeezing where as all other rock types show no squeezing.

Stresses and displacements are also obtained from FLAC and corresponding strains were obtained for all rocks. The results are shown in Figure 6(a), (b), (c) and (d) for shale and similar graphs were plotted for other rocks as well. The results are compared in Table 4 along with the results obtained by closed form solutions. It is clear that the deformation values obtained from FLAC are higher than the closed form solutions. The strains for both rocks falls under moderate squeezing category.

Rock mass response behaviour from closed form solutions and FLAC obtained for all rock masses. FLAC software is used to stabilise the tunnel at all sections with shotcrete lining and rock bolts of appropriate input parameters. After installation of support shortcrete lining is checked in bending and direct stresses and rock bolts are checked in tensile stresses.

Bending movements, axial force and structural displacements in supports for all rock types are plotted and typical results obtained for shale are presented in Figure 7(a) and (b). Comparison of results without and with support is given in Tables 4 and 5 for all sections.

Fig. 5: (a) Stress Variation (b) Deformation in Shale

Table 4: Details of Stresses

Table 5: Comparison of Results

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Ground Response and Support Measures for Pir Panjal Tunnel in the Himalayas

Fig. 6: (a) Maximum Principal Stress (b) Minimum Principal Stress (c) Y Displacement (d) Plasticity Indicator Contours for Shale

Fig. 7: (a) Axial Force and (b) Displacement in Shale

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Ground Response and Support Measures for Pir Panjal Tunnel in the Himalayas

Table 5 shows the shotcrete lining of 300 mm thickness is not safe in bending stresses in shale at section 1100/22. Therefore, the thickness of shotcrete needs to increase. After installation of supports all sections except shale are stabilised. Shale is suffering from moderate squeezing.

For such condition, forepoling and advance face stabilisation are required. Yielding support system may be required in extreme cases to prevent squeezing conditions.

5. CONCLUSIONS

The Pir Panjal tunnel in the Himalayas traverses through very diversified geology experiencing high ground stresses. Time dependent behaviour of tunnels in squeezing rock is investigated using closed form and FLAC methods. Stresses and displacements are obtained before and after the installation of support measures. Nominal shortcrete and rock bolting proves ineffective to control squeezing in shales, hence advance face stabilization suggested approaches in these sections.

REFERENCES

Dube, A.K., Singh, B. and Singh, B. (1986). “Study of Squeezing Pressure Phenomenon in Tunnel-1”, Tunneling and Underground Space Technology, Vol. 1, No. 1,
pp. 35–39.