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INTEGRATED SETTLEMENT ANALYSIS AND TUNNEL DESIGN FOR THE AIRPORT METRO EXPRESS LINE, DELHI, INDIA

Tams Megyeri1, Ana Obradovic1, Botond Ben1, Douglas Rutherford2, O P Singh3, Pramit K Garg4, Ashit .D Shah5

1Mott MacDonald Magyarorszg Kft., H-1138 Budapest, Npfrd u. 22., Hungary 2Mott MacDonald Limited, Croydon, St Anne House 20-26 Wellesley Road, United Kingdom

3Delhi Metro Rail Corporation Limited, 8 Jantar Mantar Road, New Delhi, India 4Airport Line Consultants, 8 Jantar Mantar Road, New Delhi, India

5Mott MacDonald India, A20 Sector 2, Noida, U P, India

Keywords: FLAC 2D, ground movement, monitoring

INTRODUCTION The Delhi Metro opened at the end of 2002 as the second underground rapid rail transit system in India. It consists of a combination of elevated and underground lines. The construction of this extensive network has been divided into three phases. As a part of Phase 2 which is currently under construction with a target completion date of 2010, the Airport Metro Express Line (AMEL) is being constructed connecting New Delhi Railway Station with the Indira Gandhi International Airport. It will be an approximately 27km long rapid rail transit system with both underground and elevated sections. The alignment has been divided into 9 sections covered by independent contracts. On one of the contracts, AMEL C1, Mott MacDonald is the designer for the ASH JV (Alpine Bau, Samsung and Hindustan Construction Company Joint Venture), engaged as the contractor by the client Delhi Metro Rail Corporation.

The entirely underground AMEL C1 section is 3.861km long and it comprises of two stations - New Delhi Station and Shivaji Stadium Station, and connecting bored tunnel and cut and cover tunnel along Baba Kharak Singh Marg and under Talkatora Gardens. The contract value of AMEL C1 is INR 7,746,358,700 (approximately US $155 million).

INTEGRATED SETTLEMENT ANALYSIS AND TUNNEL DESIGN As part of an integrated design process computer models were used to help design the lining of the running tunnels and the cross passage. The numerical model for the Earth Pressure Balance Machine driven running tunnels helped to inform input parameters for the settlement assessment. Instrumentation for the new structures and existing buildings was designed on the basis of the settlement analyses. The review of the monitoring instrumentation during construction forms the final part in the process.

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Tunnel Design The numerical analysis was undertaken using the program FLAC 2D Version 5.0 which employs an explicit finite difference formulation for the analysis of continua. The cross section modelled has been selected to ensure that the lining design has been checked for the most onerous ground loads as well as for the greatest cover depth (41.5m to tunnel axis level). The bored tunnels have an excavated diameter of 6.51m (including grout annulus) and are 15.15m apart (centre to centre). The ground conditions along the alignment consist of made ground, which comprises variable materials, typically less than 2.5m in thickness, overlying alluvial deposits predominantly of silt with sand layers to variable depths, overlying bedrock of quartzite and minor schist. For the analysis, the ground has been considered as entirely alluvium which represents the worst case. The alluvium was modelled as a linear elastic, perfectly plastic constitutive model for the Mohr-Coulomb failure criterion. The strength and stiffness have been adopted from the Geotechnical Interpretative Report as detailed in Table 1.

Table 1 Geotechnical properties Parameter Alluvium

Bulk Unit Weight bulk (kN/m3) 20 Porosity n (%) 50

Cohesion c (kPa) 0 Friction Angle ' () 32

Youngs Modulus E

(MPa) 3.5+2.52z* Poissons Ratio 0.2

Permeability k (m/s) 1.3 x 10-5 K0 0.8

When bored tunneling takes place within the alluvium, the tunnel boring machine applies a pressure to the tunnel face and prevents drawdown of the water table. After the tunnel lining is installed, the tunnel linings are assumed to be impermeable. Therefore full hydrostatic pressure acts on the tunnels throughout the analysis. During excavation, a certain degree of ground relaxation occurs before the lining is installed. A percentage unloading method was used to approximate the three-dimensional redistribution of stress around the two-dimensional model. After removing the elements representing the tunnel, the soil stresses around the periphery of the excavation were replaced with a set of equivalent grid point forces. These forces were then gradually reduced to a specified percentage (degree of relaxation) of their initial value whilst the model is allowed to solve. The lining elements were then introduced, the remaining grid point forces removed and the model was allowed to solve to equilibrium. The analysis of the bored tunnel was performed in several stages as shown in Figure 1:

1. The model was set up with the initial conditions and run to equilibrium. A 50kPa surcharge representing adjacent buildings is applied at the ground surface.

2. Excavation of the left bored tunnel was carried out and a percentage of ground relaxation was allowed. The pore pressures remain the same.

3. The tunnel lining was installed and the remaining percentage of ground relaxation was allowed to take place.

4. Stages 2 and 3 were repeated for the right bored tunnel.

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At each stage the model was run to equilibrium and the results were saved for future evaluation. Bending moments and axial forces in the circumferential direction have been extracted from the FLAC model at the last stage and plotted on interaction diagrams with the concrete lining capacity curve. In accordance with the Indian standards IS456:2000, concrete was factored by 1.50 and steel was factored by 1.15. The capacity of the 275mm thick running tunnels segmental lining was shown to be adequate for the axial forces and bending moments predicted to act upon it by the FLAC model. The segments are obviously subject to many other loads (e.g. from the TBM rams, handling, etc.) and these were checked in separate calculations. The volume loss of approximately 0.6% was obtained by allowing the soil to relax by 40% before lining installation. The maximum ground settlement predicted by the model was 8.5mm. These predictions were quite low, in line with the small volume losses normally achieved by a closed face TBM. In general, numerical modelling does not provide the best prediction of the shape of settlement curve and it was expected that the peak settlement could be higher in practice. In general, unless sophisticated constitutive models are used for the ground, numerical models predict wider and flatter settlement curves, although the overall volume loss predicted is reliable.

Description Illustration Set up initial conditions Applying 50 kPa surcharge

Stage 1a Excavate left bored tunnel 100-60%

Stage 1b Install tunnel lining 60-0%

Stage 2a Excavate right bored tunnel 100-60%

Stage 2b Install tunnel lining 60-0%

Figure 1 - Modelling procedure for the running tunnels

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Ground Movement Assessment The analysis of the ground movement and its effect on the structures adjacent to the tunnels is an issue of great importance, especially when undertaking a project of this scope in a heavily populated area such as New Delhi. Settlement analysis was carried out along the entire 3.861km long section of AMEL C1. Initially settlement contours were produced for the examined alignment section and this resulted in the identification of 85 structures within the 1mm settlement contour. Mott MacDonalds in-house computer software GRP (Ground Response Program) was used for the analysis. GRP calculates the vertical settlement and horizontal movements caused by tunnels, shafts and open cuts either for a grid of points (grid analysis) or along a section line of points (line analysis). The vertical settlement caused by tunnels is predicted using an inverted Gausian curve. The horizontal movements induced by tunnels are based on the equations presented by Attewell (1986). The calculation for the vertical settlement caused by a shaft is approximated by a portion of the Gaussian curve such that the point of inflexion coincides with the wall of the shaft. The GRP input data used was based on the proposed method and sequence of construction and the geotechnical characteristics of the ground. Volume loss of 1% has been assumed, which is considered conservative bearing in mind the methods of construction and the results of the earlier numerical modelling. The output file from GRP was input into the graphical software Surfer and finaly exported to AutoCad in DXF format, to produce the settlement contours depicted in Figure 2.

Figure 2 Graphical output of settlement contour prediction

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Further analysis was then performed on the 67 structures within 10mm settlement contour in order to classify them into five damage categories ranging from Negligible to Severe to Very Severe. The categories were defined by Delhi Metro Rail Corporation Limited in the projects Outline Design Specifications and were based on the work of Burland (1977) and Boscardin and Cording (1989). Using the physical properties of the structure as identified in the detailed structural survey a Mott MacDonald in-house computer software Brexis X (Building Response to Excavation Induced Settlement) has been utilised to carry out the assessment. Brexis X is embedded within the Microsoft Excel environment and it calculates the response of a building due to the introduction of excavations within the surrounding area. The analysis assumes that surface settlement caused by tunnel excavation can be predicted using an inverted Gaussian curve. Hogging and sagging zones may develop under the building and associated strains can be calculated from the settlements within these zones. The maximum tensile strain is reported and graded according to the classification given by Burland et al (1977). A graphical output is available indicating the general arrangement of structures under analysis together with the horizontal and vertical movements immediately beneath the assessed building in detail as shown in Figure 3. Brexis X can be used independently or, as in this case, it can use the output from the GRP containing the information about the settlement at the location of the building.

Figure 3 Brexis X output example

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Additional more detailed assessments were carried out for the existing Rajiv Chowk (Formally Connaught Place) Metro Station and the NDMC State Emporium Underground Car Park including consideration of worst case ground movement.

Instrumentation and Monitoring In addition to the thorough data collection, the use of sophisticated analysis software and detailed interpretation of the results, it is vital that ground movement predictions are monitored and verified. This is particularly the case for sensitive building structures such as government buildings, religious sites, hospitals etc. Monitoring regimes and the instrumentation used varies in frequency and complexity from building to building. This is usually governed by the end-use of the monitored structure, its current condition and its proximity to the tunnelling/excavation activity. The most common monitoring techniques and instrumentation equipment employed along the proposed 3.861km AMEL C1 route can be found in Table 2 and Table 3.

Table 2 General instrumentation guide for buildings

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Table 3 General building instrument types and installations

One particular case within the remit of the project where a bespoke monitoring regime was employed is Rajiv Chowk Station, one of the major interchanges of the existing Delhi metro network. The safe and effective monitoring of this strategically important structure presented unique difficulties which required close cooperation of the whole design team from the consultant through to the contractor and the client. Primary concern was passenger safety both in terms of the train movements and safety within the station facilities. This was also linked to the efficient operation of the metro system with zero or minimal interruptions. Following a detailed inspection of the station by the design and construction teams together with the client, an instrumentation and monitoring regime was devised addressing all safety and operational concerns, and also fulfilling the requirements for sufficient data collection enabling comprehensive interpretation. A fundamental aspect of the monitoring regime, to guarantee a safe and effective operation of the existing facilities whilst ensuring the uninterrupted advance of the TBM, is the appropriate setting of trigger levels for movement. In the case of Rajiv Chowk Station two separate Stage II analyses were carried out to investigate the building response (at foundation depth). The first one was made for the expected volume loss of 1% and the second was made for a higher volume loss of 2%. An important outcome of this exercise was to demonstrate that a structural integrity can be maintained even if the maximum design settlement is reached or slightly exceeded. This gave the design team peace of mind to set reasonably relaxed trigger levels. As an added safety and control measure, joint daily review meetings were recommended to the contractor in order to evaluate the recorded movements. The daily review allows the team to identify adverse trends in movements and respond if necessary.

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Construction Progress The construction of the AMEL C1 section is going well, and by February 2009 the civil and structural works have been completed in the following percentages:

New Delhi Station approximately 35% Shivaji Stadium Station approximately 55% Bored Tunnel section approximately 20% Cut and Cover Tunnel section

approximately 40%

CONCLUSIONS As an example of the modern global nature of the tunnelling industry this design was carried out by 3 teams, based in London, Budapest and Delhi. This project illustrates how state-of-the-art computer modelling can be combined with traditional engineering judgement and risk management principles to deliver modern metros. Using an integrated team simplifies the design process by internalising the interfaces within one team resulting in easier interface between the lining design and TBM specification and logical linkage between settlement prediction and instrumentation specification. As well as easing communication this speeds up the design process which is crucial for Design and Construction projects.

ACKNOWLEDGEMENTS We would like to thank Mott MacDonald Delhi Office, Mr. M L Gupta and Dr Alun Thomas for their valuable contribution to this paper. The authors gratefully acknowledge the kind permission of DMRC and ALC to publish this paper.

REFERENCES Attewell, P.B., Yeates, J. and Selby, A.R. (1986), Soil Movements Induced by Tunnelling and their Effects on Pipelines and Structures, Blackie, Glasgow. Boscardin, M. D. and Cording, E. J. (1989), Building response to excavation-induced settlement, Journal of Geotechnical Engineering, Vol. 115, N 1. Burland, J. B., Broms, B. B. and de Mello, V. F. B. (1977), Behaviour of foundations and structures, State-of-the-Art Report, Session 2, Proceedings 9th ICSMFE, Tokyo, pp. 495-546.