Biswanath Dewanjee
General Manager
Kolkata Metro Rail Corporation

The 1st Metro line in Kolkata in North-South direction was eventually the 1st rapid transit system in India which is being successfully operated over last three decades. The 2nd Metro line in East-West direction has been undertaken with majority of this metro corridor running underground through densely populated and built up part of the city including coveted crossing of river Ganges. Twin RCC segmental tunnels of 5.55m internal diameter are built in the vicinity of numerous old structures around the tunnel alignment within the influence zone. The passage of underground tunnels in vicinity of Writers Building, which is a prominent old iconic structures show cases meticulous planning and investigation involved including detailed building conditioning survey and structural impact assessment for structures in tunnel influence zone.

Geology

Sedimentation in the Bengal Basin occurred after the outpouring of Rajmahal trap in the Cretaceous period at different stages. Kolkata is located in the lower deltaic plain of the Ganga- Brahmaputra river system at an elevation ranging between 1.5m and 9m above sea level.

The city is spread along the banks of the river from north to south and situated predominantly on reclaimed wetland. The Gangetic alluvial plain consists of several hundred meters of alluvium deposits.

The alluvial deposits consist of silts, clays, sands and peats. Meticulous Geotechnical investigation is required in an underground construction project like Metro. Geotechnical investigation has been done along the tunnel alignment mostly in 50m intervals using land cable percussion in land portion and marine cable percussion in riverine section. Geotechnical investigations were done prior to award of the construction contract and confirmatory bore holes were also done after award of the construction contracts. The geotechnical profile as determined through investigation is depicted in Fig. 1 and the engineering characteristics of subsoil in the tunnel zone are put up in Table 1.

Tunnel Boring Machine

The tunnelling in Phase-II of the project was a continuous drive from Howrah Maidan to Esplanade (distance of 3.8 km) including crossing of river Hooghly. Therefore the selection of the Tunnel Boring Machine ( TBM) must encompass the requirement of the entire tunneling stretch. The soil profile encountered in the tunneling horizon is shown in Fig. 2. It is observed that 50% of the stretch is passing through Firm to stiff clayey silt. Therefore TBM to be chosen must be suitable for soft ground.

Closed face TBMs are effectively sealed from the full ground and hydrostatic pressures and suitable for soft ground tunnels due to their ability to maintain control of the ground particularly at the face of the excavation. These TBMs provide positive support to the ground and minimize the potential for problematic ground water inflows into the tunnel excavation.

Laboratory results of grain size distribution of Kolkata sub-soil in tunneling horizon is superimposed in reference guidelines graph as shown in Fig. 3 which indicates that Earth pressure balancing tunnel boring machine with soil conditioning is suitable for Kolkata subsoil.

Tunnelling In The Vicinity Of Writer’s Building

History Of The Structure: The Writers Building is the most prominent and iconic structure in the city of Kolkata. Initially designed by Thomas Lyon in 1777, this building was the 1st three storied building in eastern India.

Subsequently there were several additions to the building. Initially the building was constructed as a simple service residence of the ‘writers’ of the East India Company; after advent of British ruling, this building was converted as the administrative headquarters of the British Raj. In year 1821, addition of a 128ft long verandah with 32ft high circular columns in southern part imparted a Greco-Roman look with a portico in the central bay. This impressive first colonial building in the former capital of British Raj was further facelifted in year 1883 with the terrace lined with spectacular statues. After the independence, this building was the Administrative headquarters of West Bengal. It is a brick masonry structure with load bearing walls with a roof of each floor constructed with lime concrete supporting on steel frame. The building is presently being maintained by State Public Works Department and a major structural modification has been undertaken and the offices are shifted to another building.

Building Condition Survey: The Writers Building is now practically an unoccupied heritage building under modification with different degree of structural dis-integrity and disorientation. Considering the age of the building as well as the immense importance, thorough building condition survey was conducted for Main Block and Block-1 prior to impact assessment. As there was no digital drawing for the building, a digital plan was also prepared. Being the administrative headquarters for centuries, there were thorough engineering surveillance and maintenance and the general condition of structural health was found workable. But the natural degradation of structural material due to ageing in corrosive humid atmospheric condition with heavy rainfall resulted considerable damages in the building. From existing drawings as well as field survey, the foundation details of the building has been determined. The foundation consists of stepped brick-masonry work with bottom width of 2.5m and depth 1.8m from ground level as shown in Fig. 6.

Tunneling Scheme In The Vicinity Of The Structure: Present-day Writers Building covers a huge area in northern part of the Dalhousie Square. It has thirteen blocks out of which the Main Block and Block-1 are close to the tunnel alignment. The alignment of the tunnel vis-à-vis the location of Writers Building is shown in Fig. 7. The minimum horizontal distance from West Bound Tunnel center to Writers Building is 5.87m while the distance between centers of tunnels are 12.65m at that location. Roof of Main Block is a Georgian roof structure with Jack arch vaulted roof supported by steel I-sections The I sections of the roof of main block are running parallel to the tunnel alignment. Roof of Block-1 is a flat roof also supported by I sections which are running transverse to the tunnel alignment.

*Fig. 7: Horizontal Alignment of Tunnel in the Vicinity of Writers Building*

Structural Impact Assessment

The impact assessment on structures due to tunneling begins with the study of induced ground movement. The tensile stress in structure due to induced ground movement is analyzed thereafter to assess the resultant effect. The precautionary measures and ground improvement techniques are devised accordingly to avert any untowardly effect.

*Fig. 8: Volume Loss Components and Related TBM Parameters*

Ground Surface Settlement Due To Tunnelling

The construction of a shield tunnel leads to a larger amount of soil to be excavated than replaced by the volume of the tunnel. The amount of over excavation is quantified by the volume loss.

This volume loss is the extent of the total ground disturbance. It causes the settlement trough at the surface and in undrained conditions; the volume of this settlement trough is same as volume loss. For shield tunneling, Attewell [1] derived the volume loss into four categories, which are face loss, shield loss, tail loss and the long term radial loss after grouting. Fig. 8 shows a schematic section of the advancing tunnel shield, which depicts the three components of volume loss due to soft sub-soil shield tunneling. The first component is the ‘face loss’. It is mainly due to the inward flow of the ground from a zone of influence ahead and to the side of the face (Zone 1) into the shield. The settlement component caused by the face loss is referred to as ∂f . The rotating cutters of the simultaneously a confinement pressure is applied. In spite of the confinement, the ground tends to protrude shield remove material from the tunnel face and out of the face from a zone of influence ahead and around the tunnel face. To minimize the volume loss at the face, the shield advancement parameter must be controlled through face pressure (pf ).

In order to advance the shield and to reduce the chance of the shield being stuck and to allow steering the shield in curved alignment, it is necessary to excavate an oversized hole as the cutter diameter is slightly more than the TBM body diameter. After the cutter has passed, an annular void is created and the surrounding soil tends to move inward radially to fill the void. This movement of the soil creates a loss of ground namely ‘shield loss’ which leads to the settlement component ∂s in Zone 2. The entity of this convergence depends upon the rate of deformation of the soil with respect to the rate of tunnel advance as per Surarok[2].

Due to the slightly smaller size of the tunnel lining compared to the Shield/TBM Body, there is an annular space which is normally filled with grout. Thus there is an opportunity for the soil to displace radially onto the lining until the grout has hardened and gained enough strength to resist the earth pressure. This soil movement causes the last component of volume loss ‘tail loss’ which occur in Zone 3. Tail void loss is a function of the pressure, volume and accurateness of the injection filling of grout into the tail void around the installed tunnel lining.

A further increase of the radial convergence can be observed over a period of time. Such Long-term loss can occur due to the lining deformations after the grouting, caused by the transfer of the overburden pressure (unless time-dependent soil behavior is observed, this component is quite negligible.

The ground movement has been calculated as per established empirical formula as the settlement trough transversely to the tunnel axis can be approximately expressed by an inverted Gauss distribution curve where dimension and shape can be defined mainly through two parameters: as per Attewell et al[3] and Rankin[4] .

*Fig. 9: PLAXIS Output for Ground Settlement after Passage of Both Tunnels*

The ground movement has also been calculated by Numerical Method with more accurate FEM method using 2-D Plaxis analysis was also done to assess the settlement. The building is modelled as plate elements and live load of 5Kpa applied in each floor. For analysis of geotechnical model analysis using Plaxis, following sequence was considered:

Stress condition and water pressure initialized

Surcharge applied.
Tunnelling and installation lining for 1st tunnel,
Tunnelling and installation lining for 2nd tunnel,
Consolidation after construction completed
Change the lining properties to long term modulus;
Apply seismic acceleration
Consolidation after seismic acceleration.

Ground loss of 1.5% has been considered in analysis and relaxation of ground stress is assumed before installation of lining. A typical Plaxis output is shown in Fig. 9.

Comparison of maximum ground settlement values as derived from empirical formula and Plaxis analysis is tabulated in Table 4 which shows a maximum settlement of 36.94 mm from Plaxis analysis as compared to 42.71mm by empirical formula.

Stress Analysis In Building Due To Ground Settlement

Ground settlement as discussed above induces tensile stress in building/structures which causes distress/cracks in the building. The settlement analysis is therefore used to determine the tensile strain in the building. The approach adopted by Burland and Wroth [6] is used for analysis where the building is represented by a rectangular beam to calculate the tensile strains in the beam for a given deflected shape of the building foundations and to obtain the sagging or hogging ratio Δ/L at which cracking is initiated. The distribution of strains within the beam is determined based on mode of deformation. Two extreme modes are ‘bending only’ about a neutral axis at the center and ‘shearing only’. In general, both modes of deformation will occur simultaneously, and it is necessary to calculate both bending and diagonal tensile strains to ascertain which type is limiting. Timoshenko [7] has given the expression for the total md-span deflection of a centrally loaded beam having both bending and shear stiffness which is used along with derivation of maximum extreme fiber strain in replicated beam. The ground surface movements associated with tunneling not only involve sagging and hogging profiles but significant horizontal strains also which is analyzed as per method suggested by Bos Cardin and Cording [8]

The tensile strain induced in Writers Building due to ground movement caused by tunneling is derived using the above principles. Summary of calculated tensile Strains at critical sections for anticipated surface settlements due to tunneling operation are presented in Table 5.

*Fig. 10: Typical Plan for Surface Grouting*

Pre-Emptive Measures Adopted

Following pre-emptive measures were adopted:

Control of Tunnelling Parameters:

Maintaining steady rate of tunnelling
Maintaining proper face pressure
Proper tail skin grout pressure and proper grout volume
Rigorous secondary grout at every third ring of tunnel
Tertiary grout in case of requirement

Supporting System and Propping in Vulnerable Locations:

Vertical propping at critical load transfer locations are provided as alternative load path.
Props equipped with proper jacking facility to ensure a positive support at all times.
Staircases in particular have been supported.
To avoid new cracks and widening of already existing cracks all the openings in walls such as doors and windows have been supported
Differential settlement under the building is expected around 20-25mm. To avoid any unusual structural instability, vertical and horizontal proppings have to be installed to control the differential settlement and to restrict the propagation of crack.
Point of inflexion is potential crack developing zone where the dome of the building is located. Hence extensive supporting measures are provided in this zone right from ground floor to top floor to make sure no failure of any structural member happens.

Permeation Grouting: A typical layout of surface grouting is shown in Fig. 10. The surface grouting has been done with cement sand and water as per IRC Bridge Code. The typical water cement ratio of grout was 0.4 to 0.5 with addition of non-shrink admixtures. The grouting pressure adopted is 1 to 2.5 kg/sq. cm transverse direction using G.I. or PVC pipes of 12mm to 20mm diameter up to 2m depth from GL. The grouting pipes are drilled in such a way to avoid other utilities depending upon the situation.

Theoretical Impact Vs. Actual Impact

Adopting all these measures, the tunnelling in this zone has been successfully completed without any appreciable and notable distress. The typical plot of surface settlement along the tunnel is represented in Fig. 11 and plot of building settlement in Fig. 12.

It is observed that the maximum surface settlement was 12mm and building settlement was 10mm only which are well within the limit. The typical result of optical target fixed in plotted in Fig. 13 showing that maximum deflection

observed was only 5mm. No new cracks were developed and width of the existing cracks was not increased.

Reference

*Fig. 13: Deflection shown in Optical Targets during Tunneling in Writers Building*

Attewell, P. B.1978 “Ground movements caused by tunneling in soil.” - Proc., Int. Conf. on Large Movements and Structures, , J. D. Geddes,ed., Pentech, London : 812–948.
Chanton Surarok, 2010 Geotechnical aspects of the Bangkok MRT Blue Line Project’ – Doctoral thesis submitted in September, 2010 in Griffith School of Engineering Science, Environment, Engineering & Technology, Griffith University
Attewell, P. B., Yeates, J., & Selby, A. R,.1986 -Soil movements induced by tunneling and their effects on pipelines and structures- Blackie and Son Ltd., London
Rankine W.J., 1988 – Ground movements resulting from Urban tunneling, prediction and effects, Engineering Geology of Underground Movements 5, 71-77
O’Reilly, M. P., and New, B. M.1982- “Settlement above tunnels in the United Kingdom—Their magnitude and prediction”- ”Tunneling ’82” - London Institute of Mining and Metallurgy, 173–181.
Burland J.B. & Worth C.P, 1974- ‘Settlement of buildings and associated damage’, proceedings, conference, settlement of structures, Pentech Press, London : 611-657
Timoshenko, S., 1957- Elements of strength of materials, D. Van Nostrand Co
Boscardin, M.D & Cording J.C., 1989 Response to excavation-induced-settlement, Journal of Geotechnical Engineering, Vol CXV: 1-21
Burland J.B., Brooms B.B., de Mello V.F.B , 1977 -Behaviour of foundation & structures, state-of-the-art Report, proceedings, 9th International conference on Soil Mechanics and Foundation Engineering II, Tokyo, Japan 495-546

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Tunnelling In The Vicinity Of An Iconic Building Along Kolkata East-West Metro Route

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