Civil Engineering Construction - Case Study Example

Contents 1. Introduction……………………………………………………………………. .3 2. Geological Considerations……………………………………………………....4 3. Tunnel Stabilization and Lining…………………………………………………6 4. Dewatering……………………………………………………………………....7 5. Grouting………………………………………………………………………....7 6. Tunnel Boring Machine (TBM)………………………………………………....7 7. Removing the excavated material……………………………………………….9 8. Quality of Project….…………………………………………………………...10 9. Problems and Proposed Solutions……………………………………………...11 10. Health and Safety………………………………………………………………12 11. Conclusions and Recommendations …………………………………………...13 Civil Engineering Construction Crossrail Tunnels Introduction: Crossrail is a project to build a railway network under London. Based around an east-west tunnel there is a central section from Paddington to Liverpool Street station. The other route is the Chelsea–Hackney line. The project was approved in October 2007. The agreement regarding the full finance for the project was signed in late 2008. 10-carriage trains are proposed to run at the frequency of up to 24 trains per hour in each direction through the central tunnel section. Transport for London (which reports to the Mayor of London) and the Department for Transport (which reports to the secretary of states for transport) are responsible for the sponsorship of the project. Full cost of the project will be financed by these two departments and Network Rail, BAA and City of London will also contribute. Transport for London is drawing funding from the European Investment Bank. In 2009 the enabling work phase of the project started, demolition work took place demolishing buildings including Astoria Theatre, Royal Oak Portal Taxi Facility Demolition, Demolition works for Crossrail Bond Street Station, Demolition works for Crossrail Tottenham Court Road Station and Pudding Mill Lane Portal. The enabling work contracts were awarded to different civil engineering companies. Figure 2 Year-wise Proposed Development by http://crossrail.co.uk/library (2009) Design: Tunneling provides solutions to the increased urbanization, congestion and pollution in the present age but tunneling is not such an easy task with huge challenges encountered because the work is carried out under major urban cities. The major factors responsible for problems are: i. Varying Geology ii. An array of sewers iii. Tunnels and pipes iv. Foundations of buildings These factors make tunneling beneath large cities the most complex structure in civil engineering. Geological Considerations: Geological considerations are very important in design of any civil engineering project in general and in tunneling operation in particular. Figure 3 Central Tunnels Geology by http://crossrail.co.uk/library The important challenges for which geological considerations are to be taken into account are i. Vast uncertainty in underground behaviors of soils ii. Every feature of geological aspect should be known exactly for tunneling operation iii. Geology dominates the cost and feasibility of any tunneling project iv. Ground water reservoirs and levels should be known because it is the most important consideration v. Geology dictates the selection of system of construction (tunneling system) The important phases in geological considerations are Proposal and Scoping Process Existing Data Collection and Evaluation Geological Reconnaissance Feasibility Investigation and Design Preliminary Investigation and Design Final Investigation and Design Exploration Evaluation of Results and Geotechnical Design Identification of Cost Issues Additional Exploration to Support Cost Estimates Final Design Interaction with Designers and Cost Estimators Construction Services Preconstruction briefings and periodic meetings Reviews of submittals Construction inspection Confirmation of actual geotechnical conditions and behavior Compare actual to predicted conditions and behavior (Ground and Groundwater) Exploration during construction to solve specific geological needs Instrumentation to confirm design and for early warning of problems Problem Solving Table 1 Geological Considerations The engineering properties to be considered are i. Soil Mass Strength ii. Soil Mass Modulus iii. Soil Mass Permeability The major design considerations are i. Water: the resistance to hydrostatic pressures ii. Constructability: compatibility of construction process with the expected ground conditions iii. Shield Systems: diameter of the shield, larger than the diameter of the tunnel section iv. Lining Dimensions: the length and width of the precast concrete segments The most important consideration before the start of the tunneling operation is to know the nature of the soil. In case of soft ground tunneling, clay is somewhat easy to be tunneled into as compared to coarse grained soils (sand and gravels). The London clay, in this regard, through which much of the Crossrail’s tunnels will pass very much supports tunneling operation. This nature of the London clay allowed the engineers to construct the rail tunnels in London as early as the 1880’s. But the clay in the area itself has variable properties and the major task is to determine these fluctuations in properties by analyzing the load response of samples recovered from boreholes. The microstructure of soil reveals presence of diatoms which have very significant effect on the behavior of clay in terms of strength and compressibility. The variation of the geology is most significant between King Cross and Stratford. Clays, mainly London clay, are present in the form of mixtures with sands (thanet sand) and gravels. The geology of the project site can be summed up as ‘Made ground, terrace gravels and alluvium (gravel, sand, silt and clay), London Clay and various deposits of the Lambeth Group (sand, silt, clay) and groundwater table above tunnel roof elevations’ by Gall Zeidler Consultants (2010). The proposed system for tunneling operation in case of Crossrail is the tube tunnel rail system in which TBM’s (Tunnel Boring Machine) are used to bore 6m tunnels followed by circular precast concrete lining jointed by bolts as in cast iron lining. Tunnel Stabilization and Lining The type lining to be used in Crossrail tunnels is known as segmental lining. Soft ground tunneling usually employs this technique. Segmental lining provide a one-pass system using a shield protection during construction, providing both stabilization of the tunnel opening during construction and a permanent service lining. Segmental lining can also be used in two-pass system, as the case with Crossrail tunnels. Segmental linings have smaller diameter than that of the tunnel excavation because they are fixed inside a protective shield which is the part of the tunneling assembly. As the excavation or TBM moves forward, grout is pumped into the space between the excavated tunnel and lining. The segmental lining is bolted and in wet grounds, joints are sealed to avoid water leakage. Dry grounds can have unbolted segmental linings. Sometimes an initial stabilization system is maintained in grounds with damp conditions. A water proofing membrane is provided between the stabilization system and the inner lining some sections of the Crossrail tunnels will be constructed according to this conditions where the soil is damp. Dewatering In some of the sections in the Crossrail tunnels dewatering is required to decrease the water level and dampness of soil to protect the tubes from leakage. Dewatering is carried by means of pumps which are installed in shafts or specially bored holes for this purpose. Grouting Grouting in tunnels can be done by cement or chemicals. In Crossrail tunnels, grouting would be done by cement along with chemical admixtures which make the cement water repellent. The Crossrail tunnels can be successfully grouted because average 8% of the soil (crossrail.co.uk) passes through the sieve #200. TBM (Tunnel Boring Machine) The other name for tunnel boring machine (TBM) is mole. It is a machine used to excavate circular cross-section tunnels. This excavating operation can be carried out in different types of soil and rock conditions. Sometimes the tunnel boring machine are used to excavate tunnels of diameter as less as 1m for cable trenches, this type of TBM’s are called micro TBM’s. On the other hand the maximum diameter of excavation so far achieved using TBM’s is a little more than16m. Tunnel boring machines have replaced the conventional hand mining in practice in the soft grounds and so is the case with the Crossrail tunnels where all of the excavation operation will be carried out by eight TBM’s. TBM’s significantly reduce the cost of lining because they produce very smooth walls of the excavation. The cost of tunnel boring machines is very high but in case of large lengths the cost of hand mining as compared with the cost of construction of a TBM becomes much more so ultimately TBM keeps the engineers on the safer side from finances viewpoint. The other main advantage of TBM is the reduction in the time period of the project. The structure of the Crossrail tunnel boring machines consists of a cutting wheel called a cutter head. The cutter head has behind it i. main bearing ii. thrust system iii. trailing support mechanism The Crossrail TBM’s to be used are Earth Pressure Balance Machines (a type of TBM). Thrust cylinders are used to advance forward and concrete segments are pushed back. A combination of tungsten carbide cutting bits and carbide disc cutter makes the cutterhead. The Earth Pressure Balance Machines of the Crossrail are meant to create a balance between the earth and pressure. The design is considered to make the advancement time and time of cutting equal to each other. Bentonite and other polymers are also being used to stabilize the soil. Eight tunnel boring machines are to be used to build 6m-diameter tunnels through which the trains will run. According to estimate these TBM’s will excavate eight million cubic meters of soil working 24/7. The tunnel boring machines will advance and the precast concrete tunnel sections would be fixed behind. In some areas TBM’s will not be used. These areas are decided after deeply studying the geology of the sites. In these areas other excavation techniques are used and concrete is lined by spraying. The areas which will be constructed by this method are the larger platforms tunnels and the smaller tunnels which are to be excavated for passenger circulation from ground level to underground. The reason for this is that that TBM’s are very expensive to construct and the amount of work in stations tunnels and circulation/transition tunnels is very less that construction of separate TBM’s for these tunnels is not economically feasible. Moreover not all the station tunnels and circulation tunnels have same diameters. Another example of this sprayed concrete lining is the construction of Waterloo station during the construction of Jubilee Line extension. TBM’s are ideal for use in heavily urbanized areas like London. The TBM’s to be used in Crossrail tunnels will have their crossheads rotating at a speed of 1 to 3 rpm. The tunnel waste i.e. the excavated material will be removed at a controlled rate from the tunnel face with the help of a screw conveyor which in turn deposits the collected material on a series of belt conveyors which will take the excavated material out of the tunnel. There will be an additional assembly provided to inject foam into the face of the TBM for assistance in handling the excavated material. A mechanism involving a series of hydraulic rams at the back of the shield pushes forward the last segment of the precast concrete erected and makes the TBM move ahead. The process continues as the next precast concrete ring is erected in the shield after it has been moved forward. The mechanical and electrical support equipments are fixed on a series of trailers which are towed behind the shield. The scheme of tunneling support and lining consists of precast segments made up into rings of approximately 6 segments. All the longitudinal and circumferential joints are bolted. Removing the excavated material As the spoil will fall from the excavated face scoops will pick it up and transfer to conveyor belt. This is then transferred to the trucks at the back of the TBM to be taken outside the tunnel. 50 lorries a day are currently taking the earth away from the ongoing works on the tunnels. The lorries will transport earth from central sites including Bond Street Station, Hanover Square, Newman Street, Oxford Street, Charing Cross Road, Goslett Yard, Westbourne Park, Royal Oak and Paddington. The Crossrail excavated material will go up to 200k cubic meter per month at peak which will be transported by rail (14%), by barge (39%) and by road (46%). Clay 2.3M cubic meter Piling and Diaphragm Walling Arisings 0.68M cubic meter Sprayed Concrete Line 1.4M cubic meter Sand & Gravels 1.15M cubic meter Chalk 0.6M cubic meter Lambeth Group 0.9M cubic meter Demolition Arisings 0.27M cubic meter Subtotal 7.3M cubic meter Recovery Aggregate and hardcore -2.0M cubic meter Total for beneficial use 5.3M cubic meter Table 2 Excavated Material Quality of the Project and High Quality Materials: The following aspects make Crossrail tunnels a high quality project. 1. Environment friendly because it will decrease the hydro-carbon fueled road traffic of London 2. High quality materials ensure high efficiency decreasing the chances of delays and thus Crossrail in this respect surpasses other projects of its kind 3. Robust cost control mechanism shall be in place throughout the lifetime of the Project. 4. Environmental assurance, safety and security regimes were established to be implemented during the phases of construction, commissioning and shall be established during service operation. 5. The design of the Crossrail project complies with the applicable laws and applicable standards particularly pertaining to safety, security, interoperability, the environment and provision for the disability 6. The materials used in the Crossrail tunnels are of very high quality, some of them coming out from far off place. These include a. Sand: The sand used for the lining and the grouting mortar is with high degree of sorting coupled with high degree of rounding. Well graded sand with rounded particles and free from impurities with bulk specific gravity ranging from 1.7 to 2. The sand locally available is not of such a good quality. b. Precast linings: The precast concrete linings used are also of very good quality being water proof and frost resistant due to addition of admixtures, with high quality rubber washers used at the back and the front of the tunnel lining to seal the bolts. c. Use of PE-foam and XPS (expanded polyester) for the sealing of joints along with water tight membrane of PVC placed along each of the segment. d. High quality aggregates (crush) tested to according to BS 812 crushed using methods which mirrors natural processes of crushing, using primary water-born techniques of crushing. Problems: The Crossrail is the Europe’s largest civil engineering project, keeping in view the complex nature of the project the following are the major management and design problems to be encountered. 1. Number of industry bodies are not pleased with the project, with concerns regarding design 2. Short deadlines to be met, with operation expected to start by 2017. 3. High cost of the project i.e. £16bn. 4. High demand of man-power, Crossrail requires having around 15000 people at the peak of the construction project. 5. Requirement of relatively long working shifts. 6. Requirement of multi-disciplinary workforce (ground engineers, specialists of contaminated land, railway engineers, structure engineers and transport planners) 7. Shortage of suitable recruits. 8. Complex IT infrastructure. 9. The settlement of the buildings which are directly above the Crossrail tunnels. 10. Vibrations caused by the TBM and later caused by the passing trains during operation. 11. Contaminated excavated material. Solutions: 1. Creating awareness about the design by issuing free brochures explaining the design and management aspects to the common people and the concerned industry people. 2. Creating divisions and simultaneously working on different sites. 3. Drawing funds by providing incentives and using BOT type of contracts. 4. Providing incentives to the labor liking boarding and lodging facilities and easy visa considerations in collaboration with the government to attract labor from 3rd world nations. 5. High wages should be provided. 6. Acquiring the services of companies supplying construction professional services (CPS) 7. Establishment of a number of project offices and site offices. 8. Evaluation of the settlement and controlling the settlement of the building above the tunnels using different techniques like filling the grounds near the foundations with liquid cement compensation grouting using steel tubes etc. 9. The vibrations due to TBM are monitored using computer monitoring and to check the effect of vibrations during the operation full scale models should be tested. 10. The tunnel material to be excavated should be tested and contaminated material should be dumped accordingly. 11. Foundations of high rise buildings and important heritage buildings above the tunnels should be underpinned using new piling. 12. Underground concrete walls should also be built to restrict movements. Health and Safety: The Crossrail will provide high levels of health and safety services to its users 1. Provision of emergency medical facilities at all the stations and interchange terminals. 2. Provision of automatic fire fighting system at the terminals, stations and in the tunnels. 3. Use of environment friendly fire extinguisher powder rather than CO2 4. Provision of fire-escape system 5. Air-conditioned carriages customized by air-filtering mechanism to ensure better breathing environment for the commuters. 6. Dedicated space for wheel chair users. 7. Audio and visual information about the trains journey. 8. Overhead line disconnection system with remote transmission indicating the switching status inside and outside the tunnel portals will help maintain safe flow of the traffic. 9. Provision of orientation lighting 10. Marked emergency escape routes 11. Fire water supplied throughout the tunnels 12. Emergency telephones with position identification in the train operation centre 13. Radio facilities for emergency response organizations 14. Rescue areas and access ways (Roller Pallets) Conclusion and Recommendations: Original design methods along with special construction techniques are being implemented in the Crossrail tunnels. Due to safety, environmental aspects and space restrictions precast RCC lining is used in the tunnels. Precast piling should be used to support the foundations of the buildings which are directly above the tunneling sites. The retaining system should consist of concrete pile walls, micropiles and pre stressed anchors. The management issues should be addressed properly and the affected people should be provided proper alternatives ensuring their satisfaction, for this purpose separate department should be founded. The cultural heritage should be preserved as far as possible because the 18th century buildings above the tunnels are of importance for preserving the cultural heritage. An extensive program of geotechnical measurements need to be undertaken after the construction of each of the section to assess the behavior of the open cuts and the functioning of the retaining system, for this special instruments should be installed in specific locations. In order to improve the escape route situation, unconventional measures such as cross-passages between road tunnels and railway tunnels may also offer professional, financially feasible and efficient solutions. The responsibilities for the installation inside the cross-passages are thoroughly to be considered in the initial design and ultimately should be defined based on the possible strategies designed to overcome conceivable accident scenarios. Overall the project is an combination of conventional and modern techniques of tunneling. The management and finances issues are as complex as the design itself and need to be given importance also. The project if completed on time and within the estimated cost of the project will turn out to be an ultimate success not only for the workforce involved in this project but also will serve as a pioneer for civil engineering projects of the future. References John O. Bickel, Thomas R. Kuesel, Elwyn H. King (2002) Tunnel Engineering Handbook, CBS Publishers and Distributers C. A. Pequignot (1963) Tunnels and Tunneling, Hutchison Scientific and Technical F. G. Bell (2004) Engineering Geology and Construction, Spon Press Crossrail (2010) http://crossrail.co.uk/library [April 2010] Frederick E. Gould (2005) Managing the Construction Process, Pearson Education Ltd. R. C. Smith, C. K. Andres (1989) Materials of Construction, McGraw-Hill Co. Read More