Minimum disruption expected

The PJA on advances in pipe jacking and tunnel spoil disposal issues

Pipe jacking is a specialist tunnelling method for installing underground pipelines with the minimum of surface disruption. Primarily used for new sewer construction, it is also used for sewer replacement and relining, gas and water mains, oil pipelines, electricity and telecommunication installations, culverts and subways. The Pipe Jacking Association (PJA) has been established for more than 30 years and represents the leading contractors, pipe suppliers, and machine manufacturers in the UK pipe jacking and micro-tunnelling industry. The association is either represented on, or works closely with, all the key bodies involved with education and training, specifications and standards, and research in the tunnelling and pipe jacking industry.

These include the British Standards Institution and CEN, British Tunnelling Society, Construction Industry Training Board, the Departments of Civil Engineering at Oxford and Cambridge Universities, Transport Research Laboratory and the Health & Safety Executive.

THE TECHNIQUE

Pipe jacking is the technique of installing pipes by driving a line of them through the ground with hydraulic jacks from a prepared jacking pit. Excavation is carried out in a shield forward of the leading pipe as the pipeline is pushed out of the jacking pit and the process repeated.

Excavation can be by hand or machine depending upon the conditions. Micro-tunnelling is pipe jacking using remotely controlled machines, operated and steered from the surface, generally less than 900mm diameter, although larger diameters are also jacked remotely utilising similar techniques.

In the UK 30 years ago pipe jacking was a basic technique that miners used working inside a simple steel shield to manually excavate for the installation of modified open-cut pipes by jacking with short stroke hydraulic jacks and spacers. The pipelines were used primarily for crossing under existing structures, roads and railways in stable soil conditions.

The techniques for excavating and controlling the face, steering the shield and controlling jacking forces were very crude, with the result that the pipes installed were frequently damaged in the process and the line and level of the finished pipes were often out of acceptable tolerance. The possibility of constructing longer sections of deep pipe by pipe jacking instead of open-cut excavation had been realised but the system's quality control, particularly in variable soil conditions, held back any significant increases in this market. The advent of the latest generation of mechanised tunnelling systems reliant upon pipe jacking, which can handle either a wide range of ground conditions with one design or be custom-made for specific conditions such as rock, have proved to be of tremendous benefit to contractors and clients alike.

The resultant reduced unit costs have allowed clients to carry out more work for the given budget and, in some cases, made previously impossible jobs possible. Micro-tunnelling has gained a wider acceptance as clients and engineers, previously unfamiliar with the technology, are now specifying its use with confidence.

Micro-tunnelling machines (250-900mm diameter) are essentially miniaturised versions of full-faced tunnel boring machines. Their development generated the requirement for the remote control of all functions. These include the operation of the shield machine, jacking system, line and level control, soil removal and pipe lubrication. The new micro-tunnelling systems proved effective in a wide range of soil conditions and were reliable over lengths of 80m at 300mm diameter and approximately 200m at 900mm diameter.

These same remote control features were then adopted in larger diameter machines of 1,000-2,500mm with equal reliability and increased versatility. These larger machines can cope with an even wider range of ground conditions as well as installations of far greater single-drive lengths by adopting the use of interjacks.

As with all tunnelling processes, the geological conditions determine the most suitable excavation and face support methods. As a result, a range of machines with various excavation and ground support characteristics are now available to achieve economic installations across the entire range of ground conditions from soft, unstable soils through to solid rock.

Academic research has played a major role in the development of pipe jacking. A state-of-the-art review, carried out by CIRIA (Construction Industry Research and Information Association) and commissioned by the PJA more than 20 years ago started the ball rolling. Five areas of research were highlighted:

  • friction loads in different ground conditions,
  • load/deflection characteristics of the joints with different packing materials,
  • the effect of cyclic loading on the pipes at intermediate jacking stations,
  • the effect of lubricants in reducing friction along a pipe,
  • the development of a site investigation test suitable for the prediction of friction forces.

Subsequently, several centres of research, many of which are part of or associated with our universities, were established. The PJA, working with a number of water companies and the Department of Engineering Science at the University of Oxford, have together been carrying out fundamental research.

This Pipe Jacking and Tunnelling Research Group has to date completed five separate stages of research addressing many of the original report topics. Two of the UK's premier universities, Oxford and Cambridge, have collaborated on the current three-year research programme into the effective use of soil conditioning and lubricating agents in association with tunnelling machines. This £500,000 research project has the backing of the Engineering and Physical Sciences Research Council (EPSRC), with the PJA and industry providing a major portion of the funds. Oxford University has focused on soil conditioning and lubrication agents in granular soils and Cambridge on soil conditioning and lubrication agents in clayey soils. Field work has also been carried out on major tunnelling projects including the channel tunnel rail link. The programme is being supervised by Robert Mair, professor of geotechnical engineering at Cambridge and a founding director of Geotechnical Consulting Group, and Guy Houlsby, professor of civil engineering at Oxford University since 1991.

Primary objectives of the research were to investigate means of reducing the frictional forces between pipes and ground during pipe jacking, to investigate the use of soil conditioning agents within tunnelling machines and the interaction between the performance of tunnelling machines and the lining system, whether pipe-jacked or segmental.

A range of additives have been investigated in the laboratory and their properties assessed using compression, swelling, permeability and shear strength tests. This was followed by an analysis of soil-steel interface properties with model screw conveyors. In addition, microscopy and chemical analysis was used to examine the interaction between conditioning agents and ground on a micro-structural level. It is anticipated the research findings, which are due shortly, will enable tunnelling engineers to understand how various additives interact with different types of soil, improving the efficiency of pipe jacking operations and broadening the range of soils in which tunnelling machines can successfully operate.

Environmental issues

Shortly to be introduced EU directives on the disposal of liquid wastes will have a major impact on both the pipe jacking and tunnelling industry as well as the water industry. Currently there appears to be meaningful definition of liquid wastes. In November 2003, the EPSRC funded Network for Trenchless Technology (NETTWORK) convened an expert workshop attended by leading tunnelling contractors, consultants and academics involved in trenchless technology. The workshop developed a number of crucial research requirements for the tunnelling industry.

Research on slurry cleaning and tunnel spoil management was ranked as the most important of these - this ranking largely driven by environmental considerations. For tunnelling and many other construction processes, environmental considerations are now becoming so important as to exert a major control on the technology and the economics.

A particular challenge to the tunnelling industry relates to the management of tunnel spoil. This spoil should be re-used in other works rather than disposed to landfill. However, in practice, much material has to be landfilled because it is too wet to be re-used. The spoil from all types of tunnelling can be difficult, but spoil from slurry tunnelling, the most widely used procedure for small projects, is particularly problematic.

In slurry tunnelling a slurry is pumped into the face of the tunnelling machine where it mixes with the cut spoil. The spoil slurry mix is then pumped from the face to a surface cleaning plant. In the cleaning plant spoil and liquid are separated, the spoil for disposal and the liquid for return to the face. The liquid thus becomes a carrying fluid, transporting the spoil from the face to the surface. Machine excavation is now used for 95% of UK tunnelling and 80% of these tunnels employ slurry tunnelling. Furthermore, slurry excavation is essential in many water-bearing soils as the machine face is sealed, which prevents groundwater inflow and the risk of tunnel flooding and dewatering-induced settlement of neighbouring buildings.

Slurry tunnelling may be used for all tunnel diameters from about 0.3m diameter to much over 10m. Despite the key role of slurry tunnelling in UK construction works there has been almost no UK research on slurry systems (indeed the last significant programme of research programme is believed to have been the work on slurry cleaning funded by the then National Research and Development Corporation in the 1970s).

In addition to the problems of re-use of wet spoil, the stage-wise implementation of the EU Landfill Directive also presents a major challenge for the tunnelling industry. Under the directive, disposal of hazardous liquid waste to landfill is now banned and disposal of all liquids to landfill will be banned within two years. If any spoil is so wet as to be classified as liquid then disposal to landfill will not be permitted without prior treatment. If cement or similar materials are added to the spoil to solidify it then the resulting material may be classified as hazardous waste because of the resulting high pH. Thus treated tunnel spoil may be unacceptable, except at hazardous waste landfills - potentially involving large haulage distances as the number of such landfills will be severely limited in the future. The PJA is investigating research aimed at addressing this tunnel spoil problem at source rather than as an end-of-pipe treatment such as spoil drying.

Utility legislation

The PJA has also been at the forefront of lobbying government to introduce legislation that will reduce disruption on highways by utility installation through the use of techniques that include trenchless technology. The association sponsored a report by the highly respected TRL, formerly the Transport Research Laboratory, which concluded unnecessary disruption caused mainly by utility installation costs the public around £2B per year, around double the cost of installing pipelines in roads carried out annually by the utility sector.

The original legislation aimed at tackling this issue, the New Roads and Streetworks Act (1991) had proved ineffective and although it has being strengthened by the implementation of Section 74 of that act, which allowed for the charging of utilities for over-staying their occupation of the highway, the government is currently introducing further legislation. The Traffic Management Bill currently before Parliament will provide increased powers for local authorities over utilities street works as part of an overall strategy to co-ordinate the management of national and local roads and traffic, better co-ordination and management of works and other activities on the network and more effective powers and sanctions over utilities' street works as part of a broader agenda on tackling congestion.

The net result of this, although not a specific aim of the legislation, will be to encourage the use of no-dig techniques such as pipe jacking. With the anticipated growth in the use of trenchless systems as new legislation is introduced and the research and environmental benefits of pipe jacking become recognised, the PJA has launched a seminar programme aimed at engineers, specifiers and end-users on the benefits of pipe jacking and micro-tunnelling, together with best practice for installation. The association will supply experienced speakers to talk to target groups as part of continuing professional development programmes. As part of this initiative, two presentations have been prepared and a certificate of attendance will be issued to all participants, recognising the event has been independently certified as conforming to accepted CPD guidelines. All seminars are sponsored by the PJA and are supported with full complementary documentation to include design guides


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