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IDEAS IN TRANSIT

MEETING REPORT

Report on Transport Geotechnics, the 5th ICE East Midlands Geotechnical Group symposium and workshop held at Nottingham Trent University, 11 and 12 September 2003, by Ed Faragher, secretary, EMGG.

Transportation geotechnics upholds the whole of world civilisation. . .'

Drawing on the contribution of the Romans to world civilisation, and their recognition of the need for effective communication links, was Professor Ian Smalley's tribute to those who organised, attended and participated in the East Midlands Geotechnical Group's symposium on transportation geotechnics, held at Nottingham Trent University in September last year.

Although the EMGG had not organised the event with such a universal and historical perspective in mind, this fifth in a highly successful series of biennial symposia drew together an impressive array of experts in the field of transportation geotechnics. The accompanying exhibition provided a networking opportunity for delegates.

Keynote speaker Professor Stephen Brown of the University of Nottingham drew on his extensive involvement in pavement engineering research and its application in the road, rail and air transport sectors. He described fundamental aspects of pavement construction types;

the stress and strain regimes in the differing materials and layers; the effects of cyclic loading; and how analysis of these factors is used to inform pavement design methods.

He also outlined the range of increasingly important pavement assessment methods, such as the falling weight deflectometer, aimed at determining the 'life expectancy' of transportation infrastructure and allowing targeting of asset maintenance and replacement regimes.

He noted that assessment methods were being applied to the UK's railway infrastructure with similar aims.

Steve Corbet of Faber Maunsell then described how geosynthetics are becoming more widely used for a whole range of engineering uses, including lightweight embankment fill, separation layers, drainage provision, slope stabilisation, reinforced earth construction, erosion protection and contaminant barriers. He outlined the types of polymers used, methods of manufacture and specification and testing requirements. Examples of high profile construction projects where geosynthetics have been extensively used are Chek Lap Kok Airport, the Channel Tunnel Rail Link and the Copenhagen Metro.

Doug Allenby of Edmund Nuttall described box jack tunnelling. Use of the technique is growing as it allows tunnel construction under live transportation corridors. He described the importance of maintaining the line and level of the tunnel box and the need to minimise movement of the overlying ground surface. Examples of major box-jack projects include the new underpass at J15A of the M1 near Northampton (GE August 2002) and the tunnels on the Boston Central Artery project (GE September 2003).

Nick O'Riordan of Rail Link Engineering described the challenges of building the high speed Channel Tunnel Rail Link over the north Thames marshes in Essex, for which a piled concrete slab construction was adopted.

He outlined the loading regimes of CTRL traffic and described a range of full-scale field loading tests, during which design cyclic loadings were applied to test piles driven into the soft alluvial clay. These resulted in unacceptably large permanent settlements indicating some shortcomings in the design approach.

Further tests were carried out using different loading amplitudes on different types of piles and an empirical relationship was developed to predict pile settlement.This was validated using 3D finite element analysis and results incorporated into piled slab design, resulting in a better and more economical pile system (GE CTRL Supplement, May 2002).

The importance of shallow geotechnics in the performance of rail trackbeds was described by Robert Armitage of Scott Wilson Pavement Engineering.He described how trackbed renewal can be optimised using ground probing radar, automatic ballast sampling, laboratory assessment and the falling weight deflectometer (FWD).

These allow the overall condition of the route to be assessed; ballast quality and residual ballast life estimated; and appropriate renewal programmes targeted. Examples quoted were improving the east coast primary freight route for passenger trains and raising line speeds on part of the West Coast Main Line.

David Cudworth of Babtie Pavement Management & Engineering, described the issue of voids under concrete slab pavements, which can compromise the integrity of the UK's ageing concrete road stock and also the floors of today's large warehouse facilities. The M25 provided an example of the phenomenon of asymmetric voiding due to traffic loading.Methods of assessing the stability of concrete pavements (eg deflectograph and, now more commonly, the FWD) were covered, and analysis of the results, such as void intercept analysis.The treatment on the M25 was vacuum grouting.

Stewart Jarvis of Arup outlined the effect on the geotechnical design process of new financial and contractual arrangements for construction projects, such as design and build and the Private Finance Initiative.

Ground conditions are often the largest source of uncertainty in construction work, and these can only be estimated at tender stage, with more detailed design - post tender award - being required to quantify uncertainties with greater accuracy.

Jarvis underlined the need for effective communication with the contractor client, and the need for risk registers at tender stage to identify cost and programme issues. Once the contract is awarded, the pressures of time for collecting the necessary geotechnical data for use in detailed design was an issue for the geotechnical engineer. It was hoped the advent of early contractor involvement would bring about a better understanding of geotechnical design timescales and risk-balancing in these new project arrangements.

Matthew Raybould of Scott Wilson described the increasingly important field of geotechnical asset management as an engineering and business tool, with particular reference to rail infrastructure.

To be effective, qualitative and quantitative data at strategic and tactical levels must be analysed to provide an assessment of asset condition, and allow risk or hazard potential to be estimated. Risk assessment outputs can be used to define risk management strategies for assets, and to inform strategies for asset maintenance.

Evolutions in data collection and analysis promise further advances in this relatively new discipline.

Paul Fleming of Loughborough University spoke on developments in performance-based specifications for assessing road foundations during construction as an alternative to 'recipe' based specifications.

The important engineering parameters for pavement performance are resilient elastic modulus and resistance to permanent deformation (largely controlled by adequate shear strength). Fleming described the extensive laboratory and field studies carried out aimed at validating new field measurement techniques, for assessing insitu stiffness in particular.

A performance-based approach in road design allows for greater flexibility in design and material selection - from evaluating the contribution of the foundation properties better and thus facilitating greater use of recycled or marginal materials. It also provides greater assurance of the 'as-built' quality.

An overview of recycling in transportation was given by Sally Ellis of TRL's Centre for Sustainability. The search for alternative materials has been driven by government targets for the re-use of a range of waste streams, and assisted by taxation of aggregates and deposits to landfill.

The transportation sector and material suppliers have made developments in stabilisation (using PFA and blast furnace slag), cold recycling of asphalt planings, the crushing and regrading of wastes such as railway ballast and the use of pulverised fuel ash as a cement replacement in structural concrete.

Ellis outlined the concepts of down-cycling and up-cycling (where the products are of lower or higher utility than the pre-processed materials) and level-cycling. UK recycling tends to produce 'down-cycled' materials, whereas greater value could be achieved if 'up-cycling'was more widely adopted, she said.

A full scale trial of hydraulically bound foundation material (HBFM) was described by Steve Biczysko of Atkins ESL. This novel approach to highway construction was used on the construction of the Crick bypass, and HBFM allowed insitu soil to be treated and used in place of traditional imported granular sub base, with substantial environmental, cost and programme benefits.

Biczysko described how the native Lower Lias Clay was treated with lime and cement binders before compaction and assessed by moisture condition value. Lightweight dynamic plate devices were used as a simple quality control tool to evaluate the stiffness of the treated and compacted soil, and to ensure material consistency.

After sealing with a tack coat, the asphaltic layers were laid directly on the treated soil. The road was opened in May 2003 and its stability is being monitored using deflectograph surveys - the results of which will determine the long term stability of HBFM under traffic. A performance-based specification for HBFMs has been put forward as a model for future use of materials of this type.

The innovative use of vegetation for stabilising embankments was described by John Greenwood of Nottingham Trent University.

Vegetation offers a means of surface erosion prevention, but its use for other engineering purposes remains untested.

Greenwood described trials of a deliberately steepened embankment of the M20 planted with several types of vegetation. Some types, such as willows and alders, produced deep root systems extending into the native Gault Clay, and the tensile strength of these roots was considered to improve the stability of the slope.

The effect of water removal by vegetation was however generally masked by the seasonal variations in the water table and pore pressure changes were similarly heavily influenced by seasonal factors.

Greenwood described how the contribution of vegetation can be modelled in slope stability analysis and underlined the need for appropriate maintenance strategies to ensure that the vegetation remains effective.

The EU Ecoslopes project aims to develop better site equipment and a decision-support system to help geotechnical engineers to select and maintain effective vegetation-based slope enhancement regimes.

The EMGG takes its commitment to continuing education and professional development seriously. Recognising this, a one day workshop, organised by Paul Fleming and Matthew Frost, was held the day after the symposium.

This was aimed at providing younger engineers with high quality 'hands-on' experience and the opportunity to learn from experienced practising engineers through real case studies.

Delegates considered the issues affecting railway track construction options (with Barnaby Temple of Balfour Beatty Rail Technologies) and also got to work on a pavement assessment scenario provided by David Cudworth and his team from Babtie. The day ended with a brainstorming session aimed at understanding the causes of transportation problems in the UK and the issues surrounding an integrated transport policy, run by Steve Ison of Loughborough University.

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