Report on the BGA informal discussion on Modern Earthwork Practice, held at the Institution of Civil Engineers on 12 May 2004, by Richard Mellor of Mott MacDonald.
The design and development of modern earthworks has adapted to conform to changes in statutory, economic and social practice. High speed infrastructure and other improving technologies demand ever tighter tolerances on structural performance. Upgrades to existing earthworks must also be in line with modern practice, especially when considering the failure effects of more hazardous structures.
Changes in construction techniques, increased partnering and greater planning have had to evolve to comply with recent requirements for sustainable development. The informal meeting was devoted to giving an overview of these changing practices.
Alan Phear of Arup opened the meeting with a description of the Channel Tunnel Rail Link (CTRL) with emphasis on track bed design, mitigation against changes in underlying material stiffness and the effects of cyclical loading.
Jack Meldrum of Mott MacDonald went on to describe current practice in dam engineering, particularly considering the balance between economic and sustainable construction versus the potentially catastrophic effects of failure.
Philip Dumelow of Balfour Beatty described how recent changes to legislation now ensure that sustainable construction is often the most economical solution and as a consequence, how greater planning and earlier contractor involvement is advantageous.
Andrew Ridley of Geotechnical Observations concluded the presentations with a commentary on the effects of seasonally variable pore water pressures in clay fill embankments.
Keeping track of dynamic factors
With high axle load vehicles travelling at up to 300km/h, explained Alan Phear of Arup, the CTRL earthworks designers had to accept greater dynamic loading than would normally be required of other infrastructure projects. The track alignment had to be planned to ensure maximum passenger safety and comfort, necessitating very tight tolerances.
To ensure consistency in behaviour, the track bed required a reasonably similar stiffness over the various geological and structural obstacles encountered along the route (Figure 1). The design also had to conform to the requirements for satisfactory longevity, minimum cost and maximum re-use of excavated materials.
To meet these requirements, the CTRL track bed was designed using an empirical approach based on the International Union of Railways document UIC 719R and incorporating French TGV experience.
Within this classifiication underlying foundation material is categorised on a scale from S0 (Very Poor) to S3 (Good) but none of the material encountered during design of the CTRL classified as S3.The empirical approach ensured that the track formation stiffness was kept within tolerance by varying the thickness and type of imported fill subgrade. Confidence in the approach was confirmed by cyclical triaxial testing on selected material such as chalk.
Cuttings provided much of the fill required along the route, with the material end use based on a hierarchy governed by suitability.
The first priority was to re-use the material in railway and highway earthworks, but if it was not suitable then material was used for general landscaping. Mixing unsuitable material with quicklime allowed it to be used as general fill.
Where fill was surplus to the needs of the project, it was made available to third parties and only as a last resort was discarded as 'waste'.
Much of the earthworks fill comprised chalk or Folkestone Sand. The chalk was suitable but required careful handling while the Folkestone Sand was dense insitu but reduced in volume following compaction and was also susceptible to erosion during heavy rain.
Over consolidated clay was not used within any of the new railway embankments and was instead typically incorporated into landscaping.
Following French experience a gradual change in subgrade stiffness was required at the transition to underbridges. This was achieved using a combination of treated subballast and various types of fill, some stabilised with cement (Figure 2). The construction team carried out acceptance testing using 600mm diameter plate load tests at 40m centres along the trace. Automatic methods of recording compaction were trialled on Folkestone Sand and provided data for Quality Assurance purposes.
Requirements for modern dams
Dam engineers face a different set of challenges to those encountered on infrastructure projects, explained Jack Meldrum from Mott MacDonald, the most obvious being the requirement to design for water retention while at all times bearing in mind the potentially catastrophic outcome of failure.
Structures have to keep water in the reservoir without undergoing internal or surface erosion and must also provide a means of safely allowing floodwater to bypass the dam itself.
Few major dams are being built in Britain following the peak in construction experienced between the 1950s and 70s. Many new structures are small and typically used for irrigation and ornamental purposes. Existing dams are often over 100 years old and can present an ageing problem.
The majority of recent earthfill dams in the UK are of the homogenous or modified homogenous type. The industry has also seen an increased use of waterproof geo- membranes during the past 15 years, especially for construction of ornamental lakes or irrigation reservoirs.
Geomembranes have also been successfully used to increase the height of dams (Figure 3). Typically designs make use of 0.75mm or 1mm thick LDPE or HDPE, which are simple and quick to install, and are often used for the smaller reservoirs.
However, the membrane is only as strong as the ground that supports it and its integrity can be compromised by ground movements and unsuitable formation materials. Membranes may also be damaged by movement of the liner itself.
Groundwater or gas trapped beneath the membrane can damage the liner, such as in rapid drawdown of reservoir levels resulting in uplift pressure.
Drainage is an important component of dam design. Drains are required to both ensure water pressures within the earthwork structure do not exceed design limits and to dissipate any excess pore pressures during the consolidation process, and they must be designed to prevent internal erosion.
Drains are designed by particle grading to remain stable while stopping migration of surrounding fill into the conduit. Whereas geotextiles have been available for several decades and their use is generally widespread in earthworks, there is still a degree of conservatism in dam engineering with their use being generally limited to noncritical and maintainable areas.
It is becoming more commonly acceptable that floodwaters can overtop earthfill structures, which removes the need to build a dedicated spillway structure. Erosion of the earthfill is prevented by the use of concrete facing, reinforced grass, gabion mattresses and rip-rap or similar on both the down and upstream faces (Figure 4).
For permeable protection such as gabions or rip-rap, several bedding layers are required to prevent washout of fines from below the fill.
Impact of sustainable construction
From a contractor's viewpoint, the biggest transformation to modern earthworks practice has been the need for greater sustainability in construction.
Philip Dumelow of Balfour Beatty said it is not only the statutory requirements that are forcing change; increasingly it is recognised that the most sustainable option is often the most economic. The use of market forces has encouraged rapid switches in environmental attitudes that have outpaced the ability of traditional parts of the industry to adapt.
Sustainable construction requires a flexible approach to ensure all parties provide input.The ICE 5th Edition contract is considered restrictive, adversarial and prescriptive, while design and build and DBFO lump sum contracts are more flexible with greater contractor input but carry a greater long term risk. The early contractor involvement (ECI) contract encourages greater use of the contractor's knowledge and provides more time for planning.
Early input from all parties is a fundamental requirement of sustainable construction. A full appreciation of risk is required and good site investigation becomes essential to define earthwork materials and identify any potential contaminated land. During early stages, the alignment of infrastructure projects can be altered to balance excavation and filling along the route.
A surplus or deficit of material may still occur; factors such as taxation, local disposal sites, vehicle movements, considerate construction, design requirements, landscaping, fill material modification, local borrow pits and availability of primary materials must be considered to achieve an earthworks balance.
Dumelow described several examples where re-use or modification of material resulted in more sustainable, and consequently more economical, construction. On the Birmingham northern relief road, sand and gravel was excavated from cuttings, processed for concrete aggregate and replaced with clay from other parts of the site (Figure 5).
More generally, locally available secondary materials, such as PFA, tyres, blocks, bricks, china clay waste and slate could be suitable for use as bulk fill and replace primary aggregate.
The existing unsuitable fill on site could be improved by techniques such as lime/cement modification; primary aggregates increased in volume by mixing with sand and glass or topsoil extended with sand.
Similarly, sorting and screening of recyclable material can yield a source of aggregate with a corresponding reduction in the amount of disposal (Figure 6).
The use of recyclable material does however require increased support and co-operation from government agencies to be successful, particularly with licensing timescales and classification.
The meeting was concluded with a description from Andrew Ridley from geotechnical observations of pore water pressures measured in a clay embankment on the M23, which has a history of shallow slips.
Seasonal variations were noted, with suctions greater than 100kPa recorded at depths of 1m during the summer months and small positive pore water pressures recorded every winter (Figure 7).
Mike Turner of Applied Geotechncial Engineering asked for a comparison between lime and lime/cement stabilisation.
Phear replied that lime is typically used to improve fill while lime and cement are used to provide a structural volume of material.
Dumelow added that lime stabilised material was typically assessed using tests such as the CBR, while lime and cement was by cube strength.
Bill Lewis of Owen Williams asked if a sand blanket or geotextile was used to prevent pumping beneath the CTRL track bed. Phear replied that they were, but only in limited areas as the low cyclical stresses at the base of the subgrade negated use.
Tony Bracegirdle of GCG, and chairman of the meeting, queried if there were any upcoming changes to the current conservative design of dam drainage and if there were examples of trials on drains taken to failure.
Meldrum replied that the high potential hazard due to failure of dams meant that design would remain conservative, especially considering the ease that segregation of fines, say during transportation, could change the performance of the filter.
On trials, he added that he was aware that there were some being undertaken.
Regarding waste management licensing, Bruno Guillaume of Bechtel asked if the use of mobile plant rather than site licences could reduce the preconstruction planning time.
Dumelow replied that mobile licences are typically held by specialist contractors but works need to be done in house to be competitive.