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FEAT OF CLAY

GEOTECHNICS OF TRANSPORT

Network Rail is researching the behaviour of clay embankments to extend design life and prevent failures.

Specialist instrumentation has recently been installed at two embankments for Network Rail in its South East Territory.

One is at Pound Green on the Reading to Basing oke line and the other at Magnolia Road on the Shenfield to Southend Vic oria Line, both heavily traffiked London commuter lines. The embankments predominantly consist of London Clay derived fill, overlying insitu London Clay.

The monitoring is part of a project to improve the embankments and help alleviate poor track geometry ('rough rides') and the risk of progressive earthwork failures.

It follows consultant Arup's extensive literature review and reference document on the management of London Clay earthworks carried out for Network Rail in London North East Territory.

A similar project is being carried out in London North West Territory and it is hoped the results will help Network Rail's engineers understand the implications of removing mature trees from earthworks.

The South East Territory, which is a combination of the former Anglia and Southern Regions, has a number of high plasticity clay earthworks showing failure. These are associated with high levels of track maintenance and poor track geometry.

Derek Butcher, South East Territory earthwork and drainage engineer, explains: 'These two earth forks are over one mile [1.6km] long and clearly stan ard techniques of earthwork remediation are not economic.

'Current techniques for embankment failure preventive works range from £2500 to £7000 per linear metre, so standard approaches to remediation would cost in the region of £6M-10M per site.

'We hope to be able to remediate the most necessary locations using techniques such as piling and soil nailing, and come up with some softer, drainage or vegetation management techniques in between, ' he says. 'Vegetation clearance would be over and above that done to prevent signals passed at danger and leaves on the line.

'Most earthwork engineers know clearing earthworks of vegetation and installing drainage has some effect, however we are unsure as to the exact impact at different times of the year, response times and serviceability implications.

' For these particular earthworks, which are probably the most sensitive to seasonal movements in the south east, we hope to be able to come up with a cost effective and well engineered management plan.'

Geotechnical instrumentation will help determine why the earthworks are performing so poorly. Vegetation and seasonal variations are thought to be the main culprits.

The earthworks will be monitored for a year to determine a baseline of seasonal fluctuations. Then the effects on the embankments of different approaches to managing vegetation and drainage will be investigated.

A rain gauge has been installed at each site to enhance knowledge and use of soil moisture deficit as a means of predicting wet weather embankment failures and dry weather desiccation. Both these types of event can cause speed restrictions and delays to passengers as a result of poor track geometry.

Instrumentation is mainly being installed and monitored by Geotechnical Observations, while Contractor Geotechnical Engineering is drilling the boreholes and Arup is managing the work.

As well as inclinometers and standpipe piezometers, more sensitive instrumentation on the sites includes magnetic extensometers to monitor vertical deformations and GeO flushable piezometers capable of measuring soil suction (negative pore water pressures up to -90kPa).

Lankelma has installed a number of BAT piezometers at the Pound Green embankment using its UK11 road rail CPT rig, as well as undertaking continuous CPT, CPTU and moisture probe testing (GE April 06).

Geotechnical Engineering used its P45 and P45A rigs to carry out the investigation and to install the instrumentation.

This percussive 'slope climbing' rig is able to drill up to 100mm diameter holes to about 15m depth on slopes up to 45infinity (GE news April 06), although at the embankment sites the deepest holes go to 12m below ground level.

Butcher says use of the rig is a big improvement on window samplers, which have commonly been used to investigate steep railway earthworks over the past 15 years, but often cannot penetrate deep enough to fix instruments securely.

Arup has teamed up with Professor William Powrie and Joel Smethurst from the University of Southampton, who are providing expertise on the behaviour of infrastructure earthworks.

The university provided and installed TDR theta probes and neutron probes. These measure seasonal and spatial variations in volumetric moisture content in the embankments. The university has also taken a number of high quality block samples of the fi ll material for laboratory testing.

Arup senior engineer and project manager Chris Martin says: 'This has been an extremely challenging project, not only from the technical perspective, but also allowing for the significant logistical, commercial and health and safety implications of working in the railway environment.

We await the first monitoring results with bated breath!'

Playford Mere repair

Repair work was recently completed on an embankment for Network Rail in Suffolk.

Known locally as Playford Mere, the site carries the East Suffolk Railway between Ipswich and Lowestoft.

Construction of the East Suffolk Railway here in 1858 was a challenge because of the soft alluvial soils of the notorious Mere, which made building the approach embankment and wooden viaduct difficult.

The amount of material consumed by the Mere was substantial and the settlement so large that it was decided to relocate the railway alignment and replace the viaduct with a solid embankment on firmer ground.

This hugged the side of the River Fynn valley rather than crossing the Mere.

Work was substantially complete by 3 March 1859 and the East Suffolk Railway opened on 1 June that year. The borrow pits are still visible today, as are buttresses to the present embankment where failures occurred during heavy rain the winter after they were finished.

The location of the railway continues to cause problems for engineers. Two recent failures led to speed reductions on the line from 55mph (88.6kph) to 20mph (32kph), and left a vertical slip backscar of 3-4m high in the 6m high embankment, compromising the track support zone.

The most recent repair, consisting of piles and a regrade, has just been completed by Network Rail's East Anglia civils framework contractor, Kier.

Piles were chosen as the embankment toe is underlain by peat and alluvium. It appears locally won Norwich Crag (a sand) was used in construction of the embankment. The succession of sand embankment, alluvium and London Clay means that spring lines are common under the embankment.

Poor drainage through the embankment and softening of the toe are the probable cause of failure. The 12m long piles were driven using Giken's UP150 U sheet pile press-in method. The technique is silent and vibration free, with the added advantage that a support crane was fixed to the piled wall and not located on the boggy ground.

After piling the designer, URS, specified that the slope be regraded with Class 1A material to a safe angle of 1 in 2.

This was overlain with topsoil, seed and biodegradable geo-jute to ensure a green finish in the environmentally sensitive area.

The speed restriction, set in September 2005, was removed in March 2006. The seven-month programme included ground Research into increased loading on clay embankments Network Rail is also keen to establish the impact of increased loading caused by railway traffic on embankments built with clay.

This project will be carried out with the Rail Safety and Standards Board. It will look at porewater pressure variations and seasonal shrink-swell movements, which can cause local surface movement or desiccation. Both are exacerbated by cyclical loading from trains. The action of train loading can result in dishing of the centre portion of clay embankments, contributing to overall settlement.

Subsequent topping up of these embankments with ash (from steam train boilers) or ballast to restore track level, has resulted in granular material in pockets underlain by an impermeable clay. Water often becomes trapped in this area and pumping due to the cyclical loadings at the clay/ballast interface can result in poor track performance or failure of the slope Although deterioration of clay slopes occurs naturally over time, there has been an increase in such events over the past 10 years, particularly where there has been an increase in rail traffic loading due to improved speed or higher axle loads.

Network Rail is keen to understand whether there is a clear link between the increase in embankment damage and the increased traffic, rather than gradual deterioration associated with ageing, high rainfall and seasonal effects.

The project will look at embankment damage in two phases. Phase 1 will obtain data on the changes in the engineering properties of embankments where damage occurs due to an increase in railway live loading. Phase 2 will provide an understanding of the damage mechanisms of particular soil types and embankment geometries.

Sites are yet to be chosen. However the Gospel Oak to Barking and North London Line, which link the growing Felixstowe and new Shellhaven ports to the upgraded West Coast Main Line, are expected to see a big change in freight traffic.

The number of intermodal freight trains using the two lines is likely to have doubled by 2023. Construction materials traffic is also due to increase by 10% over the North London Line.

Network Rail South East Territory earthwork and drainage engineer Derek Butcher says: 'It is a concern that we could remediate an embankment in accordance with loadings specified in our design standard and think we have a 120-year design life, only for loadings to change in 20 years with a resulting reduction in that design life.

'The impact on embankments of cyclical loadings from the increase in freight traffic is not understood and this study will help us to prioritise which earthworks need to be enhanced first and to specify the right parameters for design.'

It is hoped the study will improve understanding of cyclical loadings on earthworks when carrying out slope stability analyses for renewals and enhancements.

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