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Contract 440

SELF-CERTIFICATION

The family of NECs places less emphasis on legal obligations and liabilities and more on greater flexibility, good project management, simpler and clearer expression and mutual trust and co-operation within the project team. To improve quality, the focus is on quality assurance plans, quality systems and responsibility for quality of the entire team.

An advantage of the NEC is the ability to refine the design with the contractor's input to suit plant, materials and conditions prevailing at the time of construction.

An example of this occurred on Contract 440 where the CTRL downline crosses an area of made ground extending to 15m below proposed rail level. The made ground was a combination of construction rubble and soft reworked Gault Clay and had been used to backfill a sandpit, part of a river valley and for landscaping on the M20 and Dollands Moor Freight Terminal.

It was anticipated that, because of previous surcharging, the made ground would be suitable for track formation. However, once it was exposed, it was clear that the material was more varied and also softer than expected.

Conventional excavation, even to partial depth, would have been difficult because of the proximity of the M20 and Dollands Moor Freight Terminal.

Disposal of the excavated material would also have been difficult.

Blackwell was able to mobilise at short notice a high energy impact compactor from Landpac Ground Engineering and a trial was undertaken.

The three-sided compactor passed over the made ground until an average induced settlement of +10mm for the last five surface passes had been achieved. Although this provided a good identifier of weak ground, only marginal improvement was achieved in the top 2m, based on the results of before and after CPT testing, probably because of the cohesive nature of the material. An alternative solution was sought.

A series of trials was carried out involving varying combinations of over-dig depths and sizes of stone in the back-fill. Final design was a 0.8m deep over-dig below formation level, backfilled with a 500mm thick layer of rock fill - 400mm size boulders processed on site from 'doggers' - table-size lumps of sandstone - from the Folkstone Beds.

This was covered with 300mm of Class 6C fill starter layer, followed by a layer of Terram 3000 geotextile at formation level and a 500mm of selected granular material to form the prepared subgrade.

Finally, a 200mm sub-ballast layer of granular material will be laid, followed by track ballast.

Balfour Beatty Major Projects site laboratory manager Ben Brown, RLE senior geotechnical field engineer Lee Parry and CA Blackwell (Contracts) contracts manager Jess Yates.

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