THE OBSERVATIONAL APPROACH TO DESIGN OF A SHEET-PILED RETAINING WALL

The Brickpit structure at the Channel Tunnel terminal at Folkestone comprises a reinforced earth embankment supporting the railway line close to the tunnel portal. A principal constraint on the design was the need to found the structure close to and substantially below the power cables which link the British and French national grids. The economic cost of disruption of these cables was extremely high; the need to minimize such disruption was therefore paramount. The area had been worked as a brickpit which was infilled with unsuitable material from the construction of the M20 nearby. Excavation of the poor material, and a retaining structure to support the temporary excavation were necessary. Although other structural forms of retaining wall would have been stiffer, there were major attractions in the use of an anchored sheet-piled retaining wall. It was, however, necessary to show that this more flexible term of construction was capable of satisfactory performance. To this end, an instrumented trial installation of a short panel of sheet piling was made within the area of the brickpit to assess movements due to excavation, pile installation and extraction. This trial was back-analysed using the CRISP finite element program, and the results were used to predict movements of the full-scale structure. Instrumentation was installed on the sheet piles and on the power cables to monitor movements and to allow adjustments to the construction procedure in the event of unacceptably high movements being recorded. The Paper describes the ground conditions and site investigations. The effects of the wall on the adjacent power cables are considered and an observational approach to construction is developed which incorporates finite element modelling, a field trial and construction monitoring.