RETROFIT EARTHWORKS TO UNSTABLE RAILWAY EMBANKMENTS

Assessment of the Railtrack Scotland Zone earthworks assets identified several embankments which were exhibiting signs of instability. In one particular case, a 20m high, 600m long embankment at Airbles on the Hamilton Circle Line, instability in the embankment slopes caused settlement of the track formation and rotation of an OHLE foundation. In order to identify the causes of instability and to design remedial works, the site was scheduled within the Railtrack Scotland Structures Renewals and Maintenance Programme for 2001-2. Site investigations identified that the embankment comprised a clay core with ash shoulders overlying soft alluvial deposits, glacial till and rockhead at depth. The design of the remedial works comprised detailed analysis of existing failure surfaces in the side slopes to assess the causes of the instability and associated residual shear parameters. At this stage feasible remedial works options were identified and assessed with regard to safety during construction, timescale and cost, with an earthworks solution being identified as the most appropriate remedial works option for the site. The detailed design stage identified the soft alluvial foundation strata to be an important consideration with a requirement for staged construction (pause clause specification) to prevent bearing failure during the works. To allow safe and economic construction of the earthworks a series of vibrating wire piezometers were installed to monitor excess pore water pressures in the alluvium. Continuous remote monitoring of the instruments provided data from which it was possible to proactively control the rate of filling without causing the unnecessary delays which often arise from pause clauses in earthworks projects. Inert industrial waste materials including ash, slag and demolition rubble were used as fill material to minimise the construction cost and environmental impact. Extensive laboratory testing and visual assessment of the materials was undertaken both prior to and during construction to ensure they complied with the geotechnical specification together with the requirements of statutory environmental bodies. The works at Airbles have recently been successfully completed by Carillion Rail at a cost of approximately #1 million. The teamwork between all parties involved in the scheme was highlighted as a significant factor in the delivery of a safe, buildable and cost effective solution to the problems which existed at the site. For the covering abstract see ITRD E122683.