Tony Gee and Partners LLP - Article - Cutting Edge Solution at Winterbourne

Cutting Edge Solution at Winterbourne

Julian Case

Senior Geotechnical Engineer, Tony Gee and Partners

The rock stabilisation scheme currently underway at Winterbourne railway cutting near Bristol has demanded the latest techniques from its designer, Tony Gee and Partners (TGP). Their design used a high definition 3D LIDAR (laser scanning) survey to provide rock mass data for stability modelling. This advanced technique has optimised the stabilisation works, so ensuring safety while minimising the visual effect on the natural environment.

	It aims to provide a coordinated approach to the maintenance and renewal of structures and earthworks in the Great Western region developing best practice and establishing best value by year-on-year cost reductions through efficiency and innovation. Tony Gee and Partners (TGP) is one of a number of design consultants working for GWESPA and has the largest share of work, primarily because it has a permanent staff presence in the shared offices in Stonehouse, Gloucestershire. The background information is that the cutting includes 970 metres of rock face (interbedded sandstones and mudstones) up to 15 m high. In 2003 a rockfall in the cutting was proof of local instability on the faces, and this threatened the operation of the London to South Wales main line. Following an emergency stability assessment of the whole cutting, it was judged there was time for an in-depth study of the rock mass before the appropriate remedial work needed to be carried out.
		Cutting edge laser scanned helped TGP optimise work at the Winterbourne Cutting, near Bristol, writes Julian Case in Ground Engineering magazine, Nov 2005. (Left) Figure 1 : LIDAR point cloud data plot from which rock joint characteristics were extracted using Split FS software.
	TGP was appointed by GWESPA (Great Western Earthwork Structures Partnering Arrangement), the partnering arrangement between Network Rail and Alfred McAlpine, to provide the remedial design for the scheme. As usual, there was an underlying requirement for TGP to produce the most efficient and cost-effective design. However the situation at Winterbourne was complicated by the exposed rock in the cutting being a geological Site of Special Scientific Interest and thus protected by law, which also required the design to limit the visual effect and physical extent of the stabilisation scheme. The initial design phase used traditional field inspection and survey techniques, which maximised the cost-benefit within a short time. Assessment of this data led to the rock faces being divided into zones of differing risk, and each zone was given initial remediation recommendations. The LIDAR survey was then commissioned by TGP as part of the detailed design for the zones of high risk, to refine the initial recommendations. The use of LIDAR data in this way is believed to be the first commercial application for railway work in the UK; indeed, this field use has helped to develop the software package, which is currently at its beta test stage.

	Figure 2 : Winterbourne cutting, near Bristol - ground anchor installation.

	The two main benefits of the LIDAR technique were that it rapidly provided very detailed structural data while minimising disruption to train movements, so it replaced the more prolonged traditional methods such as trackside line scans, or roped access within available possessions. The use of LIDAR was enhanced by two fundamental factors: the rock mass having a blocky (and thus ‘reflective’) nature, and most of the face not being obscured by vegetation. A LIDAR survey provides “point cloud” data, so for Winterbourne cutting it gave millions of digital data points that effectively reproduced the rock surface. The initial processing of the LIDAR data was carried out by Split Engineering at Arizona State University, and the interpretation of the results, including stereonets for stability analyses, etc, was by TGP engineers. The final design solution at Winterbourne cutting was a significant reduction on the initial scope of works (and thus it has reduced the eventual remediation cost), and this was a direct result of the use of the LIDAR data. However the work still involves 1400 rock anchors being installed on occasion at a rate of around 90 per track possession, and these secure a combination of high tensile ‘active’ constraint netting and lower strength passive netting. The Winterbourne stabilisation scheme is scheduled to be complete at the end of October 2005.
	Figure Credits All Figures, Tony Gee and Partners. Article originally published in Ground Engineering, November 2005