Innovative railway geotechnics

Reducing and eliminating delays to train operations is a priority to avoid inconveniences to the travelling public. Undertaking geotechnical work on slopes and structures close to the track is often time-consuming and therefore difficult to achieve in possessions.

As one of the country's leading geotechnical contractors carrying out ground investigation, ground engineering and concrete techniques work on the railway infrastructure, Ritchies is committed to exploring solutions that not only minimise costs to the client but also reduce and eliminate the impact that projects can have on train operations. Examples of this commitment may be seen in the recent jobs that the company has undertaken on cuttings and embankments.

Safe execution of ground investigations on steep slopes, which are frequently covered in loose ballast and ash, has taxed the minds of clients, consultants and ground investigation practioners. In addition, employers now take stringent measures to eliminate risks caused by manual handling and the causes of handarm vibration syndrome (HAVS) to employees. These various requirements are resulting in greater use of innovative mechanised plant and equipment.

Ritchies has always been in the forefront of developing special rigs and equipment. The recent deployment of the company's radio remote-control Sherpa rigs is a clear example of this philosophy.

Following the success of the original Ritchies-designed and manufactured Sherpa steep-slope drilling rig, the company has recently added a second one to the fleet, with a third currently being built. The first two have recently been hard at work on a railway embankment at Hook in Hampshire for Network Rail through the Southern Zone Construction Partnership framework contract.

The fact that the second and third rigs are identical to the original prototype confirms the outstanding original design and manufacture by Ritchies' Plant Department.

The mast assembly is a standard 'Terrier' windowless sampling rig. This is mounted on an articulated frame containing the power pack and hydraulics. The frame hydraulically self levels regardless of slope angle to a maximum of 50 degrees. This unit is mounted on a hydraulically driven tracked carrier.

All the functions of the Sherpa rig are radio remote-controlled. This enables safe operation on steep slopes since the operator can view the rig moving up and down the slopes from a position of safety. The unit is equipped with a winch and is small ~ enough to be transported on a fourwheeled trailer towed by a Land Rover.

The Sherpa rig has worked on contracts across the country from the south east to the far north of Scotland. It has proved to be particularly effective on steep slopes up to 45 degrees where the surface would prevent the use of any other type of rig or the use of scaffolding.

Ritchies' railway ground engineering activities continue to be an important part of the business with several recent contracts having been successfully completed.

Slope stabilisation work has been a particular skill that Ritchies has embraced with major recent contracts at Flax Bourton and Saltford near Bristol, Copyhold near Haywards Heath, and on the revised Airdrie to Bathgate line in Scotland.

The Copyhold Cutting is located near to Haywards Heath on the main London to Brighton line and is over 200m in length and up to 30m high.

A series of landslides occurred in the weathered clays leading to speed restrictions being placed on the line. Emergency responses shortly after the landslides allowed speed restriction to be removed, however Network Rail wanted to remove I the risk of further occurrences through a j permanent slope stabilisation scheme.

Initially Ritchies undertook 'Panda ~ Probing' of the slope face via rope access techniques in order to determine the weathered profile of the mudstone cutting. From this a design was developed by Tony Gee and Partners, working closely with Ritchies, in order to negate the need for possession work and allow trains to operate without restriction.

Initially the cutting was de-vegetated and Ritchies' sister organisation Nuttall undertook the re-grading of the highly weathered top section of the cutting with Ritchies specialist rope access personnel. In total 6000 tonnes of spoil was removed from the top half of the cutting using long reach excavators, reducing the slope angle from 50 to 30 degrees. Removal of 100 root balls in this re-grade area was completed again with specialist rope access techniques. Ritchies personnel on the face of the slope communicated with the long-reach drivers via a CCTV camera and microphone mounted towards the end of the excavator's dipper arm.

A 250 metre 'mid-slope barrier' to prevent debris falling down the slope was designed and constructed by Ritchies prior to the start of the main works, so allowing trains to run at linespeed (90mph) whilst the works took place, thus eliminating disruption to the travelling public.

With the slope re-graded, the main stabilisation works consisted of:

• 54 No. raking anchors and 54 No. micropiles installed along the crest, forming the basis for a concrete top anchor block
• 200 No. face nails drilled using air masts mounted on longreach excavators.
• 194 No. face and toe anchors drilled using four Terrapin rigs in-house designed rigs, supported temporary and permanent anchors
• Nine-metre face nail~ 25mm diameter GEWI@ bar grouted with 40Nmm-2 neat OPC grout.
• Bottom anchors of a combination of GEWI@ bar and 9m Geobrugg wire rope anchors again grouted with 40Nmm-2 neat OPC and formed with a surrounding thrust block.
  Once the soil nail installation was completed, the slope surface treatment consisted of:
• Top-soiling and grass seeding of the regrade area an of North American Green
  (NAG) C350 biodegradable matting to stop erosion
• Standard Maccaferri rock netting held in place by plates
• The lower slope highly weathered areas treated with Macmat-R
• Areas of relatively un-weathered ground treated by rockfall netting to contain loose material
• The Macmat-R and rock net curtain are supported by a series of vertical and horizontal steel cables, held in place by the anchor blocks.

For further information, please contact David Gibson, business development manager:

BAM Ritchies
Tel: 01275 875338
Email: David.Gibson@BAMRitchies.co.uk
Web: www.bamritchies.co.uk

Article courtesy of Railway Strategies - June 2008