Stabilisation of Tutshill North cutting

September 19th 2008

Tutshill North cutting runs northeast/southwest just to the north of Chepstow on the line to Gloucester. White Young Green was commissioned to undertake an assessment of the cutting following a history of minor soil and rock instability. The cutting is almost 1km long running through changing geology which includes Mercia Mudstone, Carboniferous Limestone and Dolomite – all presenting a variety of stability and rockfall risks throughout. After initial ground investigations and the Form A and Form B design development process, the main works contract was awarded to Alfred McAlpine Project Services.

Regrade scheme

Access constraints required a specialist geotechnical roped-access contractor. CAN Geotechnical were commissioned to carry out the geotechnical aspects of the works. Over time, the original cut through Mercia Mudstone had become over-steepened and highly weathered. This had led to numerous small scale planar failures presenting risk to the operation of the line. A wide crest at the top of the slope in this section of cut enabled the slope to be stabilised through a regrade scheme. A stable batter was achieved using long reach excavators working from the crest.

To ensure that a safe working stand-off at the top of the crest was adopted, a design was developed using slope stability analysis to model the surcharge produced by a 45t machine. This meant the excavators were working blind and were carefully guided using ’walkie talkies’ as they removed the unstable slope mantle and weathered material.

High tensile steel mesh

The new cut slope was protected from weathering with the installation of erosion control matting and the spreading of grass seed. Several drainage measures, including a crest cut off drain, inclined face drainage holes on the slope and a cess drain were installed. These measures are designed to drawdown the piezometric level within the slope to provide additional stability in the long term and to reduce weathering processes.

To complicate matters, the regraded slope interfaced with existing geological structures and a steeper limestone cut. At these locations, the slope regrade angle steepened to meet the adjoining structures and was more in line with original slope conditions which had proved to be unstable. To combat this potential area of instability, high tensile steel mesh, which provides active face restraint, was installed to overlap the edges of regrade and structures and was secured with the use of fully bonded steel ground anchors.

The rock cut through the Carboniferous Limestone was much steeper and presented rockfall hazards rather than slope instability of the Mercia Mudstone. At the feasibility stage this section of the cutting was heavily vegetated and its surface highly weathered making assessment difficult. For this reason an observational method of stabilisation was adopted where the rockfaces were devegetated and loose rock scaled before rock bolts were designed for specific features. In highly weathered areas, spray concrete was used to provide additional support.

Tightly fixed passive rockfall mesh

Passive rockfall mesh was installed over the newly scaled face to control further rock hazards. In this case, the mesh was tightly fixed to the face and reinforced with diagonal steel wire cables to profile the mesh to the face and provide additional support. This was designed to minimise future maintenance of the system by holding loose rocks in place and prevent incursion into the kinematic envelope of trains passing close to the profile of the cut face. At critical structures, such as the bank seats of bridges, an active support measure using high tensile steel meshing was adopted to ensure stability.

The works were completed over a six month construction period using several 50 hour abnormal possessions at weekends and green zone work during the week. The design encompassed a range of geotechnical processes, allowing risks to be controlled in a cost effective manner in keeping with the natural environment. The greening of the Mercia Mudstone cutting has been particularly successful, with both a visually attractive and sustainable end product. The high quality of initial Limestone scaling works allowed a significant reduction in subsequent rock bolting works, facilitating cost savings to Network Rail.

Copyright Grahame Taylor.

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