Business profile
were scraped from the cutting face and the top of the central spine using ultra- long reach excavators. The work reduced the angle of the face itself from the point where the valley gravel’s layer sits above the chalk, reducing the amount of material that sits above it and minimising the possibility of any slip failure. The finished face was also nailed and meshed to prevent any falls. O’Donovan said the scheme was ‘complicated’ but the team completed a project that the Victorians would admire.
the southern end of a 650-metre section of work is also contributing.’
A layer of gravels, stones and boulders
sits on top of the chalk strata which makes up this section of the North Downs. At the Hooley Cutting, the layer of gravel thickens, becoming less stable where the Victorian engineers cut through it and prone to landslips which have caused derailments. ‘It’s not so much the inclination of the
chalk strata that affects the slope stability but the layer of dry valley gravel above it. Dry valley gravel is just a geological term for the material - it’s anything but dry and can slip into the cutting,’ said O’Donovan, which is exactly why the stabilisation scheme has been carried out on behalf of client Network Rail.
Work at Hooley Cutting The BAM Ritchies team began work at the Hooley Cutting site over Christmas 2011 and stabilisation work took place in late 2012 into 2013. The cutting was broken down into four separate sections and the team installed the three stabilisation solutions. The four sections were: on the western
slope from the tunnel portal through to the first row of houses that teeter at the top of the cutting; then the Esso houses section a little further down the cutting; the new beam and grillage section and lastly, the central spine. In the two ‘houses’ sections the dry
valley gravels were stabilised through the use of 100mm diameter soil nails installed at depths of up to 14m at nominal 1.5m centres. The highest line of nails is encapsulated in a 430-metre crest beam installed along most of the 620m length of the cutting forming part of the new access and inspection walkway. Drilling and grouting for the 6,200 soil nails was carried out by three Rippamonte
Page 118 October 2013
hydraulic slope rigs, specially built for the BAM Ritchies team. ‘Despite its name, the dry valley gravel
material that we drilled into was neither dry nor particularly granular. It’s a real mix with the bore incapable of staying open after drilling. That’s why we grouted them up immediately,’ explained O’Donovan. Toward the northern end of the
cutting the team installed 21 new concrete columns along the face. These were crossed with the crest beam and a further beam at the lowest edge of the dry valley gravel with an intermediate concrete beam placed between the two. Due to access issues and the slope’s steep gradient, the reinforced concrete columns were placed with spray applied concrete techniques Across the railway line and in the
central spin, major earthworks were carried out to reprofile the dry valley gravel layer. Around 14,000cu metres of this layer
Crossrail In London, BAM Ritchies is heavily involved in the Crossrail project with compensation grouting works for the BAM Nuttall/Ferrovial and Kier JV’s contract, C300 - the western running tunnels currently progressing east from Royal Oak, under London’s West End and on to Farringdon just outside the City. This work, along the Crossrail route, is set to limit settlement caused by the tunnelling operations. Recently, the drilling and installation of approximately 40,000 metres of tube-a-manchettes necessary for the grout injection was completed and grout injection continues as the tunnels advance. Levels of potential settlement vary according to the different tunnel and excavation methods used. BAM Ritchies engineering manager
Owen Francis was part of the team ensuring that settlement is within agreed guidelines. ‘The maps of the route highlight every heritage structure or listed building along its length and the degrees of risk of settlement associated with the work. Obviously the project involves tunnelling beneath London, so there are plenty of issues,’ said Francis. Work is on schedule and grouting is expected to continue well into next year.
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