Stabilising Hooley Cutting Issue 96 of the rail engineer (October 2012), covered the strategic approach being used by Network Rail to manage its earthworks and drainage assets. This month’s article looks at work being carried out on a classic example of a structure that a railway engineer would consider might cause serious risks to the network.
Lying just south of Coulsdon South station on the London to Brighton main lines, Hooley Cutting is actually two cuttings, running roughly parallel, where the four track route divides into two pairs. On the Down side are the fast lines, or Quarry Lines, and on the Up side lie the Redhill, or Slow Lines. This separation is required by the presence of the Merstham and Quarry Tunnels and the topography of the area to the south of the site, the two parallel tunnels each carrying two tracks.
The sides of both cuttings are extremely steep by modern standards. There have been a significant number of cutting slips here, with two derailments caused by such events in the last twelve years. The cuttings were thus an obvious priority for remedial works.
Dry gravel that isn’t
The reason for the instability of the cuttings was more complex than just the steepness of the slopes. The Quarry Lines cutting appears to have the steeper slopes of the two, but Network Rail considered this to be a lower priority than the neighbouring Redhill Slows cutting on the basis of both the numbers of incidents which have occurred in each and the conditions found by inspection and monitoring.
The difference in behaviour between the two cuttings is caused by the geology of the site. The underlying ground is chalk, and the greater proportion of the depth of both cuttings lies in relatively competent chalk. However, throughout the site, the chalk is overlain by ‘dry river gravels’. Despite the geological name applied to them, these gravels are not necessarily dry!
The name reflects the fact that they are gravels deposited by a river or rivers that have now disappeared, or dried up. In actuality the materials may well be very wet after rain, and so become unstable. The depth of these overlying materials varies widely, and sometimes quite suddenly, from place to place. This follows from the fact that there are old river channels which were cut into the chalk long ago and were left full of sand and gravel after the river moved or retreated. This feature is evident when looking at the works, where the dry river gravels show as dark intrusions into the white chalk of the newly exposed slopes. The chalk stratum dips across the railway, meaning that the depth of gravels overlying it is greater in the case of the Redhill Slows cutting, causing that cutting to suffer from greater instability.
Visiting the site on a very wet day in the company of Network Rail’s scheme project manager Stuart Jones, his colleague, construction manager Mick Hull and BAM Nuttall’s project agent Andrew O’Donovan, it was possible to get a good view of the site from above, as this is the way into the works for all purposes. The depth of the two cuttings and the steep side slopes are immediately apparent from the Crossing Forge Lane overbridge. Despite this, and the nature of the works being undertaken, Network Rail and their contractor BAM Nuttall had contrived a safe method of working that was allowing trains to proceed below at full line speed.
How have they achieved this? The first tactic was to prepare the cutting appropriately before the start of the main works. This was done in December 2011, when catch netting was installed over the whole of the affected area of the cutting slopes on both sides of the line. In addition, the first lift of scaffolding was erected on the Up side of the line at the foot of the cutting slope. The netting was designed to catch any debris that might fall down the slopes as the main works proceeded, preventing it from falling onto the line. The scaffold lift was to become the foundation for the much higher scaffold structure which now forms the main work access to the Up side slope. It also forms an additional safety barrier preventing anything falling onto the tracks. Access to the scaffold is now via an impressively tall temporary scaffold staircase adjacent to Forge Lane bridge.
Another key preparatory task was the work undertaken by Network Rail to ensure access to the top of the cutting on the Up side throughout its 650 metre length. Here the greater proportion of the boundary is occupied by private houses and a few commercial premises. The company acquired two properties which were demolished to allow site accesses to be constructed. Temporary access to 24 other rear gardens was required as well, and Network Rail had to negotiate and agree these arrangements with the relevant parties before work could start.
Drilling, nailing and meshing
In March 2012 the main works began, with a planned duration of 12 months. 12,000 cubic metres of material is being removed from the Down side cutting slope to reduce the quantity of weak material from the top of the slope above the chalk and to flatten the angle of this slope. The Up side will receive a much lighter removal of face material in specific locations before the installation of soil nails and mesh to the slope. All of this is being carried out from the tops of the slopes using a combination of hydraulic slope-climbing drill rigs and normal, medium and long reach excavators. All of the central spine slope and most of the Up side is being permanently strengthened by a technique involving soil nails. The top tier of these is to be encapsulated in a crest beam along the top of the cutting slope, this doubling as a walkway for future access and inspection. Below this, further tiers of soil nails and meshing are being applied to the top two thirds of the slopes, stabilising the dry valley gravel material.
The soil nailing is quite unusual, as the three drilling rigs, which are powered hydraulically rather than by the more usual compressed air, have been specially built for the site by Rippamonte, the specialist Italian company, and supplied by Atlas Copco UK. They are being operated using roped access techniques whereby both rig and operator are supported from the top of the slope by approved temporary anchors. The operators are all appropriately trained and experienced for this specialist work. The hydraulic power packs sit below, on the scaffold platform. The system is very quiet in operation, even when the rigs are drilling percussively as is necessary when dealing with the lenses of flint sometimes encountered. When rotary drilling alone is in use, the rigs are hardly audible from above.
The soil nails themselves are also unusual, being hollow threaded Dywidrill bars fitted with disposable screw-on drilling bits that are left in-situ. Both bars and bits are supplied by Dywidag Systems. The reason for using hollow bars is that it is then possible to pump the required grout through the bars via flushing holes incorporated in the drill head and out through the drill bits. This means bore stability and a grout annular is always maintained, saving the need to case the holes as the drilling progresses. Installing, and later recovering, drill casings is time consuming and would be very difficult in the circumstances of this site.
BAM Nuttall’s choice of this methodology was based upon their past experience and specialist knowledge of this type of work. It means significant reductions in risk, together with appreciable savings of time and cost. Around 6,200 soil nails are to be installed during these works, some as much as 14 metres long. As this means something like 58 kilometres of drilling, it can be appreciated that even a small saving in the time taken to install and grout each nail will have a significant effect on overall timescales and cost.
The mesh being used is another innovation. Deltax mesh is being used here for the first time on a railway site in the UK. Produced by Geobrugg and supplied, like the soil nails and drill bits, by Dywidag, it has a special coating that is warranted to give better corrosion protection and a longer service life than conventional galvanising. In fact, both the soil nail system and the mesh are designed for a 60 years lifespan, important when around 18,000 square metres of mesh are to be used on the site.
On the Up side, one section of the slope has been strengthened in the past by a different system. This employed a concrete “beam and grillage” system to support the slope. A further section is to be treated similarly under the current project. Twenty-one vertical concrete columns are to be built by concrete spraying against the cutting slope at intervals over the affected length. At the location of each of these columns, before it is sprayed, soil nails are being installed to support it against the horizontal and vertical components of the forces that will act on it.
The beam and grillage framework contains vertical soil nails at the base (foundation point) and conventional raked soil nails throughout the remaining area. The grillage system will be completed by spraying horizontal concrete beams across the slope between the columns and mesh will be installed. In all, some 200 cubic metres of sprayed concrete will be applied.
Work is progressing well despite the heavy rain experienced across most of the country. However, the project has experienced some delays, and for a rather unusual reason – Roman snails! These creatures are quite impressive beasts, considerably larger than the sort of snail we are used to in the UK, and rare enough to be a protected species. It seems that they were originally brought to Britain by the Romans, who liked eating them and wanted a ready supply here. They have survived in a few places ever since, and this colony are apparently in the habit of hibernating on the slopes of the central spine of the site.
They emerged shortly after the contractor began the main works, causing excavations to be stopped for about five weeks whilst permission was obtained from Natural England and the snails were collected and relocated. Around 400 were found, and they have been removed temporarily to another suitable site on Network Rail land, to be returned to their original location upon completion of the current works.
By the time that the snails are back in their original home, in March 2013, Network Rail should have a much safer cutting with no further risk of derailment or disruption caused by slips. It seems likely that the decision to use a contractor with significant expertise and experience in similar works will have paid off well, not only through quality, cost and time benefits, but also because their expertise has ensured that the works could be undertaken with no restrictions on train speeds, without any requirement for unplanned possessions and with limited disruptive possessions of the line.
However, this won’t be the end of works at Hooley. Before too long, Network Rail will be back to stabilise the parallel Quarry Lines cutting.