It must have been a great sense of relief for everyone when Network Rail announced on Friday 13 March, three weeks earlier than the expected date, that the line between Leamington Spa and Banbury was now open and safe for trains to run. This was less than six weeks after the 350,000 tonne landslip extending along more than 150 metres of embankment had occurred at Harbury Tunnel cutting on 31 January.
The route carries more than 50 freight trains and 80 passenger trains every day and, although the two tracks were not directly affected by the new slip, it was clear that the potential for further movement was significant and therefore the route had to be closed and a solution quickly found.
Largest man made cutting
The railway, built by Brunel, came to the village of Harbury in 1847 as part of the construction of the main Oxford to Birmingham GWR line. The cutting is located to the north of the village and, at the time, it was considered to be a significant engineering feat. At over 34 metres deep the cutting was the largest man-made cutting in the world that was dug entirely by hand through Blue Lias clay. It was completed in 1852 and, as was often the case, the workers employed to build the line lived in housing constructed in the village.
There is a history of geotechnical failures associated with the cutting so it has been closely monitored for any movement for many years. When the landslip occurred, work was nearing completion on an earlier shallow, one metre deep, 20 metres wide landslip at the same site and J Murphy and Sons, Network Rail’s emergency earthworks contractor in LNW, was on site completing this work valued at £2 million, re-grading the slope, installing counterfort drains and removing debris from the toe of the cutting.
Just before Murphy had been due to start permanent works on the two-metre slip in the summer of 2014, our friend the Great Crested Newt, surfaced. The site is already a SSSI because of its butterflies and there are bats and badgers as well, so appropriate protective measures were put in place. As this took some time, this work could not start until September 2014, with the onset of much wetter conditions.
Slips were a regular occurrence
The wetter conditions did not help the stability of the cutting at all and small slips like the one being repaired have occurred on average every four or five years. During 2013/14, the rainfall in this area was very heavy. The geological mapping of the area showed that there was completely or partially-weathered Blue Lias clay, embedded with mudstone, resting on a near horizontal limestone band which was jointed to form blocks of various sizes and allowed water seepage after rainfall. However, unbeknown to the engineers involved, a vertical fault line of limestone backed by mudstone ran along the cutting parallel to, and located approximately 35 metres away from, the railway tracks.
This impermeable fault line was effectively facilitating a head of water that was ten metres higher behind the fault line than in front. After heavy rainfall, it is hypothesised that this vertical differential caused the latest slip to shift in such a dramatic way.
Karl Budge, Network Rail’s route delivery director, explained that this fault zone made its presence known as the earlier planned work was nearing completion. Small tension cracks appeared along the fault zone and initial movement of approximately 10mm per day was being monitored. The deepening of the counterfort drains helped to release the water pressure to a certain extent and stabilise the slope. However, counteracting this benefit was the previous removal of material downside of the fault line. This reduction in loading toward the toe of the embankment further exacerbated the problem and, in late January, the earth started to move.
Conventional methods for monitoring the slope’s stability had proved adequate for many years and there was a grid system of pegs and inclinometers in place. Engineers were carrying out theodolite monitoring on a regular basis, plus there was a full time watchman on hand at the time. He was the person who first noticed the initial movement over a 160 metre length of the cutting. Even though the tracks themselves had not been affected by the movement, both lines were closed as a precautionary measure.
The slip stabilised after about five days. As a precautionary measure, matting was placed over the tracks in case of any further ground movement. RJM Ground Solutions had been working with Murphy on the smaller slip site offering design advice and it was the first, along with the Network Rail Asset Management team and the project engineers, to consider the nature of the failure mechanism for this far-more-significant and disruptive incident.
The closure of the route was causing significant disruption. A number of local politicians and the managing directors of freight operators, CrossCountry and Chiltern Railways all wanted to visit the site. Mark Carne, chief executive of Network Rail, also went to see things first- hand.
Before joining Network Rail, Mark had worked in the oil industry and he recommended that the team should widen its geological outlook and approach Schlumberger International for advice. Karl thought that this move, plus peer reviews from other geological experts, was very helpful, reassuring the team that the right approach was being taken. As he pointed out, ground engineering is not an exact science.
As a team from Murphy was already on site, it was able to react immediately. Around the clock working was introduced immediately, working twelve-hour shifts. It was recognised that fatigue would be a significant risk so it was decided that the workforce should live close to site. Appropriate facilities were installed in the site compound including a cook and canteen guaranteeing good hot food around the clock. On average there were 50 people on day shift and 30 at night throughout the work and the workforce stayed in local accommodation.
An exclusion zone was immediately imposed on and below the slip. Also, the tunnel portal was deemed unsafe until further investigation. A specific safe system of work was introduced involving a rope access system and Murphy was given overall control of all site access, monitoring and controlling who and why anyone needed to enter.
There were 30/40 machines working at any one time. To ensure safe segregation of plant and pedestrians in the tight working area, two experienced and very capable Murphy foremen were put in total control of all site movement at all times, negating the requirement for multiple banksmen to be used. To assist them, Murphy ensured that there was a fulltime safety professional on site. In addition, Murphy brought in its highly skilled, directly employed personnel to bolster the site team already allocated to the project and cover the emergency.
Given the high profile media interest, and knowing that VIPs would want to visit the site, a viewing platform was constructed near to the car park thus avoiding the need for more people to walk onto the worksite whilst enabling the visitors to observe the progress being made.
Karl was very pleased to say that, throughout the whole period while the route was closed, no accidents were reported. That is definitely something to be proud of especially given the urgency of the work, the number of plant movements in such a limited area and the conditions at that time of year.
Detecting microscopic movement
A more sophisticated monitoring system was installed on the embankment slopes and around the tunnel portal. This included 80 wireless slope sensors and wall sensors for the tunnel portal. These sensors are designed to detect any microscopic movement and the information is collated by DATUM Monitoring Services. The information is passed through their Control Centre which operates around the clock transmitting information to Network Rail via an internet site.
More than 320,000 tonnes of earth has been removed from the slip area using a fleet of Moxy dump trucks and excavators provided by various plant suppliers. The aim has been to move the toe of the embankment 30 metres away from the tracks, thus removing the fault line, and to re-profile the embankment. This has been completed and now a further 150,000 tonnes of earth is also being removed from further along the cutting – work now largely complete.
Additional land alongside the cutting has been rented to temporarily stockpile about 350,000 tonnes of material, known locally as Murphy’s Mount. More than 60,000 tonnes has been sent to a waste tip located one mile away. This tip is able to handle 2,500 tonnes per day. However, with more than 560,000 tonnes in total, it will take six months to transfer the stockpiled load to this tip.
Various options were considered, including a conveyer belt system to load the spoil onto road wagons or ballast trains. However, stockpiling then sending the spoil to the local tip was considered to be the most environmentally friendly approach and is the one adopted. Network Rail is continuing to investigate other local opportunities to accelerate removal of the remaining spoil.
Vertical Access Ltd utilised a construction platform as well as roped access to enable soil nailing and netting work to take place to stabilise the ground adjacent to the tunnel portal. This was because the eastern extent of the slip had removed the end of the portal wing wall return such that support to this zone was lost.
Complementing the design approach by RJM Ground Solutions, Network Rail employed Tony Gee and Partners to carry out a review of the earthworks remediation as it was being carried out. With frequent round the table meetings on-site, Network Rail was continually appraised in detail of the evolving rationale of the earthworks remediation. This meant that reasoned assessments could be made regarding the improving condition of the earthworks as the site work progressed in order to secure the earliest safe return of rail traffic.
Responding to pressure
It was recognised early in the process that, when under such pressure, the temptation is to declare the route fit for traffic before it is a realistic option. To avoid this temptation, robust criteria for re-opening the route to trains were agreed at an early stage. Design details were signed off, revised slope angles achieved, dewatering levels met, signalling tested, track alignment surveyed and structural checks of the tunnel portal carried out. It is estimated that the final cost of the physical works on site will be in excess of £5 million.
Tony Gee and Partners carried out a stability check and designed an anchorage system for the end of the undamaged wall to replace the lost end-support. It also carried out a physical inspection of the condition of the portal wall via roped access supplemented by core drilling. This was to both assess the immediate condition of the wall and, together with other investigations, to enable a future assessment of the long-term performance of the wall to be carried out. Adjacent to the wing wall the ground has been steepened with soil nailing to stabilise this section taking the loads back into soil undisturbed by the slip movements.
Thee site team worked closely with the Chiltern Railway and CrossCountry franchises and Karl was keen to point out that the collaboration worked extremely well.
Communication was excellent and people at all levels were kept informed at all times. Local politicians were challenging yet supportive and they recognised that the time to reopen the route was when it was safe to do so and not before. The site has been likened to an open cast coal mining exercise – looking at the pictures, one can see why!
A more permanent solution still needs to be designed for the tunnel portal to complement the soil nailing that has been carried out. The cutting will be covered in topsoil and turned into grassland. Meanwhile, a road cleaner continues full time to help minimise the impact of this work on the village of Harbury and, in recognition of the invasive impact this work has had on the village, the library carpark has been completely resurfaced, one of the community initiatives undertaken.
And, just as in Brunel’s day, the temporary village and its inhabitants have moved on.