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The Ironbridge Gorge has a history of land-sliding. Formed at the end of the last ice age some 13,000 years ago, the over steepened slopes of the Gorge have been formed by incision and erosion by the River Severn. The Industrial Revolution that Ironbridge Gorge is recognised for is a major contributor to the instability of its slopes. The human influence of mining which has created voids, the loading of slopes with mine waste and the disposal of tile and brick waste along the river banks has caused them to be susceptible to river erosion and slope instability.
An increase in the frequency of flooding of the Ironbridge Gorge over the last decade and heavy periods of rainfall have increased the levels of groundwater within the slopes adding to the instability.
Telford & Wrekin Council has been proactive in addressing instability issues particularly over the last four years. Investigations have increased the understanding of the scale of the problem and revealed areas within the Gorge which show evidence of recent ground movement. These areas include the Wesley Road, The Lloyds, Lloyds Head and Salthouse Road in Jackfield. The land instability can be seen in the form of deformed and damaged roads, pathways and services or collapsed or damaged retaining walls.
Monitoring of the slopes has shown that some areas are moving at a greater rate than others. Failure of the ground in front of Lloyds Cottage will almost certainly result in the loss of road in this area. Severance of the road would ultimately result in road diversions. Further failure of the slope may result in the narrowing of the River Severn and a shift of stream flow to the southerly bank; this would undoubtedly result in accelerated erosion of the Lloyds Head area with consequential increase in instability upslope in this area.
Telford & Wrekin Council has, as a result of funding received from the European Regional Development Fund, commissioned work to implement a new innovative scheme which will stabilise 165 meters of highway at The Lloyds which is a key link road along the floor of the Gorge.
Work started in September 2006 and will involve re-profiling the road, installing 160, 24-30m long, steel-reinforced concrete piles, a new retaining structure and a positive drainage system. These stabilisation works for the road, in conjunction with the later phase of river bank stabilisation works will, once complete, have a design life in excess of 100 yrs.
The scheme is evidence of a commitment to the objectives of the World Heritage Site Management Plan on the issue of land Instability “To evidence and undertake appropriate measures to deal with existing damage caused by land instability and to minimise inconvenience from future movement.” and represents proactive management of the issue.
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Progress October 2006 to December 2006
• The works entails three main activities; drilling and grouting to stabilise the mine workings beneath the site and piling works to stabilise the highway
• A very important element of the project is the monitoring regime that has been adopted to gauge the ground movement taking place during the works. The background level of movement measured on the lower slope between the highway and the river is relatively constant at around 2mm per day
• The Listed slag block wall was removed and the outfall drainage that connects into the new headwall was constructed
• A media morning was held on 7th November 2006 to give local press the opportunity to look round the site, take photos and interview the project team
• A public exhibition was held on 25th November 2006 at Coalport Village Hall to give the public an opportunity to look at displays of the work and ask the project team questions about the project
Progress January 2007 to March 2007
• The drilling and grouting of five worked coal seams beneath the piling has been completed, drilling 170 holes down to a depth of 55m and injecting under pressure 1200m3 of liquid grout. This work will ensure that the ground beneath the piling is not affected by collapsing mine workings
• The structural heart of the project is the construction of the rotary bored, steel tube reinforced, concrete piles. The main piles are reinforced with thick steel tubes which are socketed into rock. 90 of the 144 piles are now complete.
• At the end of March work started on the fabrication of the reinforcing steel for the combined pile cap and retaining wall. The main piles are connected together at the top in 15m long bays. When complete the piles, pile cap and retaining wall will be buried.
• The ground movement monitoring has shown that there is a clear link between river water levels and rates of slope movement. Increased rates of movement occur immediately following flood events. As the flooding in Ironbridge over the Christmas/New Year period receded the ground movement was measured at up to 6mm per day. This rate was eclipsed in the spring floods when the rate rose to 15mm per day.
• The project was featured during the BBC's Countryfile programme on 25th March 2007 highlighting the point that the Lloyds is a key link within the Gorge and that the project was dealing with a combination of the natural Gorge processes and the legacy of the industrial revolution.
Progress April - June 2007
• With the grid drilling & grouting exercise being fully completed in February, the emphasis this quarter was to concentrate wholly on the structural solution adopted here at The Lloyds.
• This involves the construction of 144 reinforced concrete piles. The piling was successfully completed on 5th June 2007.
• The reinforced concrete base and retaining wall works followed as the piling works progressed. Once the base was available the retaining wall was cast. To date 9 of the 11 bases and 8 of the 11 walls have been constructed.
• During this period we have also continued to monitor the ground movement of both the upper and lower slopes. Again the output clearly shows a direct correlation between river levels and ground movement. In addition to the ground movement monitoring we are now starting to monitor the movement of the piles themselves. The information from the overall monitoring regime will prove to be an extremely useful resource when considering further instability projects.
July - September 2007
Following the successful completion of the grid drilling & grouting and piling phases, in February and June respectively, the focus during this quarter was to complete the works to affect the opening of The Lloyds itself, after almost twelve months of closure. The reinforced concrete base and retaining wall works, that connected together the lower rows of piles in 15m bays, were completed in late July. As each bay was released the ground levels behind them were built back up again using previously excavated material and the recycled hardcore that formed the crane mat for the piling cranes.
Following on behind this work, the highway and watercourse drainage were constructed beneath the highway alignment. Utility ducts and spare ducts were placed in the northern highway verge. The highway construction and pedestrian handrail were then completed, allowing The Lloyds to open to vehicular and pedestrian traffic on Friday 31st August 2007, just prior to the start of the new academic year. Throughout this period the ground movement and pile monitoring continued. The information being recorded from the manual and remotely-read inclinometers within the piles selected for monitoring, and the strain gauges attached to the outside of the piles, is building into a comprehensive and consistent body of information. The site was cleared at the end of September.
Articles and case studies:
Horizontal Themes Case Study
Insight Article, November 2006
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