Seepage through the left bank embankment at Chapel Haddlesey posed a risk of internal erosion and eventual failure of the embankment. Internal erosion is an ongoing process that will ultimately cause failure of the structure and as there is no natural process that will stop it, engineering intervention was required. We needed to address both bank instability and seepage issues to improve the condition of the flood defence embankment. Following site setup, a Tensar-designed piling platform was constructed using layers of geotextile and sub-base. Subcontractor Sheet Piling UK utilised a 110T lattice boom crawler crane* with adjustable ballast to install a total of 69 piles from the platform.
The repair consisted of two sets of sheet piles. The first set, referred to as the ‘anchor piles’, were placed 12m deep along a section on the crest of the embankment to reduce the seepage into the residential areas adjacent to the river. These anchor piles were connected via steel tie rods to the second set of 18m piles placed 16m deep along the river’s edge - the ‘river piles’. The additional 2m of river piles** above finished level was included in the temporary works design as a measure for water retaining and edge protection, before being cut to size at the end of the project.
Failed material between the two sets of piles was excavated, disposed off-site and replaced with clean fill due to the failed material being contaminated with elevated concentrations of lead, arsenic, mercury and poly aromatic hydrocarbons. The materials were replaced in the form of a 1m benched detail starting 2m below river level and stepping up towards the anchor piles, with geogrid at each step to stabilise and strengthen each layer. The material was replaced with compacted 2C clay fill in 600m layers and capped off with a 200mm layer of topsoil.
Once filled, the bank was cut to the designed gradient leaving a 4m foreshaw between the main bank crest and top bund. The bund, situated above the anchor piles, tied into the existing embankment profile and formed a further flood defence. Due to the distance between the river and piling platform a long reach excavator was used for the duration of this project. Following embankment reconstruction, the river piles were trimmed to final level and capped off with 300mm dia. rock rolls embedded in the channel to reduce the erosion on the embankment at the average river level.
The piling platform was then removed and recycled, the embankment face was re-seeded, covered in erosion protection matting and the residential areas past the crest of the embankment were reinstated to original topography, landscaped, and fencing was installed.
Throughout the project, we used a number of innovative techniques including:
River piles utilised in temporary works design – The 18m piles in the river were intentionally left long and extended 2m above river level for the duration of the project. This provided a temporary works solution to both retain water and provide edge protection for both operatives and machines.
110T prototype crawler crane – One-of-a-kind crane which was capable of reaching the required distance from piling mat to river pile location due to its length of lattice boom and adjustable ballast.
Solar powered facilities & HVO fuel– The office, canteen and toilet facilities were all run primarily off a solar system, with a backup diesel generator that would kick in when the solar charge was insufficient. Hydrotreated Vegetable Oil (or Renewable Diesel) is a fossil-free alternative to mineral diesel, resulting in up to 90% reduction in Greenhouse Gas emissions. HVO was used in all mobile plant and backup generators for the solar system. These, coupled with battery operated tools, were all part of a drive to reduce our carbon footprint and emissions local to site.
