Shaldon and Ringmore Tidal Defense Scheme Project Case study
Resin injection used to strengthen ground beneath a 163 metre long sea defense wall.
Oxford Hydrotechnics worked in conjunction with Interserve during the construction of the £3.8m Shaldon & Ringmore Tidal Defence Scheme on the southern side of the Teign Estuary, South Devon. This Environment Agency scheme will protect over 250 properties from tidal flooding, up to an event with a 1 in 1000 chance of occurring each year for the next 100 years.
As part of the Environment Agency's tidal defense scheme on the Teign Estuary, a 163 metre long section of existing tidal defence wall required stabilisation. Site constraints to the stabilisation work included; the unstable nature and poor load bearing capacity of the existing ground, the frequent exposure of the wall to marine conditions and the proximity of adjacent residential properties and a child play-park.
A number of different ground stabilisation techniques were explored by the site team, however, traditional underpinning was not possible due to the existing ground conditions and steel rod insertion would prove to be a maintenance liability, given the corrosive effects of regular seawater exposure. To overcome these constraints, our sub-contractor, Oxford Hydrotechnics, developed a new, marine resistant, resin injection ground stabilisation technique.
A low vibration, hand-held diamond drill was used to core 50mm diameter holes, vertically through the existing wall. Holes were cored to a depth of 4m at 600mm centres along the full length of the wall. A 30mm diameter, hollow fibre-glass dowel was inserted into each core hole, which is not susceptible to saltwater corrosion.
Five litres of resin was injected through the dowel to provide a 300mm cylindrical resin base. Once hardened, it was mechanically pushed 300mm into the ground by twisting the dowel, before a second 300mm high resin layer was injected. This process was repeated three times, resulting in a 900mm high, end bearing, resin pile. The core hole was then filled using NATCEM quick-setting grout, to the top of wall level.
Benefits for Use on Shaldon and Ringmore
This innovative technique provided a marine resistant ground strengthening system, allowing the existing wall to be retained and avoiding the need for conventional piling works. As such, it significantly reduced the impact of our works on the local community.
A cost comparison of the different options considered is given below:
Full wall replacement(Considered at iPAR stage)
Total Cost: £880,000 Cost Per metre: £4,755
Conventional underpinning and wall extension (Considered at construction stage)
Total Cost: £328,250 Cost Per Metre: £1,775
Resin Injection and wall extension (Actual solution adopted)
Total Cost: £472,000 Cost Per Metre: £2,550
Research and Development
The new resin injection system developed by our Oxford Hydrotechnics was called Stabila P40. It incorporates a quicker bonding accelerator than used in similar resins, to account for the marine environment. We carried out several trials of this system in the beach adjacent to the wall. Samples were taken from the trial injections after repeated exposure to seawater and strength tests were carried out. Once the results established the resin injection system achieved the required performance, we commenced installation within the wall. Further strength tests on this system were carried out during the construction phase, by excavating mini-trenches along sections of the wall.
Overall, the resin injection ground strengthening works went well and were completed on programme. Core drilling through the wall went particularly well
and was completed ahead of programme. Typically, 8 holes were cored and dowelled per day. Several instances of grout loss occurred through the existing pointing, during the final core grouting operation. To mitigate the impact of this, natural cement was used in the grout. Where possible, re-pointing was also carried out prior to grouting, to reduce grout loss through the wall.
Potential for Further Development
This system offers an innovative approach to ground stabilisation in and around tidal defences. Shaldon & Ringmore will be used as a benchmark for
durability and performance, to continue development of marine based resin injection systems.