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Lowestoft – Holding back the tide
Tuesday, May 25th, 2021
Lankelma faced extreme weather and a busy shipping calendar during critical nearshore investigations for the design of the foundations for a new moveable tidal barrier in Lowestoft in March 2021, as part of the Lowestoft Flood Risk Management Project.
In 2013, a tidal surge caused widespread flooding and damage to towns along the coast on the east of England, as the North Sea reached its highest levels since 1953. In the harbour town of Lowestoft, property was flooded and transport networks were severely disrupted. The Lowestoft Flood Risk Management (LFRM) Project aims to defend the area at greatest risk of tidal flooding and will also help support the local economy and the regeneration of the town.
Ground investigations have been carried out over several phases; the latest, in February and March this year, involved CPT testing and drilling in and around the harbour entrance, to provide geotechnical data for the design of the foundations for the tidal barrier.
Lankelma was commissioned by geotechnical consultant Tetra Tech, on behalf of Associated British Ports to carry out the overwater element of the investigation. The project encompassed 20 top-push piezocone Cone Penetration Tests (CPTs) and sunk four cable percussive boreholes, ranging from 20 to 50m below the riverbed, from its Combifloat C5 Sandpiper jack-up barge, working in up to 10m of water. Works were performed through the jack-up’s moon pool, with a cantilever frame attached to the side of the deck used when testing was carried out close to the quay wall.
A key element of the CPT work was dissipation testing. The results of dissipation testing is used to calculate the hydraulic conductivity of soils, which in turn is used to determine their settlement properties. Pressuremeter testing was also carried out, using the full displacement pressuremeter which applies a uniform pressure to the walls of the test hole by expanding a flexible membrane behind the cone tip. This measures both soil strength and stiffness parameters at strain levels as low as 0.01%. Both these tests provided important ground data for the design of the flood barrier foundations, offering an understanding of how the ground will behave under the heavy loads.
Undisturbed and disturbed samples were collected from the cable percussive boreholes, drilled using its Dando 4000 cable percussion rig, along with SPTs, permeability testing and downhole pressuremeter testing.
Not only were ground conditions challenging – including dense and blowing sands (where fine, fluidised sand flows upward into the borehole) – but Lankelma also had to deal with some unpredictable and rapidly-changing weather during the first two weeks of the project, and work around vessels moving in and out of the harbour. Work was around scheduled vessel movements and flexibility was required to account for unscheduled shipping movements in the busy harbour. The Sandpiper jack-up is a modular system and was reduced in size so it would be less disruptive to port operations and be able to manoeuvre around smaller spaces.
Despite all the obstacles – including having to adjust ways of working to ensure investigations were COVID-safe [including social distancing, PPE and rapid onsite testing] – the nearshore drilling and testing was completed on schedule.
Lankelma and Sandpiper also have history in the area, having carried out an investigation for a similar flood defence scheme further up the coast, in 2017.