Southend-on-Sea

To support the project, MACS / GPS chartered Red7Marine’s 100t deck-capacity jack-up barge, the Haven Seajack 2. The barge was used as a stable platform for the installation of new support piles and steel beams, and bracing, to provide additional support for the pier structure.

This was one of the most challenging locations we have ever had to mobilise a jack-up barge due to the lack of accurate geotechnical and environmental data available. Whilst completing a Site-Specific Assessment for the project, our team found some data which suggested reasonably firm clay at the surface, whilst others stated a rather deep layer of silty clay with limited properties in regard to its ground bearing capacity. There was also evidence of a sand layer at a depth of around 10-13m deep, followed by further clay, presenting a risk of punch-through. Risk of scour was assessed but due to the expected depth of penetration, the risk was considered as low. However, having significant penetration into clay, produced a risk during extraction of the legs.

We had initially assessed using our Haven Seaway jack-up barge due to its larger deck capacity, however after conducting a Leg Penetration Assessment (LPA), a high risk of punch-through was identified due to a layer of loose sand within the clay layer. If the legs were to penetrate through this sand layer, then there would be insufficient leg length to get the barge to a safe height. To overcome this problem, Red7Marine performed an LPA for the Haven Seajack 2, with a lighter deck load, there was less penetration. While the risk of punch-through remained on the low bound assessments, the leg length stayed above the operational limits of the barge. To best reduce risk during the event of punch-through, cyclic preloading techniques were adopted and all vibro piling operations were to be restricted if used.