The Visitors’ Centre Supporting Structure

Pier’s Progress #14

by HIP

Although the unusual constraints of the Pier site made planning this aspect of the project very complex, the actual works are, for the most part, fairly straightforward.  Whereas the original 19th century piles were manually screwed into the ground with a capstan, the new piles are lowered into place by a crane, and then driven into the seabed with a 'piling rig'.

These six new piles consist of cylindrical sections of steel 'pile casing', which, once joined together and positioned, form a vertical 'tube' that is then filled with reinforced concrete.  It is the reinforced concrete within the pile casing which forms the actual 'pile', and will support the weight of the Visitor Centre.

Firstly, to ensure each pile is correctly aligned, an open-ended 'cage' of scaffold bars is built over the pile site.  This 'cage' extends 6 metres beneath the Pier platform, and guides the pile casing as it is lowered to the seabed.

The first section of pile casing is lifted into the scaffolding 'cage' by the crane and secured.  The next section is attached, the joint is welded, and the pile casing is lowered.  This process is repeated until the pile casing is long enough to reach the seabed.  Rather like a hammer hitting a nail, the 'piling rig' is then used to gradually tap the pile casing into the seabed using a 1 Tonne (0.98 ton) weight.  As the pile casing is driven further down, several smaller sections are added, one by one.

Once the pile casing has been driven to the required depth, sand, clay, and any other material inside are removed using a special scoop-like device.  This device is then used to bore deeper into the seabed.

When this process has been completed, a steel reinforcement frame is installed inside the pile casing, extending beyond the base of the pile casing and down into the seabed.  A 'trémie tube' is then used to fill the pile casing with marine-grade concrete.  Finally, the pile casing is cut off at the necessary level and capped, ready for the steelworkers to fit the new lattice girders.

As with so much of the Pier project, the piling operation is influenced by the weather.  However, so far, the weather has been reasonable for this time of year, and all is going smoothly.