A concrete plinth and iron framework were constructed on the site over 100 years ago, which has more recently supported a navigation radar beacon signalling the rock's position to passing shipping. Due to the prevailing weather and marine conditions, the old concrete plinth was gradually wearing away and needed to be replaced by a much more robust and weatherproof structure.

Cornwall based Seacore Ltd called upon their marine construction experience to gain sufficient access to the rock for installation of a new reinforced concrete structure and navigation beacon. Seacore's jack-up platform Skate 2D was large enough to accommodate the plant and equipment required for the job while being compact enough to be jacked up on the rock itself and carry out all works without requiring repositioning.

To take advantage of more favourable weather conditions, work started during July 2001.

Jack-up positioning needed to be as precise as possible because water depth rapidly increased to 100m adjacent to the rock. Once in position, the old iron structure was dismantled and the concrete demolished down to bare rock using small excavator and rock breaker. All work at this stage of the project was tidally dependent as most of the rock was covered at high water. With all rubble removed from the site, holes were drilled into the bedrock using a small tracked drill rig. These holes would accept grouted in 'Dywidag' rock anchors.

The tower design for the new beacon used a series of precast concrete manhole rings placed one on top of another and infilled with reinforced concrete.

Construction required six pours of C50 concrete, using a 3m 3mixer on site to ensure maximum pouring time. Temporary shuttering was assembled at the base of the structure and a system of steel bars was grouted into holes drilled in the rock to form the framework of the reinforced concrete base. The first pre-cast concrete ring was cast into the base pour before the remaining concrete rings were positioned on top of each other. Reinforcing steel and rock anchors were subsequently placed before the second concrete pour inside the tower.

In order for the structure to attain sufficient strength, the rock anchors required tensioning with a hydraulic jack to exert a vertical pull of 60 tonnes. The tower was then extended to its full height of 7m before the third concrete pour and retensioning of the rock anchors. During tower construction the Skate 2D jack-up was raised to the appropriate working height to access all levels.

Once the tower reached full height a concrete platform was cast onto the top within a pre-designed bespoke steel framework which extended down to the rock base for support. The reinforcing steel framework was positioned and the fourth concrete pour took place to produce the required platform area.

Seacore personnel also assisted NLB technicians to install solar panels, batteries and handrails for the new Racon radar beacon and light. An access walkway and jetty constituted the final two concrete pours and the whole structure was coated and finished with hard wearing paint.

MJ Information No: 16701