Mark Tregelles had to get to grips with two very different elements in this project on the side of a forested hill. While quality oozes from both, the visual contrast is stark: on the one hand, a tastefully discreet museum set largely underground; on the other, a grand elevated approach to the museum that snakes through the tree canopy 14 metres above the woodland floor.
The stunning walkway presented the greater technical challenge, not least because the South African architect and engineer of its inspiration, the Boomslang bridge on the slopes of Table Mountain in Cape Town, had been appointed to design it. Fabricated in South Africa, the steel components and timber handrails and boardwalk had to be imported, brought to site in containers and then assembled onto the site-built pile and tripod construction to millimetre tolerances.
Methodical planning and collaborative efficiency were needed to manage the fraught logistics and remote relationship. Even more impressive is Mark’s leadership of the foundations solution for the 130m-long elevated steel structure on a slope in a delicate woodland environment with limited access. Every single tree on the site was protected by a preservation order, so no damage was permitted during walkway construction.
It soon became apparent that while using screw piles for the foundations may have worked well on Table Mountain they didn’t cut the mustard in Somerset. Given the large number of tree roots that had to be avoided, and the layers of rock present, screw piles were not only unable to achieve the loading requirements but were also likely to cause considerable damage to the trees’ root network.
Mark led the piling review to the successful option of micro-piling. Each pile location was hand-dug 600mm deep to avoid damaging the roots, with sleeving used to contain the grout while the pile was formed to prevent contaminated water entering the soil. The piles went in and the trees continued to flourish.
The smaller headaches of the museum building were likewise cleared up by Mark. For example, his use of a higher cement content in the concrete mix reduced the curing period for the 5m-high retaining walls and columns. It allowed the early strike and reuse of the concrete shuttering, and got the roof-slab formwork away early.