The scheme involved the construction of a subterranean structure beneath the existing garden, accommodating car parking and a recreational area including a swimming pool. The existing south wing swimming pool annex was removed to accommodate a reconfigured and lower-level swimming pool – this area was integrated with the existing south wing Orangery and also linked to the subterranean rear extension. The car park is accessed via a car lift and a box tunnel constructed beneath the existing Coach House on the north side of the building.
Being Grade I listed, the existing building remained unaltered, including its existing foundation formation level. There was concern over partial underpinning of the building. The proximity of the new works to both the south and north ends of the main building required a bespoke solution to maintain the integrity of the main house. Furthermore, the site constraints, with regards to access / egress also dictated the method of construction. For both these locations, a top down construction sequence with plunge columns was chosen. Contiguous mini pored pile walls were installed to both south and north ends. The capping slabs were then installed and the south wing excavation beneath the slab progressed from the rear garden with spoil bagged and removed. A raft foundation was then installed and the adjoining boundary wall underpinned. The coordination / phasing of the temporary and permanent works required detailed consideration.
On the north end, one of the contiguous piled wall was installed within the existing Coach House and the other, adjacent to an existing boundary garden wall. The roof of the tunnel was then installed in a phased sequence to allow the Coach House to be re supported off the tunnel roof. Immediately adjacent to the tunnel was the Grade I listed Garden House that features tiles by William de Morgan. Excavation for the tunnel progressed from the garden and a raft foundation installed to form the floor of the tunnel.
A contiguous mini piled wall was also installed to the perimeter of the subterranean car park / recreational area and to the swimming pool which extended below this area. Being remote from the house, this area was constructed in a bottom up sequence. A drained cavity system was installed throughout.
Prior to the excavation works commencing, the existing building was monitored on all elevations and monitoring was continued for the entire duration of the works with pre-defined trigger level set, should the actual movement exceed the predicted movement – including differential and tilt movements.
This was a fantastic engineering project to work on and technically challenging, owing to the juxtaposition of Debenham House, Grade I Listed, to the new subterranean work.