After several redesigns, the engineers settled on a system of 12 piles and a reinforced slab. It was a bit of a saga getting to that point, because although we are next to a pond, there was nevertheless more water in the trial borehole than the experts expected.
The initial plan by the contractors was to auger the piles, to avoid using a pile driver near an old building. However, this was vulnerable to water inflows, which proved excessive, which would have made it hard to be sure the concrete would set properly.
So we were advised to switch to steel-cased piles, with the first three metres augered. With hammered piling starting three metres below ground, it was hoped that the vibrations in the old house would be reduced. This proved the case, though we checked the house carefully every hour or so to make sure there was no damage.
The best way to test the effect was to hold a hand lightly against a wall. In fact, we were rather more worried about the brick-lined well, which we also checked regularly, because it is less than 10 metres from the nearest pile. There was no sign of damage. In the end, most of the piles were less deep than we budgeted – under 7 metres instead of 10 metres – because they quickly went into the hard grey boulder clay, which is very strong and stable. One pile went to 11 metres.
Other technical complications meant constant revisions to the foundation plans, but in the end the work was signed off by the building inspector, ahead of pouring the concrete for the reinforced raft.
The contractor was DJE Construction of Attleborough and the consulting engineers were Stroud Associates of Harkstead, Ipswich.