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Submit plansI am thinking of having a 3m x 3.5m single-storey extension built and could do with some advice regarding the foundations for it. The extension will be 1.6m away from a 2m-high wood log retaining wall that in turn supports another 2m high log retaining wall 3m back from it. The log wall is 1m deep and is concreted into the ground. My concern is that digging in front of the wall will make its foundation unstable and, in the worst case scenario, the whole wall could move forward. My builder said that one option is to dig right up to the wall and concrete the whole ground in a day. This would prevent the wall from moving forward in future due to having disturbed the ground in front of it. The other option is to use piling for the foundation, which will cost about £3,500 compared to £1,000 using the concrete method. Which method do you recommend I should use? The ground is clay soil and the garden is quite waterlogged in winter but the extension will be built in July to August.
You are right to be cautious. You shouldn’t even consider any form of new foundations so close to a retaining wall without detailed design work being undertaken by a structural engineer. He or she will require a small trial hole to be hand dug in front of the log wall to expose the foundation and to determine its depth. The engineer will record all this data, together with information about what loads the retaining wall is supporting (dead and imposed loads). See the engineer’s fee as a necessary investment as any collapse of the retaining wall (and the land it supports which may be outside of your ownership) due to negligence by you and/or your builder will not be looked on favourably by your house insurer.
