bad ground and help

21 Apr 2011
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United Kingdom
Due to the high estimates from builders we have started an extension to one of our childrens nurseries and dug the trench for the footings it is approx 12m x 6m single storey we asked the building inspector to have a look and he said the ground was soft clay and we were to dig deeper. We have dug down to 1.8m and the ground is still soft and it is getting wetter, what would anyone suggest that we do, any suggestions would be helpful. What does soft clay mean and what impact will this have on the buiding, do we just have to make the footing wider. What does soft clay mean and what effect willl this have on our extension, does it mean we have to cancel the project?
Any help or comments would be really appreciated
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Digging deeper may be an option but, without digging a trial hole or knowing the local geology, the ground might get better or worse so it's a gamble. How deep/what type are the existing foundations? It sounds like you may have already reached the water table which adds a couple complications to the build. You also have the risk of collapse of a deep trench to consider - the BCO shouldn't be telling you to dig deeper under any circumstances, only suggesting. It would be worth calling the BC dept and asking about ground conditions in that area and what others have done in the past.

I would strongly recommend employing a structural engineer to assess and test the bearing capacity of the soil, i.e. how much load it can take. From there the engineer can do simple calculations to work out whether widening the foundation will spread the load sufficiently or whether an alternative foundation type is more feasible or cheaper, like a raft or piles. You might be able to reduce the weight of the building as well, e.g. timber frame construction, in which case it would be a very poor soil that would not support the building loads. Give the calcs to the BCO and they'll be happy.
When there is a situation with say the existing house on something like 600-1000mm deep foundations and the building inspector wants the new work to exceed this depth by more than 50% for no reason other than he thinks the ground is unsuitable ..... then rather than digging down to the core, its worth spending that extra money on an engineers opinion instead, who will in most cases be able to tell the BCO to STFU.

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