Conservatories and sand pits

There’s a large housing estate being constructed to the south of Leighton Buzzard on what started off as a giant sand quarry, then became a giant infill site. So essentially the houses are constructed on a very deep layer of fill.

From an engineering point of view, the solution is relatively straightforward, if a little heavy handed and relatively expensive for a housing project. Reinforced concrete piles penetrate through the fill down to the virgin material, sometimes up to 20m deep. The piles work in two ways. Firstly the bottom acts like the legs of a table sitting on the floor (end bearing) but the fill material itself grips the shaft of the pile (skin friction). If there is sufficient skin friction, the end bearing is not required and the piles can be shorter.

But the design has to ignore the material immediately below the footings, and pretend that they span between the piles, usually 3m to 4m, but sometimes longer. This means reinforced concrete ground beams are needed to support the walls.

But what happens if the householder wants to add a conservatory. It’s all very well for the developer to install hundreds of piles for a large number of houses, but it is usually uneconomic to do this for a conservatory. There are such things as mini-piles for difficult ground conditions, but they are not appropriate for these sort of depths.

The solution is a reinforced concrete raft, about 300mm to 400mm deep. This foundation and conservatory act much like a cup and saucer. If the ground settles a bit, the raft tilts a bit and takes the conservatory without distorting it. This sort of solution is ideal for, say, a detached garage, but conservatories usually butt up against the house, so provision must be made for differential movement between the two different structures.

It can be that this solution is a bit more expensive than the “traditional” solution, but the oncost is relatively modest; usually a small percentage of the total cost