On this property which has subsidence underpinning required, we had a slight added mountain to cross in that we were also adding a loft conversion at the same time. Best practice meant that we were to focus on the subsidence underpinning required first. The unstability of the supporting soil and foundation of the property could have had catastrophic results had we proceeded with adding extra weight above ground and continued with construction vibrations and movement associated with construction.
Subsidence occurs in many soil types and can be attributed to many things like the weather for example. A wet winter followed by a hot summer followed by a wet winter on clay soil (As most of Greater London is built on) often wreaks havoc with the foundations of a property. Clay is a strange soil in that it is sticky when wet, can be rock hard when dry and when baked hard with sunlight can refuse to reform into its roiginal state. When this happens under a foundation, we have subsidence.
Usually where subsidence underpinning is required, traditional underpinning is the best way to proceed. A section at a time, we will dig under the foundation after propping the house or property structure first. This keeps the house in place whilst we start work on the structure. We then create a form into which concrete is poured – a form is literally the shape of a foundation usually built with a heavy duty plywood or steel sheeting. Once the concrete is cured, the form is removed and the support or propping is removed leaving your property well supported. The repair is virtually invisible once the removed subsoil is replaced.
Subsidence underpinning required? It is not something that one should try doing without experience, incorrect propping and shoring could result in your house collapsing!