A settlement is downward movement of that part of a site which is loaded by the weight of the structure or other material. Settlement takes place over a period of time depending upon the type of material involved. E.g.: a motorway embankment built on rather boggy ground will undergo fairly considerable settlement even at the bottom of the embankment as the boggy ground is compressed by the new weight of material.
The top of the embankment undergoes even greater settlement because of both the movement at the bottom and consolidation within the embankment itself.
A house built on well- graded sandy material will undergo little settlement, whereas one built upon a weak clay will undergo settlement over a long period of time, perhaps twenty-odd years. If peat is involved there is no knowing how long the settlement will take and in such circumstances, one is better off piling the foundations down to a depth below the influence of the peat.
What are the effects of settlement?
All structures when built undergo some settlement. The designer of the foundations seeks to size the foundations to equalise the pressure throughout a house to ensure that different parts settle by approximately equal amounts. Cracking arises when the settlement is not uniform. If one builds an extension long after the house has been built, by then the house will not settle any more but the extension may well settle.
How can I deal with settlement?
Settlement for most householders is usually experienced after an extension has been built. Once a structure has been built, some settlement can be expected, depending on the type of soil, the foundation design, etc. Before an extension is built you can decide to
- Tolerate the differential settlement and repair occasionally, but this is not recommended,
- Ask to have the foundations designed so the new extension moves only a little i.e. to design it to move differentially but marginally,
- Ask to have the foundation designed on piled foundations.
The piled foundation arrangement tolerates much less vertical movement and can be designed to minimise the risk of future subsidence. However, it may cost more, but if the alternative is a considerable amount of excavation with the soil being carried through your house, a piled solution with piles driven into the ground displacing the soil, may have other advantages. Screw piles have similar advantages, except that the operatives can determine the ultimate load bearing capacity by measuring the input torque.
If you have an extension suffering from settlement, we recommend you give us a call to check if you have subsidence or settlement and also to work out the best solution, which can be underpinning. If your house is close to a river, you may have tertiary consolidation (for consolidation read "settlement") but insurance policies are usually worded to define it as subsidence.