Settling is the process by which soil decreases in volume and becomes denser because water and air disappear from the pores, causing the ground level to drop. It occurs mainly in soft soil types such as peat and clay, and plays an important role in ground subsidence in low-lying areas.
How does settling occur?
Soft soil contains a lot of water and organic material. When the soil is loaded, drained or exposed to air, something happens:
- Drainage — with a lower groundwater level the water drains away and the soil particles press more closely together
- Oxidation — peat that rises above the groundwater wastes away because it reacts with oxygen
- Loading — the weight of fill or building presses water out of the soil (this is also called settlement or consolidation)
The result is that the soil sinks, sometimes over many years.
Application and consequences
- Foundations — uneven settling can lead to subsidence, tilting and cracks in buildings
- Fill — with newly placed sand or soil, the after-settlement of the layer is deliberately taken into account
- Ground level and gardens — the street level or terrace can sink over time relative to the pile-founded house
Points to note
- Do not lower the groundwater level unnecessarily; that accelerates settling and can rot wooden pile heads
- For new build on soft soil, allow for settlement time and possibly preloading
- In case of subsidence complaints, have the soil investigated and the foundation assessed
Related terms
- Settlement
- Groundwater level
- Foundation
- Consolidation
- Soil investigation
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