What is absolute settlement?
Absolute settlement is the total vertical displacement (subsidence) of a foundation or structure relative to a fixed reference point, such as a national datum (e.g. Ordnance Datum in the UK or NAVD88 in the US). It is measured in millimetres and indicates how much a structure has sunk as a whole since completion or a previous measurement.
Difference from differential settlement
It is important to distinguish absolute settlement from differential (uneven) settlement:
- Absolute settlement — the total subsidence of the entire structure. If a building settles evenly by 20 mm, the absolute settlement is 20 mm
- Differential settlement — the difference in settlement between two points on the same structure. This causes tilting and cracking
Uniform absolute settlement is generally less problematic than differential settlement, provided the connections to drainage, services and adjoining buildings remain intact.
Application
Absolute settlement is relevant in:
- Foundation design — the structural engineer calculates the expected settlement based on ground investigation (cone penetration tests) and the building load
- Monitoring — on major projects, settlement is measured using precise levelling or satellite-based monitoring
- Existing buildings — when foundation problems arise, absolute settlement is measured to assess the severity of the situation
Causes of settlement
- Compression of the soil — clay and peat layers are compressed under the weight of the structure
- Consolidation — water is slowly squeezed out of the soil, causing the ground to compact
- Creep — long-term deformation of the soil under constant loading
- Groundwater lowering — through dewatering or drought, the groundwater table drops, increasing effective stress in the soil
Related terms
- Foundation
- Ground investigation
- Pile foundation
- Strip foundation
- Soil mechanics
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