What is silt?
Silt is the particle fraction of soil that sits between clay and sand in size: grains of 0.002 to 0.063 mm (2 to 63 micrometres). This is the boundary used in the European soil classification standard EN ISO 14688. Soils dominated by silt are described as silty; loess is a typical wind-blown silt deposit. Silt feels floury and smooth between the fingers, neither gritty like sand nor sticky like clay.
Properties that matter for building
- Poor drainage. Water passes through far more slowly than through sand, so the ground stays wet for a long time.
- Frost-susceptible. Silt draws water up by capillary action. In freezing weather ice lenses form and the ground heaves, lifting paving and shallow foundations.
- Unstable when saturated. Saturated silt quickly loses bearing capacity and can turn almost fluid under vibration or load.
- Hard to compact. It compacts well only within a narrow moisture range; too wet and it turns to mush.
- Prone to erosion. The fine particles wash out easily, for example from slopes or around leaking drains.
Application: practical tips
- Get a soil investigation. Boreholes and probing show whether silt layers are present and how thick they are. This is essential for any foundation design.
- Do not use it as fill. Silty spoil is unsuitable as a sub-base or bedding under paving, floor slabs or foundations. Use free-draining, compactable granular material instead, unless an engineer advises otherwise.
- Keep excavations dry. The base of a trench in silt softens quickly and sides can slump. Dewatering and battering or trench support need to allow for this.
- Provide drainage. Falls and land drains stop water from sitting under paving or against foundations.
- Keep heavy plant off wet ground. Tracking over wet silt remoulds it and permanently reduces its strength.
Difference from sludge or mud
Mud or sludge is not a particle size but a deposit: the soft sediment on the bed of ditches, rivers and harbours. It may contain a lot of silt, but also clay and organic matter. Silt describes grain size; mud describes where the material lies.
Related terms
- Clay
- Loam
- Loess
- Soil type
- Soil investigation
- Drainage
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