Boulder clay, or glacial till (Dutch: keileem), is a dense, unsorted soil deposited by land ice during the ice ages. It consists of a mixture of clay, sand, gravel and sometimes large stones (boulders) pressed tightly together. As a result boulder clay is firm, hard and poorly permeable to water.
Properties
- Unsorted — fine and coarse particles are mixed together at random, unlike sand or clay sorted by flowing water.
- Dense and firm — compressed by the weight of the ice, which makes it load-bearing.
- Poorly permeable — the clay fraction makes boulder clay almost watertight, so it acts as a natural aquitard (water-retaining layer).
In the Netherlands boulder clay occurs mainly in the north and east, deposited during the Saalian glaciation.
Application and significance in construction
- Load-bearing layer — its firmness makes boulder clay a good foundation soil.
- Water-retaining layer — important for the groundwater level and when designing cellars and dewatering.
- Perched water — rainwater can stand on boulder clay, giving locally high (perched) water levels.
Points to watch
- Always establish the soil profile with a ground investigation (cone penetration test or boring); the thickness and depth of boulder clay vary greatly by location.
- When excavating boulder clay: it is hard and tough, and its poor permeability can cause water problems in the excavation.
Related terms
- Soil type
- Soil mechanics
- Ground investigation
- Foundation
- Groundwater level
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