Festschicht

Festschicht is a German term used primarily in geology and geotechnical engineering to denote a relatively hard, cohesive layer within a sedimentary sequence or soil profile that behaves as a single, fixed unit under load. The word combines fest (firm) and schicht (layer).

Such layers may form through cementation, lithification, or mechanical compaction, and may include cemented sands, claystones, or indurated strata. They can occur anywhere in the subsurface, from near-surface soils to deeper bedrock. In practice, the term is used to describe a layer with higher strength and stiffness than its surrounding material, creating a distinct mechanical boundary within the subsurface.

The presence of a Festschicht influences bearing capacity, settlement, stability of slopes and excavations, and may require different foundation design or stabilization measures. It can affect groundwater flow and the response of a soil mass to loading, vibration, or excavation activities.

Detection and assessment are typically achieved through geotechnical methods such as borehole drilling and logging, standard penetration tests, plate load tests, and geophysical surveys. The term is more common in German-language literature, and its exact scope and definition can vary by region and discipline.

See also: bedrock, indurated rock, cemented soil layers, geotechnical engineering.