Demonstrating buoyancy in waterlogged ground

Abstract

Archimedes' principle is one of the foundation subjects in physics. Normally the principle is exemplified by reference to floating boats. However, Archimedes' principle also applies to objects embedded in waterlogged ground, which can lead to the surprising result of an object weighing several tons being lifted. A practical example is presented of a 22 ton concrete water tank being lifted out of waterlogged ground. Fuel tanks embedded in the ground can be crushed by groundwater. Ground water buoyancy can be a serious problem when a large excavation for the foundation of a high-rise building is left empty before building commences. A qualitative classroom demonstration of groundwater buoyancy is described that involves immersing plastic cups in sand and adding water to observe the cups rising out of the sand. Additionally, a quantitative demonstration is described in which an empty beaker is floated on a water-filled container placed on an electronic balance placed on a scissor table. A plank of wood placed across the top of the beaker holds it in place while the scissor table is used to raise the water level around the beaker simulating rising groundwater. In view of the challenges of climate change and increasing frequency and severity of flooding, knowledge of groundwater buoyancy is useful for students.