Buoyancy

Any body wholly or partly immersed in a fluid is pushed upward by a force equal to the weight of fluid it displaces. That is Archimedes' principle, and it explains why ships float, why pontoons and caissons can be towed into place, and why empty underground tanks, basements and pipelines can rise out of the ground when the water table is high. The buoyant force acts through the centre of buoyancy, the centroid of the displaced volume.

Whether a floating body is stable depends on the metacentre. When the body tilts slightly, the centre of buoyancy moves sideways. The metacentre M is where the vertical through the new centre of buoyancy cuts the body's original centre line. If M lies above the centre of gravity G, the buoyant force and weight form a restoring couple and the body is stable. The metacentric height GM = I / V − BG, where I is the second moment of the waterplane area about the tilt axis and V the displaced volume. Wide, low bodies are stable; tall narrow ones are not.

In civil works, flotation checks are essential for empty sumps, manholes, septic tanks, swimming pools, basements and buried pipelines. The structure's weight, plus soil above it where it can be relied on, must exceed the uplift with a margin; otherwise add weight, anchors, tension piles or relief drains. Caissons for well foundations and immersed tunnel elements are floated, positioned and then ballasted to sink in a controlled way.