Structural Engineering & Statics
where the load goes
Concept

Drainage — where it appears

A system of drains, blanket and sump that lowers the water pressure under a slab or behind a wall to a level a pump can hold. It relieves the structure only while the pump runs and the drains stay open, and fines settling in them raise the head it holds year by year.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Also named here as water table — the same set of essays touches all of them, so they are one junction rather than several.

A basement is a boat. A 20 by 30 m substructure dug 6 m into ground whose water table stands 2 m down. The head on the underside of the base slab is 4.0 m, so the pressure there is 39.2 kN/m² over the whole plan — 23.5 MN of it, pushing upward. Nothing about the structure changes that number. What resists it is weight: 18.7 MN of concrete and whatever is built above, giving a factor of 0.80. The structure floats if the water reaches 2.82 m below the ground, and a base slab alone would have to be 1.64 m thick to hold it down.

A basement is a boat

Every load in this collection presses down and is resisted by strength. Hydrostatic uplift presses up, is resisted by weight, and does not care what is built on it — so the check contains no material property at all. It is a ratio of two weights, and one of them is water.

equilibrium · Uplift The silt hands the head back over decades, a season at a time. The basement is under five storeys, its water table 2.0 m above the design head and swinging ±0.5 m over the year, highest in April; pumped drains under its slab start 30 times as conductive as the ground. The head under the slab above the design head, against the years since the drains were new, for drains silting with time constants of 4, 8 and 16 years; dots, a standpipe read every October on the 8-year drains. With those drains the head reaches flotation in the wet season of year 24.1; the reading first exceeds 0.25 m in year 14.8, 9.4 years before.

The drain that fails a season at a time

A basement held at its design head by drains and a pump can fail suddenly, when the pump stops, or slowly, when fines settle in the drains and the head under the slab creeps up while the pump keeps running. The slow failure gives a standpipe read once a year a decade of warning, in its own time constants — if it is read in the wet season. And long before the silt floats the building on its own, it has made every stop of the pump more likely to.

equilibrium · Uplift

Named alongside it

The objects these essays reach for when they reach for this one.

FlotationUpliftWater tableBallastBuoyancyControl perimeterEffective depthEquilibriumFactor of safetyFree bodyHydrostatic pressureInspection

All concepts