What if a house were built on soft clay?

A house sits on its foundation. The foundation sits on the ground.

Some ground is firm, like rock. Some is soft, like clay. What if a house were built on soft clay?

Under every building is a foundation, the part that carries the building's weight down into the ground. The bottom of it, which spreads the weight out, is called a footing.

But the ground isn't all the same. Solid rock can carry a lot. Sand and gravel can carry less, and soft clay and silt less again. What happens if the same building, on the same footing, is moved from sand onto soft clay?

A shallow foundation transfers a building's weight to the soil through a footing, producing a bearing pressure q = m·g ÷ A. Building codes give presumptive allowable pressures for each class of ground, to be used where no full geotechnical evaluation is done.

This lab uses those presumptive values to ask what happens when a 300 t building on a 40 m² footing is moved from sand to clay or silt, and how large the footing must become to bring the pressure back within the allowable value.

Olly's building stands on sand. Its big, flat foot spreads out its weight.

The sand can hold it. The building is safe.

In the model, a 300-tonne building rests on a footing that covers 40 square meters of sandy ground. Its weight presses on the sand with a pressure of about 74 kilopascals (the readout shows 73.6).

The model's safe limit for sand is about 96 kilopascals, so the building uses 77% of it. Safe! In fact, the model says any footing bigger than about 31 square meters would be safe on this sand.

The bearing pressure is 300,000 kg × 9.81 m/s² ÷ 40 m² = 73.6 kPa. For sand, silty sand and clayey sand, the code's presumptive value is 2,000 psf, or 95.8 kPa, so the footing uses 77% of the allowable pressure and the verdict is safe.

The smallest safe footing on sand is A_min = m·g ÷ q_allowable = 30.7 m².

Now Olly moves the same building onto soft clay.

Clay can't hold as much. The building is now a little too heavy for it, so it sinks!

Now switch the ground to clay or silt. The building and its footing are exactly the same, so the pressure is still 73.6 kilopascals. But the model's safe limit for clay is only about 72 kilopascals.

The building now uses 102% of the safe limit. That's just over, but the model says the ground would squash, so the building would sink and might crack or tilt. Make the footing a little bigger, 42 square meters, and it's safe again. Shrink it to 13 square meters, and the model says the clay would give way.

On clay or silt, with a presumptive value of 1,500 psf (71.8 kPa), the same 73.6 kPa uses 102% of the allowable pressure; the model's rough rule classes 100–300% as settlement (sinking, cracking or tilting) and more than 300% as bearing failure.

The smallest safe footing on clay is 41.0 m², a third larger than the 30.7 m² sand needs, because 2,000 ÷ 1,500 = 1.33. At 42 m² the use drops to 98%; at 30 m² it is 137%; at 13 m² it reaches 315% and the model predicts the soil gives way.

Pressure is how hard something pushes on each bit of ground.

A bigger foot spreads the push out. Then each bit of ground gets a gentler push.

Pressure is the force divided by the area it's spread over. The building's weight doesn't change, but spreading it over a bigger footing lowers the pressure on every square meter of ground.

Soft clay can only take a gentle press before it starts to squash. So a building on clay needs a wider footing than the same building on sand, gravel or rock.

Since q = F ÷ A, the footing area needed scales inversely with the allowable pressure: A_min = m·g ÷ q_allowable. With presumptive values of 1,500, 2,000, 3,000, 4,000 and 12,000 psf, a 300 t building needs 41.0, 30.7, 20.5, 15.4 and 5.12 m² of footing on clay, sand, gravel, sedimentary rock and crystalline bedrock.

The model's limits: an even pressure under a shallow footing, one presumptive value per ground class, and a rough three-band verdict. It does not calculate how far a building sinks, how soil changes with depth and water, or deep foundations such as piles. The code itself requires a soils investigation where ground weaker than 1,500 psf is likely.

Builders make footings wider on soft ground.

A famous tower in Italy leans because its ground is soft!

Building rules really do ask for wider footings on softer ground. In the Residential Code of Ohio, a two-story house with a basement needs a footing 22 inches (about 56 centimeters) wide on clay or silt, but only 16 inches (about 41 centimeters) on sand and 12 inches (about 30 centimeters) on gravel, in the table for the lightest snow loads.

The Leaning Tower of Pisa in Italy shows what can happen when ground settles unevenly. Building began in 1173. Britannica explains that after three of its eight stories were built, its foundations had already begun settling unevenly in the soft ground.

Table 403.1(1) of the Residential Code of Ohio, based on the International Residential Code, sets minimum footing widths by soil load-bearing value. For a two-story light-frame house with a basement and a 20 psf snow load, it requires 22 inches on 1,500 psf soil, 16 inches on 2,000 psf and 12 inches on 3,000 psf. Where compressible or shifting soils are present, section 401.4.3 requires them to be removed or stabilized.

Britannica attributes the lean of the Leaning Tower of Pisa, begun in 1173, to uneven settling of its foundations in soft ground. A stabilization project began in 1990: earth was removed from beneath the foundations, and by 2001 the lean had been reduced.