Buildings
Foundations Are Chosen By The Ground, Not By The Building
What sits below a building is decided by soil, water and neighbours rather than by the design above, which is why identical schemes cost differently on adjacent plots.
Two identical buildings on neighbouring plots can differ substantially in cost before anything appears above ground. The reason sits in the soil investigation rather than in the drawings.
Ground conditions are discovered, not designed
A site investigation drills a limited number of holes and infers what lies between them. The picture it produces is an interpretation, not a survey of everything under the plot.
Made ground, old foundations, buried services and pockets of soft material sit between the boreholes and appear only when excavation starts.
Because the risk cannot be eliminated, it is priced. Contractors either carry a contingency or push the uncertainty back to the client through the contract, and either route costs money.
Bearing capacity sets the foundation type
Where competent ground sits close to the surface, loads can be spread through shallow pads or strips, which are quick to build and use relatively little material.
Where the surface layers are weak, load must be carried down to something stronger, either by end bearing on a firm stratum or by friction along the length of a pile.
The switch from shallow to piled foundations changes the plant on site, the programme and the sequence, and it happens because of the ground rather than because the building grew.
Water is the variable that complicates everything
A high water table turns a straightforward excavation into a dewatering exercise, with pumping, discharge consents and the risk of drawing down water beneath neighbouring structures.
Below the water table, a basement must resist uplift as well as carry load, because the structure behaves like a hull trying to float in saturated ground.
Waterproofing then becomes a system rather than a coating, with drained cavities and maintainable elements, since a defect below ground cannot be reached once the building is complete.
Neighbours constrain the method
On a tight urban plot the excavation edge sits close to existing structures, so the sides must be supported before digging rather than battered back to a safe slope.
Secant walls, sheet piles and propping are chosen for what they do to the neighbours, not for what the new building needs, and they are among the more expensive elements of the job.
Movement monitoring runs throughout, because the obligation is to avoid damaging adjacent property, and that obligation shapes the sequence more than structural efficiency does.
Contamination changes the disposal question
Former industrial land often contains material that cannot go to an ordinary tip, and disposal is charged by weight with the more contaminated grades costing considerably more.
Designers respond by reducing the volume dug out, which pushes schemes towards shallower basements and foundation types that displace less soil.
The building above may be unchanged, but its economics have been rewritten by what previous occupants left in the ground.
Questions readers ask
Can old car parks become housing?
Some can, mostly those with flat decks, external ramps and generous clear heights. Continuously sloping decks are rarely worth converting, and demolition often prices out as the cheaper route.
Why are car parks placed at the base of buildings?
Because building below ground is expensive and building above is structurally awkward. The base is the cheapest place to put them, and it is also the part of the building the street uses.
