How deep are skyscraper foundations?
Deep enough to reach ground that can carry the weight, which is a question about geology rather than about height. The same tower needs a different foundation in Manhattan than it does in Shanghai.
The range, and what actually sets it
A tall building's foundation usually reaches between 15 and 50 m below the street, and only a handful have gone past 100 m. The figure people expect, something proportional to the tower above, does not exist. Depth is set by how far down the first competent layer of ground lies.
That produces results which look backwards. A 300 m tower on rock a few meters under the pavement can sit on a shallower foundation than a twelve-story building on soft clay, because the rock takes the load immediately and the clay never does. Height decides width instead: a taller tower pushes more load and far more overturning moment into the ground, so the foundation grows wider or the piles multiply.
Spread footings, rafts, piles and barrettes
Where rock is at or near the surface, columns bear on spread footings: a widened concrete pad under each one, sized so the pressure stays within what the rock will take. It is the cheapest answer.
A raft, or mat, is one thick slab under the footprint, spreading the load across the building area and bridging soft spots. Chicago engineers worked out the floating raft in the 1880s, when downtown turned out to sit on soft lake clay with rock far below, and the Chicago School's towers still float on those slabs.
Piles carry the load down. An end-bearing pile stops on rock or dense gravel and works like a column in the ground. A friction pile never reaches rock; it hangs in the soil and the load transfers through skin friction along its whole surface. Caissons are the same idea at much larger diameter, and barrettes are rectangular rather than round, which gives more perimeter per cubic meter of concrete, exactly what a friction design wants.
Most supertalls use a piled raft, where both work together: the piles take most of the load and the raft above is stiff enough to tie their heads into one block and spread the core's weight across them. A pile cap does that job under a single column group.
The Burj Khalifa has no bedrock to reach
The Burj Khalifa, 828 m and 163 floors, stands on 192 bored piles 1.5 m in diameter and about 50 m deep, under a raft 3.7 m thick covering roughly 8,000 square meters. Not one of those piles touches bedrock, because Dubai has none within reach, only weakly cemented siltstones and sandstones. The entire load of the tallest building on earth is held by friction along the sides of the piles.
The groundwater there is saltier than seawater, so the concrete was mixed for chemical resistance and the raft given cathodic protection. The Burj Khalifa page covers the tower.
Petronas, and the site that moved
The Petronas Twin Towers hold the record for the deepest foundations under a building. Kuala Lumpur's limestone is riddled with cavities, and the borings found half the original plot on decayed rock and half on soft material. Rather than build across that edge, the towers were shifted about 60 m onto the soft ground and the plan to reach rock was dropped. They rest on 104 barrettes, the deepest around 114 m, under a raft about 4.5 m thick. The Petronas Towers page covers the towers themselves.
Why skylines sit where they sit
Manhattan schist lies close to the surface in Midtown and the Financial District, so columns there reach rock cheaply. Between the two the bedrock drops away under deep glacial deposits and the towers thin out accordingly: the gap in New York's skyline is a map of the rock surface. Shanghai's Pudong alluvium goes down hundreds of meters with nothing solid in between, so the Shanghai Tower needed 980 bored piles and a raft 6 m thick to do what a few footings do in Midtown.
Settlement is designed for, not prevented
Every heavy building settles, and engineers calculate how much. Uniform settlement of several centimeters is acceptable, because the structure goes down together and nothing is bent. What damages a building is differential settlement, one part dropping relative to another, which racks door frames and cracks facades.
The Millennium Tower in San Francisco is the case where the sums came out wrong. The 58-story concrete building, finished in 2008, sits on a thick mat on piles driven into dense sand roughly 24 m down, short of the much deeper bedrock. It settled around 45 cm, and unevenly, so the top leaned by well over half a meter. A retrofit costing about 100 million dollars later drove new piles to rock along two sides.
Water, dewatering and a basement that floats
Most sites have a water table above the bottom of the excavation, so the hole has to be held dry while the concrete goes in. Contractors cut a diaphragm wall or sheet piles around the site and pump the water out, watching the drawdown, because lowering groundwater under neighboring buildings makes them settle too.
Once the structure is up the problem inverts. A deep basement is a large box below the water table and water pushes up on it, so a lightly loaded one can have more buoyancy than weight and wants tension piles or ballast to hold it down. The tower normally settles that argument; a podium beside it may not. The basement is often there for reasons unrelated to structure anyway, because parking and plant rooms want floor area nobody needs a view from, so one several floors down is no evidence of a foundation that deep. The construction sequence takes over after the raft is poured, and the safety page covers the loads the finished building survives.