Chicago's bedrock at 80' is relatively close to the surface. In central London you would hope for highly compacted sand at about 50m (150') deep. Above this is London clay and a whole lot of river basin sediment. London's tall buildings are either founded on the sand with very long piles which are tricky to construct as they are subjected to groundwater at high pressure. The base of the pile is further under water than a WW2 submarine would typically operate so it's difficult to prevent the pile from collapsing while inserting concrete at the bottom. Or they rely on the stickiness of the clay layer by using large diameter (up to 2.5m/8') piles that rely on a large amount of surface area in contact with the clay. Next time you look at one of the funny shaped buildings on London's horizon, think that it could be a little bit like a 100,000 tonne water boatman insect :-)