The present foundations of the Chicago Skyscrapers

Abstract

The main characteristics of the land and the supporting rock are given, and the pattern of present performance of Chicago's deep foundations is briefly described. This city probably has some of the highest skyscrapers in the world with deep foundations resting on rock, and possibly as well, the highest building with foundations resting on soil. The construction of piles of large diameter, resting on rock and concreted dry, as prescribed by the City Code, is the weak point which sometimes makes it necessary to insert a permanent lining tube deep into the rock in order to obtain the required leaktightness, or to pump out significant quantities of water from the bottom of the excavations. The latter can cause the dragging away of earth from adjacent foundations resting on soil at a higher level, as with the drift of the phreatic level, with the consequent risk of producing appreciable subsidence, and even undermining. To overcome this problem we carried out for the first time in Chicago, a pre-sealing, by means of chemical injections, of the supporting rock of the periphery piles of some deep foundations, by drilling into the centre of each pile, which has also been useful as a means of examining the subsoil before proceeding to the perforation of each pile. This operation was carried out on the large diameter piles of the foundations of the Northern Trust Bank skyscraper, at present under construction, with very satisfactory results. The method of pre-sealing the rock used and the results obtained are described.