Engineering Wonders of the World
Volume I

THE UNDERGROUND RAILWAYS OF CHICAGO. . 361 The construction of a network of lines far below the surface of a city’s streets is in itself a difficult feat, but in the case of Chicago the engineering problems to be overcome were of the most perplexing character. The city is built on a vast deposit of soft clay, of a consistency resembling that of cheese. Heavy buildings have to be supported on piles or concrete piers, -which pierce this clay until they finally reach bedrock, from 80 to 120 feet below the street level. The weight of many of these buildings is enormous ; and while it was not contemplated that the tunnels should go under them, but under the middle through the centre of the street, and the tremendous weight resting on each side, might lead to settling and straining of the build- ings, or even total collapse. How real this danger was the construction of the New York Subway demonstrated at a later date, when several buildings were entirely wrecked be- cause of the excavations in the street which they faced. It is one of the most astonishing facts in the history of the Chicago Tunnel construc- tion that, though the work lasted seven years, though every street in the business section of the city has been tunnelled, and TypiCAL STREET INTERSECTION. Observe the peculiar arrangement of double points. of the streets on which they abutted, there was a possibility that the removal of the amount of material necessary for the bores some 60 miles of subways have been con- structed, there has not been a single claim for damages on the part of any property