=The Cofferdam Method.=--=The Van Buren Street Tunnel, Chicago River.=--According to the cofferdam method, the work is attacked at one of the shores, and the tunnel built in sections of such length as not to interfere with the flow of water or the navigation of the river. Round the entire exterior line of the first section a double-walled cofferdam is built, and strongly braced transversely, so as to withstand the pressure of the water. When the water is pumped out, a single-walled cofferdam is built within the first, leaving sufficient distance between the two to allow of the construction of the masonry. The soil is then removed within the inner cofferdam, and the tunnel constructed from the foundation. When the end of the tunnel reaches the channel end of the cofferdam, a crib-wall is erected over the end of the completed tunnel. This crib, in turn, forms the end wall of another cofferdam, built in continuation of the first, so as to allow the second section to be proceeded with, and at the same time to facilitate the removal of the cofferdams of the first section. The work goes on continuously in this way until the distant shore is reached. VAN BUREN STREET TUNNEL, CHICAGO. The Van Buren Street tunnel, built to carry a double-track street railway under the Chicago River, was completed in 1894 by the cofferdam method. The special features of the tunnel[14] are: (1) the unusually large dimensions of the cross-section of 30 ft. × 15 ft. 9 ins.; (2) its construction inside of cofferdams of great length and width; (3) the construction under some very high buildings calling for great care and very strong temporary and permanent supports. [14] “Eng. News,” April 12, 1892. The special feature of the work for our present purpose was the construction of the tunnel across the river. To accomplish this a cofferdam was built out from the west shore of the river to its middle, and the tunnel constructed within it like the building of any other structure within a cofferdam. Transverse and longitudinal sections of this cofferdam are shown by Fig. 141. As will be seen, it was a simple double-wall cofferdam, with a clear width between the walls of 58 ft., and braced transversely as shown. Inside of this a single-wall cofferdam of piles was constructed, with a clear width just sufficient to allow the construction of the masonry within it. When the tunnel end reached the channel end of the cofferdam, a crib-wall was built over the end of the completed tunnel, as shown by the drawings. This crib-wall was intended to form the end wall of another cofferdam, which was built out from the east shore, and within which the remaining half of the tunnel was built as the first half had been. The drawings show the character of the tunnel masonry and of the centering upon which it was built. [Illustration: ~TRANSVERSE SECTION OF COFFERDAM AND TUNNEL~ ~SECTION SHOWING METHOD OF CONSTRUCTING CRIB DAM.~