=Culverts.=--The purpose of culverts in tunnels is to collect the water which seeps into the tunnel from the walls and shafts. The culvert is usually located along the center line of the tunnel at the bottom. In soft-ground tunnels it is built of masonry, and forms a part of the invert, but in rock tunnels it is the common practice to cut a channel in the rock floor of the excavation. Both box and arch sections are employed for culverts. The dimensions of the section vary, of course, with the amount of water which has to be carried away. The following are the dimensions commonly employed: +----------------+--------+--------+----------+-----------+ |KIND OF CULVERT.| HEIGHT | WIDTH |THICKNESS | THICKNESS | | |IN FEET.|IN FEET.|OF WALLS |OF COVERING| | | | | IN FEET. | IN FEET. | +----------------+--------+--------+----------+-----------+ |Box culvert |1 to 1.5|1 to 1.5|0.8 to 1.2| 0.3 | |Arch culvert |1 to 1.5|1 to 1.5|0.8 to 1.2| 0.4 | +----------------+--------+--------+----------+-----------+ It should be understood that the dimensions given in the table are those for ordinary conditions of leakage; where larger quantities of water are met with, the size of the culverts has, of course, to be enlarged. To permit the water to enter the culvert, openings are provided at intervals along its side; and these openings are usually provided with screens of loose stones which check the current, and cause the suspended material to be deposited before it enters the culvert. In cases where springs are encountered in excavating the tunnel, it is necessary to make special provisions for confining their outflow and conducting it to the culvert. In all cases the culverts should be provided with catch basins at intervals of from 150 ft. to 300 ft., in which such suspended matter as enters the culverts is deposited, and removed through covered openings over each basin. At the ends of the tunnel the culvert is usually divided into two branches, one running to the drain on each side of the track. [Illustration: FIG. 48.--Refuge Niche in St. Gothard Tunnel.]