For arch culverts on very steep side-lying ground it is better to build the arch-top in steps, as shown in Fig. 72, instead of forming it parallel to the invert, or slope, of the stream-course. The level portions of the arching give a better hold for the embankment than could be obtained on a long inclined surface of brickwork or masonry. The writer has built a large number of culverts of this type for mountain streams on steep hillsides, and has found them to prove satisfactory in every way. In embankments alongside tidal rivers, or across the corners of estuaries of the sea, culverts have frequently to be so constructed that they will permit the passage of the drainage water from the land, or high side, without admitting the tidal water. This can be arranged by placing at the lower end of the culvert close-fitting hinged-flap valves opening outwards. When the tide has gone down the weight of the fresh, or land, water swings the flap-valve sufficiently open to allow of a free passage; and, on the other hand, when the tide rises, the pressure of the water against the face of the flap-valve keeps it tightly closed, and prevents ingress of the salt water. Culverts are sometimes fitted with lifting-valves or doors, which can be raised or lowered to serve irrigation purposes. The door, which works in guides, is made sufficiently heavy to fall with its own weight, and the raising is effected by means of a screwed suspension-rod working in a well-secured fixed nut. In cases of soft or treacherous ground, timber-piling or wide bed-courses of cement concrete are necessary to form firm foundations for culverts. Drains and streams which are intersected by a railway cutting have to be dealt with according to their size and their height above the finished rail level. The water from a small drain or field spring may be conducted in pipes down the slope of the cutting into the water-table, or side drain, at formation level, and will be thus carried away to the lower level at the entrance of the cutting. In many cases streams can be diverted, and the water led away to some 79 lower point without the necessity of actually crossing the railway. With a large stream, where it is essential that the water should be conveyed across the line and continue on its ordinary course, it may be carried over in iron pipes or iron trough if there is ample headway, or in iron syphon pipes where the height is not sufficient. The iron pipes or trough can be supported on masonry or brick piers, or cast-iron columns, the height from the rails to the underside of the conduit being not less than that adopted for the over-line bridges. Occasionally the pipes can be carried across on an over-line bridge, either by placing them under the roadway or on small brackets outside the parapet.