The excavation should be made to a sufficient depth so as to get below the frost line. The ground should be tamped thoroughly, and the excavation filled with cinders, broken stone, gravel, or brickbat, to within four inches (or whatever thickness of slab is to be used) of the top of the grade. The foundation should be thoroughly rammed, and by using gravel or cinders to make the foundation, a very firm surface can be secured

Fig. 76. Stone Arch Culvert. See Fig. 77 for Photographic View.

Fig. 76. Stone Arch Culvert. See Fig. 77 for Photographic View.

Side drains should be put in at convenient intervals where outlets can be secured. The foundation is sometimes omitted, even in cold climates, if the soil is porous. Walks laid on the natural soils have proven in many cases to be very satisfactory. At the Convention of the National Cement Users' Association, held at Buffalo, N. Y., January 21 to 23, 1908, the Committee on Sidewalks, Streets, and Floors presented the following specifications for sidewalk foundations:

"The ground base shall be made as solid and permanent as possible. Where excavations or fills are made, all wood or other materials which will decompose shall be removed, and replaced with earth or other filling like the rest of the foundation. Fills of clay or other material which will settle after heavy rains or deep frost, should be tamped, and laid in layers not more than six inches in thickness, so as to insure a solid embankment which will remain firm after the walk is laid. Embankments should not be less than 2\ feet wider than the walk which is to be laid. When porous materials, such as coal ashes, granulated slag, or gravel, are used, under-drains of tile should be laid to the curb drains or gutters, so as to prevent water accumulating and freezing under the walk and breaking the block."