Quite often in cities it is desirable to construct a dry basement in localities where water permeates the soil to within a few feet of the sidewalk.
In such cases it is necessary not only to make the walls and floor waterproof, but also to give sufficient thickness to the floor that the buoyant force of the water will not cause it to break through.
To make the cellar water-tight the entire area of the cellar should be covered with concrete, after the footings of the walls and piers are in, from 3 to 6 inches thick, so that the concrete will be level with the top of the footings. A narrow course of brick or stone should then be laid along the centre of the footings, as shown in Fig. 248, to form a break. Upon the top of the footings three thicknesses of tarred felt or burlap should then be mopped in hot asphalt, the felt being allowed to project 6 inches on each side. A similar layer of felt and asphalt should be laid over the footings of all piers, engine foundations, etc., and allowed to project at least 6 inches on all sides.
After the external walls are completed, and before "filling in," the projecting felting should be turned up and mopped with hot asphalt against the wall, and the entire outside surface of the wall to the sidewalk line covered with three thicknesses of felt laid breaking joints in hot asphalt and overlapping the felt that comes through the wall. For further protection this covering is also frequently plastered with 1 to 2 Portland cement mortar.
Before the completion of the building the entire cellar floor must also be covered with felt in hot asphalt, laid in at least three thicknesses, breaking joint and overlapping the felt first laid. On the top of the felt thus laid there should then be laid Portland cement concrete at least 1 inch thick for each 3 inches in depth of the water above the level of the cellar bottom, with a minimum depth of 6 inches.
The following description of the waterproofing of the basement of the Herald Building, in New York City, is given as an actual example of the above method : *
In this building the printing presses are placed in the basement, and great pains were taken to exclude moisture below grade. The footings and outside basement walls were covered with four-ply burlap mopped on solid, commencing at the inner edge of sidewalk and back over top of vault and down the outside of the wall to the bottom of the same, thence through the wall and turned up against same for connection to the waterproof course.
Beneath the surface of the entire basement, including floor of vaults, the best four-ply roofing felt was mopped on solid, and similar material was used in connection with all piers, extending in each case through the entire thickness of the pier and beneath the entire surface of foundations for boilers and machinery.
The felt was securely lapped and turned up around all walls. Above the felt 4 inches of concrete was laid in the basement and 16 inches in the boiler room.
* From the Engineering Record, July I, 1893.