When you add a basement under an entire house, build it in sections. It is costly and hazardous to excavate a large basement under an existing building all at one time. While it is usually poor economy to add a large basement to an existing house, a small basement is (8-10 feet wide, with sufficient space for a furnace) can be added without great cost or labor.
The farther the basement is under the house, the greater the amount of work involved, so locate it near one outside foundation wall. If the basement will be used for heating-plant space, locate itso that the chimney will be accessible from the basement.
Support the building girder, or still, that spans the length of the basement with I-beams or heavy wood beams. These beams replace the piers that supported the building girder before the basement was dug. The ends of the supporting beams must rest in slots in the top of the new basement walls.
A safe distance must be allowed between the basement walls and the foundation walls that are parallel to them. If the basement wall is too close to the foundation wall, pressure exerted on the soil by the foundation wall will be exerted laterally on the basement wall.
The safe distance depends on the type of soil and on the difference in vertical height between the footing of the foundation wall and the footing of the basement wall.
In loose, sandy soil, do not put the new basement wall closer to a parallel foundation wall than 1 ½ feet for each vertical foot distance between the footings of the two walls. For example, if the new footing is 4 feet lower than the old footing, the foundation wall and the basement wall must be 6 feet apart.
The safe distance per vertical foot difference in damp clay is 2 feet; in mixtures of sand, dry clay, gravel, and ordinary soil it is 1 ½ feet; in decomposed rock, cinders, or ashes it is 1 foot. If the soil becomes very wet, greater distances will be necessary and will have to be determined by an engineer.
When the side walls of the basement are built and the building girders are supported on beams, excavate the basement. Remove the earth from the excavation through an opening in the foundation wall. This opening can later become an outside door to the basement.
Build the rear wall of the basement after the excavation is completed. If the foundation of the house is entirely of piers (with or without curtain walls on the perimeter of the house), four basement walls will be needed.
When a chimney is within the area of a new basement, it must be extended to below the basement floor. During the excavation it must be supported on two or three steel beams. Support the ends of the beams on cribbing.
The exact placement of the beams and cribbing will depend on the headroom and work area under the house. Ordinarily, holes are cut in the masonry of the chimney just above the footing and the beams are inserted through the holes and blocked up. The chimney is then extended to a new footing below the basement floor. After the new chimney extension is strong enough to support the weight of the existing chimney, the beams are withdrawn and the holes are filled.
When you extend piers to lower a level, support the building girders on blocking, tear out the old piers, and build new ones. Start with a footing below the basement floor and build up. Pipe or structural steel piers are easier to build than masonry piers.
When you extend a wall or chimney to a lower level with unit masonry, you will probably have to omit the top course of units on the extension because of the unevenness of the bottom of the old footing. You can fill this space with stiff concrete. A boxlike form will keep the concrete in place until it sets.
When pouring concrete extensions, the projection of the footing is chiseled off plumb with the wall in 12- to 18-inch sections, and the concrete is poured through the chiseled out sections.
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