Foundations of Buildings and Structures

the parts of buildings and structures, usually below ground level, that serve to transfer the loads from the buildings or structures to a natural or artificial bedding (seeBEDDING OF STRUCTURES). Shallow foundations are classified as continuous types under bearing and shear walls (Figure 1,a); continuous types under a series of columns (Figure 1,b); pedestal types under walls; isolated types under columns (Figure 1,c) and, in combination with beams, under walls; combination types in the form of uniform (Figure 1,d) or ribbed slabs (under an entire structure or part of a structure); and mat, or raft, foundations (under an entire structure). Such foundations usually consist of a series of descending steps of increasing

Figure 1. Cast shallow foundations: (a) continuous under a wall, (b) continuous under columns, (c) isolated under a column, (d) combination under columns; (1) lower continuous footing of reinforced concrete, (2) foundation wall (3) column

size. The upper surface of a foundation, which separates it from the part of the building or structure lying above, is called the sill, and the lower surface, which is supported by the bedding, is called the footing. The distance from the sill to the footing is known as the height of the foundation, and the distance from a level mark on the earth’s surface to the footing is called the depth. In the upper parts of isolated foundations, known as subcolumns, a recess or socket is provided for mounting columns.

The type of foundation chosen depends on the geological, engineering, and hydrogeological conditions at the construction site, the purpose and design of the building or structure, the magnitude of the load imposed on the foundation, and the production capacities of the construction organization. The depth is chosen according to the properties and nature of the soil strata, the groundwater level (with allowance for variations during the construction and use of the structure), the magnitude and nature of the loads acting on the foundation, the depth of underground communication lines and of the foundations under machines and equipment, and the climatic characteristics of the construction region (such as the depth of seasonal freezing). The depth chosen must be sufficient to ensure the stability of the bedding and to eliminate the possibility of the earth heaving during freezing and settling during thawing. When the groundwater level in earth not subject to heaving lies a substantial distance below the surface, the footing can be laid above the freezing depth.

The dimensions of a footing are determined according to bedding conditions so that the average pressure on the bedding does not exceed a design value that depends on the nature and properties of the earth, the depth of the foundation, and the design features of the structure. In the assigning of dimensions for the footing of a foundation, allowance is made for the maximum vertical deformations (settling and raising) under which the necessary rigidity of the structures above the foundation will be retained and under which the building will still conform to engineering or architectural requirements. When substantial horizontal loads, including earthquakes, are expected or in the case of water-saturated clayey and peat soils, bedding stability must be ensured.

Design calculations for foundations are computed for strength and the size of cracks that may develop. Shallow foundations are usually monoliths made of stone materials, rubble concrete, concrete, or reinforced concrete. Continuous, combination, and raft foundations and isolated foundations under columns are usually made of reinforced concrete. The materials used must have the requisite water and frost resistance. In modern construction it is very efficient to use precast continuous foundations under building walls; they are made of standardized, reinforced-concrete pillow blocks and concrete central blocks or slabs. The pillow blocks may be laid with gaps, thus forming a discontinuous footing. Settling is less for such discontinuous footings than for continuous footings, so that the pressure can be increased by 20 to 30 percent. Precast footings under isolated columns and pillars are made up of pedestal units or several pillow blocks.

Where the groundwater level is high, foundations for buildings with basements must be waterproofed to avoid the possibility of flooding the basements. To protect foundations from the effect of corrosive groundwaters, dense concrete with special additives is used as well as oiling, cementing, and other waterproofing.

Shallow foundations are generally constructed in excavations or trenches. A common method is to pack down the area under isolated foundations or trenches under continuous foundations with tampers. This makes it possible to eliminate earthwork operations, and the bedding soil can be made more compact. About 80 percent of residential and industrial buildings have shallow foundations.

Deep foundations are laid with drill-filling and driven piles (seePILE FOUNDATION), deep piers (driven or made of casings), and caissons. Their use is recommended for weak, subsiding, swelling, and other soils having special properties and a high ground-water level, particularly in the building of bridges and deep underground structures.