A shallow foundations are often selected when the structural load will not cause excessive settlement of the underlying soil layers. In general, shallow foundations are more economical to construct than deep foundations. Common types of shallow foundations are listed in Table 6.2 and described below
- Spread Footings, Combined Footings, and Strip Footings. These types of shallow foundations are probably the most common types of building foundations. Examples of these types of footings are shown in Fig. 6.40.
- Mat Foundation. Based on economic considerations, mat foundations are constructed for the following reasons:
- Large Individual Footings. A mat foundation is often constructed when the sum of individual footing areas exceeds about one-half of the total foundation area.
- Cavities or Compressible Lenses. A mat foundation can be used when the subsurface exploration indicates that there will be unequal settlementcaused by small cavities or compressible lenses below the foundation. A mat foundation would tend to span over the small cavities or weak lenses and create a more uniform settlement condition.
- Shallow Settlements. A mat foundation can be recommended when shallow settlements predominate and the mat foundation would minimize differential settlements.
- Unequal Distribution of Loads. For some structures, there can be a large difference in building loads acting on different areas of the foundation. Conventional spread footings could be subjected to excessive differential settle-ment, but a mat foundation would tend to distribute the unequal building loads and reduce the differential settlements.
- Hydrostatic Uplift. When the foundation will be subjected to hydrostatic uplift due to a high groundwater table, a mat foundation could be used to resist the uplift forces.
- Post-Tensioned Slabs-on-Grade. Post-tensioned slabs-on-grade are common in southern California and other parts of the United States. They are an economical foundation type when there is no ground freezing or the depth of frost penetration is low. The most common uses of post-tensioned slabs-on-grade are to resist expansive soil forces or when the projected differential settlement exceeds the tolerable value for a conventional (lightly reinforced) slabs-on-grade. For example, post-tensioned slabs-on-grade are frequently recommended if the projected differential settlement is expected to exceed 2 cm (0.75 in). Installation and field inspection procedures for post-tensioned slabs-on-grade have been prepared by the Post-Tensioning Institute (‘‘Design and Construction of Posttensioned Slabs-on-Ground,’’ 2d ed., Phoenix). Post-tensioned slabs-on-grade consists of concrete with embedded steel tendons that are encasedinthickplastic sheaths. The plastic sheath prevents the tendon from coming in contact with the concrete and permits the tendon to slide within the hardened concrete during the tensioning operations. Usually tendons have a dead end (anchoring plate) in the perimeter (edge) beam and a stressing end at the opposite perimeter beam to enable the tendons to be stressed from one end. The Post-Tensioning Institute(‘‘Design and Construction of Post-tensioned Slabs-on-Ground,’’ 2d ed., Phoenix) provides typical anchorage details for the tendons.
- Shallow Foundation Alternatives. If the expected settlement for a proposed
shallow foundation is too large, then other options for foundation support or soil
stabilization mustbe evaluated. Some commonly used alternatives areas follows:- Grading. Grading operations can be used to remove the compressible soil layer and replace it with structural fill. Usually the grading option is economical only if the compressible soil layer is near the ground surface and the groundwater table is below the compressible soil layer or the groundwater table can be economically lowered.
- Surcharge. If the site contains an underlying compressible cohesive soil layer, the site can be surcharged with a fill layer placed at the ground surface. Vertical drains (such as wick drains or sand drains) can be installed in the compressible soil layer to reduce the drainage paths and speed up the consolidation process. Once the compressible cohesive soil layer has had sufficient consolidation, the fill surcharge layer is removed and the building is constructed.
- Densification of Soil. Many different methods can be used to densify loose or soft soil. For example, vibro-flotation and dynamic compaction are often effective at increasing the density of loose sand deposits. Another option is compaction grouting, which consists of intruding a mass of very thick consistency grout into the soil, which both displaces and compacts the loose soil.
- Floating Foundation. A floating foundation is a special type of deep foundation where the weight of the structure is balanced by the removal of soil and construction of an underground basement.
References:
- BUILDING DESIGN AND CONSTRUCTION HANDBOOK for Frederick S. Merritt & Jonathan T. Ricketts