Foundations are the basis for any construction object, and it is essential for any project to determine the most fitting type of foundation. Basically, all foundations are subdivided into shallow and deep ones in respect of the depth of their placement into the ground (Das, 2009, p. 1). Although shallow foundations are substantially less costly than deep ones, today deep foundations are used as they are far safer than shallow ones.
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The essence of the shallow foundations lies in the fact that they distributed the load of the whole building equally to all its volume near the ground surface. Das (2009) defines shallow foundations as “individual footings, square or rectangular in plan, that support columns and strip footings that support walls and other similar structures” (p. 1). Citing Terzaghi’s theory of shallow foundations’ loading capacity, Das (2009, p. 11) also notices that the width of a shallow foundation is approximately equal to its depth, which provides for the proper distribution of the load in relatively similar portions to all the parts of the foundations.
As contrasted to the shallow ones, the deep foundations, according to Kikuchi et al. (2007), are the foundations that are embedded into the construction ground to a much larger extent (p. 187). Deep foundations are also often referred to as pile foundations, and Kikuchi et al. (2007) argue that pile foundations, shaft foundations, reinforced, and pre-tensioned concrete foundations all fall into the category of deep foundations according to their design and expected working functions (pp. 111 – 112). Deep foundations acquire greater popularity nowadays, mainly due to security and safety reasons. Although requiring more work to be carried out and being more costly than shallow foundations, the deep ones dominate in today’s construction.
However, to understand the differences between the two foundation types, it is necessary to discuss how and where they are applied. Thus, Das (2009) argues that the shallow foundation, which he divides into spread footing, strip footings, and grade beams, are used for smaller and less important building if compared to deep foundations (p. 8). More specifically, shallow foundations are used in steel building from six to eight stories, while for concrete buildings the four to five stories limit is determined by the greater weight of concrete. Concerning weight, shallow foundations can be used only if a load for a column is up to 180 tons, the relative maximum soil pressure is 4000 pounds per square foot, and the size of the foundation itself does not exceed 12 feet in total (Das, 2009, pp. 7 – 8).
Pros and Cons
Drawing from the above said, the use of shallow foundations displays a lot of advantages and disadvantages; the former concern mainly the cost of the work and its relative complexity, while the latter touch upon the issues of safety and economic reasonability. In particular, Das (2009, p. 186) and Allen and Iano (2009, pp. 128 – 129) single out such advantages and disadvantages of shallow foundations:
|Less costly;||Limited area of use;|
|Simpler construction process;||Weight, soil, and footing limitations;|
|Easier maintenance and repair works.||Doubted safety of shallow foundations.|
Frost-Protected and Slab-on-grade Foundations
The brightest examples of shallow foundations are frost-protected and slab-on-grade ones. The former are used in Scandinavian countries and other regions where low temperatures are common. Das (2009, p. 132) notices that frost-protected foundations are material-efficient because using special insulation allows reducing the amount of concrete poured below the insulation line. Frost-protected foundations require little excavation and backfill works, while in operation they are compatible with fly-ash concrete and hydronic heating.
As contrasted, slab-on-grade foundations are used in warm climate zones. These foundations are products of mold and concrete mixture, which leaves no space for air between the ground and the foundation itself. So, slab-on-grade foundations are less vulnerable to temperature changes and other natural forces. Finally, Das (2009, p. 133) argues that slab-on-grade foundations are predominantly used on richly clay soils. At the same time, absence of space between the ground and the foundation complicates the foundation repair works and makes remodeling of a building with such a foundation almost impossible without considerable damage to the foundation.
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Deep foundations, as contrasted to shallow ones, are used for safety reasons in most cases. Allen and Iano (2009, p. 193) and Kikuchi et al. (2007, pp. 191 – 192) argue that deep foundations are applied when:
- Upper soil levels are too soft and unable of carrying earthquake produced or other unexpected loads;
- Buildings, like bridges, are subjected to potential flood conditions and need additional support that upper soil levels cannot provide;
- Liquefaction of soils is expected and the damage caused by it to the building should be prevented.
Pros and Cons
Based on the spheres of use of deep foundations, scholars like Allen and Iano (2009, p. 194) and Kikuchi et al. (2007, pp. 191 – 192) single out the following pros and cons of their use:
|Safer foundation design;||Much more costly;|
|Protection of buildings from unexpected damages;||Complicated design process involving numerous tests;|
|Greater financial rationality of their use.||Complex construction process.|
End-bearing pile and Vibroflotation-replacement Stone Columns
The most notable examples of deep foundations are end-bearing piles and vibroflotation-replacement stone columns. The end-bearing piles, as Kikuchi et al. (2007) argue, are used when the ground on which the construction takes place contains a layer of a soft soil, under which there is one or more layers of hard soil (p. 19). Accordingly, the end-bearing piles are embedded into the hard layer and provide the necessary protection for the foundation when the piles reach the layer of the soft soil.
The vibroflotation-replacement stone columns are used to prevent the liquefaction of soil as a result of an earthquake or another natural disaster. Kikuchi et al. (2007) and Allen and Iano (2009) describe the procedure of using vibroflotation-replacement stone columns in terms of digging cylindrical vertical holes later filled with gravel or rock. Stone columns are then embedded into these holes being protected from flood conditions as waters react with the gravel or rock and are dissipated without any damage to the foundation.
Thus, shallow and deep foundations are the two basic foundation types used in modern construction works. The analysis presented above allows seeing that although shallow foundations are substantially less costly than deep ones, today deep foundations are used as they are far safer than the shallow ones. Deep foundations are used in building that require greater responsibility and potentially need additional protection means, while shallow foundations are used only for small-scope construction objects. So, safety and financial reasonability are the main reasons for deep foundations to dominate in construction use nowadays.
Allen, Edward and Joseph Iano. Fundamentals of Building Construction: Materials and Methods. John Wiley and Sons, lnc. 2009. Print.
Das, Braja. Shallow Foundations: Bearing Capacity and Settlement, Second Edition. CRC Press, 2009. Print.
Kikuchi, Yoshiaki et al. Advances in deep foundations: proceedings of the International Workshop on Recent Advances of Deep Foundations (IWDPF07), Port and Airport Research Institute, Yokosuka, Japan, 2007.