Since before Rumi and long after Thoreau, foundations have served mystical poets and pragmatic writers alike as suitable metaphors for their varying artistic agendas. As perhaps it is the realm of Thoreau to instruct one in the building of abodes in the air, it is the realm of this article to delve into the deep (or shallow) world of house foundations.
The foundation is that entity of a house, which working collaboratively with the bearing layer of the soil on site, dissipates the weight of a home safely into the ground. In principle, all types of house foundations are classified as either shallow or deep.
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Shallow foundations are a type of house foundation that operate at level relatively close to the natural ground line. This is in part because the loads that they are required to transmit are comparatively small and also that the soil type has a safe bearing capacity close to the natural ground level.
This type of house foundation is employed when the load of a house is carried by columns. It is employed when the columns in a house are significantly spaced and it can be proved conclusively that no differential settlement will occur. An isolated footing dissipates the weight it is carrying onto a single footing (also known as a pad). Depending on the column load and the soil type on site, the isolated footing can be cast in the shape of a square, rectangle or circle and has varying profiles which are either sloped, stepped or simple. Because it is the simplest to implement, the isolated footing is both a time and money saver.
Combined and Isolated Footings share a similar working principle, but differ in that, a Combined footing is a type of house foundation used when structural columns occur in close proximity to each other, causing an uncomfortable situation in which the subterranean footings to overlap. This is averted by creating a footing on which all the columns in close proximity to each other sit. This dissipates their combined weight safely into the bearing layer of the soil.
The Strap Footing is a type of house foundation similar to a Combined Footing in which two or more footings are connected by a beam which is called a Strap. It is used when one column sits near a restriction such as a sewer line, compromising the shape of its footing. The column, due to an inadequate footing, will experience irregular bending moments and will in effect, need to be braced. This bracing is facilitated through the use of the strap which is connected to the column just above the ground to prevent the transfer of shear forces into the soil. This transfers the weight of an outer column onto an inner one.
This is the type of house foundation used with load bearing walls. In profile, the width of a Continuous Footing is normally two or three times the width of the wall that sits on it. Due to their continuity, strip footings transfer the load of a house structure over a greater area providing greater stability. This makes it ideal for a house in which walls, as opposed to columns are the main structural elements.
The Mat Foundation can be imaged as a type of house foundation in which an immense footing spread across the entire area of a house, bearing the weight of all the house’s walls and columns. It is used when a house’s structural loads are relatively high and the soil’s bearing capacity is low. Due to its expanse, it dissipates weight much more efficiently, reducing the stress the soil experiences per unit area. It is also economically viable when the combined area of isolated and strip footings occupies more than half the area of the house. This foundation is ideal for houses with a basement.
Deep foundations, as the name implies, are a type of house foundation that work at an immensely greater depth than shallow foundations. This is due to the surface soil’s inability to bear the weight of the house. They are also particularly useful for those house builders who tend to be unconventional in their choice of site.
Piles are in essence, columns that penetrate deep into soil, reaching the rock strata. A pile foundation is used where the soil near the surface is inadequate for bearing a house’s load. This maybe because this soil is prone to rampant erosion or that the site is located in neighbourly proximity to water bodies. Piles work by friction, and as such, the strength they provide is in direct proportion to the depth to which they are depressed. They are also the best foundation for areas prone to lateral loads such as high wind speeds and earth movements.
A Caisson is a hollow prefabricated construction of concrete that is placed in the desired location at a desired pre-excavated depth, and then filled with concrete from above, forming a solid foundation. Caissons are a type of house foundation especially used in or near water bodies and work similar to piles in that the bearing layer of the soil is beneath vast layers of unsuitable material such as peat and in most cases, water. Even though caissons can carry immense loads, they are generally the most expensive type of foundation and require a number of specialists to execute.
After our exploration of the subterranean world of house foundations, it is hoped by this article, as well as Henry David Thoreau perhaps, that you have emerged established in a new knowledge that will help you choose the type of foundation that best fits under your most ambitious of air castles.