Foundations

7/18/2019 Foundations

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FOUNDATIONS

The foundations of the building transfer the weight of the building to the

ground. While 'foundation' is a general word, normally, every building has anumber of individual foundations, commonly called footings. Usually eachcolumn of the building will have its own footing.

Since the weight of the building rests on the soil (or rock), engineers have tostudy the roerties of the soil very carefully to ensure that it can carry theloads imosed by the building. !t is common for engineers to determinethe safe bearing capacity of the soil after such study. "s the name

suggests, this is the amount of weight er unit area the soil can bear. #ore$amle, the safe bearing caacity (S%&) at a location could be Tm, or tones er s*uare metre.

This caacity also changes at different deths of soil. !n general, the deeer one digs, the greater the S%&, unless there are ockets of weak soil in theearth. To roerly suort a building, the soil must be very firm and strong. !t is common for the soil near the surface of the earth to be loose and

weak. !f a building is rested on this soil, it will sink into the earth like a shiin water. %uilding contractors will usually dig until they reach very firm,strong, soil that cannot be dug u easily before constructing a foundation.

To study the roerties of the soil before designing foundations, engineerswill ask for a soil investigation to be done. " soil investigation engineerwill drill a + or - hollow ie into the ground, and will remove samles ofthe earth while doing so. e will then send these samles to a lab to find

out the detailed roerties of the soil at every deth. Soil is usuallycomosed of strata, or different layers, each with its own set of roerties. /rilling technology today makes it easy and economical to drill to greatdeths, easily several hundred meters or more, even in hard rock.

The soil investigation team will then reare a soil investigation report thatlists the engineering roerties of the soil at regular intervals, say every meters. %ased on this deort, engineers designing the structure can decideat what deth of soil to rovide the foundations, the tye of foundations they

should rovide, and the si0e of the foundations.



1very once in a while, engineers will find fill at a site. This occurs whenhumans have reviously dug u the earth there, and then filled it back in. This haens if a *uarry was dug or a building built there reviously. Sincefill is loose and soft and cannot suort weight, engineers will dig to a dethbelow that of the fill, where strong soil is found, and construct foundationsthere.

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%roadly seaking, all foundations are divided into two categories6shallow foundations and dee foundations. The words shallow and deerefer to the deth of soil in which the foundation is made. Shallowfoundations can be made in deths of as little as 7ft (8m), while deefoundations can be made at deths of - 9 ft ( 9 -:m). Shallowfoundations are used for small, light buildings, while dee ones are forlarge, heavy buildings.

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Shallow foundations are also called spread

footings or open footings. The 'open'refers to the fact that the foundations aremade by rst excavating all the earth tillthe bottom of the footing, and thenconstructing the footing. During the earlystages of work, the entire footing is visibleto the eye, and is therefore called an openfoundation. The idea is that each footing

takes the concentrated load of the columnand spreads it out over a large area, so



that the actual weight on the soil does notexceed the safe bearing capacity of thesoil.

There are several kinds of shallowfootings individual footings, strip footingsand raft foundations.

!n cold climates, shallow foundations mustbe protected from free"ing. This isbecause water in the soil around the

foundation can free"e and expand,thereby damaging the foundation. Thesefoundations should be built belowthe frost line, which is the level in theground above which free"ing occurs. !fthey cannot be built below the frost line,they should be protected by insulationnormally a little heat from the building will

permeate into the soil and preventfree"ing.

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INDIVIDUAL FOOTINGS

!ndividual footings awaiting concreting of the footing column.

!ndividual footings are one of the most simle andcommon tyes of foundations. These are used whencolumns carry the load of the building. Usually, eachcolumn will have its own footing. The footing is <ust as*uare or rectangular ad of concrete on which thecolumn sits. To get a very rough idea of the si0e of the

footing, the engineer will take the total load on the columnand divide it by the safe bearing caacity (S%&) of the soil.#or e$amle, if a column has a vertical load of 8T, andthe S%& of the soil is 8Tm, then the area of the footingwill be 8m. !n ractice, the designer will look at manyother factors before rearing a construction design forthe footing.



!ndividual footings connected by a linth beam. 5ote that thefootings have been cast on to of beds of lain cement concrete(3&&), which has been done to create a level, firm base for the

footing.

" linth beam, a hori0ontal beam that is built at ground orbelow ground level, usually connects individual footings.

STRIP FOOTINGSStri footings are commonly found in load9bearingmasonry construction, and act as a long stri that suortsthe weight of an entire wall. These are used where entire

walls carry the building loads rather than isolated columns,such as in older buildings made of masonry.

RAFT OR MAT FOUNDATIONS=aft #oundations, also called >at #oundations, are mostoften used when basements are to be constructed. !n araft, the entire basement floor slab acts as the foundation?the weight of the building is sread evenly over the entire

footrint of the building. !t is called a raft because thebuilding is like a vessel that 'floats' in a sea of soil.



>at #oundations are used where the soil is week, andtherefore building loads have to be sread over a largearea, or where columns are closely saced, which meansthat if individual footings were used, they would toucheach other.

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PILE FOUNDATIONS " ile is basically a long cylinder of a strong material suchas concrete that is ushed into the ground so thatstructures can be suorted on to of it.

3ile foundations are used in the following situations6

8. When there is a layer of weak soil at the surface.This layer cannot suort the weight of the building,so the loads of the building have to byass this layerand be transferred to the layer of stronger soil or rockthat is below the weak layer.

. When a building has very heavy, concentrated loads,such as in a high9rise structure.

3ile foundations are caable of taking higher loads thansread footings.

There are two tyes of ile foundations, each of which works in itsown way.



End Bearing Piles

!n end bearing iles, the bottom end o t!e "ile rests ona la#er o es"e$iall# strong soil or ro$%. The load of thebuilding is transferred through the ile onto the stronglayer. !n a sense, this ile acts like a column. The keyrincile is that the bottom end rests on the surface, whichis the intersection of a weak and strong layer. The loadtherefore byasses the weak layer and is safelytransferred to the strong layer.

Fri$tion Piles

#riction iles work on a different rincile. The iletransfers the load of the building to the soil across the fullheight of the ile, by friction. !n other words, the entiresurface of the ile, which is cylindrical in shae, works totransfer the forces to the soil.



To visualise how this works, imagine you are ushing asolid metal rod of say +mm diameter into a tub of fro0enice cream. 4nce you have ushed it in, it is strong enoughto suort some load. The greater the embedmentdepth in the ice cream, the more load it can suort. Thisis very similar to how a friction ile works. !n a friction ile,the amount of load a ile can suort is directlyroortionate to its length.

!n ractice, however, each ile resists load by a$ombination o end bearing and ri$tion.

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