Foundations and Footings

At BuildingeKart, our engineers strive to maintain transparency with our clients and keep them aware of the construction nuances. As a step towards this, we bring to you various ideas and technologies about civil engineering and construction through our articles. Today's article talks about  the most crucial aspect of your building- the foundation and footing.

The foundations of the building transfer the load of the building to the ground. Hence we must determine the safe bearing capacity of soil before constructing a home. As the name suggests, “this is the amount of weight per unit area that the soil can bear”.

footing

This capacity also changes at different depths of the soil. In general, the deeper one digs the greater the soil bearing capacity, unless there is a pocket of weak soil in the earth. It is common that the soil near the surface of the earth has a weak safe bearing capacity. Hence, to properly support the building, the soil must be firm and strong. If a building is to rest on the soil, it will sink into the earth like a ship in the water. Building contractors usually dig until they reach very firm and strong soil.

Usually, every building has a number of individual foundations commonly called as footings. And each footing has a column.

All foundations are classified into two categories, namely shallow foundations and deep foundations. The names shallow and deep refers to the depth of the soil in which the foundation is made. Shallow foundations can be made in depth of 3ft. and deep foundations can be made at depths of 60 – 200ft. Shallow foundations are used for small, light buildings while deep ones for large and heavy buildings.

Shallow Foundations:

                       Shallow foundations are also called spread footing and open footings. 'Open' refers to the fact that the foundations are made by first excavating all the earth till the bottom of the footing, and then constructing the footing. During the early stages of work, the entire footing is visible to the eye. And therefore it is called as open footing.  The idea is that each footing takes the concentrated load of the column and spreads it out over a large area so that actual weight of the building on the soil does not exceed the safe bearing capacity of the soil.

There are several shallow foundations: Individual footings, strip footings, and raft foundations.

In cold climate, the Shallow foundations must be protected from freezing. This is because the water in the soil around the foundation can freeze and expand, thereby damaging the foundation. These foundations should be built under the frost line. This is the level within the ground above which freezing occurs. If they cannot be built below the frost line, they should be protected by insulation. Normally a little heat from the building will permeate into the soil and prevent freezing.

Individual Footing:

Individual footings are one of the most simple and common types of footings. These are used when the load of the building is carried by columns. Usually, each column has its own footing. The footing is just a rectangle or square on which column sits. To get the rough idea of the size of footing, we will take the total weight on the column divide it by safe bearing capacity of soil.

For example:

The column has a vertical load of 10T and sharing capacity of soil is 10T/mm2 then area of footing is 1mm2.

  • Individual footings are connected with a plinth beam, it is a Horizontal beam, and it is constructed at the ground or below the ground level.

Strip Footings:

               Strip footings are commonly found in load-bearing masonry constructions and act as a long strip for an entire wall. These are used where the building loads are carried by entire walls rather than isolated columns such as older buildings made of masonry.

Raft foundations:

Raft foundations are also called mat foundations. These are used where the soil is weak and where the columns are closely spaced.

Summary

Foundation is the part of structure below plinth level up to the soil. It is in direct contact with soil and transmits load of super-structure to the soil. Generally, it is below the ground level if some part of the foundation is above the ground level, it is also covered with earth filling. This portion of the structure is not in contact with air, light etc. hence it is called as hidden part of the structure.

Steps for constructing the foundation work:

  1. Excavation of earth work in trenches for foundation.
  2. Laying out cement concrete.
  3. Laying the footing in case of column construction.
  4. Laying Anti termite treatment.
  5. Laying brick work up to the plinth level.
  6. Laying damp proof course on the walls.
  7. Refilling of earth around the walls.

Depth of foundation:

Depth of foundation is based on the following factors

  1. Availability of adequate bearing capacity.
  2. Depth of shrinkage and swelling in case of clay soil.
  3. Depth of ground water table.
  4. Depth of frost penetration in case of fine sand and silt.
  5. Practical minimum depth of foundation should not be less than 50 cm. To allow removal of top soil and variations in ground level.
  6. Hence best recommended depth of the soil is from 1m to 1.5m from original ground level.

Precautions during excavation of earth for foundation:

The depth and width of foundation should be according to structural engineer

  1. The depth of the soil should not be less than 1m if the design is not available.
  2. The length, width, and depth of the excavation should be checked with the center line and level marked on the burjis.
  3. The excavated material should dumped at a distance of 1m from the edges.
  4. Work should be done on dry soil.
  5. Arrangement of water pump should be made for pumping out the rain water.
  6. The bottom layer of foundation should be compacted

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