FOUNDATIONS: AN EASY WAY TO DECIDE THE BEST TO USE
Foundations are horizontal or vertical structural members that support structures and transmit loads to the soil below at optimum cost. A good foundation be able to:
1. Increase structural stability
2. Distribute the weight of the structure over a large area of soil
3. Avoid unequal settlement
4. Prevent lateral movement of the structure
Globally, there are various kinds of soil with different load bearing capacities. So, depending on the soil profile, size and load of the structure, engineers chose the most suitable of the different types of foundation for the construction projects. These various types are discussed as we move on.
TYPES OF FOUNDATION AND THEIR USES
Foundations can be broadly classified as:
1. Shallow foundation
2. Deep foundation
Shallow foundation
Shallow foundations are those found near the finished ground surface. It is used in areas where the bearing capacity of the soil is high at shallow depths. Generally, the founding depth is less than the width of the footing or less than 3m. Here, the bearing capacity of the foundation is affected by the surface conditions. Foundation types under this include:
· Individual footing or isolated footing or pad foundation
It is used when the load from the structure is carried by the columns (Fig 3). Usually, each column has its own footing which may be rectangular, square or circular in shape. It is suitable when columns are not closely packed and the load of the structure is relatively low
· Combined footing
Here, two or more pad foundations are combined because the columns are close enough and their isolated footings intersect each other. The objective is to achieve uniform distribution of loads under the entire area of footing. It can also be used to prevent the foundation footing from crossing the property line (as seen in Fig 4)
· Wall footing or Strip foundation
It is used to distribute loads of structural load bearing walls to the ground. The width of the wall foundation is usually 2 to 3 times the wall of the foundation footing. It spreads the weight of the material over a wider area thus offering better stability. It is used where you have strong soil base and in non-waterlogged areas. Generally, the weaker the soil, the wider the strip.
· Raft or mat foundation
They are spread across the entire area of the building to support structural loads from columns and walls. It greatly tackles the problem of differential settlement experienced by the aforementioned foundation types. Concrete is spread over this base from the base of the foundation to the German floor, otherwise known as DPC (Damp Proof Course). They are most often used when basements are to be constructed. It is suitable in areas where the soil is sandy and loose, that is, the bearing capacity of the soil is poor, where the structure will be subjected to shocks and jerks or in waterlogged areas though in this case the building must have lesser storeys.
· Cantilever or strap footing
These are foundations that are similar to combined footing in that it consists of two or more column footings that are well connected by a concrete strap beam. The footings under the columns are act built individually and connected by strap beam. It can also be used to prevent the foundation footing from crossing the property line.
Deep foundation
The shallow foundations may not be economical or even possible when the soil bearing capacity near the surface is too low. In those cases, deep foundations are used to transfer loads to a stronger layer, which may be located at a significant depth below the ground surface. The load is transferred through skin friction and end bearing
· Pile foundation
A pile is a slender member with a small cross-sectional area compared to its length. Piles transmit loads by either skin friction or bearing. Through columns, heavy loads are transferred to hard soil strata which is much below the ground level; preventing uplift of structure due to lateral loads such as earthquakes and wind forces. It is suitable for multi-storey buildings. It is used in marshy areas and where the bearing capacity of the soil near the surface is relatively low and where the topsoil layer is compressible in nature. In addition to supporting structures, piles are also used to anchor structures against uplift forces and to assist structures in resisting lateral and overturning forces.
· Drilled shafts or caisson foundation
They are similar to pile foundations except that they are cast in-situ. It is suitable for soft clays and loose, water bearing granular soils, or where artesian aquifers exist. A caisson is a impermeable structure which can be floated to the desired location and then sunk into place to the desired level and then filled with concrete which ultimately converts to a foundation. It is mostly used as bridge piers and in structures which require foundations beneath water bodies. It reduces noise and vibration. It resists loads from structure by shaft resistance and/or toe resistance
· Pier foundation
A pier is an underground structure that transmits heavier load which cannot be transmitted by shallow foundations. It is usually shallower than pile foundations. Unlike pile, it can transfer load by bearing and not by skin friction. It is used where sound rock strata lie under a decomposed rock layer at the top, where a heavy load is to be transferred to the soil and where the topsoil is stiff clay which resists driving the bearing pile
Others foundations include:
· Benching or Stepped foundation
It offers a solution to finding a foundation for uneven (sloppy) terrain. Here, the excavation trenches are made in form of steps of equal depth and length (Fig 10). The aim is to avoid unnecessary cutting and filling thus minimizing cost. The plinth of the structure should start after the highest point of the ground. Sometimes, reinforced concrete pile is driven along the lowest base of the footing to prevent lateral movement
· Inverted arch foundation
It is used in places in weak soils and in areas where deep excavation is not possible or the load of the structure is concentrated over pillars. The arrangement of the pillars determines the span of the arches. The thickness of the arch ring should greater or equal to 30cm
· Grillage foundation
It is suitable for areas where the bearing capacity of the soil is very poor yet it is required to carry a very massive load. They are provided for the construction of stanchions.
CONCLUSION
The foundation is the most important part of a structure and serious attention must be taken to chose the best type that will support the structure. The basis for this choice will vary from site to site and from building to building. Also, cost can also be a determining factor. If the foundation fails, the whole building is going to collapse. So, engineers need to make this a top priority during construction to avoid delays in projects and ensure adequate financial management.
