All buildings, whether single or multi-storey, have to be founded on the soil. They transmit their weight and occupancy loads to the soil stratum.
Safety and serviceability of the building depend critically on the foundation: the sub-structure that distributes the load on to the soil.
The builder can choose proper material for construction of the building such as steel, RCC or timber. However, in most cases, he has to make use of the site given to him though it may have unsatisfactory soil condition. In a few cases, the soil condition may be improved by ground improvement techniques but by and large soil is accepted as available and a suitable foundation is chosen.
Soil has a wide range of characteristics depending on the type of deposit at the chosen site. Even in a given site, soil taken from two locations from the same stratum show widely varying properties. It is also affected considerably by environmental changes. Some clay which are very hard when being dry lose their bearing capacity when they are wet. Vibrations alter the characteristics of sand differently under wet and dry conditions.
Therefore, it is important to have a soil investigation done before choosing a suitable foundation. Foundations can broadly be grouped as either shallow or deep. A shallow foundation transmits structural loads to soil strata occurring at a relatively small depth. In deep foundations, the load is supported by frictional resistance around the foundation and/or end bearing when they rest on rock or hard stratum. The method of *in situ *construction of deep foundation makes it impossible to visually inspect the quality unlike shallow open foundations.
Shallow foundation: Shallow foundation must be properly located and founded at a depth where it is not affected by volume changes caused by weathering, expulsion of soil from beneath or effect of adjoining structure foundations. The depth at which it is founded should have adequate bearing capacity.
The settlement, especially the differential settlement, will affect the utility of the building and cause damage to the structure. Hence, both settlement and differential settlement should be within limits.
The above three requirements, proper location, adequate bearing capacity and permissible settlement should be satisfied individually.
Deep foundation: In situations where the soil at shallow depth is poor, the depth of foundation is increased till suitable soil stratum is met. Piles and piers are examples of deep foundations.