Question

How will you design the foundation of a structure in Swelling soil.

Answer 1

Swelling soil is mainly termed as expansive soil. This soil has the property of swellng and shrinkage in it due to presence of minerals like smectite clay. When the soil comes in contact with water it absorbs the water and swells, similarly during the summer as water evapourate soil shrinks. due to this characteristic the structure undergoes settlement developing cracks or sudden collapse. As a result it is necessary to choose proper foundation for the structure.

Foundation should be selected properly as a result load of structure is distributed and could withstand the stresses from the soil.

As a result before designing the foundation it is necessary to carry out a field visit for a reconnaissance survey of the area to understand the ground conditions. During this survey sample of soils are obtained and nearby structures too are studied for extra details. Other factors like site access and local construction experiences too are required.

From the collected soil sample properties like liquid limit, plastic index, shrinkage limit of soil are studied to obtain the swelling potential of the soil. These readings helps to determine the nature of expansive soil to choose proper foundation to avoid collapse of the structure.

Depending on the loading and the soil conditions the foundation of structure is designed.

1. in case of lightly loading building structure where displacement is 0.5 inches following are footing alternatives

a. Raft Footing - In this case thick slab is provided at bottom to minimise the vertical movement of the Soil, stabilising the structure.

b. Cushion footing - In this method swelling soil is replaced by the non swelling soil below the footing to avoid the expansion of soil thereby maintaining the stability.

c. Pad footing is used where swelling soil is low in thickness and chances of small movement may occur.

2. In case of heavy loading structure with no limit of displacement, deep foundation is mainly used. In deep foundation mainly pile footoing is used to transfer load to stable stratum.

In case of black cotton soil Under-reamed piles are suitable due to high water level in soil & less load carrying capacity. The enlarged portion called bulb of pipe help to achieve more contact surface .

Thus Design of the foundation depend on following factors

1. Soil Characteristics

2. Structure load

3. Proper selection of Foundation.