Underpinning; It’s Not a Choice, It’s a Necessity

Underpinning is the process of providing additional bearing capacity to the foundation in order to make it capable of withstanding the additional load of an added story or added walls. It is generally employed in cases wherein the foundation seems to lose its power of holding the structure for which it was laid for. Thus, it is nothing but an agent which helps in augmenting the life of the structure.

In order to grant the structure with additional load bearing capacity, strong concrete mix is put to use to make it reach a level wherein strong support can be provided to the infrastructure. There are a number of underpinning methods which are used to provide the foundations with the structural stability. These include-Mass concrete underpinning method (pit method), Traditional box underpinning, Underpinning by cantilever needle beam method, Pier, and beam underpinning method, Mini piled underpinning, Pile method of underpinning and Pre-test method of underpinning.

Which technique of undermining should be deployed depends upon how old the structure is. The three major categories are- Modern Structures-which have an age of less than 50 years, Recent Structures- which have an age between 50-150 years and Ancient Structures-which have an age of more than 150 years.

Apart from the age factor, ground conditions and foundation depth also plays a significant role in the selection of the best and appropriate undermining methods. A brief description of each of these methods is given below-

• MASS CONCRETE UNDERPINNING METHOD- Also regarded as the Pit Method, this is the traditional technique of underpinning which aims at extending the traditional foundations of the structure up till the point it reaches a level where it can get a stable support. After this level is reached via excavation, it is filled with concrete. This method is best suited for shallow foundations and is quite economical in nature.

• PILE METHOD-In this method of underpinning, piles are used to provide support to the weak foundation of the structure. This method is best suited to cure settlement in soil that may be caused due to the clayey nature or due to water clogging.

• PRE-TEST METHOD-This technique is usually put into use for strip or pad foundations and is feasible to perform in case of buildings with 5-10 stories. The pre-requisite for performing underpinning through this method is lack of noise and minimal disruptions. However, if you are looking for an apt underpinning method for raft foundation, this might not be the one for you.

• MINI PILED UNDERPINNING-This method can be employed when the load is to be transferred from the foundation to the stratum which is located at a distance of more than 5m. Mini Piled method of underpinning can be used in areas where the soil is of some variable nature.

• PIER AND BEAM UNDERPINNING METHOD- This method came into being when the mass concrete method failed to do wonders in case of huge depth of foundation. In this case, concrete beams are driven to transfer the load from the weak foundations to concrete piers. This method has proven to be expedient for nearly all types of ground conditions and may be the one that you are looking for.

While selecting the underpinning method an important aspect which needs to be given utmost attention is the extent of pressure and disturbance it will cause to the building. Different methodologies have different effects and most of these have adverse effects on the external part of the buildings. So, in order to prevent events of total collapse, a proper survey should be conducted on part of the structural engineer in order to keep track with the safety measures.

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