Underpinning

UNDERPINNING

Underpinning is a method for repair and strengthening of building foundations. There are situations where a failure in foundation or footing happens unexpectedly after the completion of whole structure. Under such a situation, a remedial method has to be suggested to regain the structural stability. The method of underpinning helps to strengthen the foundation of an existing building or any other infrastructure. These involve installation of permanent or temporary support to an already held foundation so that additional depth and bearing capacity is achieved.

TYPES OF WORKS FOR SELECTION OF UNDERPINNING METHODS

Conversion Works: The structure has to be converted to another function, which requires stronger foundation compared to existing

Protection Works: The following problems of a building have to undergo protection works:

❖ The existing foundation is not strong or stable

❖ Nearby excavation would affect the soil that supports existing footing.

❖ Stabilization of the foundation soil to resist against natural calamities

❖ Requirement of basement below an already existing structure

Remedial Works: The following remedial works requires underpinning:

❖ Mistakes in initial foundation design caused subsidence of the structure

❖ Construction on existing structure than building a new one

STRUCTURAL NECESSITY FOR UNDERPINNING

The following are the reasons to suggest underpinning method for stabilization of the substructure:

❖ The degradation of timber piles used as a foundation for normal buildings would cause settlement. This degradation of structures is due to water table fluctuations.

❖ Rise and lowering of the water table can cause a decrease of bearing capacity of soil making the structure to settle.

❖ Structures that are built over soil with a bearing capacity not suitable for the structure would cause settlement.

METHODS OF UNDERPINNING

The choice of method depends on the ground conditions and the required foundation depth. Following are the different methods of underpinning used for foundation strengthening:

❖ Mass concrete underpinning method (pit method)

❖ Underpinning by cantilever needle beam method

❖ Pier and beam underpinning method

❖ Mini piled underpinning

❖ Pile method of underpinning

❖ Pre-test method of underpinning

1. Mass Concrete Underpinning Method (Pit Method)

Mass concrete underpinning method is the traditional method of underpinning, as it has been followed by centuries. The method involves extending the old foundation till it reaches a stable stratum. The soil below the existing foundation is excavated in a controlled manner through stages or pins. When a suitable stratum is reached, the excavation is filled with concrete and kept for curing, before next excavation starts. In order to transfer the load from old foundation to new one, a new pin is provided by means of placing dry sand-cement pack. This is a low-cost method suitable for the shallow foundation.

2. Underpinning by Cantilever Needle Beam Method

If the foundation has to be extended only to one side and the plan possesses a stronger interior column, this method can be used for underpinning. The advantages and disadvantages of cantilever needle beam method are:

Advantages:

❖ Faster than traditional method

❖ One side access only

❖ High load carrying capability

Disadvantages:

❖ Digging found uneconomical when existing foundation is deep

❖ Constraint in access restricts the use of needle beams

3. Pier and Beam Underpinning Method

It is also termed as base and beam method which was implemented after the Second World War. This method progressed because the mass concrete method couldn't work well for a huge depth of foundation. It is found feasible for most of the ground conditions. Here reinforced concrete beams are placed to transfer the load to mass concrete bases or piers. The size and depth of the beams are based on the ground conditions and applied loads. It is found economical for depth shallower than 6m.

4. Mini Piled Underpinning

This method can be implemented where the loads from the foundation have to be transferred to strata located at a distance greater than 5m. This method is adaptable for soil that has variable nature, access is restrictive and causes environmental pollution problems. Piles of diameter between 150 to 300mm in diameter is driven which may be either augured or driven steel cased ones.

5. Pile Method of Underpinning

In this method, piles are driven on adjacent sides of the wall that supports the weak foundation. A needle or pin penetrates through the wall that is in turn connected to the piles. These needles behave like pile caps. Settlement in soil due to water clogging or clayey nature can be treated by this method

6. Pre-test Method of Underpinning

It is employed for strip foundation. It can be used for building with 5 to 10 storeys. Here the subsoil is made compact and compressed, in the new excavation level that gives predetermined loads to the soil. This is done before underpinning is performed. Hence reduced noise and disruption are expected. This method cannot be implemented for raft foundation.