Underpinning is a process of strengthening the foundation of a building or a structure. The entire process provides a permanent structure or a temporary support to an already held foundation. It can be done by various ways such as:
Improving the foundation system
In here, the foundation is made deeper to achieve better support from the soil beneath. Further, various underpinning experts also increase the breadth of the foundation so that the load or pressure of the building is distributed across a greater area.
Improving the soil below the foundation
This method works by improving the soil quality below the foundation. In here, building micro piles or jet grouting is done by filling up void spaces in the soil. This increases the strength of the soil from beneath and makes your building more secure.
Reasons for Underpinning the Foundation?
There are various reasons why your building might need underpinning. Some of these include:
- Change in the usage of the structure.
- Original foundation is not strong or stable.
- Soil properties might have changed or were mischaracterized during design, leading to an unsuitable foundation.
- Nearby construction necessitates soil excavation.
- Load or depth capacity of existing foundations needs to be increased to support more floors.
- Natural disasters have caused the structure to move and lead to the need of stabilizing the foundations.
- Rise or fall in the water table leading to a change in bearing capacity of the soil.
- Degradation of timber piles that were originally used for foundations.
Now that we have seen the reasons why underpinning is necessary, let’s have a look on various methods of underpinning.
What are the Different Methods of Underpinning?
The choice of the method for underpinning depends on the condition of the ground and the foundation depth requirement. It also depends upon the age of the structure and the type of work involved.
The various underpinning methods are described below:
Mass concrete undermining or pit method:
This is a traditional method that has been followed through the centuries. It involves excavation of the soil beneath in a controlled procedure in stages or pins. Concrete is poured and kept for curing when a stable soil stratum is reached. Hence, the second layer of foundation is built.
This method is suitable if the foundation is shallow (up to 50 feet deep).
This method has various advantages like simple engineering, low cost of labor; structure can be used even during construction. The major limitation of mass concrete underpinning is that it does not work with deeper foundations.
Cantilever needle method:
It is an improved version of the pit method.
It can be only used on buildings that allow the foundation to be extended only towards one side. Further, the concerned building should also have a strong interior column.
This method is advantageous because it is faster than the pit method and requires access to only one side. Further, this method is also capable of carrying a heavy load.
However, this method is not suitable for all as it is uneconomical if the foundation is deep and the access to just from one side may limit the use of needle beams.
Base and beam underpinning method:
This method involves the places reinforced concrete beams such that the load is transferred to the concrete bases constructed at strategic points. The base size and its depth is decided by the conditions of the ground.
The method is economical for depth that is shallow or less than 6 cm.
Mini piled underpinning:
Mini piled underpinning involves usage of rigs and is capable of constructing piles to depths of 15cm. These mini piles can either be augured or steel cased and the dimensions could be between 150mm to 300mm.
It is good if the soil conditions are variable, access is limited, environmental pollution aspects are significant and the structural movement is at a minimum. Further, this method also works fine when the load to be transferred from foundation to stable soil at a considerable depth exceeding 5cm.
Pile method of underpinning:
This is a method in which piles are structured on adjacent sides of the wall with a weak foundation and a needle or pin penetrates through the wall to connect with the piles.
It will be good for places where water clogging or clayey nature of soil needs to be rectified.
Expanding the resin injection method of underpinning:
A mixture of resins and hardener is injected into the foundation beneath the footings. The resins and hardener mix with the ground and expand due to chemical reactions. Hence, it fills up any void or services in the soil beneath and further injections could be used to raise and re-level the structure. Advantages are that it is clean, does not require any construction or excavation and is non-disruptive.
As is evident, there could be various situations when you might need underpinning. It is highly recommended to talk to your underpinning expert to choose the right method which is in line with your building.
Thus, if you notice any noticeable signs of distress like crack on the floors and sloping floors, do not delay, and talk to your underpinning expert to minimize the damage and loss.