It is the process of strengthening the buildings’ foundations. This could be in two ways: strengthening either a new foundation during construction or an old foundation during the renovation work.
Why underpinning is done?
It is done for several reasons which are:
During renovation to strengthen the foundation to build additional floors.
In ancient monuments to strengthen its basement to prolong its lifetime.
The old basement is not strong enough.
Due to the construction of heavy structures nearby resulting in the excavation of soil in existing buildings.
Change in the properties of the soil due to soil erosion, degradation, or change in its properties.
For stabilization, in case of loosening of the soil during natural calamities like earthquakes, drought, and
How underpinning is done?
It is done by extending the foundation either in depth or breadth. This is carried out by establishing another structure in the soil strata so that the total load is distributed across a greater area.
Methods of underpinning
Following are the different underpinning methods used for foundation strengthening.
Mass concrete underpinning method (pit method) This is a traditional method that has been followed for centuries and is a low-cost one used primarily for shallow constructions. This involves extracting the soil below the existing foundation in a controlled manner through stages or pins. When a suitable level is reached, the extracted portion is filled with concrete and curing is done. As a support, a new pin is provided by placing sand-cement binding.
Cantilever needle beam method (base and beam method) This is carried out only when there is an interior column of strong support in the foundations of the building. So, the foundation area is increased by maintaining the interior column as support.
Pier and beam method This method has been in use since the Second World War. This is like a base and beam method, in that, the base is strengthened by placing concrete beams in positions that are estimated to carry heavy loads and thereby is prone to high stress. Thus, the load is transferred along the beams.
Mini-piled method When the loads acting on the foundation must be transferred to greater depths, for instance, more than 5 m, this method is employed. Structures called mini-piles with a diameter between 150 mm (5.9 in) and 300 mm (12 in) are normally used as it is easier to anchor such piles to the existing structure. Rigs are usually used to gain access to the foundation. It is used in areas that have varying soil properties and have recurring environmental problems.
Pile method Piles are placed adjacent to each other along the walls of the basement. A needle or pin-like structure is made to connect the piles and thus acts as pile caps. This method is mainly used in cases of water-clogging in the basement or clayey formation due to heavy rains and floods.
Resin Injection During the process, a mix of structural resins and hardener is injected into the foundation by means of drilling small holes. This mixture gets expanded and thereby fills voids and crevices that exist in the foundation. If the injection is continued, the level of the foundation begins to raise up, and this is then levelled to distribute the weight evenly.
This method offers a clean and non-disruptive approach.
With the continuing advancements, more and more methods are available to strengthen the foundation. However, it is imperative to choose the desired method based on the objectives to be met. Hence, a careful selection is necessary and so a professional advice is suggested rather than the ‘do-it-yourself’ ideologies.
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