Underpinning is a process that increases the strength of an existing foundation system.
Underpinning can be done as follows:
The foundation system may not be able to support the weight of a building.
Buildings are either expanding or changing to accommodate a different type of activity that puts a greater load on the building than its bearing capacity.
The construction of a large building with deep foundations and basements next to an existing building can have a significant impact on the building.
Types of Underpinning
The purpose of underpinning is to strengthen the foundation system by expanding or joining existing foundations, as well as improving the soil properties underneath the foundations without touching the building.
UNDERPINNING METHODO 1: PIT METHOD
The following is the way it is done:
In the beginning, a temporary system of foundation is constructed around the existing one.
To reduce the load-bearing, a pit will be dug under and around the existing foundations.
Then, the concrete pit is filled with a new foundation.
Once the concrete has been set, the temporary foundations will be removed to stabilize the load onto the new foundation system.
Each section is 8′ to 12′ long (2 – 4m approx), depending on the size of the building. The length of each unit varies from 8′ to12′ (2 – 4m) depending on the size and shape of the building. The old cells are interspersed with the new to secure the system.
UNDERPINNING METHODOLOGY 2: NEEDLE BEAM METHOD
This is done in the following way :
Two micro piles at a distance are initially built with a micro piling machine.
Then, a large hole is made to catch the needle beam.
The RCC needle beams are built over the piles and inside the foundation walls.
Fill in the earth immediately after the beam has been set. The new foundation system can now handle the additional load.
This method does not affect the interior of the building.
UNDERPINNING METHODO 3: MICROPILING METHOD
The following methods are used:
The micropile is built at an angle just below the existing foundation.
Excavate the earth to the top of the pile.
Then, remove all the earth between the pile of soil and the existing foundation by hand. This will create a cone-shaped void on top of the micropile.
Concrete can be used to fill in the void. Once the micropile is in place, the existing foundation can transfer forces to it, increasing its strength.
Repeat this process on a regular basis.