Methods of Underpinning

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Underpinning refers to the process of strengthening and stabilizing the foundation of a structure. It is typically done when the existing foundation is unable to support the load of the structure or when the soil conditions have changed, causing settlement or movement. There are several methods of underpinning that can be used, depending on the specific circumstances and requirements of the project. These methods can be broadly categorized into traditional underpinning, mass concrete underpinning, mini-piled underpinning, and beam and base underpinning.

1. Traditional Underpinning:
Traditional underpinning is the most commonly used method and involves excavating sections of the existing foundation in a sequential manner and replacing it with new, deeper foundations. This method is suitable for relatively shallow foundations and can be used to increase the load-bearing capacity of the foundation or to correct settlement issues. The process typically involves the following steps:

– Excavation: The area around the existing foundation is excavated in small sections, usually one at a time, to minimize disruption to the structure.
– Installation of temporary supports: Temporary supports such as jacks or props are installed to provide temporary stability to the structure during excavation.
– Construction of new foundations: Once the excavation is complete, new concrete foundations are constructed beneath the existing foundation using traditional construction techniques.
– Transfer of load: The load from the existing foundation is gradually transferred onto the new foundations using hydraulic jacks or other lifting mechanisms.
– Repeat process: The above steps are repeated for each section until all areas requiring underpinning have been addressed.

2. Mass Concrete Underpinning:
Mass concrete underpinning involves constructing a reinforced concrete base beneath the existing foundation to increase its load-bearing capacity. This method is suitable for structures with deep foundations or where there is limited access for excavation. The process typically involves the following steps:

– Excavation: A pit is excavated adjacent to the existing foundation, and the soil is removed to the required depth.
– Construction of concrete base: Reinforcement bars are placed in the pit, and concrete is poured to form a solid base beneath the existing foundation.
– Curing and load transfer: The concrete is allowed to cure and gain strength before the load from the existing foundation is transferred onto the new base.
– Repeat process: The above steps are repeated for each section until all areas requiring underpinning have been addressed.

3. Mini-Piled Underpinning:
Mini-piled underpinning involves the installation of small-diameter piles beneath the existing foundation to transfer the load to deeper, more stable soil layers. This method is suitable for structures with deep foundations or where there are access restrictions. The process typically involves the following steps:

– Excavation: A small pit is excavated adjacent to the existing foundation, and a pile rig is positioned over it.
– Pile installation: Small-diameter piles, typically made of steel or concrete, are drilled into the ground using specialized equipment.
– Load transfer: Once the piles are installed, they are connected to a reinforced concrete beam that spans beneath the existing foundation. The load from the existing foundation is then transferred onto the piles and distributed to the deeper soil layers.
– Repeat process: The above steps are repeated for each section until all areas requiring underpinning have been addressed.

4. Beam and Base Underpinning:
Beam and base underpinning involves constructing reinforced concrete beams beneath the existing foundation at regular intervals. These beams act as load-bearing elements and transfer the load from the existing foundation to more stable soil layers. This method is suitable for structures with deep foundations or where there are access restrictions. The process typically involves the following steps:

– Excavation: A trench is excavated beneath the existing foundation, and formwork is installed to support the beams during construction.
– Construction of beams: Reinforced concrete beams are constructed in the trench, typically using traditional construction techniques.
– Load transfer: Once the beams are in place, the load from the existing foundation is transferred onto the beams, which then distribute the load to the deeper soil layers.
– Repeat process: The above steps are repeated at regular intervals until all areas requiring underpinning have been addressed.

It is important to note that the choice of underpinning method depends on various factors such as the type of structure, soil conditions, access restrictions, and project requirements. Consulting with a qualified structural engineer or foundation specialist is crucial to determine the most appropriate method for a specific project.

Top 3 Authoritative Reference Publications or Domain Names Used in Answering this Question:

1. Institution of Structural Engineers (www.istructe.org)
2. American Society of Civil Engineers (www.asce.org)
3. Construction Industry Research and Information Association (www.ciria.org)

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Arbaj Demrot is the founder of VideRime Online Learning, a leading engineering website. He did his BE Civil and M.Tech Structure from RGPV University, Bhopal and has been working as an Assistant Professor in a reputed college.

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