Underpinning is a technique used to strengthen the foundation of an existing structure by extending it to a deeper and more stable soil layer, or by replacing it with a new one.

Underpinning may be necessary for various reasons, such as:

• The original foundation is damaged, unstable, or inadequate for the current or planned use of the structure.
• The soil underneath the existing foundation has changed due to environmental pollution, natural causes, or human activities.
• The structure has undergone structural movements, such as settlement, subsidence, or heave, that cause the foundation to crack or shift.
• The structure needs to be modified, enlarged, or raised, which increases the load on the foundation.
• The structure is located near a party wall or a boundary line, and requires lateral support or protection from adjacent structures.

The history of underpinning can be traced back to ancient times, when various methods were used to repair or reinforce foundations of buildings, bridges, temples, and other structures. Some of the earliest examples of underpinning include the use of wooden piles, stone blocks, brick arches, and concrete piers. Underpinning techniques have evolved over time with the development of engineering knowledge, materials, and technology.

There are different types of underpinning methods that can be used depending on the site conditions, structural requirements, and design preferences. Some of the common types of underpinning are:

• Mass concrete underpinning: This is the traditional method of underpinning, which involves excavating a series of pits below the existing foundation and filling them with concrete. The concrete forms a new base for the foundation and transfers the load to a deeper and more stable soil layer.
• Beam and base underpinning: This is a variation of mass concrete underpinning, which involves constructing a reinforced concrete beam below the existing foundation and resting it on concrete bases. The beam acts as a support for the foundation and distributes the load evenly to the bases.
• Mini-piled underpinning: This is a method of underpinning that uses small-diameter piles (usually less than 300 mm) to support the existing foundation. The piles are driven or drilled into the ground at an angle or vertically, depending on the soil conditions and structural needs. The piles are connected to the foundation by a reinforced concrete beam or a steel frame.
• Piled raft underpinning: This is a method of underpinning that combines mini-piles and a concrete raft to support the existing foundation. The raft is a thick slab of concrete that covers the entire area under the foundation and rests on mini-piles. The raft acts as a platform for the foundation and transfers the load to the mini-piles.
• Expanding resin injection underpinning: This is a method of underpinning that uses expanding resin to fill the voids and cracks in the soil below the existing foundation. The resin is injected into the ground through small holes drilled in the foundation or near it. The resin expands and hardens, forming a solid mass that stabilizes and lifts the foundation.