Underpinning

Underpinning is a construction technique used to strengthen and stabilize the foundation of an existing building or structure. It involves extending the depth or breadth of the current foundation to ensure it rests on more stable soil or to distribute the load more effectively.

Why We Do Underpinning

Foundation Failure

If the existing foundation is no longer capable of supporting the building due to soil movement, erosion, or poor initial construction, underpinning is necessary to prevent further settlement or collapse.

Changes in Building Usage

When the use of a building changes, resulting in increased loads (e.g., converting a residential property to commercial use), the foundation may need strengthening to handle the additional weight.

Addition of New Floors

Adding extra stories to an existing building increases the load on the foundation, necessitating underpinning to support the additional weight.

Soil Conditions

Variations in soil conditions, such as subsidence, shrinkage, or expansion, can undermine the stability of the foundation. Underpinning helps to reach more stable soil layers.

Adjacent Construction

Construction activities near a building, such as excavation or new construction, can destabilize the existing foundation. Underpinning can provide additional support in these situations.

Foundation Deepening

When building a basement or lowering the floor level for additional space, the foundation may need to be deepened to support the new configuration.

How Underpinning Is Done

Several methods are used for underpinning, each suited to specific conditions and requirements:

Mass Concrete Underpinning (Traditional Method)

This is the most common method and involves excavating sections below the existing foundation in a controlled sequence and filling them with concrete. This process is repeated until the entire foundation is underpinned.

Beam and Base Method

This involves constructing a concrete beam under the existing foundation that transfers the load to a series of bases or piers installed at intervals. The beams and bases work together to support and stabilize the structure.

Mini-Piled Underpinning

Mini-piles (small diameter piles) are driven deep into the ground to reach stable soil or bedrock. The load of the building is then transferred to these piles using a system of needles or beams. This method is useful in situations with restricted access or where traditional underpinning is not feasible.

Pile and Beam Method

Similar to the beam and base method, but with the addition of piles driven deep into the ground. The load is transferred from the foundation to the piles via a reinforced concrete beam.

Cantilever Needle Beam Method

This involves installing cantilever beams, which are supported at one end by the existing foundation and at the other end by piles or piers. This method is used in situations where access is restricted to one side of the foundation.

Jet Grouting

This involves injecting a high-pressure grout into the soil to create a solid, stable base for the foundation. Jet grouting can be used to improve the bearing capacity of the soil and reduce settlement.

Process Overview

Assessment and Planning

A thorough site assessment is conducted to determine the cause of foundation issues and the most suitable underpinning method. Detailed plans are created to outline the underpinning process.

Preparation

Preparatory work includes securing the site, providing temporary supports, and ensuring safety measures are in place.

Excavation

Controlled excavation is carried out beneath the existing foundation according to the chosen method. Care is taken to avoid disturbing the structure.

Installation

The chosen underpinning method is implemented, whether by pouring concrete, installing piles, or another technique.

Reinforcement

Additional reinforcement, such as steel bars or beams, may be added to enhance the stability of the underpinning.

Backfilling and Restoration

Once the underpinning is complete, the excavated areas are backfilled, and the site is restored. Any damage to the building or surrounding area is repaired.

Monitoring and Maintenance

Post-construction monitoring ensures that the underpinning is performing as expected and that there are no further issues with the foundation.

Underpinning is a critical process in construction and building maintenance, ensuring the long-term stability and safety of structures.