What Is Mass Concreting Underpinning? Pros & Cons Of Mass Concreting Underpinning

What Is Mass Concreting Underpinning?

Mass concreting underpinning, also known as the traditional method of underpinning, is a well-established technique used for relatively shallow depths of underpinning. It is commonly employed for partial underpinning of sections of a building, especially in residential properties. The technique can be applied in cohesive or granular soils but is particularly effective in shrinkable clays.

Mass concreting underpinning involves constructing a new foundation beneath a failing section of a building by extending the existing footings down to a greater depth where stable soil with suitable bearing capacity exists. The process includes excavating individual bases in short lengths, usually not exceeding 1200mm, in a predetermined sequence to a designed depth.

After excavation, the bases are inspected for the correct stratum and absence of soft spots or tree roots. Once approved, the bases are backfilled with concrete of a specified mix. To transfer the building load to the new foundation, a narrow gap is created between the top of the base and the underside of the footing, which is filled with a sand and cement “dry-pack”. Additional reinforcement cages can be introduced using couplers for continuity between the bases.

The advantages of mass concrete underpinning include the ability to construct all underpinning from outside the building without disturbing the interior, straightforward technique requiring minimal plant and machinery, and the potential to act as a root barrier and prevent damage to other parts of the building.

However, there are disadvantages such as the need to dispose of large amounts of excavated material, importation of significant quantities of concrete, difficulty in unstable or water-logged ground, and health and safety issues associated with base depths exceeding 3.0 meters. Good site access is also crucial for effective implementation.

Key Takeaways:

• Mass concreting underpinning is a well-established technique used for shallow depths of underpinning.
• It is commonly employed for partial underpinning of sections of a building, especially in residential properties.
• This technique is particularly effective in shrinkable clays.
• Advantages of mass concrete underpinning include minimal disturbance to the interior and the ability to act as a root barrier.
• Disadvantages include the disposal of excavated material, importing concrete, challenging conditions in unstable or water-logged ground, and health and safety concerns.

Advantages of Mass Concreting Underpinning

Mass concreting underpinning offers several benefits that make it a popular choice for underpinning projects. Let’s take a closer look at the advantages of this technique:

1. Ability to Construct Bases from Outside

One of the primary advantages of mass concrete underpinning is that it allows for the construction of bases from one side of a wall. This means that all the underpinning work can be done from the outside without disturbing the interior of the building or requiring the evacuation of occupants. It provides a convenient and efficient way to strengthen the foundation without causing disruptions.

2. Close Examination of Soil Conditions

During the process of mass concreting underpinning, operatives have the opportunity to closely examine the soil conditions. They can test the soil strength using hand penetrometers or vane testers and easily identify the presence of tree roots or soft spots. This allows for a thorough assessment of the ground conditions and ensures that the underpinning is carried out in a stable and reliable manner.

3. Minimal Amount of Plant and Machinery

Mass concrete underpinning is a relatively simple technique that can be carried out by properly trained operatives. It often requires minimal amounts of plant and machinery for excavation. This not only reduces the costs associated with the project but also makes it easier to implement in areas with limited access. The simplicity of the technique also means that it can be completed in a shorter timeframe compared to more complex underpinning methods.

4. Acts as a Root Barrier and Retaining Structure

In addition to its underpinning capabilities, mass concrete underpinning can also act as a root barrier, preventing damage to other parts of the building. By increasing the base width, it can also be designed to act as a retaining structure, assisting in retro-fit basement construction. This versatility adds to the overall value and functionality of this underpinning technique.

In conclusion, mass concreting underpinning offers several advantages that make it an attractive choice for underpinning projects. From its ability to be constructed from outside the building to close examination of soil conditions, minimal plant and machinery requirements, and additional functionalities as a root barrier and retaining structure, this technique provides a practical and efficient solution for strengthening the foundation of a building.

Disadvantages of Mass Concreting Underpinning

In spite of its benefits, mass concrete underpinning does come with certain drawbacks and limitations that should be considered before implementing this underpinning technique. It’s important to understand these cons to make an informed decision.

One of the disadvantages of mass concrete underpinning is the need to dispose of large amounts of excavated material. This can add to the project’s costs and require proper planning for waste management.

Another drawback is the importation of significant quantities of concrete to construct the underpinning bases. This can increase the overall expense of the project and may require coordination with suppliers to ensure an adequate supply of concrete.

Mass concrete underpinning can also be challenging in unstable or water-logged ground. In such scenarios, the construction of excavations and bases becomes more difficult, potentially leading to delays and additional costs.

Moreover, when base depths exceed 3.0 meters, it can raise safety concerns and may not be economically viable. In these cases, alternative underpinning methods should be considered.

It’s essential to have good site access for effective implementation of mass concrete underpinning. Difficult access can make the technique more challenging and costly to execute, emphasizing the need for proper planning and site assessment.

Considering the limitations and potential challenges associated with mass concrete underpinning, it is crucial to evaluate the site conditions and consult with an experienced professional to determine the most suitable underpinning method for your specific project.