Underpinning is a critical method used in construction to repair and strengthen the foundations of existing structures. When unforeseen foundation failures occur after the completion of a structure, underpinning provides a practical solution to stabilize and support the building. This article delves into the various underpinning methods, procedures, and their applications in foundation strengthening.
Underpinning strengthens the foundation of an existing building or infrastructure by installing additional support. It is particularly valuable in emergency situations where foundation or footing failures risk the stability of a structure. By reinforcing the foundation, underpinning enhances the foundation’s depth and bearing capacity, enabling it to support the structure more effectively. Underpinning methods vary, allowing engineers to choose based on the structure’s specific requirements and the soil conditions.
Selection Criteria for Underpinning Methods
Selecting the appropriate underpinning method depends on several factors, including the age of the structure and the types of work required.
- Age of the Structure
- Ancient Structures: Over 150 years old; these may need more sensitive approaches to avoid structural damage.
- Recent Structures: 50 to 150 years old; these often require consideration of any historical preservation needs.
- Modern Structures: Less than 50 years old; these typically allow for more conventional underpinning methods due to their recent construction materials and methods.
- Types of Work Required
- Conversion Works: When a building is repurposed, a stronger foundation may be necessary to support its new function.
- Protection Works: Buildings with weak foundations, nearby excavations, or vulnerable soil may require underpinning for additional support and stability.
- Remedial Works: Used to address design or construction errors or to prevent structural subsidence, underpinning can offer a solution without demolishing the existing structure.
Structural Conditions Necessitating Underpinning
There are several scenarios where underpinning is essential to stabilize the structure:
- Timber Pile Degradation: Timber piles used in foundations are prone to degradation due to fluctuating water tables, resulting in structural settlement.
- Soil Bearing Capacity Reduction: Changes in water levels can reduce the soil’s ability to bear weight, leading to foundation settlement.
- Inadequate Bearing Capacity: If the soil beneath a foundation lacks adequate bearing capacity, the structure can become unstable.
Underpinning is advised in these situations to mitigate risks and enhance structural durability.
Assessing the Need for Underpinning
To determine if underpinning is necessary, structural engineers monitor signs of distress, such as settlement or cracking. They also conduct soil evaluations before any new construction projects to assess the soil’s ability to support new loads. These steps can sometimes prevent the need for underpinning if issues are detected early and resolved during the initial foundation design phase.
Methods of Underpinning
There are multiple underpinning methods available, each suited to specific structural needs and soil conditions. Here are some of the most common methods:
Mass Concrete Underpinning Method (Pit Method)
The traditional mass concrete method, also known as the pit method, has been used for centuries. This method involves excavating the soil beneath the existing foundation in stages until a stable stratum is reached. The excavation is then filled with concrete and allowed to cure before moving to the next section. Suitable for shallow foundations, this method is cost-effective and straightforward, although it may not be ideal for more complex foundation issues.
Underpinning by Cantilever Needle Beam Method
An extension of the pit method, the cantilever needle beam method is useful when foundation extension is needed on one side only, typically when the structure has a strong interior column. The method is faster than traditional approaches and is known for its high load-bearing capability. However, it becomes less economical when deep foundations are involved, and restricted access may limit its use.
Pier and Beam Underpinning Method
This method, also known as the base and beam method, gained popularity after World War II when the need arose for underpinning deeper foundations. It uses reinforced concrete beams to transfer the structure’s load to concrete piers or bases. This method is suitable for most ground conditions and is generally economical for depths up to 6 meters.
Mini Piled Underpinning
Mini piled underpinning is beneficial when the foundation load must be transferred to a deeper stratum, often more than 5 meters deep. This method is adaptable for various soil conditions and offers advantages in areas with limited access. It also helps reduce environmental disruptions, as small-diameter piles (150-300 mm) are used to support the structure.
Pile Method of Underpinning
In this method, piles are driven adjacent to the foundation wall to support the weak foundation. A pin or needle penetrates the wall and connects to the piles, serving as a pile cap. This approach is effective in areas with soils prone to settlement due to moisture fluctuations or clay content.
Pre-test Method of Underpinning
The pre-test method is commonly used for strip or pad foundations, especially for structures with 5 to 10 stories. This approach involves compacting the subsoil before underpinning, applying predetermined loads to test its load-bearing capability. This method reduces noise and disruption but is unsuitable for raft foundations.
Conclusion
Underpinning is a vital solution for stabilizing and reinforcing building foundations, especially in emergency situations. With various underpinning methods available, engineers can select a technique based on structural age, soil type, and required depth, ensuring that the foundation provides reliable support. The ongoing development of underpinning techniques continues to improve options for strengthening foundations, enhancing stability and safety for diverse types of structures.