This article discusses the use of underpinning as a means of repairing and strengthening building foundations. Different underpinning methods and procedures are examined, along with their applications for strengthening various types of foundations. In some cases, a foundation failure can occur unexpectedly after the completion of a structure’s sub and superstructure. In such emergency situations, remedial methods must be employed to restore the building’s structural stability. Underpinning is a useful method for strengthening the foundation of an existing building or infrastructure. This involves installing permanent or temporary support to an existing foundation, which increases its depth and bearing capacity.
Selection of Underpinning Methods
The selection of underpinning methods is determined by two key factors: the age of the structure and the nature of the works that need to be carried out. These factors are essential in determining the appropriate underpinning technique that should be used. Age plays a critical role because older structures are often more susceptible to damage and require more extensive and complex underpinning methods. On the other hand, the nature of the works involved is also essential in determining the underpinning technique. This is because different underpinning methods are better suited to different types of work, such as foundation repair or strengthening, and the stability required for the structure. Overall, selecting the right underpinning method is crucial to ensure that the structure is safe, stable, and able to withstand the test of time.
Structure categories based on its age:
The categorization of structures based on their age can be divided into three main categories: ancient structures, recent structures, and modern structures. Ancient structures are those that have an age greater than 150 years, while recent structures refer to those that are between 50 to 150 years old. On the other hand, modern structures are those that have an age of less than 50 years. This classification can be useful for studying the development and evolution of architecture throughout history, as well as understanding the preservation and restoration of these structures. By categorizing structures based on their age, we can gain insight into the cultural, social, and technological influences that have shaped the built environment over time.
Types of works for selection of underpinning methods:
The existing structure needs to be transformed into a different function that demands a more robust foundation than what is currently in place.
Protection Works
The building in question is facing several issues that require protection works to be carried out. Firstly, the existing foundation is deemed to be weak and unstable, which could pose a risk to the structural integrity of the building. Secondly, there is a nearby excavation that could potentially impact the soil that supports the existing footing, further compounding the problem. It is therefore necessary to take steps to ensure that the foundation is stabilized and made resilient against any natural calamities that may occur.
Furthermore, there is also a need to construct a basement below the existing structure, which presents its own set of challenges. To ensure that this can be done safely and securely, it is crucial to carry out the necessary protection works to address the existing issues with the foundation and soil. This will help to minimize the risk of any damage or collapse occurring during the construction process and beyond.
Remedial Works
The subsidence of a structure was caused by mistakes in the initial foundation design. As a result, it may be more feasible to work on the present structure instead of building a new one.
Structural Conditions which Requires Underpinning
There are several reasons why engineers may recommend the use of underpinning as a method for stabilizing substructures. One such reason is the degradation of timber piles that are commonly used as a foundation for buildings. When these piles degrade due to fluctuations in the water table, settlement can occur.
Another factor that can contribute to settlement is the rise and lowering of the water table. As the water level changes, the bearing capacity of the soil can decrease, causing the structure to settle. This can be particularly problematic for buildings that are located in areas with a high water table.
In some cases, settlement may be caused by the soil’s inability to support the weight of the structure. When a building is constructed over soil with a bearing capacity that is not suitable for the structure, settlement can occur. Underpinning can be used in these situations to reinforce the foundation and prevent further settlement.
Need for Underpinning
Observations can play a crucial role in determining the need for underpinning requirements. In instances where existing structures display indications of settlement or distress, it becomes imperative to conduct vertical and offset level readings on a timely basis. The frequency of such readings is determined by the severity of the settlement.
In cases where a new project involves excavation work, it is essential for professionals to conduct a thorough examination of the soil’s ability to support the upcoming structure. Based on the results of this examination, the need for underpinning can be determined. By conducting such tests beforehand, it is possible to avoid the need for underpinning after the entire structure has been constructed.
Overall, careful observation and examination of the soil’s properties and the structure’s condition are necessary to make informed decisions about underpinning requirements. By taking these steps, professionals can ensure that the structures they build are safe, stable, and capable of withstanding any potential stresses or strains.
Methods of Underpinning
Foundation strengthening can be achieved through various underpinning methods such as the mass concrete underpinning method, underpinning by cantilever needle beam method, pier and beam underpinning method, mini piled underpinning, pile method of underpinning, and pre-test method of underpinning. Despite the differences in these methods, the main idea behind all of them is to extend the existing foundation either lengthwise or breadthwise, and to lay it over a stronger soil stratum. By doing so, the load can be distributed over a greater area, ensuring a stronger foundation.
Choosing the appropriate underpinning method depends on several factors, including the ground conditions and the required foundation depth. Regardless of the method chosen, the aim remains the same, which is to strengthen the foundation by extending it and laying it over a more stable soil stratum. This ensures that the load is evenly distributed over a larger area, which can prevent foundation failure and ensure a sturdy foundation for the structure.
1. Mass Concrete Underpinning Method (Pit Method)
The mass concrete underpinning method is a traditional approach that has been used for centuries to strengthen existing foundations. It involves extending the old foundation downwards until it reaches a stable stratum. The soil below the existing foundation is excavated in stages or pins, following a controlled process. Once a suitable stratum is reached, the excavation is filled with concrete and allowed to cure before proceeding to the next stage of excavation. To transfer the load from the old foundation to the new one, a new pin is provided by placing a dry sand-cement pack. This method is known for its low cost and is typically used for shallow foundations.
When dealing with complex issues involving foundational matters, it is imperative to employ more advanced and sophisticated methods. Simply put, superior approaches must be carefully selected in order to effectively tackle intricate problems related to foundational aspects.
2. Underpinning By Cantilever Needle Beam Method
The Cantilever Needle Beam Method is an extension of the pit method and can be used for underpinning when the foundation needs to be extended only on one side and there is a stronger interior column in the building’s plan. One of the main advantages of this method is its speed, as it is faster than traditional methods. Additionally, it only requires access from one side, making it more efficient than other methods that require access from multiple sides. Another advantage is its high load carrying capability, making it suitable for underpinning larger structures.
However, there are some disadvantages to consider. One potential issue is the uneconomical nature of digging when the existing foundation is deep. This can make the method less cost-effective, and other methods may be more suitable. Additionally, there may be constraints in accessing the necessary areas, which can restrict the use of needle beams. Overall, while the Cantilever Needle Beam Method has several advantages, it is important to carefully evaluate the specific situation to determine if it is the best choice for underpinning a building’s foundation.
3. Pier and Beam Underpinning Method
The method referred to as the base and beam method gained popularity after World War II as an alternative to the mass concrete method. The latter approach proved inadequate for foundations requiring significant depth. However, the former approach is suitable for a wide range of ground conditions. This technique involves the use of reinforced concrete beams to transfer the load to either mass concrete bases or piers, as depicted in Figure 2. The size and depth of the beams are determined by the ground conditions and the applied loads.
This approach is considered economically viable for depths shallower than 6 meters. Overall, the base and beam method has been found to be a feasible and effective solution for many types of construction projects.
4. Mini Piled Underpinning
The method described is suitable for situations where loads from a foundation need to be transferred to strata located more than 5m away. It is particularly useful when dealing with soil that has varying properties, where access is limited, and where environmental pollution is a concern. The method involves driving piles with a diameter ranging from 150 to 300mm. These piles can be either augured or driven steel cased ones, depending on the requirements of the project.
5. Pile Method of Underpinning
The given method involves driving piles on either side of a wall that is supporting a weak foundation. A needle or pin is then inserted through the wall, which is subsequently connected to the piles. This needle acts as a pile cap and helps to distribute the load evenly between the piles. This technique is particularly useful in addressing settlement issues caused by soil conditions such as waterlogging or high clay content. By stabilizing the foundation using this method, the structure can be prevented from sinking or settling unevenly, which could potentially lead to structural damage.
6. Pre-test Method of Underpinning
A strip or pad foundation can be employed for building structures with 5 to 10 stories. In this construction method, the subsoil is compacted and compressed to provide predetermined loads to the soil, in the new excavation level. This is done before performing underpinning, which helps to strengthen the foundation of the building.
One of the key advantages of using this method is that it results in reduced noise and disruption during the construction process. However, it is important to note that this approach cannot be used for raft foundation.
Overall, this technique is commonly used in building construction, as it allows for the creation of a strong foundation that can support the weight of the structure. It is an effective method for ensuring the stability of a building, especially when constructing taller structures.