What is a Floating Foundation?
A floating foundation is a unique type of foundation that is designed to prevent settlement of structures built on it. The foundation is constructed by excavating the soil in a specific manner, ensuring that the weight of the structure to be built on the soil is almost equal to the total weight of the soil excavated from the ground, including the weight of the water in the soil before the construction of the structure. This type of foundation is also known as a balancing raft, and its main advantage is that it causes zero settlement to the structure, thereby ensuring its stability. In essence, a floating foundation is a specialized type of foundation that is used to support structures in areas where the soil is prone to settlement, and it is an effective way of ensuring that buildings remain stable over time.
Principle of Floating Foundation
The floating foundation, also known as a balancing raft foundation, operates on the principle of balancing the weight of the removed soil with a structure of equal weight. This design ensures that there is zero settlement of the structure. The foundation consists of a structure that is built to counterbalance the weight of the soil that has been excavated, resulting in a stable foundation with minimal settlement. This innovative approach to foundation design is commonly used in construction to provide a stable base for structures while minimizing potential settlement issues.
Consider a scenario where the ground has a water table at the top, as depicted in the picture. The ground is excavated to a certain depth below the water table, and then a building with the same weight as the removed soil and water is constructed on top. Despite the excavation being below the water table, the total vertical pressure in the soil below the foundation remains unchanged due to the balancing weight of the building. However, it should be noted that the construction of the building cannot happen immediately after the excavation. During the construction process, the effective vertical pressure under the depth of excavation may slightly increase due to the unbalanced weight of the building. As a result, such foundations can also be referred to as partly compensated foundations instead of fully floating or compensated foundations.
Suitability of Floating Foundations
Floating foundations are suitable for two types of soils. The first type is soils that possess good shear strength but face issues of large settlements and differential settlements under heavy loads. In such scenarios, using a floating foundation can considerably decrease settlement values. The second type is soils that have low shear strength and lack a hard layer of soil at a reasonable depth. In such cases, floating foundations can aid in reducing shear stresses to a satisfactory level, thus preventing settlement.
Difficulties in Construction of Floating Foundation
When building a floating foundation, it is important to take proper care to prevent any potential problems that may arise during construction. These problems can occur during various stages of the construction process, including excavation, dewatering, critical depth, and bottom heave.
During the excavation phase, it is crucial to ensure that the area is properly prepared and that any loose or unstable soil is removed before the foundation is built. Failure to do so can result in instability of the foundation and potential collapse.
Dewatering is another critical step in the construction process that requires careful attention. This involves removing water from the construction site to prevent any potential damage to the foundation. If not done properly, dewatering can lead to soil erosion and instability of the foundation.
Critical depth refers to the depth at which the foundation is no longer buoyant and begins to sink. It is important to carefully calculate and monitor the critical depth to ensure that the foundation remains stable and does not sink too deep into the soil.
Bottom heave occurs when the soil beneath the foundation expands, causing the foundation to rise. This can be prevented by ensuring that the soil is properly compacted before the foundation is built and by monitoring the soil conditions during construction.
Overall, taking proper care during the construction of a floating foundation can help prevent potential problems and ensure the stability and longevity of the structure.
Excavation
To ensure the stability and safety of a floating foundation, it is crucial to conduct the excavation process with great care. Specifically, the walls of the excavated area must be supported adequately using methods such as sheet piles, timber piles, or soldier piles. This is important to prevent the walls from collapsing, which could lead to significant structural damage and potential harm to individuals on or near the construction site. By implementing proper support measures during excavation, the overall construction process can proceed smoothly and with minimal risk.
Dewatering
Dewatering becomes necessary if the excavation depth goes below the water table. To avoid any potential problems, it is advisable to assess the water table level before beginning the excavation. However, it is crucial to exercise caution during the dewatering process as it may cause a drop in the water table, which can lead to disturbances in nearby structures.
Critical Depth
The critical depth of excavation is a limit imposed by the low shear strength of the soil, and it can be determined using two equations, one proposed by Terzaghi and the other by Skempton. Terzaghi’s equation provides a method for calculating the critical depth based on the soil’s shear strength.
The equation for calculating critical depth, denoted as Dc, takes into consideration various parameters such as shear strength of soil (s), width of foundation (B), and soil-unit-weight (γ). If the width of the foundation (B) is less than the length of the foundation (L), then Skempton’s equation can be used to determine the critical depth. Skempton’s equation is a mathematical expression that relates these parameters to calculate the critical depth for a given soil condition. It is commonly used in geotechnical engineering and foundation design to ensure the stability and safety of structures built on or in soil. By plugging in the appropriate values for shear strength, width of foundation, and soil-unit-weight, engineers can calculate the critical depth using Skempton’s equation, which helps in designing foundations that can withstand the loads and pressures exerted by the soil.
The critical depth or maximum depth of excavation can be determined using either of the two equations mentioned above, where Nc represents Skempton’s bearing capacity factor. These equations provide a means to calculate the depth at which the bearing capacity of the soil becomes critical, indicating the maximum depth that can be excavated without encountering issues related to stability or bearing capacity. By applying one of these equations, engineers and construction professionals can determine the safe and optimal depth for excavation projects, taking into consideration the properties of the soil and the bearing capacity factor provided by Skempton’s method.
Bottom Heave
Excavating soil to a certain depth can lower the pressure of the soil below, leading to the formation of heave. This heave can cause settlement issues for structures or foundations. While it may not be possible to completely prevent heave formation, there are methods that can be employed to minimize its occurrence.
One effective method to minimize heave formation during excavation is to excavate narrow trenches and promptly fill them with concrete. This approach helps to stabilize the soil and prevent heave from occurring. Another approach is to install friction piles either before excavation or after completing half of the excavation process. This can provide additional support to the soil and prevent heave formation. Lowering the water table can also be effective in minimizing heave, as it reduces the pressure on the soil. Additionally, providing lateral supports along the side walls of the excavated trench can help to prevent heave formation in those areas. These measures can be taken to ensure that heave formation is minimized during excavation projects.