Introduction

Raft foundations can effectively be used in marshy lands, particularly where the soil is marshy, soft, or lacks adequate load-bearing capacity. This article delves into the advantages of using raft foundations in such conditions and the necessary considerations and design practices.

Advantages of Raft Foundations in Marshy Lands

Distributes Load

One of the key advantages of raft foundations in marshy lands is their ability to distribute the load of the structure across a larger area. This reduces the pressure on the underlying soil and stabilizes the structure, even in areas of low load-bearing capacity. By spreading the load evenly, raft foundations prevent soil compaction and enhance the overall stability of the structure.

Reduces Settlement

Marshy lands are characterized by varying soil properties that can cause significant differential settlement. Raft foundations minimize this issue by providing a large, uniform base that ensures consistent load distribution. This reduces the risk of uneven settlement, which is a common problem in marshy areas.

Enhanced Stability

Raft foundations contribute to the stability of structures built on unstable soils. They help in preventing tilting or sinking by providing a more solid and uniform base. This is particularly beneficial in marshy lands where the soil can be unpredictable and may contain pockets of poor-quality material.

Cost-Effectiveness

In some scenarios, raft foundations can be more economical than deep foundations. The cost of excavation and piling can be substantial, especially in marshy areas. By spreading the load across a larger area, raft foundations can reduce the need for deep foundations, thereby lowering overall project costs. Additionally, the simpler construction process of raft foundations can contribute to cost savings.

Considerations for Using Raft Foundations in Marshy Lands

Thorough Geotechnical Investigation

A thorough geotechnical investigation is essential to understand the nature of the soil, including the depth of soft soil layers and the presence of groundwater. This information is crucial for designing a foundation that can effectively support the structure and withstand the unique challenges of marshy lands.

Designing the Raft Foundation

The design of the raft foundation must account for the specific conditions of the site, including the thickness of the raft, reinforcement requirements, and potential for uplift due to water table fluctuations. It is essential to consider the density and type of soil to ensure the foundation can handle the loads effectively.

Effective Drainage Solutions

In marshy lands, soil moisture levels can be a significant concern. Effective drainage solutions are necessary to prevent water accumulation around the foundation, which can compromise the stability and integrity of the structure. Proper drainage systems, such as French drains or sump pumps, can help manage groundwater and maintain a dry environment.

Special Construction Techniques

Special construction techniques may be required to build raft foundations effectively in marshy lands. These techniques can include the use of formwork, waterproofing membranes, and reinforcement materials to ensure that the foundation is constructed to the required standards. In some cases, precast elements or pre-stressed concrete may be beneficial.

What is a Raft Foundation?

A raft foundation is a monolithic reinforced concrete (RCC) slab of uniform thickness that is provided throughout the entire area below the superstructure. It acts as a structural mat that supports the above structure, often referred to as a mat foundation. Unlike deep foundations, raft foundations are shallow and can be installed at a lower depth, making them an economical choice.

Why and Where to Use Mat Foundations

Raft foundations are commonly used in scenarios where the individual footing area is more than 50% of the total area of the structure. They are particularly useful in geotechnically challenging areas where the soil has a low bearing capacity, such as marshy lands, softened soil, and sandy soil. The large area of the foundation helps to distribute the load and reduce settlement, making them ideal for equal settlement and preventing differential settlement. They are also highly effective in creating basement apartments since the entire area covering the structure can be excavated.

Features of Raft Foundation

Raft foundations are characterized by their ability to compensate for the inadequate bearing capacity of the underlying soil. They can be either partially compensated or fully compensated, depending on the specific conditions. The pressure bulb extends over a wide area, resulting in a monolithic, evenly distributed load on the ground. This reduces differential settlement and allows for safe construction in areas with varying soil properties.

Types of Raft Foundation

Raft foundations can be broadly classified into two categories based on support and structural components. The first category includes raft on soil and raft on pile, while the second category includes flat plate, flat plate slab thickened under column, two-way beam and slab, and rigid frame box structure frame. Each type of raft foundation has its unique advantages and is suited to different construction scenarios.

Bearing Capacity and Settlement of Mat Foundation

The bearing capacity and settlement of mat foundations vary depending on the type of soil. For cohesionless soil like sandy loam, the bearing capacity is primarily dependent on the width of the foundation. The Terzaghi theory and Teing's method can be used to determine the safe bearing capacity. For cohesive soil like clay, the bearing capacity is determined by Skempton's equation, and consolidation settlement is generally a concern.

Design of the Raft Foundation

Designing a raft foundation requires careful consideration of various factors. The contact pressure distribution, soil behavior, and structural load are critical to ensuring the foundation can safely handle the loads. Different methods like the rigid beam method, simplified elastic method, elastic method, and non-linear elastic method can be used, with the most common approach being the conventional design approach.

Conclusion

Raft foundations are a viable and effective solution for constructions in marshy lands, provided that proper engineering practices and site assessments are followed. Their ability to distribute load, reduce settlement, and provide enhanced stability make them an excellent choice for challenging soil conditions. By understanding the specific requirements and designing the foundation appropriately, structures in marshy lands can be built with greater stability and efficiency.