Every structure has a place to rest and is able to survive due to its foundation. The stability of any structure on the ground solely thanks to the solid foundation system. So, we can conclude that the basis of any structure performs an important function, similar to the role legs do in our bodies. The main purpose for the foundations is transfer the burden of the entire building to the ground below. There are a variety of foundations that you can select to build your home, according to the kind and quality of soil, load on the foundation, levels of water and climate conditions of moisture, and of course, your budget. There are two kinds of foundations, namely the Shallow foundation as well as Deep foundation. We will talk about the foundation called a raft that is an option for Shallow foundations.
The Raft foundation can also be referred to as mat footing. It is a footing that covers all the space under the structure and is able to support all columns. It is an enormous slab of concrete that can support many structures and wall. The slab is laid over the entire structure or at the very least a substantial portion, which decreases the pressure at which it contacts as compared to conventional trench footings or strip footings. The raft is able to connect to the deposits, and thus eliminates the risk for differential settlement.
According to ‘R. Barry’ (Author of The Construction of Buildings), Raft foundations run with two directions typically covering a space that is greater or equal to the foundation of a building or structure. They are employed to support structures that are not heavily loaded on soils that have poor bearing capacity, or when variations in soil conditions require an extensive spread of load, especially for large loads on isolated footings, in which the differences in settlements are substantial.
Raft foundation or mat is utilized to prevent settlement on soils with high compressibility by creating a raft with the weight of the structure similar to the soil’s weight dug. Raft foundations can also be utilized to support storage tanks, storage equipment chimneys, as well as other industrial structures. Raft foundations are used to support chimneys, storage tanks, and other industrial structures. Raft Foundation could experience large settlements without creating detrimental differential settlement. This is why, nearly twice the amount of settlement allowed for individual footings is suitable for Rafts. Typically, when a hard soil isn’t available within 3 to 4 meters Raft foundations are used in the event that it is extremely weak. The raft could comprise of reinforced concrete beams, with an extremely thin slab beneath.
Raft foundation carries the total weight of the building across the entire area of the building. The mechanism to distribute stress in the foundation is quite simple. Self-weight of the mat as well as the weight of the entire structure are calculated, and then divided by the surface of the foundation it covers the soil’s stress.
For mat footings, the contact area of the foundation is greater than that of any other foundation type and the load is spread over more of the area, and the stress on soil is lower and the risk of soil shear failure is also minimal.
The raft is positioned on a the hard-core soil and is usually is thickened around the edges, in particular in the case of soil that is extremely poor is present.
Raft foundations typically consist of one of the following:
(a) Solid concrete slabs reinforced with concrete that cover the entire surface of the building, and reinforced with bars that are laid out in a straight line to one another about 15 centimeters below the top mat’s surface, and a second layer that is approximately 15 centimeters above the bottom or
(b) T-beams that are inverted from reinforced concrete with the slab completely covering the foundation. The beams cross across both directions, and they intersect under support walls and columns. loads, if there are any.
The beams and slabs are fashioned into a monolithic structure that can function as a single unit. The beams are reinforced with beams for walls to support them, in the event that it is needed. Basement floors are laid on top of the beams. Prior to the basement floor being put in place over the beams, the space between beams could be filled in with cinders or other kind of material. These types of foundations are utilized on natural soils that are soft or fills where the strength that the soil has is weak and the piles are unable to be utilized to their advantage. A raft needs to be formed and proportioned to ensure that the central point of the ground bearing should as much as is possible be horizontally aligned with the gravity center of the weight.
Raft foundations can be stable as long as the soil conditions remain unaffected. The rising and falling of the water table beneath is risky for these types of foundations. When ground water pressure is expected to be present relief holes must be made in the mats to ease the pressure of water.
Types of Raft Foundation
The Raft is classified into different types depending on the factors used to classify them:
(a) Based on the Methods of Their Support
1. Raft Supported on Piles
Piled Raft foundations are generally utilized for large structures and when the soil isn’t adequate to stop excessive settlement. They have become a more sought-after option for high-rise structures. If the Raft foundation by itself doesn’t meet the requirements of design It is possible to improve the efficiency of the raft through the addition of a pile. A limited amount of piles, which are strategically placed, can increase the settlement as well as differential settlement performance as well as the capacity to bear loads that the raft can carry.
2. Buoyancy Raft / Floating Raft Foundation
Buoyancy raft also known as floating foundation is a reinforced concrete foundation constructed and placed in a manner the weight of its permanent load is similar in weight to that of soil that has been displaced or the upward the pressure from subsoil (groundwater).
(b) Based on the Structural System Adopted for the Structure of the Raft
- Plain slab rafts, which are concrete slabs with a flat surface and uniform thickness across. It can be paired with pedestals or without pedestals.
- The beam as well as the slab-raft, which can be constructed using a down standing beams or an upstanding beam systems.
- Framed raft, also known as cellular raft with walls, foundations slabs columns, and some floor slabs that work together to form a strong structure.
- For more information about the various types of raft foundation that are based on structural systems, read the article: What are the various Types of Raft Foundation?
When it is Adopted?
- When the structural load is excessive or the allowed soil pressure is not high The use of spread foundations could take up more than half the area of construction It could be more cost effective to utilize Raft foundation.
- In buildings such as chimneys silos, cooling towers or even buildings with basements where constant waterproofing is required. They also are located on floating foundations in which a strong structure such as a an adze is required to limit settlement.
- They are also employed when the soil contains layer of compressible soil, which means that the differential settlement could be a challenge to manage.
- The load of the structure is to be spread over an extensive area.
- In situations where feet may meet or overlap and it is recommended to dig up the entire site instead of digging the ground under the individual footings. (In the event of such a situation, careful consideration should be taken regarding whether footings that are individual or rafts are better value. In general, if the settlements anticipated are low feet and strips, footings in individual pieces tend to be less expensive than rafts. However, rafts will require a lot more steel to support the increased shear and bending forces because of the continuity.)
- The soil’s stress must be lessened.
- There is the possibility of a different settlement in the event that the footings are individual.
- The soil strata can be unpredictable and are surrounded by pockets of soil that is compressible.
- The basement will be built.
- Other types of footing is not able to be employed to benefit.
- Additionally, it’s a common procedure nowadays, to install mat foundations over piles to prevent settlement.
- To accomplish this there is a limit to how much piles’ weight is absorbed by the. They are referred to as piled rafts.
Advantages of Raft Foundation
- Mat footing is a solution when a shallow foundation is needed and when the soil is deteriorating.
- Mat foundations require less excavation. According to “Derek Worthing” (Author of the book The Construction of Houses), they’re a speedy method of construction that requires minimal excavation. They are sometimes used with soft clays to provide an alternative to the wide-strip reinforced foundation.
- If a raft foundation is utilized instead of pile foundation, it will help save time and also cost.
Disadvantages of Raft Foundation
- Raft foundations are prone to erosion at the edge.
- Raft foundation requires a greater amount of reinforcement in some instances and will eventually cost more.
- Expertise is required in the construction industry as when compared with other foundations.
- Attention must be paid to the mat or foundation of the raft is subjected to concentrated(point) stress.
- Raft foundations require more concrete and steel, and consequently, it is more expensive.
Structural Design of Raft Foundation
In order to ensure the design is successful the design of rafts needs to be done with careful consideration of settlements as well as capacity to bear. For instance the flat slab analogy that has been effective in numerous instances but has also caused structural failures in a number of instances due to a insufficient understanding of the fundamentals. It is evident that the analogy of a flat slab that is based on a small deflection works only if the difference in of settlements between columns is minimal. If there are differential settlements that are substantial the flat slab analysis will provide us with satisfactory results.
Design Approaches for Raft Foundation
The two methods have been employed to design rafts.
- Foundation approach that is rigid (Empirical approach, without taking into consideration the characteristics of the soil)
- Flexible foundation approaches (Considering the characteristics of the soil based on its subgrade reaction modulus or its modulus of elastic)
When using the foundation method that is rigid the raft is considered to be strong and its pressure distribution thought to be linear or uniform. The raft that is rigid is constructed in either a Rib raft floors system, (flat slab or two-way slab) or as a sequence of footings that are combined in the X and Y directions. The only thing that is considered is the capacity to bear the earth (but it is not the flexibility) is considered.
In the framework of flexible foundations we can use two models namely, the Winkler model as well as the Elastic half-space model. We can also build the whole raft as a complete system by using sophisticated software programs such as finite element analysis or analyze them as a set of beams that are solid foundations that are elastic in X as well as Y direction using elemental analysis. In any event we provide about 1.5 millimeters of extension to the entire raft (or the edge beam) around the edges to deal with the uplift as well as other edges conditions.
Steps to Construct Raft Foundation
Raft Foundations are made by following the steps:
- The excavation process begins. It is carried out to the proper depth according to the design.
- After excavation the foundation bed, it is consolidated by the ramming
- Then, a water-proof plastic sheet is placed over the ground, and the thin layer of cement plain (PCC) is poured to make a perfect flat and level foundation to be used as the foundation.
- Following this the waterproofing layer is put in place, especially if it’s an underground.
- Then, the lay-up of reinforcements is completed by placing spacers on the bed of foundation.
- Finally, the concrete is completed onto the reinforcement.
- The foundation could be strengthened through beams or ribs constructed into the construction process to give the foundation extra strength and rigidity.
- To summarize the definition, a raft foundation is one that is able to cover and extend beyond the size of a building. It is comprised in reinforced concrete. They can be constructed in areas in which the subsoil is soft clay or where an uneven settlement can occur.