A mat foundation, also known as a raft foundation, is a type of shallow combined foundation which uses a large reinforced concrete slab-on-grade that transfers the entire weight of the structure to the subgrade. It works by dispersing load over much larger area on the ground when compared to discrete shallow isolated foundations. Foundation mats usually extends over the complete plan area of the structure or building. Mat foundation can be rectangular, circular or irregular in shape depending on the structural plan or building column layout.
Foundations are broadly classified into shallow and deep foundations. Mat foundation falls under the category of shallow foundations.
How the Mat Foundation Works?
When the safe bearing capacity of soil is poor, uneven or unstable at few specific area in at a construction site a raft foundation is frequently adopted because it can evenly distributes and transfer the buildings design forces over much larger surface area when compared to isolated column footings. Since the moment of inertia of the combined group is much higher than individual footing, the contact bearing pressure is considerably reduced. Also, adopting isolated footings in these areas will be uneconomical due to large plan size of individual footings required to meet the design.
In theory, when a suitable thickness of raft or mat footing is adopted, calculating the axial bearing pressure on the soil involves dividing the total weight of the building and the self-weight of the mat by the total area of the foundation that they are covering. However, sophisticated design software are required for complex structures subject to lateral design actions causing the bending effects foundation. These software are capable of capturing the flexibility of the mat and the variations in modulus subgrade reaction of the soil.
Since raft foundations have resulting contact pressure on the soil is much less it decreases the density of reinforcement required as well as improves the stability of foundation. This improves the overall economy of structure and geotechnical design of the foundation.
Selecting a Raft Foundation
Selecting the appropriate type of foundation is one of the key tasks in foundation design and construction costing. Raft foundation is often considered when
- The soil’s geotechnical capacity to support the weight of the building is insufficient with isolated footing.
- The walls or columns are so close together or have a columns near the property line that individual footings would overlap.
- If the cost of a deep foundation (a pile foundation) exceeds that of a raft foundation
- Construction of a continuous base slab is required for basement level.
- When design loads are significant and exceeds the limiting loads for isolated footings
7 Benefits of Mat Foundation
- The raft slab footing is often used as the ground floor slab. Hence, time and money on our construction is saved by pouring the ground floor slab and foundation at the same time. Because the foundation and floor slab are combined, a raft foundation is more cost-effective.
- Mat foundation requires much lesser excavation compared the deeper excavations required for isolated footings.
- More symmetric and even distribution of loads reduces global and differential settlement if structure.
- Larger contact area provides higher sliding resistance and is efficient for hydrostatic design actions and structures subject to flood loading.
- Mat foundations are reliable solution where a shallow foundation is feasible in general but the soil quality is poor in specific areas in the construction site.
- It can function in uneven or poor ground conditions and profile.
- The raft foundation require minimal formwork and shuttering.
Drawbacks of Mat Foundation
- A substantial amount of concrete and steel is needed for a raft foundation thus it is an expensive structural component.
- Requires sophisticated modelling and analysis tools to arrive at an optimal design hence may not be feasible for minor construction works such as, of domestic homes.
- If not properly maintained, retained and stabilized, the mat foundation’s edges may erode over time.
- Laborers with skill are needed for the mat foundation.
- Point loads on a mat foundation need to be treated specifically.
7 Applications of Raft – Mat Foundations
Raft foundations are ideal where
- Soil has a poor bearing capacity
- Highly compressible and extremely deep soil
- For structures such as tanks, silos, chimneys, and large machinery
- Equipment and structures vulnerable or sensitive to differential settlement
- 70% of the structure is covered by spread footing.
- Excessive structural design loads acting on foundation
- Basements with watertight construction is required are is below the groundwater table
6 Types of Mat Foundations
- Flat Plate Mat – For relatively small, uniform column spacing, and light loads, a flat plate mat is utilized. Whenever the soil isn’t overly compressible, a mat in the shape of a flat plate is appropriate.
- Plate Thickened Under Columns – In order to cater for negative moments and diagonal shear for columns subjected to very high load, the flat plate beneath the columns is typically thickened.
- Two-way Beam and Slab – It would be more cost-effective to use a two-way beam and slab raft when the column spacing is wide and carries uneven loads. Whenever the underlying soil is too compressible, this kind of mat is especially appropriate.
- Plates with Pedestals – This mat performs the same function as a thickened flat plate underneath the columns. The mat’s bottom columns offer pedestals for use.
- Rigid Frame Mat – When columns support extremely heavy loads, this kind of mat is employed. Basement walls serve as deep beams or ribs in such a design. A rigid frame mat is used when a simple beam and slab mat’s beam depth is greater than 90cm.
- Piled Raft – In this design, piles are used to support the mat. Where there is a high-water table and highly compressible soil, this type of mat is used. This kind reduces settlement and avoid floatation.
Mat Foundations Construction Process
- Remove the dirt and dig up the soil until it is uniform and flat.
- After that, ramming is applied to compact the foundation bed.
- The soil is then covered with a sheet of waterproofing membrane.
- Pour a layer of plain cement concrete measuring 7 cm to 10cm thick after that to make a base for the foundation that is absolutely flat and level.
- Over the foundation bed, place reinforcement between spacers. Steel mesh reinforcements are placed in both directions. Required cover to reinforcement is maintained. Reinforcement resist both upward downward bending forces, the foundation usually has two reinforced meshes at the top and bottom. Rebar covers should be kept to a minimum of 50mm.
- Then the concrete is poured to the desired thickness, which for small buildings is typically between 200 and 300 mm thick.
- Subsequently, a suitable curing treatment should be used to ensure that the concrete reaches the desired compression strength.
Mat Foundations – Design Methodologies
Two main design methods for Mat foundation design are
- Conventional techniques – Make use of graphs and manual calculations
- Methods of finite element analysis – Solve the design problem using a computer program.
- Approximate Flexible Method – in accordance with circular plate theories on Winkler medium
Designing Mat Foundations Using the Conventional Rigid Approach
- Determine the total load applied to the mat foundation.
- Consider the eccentricity of the loading when calculating the pressure under each column. To calculate the pressure beneath each column, axial and bending stresses resulting from the eccentricity of the load center are taken into account.
- Verify that the applied pressure does not exceed the net allowable pressure or safe bearing pressure.
- Depending on how the mat is arranged, it is then separated into strips..
- Calculate the shear and bending moments.
- Analyze the footing to determine its effective depth. The diagonal tension shear next to columns could be used as a basis.
- Find the negative and positive bending actions and moments per unit width using bending moment diagrams.
- Determine the section’s reinforcement area for every unit width.
Finite Element Analysis of Mat Foundation
In this approach, the soil is modeled, and the design and analysis take into account its behavior. Thus soil structure interaction is considered. There are numerous ways to model soil.
Using the material properties of the soil, we can design the soil beneath the foundation. Software like Plaxis might be useful for complex soil modelling. In this kind of analysis, choosing the appropriate material properties for the soil is crucial. We could also use SAFE foundation analysis and design software to obtain the bending moments and shear forces.
Area or point springs should be used for modelling the soil. The area springs can be calculated following the guidelines therein in the Bowels Foundations analysis and design book.
The foundation can be analyzed after applying the loads at the column positions. The shear forces and bending moments can then be calculated.
Calculations for reinforcement must be made by the findings of the analysis.
There are numerous software programmes available for designing mat foundations. Some of which include ABAQUS V6.8, STAAD FOUNDATION, RISA FOUNDATION, ANSYS, and others.
Designing Mat Foundations Using Approximate Flexible Method
The soil is viewed as being equivalent to an infinite number of elastic springs in the approximate flexible method. The Winkler foundation is another name for this assumption. The term “coefficient of subgrade reaction,” or “k,” refers to the elastic constant of such hypothetical springs.
- The mat is separated into strips, with each strip being loaded by a row of columns and resisting by soil pressure.
- The mat should be very rigid and the column pattern should have fairly uniform loads and spacing.
- Due to the large number of approximations and the widespread availability of computer programmes that are comparatively simple to use, this method is not currently advised.
Key Design Guidelines for Raft or Mat Foundation
- It cannot be positioned on top soil.
- According to IS 1080, mat foundations must be a minimum of 50 cm thick. It is necessary to do this to confirm the design’s assumption that the soil does have a safe bearing capacity.
- If columns are evenly spaced apart and the loads uniform, raft foundations can be constructed with a uniform thickness.
- The minimum cover required for mat foundations is 50 mm, according to British Standards, ACI 318-14, IS 456, and Eurocode 7, among other standards. Higher cover is adopted for aggressive soils.
- The foundation must be at least 2.5 meters below the surrounding surface of the ground when built on sand.
- A depth of 0.6 meters underneath the surrounding ground is the minimum required by British standards.
- Since it is assumed that the subsoil will carry the weight of the raft directly, it is not taken into account in the structural design.
- Rafts can be economically made thicker where the columns are, and their depth needs to be sufficient to withstand shear.
- When the column spacing is asymmetrical, the raft may be ribbed.
Difference between Raft Foundation and Mat Foundation?
There is no distinction between a raft foundation and a mat foundation. Raft Foundation is same as mat foundation. Both refers to a horizontal reinforced concrete plane or slab upon which you can build a structure. When the soil’s load bearing capacity is very low or there is very little space between adjacent columns, we should use raft or Mat foundation. In these circumstances, a mat foundation is beneficial because, in contrast to other foundation types, it evenly distributes loads throughout the entire building area.