What is mat foundation?
A Mat foundation is a type of shallow foundation used to support structures. It is a large reinforced concrete slab that extends over the entire footprint of the structure. Mat Foundation is also known as a raft foundation.
It is used when the soil is weak or has low bearing capacity. It distributes the weight of the structure over a large area, reducing the stress on the soil.
Mat foundation is typically used for large buildings, bridges, and other structures.
How to design Mat foundation?
The design of a mat foundation typically involves the following steps:
a) Determine the loads: Determine the loads that the foundation will need to support, including the weight of the structure, contents, and any environmental loads like wind or earthquakes.
b) Soil investigation: Conduct a soil investigation to determine the soil type, bearing capacity, and other soil properties that will influence the design of the foundation.
c) Determine the size and thickness: Determine the required size and fitness of the foundation based on loads and soil properties.
d) Determine reinforcement: Based on the loads, sign, and thickness, determine the enforcement required for the foundation.
e) Design the details: Including the reinforcement layout, anchor bolts, and any other necessary features, designed the details of the foundation.
f) Conduct structure analysis: Conduct a structure analysis that will make sure that the foundation made is going to be able to support the loads without any type of failure or excessive settlement.
g) Revise the design: Based on the results of the structure analysis, revise the design as necessary.
h) Prepare construction drawing: Prepare a drawing of construction that will show any necessary information for the construction process and details of the foundation.
i) Construction: According to the design and construction drawings, construct the foundation and make sure that it is properly placed and reinforced.
j) Quality control: Perform quality control tests and inspections during and after construction to ensure that the foundation meets the required standards and specifications.
When to use Mat Foundation?
a) Poor soil conditions: When the soil has a low bearing capacity, high settlement, or is prone to differential settlement, a mat foundation can be used to distribute loads of the structure over a larger area, reducing the stress on the soil.
b) Large structures: When a structure is very large, such as a high-rise building or a bridge, the weight of the structure can be spread out over a larger area with a mat foundation, reducing the pressure on the soil.
c) Irregular shape: When the shape of the structure is irregular, it may be difficult to use a traditional foundation system. A mat foundation can be designed to accommodate irregular shapes and provide a uniform support system.
d) Seismic activity: In areas with high seismic activity, a mat foundation can be designed to resist lateral loads and reduce the risk of damage or collapse.
e) Expansive soil: When the soil is expansive and prone to swelling and shrinking due to moisture changes, a mat foundation can be used to provide a stable foundation that is less affected by soil movement.
In general, a mat foundation is a good choice when the soil conditions are poor or the structure is very large, or has an irregular shape. A structural engineer can help determine whether a mat foundation is appropriate for a particular project.
Minimum depth of mat foundation
In general, the minimum depth of a mat foundation is typically between 0.75 to 1.5 meters (2.5 to 5 feet) below the ground surface.
However, the actual depth may vary based on the specific site conditions and the requirements of the structure. Some of the important points are given below.
a) Frost depth: If the region where the foundation is to be constructed is subject to frost, the foundation must be placed below the frost depth to avoid heaving caused by freezing soil. The minimum depth of the foundation is typically equal to or greater than the frost depth.
b) Bearing capacity: The foundation must be placed deep enough to reach soil that has sufficient bearing capacity to support the loads imposed by the structure. The depth of the foundation depends on the strength and stiffness of the soil.
c) Differential settlement: The foundation must be placed deep enough to prevent differential settlement, which can cause structural damage. The depth of the foundation is determined by the soil conditions and the loads imposed by the structure.
d) Water table: If the water table is high, the foundation must be placed deep enough to prevent buoyancy caused by the upward pressure of the water.
Minimum reinforcement in mat foundation according to IS Code
According to IS 456-2000, the minimum reinforcement for a mat foundation shall be 0.12% of the total cross-sectional area of the foundation in each direction.
However, this minimum reinforcement may be increased to 0.15% in case of poor quality of construction or unfavorable exposure conditions.
It is important to note that the reinforcement requirements of a mat foundation may also vary based on specific design parameters, such as the loads and the soil conditions.
How to design Mat foundation with numerical step by step
Designing a mat foundation involves several steps, including determining the loads, conducting a soil investigation, determining the size and thickness of the foundation, and designing the reinforcement.
Here is an example of how to design a mat foundation with a numerical example:
Step 1: Determine the Loads
Assume we are designing a mat foundation for a building with the following loads:
Dead load (DL): 500 kN/m²
Live load (LL): 300 kN/m²
Wind load (WL): 200 kN/m²
Total load = DL + LL + WL = 1000 kN/m²
Step 2: Conduct a Soil Investigation
Assume that the soil investigation reveals the following soil properties:
Soil type: Clay
Soil bearing capacity: 150 kN/m²
Soil settlement: 50 mm
Step 3: Determine the Size and Thickness of the Foundation
Using the soil bearing capacity and total load, we can calculate the required size and thickness of the foundation. Assuming a square foundation:
Area of foundation = Total load / Soil bearing capacity = 1000 / 150 = 6.67 m²
Side of foundation = √Area of foundation = √6.67 = 2.58 m
Assume a thickness of 0.5 m
Therefore, the size of the foundation will be 2.58 m x 2.58 m x 0.5 m.
Step 4: Determine Reinforcement
Using the minimum reinforcement requirements specified in the code, we can calculate the required amount of reinforcement for the foundation:
Minimum reinforcement = 0.12% x Cross-sectional area of foundation in each direction
Cross-sectional area of foundation = 2.58 m x 2.58 m = 6.65 m²
Minimum reinforcement in each direction = 0.12% x 6.65 = 0.00798 m²
Assuming 12 mm diameter bars spaced at 200 mm c/c in both directions:
Area of one bar = (π/4) x (12 mm)^2 = 0.1131 cm²
Number of bars required in each direction = Minimum reinforcement / Area of one bar
Number of bars required in each direction = 0.00798 / 0.01131 = 7.05
Therefore, we need to use 7 bars in each direction, spaced at 200 mm c/c.
Step 5: Check for Stability
We need to check for the stability of the foundation under the applied loads. We can do this by conducting a structural analysis using a software package or manually.
The foundation should be designed such that the stresses and deflections are within acceptable limits.
Advantages and Disadvantages of Mat Foundation
a) Load Distribution: Mat foundations are effective in distributing the load of the structure over a large area. This helps in reducing the bearing pressure on the soil and prevents excessive settlement or differential settlement.
b) Stability: Mat foundations provide stability to the structure by resisting overturning and sliding forces. The large base area of the mat foundation increases the overall stability and minimizes the risk of structural failure.
c) Uniform Settlement: By spreading the load evenly, mat foundations help in achieving uniform settlement across the structure. This is particularly beneficial in areas with variable soil conditions, as it reduces the risk of differential settlement that can lead to structural damage.
d) Cost Savings: In some cases, a mat foundation can be more cost-effective compared to other deep foundation systems, such as piles or caissons. It eliminates the need for deep excavation and reduces the amount of concrete and steel required.
e) Constructability: Mat foundations are relatively easier to construct compared to deep foundations. They require less excavation and can be constructed using standard construction equipment and techniques, resulting in time and cost savings during the construction process.
a) Soil Investigation and Design: Designing a mat foundation requires a thorough understanding of the soil conditions and properties at the site. Extensive soil investigation is necessary to determine the bearing capacity, settlement characteristics, and potential soil movements. This can increase the cost and time required for the design phase.
b) Excavation and Site Preparation: Mat foundations require extensive excavation and site preparation work to accommodate the large footprint of the foundation. This can be challenging and time-consuming, especially in urban areas with limited space or in areas with difficult soil conditions.
c) Structural Limitations: Mat foundations may not be suitable for all types of structures or site conditions. They are typically used for heavy or industrial structures where the loads are evenly distributed. In cases of highly variable loads or concentrated loads, other foundation systems may be more appropriate.
d) Potential for Differential Settlement: Although mat foundations aim to achieve a uniform settlement, there is still a possibility of some degree of differential settlement. This can lead to structural distortions or cracking if not properly accounted for in the design and construction process.
e) Construction Risks: During the construction of a mat foundation, there are risks associated with pouring and curing large volumes of concrete. Proper quality control and monitoring are crucial to ensure the integrity and strength of the foundation.