When it comes to construction, a structure may have either a shallow or a deep foundation. The foundation is crucial because it distributes the building’s weight evenly over the ground and ensures the building won’t topple over. In this article, we are going to talk about all there is to know about shallow foundations, including the features of their design, the types of shallow foundations, and a number of other aspects.
See also: What is foundation, types, dimensions, functions, and historic types
What is shallow foundation?
In construction, a “shallow foundation” refers to a foundation that is less than a standard height from ground level. For these reasons, shallow foundations are more common since they are easier and more affordable to construct. Shallow foundation is often used for constructing less weighty and more compact buildings.
See also: About isolated footings
Shallow foundation: Why is it used?
Shallow foundation is used to distribute structural loads across a wide horizontal area at a shallow depth under ground level. Shallow foundation is best suited for foundations with a depth equal to or less than the foundation width.
Types of shallow foundation
There are a lot of different varieties of shallow foundations, and each one is classified according to the size, form, and general structure of the foundation.
Shallow foundation: Strip footings
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One way to support a load-bearing wall is using a strip footing. With its layout, the footings overlap with one another. A building’s stability, construction, and longevity all depend on its foundation, and a load-bearing wall’s ability to be supported by the foundation.
Shallow foundation: Isolated footings
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The individual columns that make up a structure are supported by isolated footings. This foundation may have a round, square, or rectangular layout, depending on how it is constructed. They are also designed to be thick so that the weight may be distributed across a broader area. The broad design serves to reduce the stress that the structure imposes on a given location since it distributes the weight equally throughout the footing.
Shallow foundation: Cantilever footings
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Multiple columns are supported by a single set of combined or cantilever footings. They’re used when two property lines are too near together for spread footing to be used without risking structural instability, or when two loads are likely to intersect. When used in conjunction with an internal column, a combined footing may help distribute weight more reliably.
Shallow foundation: Raft foundations
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It is possible to support many columns and walls using a raft foundation, which is a sort of slab foundation. In cases when the carrying capacity of the earth is poor, and other footings are not possible, this is what must be done. Raft foundations can bear many loads simultaneously and disperse the weight uniformly throughout the ground.
Shallow foundation: Design criteria
Certain design criteria must be met before a shallow foundation may be considered for a specific loading system. There are three prerequisites, and they are as follows:
- To correctly determine the placement of a foundation, including its position and depth, it is necessary to learn as much as possible about the site’s history and the strata under the ground. To ensure optimal performance, the foundation should be placed at a location that is immune to any potential future influences.
- To provide safety against carrying capacity, the footing must be proportioned appropriately to prevent the earth from collapsing disastrously on top of the foundation. That’s the case if the load being applied exceeds the soil’s shear strength. Therefore, it is crucial to have an in-depth understanding of the geotechnical features of the rocks and soils in question.
- Limiting a building’s settling to an acceptable level is essential for maintaining its structural integrity. The severe settlement results from the aggregation of the surrounding ground and the deformation of the soil generated by applied shear loads. Again, this calls for an in-depth understanding of the geotechnical qualities of the soil in order to predict how long it will take for the structure to settle after construction is over.
Shallow foundation design: Steps to follow
The design of footings may be accomplished by carrying out the steps outlined in the following approach:
- Determine how much weight is being supported by the footing.
- Acquire a soil sample or profiles that display the soil divisions at the location.
- Determine the highest level that the water can reach.
- Collect the relevant measures from the field as well as the laboratory tests.
- Find out the dimensions of the footing as well as its position.
- Find out how much weight the supporting stratum can sustain.
- Adjust the sizes of the footings as necessary.
- Be careful to check the contact pressure of the footing.
- Verify that the footing is stable when subjected to forces of sliding, lifting, and overturning.
- Create an estimate for both the total and the difference between the settlements.
- Develop the framework of the footing.
- Determine whether or not foundation drains, insulation, or damp proofing are necessary.
Advantages of shallow foundation
- Because less excavation is required, the labour costs associated with excavation operations may be reduced.
- The development of shallow foundations is made easier by the reduced amount of depth that is required when putting the foundation.
- The tools and machinery needed for the building of a shallow foundation are not only straightforward but also quite inexpensive.
- It is possible to create shallow foundations in a short amount of time, which not only helps save money but also helps reduce the cost of renting equipment and people.
- Building on top of shallow foundations is better for the environment and ecology since it disrupts the ground less.
- If the ground is compressible, this aids in lowering the risk of settling.
- There is no need for costly pilings.
- As a result, it is easier to estimate how shallow foundations and their supporting soil would act. Since a greater depth of soil is involved with deep foundations, the level of uncertainty increases accordingly.
Disadvantages of shallow foundation
- If the building is located near a river or the sea, there is a possibility that it may be scoured. A foundation that is too shallow cannot be utilised in such locations.
- It is not possible to utilise a shallow foundation when the water level in the subsoil is high, and it would not be cost-effective to pump the water out of the pit or canal.
- In locations where the bearing capability of the top surface soil is lower, it cannot be employed.
- It is not possible to utilise it in situations in which the weight of the framework is large, and the load on the structure is spread in an uneven manner.
Shallow foundation: Ideal depth for maximum durability
Shallow foundations need to be protected against freezing in cold climates. This is because water in the soil surrounding the foundation can freeze and expand, damaging the foundation. For this reason, it’s highly recommended to build shallow foundations below the frost line, which is the level in the ground above which freezing occurs. If the foundation can’t be built below this frost line, make sure to protect it by insulation. This way a little heat from the building can penetrate the soil and prevent damage due to freezing.
FAQs
When should a shallow foundation be used?
Smaller, lighter buildings may get away with a shallow foundation, but bigger, heavier structures, especially those being constructed on hilly terrain or in areas of poor soil, would need deeper foundations.
What are the primary obstacles in designing shallow foundations?
The sinking of layers of soil under a shallow foundation is one of the most significant issues that must be overcome when building such foundations. This phenomenon may have a significant influence on the structure's stability as well as the way it is put to use.
What is the depth limit for a shallow foundation?
A shallow foundation may be no less than 800 mm and no more than 3m in depth. In areas where the earth is strong and can readily support the weight of the building, shallow foundations may be employed.
What is the bearing capacity of a shallow foundation?
The bearing capacity is the maximum load at which either no localised failure of the subsoil occurs or failure spreads across a significant portion of the soil.