The selection of an appropriate foundation for a building depends on two primary factors: those related to the soil conditions of the ground and those related to the loads that the structure will bear. The foundation’s effectiveness is determined by the interface between the loads from the structure and the supporting ground or strata.
Each of these factors varies in nature and condition, making it necessary to carefully consider the circumstances and choose an appropriate foundation accordingly. The foundation must be selected based on the specific variations and conditions of the soil and loads to ensure the building’s safety and stability.
Foundation Selection Criteria for Buildings
1. Loads of Structure
Foundation selection is a crucial aspect of construction, and it is influenced by various factors. One such factor is the loading conditions arising from the structure. These conditions are affected by the type of building, construction material, environmental factors, and earthquake susceptibility.
The choice of construction material, such as bricks, stone, steel, and concrete, also plays a crucial role in determining the suitable foundation type. Settlement calculation of the foundation is another factor that is associated with the loads of the structure and affects foundation selection.
For low-rise buildings, shallow foundations are generally preferred as they can efficiently support the building’s weight. However, high-rise buildings require deeper foundations due to the larger loads that they must support. Deep foundations are provided because the ground at greater depths is highly compacted, which provides better support for the building
2. Soil Bearing Capacity
The soil bearing capacity is a critical factor in determining the appropriate type of foundation. Depending on the soil’s bearing pressure, a decision can be made to choose between shallow or deep foundation options. For shallow foundations of up to four stories, an allowable bearing pressure of at least 100kN/m^2 or higher is effective.
For structures taller than four stories, a raft foundation may be a viable option. However, when calculating the modulus of subgrade reaction, it must not exceed the designated limit. The modulus of subgrade reaction refers to the soil’s ability to resist deformation under a given load. By ensuring that this limit is not exceeded, the integrity and safety of the foundation can be maintained. Ultimately, the selection of the appropriate foundation type must take into account the soil’s bearing capacity and the specific needs of the structure in question.
3. Soil Types
Soil is a crucial factor in determining the foundation selection for any construction project. There are several types of soil, including sandy soil, loose soil, clayey soil, and expansive soils. The topsoil layer extends from the surface to a depth of 3m, and the subsoil layer begins after that depth. The characteristics of both layers, especially their bearing capacity, play a significant role in determining the foundation selection.
Bearing capacity refers to the soil’s ability to withstand structural loads without experiencing shear failure or unacceptable settlement. For soil with low bearing capacity, a stronger and more suitable foundation must be selected compared to soil with higher strength bearing capacity. The bearing capacity of the soil, therefore, is an essential factor in determining the foundation selection for any construction project.
3.1 Clay Soil
Clay soil is known for its high water retention capacity, but it also leads to significant expansion and shrinkage. As a result, building foundations can suffer from settlement and uplift pressure, making clay soil an undesirable option. To address this issue, applicable codes, such as the British standard, recommend a minimum foundation depth of 1m, which increases to 3m if there are trees around.
Raft or mat foundations are considered the best foundation types for clay soil. They can incorporate ribs and beams to increase stiffness. However, if a raft foundation is expensive or the imposed loads are significant, an under reamed pile may be a better option, especially if a strong soil layer is not available at a shallow depth.
To address the challenges of building on clay soil, it is recommended to collect and drain rainwater, extend the foundation to a depth where moisture fluctuation does not occur, remove weak and shallow soil layers such as black cotton soil, execute construction during the dry season if possible, and distribute structural loads as uniformly as possible.
If a shallow, firm soil layer covers a soft clay soil layer, it is advisable to use a wide, reinforced strip foundation. This approach reduces the effect of loads on the weak soil layer. For high-rise buildings and whenever uplift is expected, pile foundations are recommended.
3.2 Peat Soil
Peat soil is a type of soil that is often found near wetlands. It is characterized by its porous nature, dark brown or black color, and high compressibility. Due to its properties, it has an extremely weak load carrying capacity and can expand and shrink depending on changes in moisture levels.
To build a sturdy foundation, it is often necessary to remove the peat soil and build on a stronger stratum. In cases where the thickness of the peat soil is significant, and its removal is not feasible, alternative foundation types should be considered.
One option is to use concrete piles that extend to the firm soil layer below the peat soil. Another option is to use a pad and beam foundation, which is suitable for small projects and is placed on the firm strata below the peat soil.
If there is no firm stratum available at a reasonable depth, but there is a hard surface crust with a suitable bearing capacity of 3-4m, a raft foundation may be the best choice. By selecting the appropriate foundation type, it is possible to ensure that the structure is stable and safe, even in areas with peat soil.
3.3 Silt
Silty soil is known for its smooth texture, but it is generally not considered suitable for foundation structures. This is because it has a tendency to expand, which puts pressure on the foundation and can cause damage. Additionally, silt is known to retain moisture and does not easily drain water.
If the silt or silty clay is stiff and extends to a great depth, reinforced concrete spread and isolated pad footings are appropriate for use in foundation construction. However, it is important to ensure that the depth of the foundation is greater than the zone of erosion and the zone of swelling and shrinkage. This will help to prevent any potential damage to the foundation caused by these factors.
3.4 Sand and Gravel
Sand and gravel are popular choices for foundations because they allow water to drain through them, which prevents any structural movements. When soil and sand are compacted while moist, they can provide excellent support for foundation structures. Dry compacted gravel, or a combination of gravel and sand, is also suitable for spread and strip foundations. The depth required for such foundations is typically 700mm, as long as the ground has sufficient bearing capacity.
Fig. 4: Spread Footing in Sandy Soil
When constructing spread footings in sandy soil, it is essential to keep the foundations as high as possible, as gravel submerged in water has half the bearing capacity. In such cases, a shallow, wide strip foundation that is reinforced may be the best option.
Sand is reasonably stable when it is damp, compacted, and uniform. However, trenches may collapse, and therefore, sheet piling is often necessary to retain the ground in trenches until the concrete is poured. If loose sand extends for a great depth, it is recommended to compact it and use spread footing. Alternatively, raft, driven pile, augured pile, or cast-in-place pile can be selected without the use of compaction energy.
Fig. 4: Spread Footing in Sandy Soil
3.5 Rock
Rocks like limestone, bedrock, and sandstone have high bearing capacities, making them appropriate for supporting the foundations of both commercial and residential buildings.
3.6 Loam
Loam is considered as the ideal material for supporting foundations due to its uniformly balanced characteristics. It has the ability to maintain water at a balanced rate, which prevents it from either shrinking or expanding to an extent that could cause damage to foundations. Loam is typically composed of a combination of clay, silt, and sand, and it has a dark color and a soft, dry, and crumbly texture.
When it comes to building foundations on loam soil, isolated footing is the preferred type of foundation. The depth and area of the foundation will depend on a number of factors, including the bearing capacity of the soil, the depth of the groundwater table, and the depth of the load bearing stratum. By taking these factors into consideration, builders can ensure that the foundation is properly sized and positioned to provide maximum stability and support for the structure.
3.7 Previously Used Soil
When planning for excavation works on a site that was previously used for buildings such as residential, mining, industrial, or other structures, it is important to consider any obstructions that may hinder the process. One of these obstacles could be ground contamination, which may require additional steps to be taken before excavation can begin. These considerations should be factored in when choosing the type of foundation for the new construction.
For example, if the site has loose sand that extends to a considerable depth, a raft foundation could be selected as the best option. This type of foundation can evenly distribute the load of the new structure across the sand, providing a stable base. However, for heavier structures like multi-story buildings, a pile foundation may be necessary. This type of foundation involves driving piles into the ground to provide additional support, which is needed to bear the weight of the structure. Ultimately, the foundation selected for a construction project will depend on a variety of factors unique to the site, including the type of soil, the size of the structure, and any potential obstructions.
3.8 Mountain Soils and Forest Soils
It is common for mountain soils and forest soils to contain boulders at various depths, making pile foundations unsuitable for construction. Therefore, alternative foundation methods must be considered. For residential buildings that are one or two stories tall, isolated reinforced pad foundations are a suitable option. These foundations consist of individual concrete pads that are reinforced with steel bars and designed to support the weight of the building.
On the other hand, for single-story buildings, reinforced concrete strip foundations are more appropriate. Strip foundations are continuous concrete strips that run beneath the entire length of the building and are typically used to distribute the weight of the structure evenly across the soil. Like pad foundations, they are reinforced with steel bars to provide additional strength and stability. By using these types of foundations, construction can be adapted to suit the specific conditions and soil types present in the surrounding environment.
4. Ease of Construction
When selecting a foundation for a construction project, it is important to consider how easy it will be to construct at the site. Different types of foundations require different levels of skill and expertise from laborers. Therefore, the ease with which a foundation can be executed or constructed should be a primary consideration when making a choice. This is because the level of skill required for constructing different types of foundations can vary significantly, and the availability of skilled laborers may also vary depending on the location and the resources available. Hence, choosing a foundation that is easy to construct can help ensure that the project is completed on time and within budget.
5. Water Table Level
The selection of an appropriate foundation for construction is crucial, and the water table is a significant criterion that affects the decision-making process. It is essential to avoid placing the foundation on soil that undergoes expansion and contraction due to fluctuations in the water table. To counteract the effects caused by water and avoid any overturning moments during the early stages of construction, the foundation should be constructed on fully dry or fully wet soil.
The location of the groundwater table in relation to the foundation’s formation level is also an important consideration. If the groundwater table is below the formation level of the foundation, a shallow footing, such as an isolated or combined footing, should be selected. On the other hand, for a higher water table, a raft/mat footing would be more appropriate to provide the necessary uplift resistance.
If it is not possible to construct a shallow foundation or a raft/mat footing, then deep foundations such as micro piles or bored piles should be considered. These options will provide the necessary uplift resistance to counteract the uplift pressure caused by the high water table. By carefully selecting the appropriate foundation, the construction process can proceed smoothly, and potential problems can be avoided.
6. Adjoining Structure/Water Bodies/Slopes
When selecting a foundation type for a construction project, several parameters must be taken into account, including the presence of adjoining structures, water bodies, and slopes. If an adjoining structure’s foundation is too close to the target foundation, it can impact the choice and safety of the neighboring building. This consideration is particularly important when dealing with high-rise or older structures. It can be challenging to choose the appropriate foundation type while ensuring the safety of the adjacent building.
Other factors, such as proximity to water bodies or slopes, must also be considered. In the case of sloping grounds, isolated RCC pad foundations under stilts are typically preferred for building projects. It is also essential to note that constructing residential buildings on slopes steeper than 25 degrees is not advisable. By taking these factors into account, builders can make informed decisions about the appropriate foundation types for their projects.
7. Natural Disaster and Extreme Weather
When selecting parameters for an area, it is important to take into account its history of severe natural events or extreme weather. This means that the selection process should be based on the past record or data of natural disasters and extreme weather, which can serve as a foundation for determining appropriate parameters. By considering these factors, it is possible to make more informed decisions about the selection of parameters that are suitable for the specific conditions of the area. This is particularly important in areas that are prone to such events, as selecting the wrong parameters could have serious consequences for the safety and well-being of people and property in the area. Therefore, it is essential to carefully evaluate the available data and use it to inform the parameter selection process in a way that takes into account the unique characteristics of the area and its history of natural events and extreme weather.
8. Economic Design
The selection of a suitable foundation for a project site may involve several feasible options. However, when making the final decision, cost considerations may come into play. It is important to ensure that the chosen foundation is economical and does not exceed the budget constraints of the project. However, this should not be done at the expense of safety, strength, durability, and workmanship of the foundation.
Therefore, it is crucial to strike a balance between cost and the fundamental requirements of a foundation. While it may be tempting to opt for a cheaper foundation, this should not compromise the safety of the project. The foundation’s durability and strength should also be considered to ensure it can withstand the environmental and structural loads it will be subjected to.
Moreover, workmanship plays a critical role in ensuring the quality of the foundation. The foundation should be constructed by skilled workers who understand the design specifications and can follow the necessary safety and construction standards. This will help to avoid any costly mistakes or compromises in the foundation’s quality.
In summary, the selection of an economical foundation is essential for any project, but it should not be done at the expense of safety, durability, strength, and workmanship. A balance between cost and quality should be struck to ensure that the project can be completed successfully and within budget.