Factors Affecting Depth of Foundation
The depth of a foundation can be influenced by various factors related to soil and ground conditions. One crucial factor that affects the foundation depth is the allowable bearing capacity of the soil. It is important to determine this capacity to ascertain if soil improvement is necessary or if the foundation should be constructed at a greater depth.
In sites where clay is present, it is vital to identify the zone where shrinkage and swelling of particles occur due to seasonal weather changes. This zone can be affected by trees or shrubs located in close proximity. As a result, the foundation penetration should be performed below this critical zone.
Fine sand and silts are other soil types that can impact the depth of the foundation. These soils are prone to frost action at specific depths. Hence, the foundation must penetrate below this zone to avoid any adverse effects.
In the case of bridges and piers, the maximum depth of scouring must be taken into consideration during the foundation construction process. The foundation depth should be below this scouring depth to ensure the stability and durability of the structure.
Foundation for Different Soil Types
The minimum depth for foundation penetration below natural ground level is generally 0.5m. However, if the ground is filled up, the penetration depth should exceed the fill depth, or alternative measures should be taken with caution. If a foundation is required at a higher level, the difference between the foundation base and the excavation level must be determined. This can be achieved by filling the excavation with concrete that has an allowable compressive strength no less than the allowable bearing pressure of the soil. Alternatively, an incompressible fill material such as gravel or sand can be used. The width of the fill material should be greater than the width of the foundation to ensure load dispersion from both sides of the foundation base.
Recommendations for foundation construction vary based on the ground conditions and soil types. These recommendations should be followed to ensure a safe and stable foundation.
1. Foundation on Sloping Grounds
During the construction of a foundation on sloped ground, it is important to take into account the distance between the edge of the footing and the surface of the ground. This distance varies depending on whether the ground is made of rock or soil. For rock, the distance should be 60cm, while for soil it should be 90cm.
It is also important to ensure that the line drawn at a 30-degree angle with the footing base remains parallel to the slope of the ground. This is crucial to prevent any intersection between the lines, which could compromise the stability of the foundation. Therefore, the lines must be carefully aligned to ensure that they remain parallel to each other and do not intersect. By following these guidelines, a solid foundation can be constructed on sloped ground that is stable and reliable.
Fig.1: Foundation on sloping ground
2. Foundation near Existing Building
When constructing a new footing close to an existing one, it is important to consider the width of the wider footing (w). This width is used to determine the distance that should be maintained between the new and old footings during the construction process. The reason for this is to ensure that the new footing does not interfere with the structural integrity of the old footing. Depending on the complexity of the project, it may be necessary to conduct an analysis of the bearing capacities and settlement of the soil in order to determine the appropriate course of action.
One of the main concerns when constructing a new footing near an old one is the potential for settlement. This is particularly important when dealing with larger structures or heavier loads, as even small amounts of settlement can have a significant impact on the overall stability of the structure. By analyzing the bearing capacities of the soil, engineers can determine the amount of weight that can be safely supported by the soil in the area surrounding the footings.
Another factor to consider is the severity of the project. In some cases, such as when dealing with historical structures or buildings of significant importance, it may be necessary to take extra precautions to ensure the safety and integrity of the structure. This could involve additional analysis of the soil conditions or more frequent monitoring during the construction process.
In summary, when constructing a new footing near an old one, it is important to take into account the width of the wider footing and maintain a safe distance between the two. Additionally, analyzing the bearing capacities and settlement of the soil can help ensure the stability and safety of the structure, particularly in more complex or high-risk projects.
3. Foundation at Different Levels
The given context refers to the construction of footings on granular soils at different levels. According to the figure-2, a particular recommendation must be followed while constructing the footing at various levels. It states that the line joining the lower adjacent edges of the footing should not have a slope that is steeper than ½ or 1 vertical to 2 horizontal. In other words, the slope of the line joining the lower edges of the footing should be less than or equal to 1:2. This recommendation is crucial as it ensures the stability and safety of the structure. Following this guideline will prevent the soil from sliding or collapsing and will help maintain the integrity of the structure.
Fig.2: Foundation on granular soils
When building footings on clayey soil at various levels, it’s important to ensure that the line connecting the adjacent edges of the footing does not exceed a slope of ½ (one vertical to two horizontal). This means that the angle of inclination between the two edges of the footing should not be steeper than this limit. Failing to adhere to this guideline could result in an unstable foundation that is unable to support the structure above it. Therefore, it’s crucial to carefully measure and plan the footing layout before construction to ensure that the slope between adjacent edges remains within the recommended limit.
Fig.3: Foundation on clayey soils