Table of Contents
Shallow foundations are commonly used in construction to transfer loads from structures to the ground. Ensuring their stability against overturning and lateral movement is essential for safety and durability. Simplified calculation methods provide quick assessments to determine if a foundation is stable under expected loads and conditions.
Overturning Stability
Overturning occurs when lateral forces, such as wind or seismic activity, create moments that exceed the resisting moments of the foundation. To evaluate overturning stability, the following simplified approach is used:
Calculate the overturning moment (Mo) caused by lateral forces and compare it to the resisting moment (Mr) provided by the weight of the structure and foundation. The safety factor (SF) is given by:
SF = Mr / Mo
If SF is greater than 1.5, the foundation is considered stable against overturning.
Lateral Stability
Lateral stability involves resisting horizontal forces that could cause sliding or shifting. The main considerations include the frictional resistance and the passive earth pressure.
The simplified calculation involves comparing the horizontal force (H) to the maximum resisting force (R). The resisting force is often estimated as:
R = cN + qN tan φ + 0.5 γ B N
where:
- c = cohesion of soil
- N = bearing capacity factor
- q = surcharge
- φ = internal friction angle
- γ = soil unit weight
- B = width of the foundation
If the horizontal force H is less than R, the foundation is considered stable against sliding.
Summary of Checks
- Calculate the overturning moment and compare with resisting moment.
- Ensure the safety factor exceeds 1.5 for overturning stability.
- Compare horizontal forces with resisting forces to assess lateral stability.
- Use simplified formulas for quick evaluations during preliminary design.