How do I design a footing foundation?
All structures require foundations to stand and these foundations resist the dead load (self-weight) of the structure and any applied live loadings. Foundations can be shallow or deep, an example of a shallow foundation is a simple footing. Loads acting on the structure follow the load path into the foundations where they are resolved into the ground. The foundation must rest of hard and stable strata and a bearing capacity calculation is required to substantiate the footing. Footings are usually constructed from reinforced concrete.
Bearing capacity is the maximum pressure a soil can support before failure. Geotechnical Engineers use their understanding of bearing capacity to design foundations to safely transfer loads (such as self-weight of the structure) from building foundations into the underlying soils.
What is ground bearing failure?
A building foundations transfers two types of force into the underlying soil via bearing pressure of the soil:
- Compressive stress: Generated from forces acting perpendicular to the orientation of the soil layers, compacting the soil and squeezing it together. Compressive failure occurs when the compressive stress is greater than the compressive strength of the soil.
- Shear stress: Shear stresses act along a plane around the perimeter of the foundation and shear failure occurs when the shear stress along this perimeter exceeds the shear strength of the soil.
What is ground bearing failure?
A building foundations transfers two types of force into the underlying soil via bearing pressure of the soil:
- Compressive stress: Generated from forces acting perpendicular to the orientation of the soil layers, compacting the soil and squeezing it together. Compressive failure occurs when the compressive stress is greater than the compressive strength of the soil.
- Shear stress: Shear stresses act along a plane around the perimeter of the foundation and shear failure occurs when the shear stress along this perimeter exceeds the shear strength of the soil.
What are the bearing capacity failure modes?
There are three types of bearing capacity failure condition:
- Punching shear failure: This generally happens in loose sands, layers of strong soil underlaid by weak soil and in weak clays which are loaded slowly. Failure develops gradually in this setting due to the high compressibility of these soils. Little to no disturbance is seen at ground level during this failure mode but structures experience high levels of settlement.
- Local shear failure: This failure mode occurs in cohesionless soils and loose to medium dense soils. This method has a well defined shear surface which develops below the ground which may or may not be seen at the ground surface. Local shear failure happens gradually as the footing continues to experience settlement and movement along the shear plane.
- General shear failure: This failure mode typically occurs in dense cohesionless soils and undrained cohesive soils, general shear failure is characterised by a well defined shear plane with clear disturbances in the ground surface. This type of failure occurs suddenly and can cause significant roation of the structure.
How to calculate allowable soil bearing pressure?
Calculating the allowable ground bearing pressure beneath a foundation is possible using the Terzaghi bearing capacity factors:
The terzaghi general bearing capacity equation can be written as q_{f} = cN_{c}S_{c} + yDN_{q} + 0.5yBN_{y}S_{y}
- Define the profile of the foundation: Define the width (B) of the foundation, its depth (D) and calculate the load intensity acting on the soil at the base of the foundation (q_{f}) and additional overburden (yD).
- Define the failure zones: Terzaghi's equations only consider the general shear case and the three failure zones. The punching shear zone exists directly below the foundation. The radial shear zone exists from the edges of the foundation outwards. The linear shear zone is located outside the bounds of the radial shear zone.
Calculate the Terzaghi shape factors: The terzaghi shape factors S_{c} and S_{y} can be straightforwardly calculated using the Terzaghi shape factor tables given below:
Calculate the Terzaghi bearing capacity factors: The following table can be used to calculate the bearing capacity factors:
- Repeat the calculation: The calculation should be repeated for each soil layer beneath the foundation, considering the distribution of the load through the subgrade modulus or angle of friction to dissipate the load.
Allowable bearing pressure can be determined for both shallow and deep foundations using the Terzaghi bearing capacity equations.