knowing how to find the depth of footing is very important in the design and construction of shallow foundations. A geotechnical investigation or soil investigation report may provide the recommendation to design and construct the foundations.
However, the designer shall be aware of the facts that you used to calculate or evaluate the depth of footings. This may apply to the other types of foundations as well. Depth of excavation for combined footings, strip footings, strap footing, raft foundations, etc. also could be determined using this method.
Factors affecting Depth of Footing or Foundation
We can list some of the factors to be considered in determining the footing depth. We may know how to find depth of foundation, but the following factors are also worth considering.
- Place Foundation on Soil having Adequate Bearing Capacity
Foundations shall be constructed on the ground that has adequate bearing capacity. This shall be determined from the geotechnical investigation. However, you may consider your experience and judgment when the design and construction are done to interpret the condition of the site accurately.
- Avoid Foundation Settlement
Foundation shall not be constructed on the soil that causes the settlement. The settlement could be due to weak ground conditions such as having soils with low bearing capacity, compressible soils such as peat, etc. The designer shall make sure the necessary condition is met during the construction. Further, necessary actions shall be taken to avoid the differential settlement and consolidation settlement if such risk is there.
- Avoid Construction on Loose or Disturbed Soil
Consideration of the condition of the soil is very important when determining the depth of footing. The depth of foundations will be highly influenced by these parameters. There could be excessive settlement if the soil is loose or disturbed during the excavation.
- Groundwater Table Could be Avoided
Whenever possible, groundwater tables could be avoided in the construction of foundations. Having a groundwater table close to the ground surface may not be able to avoid in foundation construction, However, whenever it is possible, it shall be done. If we found the groundwater table, it could affect the bearing capacity of the soil, additional cost for construction work, etc. will rise.
- Soil Free from Organic Matters
Soil shall be free from organic matters. Having them in the soil could lead to foundation settlement. Deterioration of them over time could create cavities in the soil and it leads to foundation settlement. Therefore, when we determine the depth of footing, shall make sure to reach adequate depth.
How to Find Minimum Depth of Footing
It is very important to calculate the minimum depth of footing. How to calculate the depth of foundation?
Let’s discuss.
Depth evaluation can be done using Rankine’s formula. It is the most widely used equation to calculate the depth of foundation.
The following equation is Rankine’s formula.
D= P/γ [(1-sinθ)/(1+sinθ)]2
Where
D – depth of footing
P – applied pressure or allowable bearing capacity of the soil
γ – density of soil
θ – angle of repose
P – is the applied pressure or the allowable bearing capacity. When you know the applied pressure by the foundation on soil, we may use it for the calculation of the footing depth. Otherwise, allowable bearing capacity could be used as we anyway do not allow to exceed the serviceability limit stage pressure than the allowable bearing capacity.
θ – angle of repose – the steepest angle at which a sloping surface formed of loose material is stable. This depends on the soil we used.
How to Calculate Minimum Footing Depth
Data
Allowable bearing Capacity =150kN/m2
Density of soil = 18 kN/m3
θ = Angle of repose = 300
D= P/γ [(1-sinθ)/(1+sinθ)]2 = 150/18 [(1-sin30)/(1+sin30)]2 = 0.926m
Discussion
Now the depth of footing is 0.926m. This is the minimum excavation depth for the footing having pressure on the soil of 150kN/m2. If this pressure will be applied by any structure, we need to excavate around 925mm. If we construct foundations and apply this pressure, we may have a footing depth of 900mm approximately.
We also should keep in mind that there are other factors to be considered in determining the correct foundation depth. Therefore, considering all the factors, the depth of foundation should be used.
