The design of piles (cast-in-situ bored single piles) is discussed in the article. Bored piles are more commonly used in the world as a deep foundation when axial capacity can not be achieved by shallow foundations.
There are different methods available for designing piles. In all the methods, skin friction and end bearing calculations are done in the design of piles. If we can calculate the above parameters, we can easily evaluate the capacity of the pile.
Calculation of negative skin friction and normal skin friction of soil is not considered in this post.
However, the effect of the soil skin friction can be taken in to account when the pile capacity is evaluated.
Especially, when there is negative skin friction, which reduces the pile capacity, it should be considered in the calculation. Effect of the skin soil skin friction will be discussed in another article on this website.
Generally, the allowable end bearing and skin friction is obtained from the geotechnical investigations.
The report provides net allowable end bearing and allowable skin friction.
If the ultimate end bearing and ultimate skin friction is given in the geotechnical investigation report, they shall be converted into allowable capacities as we compare them with the working loads(service loads) of the structure.
Equations for Evaluating End Bearing and Skin Friction
End Bearing Capacity = (net allowable end bearing) x (cross-sectional area of pile base)
Skin Friction Capacity = (allowable skin friction) x (surface area of pile in socket length)
The surface area of the pile in socket length is calculated by multiplying the socketing length(length of the pile in the fresh rock) by the perimeter length of the pile. Generally, piles are socketing depth around the diameter of the pile unless it is specified in the geotechnical report.
Geotechnical capacity of pile = End Bearing Capacity + Skin Friction Capacity
The geotechnical capacity of the pile is compared with the structural capacity of the pile to obtain the load-carrying capacity of the pile.
The structural capacity of the pile can be evaluated from structural analysis.
The pile can be designed as a column of carrying the axial load in the soil and the rock.
When and piling is done in very soft soils like peat, it is advisable to do a structural check of the pile considering the buckling effect in the very soft medium.
As a rule of thumb, engineers used the following equation to evaluate the structural capacity of piles.
Structural Capacity of Pile = 0.25 fcu Ac
Where fcu = Characteristic cube strength of concrete
Ac = Cross-section area of the pile
Design Capacity of Pile = Lesser of Structural Capacity and Geotechnical Capacity
The article pile cap design could be referred to get knowledge about pile cap design.