A Detailed Discussion on the Design of Piles
This article discusses about the design of piles, that is, cast-in-situ bored single piles. For deep foundation it is the bored piles which are more commonly used all around, this takes place when axial capacity cannot be achieved through shallow foundations.
For the designing of piles there are a number of methods available. Skin friction and end bearing calculations are done in the design of piles in any method that is applied. Upon the calculation of the stated parameters, evaluation of capacity of pile can be done very easily.
In this article we are not considering the calculation of negative skin friction and normal skin friction of soil.
Nonetheless, during the evaluation of the pile capacity the effect of the soil skin friction can be taken in to account.
It is especially important to consider the skin friction during the calculation of pile capacity if there is negative skin friction which can lead to the reduction of the pile capacity. In this article we are not going to discuss about the effect of the skin soil skin friction.
As per usual it is from the geotechnical investigations that the the allowable end bearing and skin friction is obtained.
It is through the report that the net allowable end bearing and allowable skin friction can be obtained.
If in the geotechnical investigation report the ultimate end bearing and ultimate skin friction is providedthen it is important to convert them into allowable capacities as they are compared with the working loads which are also called the service loads of the structure.
Given below are the 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)
To calculate the surface area of the pile in socket length you have to calculate it by multiplying the socketing length(which is the length of the pile in the fresh rock) with the perimeter length of the pile.
It is usually considered that the piles are socketing depth around the diameter of the pile, that is, until it is specified otherwise 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.
With the help of structural analysis the structural capacity of the pile can be calculated.
The design of thepile can be like a column which can carry the axial load in the soil and the rock./p>
It is always advised to conduct a structural check of the pile if and when the piling is done in very soft soils like peat, the buckling effect must be considered in the very soft medium.
As a general rule, the equation given below is used to evaluate the structural capacity of piles.
Structural Capacity of Pile = 0.25 fcu Ac
Here,fcu = is the characteristic cube strength of concrete
Ac = Cross-section area of the pile
Design Capacity of Pile = Lesser of Structural Capacity and Geotechnical Capacity