This article explains the construction of piles (cast-in-place bored piles). Bored piles are most frequently used throughout the world as deep foundations when axial load-bearing capacity cannot be achieved with shallow foundations.
Several methods are available for constructing piles. For all methods, surface friction and ultimate support calculations are performed during pile design. If we can calculate the above parameters, we can easily evaluate the bearing capacity of the pile.
The calculation of negative and normal surface friction of the soil is not taken into consideration in this article.
However, the influence of ground surface friction can be taken into account when calculating the load capacity of the pile.
In particular, if there is negative friction on the surface that reduces the bearing capacity of the pile, this must be taken into account in the calculation. The effects of skin friction are covered in another article on this site.
In general, the allowable end bearing and casing friction are determined from geotechnical investigations.
The report contains the allowable net end bearing and surface friction.
If the ultimate support force and ultimate skin friction are given in the geotechnical investigation report, we need to convert them into allowable capacities by comparing them with the working loads (service loads) of the structure.
Equations to calculate end bearing and casing friction
Ultimate load capacity = (permissible net ultimate load capacity) x (cross-sectional area of pile base)
Skin friction capacity = (allowable skin friction) x (pile surface at base length)
The surface area of the pile at base length is calculated by multiplying the base length (length of pile in fresh rock) by the circumferential length of the pile. Generally, the depth of the pile base is approximately the diameter of the pile, unless otherwise specified in the geotechnical report.
Geotechnical load capacity of the pile = ultimate load capacity + surface friction capacity
The geotechnical capacity of the pile is compared to the structural capacity of the pile to determine the bearing capacity of the pile.
The structural capacity of the pile can be assessed through structural analysis.
The pile can be designed as a column to support axial loads in soil and rock.
When a pile foundation is built in very soft soils such as peat, it is advisable to carry out a structural test of the pile taking into account the buckling effect in the very soft medium.
As a general rule, engineers use the following equation to evaluate the structural capacity of piles.
Pile structural capacity = 0.25 fcu Ac
Where fcu = characteristic cubic strength of concrete
Ac = cross-sectional area of the pile
Pile design capacity = less than structural capacity and geotechnical capacity
The Pile Head Construction article could be used to gain knowledge about pile cap construction.
