Pile foundation is one of the deep foundations in which the long elongated slender structural components made up of reinforced concrete or steel or timbers are embedded or cast deep into the soil strata to transfer a load of superstructure gradually and safely to the surrounding soil layer.
Pile generally transfers the load to the ground stratum by both frictional bearing mechanism and end bearing mechanism.
Pile foundations are used where the soil is unstable and doesn’t have sufficient bearing strength. Piles can be of different shape like circular, hexagonal, rectangular, in form of a sheet (sheet piles) with or without different dimensions or thickness budging and base (under ream piles) and of different sizes i.e diameter ranging from 0.15m (in case of micro piles) to the 3m (cast-in-situ piles generally) and a length ranging from few meters to 100 meters.
The piles can be installed as driven piles by hammering precast concrete or steel piles or by drilling boreholes and casting them in-situ. Battered piles are installed at an inclination in order to resist horizontal loads too.
Piles are generally used in groups due to their thin diameter and later on covered by a common pile cap for uniform load distribution.
Condition to use pile foundation
Pile foundations are basically used where the ground strata have very unstable and low bearing soil up to a definite layer.
When the foundation usually cannot transfer the safe load directly below the foot of the substructure to the ground without exceeding the allowable bearing capacity of the soil, then pile foundations are implemented.
The pile’s foundation can transfer the load gradually throughout the substructure ground strata due to the frictional bearing mechanism.
Moreover, pile foundations are thus taken as a technique to improve the bearing capacity of the soil. And, they can be also used to prevent excessive seepage through the ground strata (especially sheet piles).
The piles are very useful to use where the groundwater table is high. So, in most of the civil construction like a dam, reservoir, bridges, etc. piles are installed for a stable base even in high water table conditions for stability and to build the structure safely.
The loose soil can be even compacted and its bearing capacity is rendered as driven piles are forced to embedded over them. The piles (frictional pile) are further used where there is the probability of both uplift and thrust pressure over the structure.
Pier foundations are basically a type of pile foundation with a higher cross-section area and lesser depth. They have genera large cross-sectional area compared to the pile and thus are constructed through drilling boreholes and casting in-situ or installing precast piers.
Their frictional bearing capacity is almost negligible thus the mechanism they transfer the load to the ground is the end bearing mechanism. The piles are similar to Shallow foundation but with excessive depth till it reaches the hard strata of soil or bedrock to transfer the load without exceeding the safe bearing capacity.
There can also be of different shapes, but generally, it is preferred to use a cylindrical pier to neglect the twisting effect. There dimensions criteria is they must exceed the diameter of 0.6m and the depth they are installed are limited to 5m – 20m.
The pier foundation is used singly as they have a large cross-sectional area, so the pier cap is individual for each pier above which the foot of column are rested.
Condition to use pier foundation
Pier foundation transfers its load through end bearing mechanism. Thus, they are only useful when the ground strata don’t have good bearing capacity only up to the upper surface but consist of hard rock bed or good bearing capacity soil layer to a certain depth below.
The compacted stiff soil and rocky bases where the drilling hole too deep for the pile are extremely difficult at such places piers are more convenient to use. At such conditions, they are more efficient to be used rather than pile foundations.
If such a rock bed is found within the depth of 5m then the pier foundation of the masonry construction is more relevant to use. And if the depth exceeds 5m then the concrete piers along with reinforcement and (sometimes even) steel pipe casing are used.
They generally have the ream flat bottom at the end to increase the end area for decreasing load intensity at the bottom layer of the pier- soil interface.
Pier foundations are useful for the structure having a relatively high structural load compare to normal residential and commercial buildings with excellent ground strata at a certain convenient depth.
Sometimes the pier is also suitably loaded over pile caps and thus also term as post foundation. This is generally useful in bridges where the bridges are too high from ground level. So up to depth ground level strata, the piles are brought, and above them, the piers are constructed.
Difference between pile foundation and pier foundation
|Description||Pile foundation||Pier foundation|
|Load Bearing and transfer mechanism||They transfer load gradually to the soil layer through the lateral surface and bottom end to the soil. So both end bearing and frictional bearing mechanism can be dominant.||Generally transfer directly to the bottom end of foundation with end bearing mechanism.|
|Uplift force with standing||Due to the frictional driving of piles, they have an excellent frictional bearing mechanism. During the downward force in piles, the upward frictional resistant force is created and during upthrust the frictional resistant changes to a downward direction.||Cannot withstand the uplift forces due to mechanism beside its self weight.|
|Horizontal forces||They are designed at an inclination in order to overcome horizontal forces.||The horizontal loads are simply counter with flexural property, and retaining capacity.|
|Sizes||Very small cross-section and higher depth. The pile have a diameter from 0.3 m to 2m with depth from 20m to 100m||Have comparatively larger cross section of diameter 0.6m – 3m and depth of 5m to 20m|
|Installation||They can be installed both by driving them through ramming & hammering and casting them through boreholes.||Due to larger cross section area they are more prefer to install by drilling the bore holes.|
|Suitability||They are very suitable where the soil has very low bearing capacity till the high depth. They are suitable for a high groundwater table.||They are suitable and economical when, the hard soil strata or rock bed are presented with lesser depth about 5-10m. They are not preferable for high ground water table.|
|Spacing||The piles are of small diameter and are frequently spaced nearly. The use of a single pile is very less, so they are used as a group of piles in which the group of piles is monolithically connected by pile cap The minimum spacing is 2.5d for end bearing pile and 3d for frictional piles. And ranges up to 6d for maximum spacing. (d is the diameter of the pile)||They are singly used and are at more spacing compared to piles. The single pier cap is provided for single piers. There spacing is 5 to 10 ft apart as required.|
|Perspective||They are considered as soil bearing capacity improving techniques.||They are considered as structural members of the structure|
|Economy||Pile foundations are a very costlier method of foundation. Thus they are used only if, the construction project is too big and massive.||Pier foundation are comparatively less economic than pile foundation.|
I hope this article on “Difference between pile foundation and pier foundation” remains helpful for you.
Happy Learning – Civil Concept
Civil Engineer – Rajan Shrestha