Vertical downwards loads (Compression):
Pile carrying capacity for vertical downward loads, comes from two things:
- The frictional resistance over their side face area
- Their bottom tip resistance (end bearing resistance)
So, piles generate friction force on their side face, plus develop strength at the tip. Tip resistance is also called “end bearing” resistance but not all piles can have both end bearing and skin resistance. For example the piles installed in fairly loose ground without touching to a firm soil or rock below, has nowhere to bear. The only source of their resistance would be through skin resistance. Or similarly, a pile can have only end bearing resistance, but no skin resistance, such as when it rests on firm soil or rock, but the soil around the pile body may be very loose, so weak that its skin resistance may be negligible. In many cases though, both end bearing and skin friction resistance can be taken into account together. The bottom tip resistance can be great, if there is a rock, weathered rock, or a very dense, strong soil layer within reach. When we say rock while talking about piles, we also mean weathered rock.
Vertical upwards/uplift loads (Tension):
Piles can also resist uplift forces on the structures, if a structure tends to tilt on a side such as by wind or earthquake, which creates uplift on some of the piles, or in the case of a hydraulic structure, where water uplift forces are significant which may create uplift force on all of the piles.
In the case of uplift, the pile tip is not pushing against anything anymore, so the tip resistance portion is useless and eliminated, and all resistance is through the friction from the sides of the pile. Using battered (inclined) piles can also help to improve uplift resistance.
Horizontal (lateral) load and moment at the upper tip:
Piles must also resist horizontal loads. Examples of horizontal loads are earthquake, wind, vehicles turning curves on bridges while exerting centrifugal forces in horizontal direction, ships bumping into marine structures, heavy trucks or ships bumping to bridges or overpasses which creates horizontal net force on the structure, and so on…
The lateral loads are carried by the lateral surface area of the pile, when it presses against the soil in which the pile is located.
Note that the vertical weight loads from above already exist for the pile, while lateral load is exerted on the pile, and in some cases, uplift forces as discussed. We simply do not show them here, for clarity, as our purpose is to only show horizontal forces clearly in this particular paragraph.
Horizontal loads acting on pile body:
Sometimes piles are used as retaining structures, to retain earth behind them. In these cases, the lateral earth pressure analysis apply. We will cover retaining structures in the next section, but we already discussed lateral earth pressure concept under soil mechanics section.
Some piles are configured such that they are also expected to resist torsion effects, applied to them from their upper tips. But for our purposes here, when we say a pile is able to resist the vertical and horizontal loads mentioned above, we will assume that they will also be able to resist torsion.
In the next post of this series, we will discuss ” Pile Groups”