Pile foundation spacing and skin friction in a pile group decides the design of pile foundation, its efficiency and capacity in any construction. The main purpose of a pile foundation is to let the transfer of load through weak soil strata (soil strata with poor bearing capacity).
The pile foundation is found to be an economical choice when soil strata at a reasonable depth is weak. The ending of the pile foundation must reach into strata that gain adequate bearing capacity. Depending upon the condition, a cluster of piles can be inserted to improve the bearing capacity.
The piles are also employed in areas where the load must be transmitted by certain frictional resistance over the depth by means of skin friction with the surrounding soil. This offers adequate shear resistance.
Pile foundation also helps in avoiding the construction of cofferdams for supporting piers in water. Here the pile will carry the load to an appreciable supporting medium below a considerable depth of the water.
The piles that are driven at an angle are called as raker piles. These are used to resist the inclined forces. The inclined forces are the effect of horizontal thrust.
Those piles that transfer the load to or through an underlying stratum by means of friction is called the friction pile. Here one of the embedded surfaces is the pile surface.
End bearing piles are piles that transfer the load to the lower stratum. Specially designed piles will transmit the load by both the means.
Suitability of Pile Foundation in Construction
The pile foundation is generally employed in the following type of soil strata:
- Area with compact or hard stratum, underlying soil is soft material, sand or clay
- The area with clayey soil with soft stratum overlying the firm layer. Here the open foundations lead to high settlement
- Dense or stiff soil with soft clay overlaid. Here the open foundations can be spaced closely to reduce the pressure that is transmitted to the soft layer
- Alternative layers of clay- soft layer and thick in nature
- Sandy strata with high water table. This brings difficulty for excavation
Spacing of Pile Foundation in Pile Group Construction
The piles must be arranged in such a way that the force exerted by one of the piles on the other is least. In the case of friction piles, this factor is very important. This is because the soil surrounding the piles are in a stressed condition. This force exertion will affect the frictional resisting capacity of the neighboring piles.
The influence lines of a cluster of piles on the surrounding ground are shown in the figure-1. The lines show the stress intensities at a point. More the distance from the pile edge surface, lesser is the stress intensities. So, this provides an idea on the minimum spacing that must be provided between the two piles.
Fig.1: Pressure Distribution represented by Influence lines in the case of End Bearing Piles
Fig.2: Pressure Distribution represented by Influence lines in the case of Friction Piles
For the convenience of driving and to provide adjustments for any error during the placement or the problems due to the coming out of the pile from the plumb causing the piles to come closer, provision of minimum spacing is employed in the case of point bearing piles. The Indian code IS 2911 have given a proper explanation on this note.
In the case of friction piles, the spacing must be such a way that the zone of the influence lines on the surrounding soil must not overlap each other. This will hence reduce the bearing values and reduce the settlement. It is hence mentioned that the minimum spacing should not be less than the diagonal dimension or the diameter of the pile.
The end bearing piles that are used in compressible soils must be spaced at a minimum of 2.5d and a spacing of 3.5d(Maximum)for those piles placed on less compressible or stiff clay soil.
The Indian Road Congress specifies a minimum spacing of 3d or a distance equal to pile perimeter for the friction piles. In the case of end bearing piles, the space between the piles that are placed adjacent must not be less than least width of the pile.
The spacing of piles as per the practice followed in the UK are based on the following formulae:
End bearing piles: Spacing S = 2.5d + 0.02L
Cohesion Piles: Spacing S = 3.5d + 0.02L
Here d is the diameter of the pile and L is its length. The standard also stipulates for pile capacity till 300 kN, the distance from the edge of the pile to the pile stem must be 100mm. For higher capacities, the mentioned distance must be 150mm.
Maximum Pile Foundation Spacing
The Maximum pile spacing should be decided by considering two factors:
- The design of pile cap
- Overturning Moments
The pile cap will be heavier with increase in the spacing between the piles. So, while choosing the spacing of the piles, pile cap design should also be evaluated.
The stability of the whole pile cluster against the action of the overturning moment must be evaluated along with the spacing of the piles.
Soil Friction Factors for Pile Foundation
The skin friction factors help in the preliminary estimation of the pile capacity. The value of the soil friction factor varies from driven to bored piles. This factor can be used only for preliminary calculations.
Before a final decision is taken, a full-scale load test must be carried out. The table-1 shows the approximate skin friction factors in saturated clay. Here Ro is the consolidation ratio.
Table.1: Skin Friction Factors of Saturated Clay
|Pile Length in meters||Clay Driven Piles Normally Consolidated||Over Consolidated Clay- Driven Pile||Over Consolidated Clay- Bored Pile|