The design, efficiency, and capacity of a pile foundation are determined by the spacing of the piles and the skin friction in a pile group. The primary objective of using a pile foundation is to enable the transfer of load through soil strata that have poor bearing capacity. Pile foundations are considered to be cost-effective when the weak soil strata lie at a reasonable depth, and the foundation’s end reaches a stratum with sufficient bearing capacity. In cases where the soil condition is poor, a group of piles can be installed to enhance the bearing capacity.
Pile foundations are utilized in various areas where the load needs to be transferred through frictional resistance along the depth by means of skin friction with the surrounding soil. This type of foundation provides adequate shear resistance. Pile foundations also offer a solution for avoiding the construction of cofferdams to support piers in water. In such situations, piles are driven deep into the ground to carry the load to a more stable supporting medium below the water’s surface.
Raker piles, which are driven at an angle, are used to resist inclined forces. These forces arise from horizontal thrust, and the piles’ angled placement helps to counteract them. Friction piles, on the other hand, transfer the load to or through an underlying stratum through friction. In this case, one of the surfaces that transfer the load is the pile surface itself.
End-bearing piles are another type of pile foundation that transfers the load to a lower stratum. Finally, there are specially designed piles that transmit the load through both friction and end bearing. The choice of pile foundation depends on various factors, including the type of soil, the size and type of structure being built, and the load-bearing capacity required. A thorough understanding of the soil and the loads to be supported is essential for designing an effective and efficient pile foundation system.
Suitability of Pile Foundation in Construction
Pile foundations are commonly used in various types of soil strata for different reasons. One of the primary reasons for employing pile foundations is to provide a stable and robust foundation in areas with a compact or hard stratum underlying soft soil, sand, or clay. These types of soil strata can be challenging for open foundations, which may lead to high settlement.
In areas with clayey soil with a soft stratum overlying a firm layer, open foundations can also lead to high settlement. In such cases, pile foundations are preferred as they provide a more stable and robust foundation, which can mitigate settlement issues.
Similarly, in dense or stiff soil with soft clay overlaid, open foundations may also pose a problem, as they can transmit excessive pressure to the soft layer. Pile foundations, on the other hand, can be spaced closely together to reduce the pressure transmitted to the soft layer, providing a more stable foundation.
In some cases, the soil strata may have alternating layers of clay and soft material, which can make it difficult to construct an open foundation. Pile foundations can be a suitable alternative, as they can provide a more stable foundation in such conditions.
Finally, in sandy strata with a high water table, excavation can be challenging. In such cases, pile foundations can be employed as they do not require extensive excavation, and they can provide a robust foundation in such challenging conditions.
Spacing of Pile Foundation in Pile Group Construction
Arranging piles in an optimal manner is crucial to minimize the force exerted by one pile on another, especially for friction piles. This is because the soil surrounding the piles is already under stress, and any additional force exerted can impact the frictional resisting capacity of neighboring piles. To determine the minimum spacing required between two piles, it is essential to consider the stress intensities at a particular point. The influence lines of a cluster of piles on the surrounding ground, as illustrated in Figure 1, reveal that stress intensities decrease with increasing distance from the pile edge surface. This information provides valuable insights into the ideal spacing that must be maintained between two piles to minimize the force exerted on each other.
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
The minimum spacing between point bearing piles is necessary for convenient driving and to accommodate any errors during placement or issues arising from pile deviation from the plumb, which may cause piles to come too close to each other. IS 2911, an Indian code, provides guidance on this matter. For friction piles, the spacing should be such that the zone of influence lines on the surrounding soil does not overlap, resulting in reduced bearing values and settlement. A minimum spacing of at least the diagonal dimension or pile diameter is required. End bearing piles in compressible soils should have a minimum spacing of 2.5d and a maximum spacing of 3.5d, whereas those placed on less compressible or stiff clay soil should have a minimum spacing of the least width of the pile. The Indian Road Congress prescribes a minimum spacing of 3d or a distance equal to the pile perimeter for friction piles. In the UK, the spacing of end bearing and cohesion piles follows the formulae S = 2.5d + 0.02L and S = 3.5d + 0.02L, respectively, where d is the diameter of the pile and L is its length. The standard also requires that, for pile capacity up to 300 kN, the distance from the edge of the pile to the pile stem must be 100mm, whereas for higher capacities, it must be 150mm.
Maximum Pile Foundation Spacing
The maximum spacing between piles in a foundation is determined by two key factors: the design of the pile cap and the overturning moments that the foundation may experience. It is important to consider both of these factors when determining the spacing between piles.
As the spacing between piles increases, the pile cap will become heavier, which can affect the overall stability of the foundation. Therefore, it is crucial to evaluate the pile cap design when selecting the spacing of the piles.
In addition, the stability of the entire pile cluster must be assessed against the impact of the overturning moment. This is essential for ensuring the foundation can withstand external forces and maintain its stability over time.
By taking into account both the pile cap design and the potential overturning moments, engineers can determine the maximum spacing between piles that will ensure the stability and durability of the foundation.
Soil Friction Factors for Pile Foundation
Skin friction factors play a crucial role in the initial assessment of pile capacity. These factors provide an estimation of the soil friction factor, which varies depending on whether the pile is driven or bored. However, it is essential to note that this factor is only suitable for preliminary calculations. To make a final decision, a full-scale load test is necessary.
The approximate skin friction factors for saturated clay can be found in Table-1. It is crucial to consider that the consolidation ratio (Ro) plays a significant role in determining these factors. Therefore, these values should be used as an initial estimate and not the final decision. Conducting a full-scale load test will provide a better understanding of the actual capacity of the pile.
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 |
| 20 | 0.3 | 0.3 | 0.15 |
| 40 | 0.2 | 0.2 | 0.1 |
| 60 | 0.15 | 0.15 | 0.08 |