Selection of pile foundations based on cost vs. other foundations for construction projects and factors affecting pile foundation cost is discussed.
Frequently, structural designer cannot reach concrete decision about whether to choose pile foundation or ordinary strip or mat foundation for the structure. Especially in the case of selection of pile foundation and conventional mat or strip foundation constructed in a deeper level to meet a soil layer with acceptable bearing capacity.
All construction conditions need to have an indicator below which the application of pile foundation prove to be cheaper compared to conventional strip and mat foundations.
So, this article will try to draw the differences by which the selection of pile foundation over other foundation types can be decided easily. This condition line is created based on the moderately detailed cost evaluation of the foundation. Factors that affect the cost of foundation would be discussed as well.
Fig.1: Pile Foundation Transfer Imposed Load to a Soil
Factors Affecting Cost of Pile Foundation
Obviously, final decision on using the type of foundation cannot be made based on the estimation of excavation volume and concrete quantity of deep mat or strip foundation in comparison with cost of piles that support the same load.
Pile foundation needs a cap which its thickness is around 45cm for two piles and 60cm to 120cm for two pair of piles. Plan dimensions of piling cap can reach up to 2100mm2 for pile with diameter of 550mm. Pile caps may be joined together using tie beams in more than one direction.
Fig.2: Pile Cap
It is claimed that, the excavation cost of pile cap, capping beams and tie beams could be two times the cost of machine excavation in moderately large column bases. Not only does the pile foundation needs structural design but also required higher degree of supervision as the construction is progressed.
Fig.5: Excavated Region for Pile Cap Construction
Therefore, the cost of excavation and concrete of pile caps and capping beams combined with the cost of piling, its design and supervision would be a discouraging factor regarding the selection of pile foundation.
Moreover, it is reported that the cost of eight-meter-long pile in combination with its cap and capping beams is greater than the construction cost of 4.5m depth column bases employing mass concrete up to the ground level.
This statement is concluded for the case where the foundations are built in dry conditions in which the cost of machine excavation can be kept low, and each type of foundation subjected to the same load. It should be known that this cost evaluation of pile and column bases conducted for sizable structures like multistory building.
However, it is proven that the cost of pile construction for small structures, for example, individual house is less than that of ordinary brick or concrete footing with a depth of 1.2m.
The construction of piles foundation for individual houses will be cheaper if the excavation can be conducted by hand and the project condition should be suitable for cost effective, uncased, mechanically augured piles.
So, pile foundation would lose the economic advantage and consequently the choice of pile foundation over conventional concrete or brick footing would be undesired and uneconomical.
Additionally, if the strip foundation is narrow strip type with the same depth i.e. 1.2m, then it is recommended to use the latter foundation type.
If the required depth of the foundation reaches water bearing sands and gravels, then it is highly recommended to use pile foundation. This is because the excavation cost in such situation is as much as two to three times the excavation cost in dry situations.
Pile foundation construction is substantially advisable if the soil at the site is significantly variable. In this case, the selection of pile foundation would be advantageous both economically and structurally.
Foundations should be selected and designed based on the soil condition at any loading station. For example, if the soil type is compacted sand, then small pad foundation can be selected and sizable isolated pad or combined pad foundation shall be accepted for compressible soil.
It should be bore in mind that such foundation construction approach would need an extensive and comprehensive soil exploration and investigation which is costly and hence undesirable.
It might be suggested to construct foundation according to the soil condition that may come up as the construction is progressed but this approach would increase the duration needed for the completion of the project due to formwork construction. This technique would prevent foundation construction planning in advance.
For such circumstances, the construction of pile foundation would eliminate the majority of foundation construction issues because design and construction activities can be arranged prior to the beginning of construction process.
Lastly, the length of the pile can be increased in the case where as the soil condition is varied instead of increasing the number of piles. As a result, the settlement that the foundation is likely to suffer can be ignored.