The article delves into the selection of pile foundations in construction projects, taking into account the cost factor and other foundation types. Often, structural designers are faced with the challenge of deciding whether to opt for a pile foundation or a conventional mat or strip foundation, especially when dealing with deeper levels of soil layers with acceptable bearing capacity. To aid in the decision-making process, a cost evaluation of the foundation is necessary, and a threshold must be established below which the use of pile foundation is more cost-effective than other foundation types. The article aims to highlight the differences between pile foundations and other types of foundations, making it easier to choose the most suitable foundation type. Additionally, the factors that influence the cost of the foundation will also be discussed.
Fig.1: Pile Foundation Transfer Imposed Load to a Soil
Factors Affecting Cost of Pile Foundation
The decision to use a specific type of foundation cannot be solely based on the estimation of excavation volume and concrete quantity for deep mat or strip foundation versus the cost of piles that can support the same load. Pile foundations require a cap with a thickness of approximately 45cm for two piles and 60cm to 120cm for two pairs of piles. The plan dimensions of a piling cap can reach up to 2100mm2 for a pile with a diameter of 550mm. To provide additional support, pile caps can be joined together using tie beams in multiple directions.
Fig.2: Pile Cap
Excavating pile caps, capping beams, and tie beams can be significantly more expensive than machine excavation for moderately large column bases, with costs potentially reaching twice as much. This is due to the additional structural design required for pile foundations and the need for greater supervision during construction. The excavation of these elements is a crucial part of the construction process, requiring careful attention to detail and expert oversight to ensure that everything is done correctly. Ultimately, the higher costs associated with pile foundation excavation are a necessary investment in ensuring the stability and durability of the structure.
Fig.5: Excavated Region for Pile Cap Construction
The cost of pile foundations can be discouraging due to the expenses associated with excavation, concrete for pile caps and capping beams, and piling design and supervision. For large structures such as multistory buildings, the cost of constructing an eight-meter-long pile with its cap and capping beams exceeds the cost of building a column base with mass concrete up to the ground level. However, for individual houses, pile foundation construction may be less expensive than conventional brick or concrete footing if excavation is conducted by hand and uncased, mechanically augured piles are used.
It is recommended to use conventional strip foundations with a depth of 1.2m over pile foundations if the former is a narrow strip type. Pile foundations are advisable when the required depth of the foundation reaches water-bearing sands and gravels, as excavation costs are much higher in these conditions. If the soil condition is significantly variable, pile foundations would be economically and structurally advantageous compared to other types of foundations. However, extensive soil exploration and investigation can be costly.
Constructing foundations according to the soil condition that may arise during construction could increase the duration of the project due to formwork construction. Pile foundation construction eliminates most foundation construction issues as design and construction activities can be arranged before the start of the construction process. Increasing the length of the pile instead of the number of piles can mitigate potential settlement issues caused by varying soil conditions.
In conclusion, foundations should be selected and designed based on the soil condition at each loading station, considering factors such as the cost of excavation, type of structure, and soil variability. While pile foundations may be more expensive for large structures, they may be the more economical and structurally sound choice for individual houses and in soil conditions with significant variability.