The Hiley formula is an empirical equation used to estimate the ultimate driving resistance of piles during pile driving. It's expressed as: R = Whη/(S + C/2)
Here's a breakdown of the components:
- R: Ultimate driving resistance in kN (kilonewtons). This is the estimated capacity of the pile.
- W: Weight of the ram in kN. The ram is the heavy weight that impacts the pile.
- h: Height of the ram fall in mm. This is the distance the ram drops before impacting the pile.
- η (eta): Efficiency factor. This accounts for energy losses during impact due to factors like hammer type and pile cap condition. It typically ranges between 0.3 and 0.5.
- S: Final set (penetration) of the pile per blow in mm. This is the amount the pile moves into the ground with each hammer blow.
- C: Temporary elastic compression of the pile, pile cap, and ground in mm. This accounts for the elastic deformation during impact.
Explanation of the Formula's Logic:
The Hiley formula is based on the principle of conservation of energy. The energy delivered by the ram (Whη) is equated to the work done in overcoming the soil resistance (R) and causing the pile to penetrate (S), while also accounting for energy lost due to elastic compression (C). The formula essentially estimates how much resistance the ground provides to the pile's movement based on the energy input and the observed penetration.
Uses and Limitations:
- Use: The Hiley formula is used as a simple and quick method to estimate pile capacity during driving. It provides a rough indication of whether a pile is achieving the desired bearing capacity.
- Limitations: It is a highly empirical formula and has several limitations:
- It assumes ideal conditions and doesn't accurately account for complex soil behavior or dynamic effects.
- The accuracy of the formula is highly dependent on the correct estimation of the efficiency factor (η) and the temporary elastic compression (C).
- It's generally considered less reliable than more sophisticated methods like wave equation analysis or pile load tests.
- It does not account for soil setup (the increase in pile capacity over time).
- It is not appropriate for all soil types.
In Summary:
The Hiley formula provides a simple estimate of pile driving resistance based on hammer energy and observed pile penetration. While useful for a quick assessment, its limitations require consideration, and more accurate methods should be used for critical applications.
