---
title: "Why dynamic pile formulae are inaccurate"
subject: "Foundation Engineering"
chapter: 8
key: "semester_5/foundation_engineering:dynamic-formula-inaccurate"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/dynamic-formula-inaccurate
site: BCT Engineering
---

# Why dynamic pile formulae are inaccurate

Foundation Engineering · Chapter 8: Pile Foundations

- Asked in: 2080 Baisakh, 3 marks
- Video: [foundation engineering | pile carrying capacity | ultimate bearing capacity | hiley's formula](https://www.youtube.com/watch?v=0QuFIopkwQQ) — Civil Engineering Exam
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/dynamic-formula-inaccurate
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-8

## Question

What are the reasons behind the dynamic method of determination of ultimate load carrying capacity of pile being inaccurate?

## Short answer

Dynamic formulae are inaccurate because energy losses are uncertain, dynamic resistance differs from static due to pore pressure and strain rate, impact is treated as rigid-body collision ignoring wave propagation, time effects (set-up, relaxation) are missed, and soil layering is ignored.

## Answer

Dynamic pile formulae (Engineering News, Hiley, Danish) estimate the capacity from the set per blow, but they are inaccurate for the following reasons.

1. **Energy losses are uncertain.** The energy lost in the hammer, cap, cushion, pile compression and soil quake cannot be assessed correctly. Efficiency is only assumed.
2. **Dynamic resistance differs from static resistance.** Pore water pressure, soil damping and strain rate during driving give a resistance that differs from the static capacity, especially in clays and saturated fine sands.
3. **Impact is treated as a rigid-body collision.** The formulae use a coefficient of restitution, which ignores wave propagation in the pile (the stress wave takes time to travel the length).
4. **Time effects.** In clays the strength after driving is regained (set-up) with time, whereas in dense saturated sands there may be relaxation, which the set at the end of driving does not show.
5. **Soil variation** along the pile (layers) and the shaft–tip load distribution are not accounted for.
6. **Only suitable for granular soil** and not for clay or long piles; the results may differ by as much as 50% or more. For these reasons a factor of safety of 2 to 6 is used, and the pile load test is preferred for important structures.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why are dynamic formulae inaccurate regarding energy?
   A. Hammer, cushion, pile and soil losses are uncertain; efficiency assumed
   B. Energy is exactly known
   C. No energy used
   D. Pile weight ignored
   Answer: A. Losses cannot be assessed correctly. (Key point: Energy losses)

2. Dynamic resistance differs from static due to:
   A. Pore pressure, damping and strain rate
   B. Colour
   C. Temperature only
   D. Pile length
   Answer: A. Especially in clays and saturated fine sands. (Key point: Static vs dynamic)

3. Formulae treat impact as:
   A. Rigid-body collision, ignoring stress wave propagation
   B. Wave motion
   C. Static load
   D. Vibration
   Answer: A. They use a coefficient of restitution. (Key point: Wave propagation)

4. In clays after driving, strength:
   A. Is regained with time (set-up)
   B. Is lost
   C. Is unchanged
   D. Becomes zero
   Answer: A. The end-of-driving set does not show this. (Key point: Time effects)

## Related in Foundation Engineering

- [Pile capacity in sand, group effect, load test](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-capacity-sand-group-load-test) — Foundation Engineering, chapter 8
- [Approaches to estimate pile capacity](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-capacity-approaches) — Foundation Engineering, chapter 8
- [Allowable load of 4×4 pile group, layered sand](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-4x4-figure-dcr) — Foundation Engineering, chapter 8
- [Allowable load of 400 mm pile, layered soil](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-400mm-layered-soil-chart) — Foundation Engineering, chapter 8
- [Pile group 4×4, two-layer clay, 2.4 m spacing](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-4x4-2-4m-layered) — Foundation Engineering, chapter 8, asked 2×
- [Pile group 16 piles of 50 cm, efficiency](https://www.bctengineer.com/question.md?key=semester_5%2Ffoundation_engineering%3Apile-group-16-piles-50cm) — Foundation Engineering, chapter 8

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