---
title: "Piles subjected to uplift loads"
subject: "Foundation Engineering"
chapter: 8
key: "semester_5/foundation_engineering:uplift-piles"
source: https://www.bctengineer.com/q/semester_5/foundation_engineering/uplift-piles
site: BCT Engineering
---

# Piles subjected to uplift loads

Foundation Engineering · Chapter 8: Pile Foundations

- Asked in: 2076 Chaitra, 2 marks
- Video: [Pile Foundation - 13 | Uplift Load Carrying Capacity of Pile](https://www.youtube.com/watch?v=G2MXtOcSQtQ) — Geotechnical & Foundation Engineering by Dr Verma
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/foundation_engineering/uplift-piles
- Whole chapter: https://www.bctengineer.com/md/semester_5/foundation_engineering/chapter-8

## Question

Describe the piles subjected to uplift loads.

## Short answer

Uplift piles resist pull-out (towers, buoyant basements) using shaft friction plus pile weight, no end bearing. Under-reamed bulbs increase resistance. Group uplift capacity is the lesser of sum of individual capacities or block weight plus perimeter shear; factor of safety 2–3.

## Answer

**Uplift (tension) piles** resist upward or pull-out forces in a structure, where the load tries to lift the foundation.

- **Uses:** transmission towers and chimneys (wind overturning), basement floors and tanks under the water table (buoyancy), anchoring of docks and offshore structures, and bridge piers resisting overturning.
- **Resistance:** only the skin friction along the shaft acts (there is no end bearing), plus the weight of the pile. It is about 70 to 80% of the friction in compression. Ultimate uplift: $Q_{up} = \alpha c_u\pi DL + W_p$ (clay) or $\sum K\sigma'_v\tan\delta\,\pi D\Delta L + W_p$ (sand).
- **Under-reamed (belled) piles** have a larger resistance from the bulb. The pile is also anchored to the cap by reinforcement for full tension.
- **Group:** the uplift capacity of a group is the lesser of the sum of individual piles, and the weight of the block of soil plus pile cap and the shear around the block.
- A factor of safety of 2 to 3 is used.

## Check yourself

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

1. What resists the pull-out of an uplift pile?
   A. End bearing at the tip plus skin friction
   B. Skin friction along the shaft plus the pile weight
   C. End bearing only
   D. Passive resistance of the pile cap only
   Answer: B. In tension there is no end bearing, so only shaft friction and the pile's self weight resist uplift. (Key point: Source of uplift resistance)

2. Compared with friction in compression, the uplift skin friction is about:
   A. Twice as large
   B. The same
   C. 70 to 80% of it
   D. 10 to 20% of it
   Answer: C. Uplift friction is taken as about 70 to 80% of the friction in compression. (Key point: Uplift vs compression friction)

3. The uplift capacity of a pile group is taken as:
   A. The greater of the sum of piles and the soil block value
   B. The lesser of the sum of individual piles and the block of soil plus cap weight and shear
   C. Always the sum of individual piles
   D. Only the weight of the pile cap
   Answer: B. The weaker of the two failure modes governs the group. (Key point: Group uplift capacity)

## Related in other subjects

- [Khosla: two piles, floor at A, B, C, exit 0.2](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akhosla-two-piles-thickness-abc-02) — Irrigation and Drainage Engineering, chapter 5
- [Khosla: uplift at A, B, thickness, exit gradient 0.2](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Akhosla-uplift-floor-thickness-fig-103) — Irrigation and Drainage Engineering, chapter 5
- [Steel block under triaxial loads: dimension and volume change](https://www.bctengineer.com/question.md?key=semester_3%2Fstrength_of_materials%3Asteel-block-triaxial-loads-change-dimensions) — Strength of Materials, chapter 3
- [Arch 32 m: two rolling loads, max +/- moment at 8 m](https://www.bctengineer.com/question.md?key=semester_4%2Ftheory_of_structures_i%3Aarch-32m-two-rolling-loads-8m) — Theory of Structures I, chapter 6