---
title: "Uplift pressure with Bligh's creep theory"
subject: "Irrigation and Drainage Engineering"
chapter: 5
key: "semester_6/irrigation_and_drainage_engineering:bligh-creep-theory-uplift"
source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/bligh-creep-theory-uplift
site: BCT Engineering
---

# Uplift pressure with Bligh's creep theory

Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks

- Asked in: 2066 Bhadra (old course), 6 marks
- Video: [Bligh's creep theory for seepage flow | part 8 (irrigation engineering)](https://www.youtube.com/watch?v=_pouhFojlDk) — NavAeon (Civil Engineering studies) 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/bligh-creep-theory-uplift
- Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5

## Question

Write with definition sketch how do you determine the pressure along the foundation of the structure and ensure safety against uplift pressure using Bligh's creep theory.

## Short answer

Bligh's theory assumes seepage follows base contact with creep length L = horizontal + 2×vertical, giving straight hydraulic gradient. Piping safety requires L ≥ C×H. Uplift pressure head at point x is hx = H(1 - Lx/L). Floor thickness for uplift safety is t = (4/3)hx/(G-1) with 33% safety factor.

## Answer

#### Bligh's creep theory

Water seeping below the structure follows the contact between the base and the subsoil (the creep path). The **length of creep** $L$ is the sum of the horizontal creep (the horizontal lengths of the contact) and the vertical creep (the lengths of the cutoffs, each counted twice because the water goes down and up):

$$
L = L_h + L_v
$$

The head loss is assumed to be proportional to the creep length along the path (straight hydraulic gradient line).

```text
   H ~~~~~~~~~|  ____________   ~~~~~~~ (d/s)
   u/s    |   |  |  floor   |  |
   ===+===|===+==+==========+==+====
     pile 1                 pile 2
   point A (start) ... point B (end of creep)
   HGL: straight line from H at A to 0 at B
```

#### Safety against piping

$$
L \ge C\,H \quad \text{or} \quad \frac{H}{L} \le \frac{1}{C}
$$

with Bligh's coefficient $C$ (e.g. 18 for light sand and mud, 15 for fine micaceous sand, 12 for coarse sand, 5-9 for sand mixed with boulders and gravel).

#### Pressure along the foundation

The head lost up to a point is proportional to the creep length up to it:

$$
\text{Head lost up to point } x = H\,\frac{L_x}{L},\qquad h_x = H - H\frac{L_x}{L} = H\left(1 - \frac{L_x}{L}\right)
$$

where $h_x$ is the residual (uplift) pressure head at the point and $L_x$ the creep length from the upstream end to it. The uplift pressure on the floor is $\gamma_w h_x$ (in kN/m²).

#### Safety against uplift

Let $t$ be the floor thickness and $G$ the specific gravity of the floor material. The floor is safe when its submerged weight balances the uplift:

$$
\gamma_w (G-1)\,t \ge \gamma_w h_x \Rightarrow t = \frac{h_x}{G-1}
$$

Usually 33% is added for safety, i.e. $t = \dfrac{4}{3}\dfrac{h_x}{G-1}$. If the thickness becomes excessive, lengthen the u/s floor or deepen the u/s pile to reduce $h_x$, or thicken the floor.

## Check yourself

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

1. In Bligh's creep theory, how is the total creep length L obtained?
   A. Horizontal contact lengths plus the cutoff depths counted twice (down and up)
   B. Only the horizontal floor length
   C. Horizontal lengths plus cutoff depths counted once
   D. Twice the horizontal length plus the cutoff depths
   Answer: A. Water creeps along the base contact and goes down and up along each cutoff, so each vertical length counts twice. (Key point: Length of creep)

2. What does Bligh assume about the loss of head along the creep path?
   A. Loss is greater near the downstream end
   B. Loss is proportional to the creep length (straight hydraulic gradient line)
   C. Loss occurs only across the cutoffs
   D. Loss is zero along the horizontal floor
   Answer: B. Bligh assumes the head loss is proportional to the creep length, giving a straight HGL. (Key point: Head loss assumption)

3. Which condition ensures safety against piping by Bligh's theory?
   A. L <= C H
   B. L >= C H
   C. H/L >= 1/C
   D. L = H/C
   Answer: B. The creep length must be at least C times the head, i.e. H/L <= 1/C. (Key point: Piping safety criterion)

4. The residual (uplift) head at a point at creep length Lx from the upstream end is:
   A. H Lx / L
   B. H (1 - Lx/L)
   C. H (L - Lx)
   D. H L / Lx
   Answer: B. Head lost is H Lx/L, so the remaining head is H - H Lx/L = H(1 - Lx/L). (Key point: Residual head at a point)

5. Floor thickness for safety against uplift (with the 33% safety increase) is:
   A. t = h_x/(G-1)
   B. t = (4/3) h_x/(G-1)
   C. t = (4/3) h_x (G-1)
   D. t = h_x/(G+1)
   Answer: B. The weight of the submerged floor gamma_w(G-1)t must balance uplift h_x, and the thickness is increased by one third. (Key point: Floor thickness against uplift)

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