---
title: "Thixotropy and flow index"
subject: "Soil Mechanics"
chapter: 2
key: "semester_4/soil_mechanics:thixotropy-flow-index"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/thixotropy-flow-index
site: BCT Engineering
---

# Thixotropy and flow index

Soil Mechanics · Chapter 2: Solids- Water –Air Relations and Index properties of soils

- Asked in: 2073 Bhadra, 2 marks
- Video: [Sensitivity and Thixotropy of Soil | Soil Mechanics | APSEd](https://www.youtube.com/watch?v=_v04D-k43WM) — APSEd
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/thixotropy-flow-index
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-2

## Question

Define thixotropy and flow index.

## Short answer

Thixotropy is the loss of strength when clay is remoulded at constant water content and the gradual regain of strength with time at rest. Flow index is the slope of the water content versus log blows curve in the liquid limit test, equal to (w1-w2)/log(N2/N1).

## Answer

#### Thixotropy
Thixotropy is the property of some clay soils to **lose strength when disturbed (remoulded) at constant water content, and regain it with time** on standing at rest. The regain is caused by reorientation of water molecules and particles in the adsorbed layer. Example: pile driving in clay softens the soil, then the strength returns after some days (pile "freeze").

#### Flow index ($I_f$)
The slope of the flow curve, i.e. the line of water content against log of number of blows in the liquid limit test. It is the change in water content per one log cycle of blows:
$$
I_f = \frac{w_1 - w_2}{\log_{10}(N_2/N_1)}
$$
A high flow index means the shear strength falls quickly with increase in water content.

## Check yourself

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

1. Thixotropy is the property of some clays to:
   A. Gain strength when disturbed
   B. Lose strength when remoulded at constant water content and regain it with time at rest
   C. Swell with water
   D. Shrink on drying
   Answer: B. Strength lost on disturbance returns with time. (Key point: Thixotropy)

2. A practical example of thixotropy is:
   A. Piping
   B. Liquefaction of sand
   C. Frost heave
   D. Pile freeze after pile driving in clay
   Answer: D. Driving softens clay but strength returns after some days. (Key point: Example)

3. Flow index is the:
   A. Water content at 25 blows
   B. Slope of the flow curve (water content vs log blows)
   C. Ip/wL
   D. Intercept of shrinkage curve
   Answer: B. If = change in water content per log cycle of blows. (Key point: Flow index)

4. A high flow index means:
   A. Soil is dense
   B. Strength is constant
   C. Soil is non-plastic
   D. Strength falls quickly with increasing water content
   Answer: D. A steep flow curve means shear strength is very sensitive to water. (Key point: Meaning of high If)

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