---
title: "Factors affecting degree of consolidation"
subject: "Soil Mechanics"
chapter: 9
key: "semester_4/soil_mechanics:factors-affecting-degree-consolidation"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/factors-affecting-degree-consolidation
site: BCT Engineering
---

# Factors affecting degree of consolidation

Soil Mechanics · Chapter 9: Compressibility of Soil

- Asked in: 2079 Jestha, 4 marks
- Video: [Factors Affecting Consolidation](https://www.youtube.com/watch?v=3P77p1aD2tE) — DTG KKTM SRI GADING
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/factors-affecting-degree-consolidation
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-9

## Question

Explain the factors that affect the degree of consolidation.

## Short answer

Degree of consolidation U depends on the time factor Tv = cv t / Hdr^2. Factors that change Tv include time, coefficient of consolidation cv (which increases with permeability and decreases with compressibility), drainage path length Hdr (doubling it quadruples time), drainage condition (one-way vs two-way), and the initial excess pore pressure distribution.

## Answer

The **degree of consolidation** $U$ is the ratio of the consolidation settlement at time $t$ to the final primary settlement (or the fraction of excess pore pressure dissipated). For one-dimensional consolidation, $U$ depends only on the time factor:

$$
T_v = \frac{c_v\, t}{H_{dr}^2}, \qquad c_v = \frac{k}{m_v \gamma_w}
$$

So the factors that affect $U$ are those that change $T_v$:

1. **Time $t$.** $U$ increases as time passes. For $U<60\%$, $T_v=\frac{\pi}{4}U^2$.
2. **Coefficient of consolidation $c_v$.** A larger $c_v$ gives faster consolidation. It rises with permeability $k$ and falls with compressibility $m_v$. Sand has a very high $c_v$; clay has a low $c_v$.
3. **Coefficient of permeability $k$.** Fine, plastic clays with low $k$ consolidate slowly because water escapes slowly.
4. **Coefficient of volume compressibility $m_v$.** A more compressible soil must expel more water, so it takes longer.
5. **Drainage path length $H_{dr}$.** Time varies with $H_{dr}^2$. Doubling the layer thickness makes the time four times longer.
6. **Drainage condition.** Drainage at top and bottom (double drainage) gives $H_{dr}=H/2$, so it is four times faster than single drainage with $H_{dr}=H$.
7. **Distribution of initial excess pore pressure.** With one-way drainage, uniform, triangular or sinusoidal distributions give different $U$–$T_v$ curves, the difference being largest in the early stages. With two-way drainage, any linear distribution gives the same curve as the uniform case.
8. **Presence of sand lenses, drains or seams.** Vertical sand drains and thin sand layers shorten $H_{dr}$ and speed up consolidation.

## Check yourself

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

1. For 1D consolidation, the degree of consolidation depends only on
   A. the time factor Tv = cv t/Hdr^2
   B. the load magnitude
   C. the unit weight
   D. the layer colour
   Answer: A. U is a function of Tv, so factors that change Tv change U. (Key point: Time factor)

2. How does cv relate to k and mv?
   A. cv = k/(mv gamma_w): rises with k and falls with mv
   B. cv = k mv gamma_w
   C. cv falls with k
   D. cv is independent of both
   Answer: A. Higher permeability gives faster consolidation, higher compressibility gives slower. (Key point: Effect of cv)

3. Doubling the layer thickness (same drainage type) makes the time to a given U
   A. twice as long
   B. four times longer
   C. half
   D. unchanged
   Answer: B. Time varies with Hdr^2. (Key point: Drainage path)

4. Double drainage compared with single drainage is
   A. twice as slow
   B. same
   C. four times faster
   D. two times faster
   Answer: C. Hdr = H/2 for double drainage, so time is one quarter. (Key point: Drainage condition)

## Related in Soil Mechanics

- [Factors affecting compaction](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Afactors-affecting-compaction) — Soil Mechanics, chapter 5, asked 3×
- [Factors affecting capillary rise](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Acapillary-rise-factors) — Soil Mechanics, chapter 6
- [Define consolidation terms](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aconsolidation-definitions) — Soil Mechanics, chapter 9, asked 2×
- [One-way vs two-way drainage time](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aone-way-two-way-drainage-time) — Soil Mechanics, chapter 9

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