---
title: "One-way vs two-way drainage time"
subject: "Soil Mechanics"
chapter: 9
key: "semester_4/soil_mechanics:one-way-two-way-drainage-time"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/one-way-two-way-drainage-time
site: BCT Engineering
---

# One-way vs two-way drainage time

Soil Mechanics · Chapter 9: Compressibility of Soil

- Asked in: 2079 Asoj, 3 marks
- Video: [Time rate of consolidation example: double drainage.](https://www.youtube.com/watch?v=Gx6ahN0RakY) — GEOTECH VIDEOS
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/one-way-two-way-drainage-time
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-9

## Question

How do two way drainage and one way drainage affect the time of consolidation if the degree of consolidation and coefficient of consolidation for that clay layer are the same?

## Short answer

For the same U and cv, consolidation time is proportional to the square of the drainage path length. Two-way drainage halves the drainage path (Hdr = H/2) compared to one-way drainage (Hdr = H), so it requires only one quarter of the time to reach the same degree of consolidation.

## Answer

Time for a given degree of consolidation comes from the time factor:

$$
t = \frac{T_v\, H_{dr}^2}{c_v}
$$

For the same $U$ (same $T_v$) and the same $c_v$, time is proportional to $H_{dr}^2$.

- **One-way (single) drainage:** water leaves through one face only, so $H_{dr}=H$.
- **Two-way (double) drainage:** water leaves through top and bottom, so $H_{dr}=H/2$.

$$
\frac{t_{two}}{t_{one}} = \left(\frac{H/2}{H}\right)^2 = \frac{1}{4}
$$

**Result:** two-way drainage needs only **one quarter** of the time required for one-way drainage. Equivalently, one-way drainage takes four times longer. For example, a clay layer that needs 8 years to reach 90% consolidation with one-way drainage reaches the same degree in 2 years with two-way drainage.

## Check yourself

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

1. For the same U and cv, consolidation time is proportional to
   A. Hdr
   B. Hdr^2
   C. 1/Hdr
   D. Hdr^3
   Answer: B. t = Tv Hdr^2 / cv with Tv and cv fixed. (Key point: Time relation)

2. The drainage path for one-way and two-way drainage is respectively
   A. H/2 and H
   B. H and H/2
   C. H and H
   D. 2H and H
   Answer: B. One-way: water leaves via one face, Hdr = H; two-way: Hdr = H/2. (Key point: Drainage paths)

3. The ratio t(two-way)/t(one-way) is
   A. 1/2
   B. 2
   C. 1/4
   D. 4
   Answer: C. (H/2 / H)^2 = 1/4. (Key point: Time ratio)

4. A clay needing 8 years with one-way drainage would need how long with two-way drainage?
   A. 4 years
   B. 2 years
   C. 1 year
   D. 16 years
   Answer: B. One quarter of 8 years is 2 years. (Key point: Example)

## Related in Soil Mechanics

- [Isochrones, one and two way drainage](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aisochrones-one-two-way-drainage) — Soil Mechanics, chapter 9
- [5 m clay, Cc 0.25, 65% consolidation](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aclay-5m-cc-025-65-percent) — Soil Mechanics, chapter 9, asked 2×
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- [Consolidation test, 90% time, k, Cc](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Aconsolidation-test-50-90-percent-time) — Soil Mechanics, chapter 9

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