---
title: "Filter requirements to control piping"
subject: "Soil Mechanics"
chapter: 7
key: "semester_4/soil_mechanics:filter-design-requirements-piping"
source: https://www.bctengineer.com/q/semester_4/soil_mechanics/filter-design-requirements-piping
site: BCT Engineering
---

# Filter requirements to control piping

Soil Mechanics · Chapter 7: Seepage through Soils

- Asked in: 2078 Poush, 2 marks; 2073 Magh, 2 marks (asked 2 times)
- Video: [Unit 2. L. No. 14: Design of Filter Material](https://www.youtube.com/watch?v=vvO7UfrhMbI) — venkatesha murthy
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/soil_mechanics/filter-design-requirements-piping
- Whole chapter: https://www.bctengineer.com/md/semester_4/soil_mechanics/chapter-7

## Question

What are the basic requirements for the design of protective filters? Explain the filter requirements for controlling piping.

## Short answer

Filters must prevent piping (retain base soil) and allow free drainage (be more permeable). Terzaghi’s rules: D₁₅(filter)/D₈₅(base) < 4–5 for retention, and D₁₅(filter)/D₁₅(base) > 4–5 for permeability.

## Answer

A protective filter is a layer of graded granular material placed between the base soil (the soil being protected) and a drain or coarse material, to allow water to escape without letting soil particles wash out.

#### Basic requirements

1. **Retention (piping) criterion:** the filter pores must be small enough to hold back the particles of the base soil.
2. **Permeability criterion:** the filter must be much more permeable than the base soil, so that no pore pressure builds up and the water escapes freely.
3. The filter must be well graded, with no fines that could migrate, and be stable (no segregation) and durable.
4. It must be thick enough, and not clog; in multiple layers, each layer must satisfy the criteria with respect to the one next to it.

#### Terzaghi's filter rules for controlling piping

$$
\frac{D_{15}(\text{filter})}{D_{85}(\text{base soil})} < 4 \text{ to } 5 \quad (\text{piping})
$$

$$
\frac{D_{15}(\text{filter})}{D_{15}(\text{base soil})} > 4 \text{ to } 5 \quad (\text{permeability})
$$

Additional (U.S. Bureau / Corps of Engineers) conditions: $D_{50}(\text{filter})/D_{50}(\text{soil}) < 25$ and the grain-size curve of the filter is roughly parallel to that of the soil. For slotted pipes, $D_{85}(\text{filter}) >$ slot width (or 1.2 times slot width).

## Check yourself

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

1. What is the main function of a protective filter?
   A. To stop all water flow
   B. To let water escape without washing out the base soil particles
   C. To increase the pore pressure in the base soil
   D. To make the base soil denser by compaction
   Answer: B. A filter lets water pass freely while holding back soil particles. (Key point: Purpose of filter)

2. Why must the filter be much more permeable than the base soil?
   A. To retain the fine particles
   B. To save material
   C. To prevent segregation
   D. So no pore pressure builds up and water escapes freely
   Answer: D. High filter permeability prevents pore pressure build-up and lets seepage leave freely. (Key point: Permeability criterion)

3. Which Terzaghi ratio controls piping?
   A. D15(filter)/D85(base) < 4 to 5
   B. D15(filter)/D85(base) > 4 to 5
   C. D15(filter)/D15(base) < 4 to 5
   D. D85(filter)/D15(base) > 25
   Answer: A. Keeping the filter D15 small relative to the base soil D85 makes the filter pores small enough to retain base particles. (Key point: Piping criterion)

## Related in Soil Mechanics

- [Basic requirements of classification](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Abasic-requirements-classification) — Soil Mechanics, chapter 3, asked 2×
- [Sheet pile flow net; graded filter](https://www.bctengineer.com/question.md?key=semester_4%2Fsoil_mechanics%3Asheet-pile-flow-net-graded-filter) — Soil Mechanics, chapter 7

## Related in other subjects

- [Protection requirements; protective system vs scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Aprotective-system-vs-scheme) — Switchgear and Protection, chapter 1
- [Evaluation vs controlling; elements of control](https://www.bctengineer.com/question.md?key=semester_7%2Fproject_engineering%3Aevaluation-vs-controlling-elements) — Project Engineering, chapter 4
- [Basic requirements of a protection scheme](https://www.bctengineer.com/question.md?key=semester_6%2Fswitchgear_and_protection%3Abasic-requirements-protection-scheme) — Switchgear and Protection, chapter 1, asked 3×
- [SC filter applications; design for 100–400 rad/s peak](https://www.bctengineer.com/question.md?key=semester_5%2Ffilter_design%3Asc-applications-bode-100-400) — Filter Design, chapter 9