---
title: "Rock mass, rock material, discontinuities"
subject: "Engineering Geology I"
chapter: 5
key: "semester_3/engineering_geology_i:rock-mass-rock-material-discontinuities"
source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/rock-mass-rock-material-discontinuities
site: BCT Engineering
---

# Rock mass, rock material, discontinuities

Engineering Geology I · Chapter 5: Structural Geology

- Asked in: 2062 Baisakh (old course), 2+6 marks
- Video: [Some Properties of Rock Discontinuity(Visit Open2study)](https://www.youtube.com/watch?v=x0DlfiLgSCM) — All you want
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/rock-mass-rock-material-discontinuities
- Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-5

## Question

Define rock mass and rock material. Explain the characteristics of discontinuities in the rock mass.

## Short answer

Rock material is the intact solid rock between fractures. Rock mass is the total in-situ rock consisting of intact blocks bounded by discontinuities, whose characteristics (orientation, spacing, persistence, roughness, aperture, filling, wall strength, seepage, number of sets) dictate mass behaviour.

## Answer

#### Rock material
**Rock material (intact rock)** is the solid rock between the discontinuities, a block of rock without any fractures, as in a laboratory core specimen. It is described by its rock type, grain size, mineral composition and the strength found from laboratory tests (UCS, point load).

#### Rock mass
**Rock mass** is the rock in the field as a whole: the intact rock plus the system of discontinuities that cut it. Its behaviour is controlled mostly by the discontinuities, so its strength is much lower than that of the intact rock.

```text
   rock mass = intact rock blocks + discontinuities
   ____ ____ ____
  |    |    |    |   each block = rock material
  |____|____|____|   gaps/lines   = discontinuities
```

#### Characteristics of discontinuities

1. **Orientation:** the attitude, given by dip and dip direction (or strike). Together with the slope it decides if a block can slide.
2. **Spacing:** the distance between adjacent discontinuities of one set. Close spacing gives small blocks and a weak mass.
3. **Persistence (continuity):** the length of the discontinuity trace; a persistent plane makes a more continuous failure surface.
4. **Roughness:** unevenness of the surface; rough surfaces give higher shear strength.
5. **Aperture:** the open distance between the walls. A wide aperture means low strength and high permeability.
6. **Filling (infill):** material between the walls (clay, sand, calcite); clay filling reduces shear strength sharply.
7. **Wall strength (weathering):** strength of the rock at the wall surface.
8. **Seepage (water flow):** water in the discontinuities.
9. **Number of sets and block size.**

These characteristics are logged in the field by scanline or window mapping and are the input for rock mass classification (RMR, Q) and slope and tunnel design.

## Check yourself

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

1. What is rock material?
   A. Intact rock between discontinuities, as in a lab specimen
   B. The whole field rock with joints
   C. A fault zone
   D. A fold
   Answer: A. Intact rock has no fractures. (Key point: Rock material)

2. Why is rock mass weaker than rock material?
   A. It has more minerals
   B. It contains discontinuities that control its behaviour
   C. It is wetter always
   D. It has no cement
   Answer: B. Discontinuities lower strength. (Key point: Rock mass strength)

3. What does roughness give?
   A. Lower shear strength
   B. Higher permeability only
   C. Higher shear strength
   D. No effect
   Answer: C. Rough walls interlock. (Key point: Roughness)

4. What does orientation with the slope decide?
   A. The mineral type
   B. The rock age
   C. The colour
   D. Whether a block can slide
   Answer: D. Dip direction relative to slope controls sliding. (Key point: Orientation)

## Related in Engineering Geology I

- [Rock mass classification and discontinuities](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification-and-discontinuities) — Engineering Geology I, chapter 5
- [Rock mass classification](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification) — Engineering Geology I, chapter 5
- [Planes of discontinuity in rock masses](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aplanes-of-discontinuity-in-rock) — Engineering Geology I, chapter 5
- [Discontinuities; Siwalik and Higher Himalaya lithology](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Adiscontinuities-siwalik-and-higher-himalaya) — Engineering Geology I, chapter 7

## Related in other subjects

- [Properties of rock mass and discontinuities](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-mass-properties) — Engineering Geology II, chapter 6, asked 3×
- [RMR rock mass classification](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-classification) — Engineering Geology II, chapter 6, asked 8×
- [Q-system of rock mass classification](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Aq-system-classification) — Engineering Geology II, chapter 6, asked 7×
- [Rock mass versus intact rock](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Arock-mass-vs-intact-rock) — Engineering Geology II, chapter 6, asked 7×