---
title: "Rock mass versus intact rock"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:rock-mass-vs-intact-rock"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rock-mass-vs-intact-rock
site: BCT Engineering
---

# Rock mass versus intact rock

Engineering Geology II · Chapter 6: Measurement, analysis and interpretation of structural geological data

- Asked in: 2079 Jestha, 1 marks; 2079 Chaitra, 0.5 marks; 2071 Bhadra, 2 marks; 2070 Bhadra, 2 marks; 2068 Bhadra, 1 marks; 2068 Magh, 1 marks; 2078 Poush (asked 7 times)
- Video: [Rock Mechanics Gate Mining Engineering Tutorials 1|Intact rock vs Rock mass Discontinuities of rocks](https://www.youtube.com/watch?v=AqVTyv3dNaM) — AVD all in one
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rock-mass-vs-intact-rock
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

Define rock mass and intact rock. Differentiate between them.

## Short answer

Intact rock is the solid, unjointed material between discontinuities, tested in the lab. Rock mass is the in-situ rock with its joints, faults and bedding planes; its strength is lower, deformbility larger, and behaviour anisotropic, governed by the discontinuties.

## Answer

**Intact rock** is the unfractured block of rock between discontinuities, small enough to be tested as a laboratory core specimen. **Rock mass** is the in-situ rock together with all its discontinuities (joints, bedding, faults, shear zones), i.e. the intact rock blocks plus the structural planes between them.

| Basis | Intact rock | Rock mass |
|---|---|---|
| Definition | Rock material between discontinuities | Intact rock plus discontinuities in the field |
| Size | Core or hand specimen | Whole outcrop, slope or tunnel section |
| Discontinuities | Absent | Present, control behaviour |
| Strength | High; given by UCS | Much lower; governed by joints |
| Deformability | Small, nearly elastic | Large, includes slip along joints |
| Permeability | Primary (very low) | Secondary through joints (higher) |
| Behaviour | Homogeneous, isotropic | Heterogeneous, anisotropic |
| Scale effect | None | Strength falls as volume grows |
| Test | Laboratory (UCS, point load) | In-situ tests, classification (RMR, Q) |

## Check yourself

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

1. What is a rock mass?
   A. A laboratory core specimen
   B. Unfractured block between joints
   C. In-situ rock together with all its discontinuities
   D. Only weathered rock
   Answer: C. Rock mass is intact blocks plus the structural planes between them. (Key point: Definitions)

2. How does strength compare?
   A. Rock mass is stronger because it is larger
   B. Both are equal
   C. Intact rock is weak; rock mass is strong
   D. Intact rock is high (UCS); rock mass is much lower, governed by joints
   Answer: D. Joints control rock mass strength so it is far below intact strength. (Key point: Strength)

3. Which is true of rock mass behaviour?
   A. Heterogeneous and anisotropic, and strength falls as volume grows
   B. Homogeneous and isotropic
   C. No scale effect
   D. Permeability is only primary and very low
   Answer: A. Rock mass has a scale effect and secondary permeability through joints. (Key point: Scale effect and anisotropy)

## Related in Engineering Geology II

- [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×
- [Geo-mechanics classification of rock mass](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Ageomechanics-classification) — Engineering Geology II, chapter 6, asked 5×
- [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×
- [Meaning of rock class and shear parameter](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-class-shear-meaning) — Engineering Geology II, chapter 6
- [Rock classes needing steel rib and shotcrete](https://www.bctengineer.com/question.md?key=semester_4%2Fengineering_geology_ii%3Armr-steel-rib-classes) — Engineering Geology II, chapter 6

## Related in other subjects

- [Rock mass classification](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification) — Engineering Geology I, chapter 5
- [Rock mass, rock material, discontinuities](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-rock-material-discontinuities) — Engineering Geology I, chapter 5
- [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
- [Identify the three rock types in the field](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-the-three-rock-types) — Engineering Geology I, chapter 4, asked 5×