---
title: "Properties of rock mass and discontinuities"
subject: "Engineering Geology II"
chapter: 6
key: "semester_4/engineering_geology_ii:rock-mass-properties"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rock-mass-properties
site: BCT Engineering
---

# Properties of rock mass and discontinuities

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

- Asked in: 2079 Chaitra, 1 marks; 2068 Magh, 2 marks; 2078 Poush (asked 3 times)
- 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_4/engineering_geology_ii/rock-mass-properties
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-6

## Question

List out the different properties of rock mass. Write down the properties of discontinuities used for rock mass classification.

## Short answer

Rock mass properties include physical (density, porosity), mechanical (strength, modulus), lithological, structural, hydrogeological, in-situ stress, and time-dependent behaviour. Discontinuity properties for classification: orientation, spacing, persistence, roughness, wall strength, aperture, infilling, seepage, number of sets, block size, weathering.

## Answer

#### Properties of rock mass
1. **Physical:** density, porosity, water content, permeability.
2. **Mechanical (strength and deformation):** compressive, tensile and shear strength, modulus of deformation, Poisson's ratio, cohesion and friction angle.
3. **Lithological/petrographic:** rock type, mineral composition, grain size, texture, weathering and alteration.
4. **Structural:** discontinuities, folds, faults and their geometry.
5. **Hydrogeological:** groundwater level, seepage, secondary permeability.
6. **In-situ stress:** magnitude and direction of stresses.
7. **Time-dependent behaviour:** creep, swelling, slaking, stand-up time.

#### Properties of discontinuities (ISRM) used for classification
| Property | Meaning |
|---|---|
| Orientation | Dip and dip direction (strike) |
| Spacing | Distance between adjacent joints of a set |
| Persistence | Length or extent of the trace |
| Roughness | Waviness and surface unevenness (JRC) |
| Wall strength | Strength of the joint wall (JCS) |
| Aperture | Opening between the walls |
| Infilling | Material in the gap (clay, calcite, sand) |
| Seepage | Water flow through the joint |
| Number of sets | Joint sets cutting the mass |
| Block size | Size and shape of blocks formed |
| Weathering/alteration | Degree of decay of walls |

## Check yourself

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

1. Which is a property of rock mass in the mechanical group?
   A. Compressive, tensile and shear strength, modulus, cohesion and friction angle
   B. Porosity
   C. Groundwater level
   D. Rock type
   Answer: A. Mechanical properties cover strength and deformation parameters. (Key point: Categories)

2. Which is a time-dependent behaviour?
   A. Density
   B. Creep, swelling, slaking and stand-up time
   C. Porosity
   D. Grain size
   Answer: B. These change with time after excavation. (Key point: Time-dependent)

3. What does spacing of discontinuities mean?
   A. Opening between the walls
   B. Length of the trace
   C. Distance between adjacent joints of a set
   D. Roughness of surface
   Answer: C. Spacing is the distance between neighbouring joints of a set. (Key point: Spacing)

4. Which pair is correct?
   A. Aperture is joint wall strength; infilling is roughness
   B. Aperture is opening between walls; infilling is material (clay, calcite, sand) in the gap
   C. Aperture is dip; infilling is strike
   D. Both mean persistence
   Answer: B. Aperture is the gap; infilling fills it. (Key point: Aperture and infilling)

## 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×
- [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×

## Related in other subjects

- [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, 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](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Arock-mass-classification) — Engineering Geology I, chapter 5
- [Identify metamorphic rock; properties of sandstone and granite](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Aidentify-metamorphic-rock-properties-of) — Engineering Geology I, chapter 4