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Scan line survey, also known as discontinuity survey, is essential for the estimation of rock mass quality in rock mechanics and underground structures.
This process involves scanning the outcropped rock surface, followed by data collection for rock mass analysis. Scan line surveys are thus useful for the preliminary design of structures supported on rocks.
Furthermore, in this survey, a horizontal line is drawn over the outcropped rock surface, and all the properties of discontinuity, such as spacing, orientation, persistence, aperture, etc., are examined. Meanwhile, all the properties are reported in the scan line survey datasheets.
After that, the analysis of all the collected data is carried out in a comprehensive manner to determine the strength, deformability, and flow characteristics of the rock mass.
In conclusion, the quantified information obtained by performing the scan line surveys over the entire exposed surface helps create a database, which can be used in the decision-making process for any engineering design project for that particular site.
Contents:
1. Assessment of Rock Joint Properties
The most important characteristics of a rock joint can be estimated using the scanline sampling technique. As an illustration, the scan line survey technique includes the estimation of the following parameters:
- Joint orientation
- Spacing between joints
- Joint Aperture
- Joint persistance
- Roughness characteristic of joint
- Joint filling
- Strength of filling material
- Water flow condition
Simultaneously, while working in the field, the parameters mentioned above should be reported in the scan line survey datasheet. For this purpose, a sample of scan line survey data is given below:
1.1 Joint Orientation
Joint orientation plays a vital role in deciding the stability of the rock block as the excessive deformation in the rock mass depends upon it. Particularly, the orientation of a particular joint set is defined by its dip, dip direction, and strike.
1.1.1 Dip
It is the angle of the discontinuity plane to the horizontal plane, ranging between 0-90 degrees.
1.1.2 Dip Direction
It is the direction of the dip angle and is generally expressed as N, NE, E, SE, S, etc.
1.1.3 Strike
It is the angle perpendicular to the dip angle, generally lying between 0 to 360 degrees.
1.2 Spacing between Joints
Spacing is defined as the perpendicular distance between the discontinuities. For a particular joint set, which includes 'n' number of discontinuities, the joint spacing would be defined as the mean spacing.
In general, the spacing between joints controls the flow of filled material, and the mode by which the rock block fails. For instance, the failure of closely spaced joints is governed by circular mode (Type of rock failure).
The International Society for Rock Mechanics (ISRM) classifies the joint spacing, as shown below:
| ISRM Suggested Description | Joint spacing (m) |
| Extremely close spacing | < 0.02 |
| Very close spacing | 0.02-0.06 |
| close spacing | 0.06-0.2 |
| Moderate spacing | 0.2-0.6 |
| Wide spacing | 0.6-2 |
| Very wide spacing | 2-6 |
| Extremely wide spacing | >6 |
1.3 Joint Aperture
The two discontinuities are generally not in complete contact, because of which, a gap between the two discontinuity surfaces exists. Therefore, the perpendicular distance between the gap of rock walls, separated by the adjacent rock blocks, is known as aperture.
Basically, a joint opening is either filled with air/water (open joint) or with infill materials (filled joint). The aperture is thereby distinguished from the width of a filled discontinuity. As a result, open or filled joints with large apertures have low shear strength.
Aperture influences particularly the flow and permeability of the rock joints and the ISRM classification for the aperture is shown below:
| Aperture | Description | Features |
| < 0.1 mm? | Very tight | Closed features |
| 0.1 – 0.25 mm | Tight | Closed features |
| 0.25 – 0.5 mm | Partly open | Closed features |
| 0.5 – 2.5 mm | Open | Gapped features |
| 2.5 – 10 mm | Widely open | Gapped features |
| 1 – 10 cm | Very widely open | Open features |
| 10 – 100 cm | Extremely widely open | Open features |
| > 1 m | Cavernous | Open features |
1.4 Joint Persistence
Joint persistence is defined as the length of discontinuity and is, therefore, measured in meter. In simpler words, the trace length of discontinuity on the exposed rock mass is known as persistence.
The shear strength of any discontinuity is majorly affected by the persistence, and hence the sliding and slipping movement of rock blocks along the discontinuity plane is also controlled by it.
The ISRM classification for persistence is as shown below:
| ISRM Suggested Description | Surface Trace Length (m) |
| Very low persistence | < 1 |
| low persistence | 1-3 |
| Medium persistence | 3-10 |
| High persistence | 10-20 |
| Very high persistence | >20 |
1.5 Roughness Characteristic of Joint
Generally, the interface between the two contacting surfaces is either rough or smooth. The aperture of the interface governs the condition of the roughness of the surface. Therefore, the joint roughness is the measure of the undulated interface surface relative to the mean size of the joint plane.
Joint roughness affects the shear strength of the joint, and the roughness of discontinuity governs the displacement of blocks. However, as the filling of material in the joint increases, the roughness of that joint reduces, thereby decreasing the probability of sliding down the discontinuity.
On the other hand, the joint roughness is measured by the undulation of the exposed rock surface in the field. Thus, each of the undulations should be classified on an arbitrary scale of one to nine. Each class of roughness is defined as per the ISRM classification, which is shown below:
| Class | ISRM Description |
| 1 | Rough or irregular, stepped |
| 2 | Smooth, stepped |
| 3 | Slickensided, stepped |
| 4 | Rough or irregular, undulating |
| 5 | Smooth, undulating |
| 6 | Slickensided, undulating |
| 7 | Rough or irregular, planar |
| 8 | Smooth, planar |
| 9 | Slickensided, planar |
1.6 Joint Filling
Two rock discontinuities are mostly filled with the same filling material, such as calcite, chlorite, clay, sand, silt, quartz, and pyrite. The shear strength properties of the joint vary depending upon the properties of filling material.
The behavior of the filled discontinuities depends on several factors; where the major ones are:
- Mineralogy of filling material
- Grading or particle size
- Over-consolidation ratio
- Water content
- Permeability
- Previous shear displacement
- Wall roughness
1.7 Strength of Filling Material
The strength of filling material is calculated using the rebound hammer apparatus. While measuring the strength of the filling material, the apparatus should be calibrated as per the site environment. Thereafter, the rebound hammer test should be performed.
The ISRM classifies the filling material based upon the compressive strength, as shown below:
| S.No. | Compressive strength (MPa) | Classification |
| 1 | 0.025 | The filling material is a very soft clay |
| 2 | 0.025-0.05 | The filling material is a soft clay |
| 3 | 0.05-0.10 | The filling material is a firm clay |
| 4 | 0.10-0.25 | The filling material is a stiff clay |
| 5 | 0.25-0.50 | The filling material is a very stiff clay |
| 6 | >0.50 | The filling material is a hard clay |
| 7 | 0.25-1.0 | The filling material is an extremely weak rock |
| 8 | 1.0-5.0 | The filling material is a very weak rock |
| 9 | 5-25 | The filling material is a weak rock |
| 10 | 25-50 | The filling material is a medium strong rock |
| 11 | 50-100 | The filling material is a strong rock |
| 12 | 100-250 | The filling material is a very strong rock |
| 13 | >250 | The filling material is a extremely strong rock |
1.8 Condition of Water Flow
ISRM suggests the following rating for the water flow in joint discontinuity:
| S.No. | Description | Rating |
| 1 | Joint discontinuity is completely dry and there is no evidence of flow of water. | 0 |
| 2 | Joint discontinuity is completely dry and there is an evidence of flow of water. | 1 |
| 3 | Joint discontinuity is damp and there is no free flow of water | 2 |
| 4 | Joint discontinuity shows free flow of water but it is not continuous | 3 |
| 5 | Joint discontinuity shows continuous free flow of water | 4 |
| 6 | The joint filling materials are consolidated and no significant flow of water. | 5 |
| 7 | The joint filling materials are damp and no free flow of water. | 6 |
| 8 | The joint filling materials are wet and occasional flow of water is present. | 7 |
| 9 | The joint filling materials are wet and continuous flow of water. | 8 |
| 10 | The joint filling materials are completely washed out with continuous flow of water. | 9 |
Frequently Asked Questions
It is advantageous in the preliminary stage of the project. With the help of the scan line survey data sheet, the strength behaviour, the deformation behaviour, and the flow pattern for a particular site can be find out.
The above-mentioned characteristics will help in the decision-making process for choosing the site for a particular project.
Read More:
How to Improve Rock Quality and Stability? [PDF]
