The Constructor

What is Scan Line Survey?

Scan line survey.

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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.

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:

  1. Joint orientation
  2. Spacing between joints
  3. Joint Aperture
  4. Joint persistance
  5. Roughness characteristic of joint  
  6. Joint filling 
  7. Strength of filling material 
  8. 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:

Scan line survey data sheet.

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.

Measuring dip direction using compass.

1.1.3 Strike

It is the angle perpendicular to the dip angle, generally lying between 0 to 360 degrees. 

Representation of dip, dip direction and strike.

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 spacing0.02-0.06
close spacing0.06-0.2
Moderate spacing0.2-0.6
Wide spacing0.6-2
Very wide spacing2-6
Extremely wide spacing>6
ISRM classification of spacing in a joint discontinuity.

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:

ApertureDescription  Features
< 0.1 mm?Very tightClosed features
0.1 – 0.25 mmTightClosed features
0.25 – 0.5 mmPartly openClosed features
0.5 – 2.5 mmOpenGapped features
2.5 – 10 mmWidely openGapped features
1 – 10 cmVery widely openOpen features
10 – 100 cmExtremely widely openOpen features
> 1 mCavernousOpen features
ISRM classification of joint aperture.
Illustration of aperture.

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 persistence1-3 
Medium persistence3-10
High persistence10-20
Very high persistence>20 
ISRM classification of joint persistence.
Illustration of joint persistence.

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:

ClassISRM Description
1Rough or irregular, stepped
2Smooth, stepped
3Slickensided, stepped
4Rough or irregular, undulating
5Smooth, undulating
6Slickensided, undulating
7Rough or irregular, planar
8Smooth, planar
9Slickensided, planar
ISRM classification of roughness in a joint discontinuity.

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:

  1. Mineralogy of filling material
  2. Grading or particle size
  3. Over-consolidation ratio
  4. Water content
  5. Permeability
  6. Previous shear displacement
  7. Wall roughness 
Illustration of joint infilling.

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
10.025The filling material is a very soft clay
20.025-0.05The filling material is a soft clay
30.05-0.10The filling material is a firm clay
40.10-0.25The filling material is a stiff clay
50.25-0.50The filling material is a very stiff clay
6>0.50The filling material is a hard clay
70.25-1.0The filling material is an extremely weak rock
81.0-5.0The filling material is a very weak rock
95-25The filling material is a weak rock
1025-50The filling material is a medium strong rock
1150-100The filling material is a strong rock
12100-250The filling material is a very strong rock
13>250The filling material is a extremely strong rock
ISRM classification of filling material in a joint discontinuity.

1.8 Condition of Water Flow

ISRM suggests the following rating for the water flow in joint discontinuity:

1Joint discontinuity is completely dry and there is no evidence of flow of water.0
2Joint discontinuity is completely dry and there is an evidence of flow of water.1
3Joint discontinuity is damp and there is no free flow of water2
4Joint discontinuity shows free flow of water but it is not continuous3
5Joint discontinuity shows continuous free flow of water4
6The joint filling materials are consolidated and no significant flow of water.5
7The joint filling materials are damp and no free flow of water.6
8The joint filling materials are wet and occasional flow of water is present.7
9The joint filling materials are wet and continuous  flow of water.8
10The joint filling materials are completely washed out with  continuous flow of water.9
ISRM rating for water flow in joint discontinuity.

Frequently Asked Questions

What is the use of the scan line survey?

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]

What are the Causes of Slope Failure?

Tunneling Failures – Causes and Remedies [PDF]

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