Spatial positioning of sidewall stations in a narrow tunnel environment: a safe alternative to traditional mine survey practice

Site application of the intersecting lines method.

The method of intersecting lines was tested in a tunnel to determine the practicality of the solution. The entire operation was completed by one person with no assistance. An instrument was placed to the side of the centre line of the tunnel and the position determined by freestation from four fixed sidewall stations. The collimation elevation of the instrument is calculated directly from the freestation observations. The minor dip method of determining the vertical angle at each position was calculated using a MS Excel spreadsheet based on the following principles:

1. 
   The required gradient of the tunnel is the Major dip D°, on the required bearing of the design α°. In this case a gradient of -2 degrees was required.
   
2. 
   The position for four gradepegs was selected not to be perpendicular to the direction of mining or directly opposite each other. The positions were marked off using the red laser pointer from the instrument. The position of the required grade pegs were observed and recorded as a Horizontal Observation (H_o°). The advantage of this method is that the position of the grade peg can be moved should the condition of the sidewall or obstructions make the required position unviable.
   
3. 
   The absolute value of the deflection angle x° between the direction of the Major dip and the grade pegs is calculated to the planned position of the new grade pegs.
   

x° = │α° - H_o°│ ( )

Table . Calculated horizontal? angles of intersection .

4. 
   A vertical line was drawn on the sidewall at each point where a grade peg is to be installed. Although it is considered good practice to keep gradepegs as close to perpendicular to the direction line as possible, it is not a requirement for the calculation or installation process when this method is used.
   
5. 
   The horizontal deflection angle x° (the angle between major and minor dip) is recorded and the minor dip d° is calculated at each point.
   

Tan d° = Cos x° * Tan D° ( )

Table . Calculated minor dip to each gradepeg.

A check calculation is performed using:

( )

where: x°+ H°=90° ( )

6. 
   Using the calculated minor dip d°, the vertical angle (VA°) to be turned off is calculated.
   

VA° = 90°- d° ( )

Table The calculation of vertical angles.

Figure The installed gradepeg
Using the calculated vertical angles VA°, the grade position is marked-off on the vertical line and the gradepeg installed.

The position of the gradepeg is surveyed in using a reflectorless measurement and stored. The co-ordinates and elevation of the point is required for the setting out of the direction line using the lines of intersection method. Measurements must include the offset of the gradepeg from the sidewall.

7. 
   The correct grade elevation is calculated from the height of the instrument above or below the required grade elevation.
   

Instrument Elevation (at peg) = Peg Elevation + Instrument Height ( )

and

Grade Elevation (at peg) GE_instr = Old Grade Elevation + (Tan D° * HD)

( )

When the sidewall station method is used the elevation of the instrument is calculated as part of the freestation method, therefore no instrument height is required to be measured.

8. 
   The difference between the Grade Elevation and Instrument Elevation
   

ΔE_GP = GE_instr - Instrument Elevation ( )

is measured up or down from the indicated collimation mark. This position is the final grade elevation point which is then installed.

9. 
   The calculation may be checked using:
   

E_GP = GE_instr + ( SD * Cos VA° * Cos x° * Tan D° ) ( )


Figure The aligned gradient and direction lines
The installation is checked by visual inspection. Both gradestrings must fall in the same visual plane and should appear as one line when viewed in the plane of the Major dip. In the figure below, three red dots (circled) indicates where the laser beam set to the major dip of the excavation intersects the gradestrings.

10. 
    The co-ordinates of the gradepegs are used to calculate the intersection points with the required direction line and tie distances are calculated:
    

Table Calculated tie distances for setting-out.

11. 
    The method is repeated for the second set of gradepegs to calculate an additional oblique line joining the two grade lines.
    

12. 
    The intersection points are marked on the gradestring and permanently indicated by a metal ferrule crimped into position. The figure indicates the initial mark made by cable tie.
    

Figure Direction line mark on the gradestring

13. 
    A polar (join) calculation is performed to check the alignment of the intersection points. The bearing and gradient must correspond with the required direction.
    

Table Check calculation on bearing and gradient.

14. 
    The indicated marks are used to align the direction line of the end for marking-up of the excavation for blasting, using a laser device.
    

15. 
    The gradestrings are permanently fixed on one side when not used. To re-install the gradestring is tensioned by a cable tie.
    

16. 
    A survey instruction is issued with tie distances calculated from each of the gradepegs to re-align the marks on the gradestrings. Standard offsets from the direction line will control the direction of development.
    

Table List of tie distances and stake out points.

3.