Tunnel breakthrough survey work is about the moment two headings meet, but the decisions that matter are made long before it. On underground mining surveys, a breakthrough error found after holing can only be described, not corrected.
Quick Answer
Where two headings are driven towards each other, each one is steered by its own control from its own connection. Verify both independently, from both ends, before the headings are close enough that a missed error would need significant rework. Agree in advance who verifies each heading, from which connection, and when the check is repeated. An independent check means a different team and a fresh connection to control.
How control is carried forward through the workings is covered in our article on underground survey control. This article starts where two lines of that control are about to meet.
Why a tunnel breakthrough survey is different
Most underground survey checks can be repeated later, but a breakthrough has a deadline set by the advancing faces, after which a check stops being useful.
Each heading is steered from its own end, using control carried from its own shaft, portal or connection. Each line of control inherits whatever error was made in its connection and in every traverse leg since. If either line has drifted, the two headings will not meet where the design says. While the faces are far apart, a correction is a small change in direction. Once they are close, the same correction means re-mining. After holing, it cannot be made at all.
One end is not enough
A check from one end tests one line of control. It cannot see an error in the other. That is why a tunnel breakthrough survey is verified from both ends, each against its own connection to control.
Key point
A breakthrough check is only useful before the headings are close. After they meet, an error can be measured but not fixed.
Two headings approaching each other?
Send us the layout and the connections. We will plan the checks from both ends.
Ask us to plan the checksWhat to agree before the breakthrough
A short written agreement, made when the headings are still far apart, removes most of the ways a breakthrough goes wrong.
| Agree before the work | What happens if it is left unagreed |
|---|---|
| Who verifies each heading's control | Each side assumes the other has checked |
| Which connection each check starts from | A check reuses the line it is meant to test |
| At what stage the check is repeated | The last check is made too late to act on |
| Who decides if the checks disagree | Mining continues while the question is open |
| How results are recorded and shared | Each team works from a different version |
Make the check independent
An independent check should be carried out by a different surveyor or team using a fresh connection to control, not by repeating the same station occupations and observations. Re-reading the original results back is a review of the arithmetic. It cannot find an error in the original observations, because it uses them again.
Check direction separately
On long headings, a small error in direction becomes a large error in position at the face. An independent orientation, such as a gyroscopic observation, can test whether a line of control has drifted in bearing. Our article on what a gyro survey measures explains what that check settles and what it does not.
Repeat the check as the faces close
One check early in the drive is not enough. Development continues after it, and every new traverse leg adds to the line of control. Agree the stages at which the check is repeated, tied to the development plan rather than to the calendar, so that the last check falls while there is still room to steer. The right stages depend on the length of the drive, the layout and the ground, and the mine's survey department should set them with the contractor's surveyors.
Record every check in one place
Each check should record the stations used, the connection it started from, the team that made it, the date and the result. Keep those records in one shared register that both teams can read. When the faces are close and a decision is needed quickly, a tunnel breakthrough survey register that both sides trust is worth more than two sets of notes that have to be reconciled first. The register also shows, after holing, which checks were made and when, which helps the next breakthrough on the same mine.
Key point
Agree who checks, from where and when, before the headings are close. Write it down and give every survey team the same copy.
Planning a decline or tunnel with two approaches?
Tell us the development plan. We will price the control and the breakthrough checks.
Get an underground survey quoteWhere survey cannot help you
A well-planned tunnel breakthrough survey protects the holing, but some problems sit outside what survey control can solve on its own.
Survey cannot correct development already driven off line. It can only show how far it has departed, so that the remaining work is steered on known positions. It cannot confirm a connection that was never checked, which is why each shaft or portal connection needs its own verification, as on shaft sinking and equipping surveys. And it cannot make the decision to stop or continue mining when checks disagree. That decision belongs to the mine, and the agreement above should name who makes it. Mine survey practice in the region has its own professional body, the Institute of Mine Surveyors of Southern Africa.
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A separate team with a fresh connection can test both lines of control.
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Breakthrough checks agreed in advance and verified from both ends, while there is still time to steer.
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