Shaft Survey Control: From Surface to Underground

Shaft survey control is what lets underground workings sit in the same coordinate system as everything on surface. On shaft sinking projects, the connection is made at the shaft station, and it has two parts that are easy to treat as one.

The question often comes up when the shaft reaches its first level and someone asks whether "the connection" has been surveyed. The more useful question is which of the two connections has been surveyed, and how each was checked.

Two transfers make up shaft survey control

Shaft survey control from surface to underground is made of a direction transfer and a separate position and level transfer, each from its own observation.

Direction

Direction must arrive underground independent of the survey lines that will later depend on it. A gyroscopic observation establishes azimuth and is used to orient the survey framework. It does not establish position, which is why our article on what a gyro survey measures separates the two.

Position and level

Position and level are carried down by their own observation. In a shaft under construction, the vertical position of the shaft centre is maintained by plumb wire or by optical or laser plummet. Each of these answers a different question from the direction transfer. Level is carried down in the same way, by its own observation, and recorded against the station it belongs to.

Why one transfer cannot vouch for the other

The two transfers come from different observations, so an error in one leaves no trace in the other.

A station can be in exactly the right place with the network facing slightly the wrong way. It can also face the right way from the wrong place. Neither condition shows up in the transfer that is correct. Every transfer should therefore be checked independently rather than accepted on a single observation. In a shaft, the cost of an unchecked error grows with depth, which is why the checking matters more here than in most survey work.

In practice, the two transfers are often observed on different days, by different methods, and sometimes by different teams. Each should have its own record: what was observed, from which stations, when, and how it was checked. A single entry that says the connection is complete hides which half has actually been tested. Good shaft survey control keeps those two records separate.

When the surface control is gone

Shaft survey control can only be carried down from control that exists, so a project that has lost its surface network must rebuild it first.

The Mindola shaft project for Mopani Copper Mines in Zambia is a recent case. Work had stopped when Covid struck in 2020, and by the time Cementation Zambia was awarded the restart, every piece of historic surface survey infrastructure had been lost to general deterioration and ground anomalies.

Start from what survives

Survey House worked from an old shaft base, the reference that remained on the ground, and built the new network out from it.

Re-establish direction independently

Multiple gyro surveys were carried out to make sure the newly built survey beacons were accurate enough to serve the project to completion.

Build permanent beacons before the connection

New permanent survey beacons were built and tied in, so the network could serve the project through to completion. The existing shaft wires were then re-surveyed, and the upper shaft was scanned, so that the new headframe would tie up with the steel already installed. That second step is covered in our article on the shaft equipping survey.

Where survey cannot help you

Careful shaft survey control fixes the reference underground, but some questions sit outside what the shaft connection can answer.

The connection does not extend itself through the workings. Once past the shaft station, control is carried further through the underground workings by its own network. A good connection also cannot correct development already driven from a poor one, and it cannot confirm a station that has been disturbed since it was observed. Mine survey practice in the region has its own professional body, the Institute of Mine Surveyors of Southern Africa, and where an existing connection is in doubt, an independent check can test each transfer separately.