Raise Boring: Where Survey Verification Fits

Raise boring programmes are usually drawn up around the rig, the ground and the production plan, with survey added wherever it seems to fit. A raise bore alignment survey has fixed places in that sequence, and moving any of them later costs more than it saves.

This article places those three points in the programme and explains what each one decides. It does not cover how a gyroscopic instrument works in general, which is in our article on what a gyro survey measures.

Where survey sits in a raise boring programme

Survey verification sits at three fixed points in a raise boring programme, and each point is tied to a decision that has to be made straight after it.

A point that is skipped does not move to later in the sequence. Its decision is simply made without the information.

Why the two levels are checked against each other first

The measurement that matters most before setup is not the collar position on its own, but the relationship between the two levels the hole will connect.

Each level can be correct within its own survey network and still be wrong relative to the other. That is why the check before setup verifies both levels against a common reference, rather than accepting each level's control at face value. At Harmony Doornkop, Survey House carried out check surveys between levels to confirm accurate raise bore holings for ventilation holes. Where the levels are connected through a shaft, the same reference often comes down through shaft survey work or through gyroscopic orientation.

The pilot hole check, step by step

The pilot hole check is the point most often pushed later in the programme, and it is the one with the least room to move.

Measure before the reamer goes on

A pilot hole's path cannot be corrected once it is drilled. It can only be measured. The reaming pass then enlarges the hole along the path the pilot has already set, so reaming will not straighten a deviation. The check therefore belongs after the pilot and before reaming, while the result can still inform a decision.

Choose an instrument that suits the drill string

The pilot is drilled through a steel drill string, which can disturb a magnetic sensor's reading. Downhole deviation surveys are therefore commonly carried out with gyroscopic instruments. Some sites still use magnetic tools where the interference has been assessed as acceptable, and that assessment should be recorded.

Agree what the result will decide

Before the survey, agree who reads the result and what happens if it is not what the design expected. A result with no agreed next step can end up reamed anyway, which turns a checkpoint into a record.

Confirming breakthrough on the target level

The final survey point confirms where the hole actually arrived, measured on the target level against the same reference used before setup.

This is what lets the mine plan the work that connects to the raise with known rather than assumed positions. The breakthrough record should state the reference used, the position found, and how it compares with the design. Underground, it then becomes part of the mine's survey record for that connection.

On a long programme, it also helps to plan survey attendance around the mine's access windows, as covered in our article on underground survey planning.

Where survey cannot help you

Survey fixes the reference and verifies the result, but some outcomes of a raise boring programme sit outside what survey can control.

Survey does not steer the drill. Deviation during drilling is a function of the drilling operation and the ground. Survey cannot correct a pilot hole that has already deviated, and it cannot make a skipped check happen after the event. Background on how a raise boring machine works is widely available, for example in this overview of the raise borer.

What survey can do is give both levels a common reference, measure the pilot while the result still counts, and confirm breakthrough so the consequences are known.