Large diameter raise boring is commonly used for developing cylindrical vertical to inclined shafts in underground mines. The circular cross section is favourable from a geotechnical perspective, and a significant benefit of raise boring is an improvement in safety with the removal of personnel to work directly in the shaft during development. However, unlike conventional shaft sinking, if ground conditions are poor, there is no generally little or no opportunity to install support until the shaft is completed. As such, shaft conditions need to be well understood prior to commencing development to determine whether the shaft can remain stable throughout the back-reaming process, or stable with pre-support. An alternative location needs to be sought if conditions in the selected location are not amenable to raise boring at the desired diameter, otherwise the shaft might collapse, and the expensive gear lost. Unsafe conditions could also arise if conditions are not amenable. The geotechnical investigation requirements will vary depending on the shaft development method and geotechnical conditions at the shaft location. For the highest confidence in results, the investigation drillhole should be located as close as possible to the planned shaft axis (within 10 m) and hole deviations should be understood when assessing the confidence of results along the length of the shaft. However, there are circumstances where 10 m can be too large, such as the presence of sub-vertical adverse geological structures which might be intersected by the raise but not by a drillhole placed along the raise axis. There would be a residual risk that an unknown subvertical structure close to the raise can lead to significant failure or falloff. If there was a history of this at the site, there might be a case to drill inclined holes to confirm that there are no adverse dykes or structures near the proposed raise, at least in the weathered zone. Although the investigation drillhole should be positioned as close as possible to the planned raise, it is important to consider that reaming cannot be conducted through drill rods if they become stuck in the hole. If that is considered to be a risk, the cover hole should be stepped off by about 5 m. Other considerations for data confidence include core loss, whether from navigational drilling (which should be avoided) or from poor ground conditions. HQ triple tube drilling should be considered throughout any weathered or weak rock zones and in all cases, careful core handling and marking of drillers’ breaks should be diligently applied. In some circumstances, the geotechnical investigation drillhole might not be required. For example, at a mine where there is already significant experience in raise drilling and the geological and geotechnical conditions are already well known (and consistent), investigation drillholes are not required for shorter and smaller diameter raises (such as escapeways). However, for any large diameter raises, the risks associated with low data confidence should be well understood if a shaft investigation drillhole is not undertaken. Comprehensive practical guidance on raise bore and shaft geotechnical analysis is presented in Mine Mentor’s module on the topic. To learn more, visit: https://mine-mentor.com/learning/