Since 2012, Opus has been working with Network Rail on its strategic plan to make the country’s railway fit to keep pace with the unprecedented growth in passenger numbers over the last 10 years. Opus undertook a review of the ‘Hidden Shaft’ information for Network Rail to identify the location of all hidden shafts used in the construction of the Welsh tunnels. 

31 of the 56 tunnels had the potential to contain hidden shafts.

Clues to their location

The starting point to locating hidden shafts is documentary evidence explains Phil Taylor (Opus Team Leader – Geo/Environmental). He says:  “We look at the original tunnel construction drawings, historical plans and even the minutes of meetings held by the individual rail companies discussing the progress of the works through construction."

The research is supplemented with a review of aerial photography as well as site walkovers. This all helps determine the location of any features that could indicate the presence of a shaft, such as depressions and spoil mounds. Essentially, depressions at surface could indicate that a shaft was backfilled and the material has compacted through self-weight over the years.  Equally spoil mounds positioned in close proximity to the tunnel could relate to the deposition of excavated material from the construction of the shaft and tunnel.

“There’s also the potential for shafts to be identified from within the tunnel itself. They can be recognised in several ways including, but not limited to, weakened sections of the tunnel’s roof, changes in brick work and a significant amount of water ingress.”

Intrusive Investigation

Once a possible hidden shaft is located an intrusive investigation is often needed to confirm the shafts and to determine any remediation that may be required. To investigate the hidden shafts from within the tunnel, small diameter coreholes can be drilled vertically within the crown at 1m spacings across the suspected area. If soft ground or void is encountered, the location can be marked to allow further investigation. If voids are located, larger diameter coreholes can be drilled to allow for CCTV examination to confirm the shaft’s condition. If the shaft backfill is such that a CCTV inspection cannot be undertaken or as a result of restrictions within the tunnel, an investigation may be undertaken at surface. Dependent upon access, investigation works, such as trenching or rotary drilling would be predominantly used.


The remediation required depends on the condition and the backfilling of the shaft. 

“A shaft could be capped at ground surface and ‘plugged’ within the tunnel to prevent possible collapse, strengthening of the tunnel could also be undertaken in the area of the shaft. Should the shaft be in good condition it could be left open to allow for future examinations or be backfilled with a light weight material. The ultimate treatment is dependent upon numerous factors and requires detailed design.”