Innovation In Structural Investigation


GBG has been commissioned by Greenwich Council to survey part of the one hundred year old Woolwich Foot Tunnel in south London in order to identify the cause and location of moisture ingress points in the spiral stairwell of the North Shaft.

Built in 1908, the foot tunnel was used as a means of crossing the Thames by dock workers, as well as providing a routing for services, including a 20” water main with risers coming up through a number of shafts. The entire tunnel is faced in white glazed tiles. Over the course of the last few years, there has been considerable rust staining running down the walls of the North Shaft. In 2001, dye testing was used, which resulted in replacement of the external drainage together with sealing of a Thames Water manifold and four vertical ducts. However, the rust staining and elevated moisture levels have persisted.

Jeff Horsman from Greenwich Council’s Strategic Planning Department said: “I was keen to employ an investigative approach which would not require disturbing the structure and potentially exacerbating the water problems.”

Engineers from GBG used non-destructive testing techniques (ndt) supplemented by targeted coring for the survey. The ndt survey indicated that the high moisture areas within the brickwork were associated with conduit and service positions. This raised concern about the condition of the brickwork generally and the possible risk of graphitisation of the caisson. To investigate this possibility further, GBG carried out a materials investigation using limited coring to further examine the caisson. However, examination of the cores showed no evidence of graphitisation or corrosion of the cast iron.

Following further investigations, GBG concluded that the high level of moisture could be attributed to the poor quality of the original brickwork of the structure which had many large voids allowing the free passage of water. They also concluded that the moisture was picking up rust from the exterior of the pipes or the various steel fixings embedded in the wall, which was then being expressed to the surface.

Jeff Horsman continued: “We have been experiencing problems with this part of the Woolwich Foot Tunnel for a number of years. Despite investigations in the past, we had been unable to locate the reason for the rust staining and, perhaps more importantly, whether this was indicative of any serious problems with the overall condition of the brickwork.

“The results from the work undertake by GBG have given us a better understanding of those specific locations where we have problems of water ingress and how it has accumulated. It has also confirmed that the construction of the tunnel is not what we had believed it to be from the as-built drawings. Overall, their results have provided reassurance that despite the surface staining, the structural integrity of the brickwork and the caisson has not been compromised.”

Greenwich Council is now continuing with its programme of remedial works on the tunnel, drawing upon the results provided in GBG’s report.

For further information, please contact:
George Ballard () or David Wilson ()

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