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04.11.2022, 12:40
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KB Surveys Ltd was recently requested to attempt to locate a secret tunnel believed to run between two buildings in the centre of Liverpool-St. George’s Hall and the County Sessions House.
These two buildings are separated by 100m and a hill gradient.
St. George's Hall building work began in 1841 on the site of the Liverpool Infirmary, next to St. Johns's church (demolished). The County Sessions House was built between 1882-1884, 100m uphill from St. Georges Hall.
It has been believed that there was a secrete tunnel constructed between the two buildings for the transfer of prisoners and those awaiting sentencing, underground and away from the public. One of the main reasons this tunnel is believed to exist is because a large amount of money was spent on bricks which have not been accounted for. It is believed that these bricks were used to build the tunnel and then the tunnel was bricked up and lost.
KB Surveys were asked to perform a preliminary investigation of the area to see if GPR could be useful, as the depths the tunnel could run reach quite deep and below average GPR capability. In practice, it was possible to follow a potentially vaulted void (marked in orange on a photo above) along the pavement for some distance at a depth of approximately 2m, so it was decided a full survey would be commissioned.
Sometimes, the work KB Surveys is asked to do can be based on nothing more than an urban legend and to investigate for the sake of curiosity.
The GPR survey encompassed the road and pavement between the two buildings where appropriate to attempt to discover any connecting tunnels running between the two buildings. The GPR used was an UTSI triple frequency antenna, focussing on 200MHz data for depth penetration in search of the tunnel. The area was gridded orthogonally to create maximum coverage.
The image above shows the results of the GPR survey using the 200MHz as the primary antenna. The deep blue line, running past the fountain, was the original target discovered on a preliminary investigation. This was believed to be the tunnel, however when looking at the data as a whole it would appear that this is a large sewer tunnel. The other colours are shallower services detected.
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Overall, it seems that the GPR data did not reveal a secret
tunnel (at least down to 4m penetration) but did map a sewer system which is
the most likely recipient of the bricks which went missing.
On further evaluation of the research, KB Surveys
concluded there is a high probability that the tunnel does not exist due to the
decades between the construction of the buildings and the fact that St.
George’s Hall was only a temporary court whilst the Sessions House was being
constructed. It seems unlikely that there is a tunnel linking the two buildings
as would need to go up hill for 100m and be constructed 40 years after St.
Georges Hall for only a limited time period.
The recommendation in the report was that any potential
tunnel entrances suspected within the buildings would have a structural GPR
survey performed to detect any voids of a tunnel entrance behind existing
brickwork.
