Yorkshire Water called upon DrainsAid to restore two deteriorating manholes next to Brotherton & Byram Primary School and Old North bypass in North Yorkshire. The repair was required to prevent pollution incidents effecting a nearby woodland. The two manholes each had a rising main system entering from separate pumping stations.
Early tests measured high levels of biogenic sulphuric acid (hydrogen sulphide – H2S) within the manholes. The H2S had corroded through the concrete rings and reinforcement, to expose the earth in the downstream manhole. The presence of dangerous gas and lack of structural integrity, meant it was unsafe to carry out a man entry for repair works in the downstream manhole.
Although access to the manholes was limited, excavating and creating a new manhole would have required trees being felled. This would have drawn out the process, and caused disrupted in the area for at least two weeks.
DrainsAid set out to restore the manholes in one week, maintaining a high level of health and safety, and causing minimal impact to surrounding woodland.
Planning & Preparation
DrainsAid liaised with the Parish Council, who owned the woodland and with the neighbouring primary school, which used the footpaths adjacent to the manholes. DrainsAid committed to preserving the natural environment and make ensure that safe alternative routes for pedestrians were established. It was agreed with the school that works would be completed outside of term time. Furthermore, plans were arranged with the Highways Authority to set up traffic management and excavate next to the bypass to access the sewer.
We monitored the H2S levels for a week prior the works to determine the atmospheric acidity within the manhole. The average reading measured 175 ppm (parts per million) and the maximum fell above the fatal exposure level.
DrainAid decided to employ the innovative Hermes M-Coating technique. This rehabilitation procedure is ideal for round and rectangular manholes with widths between 500 mm – 3,000 mm and depths up to 25 m. The technology uses a centrifugal spray and winch to coat the walls of the manhole with 100 mm of ERGELIT mortar. The centrifugal spray runs at 5000 rotations per minute and operates until the predetermined coating thickness is reached. The Ergelit mortar is specifically designed to resist H2S corrosion; laboratory trials demonstrated an increase in strength of to 50%.
This process was the most efficient and cost effective solution. It would also have less impact on the environment and did not require a manhole entry.
In preparation, two excavations were completed to open access points into the two rising mains, in order to establish overpumping and divert the waste water around the manholes. Firstly, the manholes were cleaned using dual rotating jets on an automated winch, which left a firm surface for the M-Coating ERGELIT mortar to bind to.
Method & Results
The M-Coating technique operates under a computerised control system, calculated by the total surface area, depth of coat required and depth of the manhole. The built in mixer produces the necessary mortar consistency and pumps this to the centrifugal spray operating on the automatic winch. Throughout the operation, the control system produces continuous readings of the remaining time and mortar required.
All machinery used in the cleaning and M-Coating process is contained within one vehicle. The additional supply of water, came in the form of a tanker and the bags of mortar were delivered to site.
In Church Wood the operation took a four man team, one and a half days to complete the downstream (5 m depth) manhole and half a day to complete the upstream (1.5 m depth) manhole.
The finish on the manholes was not smooth because the application technique binds the cement to the solid surface beneath and manhole entry was not possible to improve the surface.It is however, durable and resistant to future H2S corrosion.
The first layer of 90 mm of cement provides the structural strength to ensure durability. The final 10 mm layer was a H2S resistant cement compound, as a preventative measure against future H2S attack.
Downstream of the rehabilitated manholes, DrainsAid identified two manholes which have also suffered from severe corrosion from H2S and were in need of rehabilitation. A few months later, when undertaking the restoration of these manholes, costs were greatly reduced because the structures of these first two manholes was still intact.