CONTRACTOR: BALFOUR BEATTY TUNNELLING
SUB-CONTRACTOR: MURPHY AND SONS TUNNELLING
Client Issue
Prolonged downpours have overwhelmed Manchester’s sewer network, resulting in water containing diluted sewage entering local brooks and streams. The foul water could eventually flow into the Manchester Ship Canal. Over the past 5 years, United Utilities have invested £150m into sewage improvement works across Greater Manchester, with the ManTrunk tunnel being part of this scheme. Normet UK provided its first soil conditioning solution to this project, as well as supplying the backfill grout admixtures for an effective grouting solution behind the segments. The project parameters are as follows: TBM Type: Refurbished Lovat EPB Cut Diameter: 4.25 m Tunnel Length: 625 m Ground Type: Stiff impermeable clay, with a high clogging potential Shafts: 1 Launch, 1 Intermediate, 1 Reception Normet TBM Construction Chemicals used: • TamSoil 280AC (anti-clay foam) • TamCem 8BFG (retarder/plasticiser) • TamShot 10SS (liquid accelerator)
The Job / The Challenge
The soil conditioning system and the given geology were not compatible with one another, creating a very steep learning curve for the Murphy and Normet site team. EPB mode was required for the entire length of the tunnel to ensure the protection of critical surface structures. The TBM was originally designed to mine through running sands, which generally requires no separate water injection to the face
Initial advancement rates ranged from approximately 7 - 15 mm/min, with both machine torque and muck temperature being extremely high. The required earth pressure balance was also difficult to achieve and maintain and several screw blow outs had also occurred. After investigation from Normet, it was found that the soil conditioning unit was unable to provide a sufficient amount of liquid to the face to fulfill the recommended soil conditioning parameters for the given geology, based on preliminary laboratory trials.
Soil conditioning is an integral part of the EPB process, with advancement rates being heavily dependent on the adequacy of the soil conditioning system. Such a major obstacle required a large amount of site support from Normet in order to increase ring builds per shift, from 2 - 4 to the required 8 or more.
The Solution
Normet UK’s primary aim was to improve and maintain consistent advancement rates, which is one of the main drivers of overall TBM productivity. Following this was the process of optimising the consumption for performance and economy.
The solution was not only the suitable use of Normet products, but also in the service and expertise supplied by the Normet TBM team. Together with Murphy’s the team improved advancement rates from 7 - 15 mm/min to 30 - 45 mm/min. The required earth pressure balance of up to 2 bar was also achieved and maintained. From desk study and site observations, the soil conditioning system was manipulated to inject the maximum amount of additional water to the face. Further amendments to the foam parameters were also made to increase the amount of foam solution to the face, as well as reducing the volume of free compressed air within the working chamber. Finally, slow gradual reductions to the cutterhead RPM reduced the muck temperature, significantly improving soil conditioning effectiveness. The result was a plastic muck which dramatically improved TBM performance.
Murphy was extremely happy with the backfill grout design; Normet project engineering provided the backfill grout specification based on the project parameters and personal experience. Furthermore, Normet analysed the grout on site and offered regular audits to ensure the agreed specification was fulfilled.
Together with the support of the Normet TBM team, Murphy reached the target of 8 ring builds per day, and operated safely and efficiently till TBM breakthrough.