A steep learning curve to match the geology with a right soil condition system
Balfour Beatty Tunnelling
Prolonged downpours have overwhelmed Manchester’s sewer network, resulting in water containing diluted sewage entering local brooks and streams.
The foul water could risk flowing into the Manchester Ship Canal. United Utilities invested £150m into sewage improvement works across Greater Manchester over five years, including the ManTrunk tunnel project.
Normet UK provided its first soil conditioning solution in this project and supplied the backfill grout admixtures for an effective grouting solution behind the segments.
The TBM type in the project is Refurbished Lovat, the EPB Cut Diameter is 4.25 metres, and the tunnel length is 625 metres. The ground type is stiff impermeable clay, with a high clogging potential. There are three shafts: launch, intermediate and reception.
The soil conditioning system and the area’s geology were not compatible with one another, creating a 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, generally not requiring 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 difficult to achieve and maintain, and several screw blow outs had occurred. An investigation by Normet discovered that the soil conditioning unit was unable to provide enough 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 that are heavily dependent on the adequacy of the soil conditioning system. A major obstacle like this required a vast amount of site support from Normet to increase ring builds per shift, from 2-4 to the required 8 or more.
- TBM productivity improved with process and product insight by Normet UK
- Advancement rates up from 7-15 mm/min to 30-45 mm/min
- Ring builds per shift increased from 2-4 to 8 or more
- Site support and extensive trials to tackle sub-contractors’ major setback
Normet UK’s goal was to improve and maintain consistent advancement rates, the main drivers of overall TBM productivity. The process of optimising the consumption for performance and economic factors was also a major focus.
The service and expertise supplied by the Normet TBM team combined with the optimised use of Normet products proved to be effective. In cooperation with the contractor, 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 into the face. Further amendments to the foam parameters were performed to increase the amount of foam solution to the face and reduce the volume of free compressed air within the working chamber.
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.
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.
Murphy was extremely happy with the backfill grout design. Normet’s project engineering provided the backfill grout specification based on the project parameters and insight. In addition, Normet analysed the grout on site and offered regular audits to ensure the agreed specification was fulfilled.