A bentonite suspension has several jobs in slurry shield tunnelling: it forms part of the face-support system, carries excavated material through the return line and passes through treatment equipment before reuse. Selecting a grade means considering how it performs across these connected tasks.
For buyers, this makes an initial viscosity result only a starting point. A useful evaluation connects the bentonite, mixing water and excavated ground with the machine and separation plant that will handle the slurry.
Before comparing products, identify the tunnelling method and the intended use of the bentonite. In slurry-mode operation, a pressurised suspension can provide the supporting medium at the tunnel face. Earth-pressure-balance operation uses conditioned excavated material as its principal support medium. Machines capable of changing modes require the application to be defined for the proposed operating conditions.
Face-support slurry and lubrication slurry should also be specified separately. Although both may contain bentonite, their duties and evaluation requirements can differ. An inquiry described only as “TBM bentonite” leaves these questions unanswered.
State whether the material will be used in the active excavation circuit, a separate lubrication system or another construction process. This gives the supplier a clearer basis for selecting samples.
The suspension operates within a pressure-controlled excavation system. Its interaction with the ground is part of the support assessment, alongside groundwater conditions and the machine’s operating controls. Bentonite selection should therefore follow the project’s geotechnical requirements.
A laboratory viscosity reading cannot establish face stability by itself. The slurry properties and operating pressure must be assessed together by the project team.
The suspension also carries excavated soil out of the tunnel. Evaluation needs to consider the solids-laden mixture that the pumps and pipelines will actually handle, rather than fresh bentonite slurry alone.
Increasing viscosity is not a complete answer to transport requirements. Compare the fluid’s rheological behaviour with the intended solids loading, pumping arrangement and operating conditions.
At the treatment plant, excavated solids are separated from the circulating suspension. Herrenknecht Separations describes the use of screens and hydrocyclones, with additional fine-treatment equipment where required.
Some fine particles can remain after separation and accumulate during circulation. A recycled slurry may therefore behave differently from the original mixture. Grade evaluation should include the treatment system’s capabilities and the properties required before the slurry returns to service.
Fresh-water testing provides a baseline, but the project may use a different water source and encounter changing ground. Where representative materials are available, extend the trial beyond the freshly prepared suspension.
Keep preparation and measurement procedures consistent between candidate grades. Record any dilution, fresh bentonite or other additions so that their effects can be distinguished from changes caused by the ground.
The related article on water quality and hydration explains why the preparation conditions deserve attention when comparing bentonite samples.
When slurry properties move away from the agreed operating range, compare results from defined locations in the circuit. Fresh suspension, return slurry and treated slurry represent different conditions and should be identified clearly in the records.
Review the water supply, excavated material, recent additions and separation-plant operation before deciding on a treatment. A change observed after recycling needs a different investigation from poor development in the fresh mixing tank.
Formulation adjustments should remain part of the wider operating assessment. They cannot compensate for unsuitable face-pressure control or inadequate separation capacity.
A useful specification connects the supplied powder with slurry prepared under agreed conditions. Include the intended application, grade and batch identification, test methods, preparation procedure and project acceptance requirements.
For sample selection, provide:
Projects in Africa, the Middle East, Russia and South America can face different water, temperature and supply conditions. Record the actual site requirements rather than selecting a grade by regional description alone.
HERMANO offers construction bentonite samples for evaluation. Sharing the machine, ground, water and treatment details helps establish which candidate materials should enter a project-specific trial. These details can be submitted through the contact page.
