TECHNICAL | SLURRY TREATMENT
and rejuvenate the slurry before it is pumped back to
the tunnel face of the TBM. It is essential that the slurry remains in adequate quality ranges to maintain efficient excavation. Slurry treatment has direct influence on the reliability
and performance of a slurry shield tunnelling project. Inadequately dimensioned and designed STPs create bottlenecks for operation and so jeopardise time schedules and performance. The STP, in this context, stands for the complete
set-up and scope of the installation. The equipment assembly groups include the separation plant, centrifuge, filter press, mixing plant, storage tanks, conveyor belts, slurry management systems etc. The separation process itself could be divided in
two main sections separating the slurry suspension continuously from the loaded solid particles in the slurry circle. The active circuit (or primary separation) consists of the separation plants’ coarse-, desanding- and desilting cut stages, which are in continuous connection and communication through the Slurry Transport System (STS) with the TBM. The passive circuit (or secondary separation) operates in a by-pass to the active circuit and consists of ultra-fine particle separation equipment, like centrifuge or filter press.
SAFETY, PERFORMANCE AND COST ASPECTS OF STPS Talking about STPs in relation to safety factors in tunnelling operations is all about slurry quality aspects, and to monitor and maintain the rheological conditions in the given target range. A properly maintained slurry quality/drilling fluid has
a direct influence on achieving balanced supporting pressure against water and soil pressure, and so ensure a stable working face condition in the working chamber of the TBM. The suspension used in slurry operated TBMs is in most cases a bentonite suspension generated, at the jobsite surface, by mixing bentonite powder with water. Bentonite is a natural clay mineral out of the smectite
clay group and shows a so-called thixotropic structure, which achieves a unique swelling behaviour that is used during tunnelling to build up a filter cake and stabilise the working face. Besides the stabilisation effect, the bentonite slurry is also used as a lubricant and transport medium for the solid load suspension, via the STS, out of the tunnel to the STP where it is recycled and pumped back to the TBM. What starts with the detailed selection of TBM
Top: Modular design of a screen machine container to simplify re-use during jobsite handling
Centre: Screen machine, used condition during disassembly Above: Cyclones, used condition during disassembly
32 | November 2023
type should also be transferred to the STS and STP. The equipment should be considered as one system with clear interface and communication definitions. Therefore, it is recommended on each project to tender specifically the STP and its components in all details to match the TBM, and so enable maximum performance in tunnelling operations while satisfying the most demanding geological parameters. If not, a lack of performance in operation is predictable.
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