Variable-Density-TBM

Variable Density TBMs in operation

Conventional tunnel boring machines usually have one specific tunnelling method. In particularly variable geology along the tunnel alignment, these tunnelling shields can reach their technical and economic limits. Within a tunnel alignment to be excavated, for example, the ground can change from soft, water-bearing unconsolidated rock to stable ground and vice versa. Such alignments are among the most complex challenges in tunnelling. To ensure maximum safety and flexibility throughout the advance, Herrenknecht AG has developed the Variable Density TBM for such cases. This flexible machine type is designed for changing ground conditions in terms of its support and excavation method and can switch from slurry-supported to EPB mode as required. The specially developed high-density suspension expands the range of applications for slurry support into coarser soil structures.

Crusher atmospherically accessible at all times 

A mixing chamber, the "slurryfier box", with a claw or roll crusher, is installed behind the screw conveyor for slurry conveying. The crusher is atmospherically accessible at all times and compressed air is not required for maintenance.

Flexible muck removal with twin screw conveyor 

In the diameter range from around 6 m, the twin screw concept enables both the integration of a hydraulic slurry circuit in slurry-supported mode and discharge from the screw conveyor onto a belt conveyor in EPB mode. To this end, a discharge gate is integrated at the end of the first screw conveyor, which transfers the muck to a belt conveyor. In slurry-supported mode, the gate is closed and the discharged material enters the closed flushing box at the end of the second screw conveyor. There, the muck is transferred to the next slurry circuit. The slurry and muck are pumped through pipes to a separation plant on the surface.

Compact short screw conveyor for new machine concepts

The Variable Density TBM can also be combined with access for an accessible cutting wheel and a shuttle system. This makes it easier to realize machine concepts for tunnelling under high pressures. For these cases, Herrenknecht AG has developed a special compact short screw unit for the Variable Density TBM. 
Hydraulic muck removal with the two operating modes LDSM and HDSM is available for operating the TBM. The compact screw conveyor unit is integrated into the shield. In this case, use in the classic EPB mode with conditioning is not intended. The compact screw conveyor has shortened helix and screw tube lengths, which is advantageous in abrasive ground conditions. Furthermore, the crusher unit in the shield is directly coupled to the compact conveyor unit. The shortened screw does not require a support in the back-up and therefore does not restrict the erector and ring building area.

High-density suspension for difficult mixed soils 

Operating a Variable Density TBM with a high-density suspension (HD) expands the range of applications for mechanized tunnelling in unconsolidated rock. In the case of coarser soil structures, such as medium to coarser gravel with low fines content and heavily fissured or karstified subsoil, the use of standard suspensions in Mixshield operation carries the risk of uncontrolled suspension losses or bentonite blowouts. To prevent this, the density of the support medium in a Variable Density TBM can be increased by adding additional fillers. The increased density of the HD suspension reduces the vertical capillary rise of the suspension in the ground and also improves the support effect. The choice between the low-viscosity bentonite suspension (Low Density Support Medium) and the denser HD support medium significantly expands the Variable Density TBM's range of applications to include coarser geologies and represents a decisive innovative step in mechanized tunnelling. The HD suspension can be obtained either by adding it from outside or, under certain circumstances, from the ground itself.

Improved muck removal in soils with increased clogging potential 

With the Variable Density TBM, excavated material is conveyed from the invert area of the excavation chamber via a screw conveyor. The soil material is conveyed out of the chamber mechanically and hydraulically. Clogging is therefore very unlikely in this area. Depending on the geology, either a sizer (roll crusher) or a rock crusher is connected at the screw discharge, which crushes the large clods of soil for further hydraulic conveying in the pipeline. The two critical points for clogging in the Mixshield, the areas of the bulkhead opening and the invert or grid, are eliminated in the Variable Density TBM by a different design that does not include these components. This results in significantly improved material flow into and out of the chamber and less solidified clogging behind the cutting wheel. This positive aspect has been demonstrated in various projects in geologies with increased clogging potential due to higher advance rates.