Slurry TBMs have traditionally been used in granular soil, heterogeneous permeable ground, and where highwater pressures are expected. These machines are most suited for tunnelling through unstable material or saturated ground where water inflow must be stopped by using boring fluid to support the tunnel face.
Slurry machines have a bulkhead (closed face) to maintain the pressure on the face and deal with significant amount of groundwater if needed. The technology involves a complicated circulation system that moves a semi-liquid mixture (slurry) to transport excavated material.
The cutterhead acts as the means of excavation, whereas face support is provided by slurry counterpressure, namely a suspension of bentonite or a clay and water mix (slurry).
Bentonite is a highly expansive and naturally occurring clay material used during the tunnelling both as transport medium for excavated material in the slurry circuit and as a support medium for the tunnel face.
The suspension is pumped into the excavation chamber where it reaches the face and penetrates into the ground forming the filter cake. This process results in the transfer of counterpressure to the excavation face.
The charge and discharge rate of the slurry material is the main way to regulate the pressure at the working face of traditional slurry TBMs.
The main components of the slurry systems are: 1) Cutterhead, fitted out with discs, blades or teeth 2) Protective shield, containing all the main components of the machine 3) Longitudinal thrust jacks and 4) Slurry treatment plant (normally located on the surface).
Slurry treatment can be considered as a series of processes that take place to provide continuous fresh slurry for tunnelling operation. These typically include: 1) separation of tunnel spoil from the slurry 2) dewatering 3) preparation of fresh slurry 4) storage of cleaned slurry 5) slurry conditioning and 6) water management.
Slurry machines may also be equipped with a stone crusher for occasional cobbles and small boulders. Larger stones and boulders are crushed by a stone crusher before the suspended muck is sucked out at the invert and transported back to the surface plant for separation. The installation of a crusher in the excavation chamber also allows any lumps, which would not pass through the hydraulic mucking system to be broken into smaller fragments.
Some slurry machines have working chambers that are divided into two parts. These two sections are connected through a submerged wall. In this system a “bubble” of pressurised air is present in the top of the second working chamber. This arrangement allows adjustment of the counterpressure at the face independent of the hydraulic circuit, a supply of bentonite slurry and excavated ground.
When planning tunnelling projects using slurry TBMs, the optimisation of penetration rates and cutting wheel rotation speed is necessary for smooth excavation and the reduction of wear on the cutting tools.
Tool replacement is not possible at every location on the excavation path since it requires the partial or even entire lowering of the slurry level in the excavation chamber. When the slurry level is lowered, the excavation chamber is typically filled with compressed air, which enables access to the tools.
Slurry machines are particularly suited for working in the presence of pressurised groundwater and soils of varying hardness. The use of disc cutters enables the machine to excavate in rock and deals with boulders. Specialised material like polymers can be used to excavate the ground containing silt and clay.
Slurry shield TBMs are generally less suitable when operating in clay because many slurry separation plants cannot easily separate clay from slurry and therefore, the cost of frequent replacement of bentonite slurry comes substantial.
One of the biggest slurry TBMs ever built to date was used for excavating a tunnel in the southern city of Shenzhen. The machine, named Chunfeng, had a diameter of 15.8m. It weighed around 4,800 tonnes and was 135m long. Chinese company CREG was the manufacturer.
