In the relatively new technology of full-face microtunnelling, two distinct operating modes exist which are generally referred to as slurry and earth pressure balance (EPB) systems. At present in Europe and America the slurry system predominates, having a market share in excess of 90%. In contrast, in Asia, the EPB system takes approximately 30-40% of the market for microtunnelling systems. This greater use of EPB systems in Asia is a result of the prevailing ground conditions of homogeneous, soft silts and clays.

The main differences between the two systems lie in the method employed to support the tunnel face and to remove the excavated material from the chamber.

In the case of EPB shields, the primary face support is provided by the excavated material, which completely fills the excavation chamber. The spoil is removed by means of a screw conveyor. The face supporting pressure is controlled by regulation of both the TBM advance rate and the screw extraction rate. Depending upon the properties of the spoil, it may be necessary to condition the excavated material either to reduce its internal friction or increase its permeability.

In the case of slurry shields, the pressure of the fluid filling the excavation chamber provides the tunnel face support. It is the circulation of this fluid through the excavation chamber that removes the spoil from the chamber. Slurry shields are subdivided into two categories depending upon whether or not an air cushion is used for fine regulation of pressure. Where shield diameters are less than 1500mm (AVN 1500), there is insufficient room for the shield to be equipped with an effective air cushion to give fine regulation of the face supporting pressure. The regulation of the face supporting pressure is controlled solely by the slurry feed and discharge flow rates via the respective pumps.

Herrenknecht has developed a system which incorporates a compressed air buffer, providing a fine regulation over and above the pump speed control, for shields of diameters 1600mm or more. This technique (Mixshield) has been used successfully on large diameter TBMs for approximately 20 years.

The distinguishing feature of these Mixshields is a partial bulkhead separating the fluid filled excavation chamber from a pressure chamber containing an air cushion above the slurry surface. The pressure in the air cushion is regulated to within fine limits by an automatic control system. This system effectively eliminates pressure fluctuations that can occur in the standard slurry systems. In cohesive soils or rocky conditions which do not require such a sensitive support mechanism, Mixshields can be used as simple slurry machines.

Geological suitability of the systems

Slurry shields of all types can be used in highly permeable ground with high hydrostatic pressures without restriction. In coarse-grained loose gravel it will be necessary to add bentonite, or appropriate polymers, to the slurry to guarantee the formation of a filter or membrane cake to transfer the support pressure to the tunnel face effectively.

The use of slurry shields in cohesive ground conditions has often proved problematic as a result of the build up of sticky material in the excavation chamber, leading to reduced and unsatisfactory machine performance. Herrenknecht has developed a system of high pressure water injection nozzles in the crusher cone area operating at 300 bar. These high pressure water jets serve to break up the build up of sticky material and so enhance the overall production rate.

In contrast, the use of EPB machines in highly permeable ground conditions is more problematic. The ground will need to be conditioned to prevent groundwater washing through the permeable material and out of the screw, taking fine material with it which would result in surface ground settlement, and to prevent difficulties in spoil handling underground. The application of spoil conditioning additives has itself a limit where the ground becomes so permeable that conditioning is no longer effective.

In mixed face conditions and for long drive lengths, the correct conditioning of the spoil, together with the regulation of the advance speed to achieve the required face support, can prove problematic.

Suitability of the systems to spoil grain size

In the case of all tunnelling machines, boulders have to be reduced in size by the cutting wheel to allow them to pass through the wheel into the cutter wheel chamber.

In slurry machines a cone crusher is incorporated to the rear of the head, which effectively reduces the size of the spoil so that it is able to pass through the pumps and pipework to the separation plant. The cone crusher works in parallel with the excavation process and so does not affect the advance speed of the machine.

With EPB machines the size of spoil that can effectively be handled through the machine is governed by the size of the screw conveyor. The removal of the spoil from the gate exit through the tunnel will laregly depend on its consistency and may be by muck cars (rail-trucks or wheeled skips for example) or by muck pumps. The limitation of muck pumps is that they are only for soft homogeneous ground without hard particles or grains The big advantage of such pumps is the continuous production possible.

Changing track sets where rail transport is used introduces additional downtime.

Separation and disposal of excavated material

Slurry type tunnelling machines can be used in a wide variety of ground conditions, but the system requires the use of a surface separation plant which adds to the overall cost. The finer the soil grading, the more complicated and expensive the separation plant becomes. It is important, therefore, to have precise knowledge of the ground conditions, not only for optimising the tunnelling machine selection, but also for the correct specification of the separation plant and for planning muck disposal procedures.

In contrast to the slurry system, the EPB system requires no additional separation plant and excavated material can be disposed of directly. But additives that may have been necessary to condition the material can affect disposal costs significantly.

In order to select the optimum tunnelling machine for any specific project, extensive co-operation and exchange of information is required between client, contractors and the machine manufacturer, particularly in respect of the existing geological conditions. The provision of a detailed geological report is absolutely essential to the machine selection and hence the optimisation of tunnelling performance.

Related Files
The limits of EPB operation in respect of ground conditioning:
Range of earth pressure balance