The Archimedes Screw, auger, and screw conveyors are not really the same thing, even if they look like it. The device found in modern EPB TBMs has been called all three, despite being in one of the most advanced forms, in particular that it performs more than one function.
Origins
Traditionally the Archimedes Screw, auger or screw conveyor have been used to move material from one place to another, including, sometimes, the extraction of material from a mass, such as with wood or soil augers and some piling rigs. The term ‘Archimedes’ or ‘Archmedian Screw’ is perhaps best restricted to devices performing the original function of lifting water for irrigation or drainage. A close fit between the auger flight and the carrying tube facilitates the transmission of water against gravity. In the transport of more solid masses this fit is not so crucial.
Auger screw conveyors are used widely in industries requiring bulk materials handling, using both tube or trough casing, for horizontal or inclined transfer, and even in rotary screw compressors. They are best applied to semi-solid materials or to fine particle solids that act in a similar way.
Augers have been used in underground construction for many years, including for, in addition to piling, site investigation and boring under surface obstructions – auger boring. With the addition of a steering device, augers with screw conveyors form the basis of one type of microtunnelling machine. The operation of such auger machines, up to around 1800mm diameter, is basically dry with limited groundwater capabilities, but developments in technology, particular of cutterheads, has increased their hard ground capabilities.
However, what we are concerned with now is their role in the so-called EPB TBM. This type of mechanised, full-face shield machine was developed after the slurry-shield machine and has the advantages that removal of excavated spoil is simpler, and that control of the face pressure is more immediate in that there is not the comparatively long time-lag associated with surface-sited slurry pumps adjusting to changes in face pressure, chiefly due to changes in groundwater pressures. The Mixshield or Benton’Air versions of slurry-shield design have minimised this problem however.
The EPB TBM needs the correct ground conditions to work properly, although the characteristics of the excavated material can be changed by the use of small quantities of bentonite slurry or other additives; principally polymers. In this way EPB TBMs can be applied to a wide range of ground from the ideal soft material to, with right equipment specifications, hard inclusions.
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By GlobalDataDespite the frequent needs to ‘improve’ spoil by introducing additives, it is still a simpler process than slurry processing, at least in terms of the equipment required. Using an EPB rather than slurry TBM eliminates all the slurry processing and separation plant required on the surface, and cuts out the major pumping lines for handling slurry to and from the TBM. However, such high-pressure pumping arrangements are better for handling more fluid ground and for high groundwater pressures. Standard EPB TBMs are limited to ground pressures of around 3-5 bar, but recent developments on sealing have lifted this to around 10 bar as in Robbins EPBMs being used in the Emisor Oriente drainage project in Mexico City.
Construction
As with other TBMs, EPB machines advance by cutting the ground at the face by rotary action of the chosen cutting elements under the pressure generated by the forward thrust of the TBM rams. The rotary action also mixes the excavated solids with groundwater and any ‘improver’ introduced to the pressure chamber.
As well as removing this excavated soil from behind the cutterhead, the auger screw conveyor control must also cater for the need to maintain the required pressure at the face to balance the ground pressure. The speed and discharge rate – through a guillotine gate – of the auger screw is controlled to maintain correct pressure at the face, and match the spoil discharge to the advance rate of the shield. This balance, hence the term ‘earth pressure balance’, produced by conveyor control prevents over-excavation, and hence excessive loss of ground and consequent settlement. Since EPB TBMs were first developed, the screw conveyor has lengthened to facilitate balancing ground pressure with a more gradual reduction of the spoil pressure to discharge.
As indicated, the auger screw conveyor may be fitted with a guillotine gate or similar device at its discharge to help control the rate at which spoil is allowed to exit the system. Without the auger screw the operation could be likened to that of a mastic dispenser or vertical toothpaste tube in which the discharge rate is entirely dependent on the pressure applied on the material in front of the pressure plate. In the case of the TBM this would be likely to cause stability problems, particularly in mixed ground where the rate of advance might not be able to match the spoil discharge rate.
If large hard inclusions are anticipated, this will also require special attention to the design of the auger screw conveyor as well as the cutting tool and wear capabilities of the TBM cutterhead. If the conveyor tube is large enough, cobbles could be handled in the normal course of operation. Depending on power transmission requirements it may be possible to use an open-type ‘ribbon screw’ type of element without a central shaft, but the construction of the flights will need to be upgraded to handle the power transmission function without the central shaft.
For boulders and other larger hard inclusions, a stone or boulder trap will be installed near the start of the conveyor so that the boulder can drop out of the conveyor itself for removal.
In some cases the auger screw conveyor is split into two lengths as in the Robbins Mexico City machines already mentioned, better facilitating the transition from the TBM pressure chamber to discharge at atmospheric pressure. In this case the first section is inclined up to the guillotine gate and stone trap. This is of ribbon screw design at 900mm diameter and is capable of carrying boulders up to 600mm in diameter. The second section of a central shaft construction and runs across the top of the TBM back-up gantry. This length forms the transition between the high pressure of the face and the atmosphere. In this case discharge is to a series of belt conveyors.
Spoil handling
The ideal natural ground material for excavation by EPB TBM is a mixture of clay, silt and water, but this is found infrequently, requiring spoil improvement to achieve optimum performance. Obviously soil improvement is unlikely to work on hard rock inclusions as may be found in made ground, glacial deposits and strata disturbed by ground movements. In such cases there must be facility for removing hard inclusions from the spoil for disposal or crushing.
In it’s simplest form, the TBM auger, or screw conveyor, performs the initial mucking out function by extraction the spoil, usually as a paste, from the face pressure chamber of the TBM, and carrying it to its discharge end over a rail car or belt conveyor. In order to perform the function of counterbalancing ground pressure at the face, this action must be coupled to maintain this pressure but gradually reduce it until the discharge at atmospheric pressure is reached. This can be accomplished by reducing the screw drive speed to less that the optimum for spoil discharge. Similarly a ‘plug’ of moving spoil can be created within the screw conveyor to eliminate voids and form a seal against the higher pressure at one end of the conveyor. If two screw conveyors are used in tandem, this plug can be formed most easily between the two conveyors by running at different transmission rates.
Improving spoil
The handling of some difficult ground by EPBMs can be made possible or improved by the introduction of various materials to improve its characteristics. Whether foams, polymers or bentonite, these are injected in the TBM’s mixing/pressure chamber between the cutterhead and bulkhead. It must be ensured that the chemical components are acceptable to the regulatory authorities and the controller of the spoil disposal site. In some jurisdictions even bentonite is considered a contaminant. The mechanisms of the exact effects of additives are too complex to detail here. Another subject for ‘How it works’, perhaps?
Rotating double screws at different rates creates a sand plug, which aids pressure holding capacity Cutaway of an EPBM with major components. The amount of material removed is controlled by the speed of the auger conveyor (5) and the section of the opening by the drive unit
