Suitable additives may greatly enhance the performance of the slurry used in slurry-type tunnelling machines for pipe jacking, but use must be properly considered in relation to separating plant, where other additives may be used. Lubrication of pipe jacks has been found to be extremely effective in reducing jacking resistance, but to obtain maximum benefit at minimum cost it is necessary to understand fully how the lubrication works and what is the best one for different ground conditions. The purpose of this article is to increase awareness of the potential for lubrication and soil conditioning in pipe jacking and microtunnelling, and to encourage consideration of methods at the design stage rather than as remedial action.
Lubrication and conditioning may be effected by the addition of suitable agents at points throughout the tunnelling process: at the point of cut; within the cutter head; in the muck removal system; around the outside of the pipes in a pipe line formed by pipe jacking or microtunnelling; in the separation units of a slurry system; or to muck on its way to tip. This
improvement may come about in a number of ways:
– reduced wear of machine cutter head face plate and tools, and all wear parts of the muck removal system
– improved stability of tunnel face, with consequently better control of ground movements
– improved flow of excavated material through the
cutter head
– reduced cutter head power requirement
– excavated material formed into a plastic mass
– support of tunnel bore and reduction of jacking forces in pipe jacking and microtunnelling
– reduction in the friction losses in the pipes, valves and pumps of a slurry machine system
– better separation of spoil from slurry in a slurry machine system
– more acceptable spoil for disposal
The uses of lubrication and soil conditioning vary with the different types of tunnelling technology. They will find greatest application in relatively long drives in difficult ground conditions, where fully mechanised systems are in use. For short drives in firm to stiff clays, where the ground is stable and excavation is by hand tools or hydraulic excavator, soil conditioning will not be needed and the benefits of lubrication will not be justififed by the additional costs involved.
In unstable ground, such as soft clays, cohesionless soils or highly fractured rock, particularly below the water table, the excavated face will have to be supported while excavation proceeds. This can be done by mechanical support from a closed machine head through which excavated material is extracted at a controlled rate as the shield advances; by compressed air pressure; by pressure from a fluid, usually either a bentonite slurry or a slurry formed from water mixed with some of the excavated spoil; or by pressure from excavated material within the working chamber of the shield (earth pressure balance). Most tunnelling machines for pipe jacking are either open-face or slurry-type, although earth pressure balance (EPB) machines have been used, while microtunnelling machines are of either slurry or auger type.
With a slurry system, cobbles and boulders may be broken down to a size suitable for transport through the slurry pipes by a crusher unit in the head. With other types of pipe jacking system, the spoil will normally be removed in muck wagons, or occasionally by conveyor; very soft soil may be pumped, and pneumatic systems have also been developed.
Ground support
In a slurry machine, fluid pressure in the head is used to support the tunnel face. In clay soils the slurry may consist of water with a proportion of suspended clay from the excavated material which is not completely removed at the separation plant. In more permeable soils a filter cake must first be formed in the soil face to prevent the slurry from dispersing into the ground; a bentonite slurry is then normally used. Bentonite on its own may work satisfactorily in silty or sandy soils, but in coarser soils additional materials may be included to increase the slurry density, without increasing its viscosity unduly, promote the formation of a filter cake and reduce slurry loss into the ground.
EPB machines support the face with pressure from the mass of remoulded soil within the machine head chamber and screw conveyor. Beneath the water table, this can only be done if the soil is not too permeable, otherwise the water pressure cannot be resisted. It is generally reckoned that a coefficient of permeability of less than 10-6 to 10-5 m/s is needed for an EPB to function satisfactorily; in more permeable soils, conditioning agents may be used to reduce the permeability to an acceptable level. Conditioning can also produce remoulded soil of a suitably plastic (and preferably slightly compressible) consistency to allow satisfactory pressure balance to be achieved at the face.
Lubrication
Lubrication may be considered under two headings:
– reduction of friction between soil cuttings, and between the cuttings and the cutters, cutter head face and wear parts of the muck removal system (such as the auger in a microtunnelling system or screw conveyor in an EPB)
– external lubrication of jacking pipes
The greatest benefits are likely to be obtained in sandy or gravelly soil which is abrasive and highly frictional. It may also be considered for cutting of very stiff clays, where the main purpose is to keep the cuttings separate and prevent them from wetting up and tending to stick together. In the latter case it is important that the lubricant does not itself contribute to the wetting up of the clay. To be fully effective, the lubricant must be added at the point of cut; suitable provision for delivery must be designed into the tunnelling machine. Lubrication at the periphery of the cutter head, where relative soil/cutter velocities are greatest, is particularly beneficial. Suitable lubricating materials are bentonite slurry, foam, and various polymers either on their own or combined with a slurry or foam.
The use of foam is spreading rapidly in large-diameter tunnelling, and should be seriously considered for pipe jacking and microtunnelling. It has at least three potential contributions: by supporting and separating soil particles or cuttings it reduces frictional effects, particularly if combined with a lubricating oil; it reduces the permeability of coarse-grained soils and allows better control of groundwater inflow; and it provides a compressible medium which evens out pressure fluctuations in an EPB head or auger conveyor.
For lubrication between pipes and ground during pipe jacking and microtunnelling, the exact requirements vary significantly with the ground conditions. In unstable ground, particularly sands and gravels below the water table, the primary function of the agent is to provide a fluid pressure to support the excavated tunnel bore and prevent collapse of the ground onto the pipes. To do this in permeable ground requires the formation of a filter cake at the soil interface, in the same way as at the tunnel face in a slurry shield machine.
Provided the ground is stable, a tunnel cut to a slightly greater diameter than the outer diameter of the pipes will minimise contact between pipes and ground. Pipes will normally then slide along the base of the bore, generating frictional resistance in proportion to their weight. If the overcut is filled with a fluid lubricant, the pipes will become partially or completely buoyant, so that the contact forces between pipes and ground will be greatly reduced. The main resistance to forward sliding of the pipes may then be the very small shear strength of the lubricant. Pipes at neutral buoyancy in a slurry lubricant of low shear strength may experience almost negligible resistance, and very long pipe jack drives become practical. The effectiveness of slurry lubrication has been demonstrated and explained by research projects in the UK and France; see for instance the results from Milligan and Norris (1999) and Pellet and Kastner (1998) (see tables).
Where full lubrication of this kind is not required, the use of lubricating agents in sandy soils or clays of low plasticity may allow the pipe to slide on a layer of mixed soil and lubricant with a significantly reduced coefficient of friction. In stiff plastic clays the use of water-based lubricants may be counter-productive. Water from the lubricant can gradually penetrate the soil, allowing it to expand and close onto the pipe. The use of inhibiting agents in a slurry may prevent the absorption of water by the soil, or lubricants may be used which do not interact with the soil. The alternative approach of creating a relatively large overcut, so that even after swelling the clay does not close on to the pipe, has the sometimes serious disadvantage of allowing greater ground movements to occur.
Spoil removal
In a slurry machine the excavated material is transported from the tunnel face to the surface by the circulating slurry, which must have the correct properties at the required velocity in the pipe line to transport the excavated material without excessive power consumption or pipe and pump wear. Thixotropic properties may also be useful, so that if circulation is halted the excavated soil is held in suspension and does not settle out in the pipes; however excessive stiffening may cause problems with restarting flow or clogging of pumps. Bentonite slurries may be enhanced by additives, to control viscosity or gel strength, or slurries made from natural or artificial polymers may be considered. However slurry treatments must be compatible with the separation processes which may also involve chemical additives.
In an EPB machine the first stage of spoil removal is via the screw conveyor from the head chamber. Similarly to the machine head the spoil must be in a suitably plastic state to form a plug and allow controlled extrusion through the screw without excessive wear or power consumption. Where the soil in the head chamber has not reached a sufficiently low permeability, the screw conveyor offers a further opportunity for injection of conditioning agents prevent excessive flows of water. As the volume of soil is relatively small and is well confined, treatments at this stage may be used as >rapid-response = control measures when ground conditions change suddenly. The auger of a microtunnelling machine involves similar processes.
In other types of TBM the spoil is usually removed by trucks or conveyors. Handling is generally easier if the spoil is not too wet or fluid; the water content of wet spoil may be reduced by the addition of water-absorbing polymers. These biodegrade with time, releasing the water; this must be remembered when considering later spoil handling and disposal.
Lubricating and conditioning materials
A wide range of materials may be used, including bentonite slurries, natural and synthetic polymers, and foams. These are considered in detail in the review, and a useful summary is provided by Lyon (1999). While many of the materials are well known in the oil exploration industry, their use in civil engineering is in its infancy. Research on the basic properties of mixtures of soil with slurries and foams (and foamed slurries) is in progress at Oxford and Cambridge Universities. Some surprising results have already been obtained, notably on the volume change and shearing characteristics of mixtures of fine sand and foam.
Conclusions
The principal applications of lubrication and soil conditioning in pipe jacking and microtunnelling may be summarised as follows:
For slurry machines, bentonite slurries, if necessary with polymer additives to enhance formation of a filter cake, maintain dispersion and provide lubrication; polymer flocculating agents to improve performance of separation plants.
For lubrication of pipe jacks, bentonite slurries, if necessary with additives to inhibit swelling of clays, improve filter cake formation and increase lubricity. It is possible that foam might also be used.
For tunnelling with EPBM shields, a wide range of conditioning agents from water through bentonite slurries to various polymer materials and especially foams, injected at the face, in the working chamber or in the screw conveyor.
Tunnelling machines must be provided with adequate facilities for adding the agents in controlled dosages and ensuring that they are sufficiently mixed with the soil, and additives need to be correctly chosen for the types of soil encountered.
