Soil Conditioning in Tunnelling
Soil conditioning objectives
The performance of mechanised tunnelling can be improved by using soil conditioning agents. The improvement may come about in a number of ways (after G.W.E. Milligan, 2000 - see here) :
Reduced wear of machine cutter-head face tools and all wear parts of the muck removal system; Improved stability of the tunnel face and better control of ground movements; Improved flow of excavated material through the cutter head; Reduced power requirements for cutter head and to turn excavated material into a suitably plastic mass; Reduction in the friction losses in the pipes, valves and pumps of slurry machine system; Better separation of spoil from the slurry machine system; More acceptable spoil for disposal; Improved safety for personnel working in the tunnel; Enhanced properties of the soil in the pressure chamber of the EPB leading to: More uniform pressures in the working chamber; Better control of ground water inflow; Reduction in clogging of machine head chamber; Controlled flow of soil and water through the screw conveyor; Easier handling of excavated soil;
Soil Conditioning with EPBs
Soil Conditioning Agents
Soil conditioning agents are typically foams, water or oil based mixtures with bentonite clay or polymer suspensions.
In EPBs, the objective is to make soil more plastic with low internal friction and low permeability. Failure to satisfy the above criteria, result either in blockages at the cutting head, clogging or problems in transporting. Without soil conditioning agents the use of EPBs would be limited to fine-grained soils (Maidl et al, 1996).
The common materials used with EPBs are bentonites, foams and polymers. Polymers and bentonite slurries are introduced into the soil in a liquid form, usually as thixotropic fluids. It is of prime importance that the exact quantity of the additives is precisely predetermined.
Bentonite slurries used in tunnelling industry are made by mixing bentonite and water. They have thixotropic properties, forming a gel at concentrations 3-6 % per volume (Lyon, 1999a). The name bentonite is used to characterise a range of clay minerals: primarily potassium, calcium and sodium montmorillonites. Montmorillonites consist of thin flat sheets of clay particles and have the ability to absorb water and to swell. Water is absorbed onto the external and internal sheet surfaces due to their low bonding energy. Calcium ions provide a stronger bond than the sodium ones and swell less. In tunnelling practice, sodium bentonite is preferred because it is the most dispersing type, showing higher viscosity than the other types for the same slurry density. Bentonite slurries must be used as a means to enhance the carrying capacity of the slurry (Lyon, 1999b).
Polymers are made from small chemical compositions, the ?monomers? through a chemical process, in which the monomers are linked together to form large long chain molecules. Polymers are used separately or in addition to bentonite, to form suitable slurries. Some types of natural polymers such as cellulose sugars, starches and proteins can be used in tunnelling. In addition to these, synthetic polymers such as polyacrylamides (PA), carboxymethyl cellulose (CMC) and polyanionic cellulose (PAC) can be used. Synthetic polymers have been developed for petroleum drilling industry as an alternative to bentonite slurries. When used with bentonite, improve the ability of bentonite slurries to form ?filter cake? and to maintain a dispersed structure. However, polyacrylamides and their derivatives are very important as soil conditioning agents (Milligan, 2000b) and have been used particularly with EPBs with foam or bentonite as face stabilisers (Babendererde, 1998). The role of the polymers is the inhibition, dehydration and (when used with oil) the lubrication of the tunnel or shaft. Water absorbing polymers like partially hydrolysed polyacrylamides (PHPA), can be used with foam in small proportions to ?plastify? coarse-grained soils.
Reduction in power requirements for any particular advance rate is achieved through reduction of the friction between soil cuttings and between soil cuttings and the cutters and machine head. Reducing power requirements means savings in energy. Lower torque on the machine head will reduce distortion of the shield and extend the life of seals and bearings.
The lubricating or conditioning agent must be injected at the point of cut. Lubrication at the cutters nearest to the periphery of the machine face, where relative soil/cutter velocities are greatest, is particularly important.
In slurry machines, fluid pressure in the head is used to support the tunnel face. In fine grained soils the slurry may consist of water with a proportion of suspended clay (from the excavated material).
In EPBs the face is supported by the mass of remoulded soil within the machine head. In water bearing soils of high permeability (more than 10E-6 m/s) conditioning agents are needed.
Machine head (pressure chamber)
The potential advantage of the EPBs is that catastrophic collapses are not possible, provided that water inflow is controlled. This is because the excavated mass, which always counterbalances the face pressure, can not escape no matter its condition.
Two functions of soil conditioning are interested here; the reduction of wear and the prevention of clogging by recompaction of plastic clays and their sticking to surfaces within the head. It is important for EPBs to control the permeability and the producing spoil in the head chamber of suitably plastic consistency in order to allow satisfactory pressure balance to be achieved at the face.
EPBs' main field of application is in soft sensitive clays, silts and fine sands which on remoulding in the machine head readily produced a mass of soil of soft and pulpy consistency of low permeability with at most the addition of a small quantity of water. In stiff clays the pressure balance can help to reduce ground movements. The problems start with highly plastic clays where large quantities of water are needed to change the water content sufficiently and the clay is very impermeable. It is difficult to achieve a uniform consistency at the right shear strength. It may be possible to create a `rubble' of chunks of the intact clay sliding in a matrix of softened soil. Recompaction of the material into a very sticky mass which clogs up the machine head and conveyor. In stiff clays the aim is to create a rubble of intact clay rocks in a matrix of foam or polymer material which inhibits uptake of water by the clay, coats the clay blocks and enables them to slide around each other and the machine head without tending to coalesce into a mass of clay. If the matrix is compressible the pressure at head becomes less sensitive to slight differences in the rates at which material enters the head chamber and is removed from it by conveyor. Free water in permeable soil lenses can degrade the foam and in turn the clay will swell.
In slurry machines, the excavated material mixed with slurry is transported and pumped through pipes. Thixotropic properties of the slurry allow the excavated soil to stay in suspension and not settle out in the pipe in case of a sudden halt. Bentonite slurries may be enhanced by various additives or slurries made from natural or artificial polymers. The slurry must be separated from the excavated material and fine particles must be removed, otherwise the slurry carrying capacity is reduced.
In EPBs the removal is achieved through a screw conveyor from the machine head chamber. The spoil must be in a suitably plastic state to allow controlled extrusion through the screw without excessive wear. Additional additives to reduce the permeability of the soil may be necessary. The soil is well confined and treatments may be used at this stage as `rapid response' control measures when ground conditions change suddenly. Advance warning of changes in the ground conditions and proper treatment from the cutter head onwards are preferable.
Foam actionFoams are used with EPB machines in fine-grained soils. In coarse-grained soils, the permeability of the soil is the crucial parameter and should not exceed 10-5 m/s (Herrenknecht, 1994). EPB TBMs operate more effectively when the soil immediately ahead of the cutter and in the excavation chamber forms a 'plastic' plug, which prevents water inflows and ensures face support. Foam appears to integrate very well with the soil. When foam is added, the bubbles lower the density of the earth slurry and reduce the friction among soil particles. Reduction of the ground internal friction leads to a reduction in power requirements.
Foam must be added onto the face so that it mixes with the soil before the air bubbles start to disintegrate. A foam-generating unit produces foam where the foam solution is swirled up with compressed air and then is injected through nozzles in front of the cutting wheel or into the excavation chamber. Based on site experience, the conditioning unit should be mounted as closely as possible to the injection point (Cash & Vine-Lott, 1996; Mauroy, 1998). Additionally, the injection points should be as close as possible to the cutter head (Moss, 1998).
As stated in Section 1.1.3, one of the main objectives of adding foam to the face is to create an impermeable layer. In the case of bentonite slurries, this can be achieved after the consolidation of the slurry, which becomes an impermeable membrane. However, in low permeability soils, the ability of bentonite slurries to form the ?filter cake? falls (Herrenknecht, 1994). Foam can be used with EPBs, in any type of soil, provided that the final permeability of the foamed soil is over 10-5 m/s. By adding anionic-active water absorbent polymers such as PAs, the soil particles are coated, creating a three-phase system (Herrenknecht & Maidl, 1995). In front of the cutting wheel, foam displaces free pore water out of the soil and thus the polymers can be absorbed. Hence, the watertightness of the recently developed three-phase system lies considerably below that of the natural ground.
Another benefit of using foam is the increased compressibility of the soil. As a result, the bulk modulus of the ground mixture is lowered so that it is possible to control the support pressure at the tunnel face (Maidl et al, 1996). If the pressure in the excavation chamber is dropped, the gas phase within the structure will expand and the ground will deform. As the volume of soil is relatively small and well-confined in the excavation chamber, small differences in the proportion of foam:excavated soil can be used as a rapid response measure to a sudden change of ground conditions at the tunnel face.
The major advantage of using foam instead of bentonite-based conditioning agents is that a significantly smaller volume of extra liquid is added to the natural water content of the muck. This, in turn, results in a smaller volume of excavated material. As 90% of foam consists of air, which will escape entirely after only a few days, the original consistency of the ground can be restored very quickly. The other 10% of foam consists of solution which is 90-99% water and the rest, foaming agent and polymers (Maidl et al, 1996). Laboratory tests (DECON, 1996) showed that the best performance in tunnelling operations can be achieved by using a mixture of 10% solution: 90% air. The same report revealed the dependence of the foam stability on the temperature as well as the ratio of solution:air.
The foam expansion rate (FER), otherwise known as expansion ratio (ER), is an important parameter in measuring the effectiveness of the liquid concentration in producing foam. It is also important to know the foam injection ratio (FIR) or mixing ratio, which is the ratio of volume of foam over the volume of excavated soil. On site, the FIR should be varied significantly in different ground conditions. The Japanese company Obayashi provided the following formula to calculate the mixing ratio Q (Kusakabe et al, 1999):
Q = 0.5a[(60-4X0.8)+(80-3.3Y0.8)+(90-2.7Z0.8)] (%)
X = fraction 0.075 mm (proportion<fine sand)
Y = fraction 0.420 mm (proportion<coarse sand)
Z = fraction 2.00 mm (proportion<fine gravel)
a = coefficient of correction, which is related to the uniformity coefficient CU, 1< a <1.6
The above Formula, recommended by the Association of Earth Pressure Balanced Shield Method, is empirical and is based on site experience in Japan.
Environmental issues are becoming increasingly important when it comes to tunnelling practice. The trend is to use biodegradable materials such as low toxicity protein-based foams. There are some standards procedures to evaluate the toxicity and biodegradability of foams. It should be mentioned, however, that these tests are designed for fire-fighting foams, for which a large variety of types exists in the market. One of the main benefits of using foams in tunnelling is that foam dissolves with time when the air disappears and the foaming agents are biodegradable. On the other hand, polymers, which can be used with foams, degrade very slowly but recently degradable polymers based on natural materials are becoming popular (Lyon, 1999b).
However, information on the environmental impact of soil conditioning agents is limited. This is because there is no standard test procedure to assess the suitability of the existing products. The problem is becoming evident in the case of pipe jacking, where the slurry is disposed in the muck and part of it is used for lubrication and consequently will remain in the ground. Furthermore, the cost of disposal increases when the material contains either toxic or non-biodegradable materials because additional remedial treatment is required. Rapid degradation may be problematic because as Milligan (1999, p. 13) noted "if run-off enters water courses, the degradation reaction may de-oxygenate the water". Conditioning agents that are based on natural materials like guar (slimming aid), xanthan (a constituent of tomato ketchup) or locust bean gel are environmentally safe. The solvents based on oil as well as the fluorocarbons in foams are considered to be potentially dangerous.
The soil in front of the face and in the pressure chamber should be water impermeable, in order to create a state of equilibrium with the existing groundwater pressure. The inner friction and the abrasiveness of the earth slurry, which acts as the supporting medium, should be as low as possible in order to reduce the power required and wear cost. The support pressure exerted on the earth slurry by the pressure wall of the tunnelling shield thus has to act both as the effective stress in the grain structure and as pore over-pressure between the grains. Ideally the solid matter particles should be in suspended state. The soil should reveal a viscoplastic behaviour similar to that of a thixotropic fluid, so that the supporting pressure acts evenly on the face and precludes de-mixing or consolidation of the earth pap. For precise control of the supporting pressure in the pressure chamber the earth slurry should have a sufficient amount of elasticity at its disposal.
Polymer foams have turned to be considerably more advantageous than the bentonite-suspensions or high density slurries, in conjunction with EPB machines. Its aim is to reduce surface tension and enable the liquid to produce fine monocellular and double film bubbles which are united to form an interrelated polyeder network. The foam stability can be considerably enhanced through the application of polymers. When they are mixed with excavated soil in the extraction chamber, the soil particles are integrated in the foam polymer network and interlinked, and as a result a three phase system is formed. As the capillary `cohesion' acts on the grain structure, the foam becomes extremely stable in the grain structure and brings about a temporary sealing of the soil. The pore water is displaced by the foam and absorbed by polymers.
The aim is to create a stable bubble of air in the interstice and introduce a counter-balance within these bubbles against the pressure in the water-bearing soil. In addition to decrease the permeability and improve the stability of the soil, the soapy foam gives the soil a cohesive sticky feel which highly improves its excavability. The foam creates a more homogeneous material inside the chamber.
Case StudiesPeron & Marcheselli (1994) first reported the use of foam in sandy-gravelly soils for a shallow tunnel of 8.0 m diameter in Italy. The foam system was developed by Obayashi in Japan. The proportions of the foam concentrate were 1.5% foaming agent and 0.7% of cellulose polymer stabilizer in water. The proportion was foaming solution 100 l : air 600 l (@1.9 bar). The FIR was from 60 to 80% in dry soil and from 50 to 60% in water bearing ground.
Another situation where foam was used was the Valencia metro tunnels (Wallis, 1995). The tunnel was beneath the ground water table, in alluvial sands and gravels with about 15% fines and some lenses of stiff silty clay. EPB was fitted with injection ports for both bentonite and foam. After experiencing problems using bentonite as the conditioning agent, it was found that the use of foam produced a more homogenous and compressible material.. The average consumption was about 500 l of foam per m3 excavated, of which only 18 l were foaming solution, compared to 220 l/m3 of bentonite. The FIR ranged from 25 to 35%. The foam mixed with the soil reduced the power and the torque needed to turn the cutting wheel by the order of 20%. Herrenknecht and Maidl (1995) also referred to the same case as evidence of the benefits of utilising foam.
Webb and Breeds (1997) reported another successful use of foam in a tunnel, with a water head of up to 18.3 m, driven through mixed ground. Various proportions of a water-absorbing polymer were used (0.5 to 2%) together with foam and bentonite. Mauroy (1998) reported the positive effect of using foam as a soil conditioning agent in reducing cutter-head torque for a 7.7 m diameter tunnel in clay. The foam had an expansion ratio of 20 and was injected through different ports at the cutter-head and into the excavation chamber and the screw conveyor. Babendererde (1998) also referred to the same project noting a cutter-head torque reduction of over 50% and thrust force reduction from 2000 to 1200 t.
Another successful case of use of foam with polymer and bentonite (Jancsecz et al, 1999) was the construction of part of the Izmir rail transit tunnels. The 6.5 m diameter EPB was driven through a wide variety of soil conditions such as sandy silts, sands and clay under water table. In silty sand and clay foam (with bentonite) of about 300 to 500 l/m3, an ER of 6 to 10 was used. In a second drive in the same formation, due to clogging of the foam injection pipes, the soil conditioning agent was switched from foam to bentonite slurry. In sandy soil, the ER was increased from 12 to 15. In silty soil (sea-side), foam was utilised only when needed to keep the water away and to make the muck less ?sticky?. The polymer consumption varied between 0.01 and 0.5 kg/m3 of excavated soil. Bentonite was used instead of foam during the stoppages because of break-downs or maintenance. The authors concluded that the use of foam in combination with bentonite and polymer improved considerably the performance of the EPB machine in terms of productivity, cost and safety.
There are also several references on the benefits when EPBs are used with soil conditioning agents. Pellet and Castner (1998) reported the benefits of soil conditioning in also reducing the face resistance. Maidl (1999) noted the success of using EPBs with foam in the Netherlands in layered silty and clayey sands under earth pressure conditions reaching 350 kPa. Maidl and Jonker (2000) also discussed the increased flexibility and adaptability of EPBs using foam in the Netherlands. They also noted that due to high pressure, transportation by screw conveyor could not be guaranteed, which is why a mixed system with conveyor belt and slurry pipe was used. In the mass-transit tunnelling project in Singapore (Reilly, 1999), EPBs were used with foam and polymer or with foam and bentonite, under face pressure varying from 150 to 360 kPa in mixed ground. The results were an impressive reduction in required torque as well as low settlements. Melis (1999) reported that for Madrid Metro project, the overall cost of utilising EPB did not exceed that of other tunnelling methods and at the same time was faster.
Problems using foam and polymer with EPBs were reported by Doran and Athenoux (1998). The problems encountered were in glacial tills with water-bearing lenses under pressure 2.2 bar and in hard fractured clays. The main difficulty faced was to control the water content in the low plasticity clay and consequently excessive wear was recorded. In those conditions, the slurry mode of operation was considered preferable.