Soil conditioning for pipejacking and tunnelling: properties of sand/foam mixtures G.T. Houlsby and S. Psomas Oxford University, UK Abstract Soil conditioning techniques are used to alter the properties of soils to make them more suitable for excavation by tunnel boring machines. Conditioning agents include foams, polymers and bentonite slurry, either alone or in combination. Conditioning agents may have additional benefits in providing lubrication for the advancing tunnel shield, or in the case of pipejacking for the entire tunnel shaft. In spite of the now widespread use of conditioning agents in tunnelling practice, little basic information is known about how they affect the fundamental properties of soils. The purpose of this paper is to report a series of laboratory tests on mixtures of sands and conditioning agents. The compressibility and permeability characteristics of sand/foam mixtures were studied using a Rowe Cell. The shear strength characteristics of the mixtures were studied using shear box tests. The paper concentrates principally on the use of foams as conditioning agents, but also includes some results involving use of polymers and bentonite. The principal conclusions, backed up by experimental data, are:

• Foam-soil mixtures are stable at remarkably high voids ratios, even when subjected to significant stress levels (approximately 200kPa).

• Mixtures at such high voids ratios exhibit extremely low angles of friction.

Introduction Soil conditioning agents are now fairly widespread in tunnelling practice, where they have a number of applications. At the cutter head they may be used to improve excavation rates, reduce wear, reduce power requirements and improve the control of water ingress. In the chamber and screw conveyor of an earth pressure balance (EPB) machine they may improve the flow characteristics of the soils, and allow the pressure to be more easily controlled. If a pumped spoil removal system is used they may again be necessary to improve the flow characteristics. Depending on the conditioning agents used, there may then be a necessity for a separation process before spoil disposal. Conditioning agents may also interact with the process of lubrication of the advancing tunnel shield, or in the case of pipejacking the lubrication of the entire pipe string. In the latter application, effective lubrication can reduce the jacking forces, and hence reduce the need for expensive interjack stations. In a typical modern tunnelling operation it is likely that a variety of different lubrication/conditioning agents may be employed at different points in the construction process. Although much empirical information has been gathered on the effectiveness of different agents in particular applications, most of this information is highly site and application specific. It is often also related to specific proprietary brands of materials, and may be held as confidential by commercial parties. There is therefore a need for generic information on the ways that different soil conditioning agents interact with a variety of soils. In clays the desired purpose of the agents will often not be to mix intimately with the parent clay material, but instead the object will be to obtain relatively large cuttings of clay suspended within a slurry formed by the conditioning agent. In coarser grained materials (sands and gravels), however, it is anticipated that none of the original structure of the soil would be preserved, and there would be an intimate mixing of the soil with the conditioning agent. The purpose of this paper is to present data on the properties of such mixtures.

In testing soils one would first examine their most basic characteristics with simple tests, and this approach is taken here for the mixtures studied. Tests were used to determine the compressibility, shear strength and permeability (although little information was obtained on the last). The tests used were relatively small scale, using both standard and non-standard equipment. The materials most commonly used as conditioning agents are bentonite, polymers and foams. When using these agents, the interaction between foam and soil is the least well understood, so the investigations concentrated on foam-soil mixtures, although the other materials also received some attention. The results of the tests can be used to assess their suitability for the various applications described above. Equipment and procedures Foam generator Figure 1 shows the design of the foam generator. The fluid consisting of water and the foaming agent is pre-mixed to the correct proportion and placed in a 7 litre capacity cylinder. Air pressure is used to drive the fluid into an inverter, in which the fluid and air phases mix. Compressed air at a higher pressure than the fluid also enters the inverter, entraining the fluid as it exits. The fluid/air mix then passes through a “foam conditioner”, which is a cylinder packed with small plastic rings. This serves to divide the bubbles and form the structure of the foam. Finally the foam exits, with the device giving a flow of about 2 to 4 litres/minute at an expansion ratio (total volume/volume of fluid) of about 15.

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Mixing equipment The sand/foam mixtures were mixed in an industrial quality food mixer of approximately 5 litre capacity before transferring to the testing equipment. A careful audit was kept of the various components in the mix (sand/liquid/air), and these values compared with those deduced from measurements of density from sampling of the specimens tested. Cases where there was any significant discrepancy (e.g. due to some loss of fluid in transfer of the mixture to the testing cell) were rejected. Rowe Cell Compression tests were carried out in a standard 75mm diameter Rowe Cell (see Figure 2). Stiff porous discs were placed on the top and bottom of the sample so that the boundary conditions are fixed strain rather than free strain. It was not possible to measure separately the flow of liquid and air from the sample, so the degree of saturation at intermediate stages of the test is unknown. It is possible, however, to determine the degree of saturation at the beginning and end of the test. The tests

were carried out with load increments representing a multiplication of the previous load by approximately 1.414 (i.e. doubling in two load steps).

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Figure 2: Rowe cell (after Head, 19861) Shear Box Shear testing was carried out in a standard “Casagrande” type shear box, 80mm square in plan, as shown in Figure 3. Vertical load was applied by dead weights. Horizontal displacements were applied at a constant rate and the corresponding shear load measured by a load cell. Both vertical and horizontal displacements were measured.

Figure 3: Shear box (from Head, 19942) Materials Studied

Sands

Two different types of sand were used, and both were relatively uniform. The coarse sand was Leighton Buzzard silica 14/25 sand (yellow). This has a coefficient of uniformity CU = 1.3, a sub-angular grain shape, specific gravity Gs = 2.65 and mean particle diameter d50 = 0.85 mm. Minimum and maximum voids ratio for the coarse sand were 0.49 and 0.79 respectively. The fine sand was Leighton Buzzard silica DA 81DF. It is also a very uniform material, with CU = 1.4, Gs = 2.65 and d50 = 0.165 mm. Minimum and maximum voids ratio for fine sand were measured as 0.61 and 0.91 respectively.

Foaming agents

Five different types of foam agent were tested. The foam generator was able to operate with all of them, producing acceptable quality micro-foam. Early tests indicated only small differences in results on foams of comparable quality, so a standard foaming agent was used for the tests reported here. The material used was CETCO Versa VSX, which is a synthetic polymeric foam agent. It is found that the addition of small quantities of further additives can improve the quality of the foam, and this was found to be particularly beneficial in connexion with the coarse sand. After some trials, the final foam solution consisted of 3% by volume VSX Versa foaming agent, and 0.7% by volume of a mixture of VPC oil with Instapac425 and SC200 (this polymer mixture is referred to as ‘FOP’). The role of ‘FOP’ is to act as a ‘booster’ enhancing the bubble production. This mix produced foam with a stable bubble size. From a microscopic inspection, the size of the foam was in the range of 0.1 mm to 1 mm, and the expansion ration was approximately 15.

Bentonite and polymer

The bentonite used was the CETCO Hydraul-EZ, a sodium montmorillonite bentonite able to swell to about 10 times its original volume. The proportion of bentonite powder to water is critical for the rheological characteristics of the slurry. After some trials, a bentonite:water ratio of 5:95 by weight was used. In tunnelling applications, the bentonite slurry dosage should be greater than the calculated void space, to create the impermeable 'filter cake'. The theoretical porosity ( )maxmaxmax 1 een += of the sand in its loosest state was 0.476 for the fine sand and 0.441 for the coarse sand. The volume of the bentonite slurry added was then defined by sandslurry VnV maxα= . Where α is a measure of the ratio

of the slurry volume to the maximum voids volume for the sand. Values of α for the tests ranged from 0.5 to 1.9. It is convenient to use the bentonite slurry voids ratio ebs, which can be defined as the ratio of the water used (Vw) over the volume of the bentonite Vbs, so that ebs = Vw / Vbs. For the compressibility tests the bentonite slurry voids ratio varied between 17 and 31. Lyon (19973, p. 18) states that the ability of bentonite to swell is due to the presence of montmorillonite, which provides greater surface area upon which water molecules may be absorbed. The presence of calcium in the water reduces the effectiveness of bentonite because calcium ions have a higher charge valence, and therefore hold the crystal lattices of sheets more tightly, allowing less dispersion in water. The water used for the mixing was tap water with a measured pH between 6.5 and 6.8. However, better mixing was achieved when the water had a pH between 7 and 8. The presence of calcium was detected by adding a tiny proportion of ammonium oxylate. The water was treated with caustic soda (NaOH) or soda ash (Na2SO4) so that the pH reached 8. In all cases with coarse sand and in some with fine sand, a small proportion of a polymer mixture ‘WOP’ (Water:VCP Oil:Polymer) was added. In the case of coarse sands, the addition of ‘WOP’ helped to produce a more homogenous foamed soil by creating a higher viscosity fluid matrix. The initial proportions were (4:1:1) respectively, and the dosage from 0.01ml to 0.04ml of ‘WOP’ mixture per g of dry sand. The addition of this mixture was necessary in the cases where the water content of the sand was more than 27-35%. ‘WOP’ was added during mixing in the soil mixer bowl as a ‘pre-conditioner’, prior to the addition of foam. The performance of the 'WOP' mixture improved when the proportion of oil:polymer changed from 4:1 to 2:3. The final 'WOP' dosages used were 25ml for the fine sand and 50ml for the coarse sand. In the case of coarse sand, this quantity was the minimum required to achieve a homogenous mixture in the mixer bowl so that a representative slurry sample could be tested. However, when bentonite slurry was added, the effectiveness of the 'WOP' mixture decreased. A measure of the viscosity of the slurry may be made with the Marsh funnel, which has a diameter of 150mm at the top, tapering over a distance of 300mm to a smooth bore tube 50mm long with an inside

diameter 4.8 mm. Over half of the top opening is a wire screen with apertures of 1.6mm. The time (in seconds) required for a certain quantity of slurry (1500ml) to pass through the exit tube is measured. The measurement in the case of fine sand with bentonite was about 45 seconds whereas in the case of coarse sand with bentonite was about 59 seconds. Testing Programme Compression Tests Each compression test was carried out incrementally, with a factor of 1.414 between successive stress levels. During each load stage the drainage lines were at first closed, so that an “undrained” compression is measured. Such a compression would be negligible for a saturated specimen, but for the sand/foam mixtures there is a significant compression of the bubbles in the foam. Then the drainage line was opened and fluid allowed to escape from the sample. Unfortunately, it was not possible to make independent measurements of the amounts of air and liquid expelled. A typical plot of displacement against time, showing both the undrained and drained phases is given in Figure 4.

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Only the end-points for each load increment are plotted in the following figures, i.e. the voids ratio achieved at the end of the drained stage. Figure 5 shows results from tests on mixtures of foam and fine sand. The lowermost curve is for sand alone, initially prepared in a loose state. At low stress level the voids ratio lies just below the maximum voids ratio for the sand. As the stress increases the voids ratio reduces by a small amount, but the sand remains in a loose to medium density (demonstrating that for a sand compaction to high densities by application of stress alone is not possible). The upper curves are for two different tests on foam/sand mixtures. The differences between the initial voids ratios at low stress levels are entirely due to the amounts of foam added. At low stress level it is possible to make a sand/foam mixture at a voids ratio much higher than that which can be achieved for sand alone. In fact in terms of the conventional Relative Density, the initial voids ratios correspond to a Relative Density in the region of –200%. As the stress level is increased the two curves converge, indicating that at higher stresses the behaviour is not so sensitive to the initial amount of foam added. Presumably and “extra” foam is expelled from the mixture, leaving an amount that corresponds to a stable structure at that particular stress level. The tests were continued to a stress level of 226kPa, which is typical of the stress levels that may be encountered in, for instance, pipejacking operations. At this stage the voids ratio of the sand/foam mixture is still well above the voids ratio at 0% Relative Density, in fact the voids ratio corresponds to a Relative Density of between –80% and –100%. This is a remarkable finding. It was unsurprising that sand/foam mixtures could be made at high voids ratios, but it was quite unexpected that such high voids ratios could be sustained at a remarkably high stress level. Note that the sand/foam mixture has a

truly composite action which is much more than the sum of the component parts: sand by itself would have been compacted to a much lower density, and the foam by itself would have been crushed at such a stress level, but the sand/foam mixture is stable in a remarkably loose state. This may have fundamental implications for tunnelling operations. Figure 6 shows results from using other soil conditioning agents. The lowermost curve is again for sand alone. The next is for a sand/bentonite mix, with a relatively low bentonite dosage just sufficient to achieve an initial voids ratio just above maxe . The compressibility of the sand/bentonite mix is very comparable to that of the sand alone. The next curve is for a sand/bentonite/polymer mix, again at a relatively low dosage. The principal effect of the polymer is to increase the effective viscosity of the pore fluid. The sample does show, however, a somewhat higher compressibility, but this may be related to the slightly higher initial voids ratio, as towards the end of the test the voids ratio of very comparable to that of the sand/bentonite mix.

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Figure 5: Consolidation of fine sand and foam mixtures

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Figure 6: compression curves for sand/bentonite mixes

The next curve demonstrates how the compressibility is affected by the dosage of the bentonite. With a much higher dosage (sufficient to more than fill the voids even at the loosest state) a very high voids ratio mix is obtained. This is sustained until quite high stress levels, at which there is quite a sudden change of stiffness. It is likely that this observed change of stiffness may be highly dependent on the particular characteristics of the sand and the bentonite, so no very definite conclusions should be drawn. However, it may be assumed that at high bentonite dosages relatively high voids ratios will be sustained up to a certain stress level, but at sufficiently high stress levels the density would approach that of the sand alone. The uppermost curve is for a sand/foam/bentonite mixture. It starts at an exceptionally high initial voids ratio. Although the compressibility is very high, at the end of the test the voids ratio is still well above maxe , and is comparable to the voids ratio achieved in the other foamed tests. A particular feature of this test is the unloading curve, where it can be seen that the elastic rebound is quite large. This must be principally due to re-expansion of highly pressurised bubbles. It appears that the bentonite is effective in trapping such bubbles, and that the combination of the use of bentonite and foam can produce a mix which has not only a high compressibility, but also exhibits a high rebound. Clearly the effect of time is important, as it may be expected that the foam would degrade with time (as it does in air), and thus become less effective. Figure 7 shows the influence of time on a particular sand/foam/polymer mix. The uppermost curve is for a relatively rapid test (1 hour per increment) and the lower curve for a slower test (10 hours per increment). The curves are broadly similar, but it is 999999clear that with elapsed time the foam does become less effective. However, even for the slower test the voids ratio remains remarkably high up to stresses of about 80kPa. With more detailed testing it may be possible to establish a set of contours of equilibrium voids ratio with effective stress for given elapsed times since foam formation.

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Figure 7: effect of time on compressibility of sand/foam mixtures Shear Box Tests Conventional direct shear box tests were carried out on sand and sand/foam mixtures. Results of typical tests at low stress level are given in Figure 8. The solid curves show the result for a medium/loose sand. The stress/displacement curve shows a modest peak, which is associated with the small amount of dilation exhibited by the sample. The stress then levels off at a value equivalent to a constant volume angle of friction, consistent with the volume changes, which level off too. The dotted curve shows an equivalent test on a sand/foam mixture. A much lower shear stress is observed. As would be expected the sand/foam mixture compresses during shearing. However, even

though the compression approximately levels off towards the end of the test, the stress ratio still lies well below the value obtained in the sand test. Thus a sand/foam mixture exhibits a much lower constant volume angle of friction than the sand by itself. This is also a remarkable result, as it is often assumed that the constant volume angle of friction for a sand represents a lower bound, which can be relied on irrespective of the state of the sand. Figure 9 shows an equivalent pair of tests at a much higher stress level (226kPa). The principal difference lies in the test on the sand alone – this time the stress level is high enough to suppress the dilatancy, and no peak in the shear stress curve is observed. The response exhibited by the sand/foam mixture is broadly similar to that at low stress level, with again a much lower angle of friction.

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How can the two observations (a) that sand/foam mixtures can exist at remarkably high voids ratios and (b) that such mixtures exhibit remarkably low angles of friction, be drawn together? The obvious framework to use is one that addresses the influence of voids ratio on angle of friction, and this topic was treated in some detail by Bolton (19864, 19875). However, he was only addressing conventional sand, which could not exist at the voids ratios encountered for the sand/foam mixtures. Figure 10 shows a plot of angle of friction, from the direct shear box tests, plotted against voids ratio at the start of the test. The solid dots show results for fine sand. Note that samples were deliberately prepared in as loose as possible condition, and several in fact plot as looser than the normally accepted minimum density. The solid line shows Bolton’s correlation for variation of angle of friction with relative density, assuming an angle of friction at constant volume of 31o. Bolton’s correlation assumes that the angle of friction falls with increasing voids ratio, but that this reduction does not continue below cvφ , which is assumed to be attained at about a Relative Density of 20%. The evidence from the sand tests is either that the angle of friction at constant volume is in the region of 27o (unlikely for a silica sand) or that for very loose specimens (which may still be compressing at the end of the test) a lower angle of friction than cvφ is obtained in the direct shear tests with rather limited displacements.

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Figure 10: variation of angle of friction with voids ratio: fine sand

The open circles show the data for the sand/foam mixtures. Although there is a considerable scatter, they in fact confirm that the broad trend predicted by Bolton’s correlation is continued at the voids ratios looser than those that can be obtained for sand alone. The angle of friction continues to drop as the voids ratio increases, falling to the astonishingly low value of 7o. A mechanistic explanation is that the sand/foam mixture may be thought of a s consisting of two types of “particles”, firstly sand grains, and secondly bubbles, which may be thought of as frictionless and deformable particles. The more bubbles, the lower the angle of friction. One would therefore expect that there would be a correlation with the degree of saturation too. This is indeed observed to be the case (the lower angles of friction within the scatter band correspond to lower degrees of saturation), but the quantitative evidence is limited. Figure 11 shows the equivalent data for coarse sand, confirming that the above result is not simply an unusual feature of the fine sand tested. Exactly the same trends as described above are observed. Note,

however, that both the sands tested are relatively single-sized, and it is necessary to extend the result to poorly sorted materials too.

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Figure 11: variation of angle of friction with voids ratio: coarse sand Permeability Tests Permeability tests were carried out in the Rowe cell under constant head conditions. The results were only partially satisfactory, as the head losses in the filters either side of the sample were significant, and the necessary corrections to the measurements were therefore large. However, a clear indication that the presence of foam decreased the permeability of the soil to water flow was obtained. Typically the permeability was reduced about tenfold. Discussion These tests have demonstrated two main findings for sand/foam mixtures: (a) Mixtures of sand and foam are stable at remarkably high voids ratios for significant periods of time at stress levels up to 200kPa (and possibly higher, as this was the highest stress level tested). The voids ratios on initial mixing of sand and foam at low stress level were many times higher than that in the loosest state for a normal sand, and depend largely on the chosen foam injection rate. The voids ratios on compression to 200kPa are, however, also well above that in the loosest state normally encountered. The foam/soil mixture has (unsurprisingly) a high compressibility at stresses up to 200kPa, but shows a much stiffer response on unloading. It had of course been anticipated that the mixtures would have a very high initial voids ratio and compressibility: the remarkable finding from the tests was that high voids ratios could still be sustained at relatively large stresses. (b) At high voids ratios (only sustainable through the addition of foam) sands exhibit remarkably low angles of friction, with the angle of friction reducing with increasing voids ratio in a way that is consistent with previously obtained data for sands at different densities. The remarkable result here is the “extension” of Bolton’s correlation to voids ratios considerably higher than that at the loosest normal state. Bolton’s correlation suggests that loose sand exhibit an angle of friction cvφ , and that denser materials show higher angles of friction due to dilatancy. In the experiments described here foamed sands at states much looser than the normal loosest state were found to exhibit angles of

friction much lower than cvφ , which had perhaps been previously envisaged as a lower bound to the friction angle. Angles of friction as low as 6o were measured. These findings are of fundamental significance for the tunnelling industry. Firstly the finding that foam/soil mixtures can be stable at relatively high stresses is very valuable. It means that foams injected, say, at the cutting face will not simply be destroyed under high stress, but that a stable foam-soil mixture could be sustained in the chamber of an EPBM. Furthermore the relatively high compressibility of this mixture will ease control of pressure within the chamber. The low angle of friction of the foam soil/mixture has implications for reduced power consumption and reduced wear throughout the tunnelling process. Paradoxically it may also be a problem. Effective control of pressure in the chamber requires controlled removal of spoil through the screw conveyor, and this can only be achieved if there is sufficient resistance within the conveyor that the spoil does not simply flow through it. Control of foam quantities to ensure that sufficient friction is maintained will be necessary. However, once through the conveyor a low angle of friction would be very beneficial for pumped removal of spoil, which would otherwise be very difficult for coarse grained materials. The control of properties at different points in the tunnelling process suggests that addition of foams at multiple points in the excavation and spoil handling process may be desirable. Although this paper has not been directed towards the important issue of disposal of spoil, a further attraction of foams is the relatively small amounts of potential pollutants that are added to the soil for a large volume of foam. Conclusions Simple laboratory tests on sand/foam mixtures have demonstrated that they can exist at remarkably high voids ratios at stresses up to about 200kPa (and possibly higher). At these high voids ratios the sands also exhibit extremely low angles of friction. Both the high compressibility and low frictional strength of these mixtures have important implications for the use of foams in tunnelling operations. Foams also serve to reduce the permeability of a sand. Use of other soil conditioning agents (bentonite and polymers) has also been examined. Although when used alone they have a less dramatic influence on compressibility and strength characteristics, they can play an important role in stabilising and modifying a sand/foam mixture. Acknowledgements The work described here was supported by the Pipe Jacking Research Group, a consortium supported by the Pipe Jacking Association, a number of water supply companies and industrial partners. References 1. Head K.H., 1986. “Manual of soil laboratory testing. Effective stress tests.” Vol. 3, Pentech press,

London. 2. Head K.H., 1994 “Manual of soil laboratory testing. Compressibility, shear strength and

permeability.” 2nd edition, Vol. 2, Pentech press, London. 3. Lyon J., 1997. “Drilling manual”. CETCO Europe. 4. Bolton M.D., 1986. “The strength and dilatancy of sands.” Geotechnique Vol 36, No 1, pp. 65-78. 5. Bolton M.D., 1987. “The strength and dilatancy of sands. Discussion” Geotechnique Vol 37, No 2,

pp. .219-236.