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NO DIG BERLIN 2017

Symposium and Exhibition 28 – 31 March

Paper 1-3

THE ROLE OF LUBRICANT SLURRIES AND EFFECTIVE STRESS IN SKIN FRICTION RESISTANCE REDUCTION IN MICROTUNNELLING Dr Ciaran C. Reilly1 and Dr Trevor L.L. Orr2, 1Ciaran Reilly & Associates, 2Trinity College Dublin The distance to which a pipeline may be installed using microtunnelling or pipe jacking techniques in one continuous drive depends mainly on the resistance to the jacking force that develops due to skin friction resistance between the advancing pipe and the soil. Often, bentonite or polymer lubricant slurries are injected into the soil from ports on the surface of the jacking pipe with the intention of reducing the skin friction resistance. Field studies have shown that this procedure can achieve reductions in skin friction resistance of up to 90%. This paper presents the results of physical modelling carried out at Trinity College Dublin to investigate the interface between coarse- and fine-grained soil and concrete jacking pipes, with and without the presence of a lubricant slurry in the interface. It was found that the maximum benefit was achieved when the lubricant slurry blocked the pores in the soil and, with the pores blocked, allowed the fluid pressure within the slurry to transfer to the soil skeleton, reducing the radial effective stress acting on the pipe barrel. Reductions of skin friction resistance of over 90% were found to be achievable in this way. The application of this research may allow for increased lengths of drives in microtunnelling projects through more effective lubricant slurry application, and a reduction in the use of costly and time consuming intermediate jacking stations. Introduction The distance to which a pipeline may be installed using microtunnelling or pipe jacking techniques in one continuous drive depends mainly on the resistance to the jacking force that develops due to skin friction resistance between the advancing pipe and the soil. The skin friction resistance is the resistance developed over the surface of the pipeline as it advances through the ground, as shown in Figure 1.

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Figure 1 – The jacking force and the main sources of resistance to jacking force.

The provision of an overcut, which is an annular gap around the product pipes created using a pipe jacking shield of a diameter greater than that of the product pipes, is the primary means of reducing the skin friction resistance on a pipeline. Further significant reductions are possible through the injection of a lubricant slurry through ports in the skin of the product pipes. A typical arrangement of lubricant injection ports on a product pipe is shown in Figure 2. Pipe jacking lubricants in common usage are bentonite- or polymer-based slurries, mixed in a grout mixer on the surface and delivered through a network of pipes by a semi-automated or an automated delivery system, illustrated in Figure 3. The lubricant slurry injection process is usually guided based on “common sense and experience” (Borghi, 2006) or prescribed injection volumes (Ulkan, 2013).

Figure 2 – Typical arrangement of lubricant slurry injection ports (Source: Pipe Jacking

Association).

Figure 3 – Automatic lubricant slurry injection system (Source: Herrenknecht AG).

Field and laboratory studies have shown that this procedure can achieve reductions in skin friction resistance of up to 90% (Çetin et al., 2011, Norris, 19982, Schoesser et al., 2011, Pellet-Beaucour and Kastner, 2002, Namli and Guler, 2016, Staheli, 2006, Curran, 2010, Curran and McCabe, 2011, Shou et al., 2010). However, the exact mechanism by which these slurries act is not well understood. Four possible friction-reducing mechanisms are:

1. The lubricant slurry forms a lubricating boundary layer between the soil and the pipe.

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2. The lubricant slurry mixes with the soil to form a layer of material with a lower angle of friction.

3. The lubricant slurry fills up the overcut, stabilising it and making the pipeline partially or fully buoyant.

4. The lubricant slurry permeates into the soil until a filter cake forms in the soil thereby reducing the effective stress on the pipeline.

These possible mechanisms are illustrated in Figure 4.

Figure 4 – Mechanisms of skin friction reduction due to lubrication in pipe jacking

Laboratory testing Interface friction testing has previously been carried out with the aim of investigating the effects of lubricant slurries in pipe jacking in a wide range of apparatus (Phelipot et al., 2003, Staheli, 2006, McGillivray, 2009, Shou et al., 2010, Namli and Guler, 2016). The present research utilised the direct shear apparatus in a conventional manner and a novel modified triaxial testing apparatus to measure the skin friction resistance in the interface between soils and concrete jacking pipes, with and without the presence of lubricant slurries in the interface. The advantage of the tests using the modified triaxial apparatus was in the ability of the apparatus to replicate conditions in the field in the interface between a pipe and the soil. The first series of experiments presented are conventional direct shear measurements of the interface friction between one coarse-grained soil, one fine-grained soil and a concrete specimen both in the presence and absence of unpressurised pipe jacking lubricant. The tests were carried out generally in accordance with BS 1377-7 Part 4 (1990) except that a specimen of concrete (or in a small number of cases, of Perspex) was prepared to fit in the bottom half of the shear

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box. The top half of the direct shear specimen consisted of the soil under test. The test arrangement is shown schematically in Figure 5.

Figure 5 – Interface friction testing in direct shear.

Where a lubricant slurry was used, a layer of the slurry was placed on top of the concrete specimen before the soil specimen was gently placed. Every effort was taken to ensure the layer of lubricant slurry remained intact prior to applying the normal stress via a hanger and weights. In addition, a select number of tests were carried out where a specimen of smooth Perspex was fitted in the bottom half of the shear box, to measure the influence of the roughness of the interface on the shear stress response. While direct shear testing allowed the modelling of the mechanisms of unpressurised slurry in pipe jacking, it was not possible to simulate accurately the mechanisms of pressurised slurry using these test methods. A modified triaxial testing apparatus and procedure were developed so that the shearing resistance on the interface between a sand specimen and a concrete specimen of similar surface roughness to a jacking pipe could be measured, while allowing for controlled injection of lubricant slurry into the interface during shearing. The modified triaxial tests were carried out using a modified triaxial testing apparatus where the test was carried out largely in accordance with BS 1377-8 (1990) but subject to modifications based on methods developed by others to determine the residual shear strength of clay specimens in triaxial compression (Chandler, 1966, Meehan et al., 2011). The tests were carried out in a triaxial cell intended for the compression testing of 38mm diameter specimens that was modified by the fitting of a laterally-translating top cap, a concrete specimen with a preformed failure plane and a lubricant slurry injection system. The apparatus as used to obtain the results presented here is shown schematically in Figure 6 and is discussed in greater detail in Reilly and Orr (2017).

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Figure 6 – Modified triaxial testing apparatus.

The triaxial tests were carried out in the drained condition by setting a known back pressure in the pore water circuit and allowing drainage during shearing. This back pressure also ensured that the sand specimens remained saturated. To model the various phases of the pipe jacking process, triaxial interface shear testing was carried out in four phases:

1. Tests without injected lubricant slurry to calibrate the testing apparatus with the direct shear testing previously carried out

2. Static lubricant slurry injection, carried out to simulate the intermittent injection of the lubricant slurry during a pipe jack, for example after a stoppage. Here, a measured quantity of slurry was injected into the interface under a known pressure and the resulting excess pore pressure at the top of the specimen was allowed to reach equilibrium with the pre-set back pressure prior to shearing commencing

3. Dynamic lubricant slurry injection, carried out to simulate the regular injection of slurry while a pipe jack is being advanced. A measured quantity of slurry was injected into the interface under a known pressure while shearing was ongoing and the dissipation of pore pressure with time was measured.

4. Constant pressure lubricant slurry injection, carried out to simulate continuous injection during a pipe jack. Slurry was injected into the interface at a known, constant pressure while shearing was ongoing.

Lubricant slurries studied

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Two lubricant slurries, one bentonite- and one polymer-based, were used in this study. Each was mixed with potable water at concentrations typically used in practice. The bentonite-based slurry was CETCO Hydraul-EZ, which is anecdotally the most commonly used pipe jacking lubricant slurry in the UK and Ireland. The Hydraul-EZ slurry was mixed at 4% concentration, when it had a Marsh Funnel time of 50 seconds, and at 8% concentration, when it was not practical to measure its March Funnel time. The polymer slurry utilised was MX Liquid Polymer produced by Mudtech Ltd. The results of tests using the polymer slurry are not reported here, but are presented in Reilly (2014) and Reilly and Orr (2017). Results of tests with fine-grained soil Two drained direct shear tests were carried out on specimens of an undisturbed fine-grained glacial lodgement till with the intention of measuring the shearing response in the interface between the soil and rough concrete, with and without the presence of 4% Hydraul-EZ lubricant slurry in the interface. Specimens for testing were cut from block samples of glacial till retrieved from a depth of 2m below ground level from a site in County Down, Ireland, where the glacial till was above the water table and would have been at an in-situ vertical effective stress level of approximately 40kPa. This test was largely qualitative in nature and was intended to show the general behaviour rather than derive quantitative parameter values. The material was described as a firm to stiff light brown slightly sandy slightly gravelly clay with occasional cobbles and occasional boulders, with the gravel fraction varying from sub-angular to sub-rounded. A typical particle size distribution for the glacial till is shown in Figure 7. It can be seen to display a grading curve characteristic of many Irish glacial tills that is almost linear when plotted on a semi-log scale. The glacial till had a plastic limit of 23.9%, a liquid limit of 58.2%, a plasticity index of 34.3% and a natural water content of 25%, just above the plastic limit. All tests were carried out at a normal stress of 60kPa as it was necessary to confine the overconsolidated clay at a pressure equal to or in excess of the stress it had experienced in the field (estimated at 40kPa) to prevent swelling. The lubricant slurry was applied to the concrete as a layer approximately 2mm thick. Following the initial consolidation phase, the specimens were sheared through a horizontal displacement of 14mm before the shear box carriage was returned to its starting position, where the specimen was allowed consolidate for at least 24 hours before the next shearing phase was commenced. Shearing was carried out at a rate judged slow enough to allow full drainage. The rate of shearing was 0.00975mm/minute so that each individual shearing phase (horizontal displacement of 14mm) took approximately 24 hours.

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Figure 7 – Particle size distribution of test soils.

The results of the two direct interface shear tests carried out on the glacial till/concrete interface are shown in Figure 8. In all there were eight shearing phases for each test set up, with lubricant slurry present and without.

Figure 8 – Shear stress plotted against cumulative displacement for drained direct shear tests on

the fine-grained glacial till/rough concrete interface with and without a lubricant layer

The plot in Figure 8 shows that the shearing behaviour was quite variable with local peaks. This has been attributed to the variable nature of this glacial till, which contains 20% by mass of particles greater than 1mm in size. It should be noted that the increase in stress at the end of Day 29 of the test involving the lubricant slurry layer has been disregarded in calculating the residual shear stress as it may be due

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to movement of coarse grains within the sample. It is inferred from the graphs in Figure 8 that, for drained shearing conditions, the presence of an unpressurised lubricant slurry layer has no beneficial respect in terms of reducing the shear resistance along the interface between the fine-grained glacial till and the concrete, i.e. it does not act as a lubricant in this case. Results of tests with coarse-grained soil Two sands were used for testing, a fine to medium silica sand known as IGB sand and a medium to coarse crushed limestone sand known as Banagher sand. Both sands were chosen based on their ready availability in the research laboratory. A limited selection of tests performed are discussed here while a complete account of the testing carried out is available in Reilly (2014) and Reilly and Orr (2017). IGB (Irish Glass Bottle) sand is a naturally occurring silica sand obtained from the former Irish Glass Bottle Company plant in Dublin, where it was used to make glass. It originated in Belgium. It is an even-graded white silica sand composed of subrounded to subangular grains. Banagher sand is composed of the fraction passing a 2mm sieve of the concrete-making sand used in the Civil Engineering research laboratories in Trinity College Dublin. It is an even-graded, medium to coarse, grey, sand with variable grain shapes. The particle size distributions of IGB and Banagher sand are shown in Figure 7. The constant volume friction angles in triaxial compression and direct shear, the minimum, maximum, and as tested dry densities, the values of D10, D50, and coefficients of uniformity CU for both sands are

presented in Table 1.

Table 1 – Properties of test sands.

Sand Φcv,ds Φcv,tx γd,min

(kN/m3)

γd,max

(kN/m3)

γd,test

(kN/m3)

D10 (mm)

D50

(mm) C

U

IGB 31.0° 34.0° 14.2 16.4 14.6 0.164 0.23 1.60

Banagher 36.2° 38.0° 14.1 17.3 15.7 0.16 0.72 2.64

Results of direct shear tests performed using IGB sand are summarised in Figure 9. As seen from the Figure, it was found that the presence of an unpressurised layer of bentonite-based lubricant slurry has either no beneficial effect or a small beneficial effect in reducing the shear resistance on the coarse-grained soil/concrete interface. The interface friction resistance between rough concrete and IGB sand is unaffected by the presence of a 2mm layer of 4% Hydraul-EZ and reduced by 12.5% when the concrete interface is coated in a 2mm layer of 8% Hydraul-EZ.

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Figure 9 – Direct shear testing results for IGB sand shearing internally and against a lubricated

and unlubricated rough concrete interface and a Perspex interface.

Figure 10 shows that while 4% Hydraul-EZ had no measurable effect on the friction resistance, it did cause the shear stress response to behave in a contractive manner, i.e. peak shear stresses were avoided during shearing. It was therefore unusual to find that a peak value of shear stress was apparent in shearing when there was 8% Hydraul-EZ in the interface, as shown in Figure 11. This may be because some time was allowed for consolidation of the specimens where 8% Hydraul-EZ was applied to the interface and a denser soil structure may have resulted near the interface. Similar behaviour was seen in the case of Banagher sand, with a 3.3% reduction in the apparent angle of interface friction in tests where 8% Hydraul-EZ was applied to the interface.

Figure 10 – Results of shear strength interface testing for IGB sand and 4% Hydraul-Ez

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Figure 11 – Results of shear strength interface testing for IGB sand and 8% Hydraul-Ez

A select number of qualitative results of tests carried out using the modified triaxial testing apparatus are presented in Figure 12, Figure 13, and Figure 14. Test results are further discussed in Reilly and Orr (2017). Figure 12 presents results for tests involving IGB sand and 4% Hydraul-EZ where the lubricant slurry was injected into the interface prior to the commencement of shearing but after steady state pore pressure had been achieved within the triaxial test specimen. This is referred to as static lubricant slurry injection and was carried out to simulate intermittent slurry injection, for example after a stoppage. It is seen that the tests where lubricant slurry was injected present noticeably less shearing resistance, as represented by the deviator stress, than tests without slurry injected. A similar trend was observed for Banagher sand.

Figure 12 – Deviator stress plot for testing where 4% Hydraul-EZ was injected into IGB sand prior

to shearing to simulate intermittent lubricant slurry injection.

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Figure 13 presents results for tests involving Banagher sand and 4% Hydraul-EZ where the lubricant slurry was injected into the interface after the test had progressed to 4% axial strain, termed dynamic lubricant slurry injection. These tests were carried out to simulate the regular injection of slurry while a pipe jack is being advanced. It was possible to reduce the shearing resistance of the interface, as represented by the deviator stress, to less than 10% of the expected value through lubricant slurry injection, in this case at pressures 5 times the hydrostatic pressure for test BH4-4-1 and 10 times the hydrostatic pressure for test BH4-4-2. The slow rate of increase in deviator stress with shearing following the cessation of injection, which is similar to the rate of increase of deviator stress without lubricant slurry, shows the existence of a “filter cake” effect, in that a pressure gradient was maintained within the specimen between the injection port and the drainage lines to the top cap. This test was not repeated for IGB sand.

Figure 13 – Deviator stress plot for testing where 4% Hydraul-EZ was injected into Banagher

sand during shearing and for comparison the deviator stress without slurry injection.

Figure 14 presents results for tests involving Banagher sand and 4% Hydraul-EZ where the lubricant slurry was injected into the interface continuously during shearing, termed constant pressure lubricant slurry injection. These tests were carried out to simulate continuous slurry injection during a pipe jack. Lubricant slurry was injected into the interface at a known, constant pressure equal to the cell pressure while shearing was ongoing and it is seen that a very low shearing resistance was mobilised in the interface, as represented by the deviator stress. This is because the pore water pressure at the interface is elevated, reducing the normal effective stress from the soil acting on the rough concrete, and hence reducing the shearing resistance.

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Figure 14 – Constant pressure water and lubricant injection with Banagher sand

While apparently lower deviator stresses were recorded in the test with the injection of water rather than with the injection of Hydraul-EZ, it is considered that this difference is due to experimental variation at the very low stress levels encountered. This test was not repeated for IGB sand. Conclusion The results of a series of laboratory tests carried out to study the effects of unpressurised and pressurised lubricant slurries on the interface shearing resistance between two soils, one coarse-grained and one fine-grained, and a rough concrete surface which closely matched the surface roughness of commonly used concrete pipe jacking pipes have been presented. While the findings of the research have not been verified in the field, the main outcomes of the research were as follows:

1. Tests carried out using the novel triaxial interface shear testing apparatus showed that pressurisation of the lubricant slurry applied in the interface between sand and pipe jacking pipes equal to the cell pressure, which is equivalent to the hydrostatic pressure in practice, is required to bring about the reductions in the magnitude of shearing resistance observed during field studies, i.e. greater than 90%. It was concluded that the mechanism of action is that of local effective stress reduction in the interface. The lubricant slurry blocks the pores in the soil, and with the pores blocked, the fluid pressure within the slurry is transferred to the soil skeleton.

2. It was found that the presence of lubricant slurries, at the volumes commonly introduced during pipe jacking, did not alter the physical character of the interface, i.e. change the angle of interface friction, between the soil and the pipe, to any significant degree, and hence it was concluded that lubricant slurries do not cause lubrication in the classical sense of reducing the angle of friction but rather cause a reduction in the effective normal stress and thereby a reduction in the shear resistance.

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3. It was noted that the presence of an unpressurised layer of lubricant slurry had only a small beneficial effect in reducing the shear resistance in the coarse-grained soil/concrete interface and no discernible effect in the case of the fine-grained soil/concrete interface. The shearing against a smooth interface mobilised a lower shearing resistance than shearing against a rough interface coated with unpressurised lubricant slurry.

4. It is shown that the main beneficial effect of injected lubricant slurry is the

reduction in radial effective stress acting on the pipe barrel through the formation of a filter cake and the generation of excess pore pressure in the soil near the interface, with boundary film lubrication playing a minor role. Further, the ability of a lubricant slurry to form a filter cake in the soil close to the pipe surface is shown to be beneficial in retaining an elevated pore pressure when the source injection pressure is reduced.

5. The main conclusion and recommendation towards practice deriving from the

research is that pipe jacking lubricant slurry should be applied under conditions of controlled pressure rather than controlled volume to gain the maximum beneficial reduction in skin friction resistance in coarse-grained soils. A similar effect is expected in fine-grained soils.

6. Field validation of this research may lead to benefits in allowing slightly less

conservative values for skin friction resistance be adopted in the design of microtunnelling projects, leading to potentially longer drives and the reduced use of intermediate jacking stations.

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