21 - 26 April 2018 Dubai International Convention

& Exhibition Centre, UAE

ITA - AITES WORLDTUNNEL CONGRESS

POSTER PAPERPROCEEDINGS

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Variations of Atterberg Limits of Clays and Their Chemical Mechanism Due to Soil Conditioning

Pengfei Liu1, Shuying Wang2 and Haibo Wang3 1Ph.D. Candidate, School of Civil Engineering, Central South University,

Changsha, 410075, Hunan, People’s Republic of China,Email: pengfeiliu@ csu.edu.cn

2Associate Professor, School of Civil Engineering, Central South University, Changsha, 410075, Hunan, People’s Republic of China,

Email: sywang @csu.edu.cn (corresponding author)3Master Candidate, School of Civil Engineering, Central South University,

Changsha, 410075, Hunan, People’s Republic of China,Email: 2564943521@qq.com

ABSTRACT

To determine of ideal type and content of soil conditioners for earth pressure balance (EPB) shield tunneling in clayey ground, this study investigates effects of soil conditioners on Atterberg limits of clays and conditioning mechanism based on their chemical effects. The applicability of soil conditioners (including foam and dispersant) was determined based on the changes of Atterberg limits of the bentonite with addition of the soil conditioners, and then the Zeta potentials and repulsive energy among particles with different dispersant contents were measured to analyze chemical conditioning mechanism of the dispersant. The study shows that it was difficult to achieve ideal effect only by using anionic-cationic foam individually. The dispersant decreased the liquid limit and plasticity index of the clay, thus it can decrease the shear strength. Therefore, the dispersant is more suitable for clay conditioning. With the increase in dispersant content, the Zeta potential decreased continually (i.e., negative charge increased) in the bentonite solution. The mean barrier potential Vmax/R of clayey particles got higher with an increase of dispersant content, thus it is more difficult for the clays to be agglomerated and easier to disperse.

Keywords: EPB shield; Soil conditioners; Atterberg limits; Chemical Mechanism

1. INTRODUCTION

When earth pressure balance (EPB) shield passes through clayey ground, clayey muck sticks easily to its cutter board, chamber bulkhead and screw conveyer, resulting in high torque, non-smooth muck discharging and low tunnelling rate. The most effective approach to avoid soil clogging is soil conditioning which requires to inject soil conditioner to shield muck, and thus it is important for EPB shield tunneling to study mechanical behavior of the conditioned muck. Some scholars presented different testing methods to study the mechanical behavior of the conditioned muck. The shear strength of the conditioned muck can be determined with the direct shear tests (Houlsby et al., 2001; Peña Duarte, 2007,). Its undrain shear strength under the atmosphere pressure can be measured in the shear vane apparatus (Merritt, 2005; Peila et al, 2015). In addition, some researchers developed new equipments to study the shear behavior of the conditioned muck. Messerklinger et al. (2011) and Zumsteg et al. (2012) used special shear vane apparatus to simulate the stress state of muck in shield chamber, and they measured the undrained shear strength of the conditioned muck under different stresses and shearing velocities. Qiao (2009) developed apparatus to measure the internal friction angle of soil and study its variation due to soil conditioner. Using large vane device, Merrit (2005) measured the undrained shear strength of

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muck before and after being conditioned, and found that they were not different significantly with those obtained from the normal shear vane apparatus. Xiao et al. (2016) carried out triaxial tests in unconsolidated-undrained condition to study shear strength of gravely sand conditioned with foam and bentonite slurry. The previous method to test soil behavior can evaluate the effect of soil conditioner on shield muck. However, they are not convenient and simple for field application and cannot reveal conditioning mechanism in fundamental soil mechanics.

The liquid limit represents the water content limit of clay between flowable and plastic states, and the plastic limit defines the water content limit of clay between plastic and hard states. With actual water content, soil fluidity can be evaluated. Additionally, the liquid and plastic limits can indirectly show the shear strength of clay. The reduction of liquid limit and plastic limit represents that the water content of soil with undrained shear strength of 1.7 kPa and 110 kPa reduces (Zhou and Zhang, 1985). When two soils with different Atterberg limits have identical water contents, the soil with lower liquid and plastic limits has lower undrained shear strength. In other words, the reductions of liquid and plastic limits indicate the reduction of undrained shear strength, vice versa (Feng, 2002). So far, few researchers have studied the effect of soil conditioner on Atterberg limits of conditioned soil. Merrit (2005) measured the variation of liquid and plastic limits of kaolinite due to addition of polyacrylamide solution. Spagnoli (2011) studied the effect of fluid dielectric constant on liquid limit. Ye et al. (2016) determined the effect of foam injection ratio on Atterberg limits of muck developed for shield passing through the argillaceous siltstone. Although some findings on the variation of Atterberg limits due to soil conditioning has been presented, there are no research results revealing the different effects of different soil conditioners on Atterberg limits and their chemical mechanism.

This paper studies the variations of Atterberg limits of bentonite due to the addition of different soil conditioners, and then the suitability of soil conditioner to ground which is passed through by EPB shield is proposed. The Zeta potential was measured on the solution of soil conditioned with dispersant, and the repulsive energy among bentonite particles was calculated based on the DLVO theory. The conditioning mechanism of the dispersant was studied from the chemical and microstructure views.

2. TESTING MATERIALS AND APPROACH

The sodium bentonite was adopted as testing materials, and its mineral composition included Na-bentonite (48.8%), Ca-bentonite (14.1%), soda feldspar (28.3%), plagioclase feldspar (5.5%), quartz (2.5%) and calcite (0.8%). Two conditioning agents were tested, including hexametaphosphate and foaming agent. As a dispersant, the hexametaphosphate included sodium hexametaphosphate (73~74%), silica gel (4~5%), N-Dimethyltetradecylamine (18~19%), tetradecyl polyoxyethylene ether (2~3%), Lauryl chloride (2~3%), lauryl alcohol ether-8 (1~2%). Each sodium hexametaphosphate anion had 30-90 radical groups of PO3 (Liu et al., 1993), which may form a long chain similar to the polymer . The foaming agent included water (93~94%), dodecyl trimethyl ammonium chloride (3.0~3.5%, cationic surface active agent), sodium dodecyl sulfate (1~1.5%, anionic surface active agent), lauryl chloride (0.2~0.3%, non-ionic surface active agent), silicone oil (1~2%, foam stabilizer) and others (1.2~2.4%), and so it was a kind of anionic-cationic surfactant compound foaming agent.

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Fall cone approach was adopted to measure Atterberg limits of the sodium bentonite and its mixtures with conditioning agents. The dry bentonite was mixed with a distilled water content of 200% and then left there 24 hours. The foaming agent solution was made up with a concentration of 3%. It was pumped into foam gun with simultaneously air, and the foam expansion ratio and half-dissipation was determined to 21 and 16 min. The solution concentrations were made up 10% dispersant. The Atterberg limits were determined after the wet bentonite was mixed respectively with each conditioning agent. The conditioning agent ratio was defined as the ratio of conditioning agent mass to dry soil mass. In field, EPB shield may stay in tunneling state or be stopped for construction management or handling problems. Thus, the mixtures of the wet bentonite with dispersant stayed there for 0h, 5h, 29h, and 53h before the fall cone tests were carried out to measure their liquid and plastic limits.

The Zeta potential of the solution of the bentonite conditioned with the dispersant was determined using a nanoparticle size potentiometric analyzer (Malvern Zetasizer Nano ZS90). The dispersant was diluted to 1%, and then the bentonite was mixed with distilled water to obtain soil solution. Then, the soil solution was mixed with the dispersant solution. Thus, the final solution included the water from the initial soil solution and the dispersant solution. The ratio of bentonite mass to distilled water mass was control to 1:1000 for each soil solution. After each soil solution was mixed enough and stayed there for 5 min. The Zeta potential was tested on each specimen collected from the upper surface solution.

3. VARIATIONS OF ATTERBERG LIMITS DUE TO CONDITIONING AGENTS

3.1 Effect of Foam on Atterberg Limits

Figure 1 shows the variations of Atterberg limits with foam injection ratio (FIR). With an increase in the FIR, the liquid limit decreased but the plastic limit changed slightly. Generally, the influences of foam on liquid and plastic limits were limited. The main reason could be that the foam almost dissipated completely because of the strong water absorption of the bentonite. Additionally, although the anionic surface active agent in the foam can increase the electrostatic repulsion and decrease the connection forces among the soil particles, the existing of cationic surface active agent in the tested bentonite reduced the effectiveness of the anionic one. The amount of anionic surface active agent was low in the foam, as presented in the previous section. Thus, for the clay with high cohesion, the conditioning agent with only anionic-cationic surfactant compound foaming agent cannot have excellent conditioning effectiveness.

Figure. 1 Changes of Atterberg limits of the bentonite with foam injection con-tent

Figure. 2 Changes of Atterberg limits of the bentonite with waiting time after adding dispersant

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3.2 Effect of Flocculant on Atterberg Limits

Figure 2 shows the variations of Atterberg limits with waiting time after mixing bentonite with a dispersant content of 4%. The liquid limit reduced obviously but the plastic limit changed slightly after the waiting time increased from 0h to 5h. With more increase in the waiting time, the liquid limit did not increased significantly. Thus, the waiting time of 0h and 5h was selected to study the effect of conditioning agent, respectively for the shield tunneling and stopping states.

3.3 Effect of Dispersant on Atterberg Limits

Figure 3 shows the variations of Atterberg limits with dispersant content once after mixing the bentonite with dispersant. The liquid limit almost decreased with an increase in dispersant content, especially from 0% to 2%. However, the plastic limit and plasticity index did change constantly, since there was much water bound among the bentonite particles. The dispersant cannot respond enough with the bentonite to release the water in the short time. With the waiting time of 5h after mixing the bentonite with the dispersant, the liquid limit, plastic limit and plasticity index decreased with an increase in dispersant content, especially from 0% to 4%.

From Figure 4, it can be found that the change of liquid limit was obviously higher than that of plastic limit, indicating that the conditioning agent had more significant influence on shear strength for the soil with a water content closed to the liquid limit than with that closed to the plastic limit. In other words, the conditioning effectiveness was more significant for the soil with a higher water content. Zumsteg and Puzrin (2012) presented that the effectiveness of high dispersant polymer was more obvious for the softer soil (with higher water content). However, they did not study the effect of dispersant on the Atterberg limits.

In a summary, using the foam generated from the anionic-cationic surfactant compound foaming agent as conditioning agent, the sodium bentonite cannot be conditioned effectively for EPB shield tunneling mainly due to the easy dissipation of foam and the existing of cationic surface active agent. The dispersant can obviously decrease liquid limit and plasticity index, resulting in a decrease of shear strength. The purpose of soil conditioning for the bentonite is to reduce the connection forces among soil particles and the cohesion, and so the dispersant is more suitable to be used as conditioning agent for EPB shield tunneling in high-cohesion soil ground.

Figure. 3 Changes of Atterberg limits of the bentonite with dispersant content once after adding dispersant

Figure. 4 Changes of Atterberg limits of the bentonite with dispersant content at 5 hours after adding dispersant

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4. THE CONDITIONING CHEMICAL MECHANISM OF DISPERSANT

4.1 Variation of Zeta Potential due to Dispersant

Figure 5 shows the variation of Zeta potential of the sodium bentonite with the dispersant content. The Zeta potential decreased with an increase in dispersant content. Compared Figure 5 and Figure 4, it is found that both the Zeta potential (i.e., the negative charges increased) and liquid limit decreased with an increase of dispersant content. The decreases were more significant with an increase of dispersant from 0% to 4% than with that from 4% to 10%. The increase of Zeta potential indicated the increases of negative charge on the soil particles and the electrostatic repulsion among the soil particles, and so the Atterberg limits decreased. Thus, the decreases of Atterberg limits were induced thanks to the decrease of Zeta potential and increase of electrostatic repulsion.

4.2 Variation of Repulsive Energy due to Dispersant

Based on the DLVO theory, the dispersant increases repulsive energy among soil particles to achieve its dispersing function, and total potential energy among soil particles is equal to the summation of Van der Waals’ force potential energy and electrostatic potential energy. The total potential energy (VT/R) was calculated for the different soil solutions based on the DLVO theory (Novich, 1984; Qiu et al., 1993). Figure 8 shows the variation of the total potential energy (VT/R) among soil particles in unit diameter with dispersant content. The peak (Vmax/R) in the curve for each dispersant content is called as potential barrier, which will be overcome if the particles want to connect each other. As the dispersant content increased, the potential barrier increased, resulting in the more difficult agglomeration and easier dispersion of particles. Thus, the decrease of Atterberg limits due to the dispersant can be explained further with the repulsive energy. The phosphate ion in the hexametaphosphate used as the dispersant were stuck to the sides and corners, which has positive charge, of the flaky clay. The negative charge was increased, resulting in a decrease of the Zeta potential and increase of the repulsive energy. Thus, the soil particles were dispersed, and so the Atterberg limits was decreased due to the dispersant.

Figure. 7 Changes of Zeta potential with dispersant content

Figure. 8 Changes of repulsive energy among bentonite particles under dif-ferent dispersant contents with particle distance

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5. CONCLUSIONS

(1) The foam generated from the anionic-cationic surfactant compound foaming agent had limited effect on the Atterberg limit of the sodium bentonite, since the bentonite had strong adsorption capacity of water to make foam dissipate easily and the existing of canonic surface active agent reduced the dispersing function of anionic surface active agent to the clay particles. Thus, the foam generated from the anionic-cationic surfactant compound foaming agent is not good conditioning agent for EPB shield tunneling in the clayey ground.

(2) The liquid limit and plasticity index decreased with an increase of dispersant content, especially from 0% to 4%. Thus, the dispersant is more suitable to be used as a conditioning agent to avoid clay clogging during EPB shield tunneling.

(3) With an increase of dispersant content, the Zeta potential decreased (i.e., the negative charges increased) on the bentonite particles. It can be found that the change of Zeta potential had good correlation with that of liquid limit and plasticity index with dispersant content. With an increase in dispersant content from 0% to 4%, both of them decreased more significantly. The potential barrier (i.e., the repulsion energy) increased with an increase of dispersant content, resulting more difficult agglomeration and easier dispersion of particles.

ACKNOWLEDGEMENTS

The financial supports from National Key R&D Program of China (No. 2017YFB1201204) and National Natural Science Foundation of China (No.51778637) are acknowledged and appreciated.

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