EFFECT OF SOIL COMPOSITION ON ELECTROKINETIC GROUTING

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1 IGC 2009, Guntur, INDIA EFFECT OF SOIL COMPOSITION ON ELECTROKINETIC GROUTING L.S. Thakur Lecturer, Applied Mechanics Department, Faculty of Tech. & Engg., M.S.U., Vadodara , India. V.H. Padhiar P.G. Student, Applied Mechanics Department, Faculty of Tech. & Engg., M.S.U., Vadodara , India. D.L. Shah Professor, Applied Mechanics Department, Faculty of Tech. & Engg., M.S.U., Vadodara , India. ABSTRACT: Sometimes it is difficult to find an ideal place for construction purpose, but if it cannot be avoided, than, such cases necessitates use of various ground improvement techniques. Electrokinetic grouting is one such process that has shown a great potential for remediating soil with low shear strengths as well as low permeability and strengthening of deep seated soil layers under the existing structure, using combined effect of electric, chemical and hydraulic potential for treatment. The work presented in this paper is aimed for understanding various factors that play a role in grouting various types of soils using electrokinetics such as soil composition, grout concentration and voltage gradient using laboratory scale models. Maximum increase in strength and decrease in permeability was observed for grout with 25% sodium silicate. The study of effect of soil composition showed maximum percentage improvement in shear strength for black cotton soil samples. 1. INTRODUCTION If one goes to see, a very small part of the total available land is available for erection of structures and for a country with a population of over 100 million, land area is becoming scarce day by day. So a need arises that one should utilize the lands for construction; residential and industrial mainly; which were considered unfit for structural erections and were rejected earlier. Moreover, the numerous existing structures are also needed to be treated for their foundation problems such as low bearing capacity of soil, etc. as they were built on soil with poor geotechnical properties. For this purpose, stabilization of soil needs to be carried out. Although many different in-situ soil stabilization techniques are available, electrokinetics offers a number of advantages over other methods. Electrokinetics is a process that has shown a great potential for remediating soil with low shear strengths and strengthening of deep seated soil layers under the existing structure. 2. LABORATORY EXPERIMENTS The present investigation was performed on three different soils; the first was procured from the Institute (Silty soil), the second soil procured from Karjan (Black cotton soil) whereas the third sample was procured from Hazira, Surat (Sandy Silt) (Table 1). Three laboratory scale models were prepared using 10 mm waterproof plywood sheet having dimensions as 60 cm length, 30 cm width and 20 cm height. PVC flooring material was used to prevent the wood from being corroded and soiled due to various chemicals to be used. Acrylic sheet 1.5 mm thick was used to create partitions for the chemical grout chambers and sand filter chambers. Two 10 mm diameter and 30 cm long carbon electrodes were placed in each anode and cathode compartments, connected in series by copper wire, and finally connected to an AC-DC converter unit (Fig. 1). Table 1: Property of Various Soils Properties Silty soil Black cotton soil Sandy silt Specific gravity Liquid limit (%) Plastic limit (%) NP Shrinkage limit (%) Free swell (%) NS OMC (%) MDD (gm/cc) Coefficient of Permeability (cm/s) Sodium Content (mg/kg) The soil mass was compacted in the models using hand compaction at 85% of their respective maximum dry densities keeping the moisture content on wet side of OMC. Duration of all tests was kept as. The voltage was kept 25 volts DC and concentration of sodium silicate (Na 2 SiO 3 ) at cathode was kept 25% and concentration of calcium chloride (CaCl 2 ) at anode was kept 2%. Readings for voltage, current, ph, ambient temperature, temperature at 392

2 surface and at depth etc were being noted at regular interval of time. Anode compartment filled with calcium chloride Graphite electrode Sand filter A.C.-D.C. converter Graphite electrode Sand filter Cathode compartment filled with sodium silicate Fig. 1: Schematic Diagram of Laboratory Model 3. RESULTS 3.1 Silty Soil Figure 2 shows variation of ph along the length of the soil sample noted near anode, at mid-section and near cathode for silty soil sample. During the initial days, ph shows values on the alkaline side reducing as time progresses to a minimum value of 8 but increases again when the consumption of the solution increased after the second week. Then it stabilizes and remains more or less around 11 throughout the experiment. Reading for temperature along the length of the sample as well as ambient temperature was noted at a fixed time every day (Fig. 3). Figure 4 denotes the variation of voltage at different sections along the length of the sample. During the first two days, high voltage was noted. But then it gradually started to decrease and thus stabilized after two weeks which suggest that the resistance of the mass has almost achieved a constant value suggesting homogenous flow of grout in the soil. Figure 5 depicts variation of current and current density with respect to time. The current density was unstable during the first week, but then stabilized and remained constant again suggesting uniform flow of grout. Voltage in Volts Fig. 4: Variation of Voltage at Different Sections along Length of Sample ph Fig. 2: Variation of ph along the Length of Sample Temperature in Degree C Time in days Fig. 3: Variation of Temperature at Surface and Ambient Temperature with Time Current Density (in Amp/sq.cm) Current Density 0.3 Current Fig. 5: Variation of Current and Current Density The shear strength of the virgin soil with same moisture content was kg/cm 2 before the start of the electrokinetic grouting experiment. It started increasing because of gel formation of chemical grout into the soil, flowing due to electro-osmosis effect under the influence of applied electric field. The average shear strength was noted 3 times more than the original strength after. Figure 6 shows the variation in shear strength of soil along the cross section of the model with increase in duration of experiment. The maximum shear strength obtained was at the cathode section, measured as kg/cm 2. Current (in Amp) 393

3 Shear strength in kg/sq cm Fig. 6: UCS of Silty Soil at Various Time Duration (Silty Soil) comparison for permeability which shows noticeable reduction in permeability of soil due to grouting. Permeability reduction is more at mid section than at anode and cathode similar to sandy soil as mentioned above, with average reduction of 81%. Shear strength in kg/sq cm Three undisturbed samples collected from near anode, at mid section and near cathode were tested for permeability. Figure 7 compares permeability of soil after electrokinetic grouting experiment with that of virgin soil. It showed noticeable reduction in permeability of soil due to grouting. Permeability reduction is more at mid section than near anode and near cathode. On an average, there was 76% reduction in permeability. This result supports flow of calcium chloride and sodium silicate into the soil and formation of gel due to applied electric potential. 1.8E-6 1.6E-6 1.4E-6 1.2E-6 1.0E-6 8.0E-7 6.0E-7 4.0E-7 2.0E-7 0.0E+0 Fig. 7: Permeability of Soil Before and After Electrokinetic Grouting (Silty Soil) 3.2 Sandy Silt Figure 8 shows comparison of shear strength of sandy silt at various intervals of time. It shows nominal increase in strength at the end of the first week, but increased gradually, with the maximum shear strength showing about 72% increase with an average value of 63%. Figure 9 shows 0.00 Fig. 8: UCS of Silty Soil at Various Time Duration (Sandy Silt) 2.5 E-2 2.0E-2 1.5E-2 1.0E-2 5.0E-3 0.0E+0 After Electrokinetic Grouting Fig. 9: Permeability of Soil Before and After Electrokinetic Grouting (Sandy Silt) 3.3 Black Cotton Soil The shear strength of black cotton soil shows a considerable increase after electrokinetic grouting with a major part of it being gained in the last two weeks, with an average increase of about 11 times (Fig. 10). The comparison of permeability of soil before and after electrokinetic grouting experiment is shown in Figure 11. Average decrease in the initial permeability of soil was found to be 24.41%. Here the overall decrease in permeability obtained was less compared to the other two experiments. (Fig. 11) 394

4 Shear strength in kg/sq cm 2 Fig. 12: Comparison of UCS for Different Soils Fig. 10: UCS of Silty Soil at Various Time Duration (Black Cotton Soil) After Electrokinetic Grouting Fig. 13: Comparison of Permeability for Different Soils Fig. 11: Permeability of Soil Before and After Electrokinetic Grouting (Black Cotton Soil) Figure 12 shows the comparison of UCS for all three soils selected for the study, the maximum increase in the shear strength was observed when black cotton soil was treated with electrokinetic grouting technique. By comparison, 75% of the total increase in shear strength was in black cotton soil compared to other two soils i.e. silty soil and sandy silt. The minimum increase in shear strength (0.6 times) which was 4% by comparison, was noted for sandy silt. The comparison of permeability for the same is shown in Figure 13. The maximum decrease in the permeability was observed for sandy silt soil which is 81.14% average and 45% compared to the other two soils. The minimum decrease in permeability comparatively was noted when black cotton soil was treated with the electrokinetic grouting technique which was 24.41% in average and 13% comparatively. This may be due to the highly expansive nature of the black cotton soil. Figure 14 shows the comparison of the sodium content which has increased many folds indicating proper grouting of all soils used. Fig. 14: Comparison of Sodium Content for Different Soil after Test 4. CONCLUSIONS It can be concluded from the work that electrokinetic grouting is very effective method for grouting soils with very low permeability. It is also seen that it work more efficiently when used for clayey types of soils. The strength increase is maximum when the selected grout mix used with black cotton soil. The decrease in permeability for black cotton soil is relatively less since the permeability before test itself is very low. Fine silty sands can be also easily grouted with a considerable decrease in their permeability as well as appreciable increase in the strength. REFERENCES Alshawabkeh A.N. and Açar Y.B. (1996). Electrokinetic Remediation II: Theoretical Model, ASCE Journal of Geotechnical Engineering, 122(3): Esrig M.I. and Gemienhardt J.P. (1967). Electrokinetic Stabilization of an Illite Clay, Journal of the Soil Mechanics and Foundations Division, Proceedings of the American Society of Civil Engineers, 93(SM3): Lo K.Y., Inculet I.I. and Ho K.S. (1991). Electroosmotic Strengthening of Soft Sensitive Clays, Canadian Geotechnical Journal, 28: Micic S., Shang J.Q. and Lo K.Y. (2001). Electrokinetic strengthening of a Marine Sediment Using Intermittent Current, Canadian Geotechnical Journal, 38:

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