Improvement of clayey soil by electro

Transcription

1 International Conference on GEOTECHNIQUES FOR INFRASTRUCTURE PROJECTS 7 th & 8 th bruary 17, Thiruvananthapuram Improvement of clayey soil by electro osmosis Reshma R J Department of Civil Engineering Marian Engineering College Trivandrum, India reshmarj591@gmail.com Mrs Tara Leander Assistant Professor Department of Civil Engineering Marian Engineering College Trivandrum, India Abstract Electro osmotic flow is caused by the Coulomb force induced by an electric field on net mobile electric charge in a solution. The electro kinetic phenomenon occurs when a direct current is applied to fine grained saturated soil. Electro osmosis is a ground improvement method for soft clayey soil to increase soil shear strength and accelerate consolidation. Electro osmosis is being tested in laboratory and found out it is a success in the places where conventional stabilization and consolidation process can t be used like underneath building foundation, roads, railways, pipelines, tunnels etc.. due to disturbance and more time consumption. A testing apparatus was set to investigate the efficiency of electro osmosis on fine grained soil by using different electrodes of uminium, Iron, Stainless and. A constant voltage of 1V was applied for days and a reverse polarity of 1V was applied for days. The dewatering process and flow of current were monitored and the shear strength was measured before and after electro osmosis on both and sides. The results revealed that the strength improvement and amount of water removed depends on type of electrode used. It has been seen that the ph, electrical conductivity, index properties changed during the test. Keywords- electro osmosis, electrode. INTRODUCTION l civil engineering constructions are carried out on soil. The behavior of structures thus depends on the properties of the soil on which they are constructed. For structures to be safe and sound, they are to be built on good soils. Soft clay deposits, which are inherently very low in strength and very high in compressibility, are widespread in coastal and low land regions. In recent years the scale of design in such soft deposits has increased tremendously as a result of extensive urbanization and industrialization. Conventional remediation methods have been known successful in minimizing several damages, however, they are time-consuming and may be difficult to implement in some existing structures. In this regard, electrochemical or electro-kinetic treatment method can be used as an alternative soil treatment method for remediation of those deficiencies underneath building foundations, roads, railways or pipelines. The use of this technique involves an approach with minimum disturbance to the surface while treating subsurface contaminants and improving the engineering characteristics of subsurface soils. The use of electro osmotic treatment which is a comparatively new methodology is being investigated in some parts of the world for the potential application through several laboratory experiments to verify the versatility and effectiveness of this technique in practice as a viable in-situ soil remediation and treatment method. I. EXPERIMENTAL INVESTIGATION A. Materials used The soil samples collected from water logged agricultural areas of appuzha district, Kerala has been used for conducting electro osmosis. The properties of the soil were determined using the standard test procedures in the laboratory as per IS specifications. The properties of clay have been given in Table 1 Table 1 Properties of clay Parameters Initial water content (%) Specific gravity Liquid limit (%) Plastic limit (%) Plasticity index Shrinkage limit (%) Sand (%) Silt (%) Clay (%) Optimum moisture content (%) Maximum dry density (g/cc) Value

2 B. Experimental setup The layout of apparatus in which the samples were placed is shown in the Fig. 1. The apparatus was made by ply wood and the sample tube of cm height and 6cm diameter itself was used as electro osmotic cell since it was hydraulically impermeable and not electrically conductive. Another advantage of using the sample tubes as the electro osmotic cell was that the samples did not need to be transferred to another cylinder which would have increased the risk of disturbing the samples before treatment. The apparatus consisted of a wooden support structure that held the sample tube in place. Filter stones were placed at the top 5 mm and the bottom 1 mm. Before the sample was placed into the apparatus, approximately cm clay was pressed out from the bottom of the tube. This was done to make room for the electrode and the filter stone at the top of the sample tube. The electrodes were made of perforated metals such as aluminium, copper, iron and stainless steel of 5.3 cm diameter. The electrodes were perforated allowing water and gas to pass through them to ensure that build up of gas or pore pressure do not affect the treatment. A variable direct current power supply of 1V is used to provide the samples with a direct current. Fig. 1 Layout of electro osmosis setup C. Sample preparation and testing procedure The soil samples were prepared in a tray by mixing the soil of 7g. The samples were mixed to particular water content close to the liquid limit of the soil samples. The cylinder was filled in three layers. The electrodes were inserted in the suitable locations i.e., on top and bottom of cylinder. Porous stones are provided at both sides for the drainage of water collected near the region. The setup is illustrated in Fig.. Current was passed at a potential gradient of 5 V/m for a period of four days, until most of the water was drained. Fig. Test setup of electro osmosis Then a reverse polarity of 5 V/m was applied for another two days till no further drainage occurs. After the process was complete, the mould was taken out of the tank and subjected to water content determinations, index property determinations and unconfined compression tests. A Milliammeter was connected to measure the flow of current II. RESULTS AND DISCUSSIONS Various tests were conducted in accordance with the standard procedure to determine the amount of dewatering and strength characteristics of soil after electro osmosis. The results obtained from various tests are discussed below

3 A. Physical properties Table Index properties of soil after electro osmosis Electrode Untreated soil Liquid limit(%) Plastic Limit(%) Plasticity index(%) uminium Iron Table shows the change in physical properties of soil after electro osmosis with different electrodes. The liquid limit decreases for all the electrodes, more in copper and least for stainless steel while the plastic limit decreases by % and is high in uminium. The plasticity index got decreased from 3.8% to 9.9% and is maximum for copper. The reactive electrodes uminium, and Iron show high difference than inert electrode of Stainless steel. For copper electrode the soil classification changes from high plasticity silt (MH) to intermediate plasticity silt(mi) according to Unified Soil Classification system. For instance electro osmosis can change the soil plasticity. B. Unconfined compressive strength The water content before electro osmosis was near to liquid limit where shear strength was negligibly low. After electro osmosis the shear strength of the soil has been increased considerably because of dewatering due to electro osmosis and the electrolysis of the electrodes causes ions get attached to the soil elements also increases the shear strength. The Unconfined compression test was done to find out the strength of soil both on and side. A greater shear strength of.6 kg/cm was obtained at side of electrode and the least is shown at side of aluminium and stainless steel electrode. The improvements in shear strength compared to untreated soil is shown in the Fig. 3. UCS (kg/cm ).6.. Cu Soil Fig. 3 UCS of soil at and side after electro osmosis C. Water content The water content, w, decreased through the whole length of sample. The percentage dewatering at each day of electro osmosis is shown in the Fig.. About 38% % of water is removed during treatment. The dewatering seen more at side than side in every electrode type and the dewatering seem to be more in electrode. The amount of water dewatered at the sixth day of treatment at the side of uminium, Iron, and electrodes were 5.5, 16.6, 16.9, 36.5 ml and at the side were 195.5, 17, 18.97, 16.9 ml respectively. The amount of dewatering was less in electrode and high in copper electrode. The amount of dewatering during the days of treatment is shown in Fig. 5. Water removed (%) 8 6 Fig. Rate of dewatering during electro osmosis Water removed (ml) Days of treatment Fig. 5 Amount of water removed during electro osmosis D. Applied current and voltage Cu The applied current and voltage are functions of the electrical conductivity of the electrodes and the enveloping formation and the geometry of electrodes (thickness. Spacing and dimension). They also depend on contact resistance between the power cable and the electrodes. The flow of current in most of the electrode seems to be more at the 5 th hour of treatment and decreased considerably. This is due to the increase in resistance due to the dewatering through electro osmosis. The magnitude of voltage loss depends up on soil properties, applied voltage, water resistance and electrode material. The Fig.6 shows the variation of applied current with time. 3

4 Current (A) Fig. 6 Variation of current with time E. Electro osmotic permeability Fig. 7 shows the electro osmotic permeability for various electrodes. The amount of out flow of water from the soil during electro osmosis can be defined by the following relationship (Mitchell, 1993) Q = K e E A (.1) Where q is the electro osmotic flow rate, A is the cross sectional area normal to the direction of flow, E is the electric field intensity and Ke is the electro osmotic permeability. The results shows that the values of K e ranges from m /SV for 1V Electro osmotic permeabili ty (k e ) (m /Sv) 1-9 Fig. 7. Variation in electro osmotic permeability with each electrode F. ph of soil Time (minutes) Days of treatment The soil used for testing has a ph of 6.8 which is neutral neither acidic nor alkali nature. As the electrodes uminium Iron and are reactive metals and solid conductor the process of oxidation and reduction may occur. Oxidation occurs at electrode and reduction takes place at side. Reduction is by definition is the gain of electrons where as oxidation is by the loss of electrons. In addition to oxidation and reduction takes place at and, electrolysis of water also takes place at and. After electro osmosis the electrode at seemed to be corroded and at the electrode seem to be thicker because of atom deposit at the. The ph of aluminium electrode at side is seemed to be more than all electrodes. And also the ph at side of Iron electrode is seemed to be less in all electrodes. Fig.8 shows the variations of ph of soil after treatment at and with untreated soil. Fig. 8 Variation in ph at and sides III. CONCLUSIONS One dimensional electro osmosis experiments were conducted with varying electrode materials including three reactive metals uminium, Iron, and an inert material Stainless steel. After electro osmosis treatment the water content, Atterberg s limits, ph of soil, Shear strength and variation of current were measured. The results were obtained as For all the four types of electrodes the water content, liquid limit and plasticity index seem to be decreasing and an increase of plastic limit and shear strength. This is more in copper electrode than all other electrodes. The variation of current seems to be decreasing by days of treatment. The electro osmotic permeability also monitored and it seemed to be more in electrode. ph The ph of soil were monitored during testing and is high at side due to reduction of ions and less in side due to oxidation of ions. The results from the experiment show that the electro osmosis can drain and strengthen the clays, in line with numerous other studies and field applications. Overall it can be concluded that electro osmosis is an effective method in dewatering of soil where conventional methods can t be done. It also reduces the time consumption References [1] Abdul Azad Sahib, I Bushra,(1), Suitability of electro osmosis in Kuttanadu clay, Proceedings of Indian Geotechnical Conference 1, 18- [] Derek V. Morris, Sydney and Cladwell, (1985)., Improvement of sensitive silty clay by electroosmosis, Canadian Geotechnical Journal,, 17-.

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