Electroosmotic Chemical Treatment Research Articles

Experimental investigation of influence of calcium ion (Ca2+) and pH on the shear strength of Taipei silty clay

ABSTRACT Electroosmotic chemical treatment is a technique for improving the strength of soft soil. This study examined the effects of injection duration, Ca2+ions concentration, and pH distribution on the shear strength of Taipei silty clay. Experimental tests were conducted by injecting calcium chloride (CaCl2) solution into Taipei silty clay during electroosmosis for various durations. After testing, the water content, pH, Ca2+ ions concentration, injection volume, drainage volume, and cone resistance were measured. The results indicated that the injection volume, drainage volume, Ca2+ ions concentration, and strength of the silty clay were low compared with the same measurements for kaolinite because Taipei silty clay consists mainly of illite, which results in the soil having a lower coefficient of electroosmotic permeability. The treatments of 12–48 h were insufficient for improving soil strength. However, the 120-h 1.5 M C120 injection treatment resulted in a moderate increase in soil strength due to consolidation and Ca2+ ions precipitation. By comparing our results with those of studies on kaolinite, we established a suitable injection procedure, which includes solutions for treatment duration and concentration of CaCl2 solution (i.e. 1.5 M C48, 0.2 M C120, and 0.75 M C120).

Effect of anode condition on electro-osmotic consolidation combined with vacuum preloading

Electro-osmotic consolidation combined with vacuum preloading is a potential technology to dewater the soft soils and slurry. The anode condition has a significant effect on dewatering efficiency during operation, which is not adequately addressed. In this study, laboratory tests were conducted to study the effect of different boundary conditions of anode on the soil-improvement result using the combination of electro-osmotic consolidation and vacuum preloading, then to obtain the best combination mode. During the physical modeling tests, the soil mass was treated by vacuum preloading first and then further processed by the combined method. Three model tests were conducted to study the effect of different vacuum conditions on electrode, while CaCl2 solutions were injected at the anode in another two tests to study the effect of the electro-osmotic chemical treatment. The discharge of water and electrical current were measured during the tests, and the soil strength were measured after the implementation of the tests. The results indicated that the combined method was more effective to remove pore water from the soil mass compared with the traditional vacuum preloading. Although the vacuum preloading and electro-osmosis drove the water flow in the opposite direction near the anode, the vacuum pressure applied on the anode prevented the soil cracks, which increased electrical current obviously and removed 15% more pore water. The injection of CaCl2 solution at the anode showed slight effect on dewatering process, however, the bearing capacity was increased significantly.

Performance Improvement in Pile Anchor System for Deep Foundation Excavation Using Electroosmotic Chemical Treatment

Anchoring force is vital to ensure the acceptable performance of a pile anchor system when supporting deep foundation excavation. The soft soil has several physical properties, such as low shear strength, high water content, large void ratio, and high flowability. Traditional grouting and anchoring techniques have demonstrated technical limits to deal with these soil properties in engineering projects, and accordingly, the anchoring force in the pile anchor system is difficult to meet design requirements. This paper conducted an experimental investigation on the performance improvement in a pile anchor system using the electroosmotic chemical treatment method, with an emphasis on the deep foundation application. Experimental tests and field studies were designed to enhance anchor capacity of a pile anchor system using self‐designed devices. The laboratory experiments utilized a simplified anchor system in which anchors were designed as the electrodes to conduct the electroosmotic chemical treatment and consolidate the soft marine soil collected from the project site. In addition, static load tests were conducted on the tested soil to measure the anchoring force. Finally, parametric analyses were performed to investigate effects of several parameters on anchoring force in terms of the ultimate pull‐out capacity of the anchor, identifying critical parameters for the field study. Based on laboratory test results, field studies were carried out in the Yingkou city. The results from field studies were compared with laboratory test results to validate feasibility of electroosmotic chemical treatment for a pile anchor system.

Cohesive Strength Improvement Mechanism Of Kaolinite Near the Anode During Electroosmotic Chemical Treatment

AbstractInjection of CaCl2 and Na2SiO3 solutions into clay suspensions during electroosmosis often improves the cohesive strength of clays near the anode and cathode, whereas the cohesive strength of clays between the electrodes remains weak. Although the main improvement mechanism for the cohesive strength of clays near the cathode was demonstrated to be a pozzolanic reaction (formation of calcium silicate hydrate cement), the mechanism of improved cohesive strength near the anode is still not understood. The objective of the present study was to investigate the mechanism for the improvement of cohesive strength near the anode and, thus, make it possible to determine a way to enhance the range in improvement using kaolinite as the test clay. The test was performed by first injecting CaCl2 solution during electroosmosis until the optimum volume of CaCl2 was attained. This was followed by treatment with Na2SiO3 solution for different lengths of time. The results indicate that the anode region after treatment was acidic (pH = 4) because the electrolysis of water causes acidification near the anode. As Na2SiO3 solution was injected through the anode, the mechanism of cohesive strength improvement of the treated clay near the anode was attributed to the silicic acid polymerization effect provided by the Na2SiO3 solution. The silicic acid may link the clay particles together to form a gel network in a low pH environment. The clay gel network structure developed rigidity as the water content was reduced. In addition, as the volume of injected Na2SiO3 solution was increased, the cohesive strength near the anode also increased.

A novel electroosmotic chemical treatment for improving the clay strength throughout the entire region

Abstract Electroosmotic chemical treatment can be used to improve clay strength. However, this improvement is often limited to regions near the anode and/or cathode, while the areas between the electrodes remain weak. The objective of this study was to develop a suitable method to improve the clay strength throughout the entire sample. The characteristics of the test clay were studied by varying the concentration of the calcium chloride (CaCl 2 ) solution, injecting a potassium hydroxide (KOH) solution to neutralize the acidic conditions in the anode area, using a fresh sodium silicate solution in the anode compartment, and constantly injecting deionized water after injection of the sodium silicate solution. The results indicated that an increase in the concentration of the CaCl 2 solution reduced the efficiency of injecting the sodium silicate solution in the subsequent treatment stage. The use of a fresh sodium silicate solution may increase the efficiency of injecting the sodium silicate solution into clay, thereby extending the improvement range. When deionized water was injected constantly after injection of the sodium silicate solution, the Ca 2 + ions in the clay migrated toward the cathode during treatment and, simultaneously, a highly alkaline condition formed near the cathode, leading to an enhancement of the pozzolanic reaction and a significant increase in the clay strength near the cathode. Based on these results, an ECT test was conducted using a fresh sodium silicate solution and constant injection of deionized water. The results indicated that the strength of the entire clay sample was improved.

A study on the modification of electroosmotic consolidation theory using electric potential distributions

Electroosmotic chemical treatment is used for ground improvement. By applying electroosmotic characteristics, water discharge through soil pores is induced; concurrently, electricity is used to facilitate the uptake of a chemical solution by the soil, thereby achieving ground improvement. This approach involves complex electrochemical mechanisms, particularly near the anode and cathode; thus, various electrochemical reactions, including electrode corrosion, electrolysis, hydrolysis, temperature increases, deposition, ion exchange, and cementation, will occur, producing losses in the applied electric potential. Previous research has demonstrated that these problematic losses in potential during electroosmosis are particularly large near the anode and cathode. However, traditional electroosmotic consolidation theory is derived from the assumption that no loss in applied potential will occur, and therefore, this theory does not address the observed issues. In this study, the results of a year of laboratory tests are presented and are placed in the context of the relevant literature; these results indicate that there are many differences between actual potential distributions and the assumptions of traditional electroosmotic consolidation theory due to the existence of losses in applied potential. These results from data compilation and experimental tests are utilized to establish a potential distribution correction model curve of electroosmotic consolidation.

Electro-Osmotic Chemical Treatment for Marine Clayey Soils: A Laboratory Experiment and A Field Study

Abstract An electro-osmotic chemical treatment (ECT) method for marine clayey soils was investigated in this study using self-designed laboratory equipment. According to related studies, electrolytes are an important factor for improving the bearing capacity of marine clayey soils. This study focused on improving the mechanical properties of marine clayey soils by using ECT and analyzed the effects of using a salt electrolyte solution on soil strength. The design of the ECT procedure was also investigated in this study. In addition, this study considered various practical issues, including the current, voltage, and bearing capacity of soils before and after testing. The laboratory testing results demonstrated that the ECT technique is an effective method for improving marine clayey soils and that the concentration of the salt solution obviously influences improvements of soil strength. Furthermore, based on laboratory results, a field study was performed in the Dong-gang Business District, Dalian City. The results from the field study were compared with those from the laboratory, and the feasibility of using ECT for marine clayey soils was further analyzed. An empirical formula for the strengthened zone was also developed for strengthening marine clayey soils to meet construction requirements. This study provides some references and suggestions for the practical application of the ECT technique for marine clayey soils.

Mechanism of soil cementation by electroosmotic chemical treatment

The injection of calcium chloride solution through the anode during electroosmosis has been shown to be effective in strengthening soft clay. The objective of this study was to investigate the mechanism responsible for this improvement using a kaolinite. Several types of samples were prepared, including a sample directly taken from the clay near the cathode after the completion of an electroosmotic chemical treatment test and samples of kaolinite mixed with different concentrations of calcium ion solutions and cured for various times. A mechanical strength test and chemical and mineralogical analyses were performed to examine the properties and compositions of the samples. Results showed that some crystalline products could be formed in an alkaline environment of pH around 10 or after a short treatment time. The products filled some of the clay voids, causing a moderate increase in clay strength. In a strongly alkaline environment, calcium silicate and/or aluminum hydrate would be formed with a longer curing time, leading to a substantial increase in clay strength. Therefore, raising the concentration of the injected calcium chloride solution, enlargement of the applied electric voltage and extension of treatment time could result in greater clay strength.

A study of the effects of electrode spacing on the cementation region for electro-osmotic chemical treatment

Previous laboratory studies have shown that injection of the calcium chloride solution can cause soil to be cemented near the cathode during electro-osmosis. In order to predict the size of the cementation region, a series of electro-osmotic chemical tests was conducted using different electrode sizes, electric voltages, and electrode spacing. After the tests, the concentration of calcium ions and pH values over the entire sample were analyzed and the cementation near the cathodes was inspected. Results show that the contours of the Ca2+ concentration, contours of the pH values and the cementation region were consistent to each other. Cementation often occurs near the cathode in an alkaline environment where the pH values are higher than 8.3 to 8.7. The cementation region was highly related to the applied voltage and electrode spacing but slightly related to the size and shape of the cathodes. When the electric field intensity was amplified by N times, the radius of the cementation region was increased by N times. This relationship can be used to predict the cementation region around the cathode and to design a rational spacing between two cathodes for multiple pairs of electrodes.

Electro-osmotic chemical treatment of clay with interbedded sand

Electro-osmotic chemical treatment is a ground improvement method that injects chemical solutions into the soil during electro-osmosis to increase soil strength. However, previous studies have shown that the electro-osmotic chemical treatment method is inadequate when applied to clay with interbedded sand. The reason was that the injected chemical solutions may leak from the interbedded sand during electro-osmosis, resulting in poor improvement of the clay. In this study, a small-scale test cell (0·23 m wide, 0·44 m long, 0·32 m deep) with a pair of electrodes were designed to develop an appropriate electro-osmotic chemical treatment process for clay strata with sand layers. A clay stratum overlying a sand layer was used to make the conductivity of the sand layer relatively high. The results show that the permeability coefficient of the interbedded sand could be effectively reduced to 10−5–10−6 m/s by first injecting sodium silicate into the sand by pressure, followed by injecting calcium chloride by electro-osmosis. The calcium chloride could therefore effectively be flowed in the clay during electro-osmosis after decreasing the permeability coefficient of the interbedded sand, resulting in strength improvement of the clay. The strength of the clay was increased by up to 20 times the original strength near the cathode region.

A novel technique of harmonic waves applied electro-osmotic chemical treatment for soil improvement

Previous studies have shown that the application of direct current (DC) during electro-osmosis with injection is effective in strengthening soft clay. However, the DC can be obtained from an alternating current supply by use of a current-switching arrangement called a rectifier and a filter. In this study, a novel technique of using the half wave rectification and full wave rectification (Harmonic waves) during electro-osmosis with injection was developed to expand the different types of power supply and to achieve an economic and practical goal. Owing to the reduction in the filter component for the half wave rectification and full wave rectification, the price of a power supply may be reduced. Results of this study indicate that an effective improvement was observed in the circular experimental cell by using a half wave rectification or full wave rectification on the soil's mechanical properties such as CPT. In particular, the improvement effect by using the full wave rectification is close to that using the rectification and filter in terms of region of improvement and average cone resistance. The confirmative experimental results also show that the improvement in a rectangular experimental cell by using the full wave rectification is close to that using the rectification and filter in terms of region of improvement and average cone resistance. Hence, the technique of the full wave rectification may be economically and practically applied in electro-osmotic chemical treatment for soil improvement.

A novel electroosmotic chemical treatment technique for soil improvement

Previous studies have shown that injection of chemical solutions through the anode or cathode during electroosmosis is effective in strengthening soft clay. However, the region of improvement is mostly limited to the anode or cathode region. A novel technique of installing a relay pipe between the anode and the cathode was developed to expand the region of electroosmotic improvement. The natural Taipei silty clay, containing 12% clay, 86% silt and 2% sand, was used in this study. Results of this study found that electroosmosis with injection through the relay pipe was superior to that with injection through the anode, nearly 1.5 times in terms of cementation area and 2 times in terms of average cone resistance. With injection through both the anode and the relay pipe, the cementation area nearly covered the entire specimen and its average cone resistance could be raised to nine times more than that of the untreated soil. Moreover, for injection through both the anode and the relay pipe, the increase of injection times of the sodium silicate solution had a better improvement effect, compared with the increase of injection times of the calcium chloride solution.