Study on the Effect of Low-Temperature Anode Filled with FeCl₃ Solution on Electro-Osmotic Reinforcement of Soft Clay

Abstract

In order to study the effect of FeCl₃ solution on the current, soil pH value, drainage volume, anode potential difference, energy consumption, and resistivity of electro-osmosis consolidation of soft clay with different moisture contents under a low temperature environment, we carried out 31 sets of model tests under different working conditions by using a self-made device and an improved Miller Soil Box. The test results showed that, under the same conditions, although the energy consumption coefficient of electro-osmosis had little change in the low temperature environment, the current, displacement, and electro-osmosis coefficient were obviously reduced, and the resistivity and anode potential difference was greatly increased, indicating that the low temperature environment reduces the efficiency of electro-osmosis of soft clay. After the anode was filled with FeCl₃ solution, the current, water displacement, and electro-osmotic coefficient were clearly increased; the anode potential difference and electro-osmotic energy consumption coefficient were clearly decreased; and the resistivity was reduced to a certain extent, which indicates that the anode filled with FeCl₃ solution is very beneficial in improving electro-osmotic efficiency and in saving energy. The electro-osmotic discharge after the anode filled with FeCl₃ solution at low temperature was clearly higher than that after the anode was filled water at normal temperature, indicating that the effect of the anode filled with FeCl₃ solution on electro-osmosis is greater than that of temperature. In the process of electro-osmosis of soft clay, the better the electro-osmosis effect, the smaller the pH value of the anode and the larger the pH value of the cathode. The pH value of the cathode and anode can be used as one of the indexes to reflect the electro-osmosis effect.

1. Introduction

Due to the low bearing capacity and high compressibility of soft clay, soft clay foundation is often encountered in engineering construction, and it needs to be reinforced. Compared with the commonly used soft clay foundation reinforcement methods, the electro-osmotic consolidation method has the advantages of low noise, good reinforcement effect, and relatively simple construction equipment. The electro-osmotic consolidation method is considered to be a soft clay reinforcement method with good development prospects. However, there are many influencing factors of electro-osmosis. At present, many influencing factors of electro-osmosis have not been fully considered and studied, and the efficiency of electro-osmosis in strengthening soft clay can be further improved. Therefore, it is of great significance to further study the influencing factors of electro-osmosis.

At present, scholars at home and abroad have conducted some related research on the influencing factors of electro-osmosis, including the influence of electrode materials [1,2,3,4], grouting solution [5,6,7,8,9,10], external load [11,12], electrifying mode [13,14,15,16,17,18,19,20], soil properties [21,22,23,24,25], and other factors on electro-osmosis consolidation. Lianwei Ren et al. [5,6,7] verified through model tests that Na₂SiO₃ and CaCl₂ solutions as grouting solutions can effectively improve the drainage rate of soft clay during electro-osmosis. Qingpeng Feng et al. [8] carried out the model test of soft clay reinforced by CaCl₂ solution. The results show that CaCl₂ solution can effectively control the decreasing trend of current and improve the shear strength of soil samples near the anode after electro-osmosis. Asavadorndeja [9] verified by model test that the strength of soft clay can be greatly improved after adding Ca²⁺-containing solution and electro-osmosis. Mohdelhassan [10] verified by model test that filling Al₂(SO4)₃ solution in soft clay can effectively improve the electro-osmosis efficiency. At present, few scholars have studied the influence of anode filled with FeCl₃ solution on soft clay electro-osmosis. During winter construction, low temperature will reduce the migration rate of ions during electro-osmosis, which is another important factor affecting the electro-osmosis consolidation of soft clay. However, there is no relevant research by scholars. Therefore, it is necessary to study the influence of low temperature environment and anodic filled with FeCl₃ solution on electro-osmosis of soft clay via an experiment.

In this paper, the electric current, pH value, and displacement of soft clay with water content of 35–55% in a normal temperature environment and low temperature environment were measured by a self-made device, and the corresponding electric permeability coefficient and energy consumption coefficient were calculated. The resistivity and potential difference of an anode without grouting, filling water, and FeCl₃ solution were measured at normal temperature by improved Miller Soil Box, and resistivity and anode potential difference after anode grouting and FeCl₃ solution filling at low temperature were measured, as well as anodic potential difference and current in frozen soft clay with 40% moisture content. According to the test results, the effects of anode grouting Fecl₃ solution on the current, pH value, drainage amount, anode potential difference, electrical permeability coefficient, energy consumption coefficient, and resistivity of soft clay during electro-osmosis at low temperature were analyzed in order to provide a reference for engineering practice.

2. Test Method

A series of complex chemical and physical changes will occur when electro-osmosis is used to reinforce soft clay [26,27]. This includes at least the following three aspects:

(1)

Under the action of electric current, pore water in soft clay seeps from anode to cathode, and then discharged from the cathode. Filling the FeCl₃ solution at an anode can strengthen this process.

(2)

Electro-osmosis is accompanied by electrolytic reaction:

2H₂O − 4e⁻ → O₂↑ + 4H⁺ (anode)

2H₂O + 2e⁻ → H₂↑ + 2OH⁻ (cathode)

At the same time, H⁺ generated by an anode reacts with the metal of the anode:

Al − 3e⁻ → Al³⁺

(3)

In the process of electro-osmosis, metal ions react with OH⁻ to produce chemical substances with cementation, such as Al(OH)₃ and Fe(OH)₃, which further improve the strength of soft clay.

The schematic diagram of the principle of electro-osmotic reinforcement of soft clay is shown in Figure 1.

2.1. Test Equipment and Materials

In this experiment, aluminum sheets with a thickness of 1 mm were used as anode and cathode materials. The FeCl₃ solution was prepared from FeCl₃·6H₂O crystal with a concentration of 400 g/L; the water used was ordinary tap water; the soil box used in the self-made device was a cylindrical plastic box with an inner diameter of 9.2 cm, a height of 7.5 cm, and a volume of 500 cm³. The soil box and the cathode material were covered with small holes for drainage; the improved Miller Soil Box was 225 mm long, 150 mm wide, and 70 mm high. We used 25 V or 30 V DC power supply, an ammeter with an accuracy of 1 mA, a voltmeter with an accuracy of 0.01 V, and a pH meter with a resolution of 0.01.

The soft clay used was taken from the construction site in Hangzhou. The soil sample was gray. The physical indexes of undisturbed soil are shown in

Table 1. Physical and mechanical indexes of undisturbed soil.

Moisture Content	Unit Weight of Soil	Void Ratio	Specific Gravity of Soil	Liquid Limit	Plastic Limit	Cohesive Strength	Internal Friction Angle
ω	γ	e0	Gs	ωl	ωp	c	φ
(%)	(kN/m3)	(%)		(%)	(%)	(kPa)	(°)
45.0	17.2	1.258	2.73	43.2	23.6	13.7	9.4

.

2.2. Test Conditions

In order to study the influence of low temperature environment and anode filled with FeCl₃ solution on the electro-osmosis of soft clay with different water content, we designed 31 working conditions in this experiment, as shown in

Table 2. Test conditions.

Test Number	Moisture Content (%)	Environment	Grout	Test Number	Moisture Content (%)	Environment	Grout
1	35	normal temperature	FeCl3 solution	17	40	low temperature	filled with water
2	40	normal temperature	FeCl3 solution	18	45	low temperature	water
3	45	normal temperature	FeCl3 solution	19	50	low temperature	water
4	50	normal temperature	FeCl3 solution	20	55	low temperature	water
5	55	normal temperature	FeCl3 solution	21	35	normal temperature	/
6	35	normal temperature	water	22	40	normal temperature	/
7	40	normal temperature	water	23	45	normal temperature	/
8	45	normal temperature	water	24	50	normal temperature	/
9	50	normal temperature	water	25	55	normal temperature	/
10	55	normal temperature	water	26	35	low temperature	/
11	35	low temperature	FeCl3 solution	27	40	low temperature	/
12	40	low temperature	FeCl3 solution	28	45	low temperature	/
13	45	low temperature	FeCl3 solution	29	50	low temperature	/
14	50	low temperature	FeCl3 solution	30	55	low temperature	/
15	55	low temperature	FeCl3 solution	31	40	freeze	/
16	35	low temperature	water

. The experiments of measuring parameters such as current, pH value, and drainage by self-made device include working conditions 1–20; the experiments of measuring resistivity and anode potential difference with improved Miller Soil Box included working conditions 1–15 and 21–31.

2.3. Test Scheme

The indoor temperature during the test was about 30 °C. The temperature of soil sample after 4 h in the refrigerator refrigeration layer was about 1–4 °C. The temperature of the soil sample placed in the refrigerator refrigeration layer for 12 h was about −1 °C, and was frozen.

2.3.1. Test Scheme of Self-Made Device

(1)

Soil sample preparation: ① After the undisturbed soil taken from the construction site was sunburned, ground into powder, sieved, and dried, the soil sample was added to the corresponding amount of water according to the moisture content of 35%, 40%, 45%, 50%, and 55%, and fully stirred and put into the soil box; ② according to the grouting condition, 2 mL FeCl₃ solution or water was poured into the anode; ③ according to the environmental conditions of the working conditions, the earth-packing box containing soil samples was sealed with preservative film and placed in the refrigerator refrigeration layer or at room temperature for 4 h.

(2)

Electro-osmosis: We fixed the upper part of the soil-filled box on the cross bar; placed the funnel and measuring cylinder for receiving water at the lower part; and then connected the 25 V DC power supply, ammeter, and soil-filled box as required for electro-osmosis, and set up the control group at the same time, as shown in Figure 2.

(3)

Data reading: In order to maintain the low temperature environment of frozen soil samples, we set the duration of electro-osmosis to 1 h. The data that we needed to read included: ① reading the current readings every 10 min; ② reading the drainage volume in the measuring tube after electro-osmosis.

(4)

pH value measurement: For soil samples with water content of 40%, we measured the pH values of soil samples near the anode, middle, and cathode after electro-osmosis. The measuring steps of soil pH value are as follows: ① We weighed 20.0 g soil sample into beaker, added 50 mL distilled water, and shook it violently for 2 min. ② The electrode of the pH meter was inserted into the suspension of the sample. The electrode probe was immersed in 1/3–2/3 of the vertical depth of the suspension under the liquid surface, and the sample was gently shaken. After the reading was stable, the pH value was recorded. ③ After each sample was measured, we rinsed the pH meter electrode with water immediately and sucked the water outside the electrode with filter paper, and then measured the next sample.

The soil sample after electro-osmosis is shown in Figure 3, and the measuring process of pH value of soil sample is shown in Figure 4.

2.3.2. Improved Miller Soil Box Test Scheme

(1)

Preparation of soil samples: ① After the undisturbed soil taken from the construction site was exposed to the sun, ground into powder, screened, and dried, the soil samples were added with corresponding amounts of water according to the water contents of 35%, 40%, 45%, 50% and 55%, and then put into the improved Miller Soil Box after fully stirring. ② According to the grouting conditions of working conditions, we filled 2 mL FeCl₃ solution or water or no grouting at the anode. ③ According to the environmental conditions of the working conditions, we sealed the improved Miller Soil Box with soil samples with a preservative film and placed them in the refrigerator refrigeration layer or at room temperature for 4 h; the soil sample in condition 31 and the improved Miller Soil Box needed to be placed in the refrigerator refrigeration layer for 12 h.

(2)

Electro-osmosis: We connected the 30 V DC power supply, voltmeter, and ammeter as required, as shown in Figure 5.

(3)

Data reading: For working conditions 1–15 and working conditions 21–30, we measured the current and anode potential difference 5 mm away from the anode; for working condition 31, the anode potential difference and current were measured at 5 mm from the anode in the first 15 min per minute. The measurement of anode potential difference is shown in Figure 6.

3. Analysis of Test Results

3.1. Current Change

The current intensity directly reflects the migration speed of ions in the process of electro-osmotic consolidation. The changes of current with time under different working conditions are shown in Figure 7, Figure 8, Figure 9 and Figure 10.

By comparing Figure 7 with Figure 10, we can see that

(1)

Under the same conditions, at the initial stage of electro-osmosis, the current of soil samples at room temperature was significantly greater than that at low temperature, indicating that low temperature was not conducive to electro-osmosis. In the later stage, the current of soil samples with low temperature was greater than that of soil samples under normal temperature, which was due to the more drainage of soil samples under normal temperature and the narrowing of the temperature gap between soil samples caused by electro-osmosis heating.

(2)

Under the same conditions, the current after the anode filled with FeCl₃ solution was greater than when that anode was filled water in each time period. Although the initial current after the anode was filled with FeCl₃ solution at low temperature was less than that after the anode was filled with water at normal temperature, with the progress of electro-osmosis, the current after the anode was filled with FeCl₃ solution at low temperature will be gradually greater than that after the anode was filled with water at normal temperature, which shows that the effect of FeCl₃ solution on electro-osmosis was greater than that of temperature.

(3)

The peak value of current after anode filled was at the beginning of electro-osmosis, which was consistent with the experimental results of Wang Liangzhi and others [28]. Except for working condition 5, after the anode was filled with FeCl₃ solution, the peak current occurred after a period of electro-osmosis, which was consistent with the current change result of Ren Lianwei et al. [6] after the anode was filled with CaCl₂ and Na₂SiO₃ solution. This current change result showed that after the anode was filled with FeCl₃ solution during electro-osmosis, the electro-osmosis effect first gradually became stronger, then gradually decreased, which was not one step at a time.

(4)

At room temperature, the peak current observed under the condition of 55% moisture content of soil sample and FeCl₃ solution anode filled was at the beginning of electro-osmosis, which may have been due to the high moisture content of the soil sample. After the FeCl₃ solution anode was filled, the current reached the peak in a short period of time. In the test, due to the relatively long time interval of current observation, no real peak current was observed.

3.2. pH Change

The electro-osmosis process was accompanied by electrolytic reaction, and therefore the pH value of soil samples at different positions after electro-osmosis was different.

For soil samples with water content of 40%, the pH value of anode soil samples was 5.50 after 2 mL FeCl₃ solution was poured into the anode before electro-osmosis. The pH value of the anode soil sample was 8.66 after 2 mL of water filled into the anode. The pH values of soil samples at different positions under different working conditions after 1 h electro-osmosis are shown in Figure 11.

According to Figure 11:

(1)

After electro-osmosis, the anode soil sample will be obviously acidic, and the cathode soil sample will be obviously alkaline, which is mainly due to the electrolytic reaction in the process of electro-osmosis, which produces H⁺ near the anode and OH⁻ near the cathode. The middle soil sample would be weakly alkaline because the original soil sample was weakly alkaline.

(2)

At room temperature, after the anode was filled with FeCl₃ solution, the electro-osmotic drainage of soil sample was the largest, the pH value of the anode was the smallest, and the pH value of the cathode was the largest. At low temperature, the electro-osmotic drainage of the soil sample after the anode was filled with water was the least, the pH value of the anode was the largest, and the pH value of the cathode was the smallest. The results show that the pH value of the cathode and anode can be used as one of the indexes to reflect the electro-osmosis effect. The smaller the pH value of the anode and the larger the pH value of the cathode, the better the electro-osmosis effect.

3.3. Analysis of Displacement and Electric Permeability Coefficient

Considering that the gravity factor may affect the drainage of soil samples, we set up a control group during the experiment. It was found that only the soil samples with 55% moisture content in low temperature had several drops of drainage in the experiment, which may have been caused by the moisture in the surrounding air being cold, and thus the gravity factor can be ignored. Gravity has no effect on the displacement of soil samples because the water permeability of soft clay itself is very poor.

3.3.1. Displacement

The amount of water discharged from soft clay during electro-osmosis is an intuitive reflection of the electro-osmosis effect. The variation of electro-osmotic drainage of anode irrigation and FeCl₃ solution under a normal temperature environment and low temperature environment with soil moisture content is shown in Figure 12.

As can be seen from Figure 12:

(1)

Under the same conditions, the anode filled with FeCl₃ solution can effectively increase the drainage of soil samples during electro-osmosis. At normal temperature, the anode filled with FeCl₃ solution can increase the drainage of soil samples by 6.0–15 mL more than the anode filled with water, and the proportion of drainage increase was 35.3–350.0%, which is equivalent to the increase of drainage after Ren Lianwei et al. [7] added 1.5 mol/L CaCl₂ solution into soft clay. Under the low temperature condition, compared with the anode filled with water, the anode filled with FeCl₃ solution was able to increase the drainage of soil samples by 7.0–13.5 mL, and the proportion of increasing drainage was 46.7–356.5%. When the water content was 35%, the effect of anode filled with FeCl₃ solution was the best, and the proportion of increasing drainage was the largest.

(2)

Under the same conditions, the low temperature will obviously reduce the drainage of soil samples during electro-osmosis. When FeCl₃ solution was injected into the anode, the drainage of soil samples at low temperature was reduced by 3.0–7.0 mL compared with that at normal temperature, and the reduction ratio of drainage was 19.0–22.8%. Compared with the normal temperature, the soil sample with drainage at low temperature was reduced by 0.7–6.0 mL under the anode filled with water, and the reduction ratio of drainage was 23.3–44.1%.

(3)

When the moisture content of soft clay was the same, the electro-osmotic discharge after the anode was filled with FeCl₃ solution at low temperature was obviously higher than that after the anode was filled with water at normal temperature, which indicates that the influence of the anode filled with FeCl₃ solution on electro-osmosis was greater than that of the temperature.

3.3.2. Electro-Permeability Coefficient

The electro-permeability coefficient represents the seepage velocity of clay under unit potential gradient. According to Esrig’s paper [24], the formula of electric permeability coefficient is:

$$k_e=\frac{VL}{AUt}$$

(1)

In the formula: $k_e$ is the electric permeability coefficient; V is the displacement during the test; U is the applied voltage; L is the height of the soil box; A is the bottom area of the soil box; t is the test duration.

According to Equation (1), when the water content of soil sample was 35–55% at normal temperature, the electro-permeability coefficient was 1.692 × 10⁻⁵–4.951 × 10⁻⁵ cm²·s⁻¹·V⁻¹ after the anode was filled with FeCl₃ solution; the electro-permeability coefficient was 3.761 × 10⁻⁶–3.071 × 10⁻⁵ cm²·s⁻¹·V⁻¹ after the anode was filled with water. When the water content of the soil sample was 35–55% at low temperature, the electro-permeability coefficient was 1.316 × 10⁻⁵–4.011 × 10⁻⁵ cm²·s⁻¹·V⁻¹ after the anode was filled with FeCl₃ solution; the electro-permeability coefficient was 2.883 × 10⁻⁶–2.319 × 10⁻⁵ cm²·s⁻¹·V⁻¹ after the anode was filled with water.

The results show that the higher the water content, the greater the electric permeability coefficient. Low temperature will reduce the electric permeability coefficient of soil samples; anodes filled with FeCl₃ solution can effectively improve the electric permeability coefficient of soil samples.

3.4. Energy Consumption Analysis

The energy consumption coefficient C represents the energy needed to discharge 1 L water in the same period of time. According to Li Yiwen’s paper, the energy consumption coefficient formula is

$$C=\frac{{{\displaystyle \int }}_{t_1}^{t_2}U{I}_t{d}_t}{Q_{t_1-t_2}}$$

(2)

In the formula, $Q_{t_1-t_2}$ is the volume of electro-osmotic drainage in time t₁–t₂, U is applied voltage, and I_t is the current value of the soil at a certain time from t₁ to t₂.

According to the Equation (2), the energy consumption coefficient under each working condition is calculated as shown in Figure 13.

According to Figure 13, when the water content of soil sample was 40–55%, the energy consumption coefficient of electro-osmosis under different working conditions had little difference. When the water content of the soil sample was 35%, the average energy consumption coefficient under different working conditions was quite different. Anodes filled with FeCl₃ solution can effectively reduce the energy consumption coefficient of electro-osmosis, and at normal temperature, the energy consumption coefficient of electro-osmosis with an anode filled with FeCl₃ solution was reduced by 0.433 kW·h·L⁻¹, with a decrease of 46.6%. The electro-osmotic energy consumption coefficient of FeCl₃ solution injected into a low temperature anode decreased by 0.222 kW·h·L⁻¹, with a decrease of 29.5%. Temperature had little influence on the electro-osmosis energy consumption coefficient.

3.5. Resistivity Change

Resistivity is used to represent the resistance characteristics of soil, and the resistivity formula [22] is

$${\rho }_0=R\frac{BH}{L}$$

In the formula, ${\rho }_0$ is the resistivity of the soil sample, R is the average resistance measured in the test, B is the width of the test container, H is the height of the test container, and L is the length of the test container.

The water content of soft clay was 35–55%, and the change of resistivity without grouting and filled with water and FeCl₃ solution near the anode at normal temperature is shown in Figure 14. The resistivity changes near the anode without grouting and FeCl₃ solution filling at low temperatures are shown in Figure 15.

According to Figure 14 and Figure 15, one can see that

(1)

Low temperature can significantly improve the resistivity of soft clay. When the anode was not grouted, the resistivity of soft clay under a low-temperature environment increased by 1.513–4.449 Ω·m compared with that under a normal temperature environment, and the resistivity increased by 29.7–93.4%. When the FeCl₃ solution was poured into the anode, the resistivity of soft clay at low temperature increased by 1.757–3.999 Ω·m compared with that at normal temperature, and the resistivity increased by 37.9–87.1%.

(2)

Whether at normal temperature or low temperature, anode filling with FeCl₃ solution can effectively reduce the resistivity. At normal temperature, when the water content was 35%, the resistivity of FeCl₃ solution decreased the most, by 0.455 Ω·m, with a decrease of 8.9%; at low temperature, when the water content was 40%, the resistivity of FeCl₃ solution decreased the most, by 0.699 Ω·m, and the decrease was 7.7%.

(3)

An anode filled with water cannot effectively reduce the resistivity at normal temperature, especially when the moisture content of soft clay is high, and even the resistivity will be increased.

3.6. Variation of Anode Potential Difference

Anode electro-osmosis difference indicates the loss of potential near the anode during electro-osmosis. The larger the anode potential difference, the more unfavorable it is to electro-osmosis.

3.6.1. Influence of Low Temperature on Anode Potential Difference

The moisture content of soft clay was 35–55%. The change of anode potential difference is shown in Figure 16 when there was no grouting, when we filled with water or FeCl₃ solution near the anode at normal temperature, and when there was no grouting and FeCl₃ solution was filled at low temperature.

As can be seen from Figure 16,

(1)

Except for the water content of 35%, the anodic potential difference at low temperature was significantly higher than that at normal temperature without grouting, with an increment of 0.15–0.40 V and an increase of 26.7–70.2%, indicating that low temperature is not conducive to electro-osmosis. When the water content was 35%, the reason why the anode potential difference at normal temperature was higher than that at low temperature requires further analysis.

(2)

Whether at normal temperature or low temperature, the anode filled with FeCl₃ solution can effectively reduce the anode potential difference, and the reduction of anode electro-osmosis difference was 0.20–1.40 V, which effectively improved the electro-osmosis efficiency. When the moisture content was 35%, the decrease of anodic potential difference of FeCl₃ solution was the largest, with a decrease of 1.40 V at normal temperature and a decrease of 75.7%, and a decrease of 0.80 V at low temperature and a decrease of 54.1%, indicating that the anode filled with FeCl₃ solution was the optimal choice when the moisture content was 35%.

(3)

After the anode was filled with FeCl₃ solution, in general, the anode potential difference at normal temperature was higher than that at low temperature, but there was little difference between them, which indicates that the anode filled with FeCl₃ solution can effectively reduce the influence of low temperature on the anode potential difference.

(4)

The anode filled with water can also reduce the potential difference of anode to a certain extent, because the water used was tap water and it contained certain conductive ions, but the effect of an anode filled with water was far less than that of an anode filled with FeCl₃ solution.

3.6.2. Influence of Freezing on Anode Potential Difference

As time progresses, the frozen soft clay will melt due to the high ambient temperature, and thus only the anode potential difference and current in the first 15 min of working condition 31 were measured here.

The change of anode potential difference of frozen soft clay (water content 40%) with time is shown in Figure 17, the change of anode potential difference of frozen soft clay (water content 40%) is shown in Figure 18, and the change of current of frozen soft clay (water content 40%) after electrifying is shown in Figure 19.

As can be seen from Figure 17, Figure 18 and Figure 19,

(1)

When freezing, the potential difference of the soft clay anode was extremely large, reaching 12.13 V at the initial stage, accounting for 40.4% of the total potential; the current was 0 mA, and the resistivity was extremely large, which is very unfavorable for electro-osmosis.

(2)

With the continuous conduction of electricity, the anode potential difference decreased rapidly. Although the reduction of potential difference per minute was repeated, the overall reduction of potential difference became increasingly smaller. The current was increasing. The reason for this phenomenon was that a certain amount of heat was generated during the electro-osmotic process, and the frozen soft clay was gradually melted. The reason for the constant current in a period of time shown in Figure 19 was limited by the accuracy of the current meter.

(3)

When adding soft clay by electro-osmosis in frozen state, we recommend melting the soft clay first and then carrying out electro-osmosis, so as to improve the efficiency of electro-osmosis.

4. Conclusions

(1)

Compared with the normal temperature environment, although the energy consumption coefficient of electro-osmosis in a low-temperature environment had little change, the current, water displacement, and electro-osmosis coefficient were obviously reduced, and the resistivity and anode potential difference were greatly increased, which indicates that the low temperature environment can reduce the electro-osmosis efficiency of soft clay, and the construction of electro-osmosis reinforcement of soft clay should be carried out at a higher temperature as far as possible.

(2)

The current, drainage volume, and electro-permeability coefficient increased significantly, while the resistivity, anode potential difference, and electro-osmosis energy consumption coefficient decreased significantly after the anodic FeCl₃ solution was applied in electro-osmosis. The results showed that anodic FeCl₃ solution was beneficial to improving electro-osmosis efficiency and energy saving in the construction of electro-osmosis reinforcement of soft clay, and the effect of anodic FeCl₃ solution was the best when the water content was 35–40%.

(3)

The pH value of the cathode and anode can be used as one of the indexes to reflect the electro-osmosis effect in the process of electro-osmosis of soft clay. The smaller the pH value of the anode and the larger the pH value of the cathode, the better the electro-osmosis effect.

(4)

There was a great difference between resistivity and anode potential in electro-osmosis of frozen soft clay. We suggest taking measures to melt soft clay before electro-osmosis reinforcement, and then conducting electro-osmosis, so as to improve the efficiency of the electro-osmosis.

Because the temperature was difficult to control during the test, the low temperature mentioned in this paper refers to the lower temperature above 0 °C. This experiment did not consider the influence of additive concentration and temperature gradient, and it still requires further improvement in future experiments and research.