To reveal the moisture migration mechanism of the unsaturated red clays, which are sensitive to water content change and widely distributed in South China, and then rationally use them as a filling material for highway embankments, a method to measure the water content of red clay cylinders using X-ray computed tomography (CT) was proposed and verified. Then, studies on the moisture migrations in the red clays under the rainfall and ground water level were performed at different degrees of compaction. The results show that the relationship between dry density, water content, and CT value determined from X-ray CT tests can be used to nondestructively measure the water content of red clay cylinders at different migration time, which avoids the error reduced by the sample-to-sample variation. The rainfall, ground water level, and degree of compaction are factors that can significantly affect the moisture migration distance and migration rate. Some techniques, such as lowering groundwater table and increasing degree of compaction of the red clays, can be used to prevent or delay the moisture migration in highway embankments filled with red clays.
Red clays are widely distributed in South China, which are unsaturated and highly sensitive to moisture change. With the rapid increase of the highway constructions, many highways are being built on the red clay subgrade. The existing subgrade treatment methods of replacing the red clays with high quality filling materials have tended to be unacceptable due to the increasing consciousness of the environmental protection and natural resource conservation. Accordingly, using the red clays as a construction material of highways becomes economically attractive, whereas the characteristic of red clays mentioned above makes its application to highway construction become a major issue [
Since the red clays are moisture dependent, when the water content is different from the surrounding conditions, the moisture migration will happen sharply due to the water potential [
Studies of the moisture migration in soils have been performed based on the soil water potential concept. There are many methods to investigate the moisture migration, such as laboratory experiments [
The objectives of this paper are to develop a nondestructive method to measure the water content of the red clays using X-ray CT and to investigate the mechanisms of the moisture migration in the red clays by monitoring the moisture content at different migration time using this nondestructive method. The paper is organized as follows. The forthcoming section introduces the procedures of X-ray CT scanning tests of red clay cylinders with three different degrees of compaction at 90%, 93%, and 96%. The relationship among the dry density, water content, and the CT value is proposed and validated. After that, the influential factors of the moisture migration are evaluated. The moisture migration tests are performed on the soil cylinders with the optimum water content of 12.9% at different conditions including degree of compaction, ground water, and rainfall. The soil cylinders at different migration time are scanned using X-ray CT and their water contents are calculated using the correlation among the dry density, water content, and the CT value developed in the previous section. In the following, the effects of the rainfall, ground water, migration time, and degree of compaction on the moisture migration of the red clay cylinders are evaluated. The final section summarizes the major findings of this study.
The soil samples were taken from the field site of Nanchang-Zhangshu highway widening project in Jiangxi Province. Their natural water content is 18.2%; the liquid limit and plastic limit are 44% and 22%, respectively. According to the compaction test, the optimum water content and maximum dry density are 12.9% and 1.949 g/cm3, respectively. Particle size analysis shows that the 0.075 mm passing percentage of the soil samples is 78%. So, the soil sample was categorized as low liquid-limit clay (
According to the specification of the degree of compaction of an embankment in China, the target degree of compaction of the soil cylinders is 96%, 93%, and 90%, with corresponding target density of 1.871 g/cm3, 1.812 g/cm3, and 1.754 g/cm3, respectively. When molded, the target water contents of the cylinders include 0%, 3%, 6%, 9%, and 12% at each degree of compaction.
In order to fabricate soil cylinders at the target degree of compaction and water content, a laboratory designed splitting mold with a diameter of 7.2 cm and a length of 7.0 cm was used. The cylinders were divided into five layers for molding. After filling each layer, it was compacted using a metal bar to achieve an initial degree of compaction, and then the next layer was filled and compacted till all the layers were finished. Subsequently, the completed soil cylinder was compacted until the designed height was reached, which corresponds to the designed degree of compaction. It was found that the soil cylinders with the water content below 6% were unable to be demolded. Therefore, the columns molded at the water content of 6% were dried in an oven at 30°C and weighted until the target water content was reached. After that, the cylinders were sealed for 15 days, before the X-ray CT scanning, and rotated by 60° every two days to ensure that the water was fully balanced. To verify the uniform distribution of the moisture in the soil cylinder, the conditioned cylinder was cut into three slices and the moisture content along the length was measured at three different locations of each slice. It was observed that the water content of the soil cylinder was uniformly distributed in the sample and the average water content was close to the molding moisture content.
Figure
Configuration of X-ray CT scanning test.
X-ray CT scanning configuration
X-ray CT test set-up
Based on the measured average CT value of the scanned sections, a correlation was established between the water content and the CT value which is a linear relationship as shown in Figure
Relationship between water content and CT value at different degrees of compaction.
To comprehensively consider the effects of the dry density and water content on the CT value, a correlation between CT value, water content, and dry density for the red clays was built as follows:
In order to validate the relationship in (
Validation of the relationship between water content, dry density, and CT value.
CT value | Dry density/(g/cm3) | Calculated water content/% | Measured water content/% | Difference value/% |
---|---|---|---|---|
4.744 | 1.720 | 10.55 | 10.1 | −0.45 |
5.073 | 1.855 | 8.750 | 9.1 | 0.35 |
5.110 | 1.788 | 15.71 | 16.25 | 0.54 |
To investigate the moisture migration in red clays, moisture migration tests were developed and the water content at different migration time was monitored using X-ray CT scanning test and the relationship in (
To address the precipitation and capillary action in South China, rainfall and ground water level were simulated, which are depicted as follows.
According to the precipitation record in South China, a moderate rainfall of 3.5 mm/h was selected. Figure
Rainfall simulation test on red clay cylinder specimen.
Schematic model
Experimental apparatus
In order to simulate the ground water level below the red clay subgrade, the cylinder specimens were put in a glass box filled with fine sands, which were saturated, as shown in Figure
Ground water simulation tests on red clay cylinder specimen.
Schematic model
Experimental apparatus
The soil cylinders with the heights of 30 cm and 45 cm for the rainfall and ground water level simulation tests were fabricated using the same molding method presented in the previous section. After this, the soil cylinders were sealed for 15 days and rotated 60° every two days to balance their moisture content. At different migration time, the soil cylinders were scanned using the X-ray CT system mentioned above for every 2 cm height and water contents were calculated using (
Details of test parameters.
Factors | Variable | Height of cylinders/cm | Degree of compaction/% | Time |
---|---|---|---|---|
Rainfall | Intensity of 3.5 mm/h@25°C | 30 | 90, 93, 96 | 12 h, 24 h, 48 h |
Ground water | @25°C | 45 | 93 | 1 d, 3 d, 7 d |
Figure
Distribution of water content in the rainfall simulation test.
Moisture along the length of soil cylinder
Sectional moisture content
Moisture contents at different migration time in ground water level simulation test are shown in Figure
Distribution of water content in the ground water level simulation tests.
Moisture along the length of soil cylinder
Sectional moisture content
In order to investigate the effect of the degree of compaction on the distribution of water content in the red clay cylinders, Figure
Migration rate of rainfall test with different compaction degrees.
The measurement of moisture content in an unsaturated red clay cylinder was studied using X-ray CT tests and the moisture migration simulation tests at rainfall and ground water level. Some major conclusions may be drawn as follows: A relationship was developed and validated among the X-ray CT value, water content, and dry density for the red clays. Based on this relationship, one specimen is sufficient to determine the water content at different migration time. The efficiency of moisture migration tests is significantly improved and the sample-to-sample variations are greatly reduced. Rainfall density, ground water level, and degrees of compaction have significant influence on the moisture migration in the red clays. With the elapse of the migration time, the moisture diffuses to a further distance. With the increase of the degree of compaction, the channels for moisture migration become narrow and the moisture migration rate decreases. According to the test results, some construction methods, such as lowering ground water table and increasing degree of compaction of the red clays, can be used to reduce or avoid the distresses, such as slope collapse and pavement cracks, resulting from moisture migration of the red clays. The effect of ground water level and rainfall on the moisture migration was studied separately and only one-dimensional model tests were conducted in this study. Actually, the moisture migration in embankment is a three-dimensional problem, which is affected by ground water level and rainfall simultaneously. Therefore, their combined influence on the moisture migration should be analyzed further and a three-dimensional model test should be also used.
The authors declare that there is no conflict of interests regarding the publication of this paper.
The authors acknowledge the National High Technology Research and Development Program of China (2012AA112504), National Natural Science Foundation of China (51108048 and 51478054), and Jiangxi Communications Department Program (2013C0011) for the financial support. The authors also gratefully acknowledge the financial support from China Scholarship Council.