Expansive soils have great volume change potentials with water content changes, which is problematic to facilities. Great efforts have been spent on finding proper methods to stabilize expansive soils, but these stabilizers all had limitations. The Polyvinyl alcohol (PVA) and K2CO3 combination was proposed in this paper. Free swell tests, oedometric tests, unconfined compression tests, and direct shear tests were performed to investigate the effectiveness of the PVA and K2CO3 combination to control the volume change and increase the soil strength. Microstructures of the natural expansive soil and the stabilized soil were also studied with SEM photos. SEM photos showed a homogenous and dense microstructure after stabilization. In addition, a laboratory soil column model was built to study the ability of this stabilizer combination to stabilize expansive soils by directly spraying the solution on the ground surface. All these test results show that the combination of PVA and K2CO3 is able to effectively stabilize the natural expansive soil and increase the shear strength. It is possible to directly spray the stabilizer solution on the soil surface to form a relatively thick layer of the stabilized expansive soil.
Expansive soils have large volume change potentials with water content changes, causing problems such as slope failures (e.g., [
Stabilizers involving chemical reactions with expansive soils used were more popular. Lime was very often used as an expansive soil stabilizer (e.g. [
However, these stabilizers are nonsoluble or have low solubility to water. Due to complicated site conditions in the field, other than the soil filling projects, it is hard to directly mix field soils with stabilizers as that in the laboratory. Therefore, it is necessary to find a new stabilizer with high solubility to water, and possibly, this new stabilizer solution could be directly sprayed to the ground surface to stabilize the expansive soil in a considerable depth. K+ can be considered as a great swelling inhibitor because of the lowest hydration energy [
The free swell potential is a very important property of expansive soils. Three groups of free swell tests, firstly proposed by Holtz and Gibbs [
Oedometric swell tests were conducted with vertical swelling only from the top of samples. Devices used in oedometric swell tests included the oedometer, the ring-sampler with the diameter of 61.8 mm and the height of 20 mm, and the displacement gauge with the maximum reading of 10 mm. K2CO3 powders having weights of 1%, 3%, and 5% of the dry expansive soil were prepared, and PVA powders having weights of 0.1%, 0.3%, and 0.5% of the dry expansive soil were prepared. Then, these powders were mixed together and dissolved into water. 16 solutions were prepared with different ratios of K2CO3 and PVA. Finally, these 16 solutions were mixed with expansive soil samples, and then cured for 24 hours with the water content of about 20% which was the minimum average water content in dry seasons, and the samples were collected from the ground surface to the depth of 2.5 m near the highway construction field. Therefore, the mass of the mixture (
In the loaded oedometric swell tests, samples were immediately put onto the oedometer and loaded with 50 kPa from the top. Then, when the vertical strain rate was smaller than 0.01 mm per hour, distilled water was poured into the sample container on the oedometer with the water surface 5 mm higher than the sample top surface. The sample height was recorded every two hours until the difference of two adjacent readings was smaller than 0.01 mm.
In the free oedometric swell tests, ring samples were immediately put onto the consolidometer and the sample container was filled with distilled water, and the water surface was set to 5 mm higher than the sample top surface. The sample height was recorded every two hours until the swelling strain rate was smaller than 0.01 mm every 6 hours.
Because stabilized expansive soils will be used in the field construction projects, compression strength is always the most important property. Unconfined strength tests were performed to determine compression strengths of samples with different curing periods. The natural expansive soil used in tests had the dry natural density of 1.6 g/cm3 and the maximum dry density of 1.871 g/cm3. The samples were mixed and stabilized with PVA with the weight of 0.5% of the dry natural expansive soil and K2CO3 with the weight of 3% of the dry natural expansive soil. The stabilized samples and natural expansive soils were sealed in the same environment for 24 hours with the water content of approximately 20%. Then, 16 samples of stabilized soil and 16 samples of natural expansive soil were prepared by a hydraulic stripping machine with the diameter of 5 cm and the height of 5 cm. These samples were cured in wet sand for 7 days, 14 days, 21 days, and 28 days, respectively. Finally, they were compressed with the axial strain rate of 0.4 mm/min without any lateral constrains, making sure that they failed within 7–14 min.
The drying-wetting cycles simulate changes of water content in the field with seasonal weather changes. The stabilized soil samples had PVA with the weight of 0.5% of the dry natural expansive soil and K2CO3 with the weight of 3% of the dry natural expansive soil. The samples were prepared with ring-samplers with the diameter of 61.8 mm and the height of 20 mm. In the tests, the water contents were varying in the range between 19% and 25%, simulating the partial drying and partial wetting. After each drying-wetting cycle, photos of the samples were taken and the sizes of cracks were measured with a digital caliper.
The undrained shear strengths of the samples before drying-wetting cycles and after 2 and 4 drying-wetting cycles were tested with direct shear tests. During shearing, the vertical pressures were set to 100 kPa, 200 kPa, 300 kPa, and 400 kPa. Then, samples were sheared with the strain rate of 1.2 mm/min and without water drainage, so they could fail within 3–5 mins.
A soil sample was scanned with the scanning electron microscopy (SEM). The SEM device model was XL-30 which was produced by the EDAX company. The soil sample had PVA with the weight of 0.5% of the dry natural expansive soil and K2CO3 with the weight of 3% of the dry natural expansive soil. The soil sample was cured for 7 days, and then dried and cut into small pieces with one smooth side sticking to the metal plate.
In the infiltration test, a soil column was built and shown in Figure
Photos of the soil column: (a) side view of the soil column; (b) plan view from the top of the soil column.
Free swell tests were performed to study the soil swelling potential. Free swell index (FSI) was calculated with the following equation:
The free swell index values for samples with: (a) K2CO3 only; (b) PVA only; (c) 3% of K2CO3 and different amounts of PVA.
Free oedometric swell tests were conducted with free swell from the top, and the lateral swell of the samples was not allowed. In the loaded oedometric swell tests, the lateral swell was also prohibited, but 50 kPa vertical load was added to the top of the sample. In order to quantify the swell behavior, oedometric swell index (OSI) was calculated as follows:
Oedometric swell test results (a) with a 50 kPa vertical load; (b) without vertical load.
In order to investigate the effect of PVA and K2CO3 on the strengths of samples, unconfined compression tests were performed. The photos of the samples before and after tests are shown in Figure
Photos of the samples before and after the unconfined compression tests.
To quantify the effect of PVA and K2CO3 on sample strengths, the unconfined compression strengths versus different curing period are plotted in Figure
Unconfined compression strengths for samples with different curing periods.
The majority of strength in expansive soil is from matric suction which is directly related to the water content [
Photos of cracks on natural expansive soil samples with different numbers of drying-wetting cycles. (a) 1 cycle. (b) 5 cycles. (c) 8 cycles. (d) 9 cycles.
Photos of cracks on stabilized soil samples with different numbers of drying-wetting cycles. (a) 1 cycle. (b) 7 cycles. (c) 13 cycles. (d) 19 cycles. (e) 25 cycles. (f) 31 cycles.
To quantify the effect of drying-wetting cycles on the strengths of both natural expansive soil and stabilized soil samples, direct shear tests were performed for samples before and after 2 cycles and 4 cycles of drying-wetting. Test results are shown in Figure
Direct shear strengths for samples with cycles of drying-wetting.
Volume change during drying-wetting cycles is also a very important property to investigate. Figure
Axial deformations during swelling-shrinking cycles: (a) the stabilized soil sample; (b) the natural expansive soil sample.
The microstructure of the natural expansive soil and the stabilized expansive soil is directly related to the macroscopic swelling potential. Therefore, the scanning electron microscopy (SEM) was used to take photos of the soil microstructures shown in Figure
SEM micrographs of the samples with and without stabilization. Note: the SEM images were taken in 2011.
One of the main goals of this work is to find a stabilizer which can be directly sprayed onto the soil surface and stabilize the natural expansive soil in the field simply by infiltration vertically through the small cracks and then horizontally into soil blocks. Then, the soil in the surface layer can be stabilized as a protection layer, and subsequently, it will help to resist drying-wetting cycles and rainfall events, keeping the moisture content constant in the deeper soil layer. Finally, the soil slope stability will be increased.
After spraying, the soil samples at the depths of 30 cm, 60 cm, and 90 cm were collected, and the free swell test results are shown in Figure
Free swell index at different depths.
Oedometric swell index at different depths: (a) 50 kPa loaded oedometric swell tests; (b) free oedometric swell tests.
The shear strengths at the depths of 30 cm, 60 cm, and 90 cm were also tested by direct shear tests with the vertical loads of 100 kPa, 200 kPa, 300 kPa, and 400 kPa. The test results are shown in Figure
Shear strengths at different depths.
In this paper, polyvinyl alcohol and potassium carbonate were used to stabilize the expansive soil. Free swell tests, oedometric swell tests, unconfined compression tests, and direct shear tests were performed to study the swelling potential and shear strength of the stabilized soil. The microstructures of the stabilized soil were also studied with SEM photos. Finally, a laboratory model with a 1 m high soil column was built to simulate the stabilization of field soil by directly spraying the stabilizer solution onto the soil surface. Some conclusions can be summarized as follows: With the free swell test results, it shows that it is more effective to stabilize the expansive soil with the combination of PVA and K2CO3 than to do it with only PVA or K2CO3 individually. Oedometric test results confirmed the ability of the combination of PVA and K2CO3 to control the volume change. Unconfined compression test results showed that the stabilized soil samples had much higher strengths. After many drying-wetting cycles, the strength and volume changes of the stabilized soil samples were well controlled. SEM photos showed that the stabilized soil sample had more homogenous and denser microstructures than the natural expansive soil sample. The laboratory soil column model confirmed the ability of the stabilizer solution to form a thick protection layer to resist volume changes during drying-wetting cycles and keep the water content constant in the lower layer.
Finally, it can be concluded that the PVA and K2CO3 combination is able to serve as an effective stabilizer of expansive soil, and more importantly, it is possible to stabilize field soils by directly spraying the stabilizer solution on the soil surface, forming a thick protection layer.
The data used to support the findings of this study are included within the article.
The authors declare that they have no conflicts of interest.
This work was supported by the National Natural Science Foundation of China (NSFC no. 51778211) and the Natural Science Foundation of Jiangsu Province (Grant no. BK20171434).