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Article

Experimental PIV Radial Splitting Study on Expansive Soil during the Drying Process

by
Shun Yu
1,
Fangchan He
2,3 and
Junran Zhang
4,*
1
Henan Jiaotou Zhongyuan Expressway Zhengluo Construction Co., Ltd., Zhengzhou 450000, China
2
Henan Provincial Water Conservancy Research Institute, Zhengzhou 450003, China
3
College of Water Conservancy and Civil Engineering, Zhengzhou University, Zhengzhou 450001, China
4
Henan Province Key Laboratory of Geomechanics and Structural Engineering, North China University of Water Resources and Electric Power, Zhengzhou 450046, China
*
Author to whom correspondence should be addressed.
Appl. Sci. 2023, 13(14), 8050; https://doi.org/10.3390/app13148050
Submission received: 22 June 2023 / Revised: 6 July 2023 / Accepted: 8 July 2023 / Published: 10 July 2023
(This article belongs to the Special Issue Geo-Environmental Problems Caused by Underground Construction, 2nd Edition)
Browse Figures
Versions Notes

Abstract

:
Expansive soil is prone to shrinkage and cracking during the drying process, leading to strength and permeability problems that exist widely in water conservancy projects and geotechnical engineering, including foundation pits and cracks at the bottom of channels and slopes. Such problems are closely related to the tensile strength of the soil. In this study, Nanyang expansive soil is taken as the research object and radial splitting tests were performed using a particle image velocimetry (PIV) test system on both undisturbed and remolded expansive soil during the drying process. The results indicated that the load–displacement curve of the undisturbed and remolded expansive soil specimens showed a strain-softening phenomenon and that the peak load increased with decreasing water content. Under the same other conditions, the peak load of the remolded expansive soil specimen was higher than that of the undisturbed soil specimen, with the undisturbed soil specimen having distinctive structural and fractural features. The load–displacement relation curve, displacement vector field, and fracture characteristics had an obvious one-to-one correspondence in the stage division. The compression deformation stage, crack development stage after the peak value, crack maturity stage, and failure stage could be observed via the PIV technique. Moreover, the fracture characteristics of the remolded specimens were more regular than those of the undisturbed specimens. The above research results provide a scientific basis for the design and construction of geotechnical engineering related to expansive soil.

1. Introduction

The characteristics of expansive soil include water absorption expansion, water loss shrinkage and repeated expansion and shrinkage deformation [1], water immersion bearing capacity attenuation, and dry shrinkage crack development. The properties of expansive soil are extremely unstable. Expansive soil is prone to shrinkage and cracking during the drying process, leading to strength and permeability problems that exist widely in water conservancy projects and geotechnical engineering, including foundation pits and cracks at the bottom of channels and slopes. Such problems are closely related to the tensile strength of the soil [2,3,4].
As a result of seasonal and climatic influences, the structure of expansive soils change, which directly changes the strength of the soil. Because the strength of expansive soils is an important parameter that can determine the stability of projects such as buildings and slopes, it is essential to analyze the factors influencing the soil strength. During drying, the strength of an expansive soil is affected by its water content and dry density. Basma et al. [5] performed drying–wetting cycle tests on remolded samples of expansive soil, and their results showed that the drying–wetting cycles changed the soil microstructure. Morris et al. [6] concluded that the strength of an expansive soil first increases and then stabilizes with the number of drying–wetting cycles. By analyzing the change in the pore volume, Lu et al. [7] found that drying–wetting cycles destroy the intergranular connection and soil structure, resulting in an attenuation of the soil strength. Using laboratory tests, Ortigao et al. [8], Khemissa et al. [9], and Abdulhadi et al. [10] found that the strength of expansive soils is affected by overconsolidation. Terzaghi [11] pointed out that cracks, as structural characteristics of overconsolidated soil, affect the strength of cohesive soils. Skempton [12] stated that when the stress on the soil exceeds the peak value of its shear strength, the crack tip causes greater stress concentration and soil damage. Zhang et al. [13,14] examined the hydromechanical behavior of expansive soils related to suction and the suction history and investigated the strength and stress–strain behavior of expansive soils under suction control using triaxial shear tests. Gu et al. [15] used the resistivity method to measure the crack depth of an expansive soil slope. Lin et al. [16] used the discrete element method to study the microstructure of expansive soils, considering the influence of different confining pressures on the internal cracks of specimens. Wang et al. [17] investigated the entire process of crack development in expansive soils under the action of water and external forces, using computerized tomography scanning combined with physical experimental parameters to reveal the effect of crack development on the macroscopic deformation of expansive soils. Singh et al. [18] used image analysis techniques to quantify dry cracks on the surfaces of specimens.
However, there are few comparative studies concerning tensile strength and crack development during the drying processes of undisturbed and remolded expansive soils. Therefore, further study is necessary to identify the relevant change rule and its internal mechanism.
In general, soil crack development is closely related to the tensile strength of the soil. Methods of measuring the tensile strength of soil primarily include the direct tens test [19,20,21], the radial splitting test [22,23], the bending beam test [24], and the ring specimen method [25]. The use of the radial splitting test to perform tensile strength tests is feasible because of its simplicity and ease of operation [26]. In addition, particle image velocimetry (PIV) technology has become a well-established technique in the field of geotechnical model tests and displacement field calculations. Liu et al. [27,28] developed a foundation-bearing capacity model test system based on PIV technology and measured non-interference relative displacement on a test model constructed using transparent soil. Iskander et al. [29] conducted triaxial compression, consolidation, and permeability tests on prepared transparent soil. Ahmed et al. [30] analyzed tunnel-induced ground motion using a transparent soil model and PIV technology. Baba et al. [31] used PIV technology and a model test to analyze the influence of the slope hydraulic stress state on gravity creep deformation. Kashan et al. [32] used PIV technology to track the hydrodynamic behavior of fine-grained sediments with different hydraulic characteristics. Qi et al. [33] derived the displacement field using PIV technology to study soil deformation during the process of casing jacking and measured the soil deformation caused by the belled pile. White et al. [34] investigated the penetration characteristics of piles in sandy soils based on the PIV technique. Zhang et al. [35] used a PIV device to divide the radial splitting experiment process into four stages; the feasibility of the test device was demonstrated, but the object of study was remolded expansive soil, which did not involve undisturbed expansive soil.
Accordingly, in this study, Nanyang expansive soil was selected as the research object. We conducted a series of radial splitting tests on undisturbed and remolded expansive soil specimens during the drying process using the PIV test system and a self-developed splitting test apparatus to further examine the tensile strength change pattern and the crack development pattern of expansive soils during the drying process and their intrinsic mechanism. Our goal was to provide a scientific basis for the construction of geotechnical engineering related to expansive soil.

2. Materials and Methods

2.1. Materials

The expansive soil used for the test was collected from Nanyang City, Henan Province, from a depth of 6 m. The basic physical properties of the expansive soil samples are shown in Table 1. The samples have the same particle grading curve (Figure 1) and mineral composition as those used in the study of He et al. [36]; the primary mineral content of the expansive soil was 74%, the main component of which was quartz. Clay minerals accounted for 26%, of which the main clay mineral composition were illite and montmorillonite. The contents of illite and montmorillonite were similar—about 11% and 14%, respectively. The tested soil was a weakly expansive soil, according to the swelling classification criteria. It was a low-liquid-limit clay (CL) soil, according to the results of the particle analysis test and the liquid–plastic limit test.

2.2. Sample Preparation

When preparing the undisturbed specimen, we first cut the appropriate size of the square undisturbed specimen with a tool. Then, we placed the ring knife on the undisturbed soil sample, pressed the ring knife, and finally smoothed the two surfaces and removed the sample with a gasket. Then, the undisturbed specimen was weighed and the volume was measured. The dry density of the undisturbed specimen was 1.69 g/cm3, and the water content was 20%. To investigate the effect of different water contents on the radial splitting strength of the undisturbed and remolded expansive soils during the drying process, the remolded specimens were prepared in such a way that the dry density of the soil was controlled at 1.69 g/cm3 and the initial water content was controlled at 20%. The samples were made using the jack static pressure method. There were five specimens each of the undisturbed and remolded expansive soils, with diameters (d) of 61.8 mm and heights (h) of 20 mm. After the specimens were prepared, they were naturally air-dried to different water contents in an environment with a room temperature of 25 °C. The actual dry density and the dry target density were different, because the shrinkage degrees of the undisturbed and remolded expansive soils caused by the different water contents during the drying process were different. The actual water content and the actual dry density of the remolded and undisturbed specimens at the end of the experiment are shown in Table 2.

2.3. Testing Apparatus

The radial splitting test equipment, based on PIV technology, consists of a photo acquisition system and a loading system, as shown in Figure 2. PIV technology was used to obtain relatively continuous displacement pictures through the camera, and image processing software was used to process the two pairs of objects before and after moving, and finally to obtain a soil deformation vector map [27,28]. The photo acquisition system included a high-speed industrial charge-coupled device (CCD) camera, a light source, and the Davis 8.0 series software StrainMaster Function module. StrainMaster is a set of professional software for strain measurement and stress analysis, with user-friendly, flexible, and powerful features. The software provides a variety of algorithms, which can calculate the stress-strain field, the displacement field, the velocity field, and other results of the acquired image, and display the results in the format of a contour map, a cloud map and a vector map. In addition, the light source has a large impact on the photography of its PIV, so it is useful to use a wide-source LED lamp. The initial size of the specimens used in the test was d0 = 6.18 cm and h0 = 2 cm. The loading system used in the test was the CMT4000 electronic universal testing machine, produced by the Mester Company in the United States [37], which can set parameters, as needed, to automatically control constant speed loading and constant speed displacement.

2.4. Test Procedure

According to the procedures set out in references [35,37], the radial splitting test was performed at a constant speed, and the tensile load and displacement were monitored during the test, which was divided into the following four stages.
(a)
First, we ensured that the instrument sensor was well connected to the data acquisition system. Then, we placed the sample, which was naturally dried to a certain moisture content, on the base, as shown in Figure 1. We adjusted the universal testing machine button manually, in such a way that the upper platform was in contact with the specimen. We set the parameters of the loading equipment to displacement control and a loading rate of 1.4 mm/min.
(b)
We adjusted the position of the floodlights and the CCD camera, and adjusted the focus of the CCD camera to ensure that the clarity of the camera and the field of view were at their optimum.
(c)
We started the PIV measurement system, and calibrated the camera with the specially designed calibration plate. The criterion for completion of calibration was that the calibration coefficient was less than 0.3. After calibration, we set the shooting frequency to 7 photos/s. The test was completed after the specimen displayed obvious damage.
(d)
At the end of the test, we obtained a load–displacement curve and selected images for the beginning and end moments of each deformation stage during the damage of the specimen. We compared and analyzed these images using the PIV image analysis system, which comes with the appropriate test equipment to obtain a map of the soil deformation field and the displacement vectors of the specimen during crack development.

3. Results and Discussion

3.1. PIV Radial Splitting Test Results

As shown in Figure 3, the load–displacement curve of the radial splitting test of the undisturbed and remolded expansive soils with different initial water contents during the process of drying can be divided into four stages. The OA section was formed by the stress concentration of the upper and lower pressure heads at the contact part of the specimen. The AB section, in which the stress was transmitted into the specimen, was similar to the linear stage of the uniaxial compression test curve, which showed good linear characteristics. In the BC section, after the peak value was reached, the specimen began to display obvious cracks and the curve decreased rapidly. In the CD section, the specimen had a large radial splitting crack. The specimen then exhibited some residual strength. As the stress increased, the crack continued to expand until it penetrated the specimen, eventually leading to the complete destruction of the specimen.
The load–displacement curves of the specimens with different water contents all showed a strain-softening phenomenon, and this phenomenon became increasingly significant with decreasing water content. The ultimate load decreased with increasing water content. The reasons for the above phenomenon were as follows: During the drying process of the soil, the water content decreased, the suction increased, and the expansive soil shrank, which led to an increase in the splitting strength of the expansive soil. The undisturbed specimens, with 20% water content, showed no obvious softening after the peak value in the load–displacement curve because of the higher water content, and the displacement required to reach the peak strength was greater, requiring continued compression to provide the plastic deformation necessary to break the samples. The above test phenomenon was inconsistent with the test results obtained by Zhang at al. [35], due to the different production methods of the specimens.
The crack propagation process and the displacement vector field of the splitting failures of the undisturbed and remolded specimens with different water contents are shown in Figure 4 and Figure 5, respectively. Darker colors in the figures indicate greater displacement and greater deformation results in more obvious cracks. After the load passed through the approximately linear AB section, no obvious cracks appeared when the peak stress point B was reached; small cracks primarily appeared at the top and bottom of the samples. After point B, the load decreased to point C and the splitting crack appeared on the splitting surface; then, the load dropped sharply to point D. The crack development diagrams and the displacement vector fields of points B, C, and D are shown in Figure 4a–e and Figure 5a–e, respectively.
From the displacement vector field of the undisturbed specimen, it could be seen that the plasticity of the soil caused the specimen to undergo compression deformation at point B; however, no obvious cracks appeared and the displacement was primarily concentrated on the top of the undisturbed specimen. There were obvious cracks at point C after the peak value. The deformation of the undisturbed specimens with water contents of 7.5%, 17.5%, and 20% was primarily concentrated in the right half of the specimen, and the deformation developed with the crack to the lower right; the above phenomenon may have been caused by an uneven specimen or by stress concentration when the specimen was in contact with the pressure plate. The deformation of the undisturbed specimen with a water content of 11% was primarily concentrated at the top of the sample. The deformation of the undisturbed specimen with a water content of 15% was primarily concentrated in the left half of the specimen, and the deformation developed with the crack to the lower left. The load dropped to a trough at point D; at this point, the cracks coalesced and some of the undisturbed specimens experienced falling soil blocks. Because of the large displacement of the damaged part of the specimens, the displacement vector field obtained by the PIV analysis system was a blank region; this experimental phenomenon was consistent with the results obtained by Zhang et al. [35]. This phenomenon was increasingly obvious with increasing water content, as the specimen water content value increased and the time required to complete stages B–D became shorter and shorter.
From the displacement vector field of the remolded specimens, it can be seen that, at point B, the remolded specimens experienced compressive deformation without obvious cracks and the displacement was primarily concentrated at the top of the specimen. There were obvious cracks at point C after the peak value. The cracks in the remolded specimens with different water contents were primarily distributed in the center of the specimen, along the radial direction and approximately in a straight line. During the splitting process, the radial displacement of the sample was the largest, with almost no displacement to either side. The load at point D dropped to a minimum, the crack was radially penetrated, some of the undisturbed specimens were split along the radial direction, and the displacement vector field was a blank area. A comparison of the crack propagation process and the displacement vector field of the splitting failure for the undisturbed and remolded specimens with the same water content was consistent with the analysis of the load–displacement curves of the undisturbed and remolded specimens with the same water content. A comparison of the splitting damage fracture extension process and its displacement vector field for undisturbed and remolded specimens with the same water content was consistent with the analysis of the load–displacement curves under the same water content conditions. The failure process of the undisturbed specimens was more rapid and obvious because of their structure. The crack distribution of the remolded specimens was more uniform, and the displacement was concentrated in the radial direction within the specimens. The undisturbed specimens had a structurally uneven structure, while the remolded specimens had a more uniform and artificially prepared structure, which led to the above phenomenon.

3.2. Discussion of the Intrinsic Mechanism of the Test Results

To explore the intrinsic reasons for the lower tensile strength of the undisturbed specimens compared with the remolded samples under the radial splitting test, a three-dimensional plot of the variation in the tensile strength with the dry density and moisture content was plotted using Origin, as shown in Figure 6. Here, the x-axis represents the water content, the y-axis represents the dry density, and the z-axis represents the tensile strength. Note that when the x = 17.5 and y = 1.69 planes (the 17.5% water content plane and the 1.69 g/cm3 dry density plane, respectively) were established, the tensile strength of the undisturbed specimens was significantly lower than that of the remolded specimens. Therefore, the structure of the undisturbed specimens was assumed to affect its strength.
Comparing the images of the undisturbed and remolded specimens with w = 17.5%, as shown in Figure 7, it is obvious that the surface of the undisturbed specimen has depressions, whereas the surface of the remolded specimen is smoother. From this observation, it can be assumed that the strength of the undisturbed specimen was affected by its own structural nature (the presence of internal fissures).
According to the microscopic test results of ref. [36], during the drying process, the shrinkage degree of a specimen affects its strength. The cumulative mercury-intrusion-volume curve and the pore-size-distribution-density curve of undisturbed and remolded expansive soil were compared and analyzed. Because the undisturbed soil had a primary structure, there was a certain proportion of large pores or primary cracks between the aggregates (Figure 7a); after the remolded soil was crushed and sieved, the large pores between the aggregates disappeared and the pores in the aggregates gradually decreased, or even disappeared, during the drying process—mainly the inter-particle pores and small aggregates (Figure 7b). Therefore, the above radial splitting test results can be reasonably explained: because of their structural and fissure properties, the peak loads of undisturbed expansive soil specimens are lower than those of re-molded specimens and the crack characteristics of the remolded specimens are more regular than those of the undisturbed specimens under the same water-content conditions.

4. Conclusions

A PIV test system was used to conduct radial splitting tests on undisturbed and remolded expansive soils with different water contents during the drying process, and the internal mechanism was analyzed according to the microstructure. The main conclusions are as follows.
(1)
During the drying process, the water content has an important influence on the splitting strength of expansive soil specimens. The load–displacement relationship curve shows the strain-softening phenomenon and, with decreasing water content, the peak strength increases.
(2)
In the process of the radial splitting test, there was an obvious one-to-one correspondence between the load–displacement relationship curve, the displacement vector field, and the crack characteristics in the stage division. With the help of PIV technology, the compression deformation stage of the radial splitting test, the stage of crack development after the peak load, and the stage of crack development maturity until breakthrough failure could be observed.
(3)
The undisturbed soil has a primary structure, and there is a certain proportion of large pores or primary cracks between the aggregates. After the remolded soil was crushed and sieved, the large pores between the aggregates disappeared and the pores in the aggregates gradually decreased or even disappeared during the drying process. Because of their inherent structural and fissure characteristics, the peak loads of undisturbed expansive soil specimens were lower than those of remolded specimens and the crack characteristics of remolded specimens were more regular than those of undisturbed specimens under the same water-content conditions.
(4)
The above research results provide the scientific basis for the construction of geotechnical engineering related to expansive soil.

Author Contributions

Conceptualization, S.Y., F.H. and J.Z.; methodology, S.Y., F.H. and J.Z.; data curation, S.Y., F.H. and J.Z.; writing—original draft preparation, S.Y. and F.H.; writing—review and editing, J.Z.; visualization, S.Y. and F.H.; supervision, J.Z.; project administration, S.Y., F.H. and J.Z.; funding acquisition, S.Y., F.H. and J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This study was financially supported by the National Natural Science Foundation of China (Grant No. 41602295), the Foundation for University Key Teachers by the Ministry of Education of Henan Province (Grant No. 2020GGJS-094), the Postgraduate Education Reform and Quality Improvement Project of Henan Province (YJS2023AL004) and the Water conservancy science and technology project of Henan Province (Grant No. GG202240).

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

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Applsci 13 08050 g001 550
Figure 1. Grading curve of Nanyang expansive soil.
Figure 1. Grading curve of Nanyang expansive soil.
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Figure 2. Schematic diagram of the test apparatus.
Figure 2. Schematic diagram of the test apparatus.
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Figure 3. Load–displacement curves: (a) undisturbed specimens; (b) remolded specimens.
Figure 3. Load–displacement curves: (a) undisturbed specimens; (b) remolded specimens.
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Applsci 13 08050 g004a 550Applsci 13 08050 g004b 550Applsci 13 08050 g004c 550
Figure 4. Crack propagation and displacement vector fields of the splitting failure for undisturbed samples with different water contents.
Figure 4. Crack propagation and displacement vector fields of the splitting failure for undisturbed samples with different water contents.
Applsci 13 08050 g004aApplsci 13 08050 g004bApplsci 13 08050 g004c
Applsci 13 08050 g005a 550Applsci 13 08050 g005b 550
Figure 5. Crack propagation and displacement vector fields of the splitting failure for remolded samples with different water contents.
Figure 5. Crack propagation and displacement vector fields of the splitting failure for remolded samples with different water contents.
Applsci 13 08050 g005aApplsci 13 08050 g005b
Applsci 13 08050 g006 550
Figure 6. Three-dimensional diagram of the tensile strength with respect to the dry density and water content.
Figure 6. Three-dimensional diagram of the tensile strength with respect to the dry density and water content.
Applsci 13 08050 g006
Applsci 13 08050 g007 550
Figure 7. Comparison of the undisturbed and remolded specimens with w = 17.5%: (a) undisturbed specimens; (b) remolded specimens.
Figure 7. Comparison of the undisturbed and remolded specimens with w = 17.5%: (a) undisturbed specimens; (b) remolded specimens.
Applsci 13 08050 g007
Table
Table 1. Basic physical properties of the Nanyang expansive soil.
Table 1. Basic physical properties of the Nanyang expansive soil.
Specific
Gravity Gs		Liquid Limit
wL/%		Plastic Limit
wp/%		Plasticity Index IpUndisturbed Soils Dry
Density ρd/(g/cm3)		Undisturbed Soil Water Content w/%Free Swelling Ratio δf/%
2.747.4325.2822.151.692052
Table
Table 2. Target water content, actual water content, and actual dry density of the undisturbed and remolded specimens.
Table 2. Target water content, actual water content, and actual dry density of the undisturbed and remolded specimens.
I1I2I3I4I5R1R2R3R4R5
Target water content w/%20.017.515.011.07.520.017.515.011.07.5
Actual water content w/%20.819.217.015.912.420.517.614.811.08.4
Actual dry density ρd (g/cm3)1.691.721.681.771.891.701.791.831.911.88
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Yu, S.; He, F.; Zhang, J. Experimental PIV Radial Splitting Study on Expansive Soil during the Drying Process. Appl. Sci. 2023, 13, 8050. https://doi.org/10.3390/app13148050

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Yu S, He F, Zhang J. Experimental PIV Radial Splitting Study on Expansive Soil during the Drying Process. Applied Sciences. 2023; 13(14):8050. https://doi.org/10.3390/app13148050

Chicago/Turabian Style

Yu, Shun, Fangchan He, and Junran Zhang. 2023. "Experimental PIV Radial Splitting Study on Expansive Soil during the Drying Process" Applied Sciences 13, no. 14: 8050. https://doi.org/10.3390/app13148050

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