Expansive Clay Cracking Behavior through Digital Image Correlation

. Expansive soils may present cracks arising from the drying process and their evolution can cause irreparable damages to engineering projects. Investigating this phenomenon is vital to understanding its geomechanics. The objective of this article is to present numerical modelling of the formation and propagation of cracks in expansive soil. A desiccation experiment was therefore carried out using an expansive silty clay from Paulista, in northeastern Brazil. The drying process was monitored by measuring the temperature and relative humidity of the air, as well as by capturing images with a camera. The digital images were correlated using the Ncorr numerical tool in MATLAB. As a result, this study made it possible to conclude that the soil cracking dynamics presented a non-orthogonal pattern during the dryness test, while the image treatment made it possible to observe the tendency of cracks to appear and propagate on the soil surface, allowing for the detection of crack growth and propagation trends.


Introduction
Expansive soils are complicated for civil engineering structures all over the world to deal with, potentially causing environmental and socioeconomic damage. This type of soil is often found in arid and semi-arid regions, where the moisture range is large, and where evapotranspiration exceeds volumetric precipitation.
Due to dryness from seasonal environmental variance, the formation and propagation of cracks in the soil occur naturally, as the soil loses water content and its volume reduces, with behaviour alternating between wet and dry seasons. During the dry season, the soil retracts and cracks emerge at the surface, changing the soil's mechanical and hydraulic properties [1,2,3].
More recently, image analysis techniques have proved to be powerful tools through which several geometric and morphologic parameters such as crack width, length, area, angle, and distribution characteristics can be determined effectively [9][10][11].
In addition, some modelling and theoretical studies on desiccation cracking have also been investigated. However, the essential mechanism of desiccation cracking is still not well understood, and the ability to predict the initiation of cracking and subsequent crack network propagation also faces several challenges [12][13][14][15][16].

Soil
The soil investigated in this article is from Paulista, in northeastern Brazil, and its physical properties are presented in Table 1. Free swell index (%) 25 The soil had a high liquid limit value of around 74%, with a plastic limit of 26%, giving a plasticity index of about 48. According to the Unified Soil Classification System (USCS), the soil was classified as high plasticity clay (CH).

Experimental Procedure
The experimental approach combined a classical study of cracks with the determination of local deformations and displacements during drying using digital image correlations (DIC).
The experimental device is illustrated in Figure 1. The soil was initially prepared with a water content equivalent to its liquid limit, and a void ratio of 2.04.
Then, a flat glass capsule (15.2 mm deep and 147 mm in diameter) was filled with a thin layer of the soil. No lubrication, such as Vaseline, was used on the glass capsule, because it can have an effect on the development of crack and their pattern.
The equipment consisted of computer software, four lights, and a digital camera. Pictures of the soil surface were taken at regular intervals of 10 minutes during desiccation. Structural inspections typically depend on professional practice, which can cause details to be missed or even faults to be detected. [15] Digital Image Correlation (DIC) therefore assists in detecting displacements on the sample surface, indicating tensile and compression strain occurring in the material [15].
This study uses a DIC analysis of desiccation in order to identify the conditions that lead to crack formation at an early stage. It also shows the main features detected only by DIC during the test and those seen by both DIC and the naked eye.

Desiccation Experiment
Some images were selected from the laboratory experiment to show the behaviour of the expansive clay during drying. The images for selected time steps during the desiccation experiment are shown in Figure 2. Figure 2.a shows that there were no cracks on the surface until 3 h 20 min, while at 5h (Figure 2.), a crack had formed on the left side of the sample. In this test configuration, it can be seen that cracks initiated within the sample, while later, at 8 h 20 min (Figure 2.c), cracks concentrated near the edge. In Figure 2.d, cracks that developed in the middle of the sample formed at orthogonal directions, in the form of a "T", while near the edge, they were shaped more like a "Y".
This behaviour continues until Figure 2.f. In Figure  2.g, however, a Y-shaped joint has formed on the left side of the sample, where a secondary crack connected to a primary one.  Figure 2.h shows that, after 21 h 40 min of desiccation, no new cracks formed. However, cracks continued to propagate by increasing in length and width.
Overall, in Figure 2, it can be seen that cracks emerged initially in the middle of the sample, and later near the edges. As these cracks developed and propagated in the sample during drying, the sample was also seen to shrink and peel away from the edge of the glass capsule.

Digital Image Correlation Technique
Digital Image Correlation (DIC) is an optical nondestructive technique for measuring strain and displacement [17]. Compared to other techniques, DIC is simple, cost effective, and accurate, leading to a great number of potential applications. [18] This method works by comparing the positional changes on the speckle pattern of digital images from different stages of a sample deformation. The system can measure the surface displacement by tracking the speckle position, building up strain map. It requires no special surface preparation in many cases that the sample surface has sufficient image texture for DIC [18].
Due to its simplicity and efficiency it has been used for a variety of engineering investigations, such as measuring thermal expansion and distortion of electronic components, measuring the mechanical properties of nuclear graphite, measuring displacement and the opening of cracks in rail and road bridges, among others [18].
This technique helps to see strains in structures that are invisible to the naked eye, which could help to predict structural deformation [18].
In this study, some stages of deformation during the desiccation test were selected in order to help see features that were missed by the naked eye.  In Figure 3.II, obtained from Figure 2.b, after a crack appeared, strain concentrated near the edge, indicating a concentric shrinkage.

DIC Analysis
In Figure 3.III, the reference image was from 5 h (Figure 2

Conclusions
This article presented a desiccation test of expansive clay from Paulista, in northeastern Brazil. The soil volume shrank as it dried. It was verified that the crack pattern was similar to those found in the literature.
Also, to assist the study of crack formation and propagation, a DIC analysis was carried out, which showed that it is possible to verify the positions where cracks wills form and their likely direction of propagation within the sample.