Desiccation cracking behavior of discrete fiber mixed with clay material

Cracks in soil provide significant preferential pathways for contaminant transport and rainfall infiltration. Water exchange between the soil matrix and crack is crucial to characterize the preferential flow, which is often quantitatively described by a water exchange ratio. The water exchange ratio is defined as the amount of water flowing from the crack into the clay matrix per unit time. Most of the previous studies on the water exchange ratio mainly focused on cracked sandy soils. The water exchange between cracks and clay matrix were rarely studied mainly due to two reasons: (1) Cracks open upon drying and close upon wetting. The deformable cracks lead to a dynamic change in the water exchange ratio. (2) The aperture of desiccation crack in clay is narrow (generally 0.5 mm to 5 mm) which is difficult to model in experiments. This study will investigate the water exchange between a deformable crack and the clay matrix using a newly developed experimental apparatus. An artificial crack with small aperture was first fabricated in clay without disturbing the clay matrix. Water content sensors and suction sensors were instrumented at different places of the cracked clay to monitor the water content and suction changes. Results showed that the water exchange ratio was relatively large at the initial stage and decreased with the increasing water content in clay matrix. The water exchange ratio increased with increasing crack apertures and approached the largest value when the clay was compacted at the water content to the optimal water content. The effective hydraulic conductivity of the crack-clay matrix interface was about one order of magnitude larger than that of saturated soil matrix.

Investigation of the damage induced by desiccation and heating of Tournemire argillite using digital image correlation

Bagasse is the residue after juicing sugarcane, and bagasse is a recyclable biological resource that can be used in many ways. Under arid climatic conditions, the clay material shrinks and loses water. Many crisscross drying shrinkage cracks formed on the surface and inside the soil will affect the stability of the soil. In this article, the cracking characteristics of clays with different bagasse contents during the evaporation process have been studied. The cracks were extracted and calculated by digital image processing technology, and the crack characteristics of samples with different bagasse contents were studied by fractal dimension and crack entropy. The results show that when moisture content maintains 34%, the clay material forms only one main crack without bagasse, forms no crack with bagasse content of 9%, and forms a crack network with the bagasse content of 3%. Adding bagasse to the clay can advance the cracking time of the clay, and different contents of bagasse have different effects on the cracking time of the clay. Among these five experimental groups, the clay was affected the most with 3% bagasse content, while the least effect happened with 6% bagasse content. The time can be advanced when the clay crack entropy appears and increases the size of the crack entropy.

Physical modeling of desiccation cracking in plastic soils

Carbon fiber is a common waste building material, but its effect on the drying and cracking properties of clay materials is unknown. In this paper, crack rate and fractal dimension are used to characterize the influence of waste carbon fiber materials on the development of soil cracking. With the rise in carbon fiber content to 0.2%, 0.4% and 0.6%, the crack rate of soil cracking decreased by 7.9%, 17.3% and 23.3%, respectively, while the fractal dimension of soil cracking decreased by 2.4%, 8.7% and 21.2%, respectively. Accordingly, the critical moisture content of the soil samples increased by 33.2%, 110% and 151%, and the time of the soil constant evaporation stage decreased by 5.1%, 13.8% and 34.5%, respectively. When carbon fiber is combined with soil, carbon fiber will increase the interface bonding strength, friction and interlocking force, effectively inhibiting the cracking of soil, and it provides a channel for water transport in the soil in the early stage.

In this study, the desiccation cracking behavior of lateritic soil caused by drying and wetting cycles was investigated and effective methods for mitigating crack development were proposed. Direct mixing and spray coating methods based on different additives were used to modify lateritic soil. Cyclic wet–dry tests were performed to analyze the influence of wet–dry cycles on the cracking behavior. Subsequently, uniaxial tensile tests were conducted to examine the strength degradation caused by crack formation and strength enhancement by additives. In addition, the modification mechanisms were revealed using electron microscopy. The results demonstrated that desiccation cracks developed significantly during the drying process, with some cracks closing upon wetting. However, most of the cracks reopened and expanded further during subsequent drying, leading to a steady increase in the crack rate during the wet-dry cycles. When using the direct mixing method, lignocellulose was the most effective additive for enhancing the crack resistance of lateritic soils. The optimal crack resistance was achieved with a lignocellulose content of 0.75%, resulting in an 18.2% increase in tensile strength. Conversely, when employing the spray coating method, PAC was found as the optimal additive, with a desirable concentration of 1.5–6.0%.

This study investigates the complex interplay between wetting–drying (W-D) cycles and initial compaction states on desiccation cracking and the mechanical behavior of different clayey soils. Natural CH, CL, and ML soils, distinguished by their chemical composition and plasticity, are subjected to a meticulously designed experimental program. The specimens are remolded at various initial compaction states, including the optimum moisture content (wopt) having maximum dry density (γdmax), and wet and dry sides of the compaction curve having identical initial dry density (γd0). Subsequently, they undergo multiple W-D cycles, systematically documented through cinematography. Mechanical response is assessed after different W-D cycles. It is observed that desiccation cracking within both CL and CH initiates after the first W-D cycle, intensifying rapidly after the second cycle and reaching an optimal cracking state after the third cycle. The crack analyses indicate a transition from surface cracking to deeper-seated cracks with an increase in W-D cycles. CH soil, characterized by a 2:1-layered clay mineral with a high propensity for swelling and shrinkage, exhibits elevated desiccation cracking at high w0 for identical γd0. Notably, CH soil exhibits maximum cracking at the wopt and γdmax. In contrast, CL soil, characterized by a 1:1-layered clay mineral, displays an inverse response across all compaction states, and ML soil, characterized by a scarcity of clay mineral, shows insignificant cracks. This disparity in behavior is closely attributed to clay mineralogy and microstructure, which define the underlying mechanism responsible for the generation of internal stresses in the soil structure induced by moisture fluctuations causing desiccation cracking. Stiffness and unconfined compressive strength (qu) of CH and CL increase and compressibility decreases as w0 increases after undergoing W-D cycles due to the volume shrinkage response of specimens. Meanwhile, for a particular compaction state, strength decreases while compressibility increases with increasing W-D cycles.

Coupling effects of interfacial friction and layer thickness on soil desiccation cracking behavior

Secure storage of nuclear spent fuel (NSF) is of great concern for protecting public health and safety. The preferred long-term solution is underground containment in geological repositories, where one or more engineered barrier materials (EBM) encapsulate the NSF and separate it from the natural rock. Bentonite clay is commonly used as an EBM due to its many advantageous properties including low hydraulic conductivity, which ensures limitation of water infiltration to the system and the subsequent risk of corrosion in NSF canisters. However, bentonite clay subjected to heating from nuclear decay may form desiccation cracking. This study conducted disk-shaped free shrinkage tests and ring-shaped restrained shrinkage tests of bentonite clay samples reinforced with basalt microfibers. Digital image correlation was used as a noncontact full-field displacement measurement to track the time-evolving shrinkage and desiccation cracking phenomena and make quantified comparisons between plain bentonite and bentonite with varying contents of basalt microfibers (i.e., 0.0, 0.5, 1.0, and 1.5 % wt.). Results indicate that plain bentonite and basalt microfiber-reinforced samples showed similar free shrinkage behavior, while desiccation cracking behavior was significantly altered by adding basalt microfibers. Microfiber reinforcement effectively reduced major cracks through a “crack-bridging” effect while causing minor cracks to initiate earlier and at higher moisture contents than plain bentonite. Results infer that reinforcing plain bentonite with inorganic microfibers can potentially control desiccation cracking, leading to safer and improved nuclear waste management.

Crack patterns formed due to desiccation of clay or similar materials showdistinctive reproducible patterns. If one measures the cumulative areaAcum covered by thecracks with widths ≥Wmin, then Acum plottedagainst Wmin shows a typical reproducible shape. In a log–log plot, this curve has two roughly linearregions with different slopes. For a polypropylene (PP) substrate, there is a sharp changefrom a nearly horizontal line to a very steep line, whereas for a glass substrate,which is smoother, there is a gradual changeover between the two regions. Wepropose a simple one-dimensional spring chain model, in which reducing the naturallength of the springs corresponds to the desiccation process. Springs may break, orslip against the substrate to accommodate strain beyond a specified threshold.The model successfully reproduces the successive stages of crack formation andbehaviour of the cumulative area curve, as observed in experiments. The difference inthe qualitative nature of the patterns on smooth and rougher substrates is alsoobtained.

Laboratorial tests were conducted on soil to investigate the desiccation cracking behavior at different temperatures (22, 60 and 105 °C). The initiation and propagation of desiccation cracks during drying was monitored using a digital camera. By applying image processing technique, the geometrical parameters of the crack networks were quantitatively determined. The distribution characteristics of crack width, length and clod area were described by employing statistical method. In addition, the surface crack ratio Rsc (the surface of cracks to the total surface) was introduced to quantify surface cracking extent at different water contents. The results show that soil desiccation cracking behavior and the geometrical characteristics of crack networks depend strongly on temperature. On the whole as the temperature increases, crack initiates earlier and the extent of cracking also increases. The final surface crack ratio, number of clods per unit area, average length of cracks and average width of cracks increases with increasing temperature, while the average area of clods, number of nodes per unit area and number of crack segments per unit area decreases with increasing temperature. The statistic results of the crack parameters indicate that the increase of temperature increase the probability of initiating longer and wider cracks and smaller clods during drying.

Stabilization using natural fiber is an inexpensive technique to increase the properties of weak soil. Soil swell-shrinkage characteristics cause engineering problems or damage the existing structures. The effects causing damage are mainly the demolition of buildings, roads and pipelines in uncropped soils and the leaching in landfills through desiccation cracks. So a better comprehension of the desiccation cracking process is essential. This study comes out with a natural material, areca fiber. Areca is accessible adequately in so many regions all over the world, yet its application in geotechnical studies has not been investigated broadly. In this current study, the appropriateness of areca fiber in the stabilization of weak soils is shown through experimental examinations. Bottom ash (BA) is used as a stabilizing agent along with areca fiber. The tests conducted are compaction tests, unconfined compression strength (UCS) tests and a series of wetting–drying cycles. Experiments are conducted in the laboratory on the crack parameters, like crack density factor (CDF) and crack intensity factor (CIF) and characteristics of desiccation cracks on the surface of soil at different percentages of areca fibers. The image processing technique (DIA) is used to quantitatively analyze the morphology characteristics of crack patterns formed at each drying path.KeywordsDigital image analysisAreca fiberUnconfined compressive strength

Experiment evidence on the temperature dependence of desiccation cracking behavior of clayey soils

Improvement of the crack resistance of clayey soils by fiber reinforcement was investigated using initially saturated and fiber-reinforced soil specimens subjected to desiccation. An image-processing technique was used to quantitatively describe the effect of fiber addition on the geometrical and morphological characteristics of crack patterns. The results show that the soil desiccation cracking behavior was significantly influenced by fiber inclusion: the crack resistance was significantly improved and the amount of desiccation cracks was significantly reduced by fiber addition. Generally, the surface crack ratio (surface of cracks to total surface), number of clods, average length and width of cracks, and crack network connectivity decreased with increasing fiber content, while the average area of clods, number of nodes per unit area, number of crack segments per unit area, crack density, and specimen integrity increased. During crack propagation, the surface crack ratio increased with decreasing water content and finally reached stabilization. Comparison between the surface crack ratio of the natural soil specimen and that of the fiber-reinforced soil specimen showed that the former was always higher than the latter. The fiber length was found to have an insignificant effect on the soil desiccation cracking behavior.

Bentonite, when used as buffer/backfill material in the deep disposal of high‐level radioactive waste (HLW), could undergo desiccation shrinkage or even cracking due to the heat released from HLW, impairing the efficiency of the barrier system. Furthermore, in‐service buffer materials are inevitably in contact with the groundwater, which sometimes contain high salt concentrations. The groundwater salinity may modify the properties of bentonite and hence affect the process of desiccation and its performance. To investigate this effect, in this study, a series of temperature‐controlled desiccation tests was conducted on compacted specimens of Gaomiaozi (GMZ) bentonite preliminarily saturated with two different saline solutions (NaCl and CaCl2) at the concentration varying from 0.5 to 2.0 mol/L. The experimental results indicated that, as the concentration of saline solution increases, the initial saturated water content of bentonite decreases, whereas the residual water content at the completion of the desiccation test increases. The water evaporation rate is reduced for the specimens saturated with a high‐concentration saline solution, and CaCl2 has a more significant influence on water evaporation than NaCl. The evolution of cracks on the sample surface during the desiccation process can be divided into four stages: crack growth, maintenance, closure, and stabilization; an increase in the salt concentration effectively inhibits crack development. It was shown that the infiltration of saline solutions alters the microstructure of bentonite by changing the arrangement of clay particles from a dispersed pattern to more aggregate state, which results in a decrease in shrinkage strain and shrinkage anisotropy.