Crack Aperture Research Articles

Influence of crack aperture and orientation on the uniaxial compressive behavior of rock

The pre-existing cracks with various geometric and physical properties significantly affect the mechanical behaviors of rock mass. In this study, uniaxial compressive tests are performed on numerical models of specimens containing a single pre-existing crack with different apertures and orientations based on PFC 2D to shed light on the impact of different crack apertures and orientations. The results reveal that the uniaxial peak strength and elastic modulus tend to decrease with increasing apertures, but they tend to increase with increasing inclination angles. Crack apertures can affect the stress intensity factor in the vicinity of the pre-existing crack tips, which further influences the crack initiation and crack type. Furthermore, shear cracks are more prone to occur in a pre-cracked specimen with a larger crack aperture and larger crack orientation. The crack initiation stress and strain initially show a slight decrease until the inclination angle reaches 45°, and then significantly increase with the further increase of angle from 45° to 90°. The microcracks occur earlier and easier when the inclination angle is 45°. As the inclination angle of the pre-existing crack increases, shear-sliding along the crack becomes more prominent, resulting in the formation of coplanar secondary cracks. The macro failure patterns switch from tensile failure to shear failure with increasing crack inclination angle, which is in good agreement with trend lines of displacement in the displacement field.

Microbial Mineralization-Based Rapid and High-Strength Simultaneous Repair of Surrounding Rock Fracture Zones and Lining Cracks

Grouting technology based on microbial mineralization represents a novel approach to enhancing the properties of rock and soil. Widely studied for its combination of high efficiency and environmental friendliness, this method improves the strength and permeability of rock and soil. In this study, a novel approach involving bioslurry + filling particles was proposed to reinforce sand columns. It was observed that the addition of filling particles provided new nucleation sites for crystallization, significantly enhancing the cementation effect of sand particles. After 3 days of grouting treatment, sand columns containing 10% filling particles exhibited an order of magnitude reduction in permeability with the unconfined compressive strength (UCS) reaching 2.5 MPa, more than twice that of sand columns reinforced with bioslurry alone. Additionally, this study presented a method for the simultaneous repair of sand particles and concrete cracks. Results indicated that after 3 days of the grouting treatment, the interfacial shear strength between sand columns and concrete reached 100 kPa. Moreover, for concrete with crack apertures less than 1 mm, the permeability coefficient was reduced by over 80%, while the recovery ratio of the splitting tensile strength reached 64.2%. These findings highlight the potential of microbial mineralized grouting technology in addressing practical challenges, providing a foundation for the rapid and high-strength simultaneous repair of tunnel-surrounding rock fracture zones and lining cracks.

Experimental Study on Water-Plugging Performance of Grouted Concrete Crack.

In this paper, ordinary Portland cement, ultrafine cement, polyurethane, and epoxy resin were selected as typical grouting materials. Grouting simulation tests were first conducted to prepare the grouted concrete crack sample. The effect of concrete crack parameters (i.e., crack aperture and roughness), grout water-cement ratio, and grouting pressure on the water-plugging performance of different grouting materials was explored through the impermeability test. The microstructure of grouted concrete cracks was analyzed by means of scanning electron microscopy (SEM) and computed tomography (CT), and the difference in water-plugging performance of different grouting materials was explained at the micro level. The results show that the impermeability of the four grouting materials was ranked as follows: Epoxy resin > polyurethane > ultra-fine cement > ordinary Portland cement. The concrete cracks grouted by epoxy resin have the highest plugging failure water pressure and the lowest permeability, which is the optimal grouting material. The effectiveness of crack grouting in water-plugging was directly proportional to the grouting pressure, provided the pressure did not exceed a certain value. When the pressure surpassed the threshold, the increase in pressure did not have a significant impact on the water plugging performance. For the two cement-based materials, the threshold pressure was 1 MPa, while for the other two chemical grouts, it was 2 MPa. The two cement-based grouts with a water-cement ratio of 0.8 showed optimal water-plugging performance. The water-plugging performance of ordinary Portland cement paste, ultra-fine cement pastes, and polyurethane grout was negatively correlated with crack aperture and positively correlated with crack roughness. However, the water-plugging performance of epoxy resin grout was not affected by crack aperture or roughness.

Effect of fully-grouted bolts on the failure behaviors of mine pillars: Insights from block-based FDEM modeling

In this study, the Voronoi-based breakable block model (VBBM) based on the combined finite-discrete element method (FDEM) is used to investigate the failure mechanism of mine pillars and the rock-rockbolt interactions. The mechanical parameters of grout-rock and grout-bolt interfaces of a fully-grouted bolt are calibrated via numerical pull-out tests. Then, uniaxial compression tests are carried out on the reinforced pillars. The kinetic energy release rate (KERR) and crack aperture (CA) are used to evaluate the reinforcement effect. The results revealed that the VBBM can effectively characterize the damage failure mechanisms of spalling on the pillar surface and conjugate shear failure in the core zone of the pillar and reproduce the pre-peak nonlinear deformation and post-peak pseudo-ductile behaviors. The passive confining pressure generated by rockbolts is activated only when the rock mass produces sufficient deformation, and the constraints generated by different numbers of rockbolts can only affect the post-peak behavior of the pillar. There is a gradient feature in the internal deformation of the pillar from the shallow surface to the core. If the rockbolt density meets the strain demand, then rockbolts can delay the inward expansion of cracks by generating local constraints and can effectively fix the spalling rock blocks. With increasing support pressure, the CA and KERR exhibit a decreasing trend and strong power-exponential relationships with support pressure, indicating that there is a transition interval of support pressure and an optimal value for setting the rockbolt density.

Analysis of the stability of cracked soil slopes using a numerical model incorporating stochastic theory

Desiccation crack is a prevalent natural phenomenon that plays a significant role in the stability of soil slopes. In this study, a hydromechanical coupling model incorporating a layer of stochastic cracks is developed for analyzing cracked soil slopes. To properly consider the anisotropy and spatial variability of desiccation cracks, three crack indices are generated through cross-correlated random fields via Cholesky decomposition. The seepage and mechanical behavior of a cracked slope are analyzed by adjusting stochastic parameters and rainfall conditions. Applied to the Ningzhen Mountains area in China, the model investigates the stability of slopes under various annual meteorological conditions. The results indicate that neglecting the spatial variability of cracked layer properties can lead to inaccurate assessments of instability risks at the base and water accumulation at the top of slopes. During heavy rainfall, slopes with deeper (up to 5 m) and weaker cracked layers often show a roughly planar sliding morphology. Moreover, the uncertainties in crack depth have the most pronounced influence on the uncertainties of the slope stability, more than horizontal permeability or crack aperture. The average crack aperture’s influence on slope stability depends on the relationship between crack infiltration rate and rainfall intensity.

Permeability evolution in fine‐grained Aji granite during triaxial compression experiments

AbstractTriaxial compression experiments were carried out on samples of fine‐grained Aji granite to measure the evolution of permeability during deformation prior to failure under confining pressures of 20 and 40 MPa. During the initial stages of deformation, a small decrease in permeability was observed, due to the closure of pre‐existing microcracks; permeability then increased with increasing differential stress. During deformation, permeability varied by up to two orders of magnitude, and we observed a small pressure dependence, with a larger variation observed at 20 MPa than at 40 MPa. This suggests that more cracks developed during brittle deformation under the lower confining pressure. The observed increase in permeability during our experiments was approximately proportional to inelastic volumetric strain, which corresponded to the volume of dilatant cracks. On the other hand, prior to brittle failure, we observed a further increase in permeability that was greater than the inelastic volumetric strain, suggesting crack aperture opening accelerated at this stress level (>∼80%). The permeability enhancement resulting from the crack dilation affects the gas‐sealing capacity of the fluid‐saturated caprock. Our experimental findings would be beneficial for the safety assessment in applications such as the long‐term storage of various gaseous wastes.

Comparative verification of hydro-mechanical fracture behavior: Task G of international research project DECOVALEX–2023

Numerical simulations become a necessity when experimental approaches cannot cover the required physical and time scale of interest. One of such area is a simulation of long-term host rock behaviors for nuclear waste disposal and simulation tools involved in the assessment must go through rigorous validation tests. The DECOVALEX project (Development of COupled models and their VAlidation against EXperiments) is dedicated to this purpose by international participants.aawww.decovalex.org. This work is part of the ongoing phase DECOVALEX–2023 (D–2023, Task G) particularly aiming to simulate fracture behaviors under various conditions. Here, we cross-verified a variety of numerical methods including continuous and discontinuous approaches against four benchmark exercises with emphasis on numerical accuracy and parameterization of the various numerical approaches. The systematic inter-comparisons of test cases highlight advantages and disadvantages of the different numerical models. Numerical details on discretization effects (e.g. mesh density and orientation) and domain size were investigated in detail for practical applications. It became evident that meticulous attention to mesh resolution and domain size is imperative for achieving accurate numerical simulations, even for static cracks. Moreover, when comparing numerical methods to closed-form solutions for static cracks, all models successfully reproduced the maximum crack opening but encountered challenges near the crack tips. Finally, the paper discusses how to convert between and therefore compare parameters of various numerical approaches. Our benchmark studies reveal that each model necessitates a distinct number of parameters, even in simple scenarios like static crack aperture benchmarks. It is generally more practical to employ fewer parameters to mitigate model over-parameterization and enhance experimental feasibility.

Effect of water on the rock strength and creep behavior of green mudstone

The weakening of rock mass after immersion substantially affects slope stability. Considering the mudstone in the north slope of the Fushun West Open pit as the research object, we investigated the failure and creep characteristics of mudstone under different immersion times and confining pressures. For this, we used a self-developed experimental apparatus that applies biaxial compression loading in a water immersion environment. Water significantly changed the strength and failure mode of the mudstone. The increase in confining pressure limits the expansion of the crack aperture, reduces the rock strength-weakening coefficient, and delays the rock weakening time. The change in cohesion is primarily responsible for the weakening of shear strength during immersion. Based on the creep test results, the creep curve of the soaking specimen was identical to that of the dry specimen in the initial phase. The weakening effect of water on the specimen increased as the soaking duration increased, due to which the creep curve of the soaking specimen gradually approached that of the saturated specimen. Water entered the microcracks and reacted with the rock interior, accelerating rock failure. Consequently, the soaking specimen had the lowest stress level for creep failure and the shortest failure time. The results of this experiment provide a reference for the long-term stability of the flooded slope.

Depth-dependent hydraulic conductivity tensor for jointed granite

Stress-dependent hydraulic conductivity tensor is formulated in a closed form on the basis of Oda's permeability tensor and Bianchi & Snow's proposal for change of the crack aperture. Using a simple cracked model, a general equation is derived to obtain the hydraulic conductivity tensor as a function of depth and the coefficient of earth pressure at rest. The general trend of the equation is consistent with the in-situ hydraulic conductivity values measured at various sites. Conductivity itself does not have any practical meaning unless the state of stress is concurrently specified. Anisotropy in the hydraulic conductivity tensor is mainly related to the state of stress given by the earth pressure coefficient rather than the crack geometry, particularly in the cases where three orthogonal sets of joints are observed. Anisotropy is magnified in two dimensions because of excessive confinement of the flow direction. Additional procedures should be performed when the results obtained from two-dimensional analyses are generalized for three-dimensional real sites. The hydraulic conductivity K0 at the earth surface and γ′H for controlling the aperture of cracks are proposed as index measures for characterizing a given site from a comprehensive point of view.

Concurrent Active Acoustic and Deformation Monitoring of Hydraulic Fracture in Laboratory Experiments

The results of laboratory experiments on concurrent active acoustic and deformation monitoring of a hydraulic fracture are presented. The experiments were carried out in a model material based on gypsum. For comparison, reference experiments were carried out to study ultrasonic waves propagation through a liquid-filled gap of controlled width between two precision glass plates. The purpose of the experiments was to study the dependence of the amplitude of the ultrasonic wave passing through the crack on the magnitude of its opening. In these experiments, a circular hydraulic fracture was created, the plane of which was perpendicular to the axis of the cylindrical sample. A cased injection well was located along the same axis, ending at the middle of its height. The sample was placed between two aluminum alloy disks equipped with piezoceramic transducers built into them, operating both in the emitter and receiver modes. Through the channel in the lower disk, the working fluid was supplied to the fracture. The sample was saturated with pore fluid through the upper disk. The entire assembly was placed in a hydraulic press providing a constant compressive force. The crack opening value varied depending on the flow rate of the fluid supplied to the center of the crack and was measured by the relative change in the distance between the disks of the assembly using inductive displacement transducers. Experiments were also carried out simulating a fracture filled with proppant. In this case, the crack aperture varied depending on the applied vertical force on the sample. Based on the results of experiments carried out under various conditions, the dependences of the amplitude of the ultrasonic wave that passed through the crack were plotted. It has been experimentally established that sound attenuation in the hydraulic fracture, which has a natural roughness, is two times lower than in the gap between precision glass plates. The obtained results will make it possible to estimate the value of hydraulic fracture opening in laboratory experiments conducted on larger samples using active acoustic monitoring.

Effects of dynamic changes of desiccation cracks on preferential flow: experimental investigation and numerical modeling

Abstract. Preferential flow induced by desiccation cracks (PF-DC) has been proven to be an important hydrological effect that could cause various geotechnical engineering and ecological environment problems. Investigation on the PF-DC remains a great challenge due to the soil shrinking–swelling behavior. This work presents an experimental and numerical study of the PF-DC considering the dynamic changes of desiccation cracks. A soil column test was conducted under wetting–drying cycles to investigate the dynamic changes of desiccation cracks and their hydrological response. The ratios between the crack area and soil matrix area (crack ratio), crack aperture and depth were measured. The soil water content, matrix suction and water drainage were monitored. A new dynamic dual-permeability preferential flow model (DPMDy) was developed, which includes physically consistent functions in describing the variation of both porosity and hydraulic conductivity in crack and matrix domains. Its performance was compared to the single-domain model (SDM) and rigid dual-permeability model (DPM) with fixed crack ratio and hydraulic conductivity. The experimental results showed that the maximum crack ratio and aperture decreased when the evaporation intensity was excessively raised. The self-closure phenomenon of cracks and increased surficial water content was observed during low-evaporation periods. The simulation results showed that the matrix evaporation modeled by the DPMDy is lower than that of the SDM and DPM, but its crack evaporation is the highest. Compared to the DPM, the DPMDy simulated a faster pressure head building-up process in the crack domain and higher water exchange rates from the crack to the matrix domain during rainfall. Using a fixed crack ratio in the DPM, whether it is the maximum or the average value from the experiment data, will overestimate the infiltration fluxes of PF-DC but underestimate its contribution to the matrix domain. In conclusion, the DPMDy better described the underlying physics involving crack evolution and hydrological response with respect to the SDM and DPM. Further improvement of the DPMDy should focus on the hysteresis effect of the soil water retention curve and soil deformation during wetting–drying cycles.

2D DIC from single moving camera by homographies compensation

The use of unmanned aerial vehicles in combination with vision-based measurement techniques has increasing interest in structural health monitoring. When displacement fields must be measured, Digital Image Correlation is one of the most popular techniques for image analysis. However, when images are taken from a single camera mounted on a drone, DIC gives incorrect results, since the camera movement creates apparent displacements and deformations. In the paper a technique for the compensation of these perspective distortions is proposed for planar surface case. The technique uses homographies estimated on a limited region of the measurement target assumed undeformed. Tests on numerically generated images have been performed to validate the proposed algorithm and to study the effect of the non-complete compliance with theoretical assumptions. Model for compensation error prediction is also proposed. Finally, some experimental tests considering real targets have been performed, obtaining good performances for the crack aperture case study.

Understanding intrinsic healing process in cementitious cracks through modeling and experiments

This study aims to understand the intrinsic healing process along crack depth in cementitious materials with various physical and chemical parameters. A new, coupled transport-thermodynamics model was formulated to predict crack wall profile change with time due to healing. The model considers the time-dependent interaction among ions transport through crack depth, healing products precipitation along crack walls, and local alteration of crack transport properties. The model was validated by controlled experiments quantifying the 3D crack aperture field before and after healing. Using X-ray computed microtomography, scanning electron microscopy and chemical analysis, the experiments uncovered and quantified the non-homogenous healing ratios and chemical compositions along crack depth. Parametric studies further elucidated how different physical (crack width, geometry, flow rate) or chemical (unhydrated cement ratio, ion concentration) parameters affect the healing profile change with time. The results shed light on the intrinsic healing mechanisms and robustness in cementitious materials.

A multi-resolution approach to hydraulic fracture simulation

We present a multi-resolution approach for constructing model-based simulations of hydraulic fracturing, wherein flow through porous media is coupled with fluid-driven fracture. The approach consists of a hybrid scheme that couples a discrete crack representation in a global domain to a phase-field representation in a local subdomain near the crack tip. The multi-resolution approach addresses issues such as the computational expense of accurate hydraulic fracture simulations and the difficulties associated with reconstructing crack apertures from diffuse fracture representations. In the global domain, a coupled system of equations for displacements and pressures is considered. The crack geometry is assumed to be fixed and the displacement field is enriched with discontinuous functions. Around the crack tips in the local subdomains, phase-field sub-problems are instantiated on the fly to propagate fractures in arbitrary, mesh independent directions. The governing equations and fields in the global and local domains are approximated using a combination of finite-volume and finite element discretizations. The efficacy of the method is illustrated through various benchmark problems in hydraulic fracturing, as well as a new study of fluid-driven crack growth around a stiff inclusion.

Synergistic Effects between Carbonation and Cracks in the Hardened Cement Paste

This paper investigates the synergistic interaction between carbonation and microcracks in hardened cement pastes. Ordinary Portland cement pastes with three different water/cement ratios of 0.4, 0.5 and 0.6, hydrated for 28 days with crack apertures ranging from 10 to 150 μm were subjected to accelerated carbonation in a climate chamber that controls the targeted relative humidity (65 and 75%), CO2 concentration (0.3 vol.% and 1 vol.%) and temperature (20 °C). Mercury intrusion porosimetry, N2-adsorption and thermogravimetry analysis were used to quantify the carbonation-induced changes in pore size distribution, porosity and phase assemblages. Additionally, the changes in crack apertures were followed during carbonation. The results indicated that cracks within the investigated range facilitate the carbonation along the crack surface due to a faster gas diffusion process. The cracks with apertures below 50 μm increase the carbonation depth at least by a factor of two for all studied w/c and environmental conditions. We observed a constant increase in crack openings during carbonation and its linear relationship with the amount of precipitated calcium carbonate. Due to depletion of CH and decalcification of C-S-H close to the sample surface, the crack aperture increase becomes limited. Therefore, the crack apertures do not increase further after the material around the crack is carbonated. It was also found that the densification of the carbonated cement matrix coexists with large capillary pores (>50 nm) or cracks, especially for the low w/c samples. The shift in pore size distribution from about 100 nm towards smaller pores (4.5–50 nm) and a decrease in gel pore fractions after 28 days of carbonation also indicate a simultaneous calcium carbonate precipitation in meso/capillary pores and decalcification of C-S-H at the nanoscale.

Mechanical response and strain localization in coal under uniaxial loading, using digital volume correlation on X-ray tomography images

A comprehensive analysis of crack initiation and propagation in coal is critical for understanding its mechanical behavior and its impact on permeability. The finite element-based digital volume correlation (DVC) of time-resolved X-ray tomography images under cleat-parallel uniaxial compression provides 3D incremental strain fields to understand the evolution and propagation of cracks. In this study, DVC coupled with quantitative image analysis was used to understand the role of cleats in guiding and initiating cracks in coal. Simulation of fluid flow through coal at different loading segments provides dynamic quantification of evolution in permeability. After each loading stage, quantification of strain eigen vectors and image analysis indicates appearance, coexistence, and coalescence of tensile and shear cracks guided by the organic matter in coal. We established that constructive interference between tensile and shear crack systems enhances permeability and decreases tortuosity by almost 3x and 7x respectively in coal by widening crack aperture, however, destructive interference is likely to lead to matrix pulverization and clogging of larger crack apertures. During compression, while one part of the coal matrix shows extension perpendicular to the loading direction, another part shows slippage parallel to the loading direction. Based on the variation in crack propagation behavior, four distinct crack morphology zones were identified and discussed. We found that the tortuosity of the cracks decreases exponentially, while permeability and crack volume fraction increase exponentially up to 2.8 μm2 and 28.6% respectively with increasing load. The box-counting-based fractal dimension for the cracks shows a steady increase from 1.58 to 1.78 with loading due to the evolution of existing cracks, and the formation of new cracks.

Real-Time Measurement of Mechanical Behavior of Granite During Heating–Cooling Cycle: A Mineralogical Perspective

Changes in mechanical properties of rocks at high temperatures are significant to various scientific and engineering problems, such as efficient exploration in enhanced geothermal systems (EGS) and rock mass stability in radioactive waste disposal. However, a comprehensive understanding of changes in rock mechanical behaviors at high temperatures and corresponding mechanisms, especially granites, remains unclear, which calls for the examination of micromechanical properties of the constituent minerals. This work investigates four representative minerals in granites, including quartz, plagioclase, amphibole, and biotite. The micromechanical properties of these minerals in the heating–cooling cycle were characterized in real time by using the nanoindentation technique. The morphology of samples was observed not only under real-time cyclic heating–cooling conditions, but also after thermal treatment. Subsequently, changes in structure of the minerals at elevated temperatures based on X-ray diffraction (XRD) were studied. Thermogravimetric analysis coupled with Fourier’s transform infrared spectroscopy (TG-FTIR) was performed to examine physio-chemical alterations of minerals at high temperatures. The experimental results revealed that the reduced modulus and hardness of quartz decreased during heating and subsequently increased during cooling. During the heating–cooling cycle, the hardness of plagioclase varied similarly to that of quartz, but its reduced modulus increased up to approximately 300,^{circ }hbox {C} followed by a decrease during heating. Unlike the net decrease of modulus of quartz after exposure to the heating–cooling cycle, feldspar showed a net increase of modulus. The micromechanical properties of biotite were found to rise significantly during heating, followed by a decrease during cooling. XRD results suggested the alteration of mineral structure, including spacing of lattice planes, crystallinity, and phase transitions, which controlled the above changes in mechanical behaviors of quartz, amphibole, and plagioclase. However, the aperture of open cracks along the cleavage in biotite determined its micromechanical properties during heating and cooling. Thermally-induced microcracks were observed during heating; whereas, closure of pre-existing cracks and initiation of microcracks were found during cooling. The real-time measured micromechanical properties of minerals could relate to the macromechanical behaviors of rocks under heating–cooling conditions based on thermally-induced microcracks.

An XFEM-based approach for 3D hydraulic fracturing simulation considering crack front segmentation

Hydraulic fracturing is a commonly adopted and effective well stimulation technique in the oil and gas extraction area. Under a mixed I/III loading condition, the crack front segmentation (i.e., the parent crack segments into echelon-shaped daughter cracks) usually occurs, which highly complicates the paths of fluid-driven cracks. This paper presents an efficient numerical model for 3D hydraulic fracturing simulation considering crack front segmentation on basis of the extended finite element method (XFEM). Solutions of the momentum balance equation and the fluid flow equation are simultaneously determined by the Newton-Raphson method along with a reduction technique. In the XFEM framework, a robust local mesh-refinement scheme of the tip-enriched elements is designed to enhance the resolution of the near-front stress field which is crucial for the determination of crack segmentation and propagation behaviors. The locally refined tip-enriched elements are then divided into a series of tetrahedra to perform high-accuracy numerical integration. After verification of the proposed approach, the effects of several critical parameters in hydraulic fracturing treatments are investigated. Results show that crack front segmentation has significant effects on the resulting crack paths and crack aperture distribution. The propagation of hydraulic fractures will be depressed on account of the stress shadow induced by overlapped segments, leading to higher pumping pressure compared to the case without considering front segmentation. The sensitivity analyses indicate that larger elastic modulus of rock formation, larger fluid viscosity, higher fluid pumping rate, and smaller fluid leak-off coefficient can alleviate the influence of crack front segmentation on the pumping pressure. Larger elastic modulus, larger fluid viscosity, higher fluid pumping rate, and greater fluid leak-off coefficient lead to smaller twisting angles of the segments and smaller overlapping ratios.

Factors Affecting Shrinkage Crack Development in Clay Soils: An Experimental Study

ABSTRACT Clay soils are widely used in constructing embankment dams, levees, highway embankments, sanitary landfills, and hydraulic barriers where they are compacted at maximum dry density and optimum water content. These structures are exposed to many cycles of wetting and drying during their service life, resulting in volume changes and developing shrinkage cracks. This study investigated the effect of density, water content, drying temperature, layer thickness, plasticity index, and multiple wetting and drying cycles on shrinkage crack parameters (length, aperture, and area). Five samples each of low-plasticity clay, medium-plasticity clay, and high-plasticity clay at water contents on both sides of optimum water content were compacted and then oven dried at temperatures of 10°C, 20°C, 30°C, 40°C, and 50°C. Upon complete drying, crack length, aperture, and area were digitally measured. Additionally, we saturated uncompacted clay layers of varying thicknesses (5, 7, 10, 20, and 30 mm) of the three clay types and oven dried them at the listed temperatures to investigate the effect of layer thickness on shrinkage crack parameters. The length, aperture, and area of the cracks were correlated with the influencing factors. Among the compacted samples, only high-plasticity clay samples developed shrinkage cracks, exhibiting an increase in crack length, aperture, and area with an increase in dry density and water content. For the uncompacted samples, crack length and crack area decreased with increasing layer thickness, whereas crack aperture increased.

Predicting evaporation from moist, cracked soil, based on near‐surface wind speed, crack width and crack distance

AbstractEvaporation from both cracked and uncracked soil surfaces under moist conditions was measured as a function of near‐surface wind speed, with wind speeds ranging between 0 and 5 ms−1. Crack aperture width and distance between neighbouring cracks ranged from 2 to 20 mm and 40 to 120 mm, respectively, while crack depth was kept constant at 50 mm. Experiments were carried out in a wind tunnel with a 1 m × 1 m × 2.5 m test section, using large‐scale (50 cm diameter, 15 cm depth) soil samples. Sixty‐six combination of crack properties and wind speed were considered. Resulting evaporation data were used to assess the shape of the relationships between evaporation rate, wind speed conditions and crack properties. A semiempirical model for estimating evaporation from cracked soil, as a function of wind speed, crack width and crack distance, given evaporation as a function of wind speed from uncracked soil, was developed. Results showed that evaporation from cracked soil under moist conditions increased by 60%–65% compared to uncracked soil, for the wind speed and crack properties given above. Evaporation from crack apertures was up to eight times higher than from an uncracked surface under the same wind conditions. Results further indicated that for constant wind speed and crack distance, there exists a crack width at which evaporation from the soil is at maximum. The semiempirical model based on exponential and power function relationships, was able to very closely match the experimental data and is further expected to be able to approximate evaporation under natural conditions.Highlights Near‐surface wind speed is strongly affected by presence of cracks Cracking in moist soils can increase evaporation up to 65% over uncracked soil There exists an intermediate crack width at which evaporation is at a maximum Evaporation may be estimated based on wind speed and crack properties