Soil Shrinkage Research Articles

Shrinkage and Consolidation Characteristics of Chitosan-Amended Soft Soil—A Sustainable Alternate Landfill Liner Material

Kuttanad is a region that lies in the southwest part of Kerala, India, and possesses soft soil, which imposes constraints on many civil engineering applications owing to low shear strength and high compressibility. Chemical stabilizers such as cement and lime have been extensively utilized in the past to address compressibility issues. However, future civilizations will be extremely dependent on the development of sustainable materials and practices such as the use of bio-enzymes, calcite precipitation methods, and biological materials as a result of escalating environmental concerns due to carbon emissions of conventional stabilizers. One such alternative is the utilization of biopolymers. The current study investigates the effect of chitosan (biopolymer extracted from shrimp shells) in improving the consolidation and shrinkage characteristics of these soft soils. The dosages adopted are 0.5%, 1%, 2%, and 4%. One-dimensional fixed ring consolidation tests indicate that consolidation characteristics are improved upon the addition of chitosan up to an optimum dosage of 2%. The coefficient of consolidation increases up to seven times that of untreated soil, indicating the acceleration of the consolidation process by incorporating chitosan. The shrinkage potential is reduced by 11% after amendment with 4% chitosan and all the treated samples exhibit zero signs of curling. Based on the findings from consolidation and shrinkage data, carbon emission assessments are carried out for a typical landfill liner amended with an optimum dosage of chitosan. In comparison to conventional stabilizers like cement and lime, the results indicate that chitosan minimized carbon emissions by 7.325 times and 8.754 times, respectively.

Mechanical and thermal properties of mud dauber nests under atmospheric drying

Mud dauber wasps construct soil nests to protect their offspring from predators, extreme temperatures, and rainwater. The mechanical and thermal properties of these nests are important for the reproductive success of mud daubers. The previous study showed that the high densities and strengths of mud dauber nests were due to the repetitive tapping and atmospheric drying used by mud daubers during nest construction. This study investigated the effect of atmospheric drying on the mechanical and thermal properties of mud dauber nests. The soil shrinkage curve, elastic modulus, suction stress characteristic curve, soil water retention curve, shear strength, and thermal conductivity function of mud dauber nest soils were measured by performing drying cake tests, direct shear tests, unconfined compression tests, and thermal conductivity measurements. This study showed atmospheric drying could increase Young’s moduli (from hundreds to thousands of kPa), the magnitudes of suction stress (up to 64 kPa), and shear strengths (e.g., unconfined compressive strength increased up to 2100 kPa) of mud dauber nests. The thermal conductivity was reduced by 47% due to atmospheric drying. Investigation of mud dauber nests under atmospheric drying could provide insights and inspiration to improve human manufacturing and manipulation of soils for earthen building construction.

Reviewing and analyzing shrinkage of peat and other organic soils in relation to selected soil properties

AbstractPeat and other organic soils (e.g., organo‐mineral soils) show distinctive volume changes through desiccation and wetting. Important processes behind volume changes are shrinkage and swelling. There is a long history of studies on shrinkage which were conducted under different schemes for soil descriptions, nomenclatures and parameters, measurement approaches, and terminologies. To date, these studies have not been harmonized in order to compare or predict shrinkage from different soil properties, for example, bulk density or substrate composition. This, however, is necessary to prevent biases in the determination of volume‐based soil properties or for the interpretation of elevation measurements in peatlands, in order to predict carbon dioxide emissions or uptake caused by microbial decomposition or peat formation. This study gives a comprehensive overview of shrinkage studies carried out in the last 100 years. Terminology and approaches are systematically classified. In part I, the concepts for shrinkage characteristics, measurement methods, and model approaches are summarized. Part II is a meta‐analysis of shrinkage studies on peat and other organic soils amended by own measurement data obtained by a three‐dimensional structured light scanner. The results show that maximum shrinkage has a wide range from 11% to 93% and is strongly affected by common soil properties (botanical composition, degree of decomposition, soil organic carbon, and bulk density). Showing a stronger correlation, bulk density was a better predictor than soil organic carbon, but maximum shrinkage showed a large spread over all types of peat and other organic soils and ranges of bulk density and soil organic carbon.

In-situ measuring and predicting dynamics of soil bulk density in a non-rigid soil as affected by tillage practices: Effects of soil subsidence and shrinkage

Non-rigid soils (e.g., Vertisols) present dynamics of bulk density (ρb) due to high shrinkage and swelling. However, the in-situ measurement and prediction of the dynamic of ρb in non-rigid soils are still great challenges. The objectives were to (1) evaluate the performance of the combined soil moisture and thermal property sensors in estimating in-situ ρb dynamics under different tillage practices, (2) and establish mathematic equations to predict the ρb dynamics associated with soil subsidence and shrink-swelling processes during wetting and drying cycles. The in-situ ρb monitoring and periodical intact soil core sampling were conducted in the 0–10 cm and 10–20 cm layers in a Vertisol under three tillage treatments, containing no-tillage (NT), rotary tillage (RT) and deep ploughing (DP). Our results showed that the dual-sensor combination provided accurate ρb estimates in the field over 2021–2022 year (R2 > 0.487, RMSE < 0.177 g cm−3), except for the early stage after deep tillage. The ρb dynamics in the 0–20 cm in the NT and the 10–20 cm layer in RT treatment were mainly caused by shrink-swelling. Whereas the ρb dynamics in the 0–10 cm and 10–20 cm in the DP and the 0–10 cm layer in RT treatment were predominantly determined by soil subsidence first and then shrink-swelling when the accumulative rainfall (Pt) reached 131.8 mm, 186.1 mm, and 79.3 mm, respectively. The ρb dynamics during soil subsidence were well-fitted by an exponential equation related to accumulative rainfall (R2 > 699, P < 0.01), while the ρb dynamics during shrink-swelling were well-fitted by a newly proposed SSCρb equation derived from the Peng and Horn soil shrinkage model (R2 > 589, P < 0.05). Combined with the long-term monitored rainfall and soil moisture, The SSCρb equation and the two-stage equation involving subsidence and SSCρb exhibited good prediction of ρb dynamic from 2017 to 2022 (R2 > 0.453, RMSE < 0.070 g cm−3). The soil subsidence and shrink-swelling process accounted for 3.32%− 12.5% and 2.84%− 14.8% of the ρb variation in tilled non-rigid soils, respectively. Our results demonstrated that the dual-sensor combination can be applied for field ρb monitoring in non-rigid soil. The proposed two-stage equation has great potential for predicting the field dynamics of ρb.

Laboratory modeling of thermal and temporal cracking in swelling clays

Crack development in a changing environment is the controlling factor for the stability and strength of expansive soils. Expansive soils exhibit large volumetric changes with changes in their moisture conditions that may occasionally lead to reduced bearing capacity and foundation failures. This study purports to model crack initiation and its spatial progression in relation to the moisture content and drying period, respectively. Volumetric soil shrinkage is determined using high-definition digital camera imaging and Vernier scale methods, while the soil settlement under vertical shrinkage deformation could be captured through a tensile stress model for soils. It was revealed that a small change in suction could trigger crack initiation, which would propagate further under different environmental conditions. Furthermore, it was observed that the crack volume increased rapidly at specific moisture content and could penetrate as deep as 1 m after nearly 1.5 months that is fully consistent with the current model predictions. A comparison between the performance of the model proposed in this study and that of two existing models shows that the former predicts the vertical shrinkage strain values in closer agreement with those observed experimentally and is less conservative than those predicted by both models. Nevertheless, the findings from this study could be used to quantify the detrimental behavior of expansive soil present in pavement subgrades and shallow foundations for lightweight structures.

Extracting the 3D spatial structure of soil cracks in the North China Plain from paraffin casting and exploring the developmental patterns of vertical cross-sectional morphology of the cracks

Cracking from soil desiccation is a common phenomenon. Understanding the structure of cracks is highly important in the research of soil moisture and heat transportation, as well as the strength of soil structure within soil. There is limited research on the three-dimensional (3D) spatial characteristics of soil desiccation cracks. Furthermore, the absence of a dependable and precise in-situ technique for extracting and digitizing the three-dimensional crack structure means that the commonly employed simplified crack models (such as V-shaped and cuboid-shaped) have received limited testing. This study examines the feasibility of extracting the 3D structure of cracks by combining in-situ paraffin casting in sandy loam soil of North China Plain with a 3D laser scanner. Parameters (length (L), width (W), depth (D), depth to width ratio (C)), as well as the vertical cross-section profile (W ∼ D) of each crack, were used to quantitatively describe the structure of cracks. Our study demonstrated that digitalized 3D cracks with 5 μm spatial resolution can be obtained quickly by paraffin infusion combined with a 3D laser scanner. Our result based on 18 crack cross-section profiles also demonstrates that W ∼ D do not follow either the V-shaped crack model or the cuboid-shaped crack model. All of the 18 normalized vertical profiles of cracks obey the power function relationship (R2 = 0.850) rather than the linear relationship of the V-shaped model (R2 = 0.794). The shape of the soil moisture profile is convex, which contradicts the concave profile of the observed vertical cracks. Even after combining the Soil Shrinkage Characteristic Curve (SSCC) with the soil moisture profile, we still failed to replicate the observed concave crack profile. The observed crack morphology in experiments can be replicated by both the elliptical model and the parabolic model of fracture mechanics, albeit with less predictive accuracy when compared to the power function model. This suggests that the current understanding of soil desiccation cracking in soil science is deficient, as it lacks appropriate methods and theoretical mechanisms for investigating soil cracking.

Evaluating innovative direct and indirect soil suction and volumetric measurement techniques for the determination of soil water retention curves following drying and wetting paths

Soil Water Retention Curves (SWRC) are a main characterising component in constitutive models developed for describing unsaturated soils behaviour. Their determination requires establishing a combination of accurate soil matric suction measurements and volumetric measurements. While direct suction methods are always preferred, current methods, such as high capacity tensiometers, are limited to a measuring range of only 1.5 MPa. In this work, the novel Northumbria High Capacity Tensiometer, with an extended measuring range of up to 3.5 MPa, is used in a simplified continuous-drying setup, with proposed modifications to enhance the equalisation times and volumetric measurements. The results are then compared and validated with direct and indirect methods in both drying and wetting paths. At the same time, different volumetric measurement techniques were assessed in order to establish an understanding of Soil Shrinkage Curves (SSCs) that describe the moisture-voids relationship during drying and wetting and has the potential to be used for indirect determination of volumetric dimensions of soil specimens. Hysteresis was observed in the water retention behaviour but not in the volumetric behaviour. An established SSC model is proposed in this work and is fitted to experimental data in order to indirectly estimate volumetric measurements from the water content of small-scale specimens. The proposed novel approach standardises volumetric measurements determination, leading to uniform and complete-range SWRCs.

Effects of MWCNTs content and dry density on hydro-structural characteristics of bentonite during drying and wetting

Bentonite has been an essential component of geosynthetic clay liners due to its low hydraulic conductivity, which has been used as a barrier against hazardous pollutants in landfills. During the drying and wetting processes caused by high temperatures and leachate in landfills, the hydro-structural characteristics of bentonite vary as the moisture content change. This article investigates the effect of dry density and multi-walled carbon nanotubes (MWCNTs) content on the soil shrinkage characteristic curve (SSCC), the soil free-swelling characteristic curve (SFSCC), the soil water characteristic curve (SWCC), and microstructure of bentonite during the drying and wetting processes. Experimental results indicate that the addition of MWCNTs can slow down the volume variation of bentonite at the normal shrinkage stage and at the initial swelling stage and improve the water retention capacity in the low suction range. Meanwhile, reducing bentonite’s dry density can increase the void ratio during drying, decrease the void ratio during wetting, and improve the water retention capacity in the low suction range. Dry density and MWCNTs content both have no influence on the SSCC at the residual shrinkage stage and the zero shrinkage stage, the SFSCC at the residual swelling stage, and the SWCC at the high suction range.

Continuum and discrete element modelling for describing coupled hydro-mechanical effects of earthworm burrow coatings on soil shrinkage

Structured soils with earthworm burrows are locally heterogeneous due to coatings along these biopore walls that are superimposed on the inter- and intra-aggregate pore network of the soil matrix. The partially compacted finer-textured and organic matter-rich coatings can limit the flow exchange between the macropores and the soil matrix during preferential flow. Still unknown are dynamic hydro-mechanical interrelations in coating and matrix domains that affect stress–strain behaviour at the macroscopic scale. Such hydro-mechanic interactions may be described with the discrete element method (DEM) coupled with a two-phase pore finite volume (2PFV) approach if relevant pore structures are represented in the model. The objective was to develop a coupled DEM-2PFV model together with a parameterization procedure. Major task was to create a parameterization procedure to calibrate micro parameters of the model by macro properties quantified from drainage experiments of soil samples with earthworm burrow wall coatings.The solid phase was modelled by particle aggregation creating inter and intra-aggregate pore network for the soil matrix in a cube of about 5 cm edge length and one side with the coating structure. This DEM model was coupled with a 2PFV model to simulate hydro-mechanic effects during drainage. Sand box drainage experiments were carried out on soil matrix and biopore samples with laser surface elevation measurements to obtain the mechanical stress–strain macro parameters necessary for model calibration. The poly-dispersed DEM-2PFV model was able to describe effects of two-phase air–water flow on stress–strain macro parameters. The micro parameters (i.e., particle stiffness and bond strength) of the pore scale model were obtained from macro parameters of the primary and secondary stress–strain stages by training a random forest meta-estimator. The model was able to reproduce the pore network of coating material and the inter- and intra-aggregate pore network of the matrix that are dynamically changing with the effective stress. The machine learning model revealed that the bond strength among particles within aggregates governed the shrinkage of soil matrix, while the particle stiffness of the coating material reduced the susceptibility of aggregate breakage producing a more stable inter-aggregated pore network during the drainage process. This study confirmed that coating material present in biopore surface increases the horizontal soil hydro structural stability. The microscale hydro-mechanic modelling can be useful for finding flow exchange parameters inputs for upscaled models and correlating pore-scale parameters to experimentally determined stress–strain macro parameters.

A study on utilization of landfill mined soil like fraction for sustainable reduction in swelling and cracking potential for expansive and marine soils

Landfill mined soil like fraction (LMSF) is derived from mining of legacy landfill waste after removal of larger fraction such as construction debris, clothes, plastics and other recyclables, and accounts for significant proportion of the total legacy landfill waste. LMSF has found applications as landfill cover material; however, owing to its large quantity in many cities, there is a need to explore more applications for its utilization. In this context the present study explores the role of LMSF (filler replacement) with/without ultrafine slag, US (primary admixture) and lime, L (activator) in controlling the shrinkage and cracking of marine and expansive soils during drying. The ratio of the area of crack and shrinkage to total area of specimen, defined as crack and shrinkage intensity factor (CSIF), has been utilized for quantification of the cracking and shrinkage characteristics of soils with/without stabilization. Further, the effect of soil stabilization on the swelling potential (viz., free swell index and swelling pressure) has been evaluated for the soils. In the present study, complete crack mitigation for expansive soil has been achieved for the combination: 40%LMSF (<425 μm) + 50% expansive soil + 8%US + 2%L, while for the marine soil it has been achieved for combination: 40% LMSF (<425 μm/<2 mm) + 50% marine soil + 8.5%US + 1.5%L. The study findings are useful in the context of utilizing LMSF with sustainable admixtures for reducing cracking and swelling potential of soils for various infrastructure applications such as natural and stabilized slopes, embankment, canal bed and ponds.

Pore shrinkage capacity of Shajiang black soils (Vertisols) on the North China Plain and its influencing factors

Different pore sizes present different pore shrinkage capacities in a nonrigid soil. However, the shrinkage capacities of different pore sizes and their influencing factors are not clear. We aimed to quantify the shrinkage capacities of different pore sizes (large pores, > 50 μm; medium pores, 0.2–50 μm; fine pores, < 0.2 μm) and determine how soil properties impact soil shrinkage capacity at the regional scale. Two sampling transects from west to east (360 km long, 35 samples) and from north to south (190 km long, 29 samples) were selected to investigate soil shrinkage capacity and physicochemical properties of at 0–20 cm depth in the Vertisol (locally known as Shajiang black soil) region of the North China Plain. The results showed that soil total shrinkage capacity, indicated by the coefficient of linear extensibility (COLE), had a mean value of 0.041–0.051 in the west-east and north-south transects. Large pores had higher pore shrinkage index (PSI) values (0.103–0.109) than medium (0.077–0.096) and fine (0.087–0.091) pores. The PSI of fine pores showed a fluctuating increasing trend from northwest to southeast, and the fine pore shrinkage capacity determined the COLE (r2 = 0.789, P < 0.001). The PSI of large pores had a significant relationship with soil bulk density (r = 0.281, P < 0.05) and organic carbon (r = -0.311, P < 0.05), whereas those of medium and fine pores were correlated with soil clay content (r = 0.381 and 0.687, respectively, P < 0.001). In addition, the PSI of fine pores was also correlated with montmorillonite content (r = 0.387, P < 0.01). It can be concluded that the PSI of large pores is related to anthropogenically influenced soil properties with low stability, whereas those of medium and fine pores are related to pedogenic properties. The high variability in anthropogenic and pedogenic factors explains the spatial pattern of Vertisol shrinkage capacity on the North China Plain.

Laboratory study on shrinkage and cracking behavior of historic earthen plaster

Grottoes are widely distributed along the ancient Silk Road in Northwest China, and wall paintings are among the most precious cultural relics in these grottoes. On-site investigation of some grottoes has revealed that these wall paintings are painted on earthen plaster and that most of them have a double-layer structure of plaster, i.e., a base coarse plaster layer (with wheat straw added) and a surface fine plaster layer (with hemp fibers added). Meanwhile, studies regarding the improvement in shrinkage and cracking behaviors via ancient plaster craftsmanship have been scarce. To investigate the specific effects of different material compositions on the shrinkage and cracking properties of plaster during desiccation, a series of laboratory shrinkage and cracking tests is conducted on plaster specimens composed of Dengban clay and quartz sand, vegetal fibers, and a potential restoration material (i.e., calcined ginger nut), respectively. The results show that adding sand and vegetal fibers can significantly increase the shrinkage limit of Dengban clay and inhibit its volume shrinkage and that vegetal fibers play an important role in the inhibition of surface cracking on plaster. Sand inhibits shrinkage by forming a skeleton in soil and reducing the formation of primary cracks. Fibers inhibit soil shrinkage by overcoming the suction between soil particles and preventing the formation of primary cracks. However, wheat straw cannot deform with the shrinkage of soil (e.g., hemp fibers); consequently, many subcracks appear around the wheat straw on the surface of coarse plaster. As a typical lime-based restoration material, calcined ginger nut paste presents slight shrinkage and a low surface crack ratio during desiccation, which demonstrates its good application prospect in plaster restoration.

Comparison of Costus dewevrei De Wild. and T. Durand Admixture with Lime and Cement in Soil Stabilization

The study investigated the use of cement and lime as composite mixture with Costus dewevrei De Wild. &amp; T. Durand as for soil stabilization aimed at improving the properties of expansive soils used for road pavement. The maximum dry density (MDD), optimum moisture content (OMC), consistency limits, California bearing ratio (CBR) and unconfined compressive strength (UCS) of the soil were subjected test to ascertain the performances of the cement and lime with bagasse composite. The results showed that the composite materials improved the soil properties, but the maximum dry density (MDD), optimum moisture content (OMC), liquid limit (LL), plasticity index (PI), California bearing ratio (CBR) and unconfined compressive strength (UCS) of the expansive soil stabilized with cement and bagasse ash composite were greater than the soil samples stabilized with lime and bagasse ash composite. Meanwhile, the value of plastic limit (PL) obtained from the soil sample stabilized with lime and bagasse ash was higher than the value recorded in the soil sample stabilized with cement and bagasse ash. The optimum values UCS and CBR were recorded at 8% combined proportion of bagasse ash with cement and lime. Therefore, the improvement recorded in the soil properties proved that the combined effect of bagasse ash obtained from Costus dewevrei De Wild. &amp; T. Durand and cement or lime, is effective and can be applied as stabilization material to reduce shrinkage and swelling of expansive soil that often lead to road pavement failure.

Peridynamics for mechanism analysis of soil desiccation cracking: Coupled hygro-mechanical model, staggered and monolithic solution

Soil shrinkage cracking is a hygro-mechanical coupled phenomenon and it can significantly weaken the mechanical properties of soil and cause various potential natural disasters. This study proposes an improved bond-based peridynamics (PD) formulation, based on a comprehensive review of the hygro-mechanical peridynamic models. The proposed model with a certain influence function can be recast as an existing bond-based PD diffusion model. Also, an attenuation kernel function and discretized micro-modulus are adopted to revise the bond force density in the prototype micro-elastic brittle (PMB) model. For the numerical implementation, both monolithic and staggered schemes are adopted for the coupled hygro-mechanical problem. Specifically, the coupled stiffness matrix in the discretized equilibrium equation is constructed by using the Lagrange multiplier method for the monolithic scheme. While the direct integration method of explicit dynamics with an artificial damping term is adopted for the staggered scheme. For modeling of soil drying and cracking, two-dimensional soil rings, and two- and three-dimensional soil strips are simulated and demonstrated. The soil ring model verifies the accuracy of the coupled moisture-soil bond-based PD model through an explicit staggered scheme, and explores the effect of the relative thickness of rings on the number of cracks. The implicit monolithic solution is adopted to analyze the mechanisms of soil strip cracking. A detailed discussion on the scheme for controlling the maximum number of bond breakage is provided to guarantee convergency and accuracy with optimal efficiency. The soil strip cracking pattern and upward curling deformation agree well with experimental observations. The results show that the hygro-mechanical coupled peridynamic model has great advantages in solving soil drying and cracking. The present work shows a potential way in soil desiccation cracking investigation for real-scale simulation and fine mechanism exploration.

Effects of Climate Change on Geotechnical Infrastructures — state of the art

Geotechnical infrastructures, like slopes and embankments, retaining walls, foundations, engineered landfills, overburden dumps, and pavements, get continuously exposed to various environmental factors which are climate dependent. Fate/stability of these infrastructures due to extreme and abrupt change in precipitation, temperatures, humidity, and wind/airflow is quite questionable. Some of the issues related to climate change on soils include increase in infiltration rate, high pore-water pressure, decrease in effective stress, soil liquefaction, seepage failures, frost heaving, changes in soil suction potential, swelling and shrinkage in fine-grained soils, differential settlement, and damage to vegetation cover and thereby causing slope failures, waterlogged conditions, floods, soil erosion and/or internal erosion of fines, damage to landfill liners and soil covers, desertification, desiccation cracks on the ground surface, and groundwater table pollution due to contaminant migration. Therefore, studies on the impact of climate change on geotechnical infrastructures have gained attention of many researchers in the recent times. In present study, an up-to-date review of the works related to the influence of various climatic factors on geotechnical properties and thereby on geotechnical projects is carried out. Topics related to climate data downscaling using global climate models (GCM), climate change-induced slope instability, acid rains, desiccation cracking in soils with changing temperatures, impacts of dry-wet cycles and freeze-thaw cycles, and vegetation effects on soils are emphasized in this paper. Furthermore, to address the challenges, need-based research related to resilient infrastructures, thermo-hydro-mechanical models, bioremediation methods, innovative sustainable composite materials, and incorporation of climatic factors in design is highlighted and discussed.

Influence of compaction on the soil shrinkage and swelling curves

The behaviour of soils undergoing deformation due to changes in water content can be described using Soil Shrinkage Curves (SSCs). The SSCs are normally influenced by several factors as the soil’s fabric, different compaction levels, residual void ratio, and the shrinkage limit. In this study, shrinkage curves at different compaction levels were obtained using both wax coating and image analysis as volumetric quantification methods. Both techniques returned consistent results. To fit the obtained experimental data, a single equation is proposed with fitting parameters pertaining to the physical characteristics of the soil samples used. The use of Mercury Intrusion Porosimetry (MIP) is also incorporated to further analyse the structure of the soils being studied. The effect of the drying path on the structure was more apparent in the size range of 100-2000 nm. The effects of compaction on soil structure and shrinkage behaviour were also discussed in the study. The presented work provides valuable insights on the effect of different compaction levels on the soil shrinkage behaviour.

The analysis of clay soil shrinkage

The paper presents the analysis of the capillary and water-colloidal theories of clayey soil shrinkage. The method by which the volumetric shrinkage of the clay soil was determined is that the soil sample is made conical, with an angle at the top of 45-75º, and it is dried in a container geometrically similar to the sample with permeable walls. With kaolinite clays, the shrinkage process has a duration of one week, with hydromicaceous clay of around two weeks, and with montmorillonitic clays three weeks. There is a parabolic dependence between the duration of clay shrinkage and the specific surface area of clay. A series of experiments and calculations have been performed, based on Kulchitsky-Usyarov clay soil model, to determine the limit values of capillary pressure in montmorillonite, hydromicaceous and kaolinite clays using the Laplace formula. At the limit of normal shrinkage, the total porosity for Ca-montmorillonite is 0.45, for Ca-hydromica - 0.25, for Na-kaolinite - 0.385. Calculations showed that during shrinkage the capillary pressure reaches: for Ca-montmorillonite – 48.6 MPa; for Ca-hydromica 8 MPa; for Na-kaolinite - 5.2 MPa. The results obtained practically coincide with the results obtained by K. Terzaghi. Studies on the cyclic “moisture-drying” of clays selected at an experimental site in Ujar (Azerbaijan) showed that after the first “swelling-shrinkage” cycle, with secondary moistening, the swelling of the samples was 12-18 times higher than the primary value.

Model validation using induced tensile stress during cracking process measured with desiccation stress tests

Tensile stress is internally induced when soil shrinkage due to desiccation is restricted, and cracks occur when the induced tensile stress reaches tensile strength. Various numerical models have been proposed to predict crack initiation; however, the validity of the computed internal stresses during the cracking process has not been assessed, although the internal stresses directly influence the crack initiation. The aim of this study was to demonstrate the effectiveness of a laboratory-based desiccation stress test, which measures the induced tensile stress until crack initiation. Desiccation stress tests were performed on unsaturated sandy soil with two different water contents. The measured tensile stress until cracking was numerically simulated using a hypo-elastic model based on the skeleton stress considering the suction effects. The computed tensile stress was consistent with the measured values throughout the cracking process in both cases, with different initial water contents. Stress-based validation using the results of the desiccation stress test demonstrated that the proposed constitutive model yields reliable estimations of the internal tensile stress in unsaturated soils until crack initiation.

In-situ assessment of soil shrinkage and swelling behavior and hydro-thermal regimes with a thermo-time domain reflectometry technique

The shrinkage and swelling behaviors of clayey soils could lead to cracks and erosions inducing severe damages to agricultural lands and engineering projects. Thus, improving the understanding of shrink-swell characteristics is crucial to nonrigid soils. However, it remains difficult for in-situ monitoring of soil shrink-swell processes continuously and nondestructively. The objective of this study was to present a thermo-time domain reflectometry (thermo-TDR) technique for determination of soil shrinkage and swelling dynamics and hydro-thermal regimes in-situ during wet-dry cycles. A field experiment on two clayey soils (a paddy soil and an alluvial soil) was conducted to evaluate the performance of the thermo-TDR method for in-situ determination of soil volume changes and associated hydraulic and thermal properties during wet-dry cycles. The results showed that both soils experienced considerable shrinking and swelling processes with maximum vertical deformations of 2.4 and 1.8 cm for the paddy soil and alluvial soil, respectively, and with maximum horizontal deformations of 370 and 280 cm2 for the paddy soil and alluvial soil, respectively, during a 36-day period. The thermo-TDR method provided volume change values (shown as the bulk density) agreed well with independent core-sampling measurements with an average RMSE of 0.15 Mg m−3 and a R2 of 0.46. A better agreement was observed between thermo-TDR measured bulk densities and values from the ruler-image processing method with an average RMSE of 0.07 Mg m−3 and a R2 of 0.77. Besides, the thermo-TDR method gave consistent in-situ soil shrinkage curve and water retention surface measurements by combining with soil matric potential sensors. Unlike rigid soils, the paddy and alluvial soils showed complicated hydro-thermal regimes in response to the shrink-swell processes. We conclude that the thermo-TDR technique is a reliable tool for continuous in-situ determination of soil shrink-swell processes. Future studies should focus on development of comprehensive soil heat and water transport models accounting for both moisture content and volume changes.

Effect of biochar and attapulgite amendment on saline soil hydraulic parameters and shrinkage

AbstractSaline soil reduces soil productivity and exacerbates food security problem. Therefore, it is increasingly necessary to find sustainable farming practices for soil and water conservation. In this regard, conditioning the soil with biochar (BC) or attapulgite (ATP) has been proved as a feasible method of improving water conservation and soil hydraulic characteristics. However, the interactive effect of BC and ATP on saline soil is still unclear. The objective of this study was to investigate the effect of BC and/or ATP on water retention, pore size distribution, hydraulic characteristic parameter, and shrinkage strain of saline soil through laboratory trials. For this purpose, BC and/or ATP were added to the loam soil at three levels of 0%, 2%, and 4% by weight, and saturated in the sodium chloride solution with a concentration of 5 g L−1. Results showed that the Van Genuchten model accurately fitted the obtained soil water characteristic curve (R2 &gt; 0.99). BC and/or ATP amendments increased saline soil water retention capacity compared to the unamended soil. Soil water retention capacity increased as the ATP application rate increased. The saturated water content, field water holding capacity, soil microporosity, and soil retention effect of 2%BC combined with 4%ATP were higher than other treatments. BC facilitated the reduction of soil shrinkage, and ATP increased soil shrinkage. Axial shrinkage strain was negatively correlated with the application rate of BC, and positively correlated with ATP. 4%BC combined with 2%ATP had the least shrinkage strain. The present results could serve as a theoretical foundation for soil amendments to improve soil quality.