Multiphase modelling of desiccation cracking in compacted soil

PhD ThesisThe development of cracking as a result of desiccation is increasingly under investigation. This work is set within the context of climate change effects on surface processes influencing infrastructure slope stability. The inherent changes to the mechanical and hydrological behaviour of clayey soils subjected to desiccation are significant. The preferential transmission of water due to cracking is widely cited as a source of strength reduction that leads to infrastructure slope failure. In order to gain a better understanding of the cracking mechanism in typical compacted fill conditions, finite difference continuum modelling has been undertaken using FLAC 2D. The two-phase flow add-on has enabled the unsaturated behaviour of the desiccating soil to be included within the mesh. Physical behaviour observed in laboratory experiments has informed the development of the numerical model by allowing better constraint of boundary conditions. Model development has featured the inclusion of several non-linear processes that are fundamental to the changing soil response during drying. The influence of significant parameters has been identified and by means of a varied experimental program, the design, manufacture and testing of a laboratory test apparatus and procedure to define the tensile strength of compacted fills under varying saturation conditions was undertaken and subsequently validated. The factors affecting crack initiation and propagation have been investigated via parametric study. This demonstrated the significant influence of basal restraint on the generation of tensile stresses conducive to cracking and the fundamental importance of the tensile strength function within the proposed modelling methodology. Experimentation with the shape of the SWRC has shown the model to be very sensitive to the hydraulic properties of the material with not only the occurrence of primary cracking being affected but also the development of the desiccated crust. The findings of this work are relatable to the incorporation of desiccation effects in the development of coupled hydrological-mechanical continuum models where atmosphere-soil interactions are increasingly significant.Newcastle University with contribution from the EPSRC project, iSMART

Soil water dynamics and response of cowpea to water availability under moisture irrigation.

Doctoral Degree. University of KwaZulu-Natal, Pietermaritzburg.Increasing population, urbanization and industrialization has put pressure on the irrigation sub-sector to produce more yield using less water i.e. improving crop water productivity (WP). This can be achieved through the adoption of efficient irrigation systems such as micro-irrigation. Moistube irrigation (MTI) is a relatively new technology like subsurface drip irrigation (SDI) but with a semi-permeable membrane whose nanopores emit water in response to applied pressure and soil water potential. Being a new technology, there is little information regarding its hydraulic characteristics and soil water distribution which are necessary for its design, operation and management. Furthermore, the response of crops under a variety of soils and environmental conditions under MTI has not been covered extensively. Therefore, this study aimed at determining the hydraulic and clogging characteristics of MTI. The effect of soil texture on the soil water dynamics of MTI was also determined. Finally, the response of cowpea, an important but neglected African indigenous legume, to varying water regimes under MTI was also determined. This study was based on the hypothesis that cowpea responds favourably to water regimes under MTI. The study was accomplished through laboratory, field experiments and agro-hydrological models. AquaCrop and HYDRUS 2D/3D were chosen for this study due to their reliability in predicting crop yield responses to water availability and soil water dynamics respectively. The laboratory experiments were conducted in soil bins to determine the soil water dynamics of MTI under sandy clay and loamy sand soils which were used to calibrate the HYDRUS 2D/3D model. The hydraulic characteristics were determined at a pressure of between 10 kPa and 100 kPa while the effect of suspended and dissolved solids was determined under a pressure of 20 kPa and 30 kPa. The field experiments consisted of glasshouse and tunnels to examine the response of cowpea to full and deficit irrigation of MTI with SDI as the control. The results were used to parameterise and validate the AquaCrop model. Finally, HYDRUS 2D/3D and AquaCrop were coupled to draw into the strengths of the individual models and used to simulate the water use of cowpea under MTI in two agro-ecological zones in South Africa. The results showed that the discharge – pressure relationship of Moistube followed linear and power functions. It was also established that suspended solids had severe clogging effect than dissolved solids. In the soil bin experiment, simulated water contents closely matched (R2 ≥ 0.70 and RMSE ≤ 0.045 cm3 cm-3) the observed values in all the points considered for the two soil textures. The model slightly under-estimated or over-estimated the soil water content with percent bias less than 15.6%. There was no significant difference (p > 0.05) between the soil water distribution in lateral and downward direction for both sandy clay loam soil and loamy sand. However, the soil water content upward of the Moistube placement depth was significantly lower (p 0.05). Water loss through drainage was significantly higher (p < 0.05) under SDI than MTI in loam while it was negligible in clay for both irrigation types. Drainage increased with increased Moistube placement depth. The interaction between the distribution of root water uptake and the soil water distribution indicated that a suitable placement depth for cowpea under MTI was 15 cm in loam and 20 cm in clay. There were no significant differences (p > 0.05) in the yield response of cowpea between MTI and SDI but the latter performed better under deficit irrigation conditions. AquaCrop model was parameterized and tested successfully under full and deficit irrigation. The results indicated the model simulated the canopy cover (CC) very well with R2 ≥ 0.85, RMSE ≤ 24.5%, EF ≥ 0.45, and d ≥ 0.87. The simulated water content closely matched the observed with R2 ≥ 0.61, RMSE ≤ 11.3 mm, EF ≥ 0.51, and d ≥ 0.86 indicating that the model reasonably captured the soil water dynamics. Generally, yield and biomass were simulated satisfactorily by the model with R2 of 0.84 and 0.88, and RMSE of 282 kg ha -1 and 1307 kg ha -1, respectively, during parameterisation. Similarly, during model testing the model performance was very good with R2 of 0.96 and 0.99, and RMSE of 165 kg ha -1 and 798 kg ha -1 for yield and biomass, respectively. The highest WP was achieved under 70% ETc (crop water requirement) and 40% ETc for yield and biomass, respectively. Having successfully calibrated and tested the HYDRUS 2D/3D and AquaCrop models, the two were used symbiotically to simulate the water use of cowpea in two environments characterized by clay and sandy soils. The crop characteristics were obtained using AquaCrop while HYDRUS 2D/3D was used to generate optimum irrigation schedules and the soil water balance. Thereafter, the water use and yield of cowpea was determined. The average grain yield and biomass were 2600 kg ha-1 and 10000 kg ha-1, respectively, with the difference between the two sites being less than 5% under both SDI and MTI. The water use and WP varied from 315 mm to 360 mm and 0.67 to 1.02 kg m-3, respectively, under the two irrigation types at the two sites considered. The WP was higher under SDI than MTI, but the differences were less than 10%. This showed that cowpea responded similarly under MTI and SDI. Further research is needed on the determination of the clogging characteristics due to fertigation. Finally, more field experiments under other environmental conditions need to be carried out to validate the results of this study

Improving infiltration modelling for crusting soils

Infiltration and surface runoff modelling is important in many disciplines including environmental engineering, mine site rehabilitation, ecology and agronomy. Major errors in modelling can occur when the throttling effect of soil surface crusts, which reduce infiltration and subsequently increase surface runoff, are not considered. The aim of this project was to improve the modelling of infiltration on crusting soils by measuring the density of the soil surface crust. A rainfall simulator was used to create a surface crust on soils (Sodosol and Chromosol) susceptible to surface crusting. The surface crusts resulted in greater than 90 per cent of applied rainfall becoming runoff. Several methods of measuring the crust density were trialled, with X-ray micro Computed Tomography (X-ray CT), when combined with the traditional soil core method, found to be the most accurate and reliable. The HYDRUS-1D software application was used to model infiltration. Measured soil parameters, including crust density, and applied rainfall rates were used as model inputs. The inclusion of crust density into HYDRUS-1D resulted in insignificant improvements to modelling accuracy. Inverse modelling identified that this was as a result of HYDRUS-1D predicted saturated hydraulic conductivity values being three to four orders of magnitude larger than obtained from the inverse solution. The application of average crust hydraulic parameters, obtained from the set of inverse solutions, was found to provide a close approximation of infiltration rates— once surface runoff had commenced — and cumulative infiltration. The findings of this project indicate that the use of average surface crust hydraulic parameters could provide major improvements in infiltration modelling accuracy using HYDRUS-1D without the requirement for additional sampling and analysis during field surveys. Further experimentation on a broader range of soils under differing vegetation regimes is required to validate the conclusions of this project

Investigating strain localisation in clay using mica markers and X-ray computed tomography

Mass movements in clay deposits result in damage to infrastructures and buildings with significant social, economic, and environmental consequences. These processes are characterised by strain localisation, a complex process to investigate experimentally and model. Strain localisation in clays is particularly worrisome and possess huge destructive capabilities because clay is characterised by low shear strength. Conventional laboratory tests are essentially a post-mortem destructive analysis of localised deformations, and do not account for the fundamental physics of soil behaviours. Hence the development of 4-dimensional (4-D) non-destructive imaging approaches to study soil mechanical behaviour. However, due to the small size of clay particles compared to achievable X-ray computed tomography (X-ray CT) resolution, it has not been possible to directly evaluate particle scale clay micromechanics non-destructively using 4-D imaging techniques. This thesis presents a novel technique involving the use of plate-shaped (“platy”) muscovite mica marker for the evaluation of the initiation and propagation of strain localisation in kaolin. First, an investigation was carried out to understand the suitability of the use of mica particle markers for the study of clay by carrying out both chemical and mechanical characterization of mica. Subsequently, sample preparation techniques were experimented to understand the appropriate sampling approach with least microstructure disturbance. Furthermore, a novel miniature triaxial cell instrumented with a high capacity tensiometer and a novel platy particle matching algorithm were developed for the study of mica marker particle kinematics within kaolin. Kinematic analysis (displacement and rotation) of mica particle markers within the kaolin sample was then carried out. The results presented in this thesis demonstrated that (i) The particle configuration of silt sized muscovite samples consistently varied (dispersive and non-dispersive) with pore-water chemistry, regardless of whether the samples being tested were suspension sediments or compacted samples. (ii) By adding both silt sized muscovite or sand sized muscovite to kaolin for up to 30% sand sized muscovite or silt sized muscovite, the compressive behaviour is still clay-dominated. Similarly, the addition of mica (up to 30%) to kaolin does not significantly affect its hydraulic conductivity of kaolin. However, the shear strength characteristics of kaolin may significantly change by the addition of about 2.5-30% of silt sized muscovite or sand in the low normal stress (<100 kPa) but not at higher stress regime. (iii) PLATYMATCH (algorithm developed in this thesis) can effectively match platy particles in consecutive sample scans when adequately registered and the particles adequately segmented. (iv) A conceptual model of the initiation and propagation of strain localisation in kaolin was developed. The findings of this thesis implies that there is the potential to use platy mica particle marker images for early (pre-peak shear strength) detection of the initiation and propagation of strain localisation in kaolin and this may possibly be useful in enhancing available constitutive models such as the double scale constitutive model for improved clay behaviour prediction.Mass movements in clay deposits result in damage to infrastructures and buildings with significant social, economic, and environmental consequences. These processes are characterised by strain localisation, a complex process to investigate experimentally and model. Strain localisation in clays is particularly worrisome and possess huge destructive capabilities because clay is characterised by low shear strength. Conventional laboratory tests are essentially a post-mortem destructive analysis of localised deformations, and do not account for the fundamental physics of soil behaviours. Hence the development of 4-dimensional (4-D) non-destructive imaging approaches to study soil mechanical behaviour. However, due to the small size of clay particles compared to achievable X-ray computed tomography (X-ray CT) resolution, it has not been possible to directly evaluate particle scale clay micromechanics non-destructively using 4-D imaging techniques. This thesis presents a novel technique involving the use of plate-shaped (“platy”) muscovite mica marker for the evaluation of the initiation and propagation of strain localisation in kaolin. First, an investigation was carried out to understand the suitability of the use of mica particle markers for the study of clay by carrying out both chemical and mechanical characterization of mica. Subsequently, sample preparation techniques were experimented to understand the appropriate sampling approach with least microstructure disturbance. Furthermore, a novel miniature triaxial cell instrumented with a high capacity tensiometer and a novel platy particle matching algorithm were developed for the study of mica marker particle kinematics within kaolin. Kinematic analysis (displacement and rotation) of mica particle markers within the kaolin sample was then carried out. The results presented in this thesis demonstrated that (i) The particle configuration of silt sized muscovite samples consistently varied (dispersive and non-dispersive) with pore-water chemistry, regardless of whether the samples being tested were suspension sediments or compacted samples. (ii) By adding both silt sized muscovite or sand sized muscovite to kaolin for up to 30% sand sized muscovite or silt sized muscovite, the compressive behaviour is still clay-dominated. Similarly, the addition of mica (up to 30%) to kaolin does not significantly affect its hydraulic conductivity of kaolin. However, the shear strength characteristics of kaolin may significantly change by the addition of about 2.5-30% of silt sized muscovite or sand in the low normal stress (<100 kPa) but not at higher stress regime. (iii) PLATYMATCH (algorithm developed in this thesis) can effectively match platy particles in consecutive sample scans when adequately registered and the particles adequately segmented. (iv) A conceptual model of the initiation and propagation of strain localisation in kaolin was developed. The findings of this thesis implies that there is the potential to use platy mica particle marker images for early (pre-peak shear strength) detection of the initiation and propagation of strain localisation in kaolin and this may possibly be useful in enhancing available constitutive models such as the double scale constitutive model for improved clay behaviour prediction

Weather-driven clay cut slope behaviour in a changing climate

Long linear earthwork assets constructed in high-plasticity overconsolidated clay are known to be deteriorating due to long-term effects of wetting and drying stress cycles as a result of seasonal weather patterns. These stress cycles can lead to shallow first-time failures due to the mobilisation of post-peak strength and progressive failure. Design requirements of new earthworks and management of existing assets requires improved understanding of this critical mechanism; seasonal ratcheting. Incremental model development and validation to allow investigation of multiple inter-related strength deterioration mechanisms of cut slope behaviour in high-plasticity overconsolidated clay slopes has been presented. Initially, the mechanism of seasonal ratcheting has been considered independently and a numerical modelling approach considering unsaturated behaviour has been validated against physical modelling data. Using the validated model, the effects of slope geometry, design parameter selection and design life have been considered. Following this, an approach to allow undrained unloading of soil, stress relief, excess pore water pressure dissipation, seasonal ratcheting and progressive failure with wetting and drying boundary conditions has been considered. Hydrogeological property deterioration and the potential implications of climate change have been explored using the model. In both cases the serviceable life of cut slopes is shown to reduce significantly in the numerical analyses. Finally, a model capable of capturing hydrogeological behaviour of a real cut slope in London Clay has been developed and validated against long-term field monitored data. Using the validated model, a climate change impact assessment for the case study slope has been performed. The numerical analyses performed have indicated that seasonal ratcheting can explain shallow first-time failures in high-plasticity overconsolidated clay slopes and that the rate of deterioration of such assets will accelerate if current climate change projections are representative of future weather

Random finite element method prediction and optimisation for open pit mine slope stability analysis

Inherent soil variability can have significant effects on the stability of open-pit mine slopes. In practice, the spatial variability of materials is not commonly considered as a routine component of slope stability analysis. The process of quantifying spatially variable parameters, as well as the modelling of their behaviour is often a complex undertaking. Currently, there are no large-scale commercial software packages containing in-built methods for modelling spatial variability within the Finite Element environment. Furthermore, conventional Limit Equilibrium Methods (LEM) incorporating spatial variability are unable to consider the stress/strain characteristics of these materials. The following research seeks to accurately model the slope mechanics of spatially variable soils, adopting The Random Finite Element Method (RFEM) developed by Griffiths and Fenton (2004) to determine slope failure mechanisms and safety factors. Techniques are developed to produce a set of optimised Random Finite Element Method simulations using the Monte Carlo Method. Additionally, random field analysis techniques are investigated to compare and categorise soil parameter fluctuation, providing a direct relationship between random field properties and slope failure surfaces. Optimisation and analysis techniques are implemented to examine the effects of cross-sectional geometries and input parameter distributions on failure mechanisms, safety factors and probabilities of failure. Cross-sectional RFEM analysis is performed in the Finite Element Method (FEM) software package Abaqus, with the techniques of this research demonstrated for a large open-pit brown coal mine located in the state of Victoria, Australia. The outcome of this research is a comprehensive procedure for optimised RFEM simulation and analysis.Doctor of Philosoph

Novel Approaches in Landslide Monitoring and Data Analysis

Significant progress has been made in the last few years that has expanded the knowledge of landslide processes. It is, therefore, necessary to summarize, share and disseminate the latest knowledge and expertise. This Special Issue brings together novel research focused on landslide monitoring, modelling and data analysis