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Slope stability analysis of a landfill subjected to leachate recirculation and aeration considering bio-hydro coupled processes
AbstractDuring the operation of landfills, leachate recirculation and aeration are widely applied to accelerate the waste stabilization process. However, these strategies may induce high pore pressures in waste, thereby affecting the stability of the landfill slope. Therefore, a three-dimensional numerical analysis for landfill slope stability during leachate recirculation and aeration is performed in this study using strength reduction method. The bio-hydro coupled processes of waste are simulated by a previously reported landfill coupled model programmed on the open-source platform OpenFOAM and then incorporated into the slope stability analysis. The results show that both increasing the injection pressure for leachate recirculation and maximum anaerobic biodegradation rate will reduce the factor of safety (FS) of the landfill slope maximally by 0.32 and 0.62, respectively, due to increased pore pressures. The ignorance of both waste biodegradation and gas flow will overestimate the slope stability of an anaerobic bioreactor landfill by about 20–50%, especially when the landfilled waste is easily degradable. The FS value of an aerobic bioreactor landfill slope will show a significant reduction (maximally by 53% in this study) when the aeration pressure exceeds a critical value and this value is termed as the safe aeration pressure. This study then proposes a relationship between the safe aeration pressure and the location of the air injection screen (i.e., the horizontal distance between the top of the injection screen and the slope surface) to avoid landfill slope failure during aeration. The findings of this study can provide insights for engineers to have a better understanding of the slope stability of a bioreactor landfill and to design and control the leachate recirculation and aeration systems in landfills.
Abstract Ensuring wells’ cement mechanical integrity (CMI) is of paramount importance for the success of a thermal project. Failed cement sheaths can lead to loss of production, environmental pollutions, or even to well abandonment. Over time, CMI software applications have been developed to design wells that do not leak. However, their efficiency depends not only on if their equations are verified, but also on how the models are validated versus wells’ downhole conditions. Unfortunately, most CMI tool designers have focused on only verifying if the models are mathematically correct, checking what is the time required for a simulation, and improving how are the simulations reported to the user. Typically, little time is dedicated on validating that the correct model is used for the specific well. This foresight has led to non-predictive CMI tools, which do not allow optimizing well designs. The authors have been involved for more than 15 years in developing and validating CMI models. They have shown the importance of simulating the cement hydration to evaluate the state of stress in the cement after it has set. They also have highlighted how the plastic behavior of the cement design can lead to opening micro-annuli at the cement-sheath's interfaces. Recently the authors have started theoretical work in the area of the cement integrity of high and ultra-high temperature wells and how these temperatures, either naturally occurring or induced, could affect the cement's mechanical integrity. The work has focused on modeling the increase in pore pressures, the opening of micro-annuli at the cement sheath's boundaries, and the phase changes which take place in the cement when it is heated to high temperature values. To date this work showed that heating cement up to 250°C can result in pore pressures larger than 100 MPa unless if the pore pressures can be released. This work has also identified three mechanisms that can lead to such release of pore pressures: 1) During cement hydration, due to the water consumption by the chemical reactions, 2) When a micro-annulus opens due to the large pore pressures, therefore allowing venting the pressures to the surface or to a downhole reservoir, and 3) When a change of phase occurs in the cement when heated to more than 110°C, as this leads to the creation of additional porosity in the cement. All this means that the cement sheath should not be simulated as a closed system, but rather as an open thermo-hydro-chemo-mechanics. How these features impact CMI has never been studied before even if they can explain why some cement designs lead to tight cement sheath and other to leaking ones. This paper highlights the work that has been done and when these conditions should be considered, and if it is feasible to design cement sheaths that do not fail, even at very high temperatures.
Abstract Most lab-scale acidizing experiments are performed in core samples with 100% water saturation conditions and at pore pressures around 1100 psi. However, this is seldom the case on the field, where different saturation conditions exist with high temperature and pressure conditions. Carbon-di-Oxide (CO2), a by-product evolved during the acidizing process, is long thought to behave inertly during the acidizing process. Recent investigations reveal that the presence of CO2 dynamically changes the behavior of wormhole patterns and acid efficiency. A compositional simulation technique was adopted to understand the process thoroughly. A validated compositional numerical model capable of replicating acidizing experiments at the core-scale level, in fully aqueous environments described in published literature was utilized in this study. The numerical model was extended to a three-phase environment and applied at the field scale level to monitor and evaluate the impacts of evolved CO2 during the carbonate acidizing processes. Lessons learned from the lab-scale were tested at the field-scale scenario via a numerical model with radial coordinates. Contrary to popular belief, high pore pressures of 1,000 psi and above are not sufficient to keep all the evolved CO2 in solution. The presence of CO2 as a separate phase hinders acid efficiency. The reach or extent of the evolved CO2 is shown to exist only near the damage zone and seldom penetrates the reservoir matrix. Based on the field scale model's predictions, this study warrants conducting acidizing experiments at the laboratory level, at precisely similar pressure, temperature, and salinity conditions faced in the near-wellbore region, and urges the application of compositional modeling techniques to account for CO2 evolution, while studying and predicting matrix acidizing jobs.
<p><b>Increases in rainfall-induced landsliding following large earthquake are well documented but the time frames over which this heightened hazard persists in the land scape remains poorly understood. Whilst it is well known that the presence of failed and partially slopes after earthquakes significantly reduces the rainfall thresholds required to activate slope movement, their failure susceptibility during specific storms and how this changes through time remains poorly studied. To improve knowledge in this field requires well documented slope failures following earthquakes and a detailed understanding of their potential failure mechanisms when pore pressures are elevated in the slope. The 2016 Mw 7.8 Kaikōura earthquake provides a unique opportunity to study how rainfall events following the earthquake may impact the timing and mechanisms of landslide reactivation. </b></p><p>This study conducted a suite of specialist triaxial cell experiments, designed to replicate varying rainfall scenarios on remoulded samples collected from two sites where numerous earthquake-induced landslides were recorded in similar Late Cretaceous to Neogene sediments with similar physical properties (the Leader Dam Landslides (LDL) and the Limestone Hill landslide (LHL)). In each experiment rainfall events were simulated using a series of different pore pressure scenarios (increases and decreases in mean effective stress) at representative field stress conditions whilst monitoring material deformation behaviour. </p><p>The results demonstrate that both the deformation behaviour and pore pressure required to generate failure were influenced by the previous changes in pore pressure. Samples subjected to stepped increases in pore pressure were subject to greater pre-failure deformation (dilation) and subsequently failed at lower pore pressures (higher mean effective stress) when compared to samples subjected to linear increases in pore pressure. In addition, increases in the rate of pore pressure also increased the amount of pre-failure deformation allowing failure to occur when pore pressures were lower. In contrast a sample subjected to both increases and decreases in pore pressure underwent pre-failure densification and subsequently required a larger increase in pore pressure to fail. The results demonstrate that landslide reactivation is influenced by a number of factors including the amount and rate of previous changes in pore pressure and the slope drainage history. </p><p>The results provide new insights into why landslide susceptibility may remain elevated for prolonged periods of time (e.g. decades) in the landscape as well as why the rainfall thresholds for site specific failures during storms may be difficult to predict. </p>
<p><b>Increases in rainfall-induced landsliding following large earthquake are well documented but the time frames over which this heightened hazard persists in the land scape remains poorly understood. Whilst it is well known that the presence of failed and partially slopes after earthquakes significantly reduces the rainfall thresholds required to activate slope movement, their failure susceptibility during specific storms and how this changes through time remains poorly studied. To improve knowledge in this field requires well documented slope failures following earthquakes and a detailed understanding of their potential failure mechanisms when pore pressures are elevated in the slope. The 2016 Mw 7.8 Kaikōura earthquake provides a unique opportunity to study how rainfall events following the earthquake may impact the timing and mechanisms of landslide reactivation. </b></p><p>This study conducted a suite of specialist triaxial cell experiments, designed to replicate varying rainfall scenarios on remoulded samples collected from two sites where numerous earthquake-induced landslides were recorded in similar Late Cretaceous to Neogene sediments with similar physical properties (the Leader Dam Landslides (LDL) and the Limestone Hill landslide (LHL)). In each experiment rainfall events were simulated using a series of different pore pressure scenarios (increases and decreases in mean effective stress) at representative field stress conditions whilst monitoring material deformation behaviour. </p><p>The results demonstrate that both the deformation behaviour and pore pressure required to generate failure were influenced by the previous changes in pore pressure. Samples subjected to stepped increases in pore pressure were subject to greater pre-failure deformation (dilation) and subsequently failed at lower pore pressures (higher mean effective stress) when compared to samples subjected to linear increases in pore pressure. In addition, increases in the rate of pore pressure also increased the amount of pre-failure deformation allowing failure to occur when pore pressures were lower. In contrast a sample subjected to both increases and decreases in pore pressure underwent pre-failure densification and subsequently required a larger increase in pore pressure to fail. The results demonstrate that landslide reactivation is influenced by a number of factors including the amount and rate of previous changes in pore pressure and the slope drainage history. </p><p>The results provide new insights into why landslide susceptibility may remain elevated for prolonged periods of time (e.g. decades) in the landscape as well as why the rainfall thresholds for site specific failures during storms may be difficult to predict. </p>
Highway embankments are important structural elements in modern road infrastructure. If such a construction is built on cohesive low-permeability soils, it is necessary to perform a prediction of long-term settlements and excess pore pressures. The paper presents a numerical analysis of an instrumented embankment constructed in the Czech Republic using the finite element method. Two alternative constitutive models were employed throughout the analysis: standardly used linear elastic perfectly plastic model and elastoplastic model with volumetric and shear hardening with stress-dependent stiffness. A construction sequence was modelled in detail including durations of partial construction stages. Both the settlements of subsoil (in short-term and long-term conditions) and excess pore pressures measured in multiple depths were evaluated and compared with predictions. Results employing a more complex constitutive model show a reasonably good agreement with measurement both in terms of settlements and pore pressures. The application of a perfectly plastic constitutive model leads to an overestimation of settlements.
