Sediment incipient motion is the first step of the whole process of sediment transport. However, previous numerical works simplified the seabed surface as a type of impermeable and rigid boundary, ...and ignored the effect of seepage flow on the mobility of bed particles. In this paper, to reveal the physics behind sediment incipient motion around a free spanning pipeline, an integrated numerical model, coupling the SST (Shear-Stress Transport) turbulence model with the porous seabed model, was proposed. Numerical studies showed that with the periodic formation and shedding of vortices around the pipeline, both the oscillatory and residual excess pore-pressures developed within the seabed. In some cases, the vertical gradient of excess pore-pressure (seepage force) had a significant impact on the mobility of bed particles around the pipeline. It was found that lower saturation degree and seabed permeability would remarkably increase both the oscillatory and residual seepage forces, and thus enhance bed particle mobility. While for a sandy seabed with smaller soil shear modulus, the oscillatory seepage force was only slightly reduced, but a high residual seepage force would be generated with time. This could lead to obvious decrease in the submerged weight of bed particles, making them more easily dragged away from the seabed.
•Propose an integrated numerical model to couple the turbulent flow and porous seabed models.•Study the excess pore-pressure responses in the seabed beneath the suspended pipeline.•Quantify the effect of seepage flow on the mobility of bed particles.•Study the effects of seabed parameters on the flow-induced seepage force.
During an Enhanced Geothermal System (EGS) experiment, fluid is injected at high pressure into crystalline rock, to enhance its permeability and thus create a reservoir from which geothermal heat can ...be extracted. The fracturing of the basement caused by these high pore‐pressures is associated with microseismicity. However, the relationship between the magnitudes of these induced seismic events and the applied fluid injection rates, and thus pore‐pressure, is unknown. Here we show how pore‐pressure can be linked to the seismic frequency–magnitude distribution, described by its slope, theb‐value. We evaluate the dataset of an EGS in Basel, Switzerland and compare the observed event‐size distribution with the outcome of a minimalistic model of pore‐pressure evolution that relates event‐sizes to the differential stressσD. We observe that the decrease of b‐values with increasing distance of the injection point is likely caused by a decrease in pore‐pressure. This leads to an increase of the probability of a large magnitude event with distance and time.
Key Points
The b‐values of induced earthquakes decrease with distance from the injection point
We propose a pressure‐driven geomechanical model to explain our observation
We estimate the probability of induced large magnitude events in space and time
Existing simplified procedures for evaluating soil liquefaction potential or for estimating excess pore pressures during earthquakes are typically based on undrained cyclic tests performed on ...saturated soil samples under controlled loading and boundary conditions. Under such conditions, the effect of excess pore pressure (ue) dissipation and redistribution to neighboring soil layers cannot be accounted for. Existing simplified procedures treat liquefiable layers as isolated soil layers without any boundary conditions even if dense and loose layers are very thin, permeable, and adjacent to each other. However, redistribution is likely to increase and decrease ue in the neighboring dense and loose layers respectively. Until now, no procedure short of fully coupled numerical analysis is available to estimate the importance of redistribution. This paper presents an approximate analytical procedure for assessing the effects of ue redistribution in (1) soil layers that would have liquefied if they were undrained, and (2) soil layers that would have not liquefied even if undrained. It is found that a layer that is initially assumed liquefied under undrained conditions might not even liquefy accounting for the ue redistribution to neighboring layers. On the other hand, a layer initially assumed to not liquefy can develop significant ue and can even liquefy due to pore pressure migration from the neighboring layers. Thus, accounting for redistributed ue is important for liquefaction consequence assessment quantification, particularly in systems that span the depth of these effects like deep foundations. Migration of u toward the tip of a pile can reduce its capacity, even if the tip is embedded in a dense sand layer. On the other hand, if redistribution can result in the reduction of ue in initially assumed liquefied layers, risks associated with liquefaction might be avoided. A criterion is also developed to evaluate the thicknesses of a layer below which redistribution could prevent liquefaction even if the layer is deemed liquefied according to the existing liquefaction-triggering procedures. Finally, the proposed procedure is illustrated by application to selected shaking events of centrifuge tests involving liquefaction of layered soil profiles. The predictions from the procedure matched the centrifuge test results reasonably.
This paper presents the results of an experimental investigation of the complete sequence of sediment behaviour beneath progressive waves. The sediment was silty with d50 = 0.060 mm. Two kinds of ...measurements were carried out: pore‐water pressure measurements (across the sediment depth), and water‐surface elevation measurements. The process of liquefaction/compaction was videotaped from the side simultaneously with the pressure and water‐surface elevation measurements. The video records were then analysed to measure: (i) the time development of the mudline, (ii) the time development of liquefaction and compaction fronts in the sediment and (iii) the characteristics of the orbital motion of the liquefied sediment including the motion of the interface between the water body and the sediment. The ranges of the various quantities in the tests were: wave height, H = 9–17 cm, wave period, T = 1.6 sec, water depth = 42 cm, and the Shields parameter = 0.34–0.59. The experiments reveal that, with the introduction of waves, excess pore pressure builds up, which is followed by liquefaction during which internal waves are experienced at the interface of the water body and the liquefied sediment, the sequence of processes known from a previous investigation. This sequence of processes is followed by dissipation of the accumulated excess pore pressure and compaction of the sediment which is followed by the formation of bed ripples. The present results regarding the dissipation and compaction appear to be in agreement with recent centrifuge wave‐tank experiments. As for the final stage of the sequence of processes (formation of ripples), the ripple steepness (normalized with the angle of repose) for sediment with liquefaction history is found to be the same as that in sediment with no liquefaction history.
Summary
This paper presents a numerical scheme for fluid‐particle coupling that uses the discrete element method by taking into consideration solid deformation and pore pressure generation. A new ...water particle element is introduced to calculate pore water pressure due to porosity changes. The water particle element has the same size and shape as the solid element and experiences the same amount of deformation. On the basis of the effective stress principle at the element contact, the total force is equal to the sum of the force transmitted through the solid element contact and the water particle force due to pore water pressure. Analytical solutions of traditional soil mechanics problems, such as isotropic compression and consolidated triaxial undrained test, are used to quantitatively validate the proposed model. The numerical results show good agreement between the model and the analytical solutions. The model therefore provides an effective method to calculate pore pressure in a porous medium in discrete modeling.
With rapid urbanisation, the amount of construction solid waste is increasing, especially in developing countries, where landfill still dominates waste treatment. Lack of proper management and ...monitor, many landfills experience high landslide risk, and a typical case is the Guangming landfill landslide in Shenzhen, China on 20 December 2015. The main problem is that detailed field investigation for each landfill requires too much time and effort. Consequently, a satellite-based analysis method is identified and used to efficiently assess landslide susceptibility of landfill. The core steps include investigation of the topography and climate of the landfill site and dynamic evaluation based on satellite data. Taking the Guangming landfill as an example, rapid filling and water catchment are taken into account with GIS, which generate high excess pore pressures and eventually trigger the gentle landfill slope to failure. The results show that the satellite-based analysis method can obtain the key factors of the landfill landslide.
Geotechnical problems with total stress changes occurring rapidly relative to the soil consolidation time can be conveniently addressed using an effective stress-based approach. For saturated states, ...analytical formulations of pore-pressure coefficients adopting coherent effective stress concepts exist. These formulations allow the induced pore water pressure variations and the corresponding effective stress changes to be determined. For unsaturated states, there are no analytical formulations of pore-pressure coefficients that adopt a suitable effective stress concept. This paper presents analytical derivations of pore-pressure coefficients for unsaturated elastic isotropic soils using the generalized effective stress (valid for saturated and unsaturated states) for different total stress changes. The model performance is described, and flowcharts to be adopted in engineering practice are provided. Compared with an existing approach that is not based on the effective stress, the proposed one requires fewer constitutive parameters (one of which, conveniently, is the soil bulk modulus), allows to analytically obtain the existing expressions for saturated states when the degree of saturation is 1 and to readily determine the retention curve under undrained conditions. Existing experimental data were analysed, and satisfactory interpretations and predictions of the experimental results were obtained. The proposed framework is a valuable tool for undrained mechanical analyses of unsaturated soils under different practical applications.
•The gas production from hydrate dissociation accounting for about 85%.•Analysis of the radius of the hydrate dissociation area in this production test.•Five-year prediction of long-term influence ...radius of pressure drop.•Exploration of favorable areas for the formation of secondary hydrates.•Analysis of the response of pressure and temperature of hydrate reservoirs.
The first offshore natural gas hydrate production test of China in 2017 has proved the feasibility of hydrate exploitation from clayey-silt reservoirs, which possesses the highest reservoirs than other types of hydrate resources. However, owing to the absence of monitoring wells in this production test, the hydrate dissociation behavior cannot be analyzed through pressure and temperature changes of hydrate reservoirs. This paper focuses on the simulation study on the detailed response of the temperature and pore pressure of hydrate reservoirs of Well SHSC-4 during the gas production by depressurization. Meanwhile, it highlights the analysis of favorable areas for the formation of secondary hydrates and the influence of the secondary hydrates on pressure and temperature field of hydrate reservoirs. The simulation results indicate that in the first 60 days, the hydrate reservoirs feature a dissociation radius of about 5 m, and the gas production from hydrate dissociation accounts for about 85%. After 1 year, 2 years and 5 years of hydrate exploitation, the influence radius of low-pressure area (<10 MPa) is 15 m, 16 m and 17 m, respectively, suggesting that the hydrate reservoirs have higher gas production efficiency in the first year. Furthermore, the temperature and pressure of hydrate reservoirs are not favorable to the formation of secondary hydrates in the first 60 days. In long-term production, secondary hydrates are mainly formed at hydrate dissociation front. This can increase the pore pressure and further decrease the effective stress in the local areas of hydrate reservoirs, thus affecting mechanical stability of the local hydrate reservoirs.
The injection of large volumes of pressurized water and grout into the subsoil during jet grouting generates a sudden increase in excess pore water pressure. This study proposes a theoretical ...approach to evaluate the variation in excess pore water pressure caused by the installation of a jet grouting column in clay, accounting for the chronological sequence of construction. The jet grouting column installation is simulated through the undrained expansion of a series of spherical cavities. Partial dissipation during the construction process is considered due to the gradual installation of the grouting columns. The relationship between the ultimate cavity radius (au) and the radius of the jet grouting column (rc) is established to represent the influences of both jetting parameters and soil properties on the generated excess pore water pressure. The proposed model is validated using two case studies, one conducted in Singapore marine clay and the other in Shanghai soft clay.
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