Responses of Laterally Loaded Single Piles Subjected to Various Loading Rates in a Poroelastic Soil

Groundwater table has an important role in soil–structure interaction problems. However, analysis of laterally loaded single piles has often been conducted by solely considering the mechanics of the soil skeleton or decoupling the interactive mechanics of the soil skeleton and the fluid flux; in other words, most analyses were performed without taking into consideration the coupling effect between the soil skeleton and the fluid flux. To improve our understanding of the hydromechanical coupling effect on laterally loaded single piles, a series of finite element study on laterally loaded single piles in saturated porous media was conducted. The effect of pile cap geometries, cap widths, cap embedment depths, and pile lengths, on the response of laterally loaded single piles was also studied. The loading condition of the pile was found to have a significant effect on the generation of excess pore-water pressure. The lateral displacement and bending moment computed at the maximum excess pore water pressure, which in turn, is equivalent to an undrained analysis, produced the minimum responses among all the other loading conditions. The effect of pile cap geometries was found to be much less significant than anticipated.

Effect of Deep Mixing Grid Spacing On The Settlement of Liquefied Soil, Using Numerical Approach

Liquefaction occurs in a loose and saturated sand layer, induces quite large damages to infrastructures, the importance of liquefaction mitigation has been emphasized to minimize earthquake disasters for many years. Many kinds of ground improvement techniques based on various improvement principles have been developed for liquefaction mitigation. Among them, deep mixing method with grid pattern was developed for liquefaction mitigation in the 1990s, where the grid of stabilized column walls functions to restrict the generation of excess pore pressure by confining the soil particle movement during earthquake. In this study, a parametric study of the grid-form deep mixing wall is performed using numerical modeling with GID+OpenSees interface V2.6.0. The finite element method with a three-dimensional analysis model can be used to estimate the foundation settlement over liquefiable soil layer. The validity of the developed model was evaluated by comparing the results obtained from the model with the results of numerical studies and the experimental centrifuge test to investigate the effect of deep mixing grid wall on the settlement and generation of excess pore pressure ratio of liquefiable soil. Based on the analysis, the settlement for improved soil was 69% smaller than the settlement for unimproved soil. The results also indicated that the grid wall space, relative density, and stiffness ratio between soil-cement columns and enclosed soil plays an important role in the occurrence of liquefaction and volumetric strains.

Analysis of Settlement Behaviour of Soft Ground Under Wide Embankment

An elastoplastic numerical model for calculating the consolidation settlement of wide embankment on soft ground is established using PLAXIS finite element software to investigate the settlement behaviour of soft ground under the wide embankment. The distribution rules are analysed and compared to narrow embankments, such as surface settlements of ground and embankment, lateral displacement of soft ground at the foot of embankment slope and excess pore pressure in soft ground. The influence rule of elastic modulus of soft ground on the settlement of soft ground under wide embankment is discussed. The results show that the settlement distributions of wide and narrow embankments on soft ground are “W” and “V” shapes, respectively. The maximum settlement of wide embankment is near the foot of the embankment slope, which is unequal to the settlement at the centreline of the embankment. The lateral displacement distribution rules of soft ground are both “belly” shaped at the foot of two types of embankments slope. However, the lateral displacement of the wide embankment is larger in each corresponding stage. During the construction period, the excess pore pressure in the soft ground under the wide embankment is much higher than that of the narrow embankment, so the post-construction consolidation time of the wide embankment is longer. Moreover, the macroscopic settlement rule of the wide embankment is still the same with the increase of elastic modulus of soft ground.

Experimental Investigation on the Behavior of Gravelly Sand Reinforced with Geogrid under Cyclic Loading

In view of the dynamic response of geogrid-reinforced gravel under high-speed train load, this paper explores the dynamic characteristics of geogrid-reinforced gravel under semi-sine wave cyclic loading. A number of large scale cyclic triaxial tests were performed on saturated gravelly soil reinforced with geogrid to study the influence of the number of reinforcement layers and loading frequencies on the dynamic responses of reinforced gravelly sand subgrade for high speed rail track. The variation of cumulative axial and volumetric strains, excess pore pressure and resilient modulus with number of loading cycles, loading frequency, and reinforcement arrangement are analyzed. The test results reveal that the cumulative axial strain decreases as the number of reinforcement layers increases, but increases with loading frequency. The resilience modulus increases with the number of reinforcement layers, but decreases as the loading frequency increases. The addition of geogrid can reduce the excess pore water pressure of the sample, but it can slightly enhance the rubber mold embedding effect of the sand sample. As the loading frequency increases, the rubber mold embedding effect gradually weakens.

Mechanisms of rainfall-induced landsliding in Wellington fill slopes

<p>Recent landslides from Wellington fill slopes have occurred as potentially hazardous, mobile debris flow-slides with long runouts during heavy rainstorms. Globally, catastrophic landslides from fill slopes are well documented, and in many instances their rapid failure and long runout suggests that their shear zones may be subject to liquefaction. Various generations of fill slopes throughout Wellington, and urban New Zealand, have been constructed using different practices and at variable scales. Despite this, very few laboratory based studies to determine how different fill slopes may perform during rainstorms have been attempted, as conventional laboratory tests do not adequately simulate the failure conditions in the slope. This study uses a novel, dynamic back-pressured shear box to conduct rapid shear and specialist pore pressure inflation tests in order to replicate rainfall induced failure conditions in fill slopes with different consolidation histories and particle size characteristics. During each test, excess pore-water pressures and deformation were monitored until failure in order to determine the failure mechanisms operating. This study demonstrates that the failure mechanisms in fill slopes are strongly influenced by the consolidation history and particle size characteristics of the shear zone materials. In over-consolidated and fine grained (< 0.4 mm) fills where cohesion is present, brittle failure was observed. In these materials, failures occur more rapidly but require much higher pore-water pressures to initiate. Conversely, normally-consolidated fill slopes constructed from coarser material (0.4 - 2 mm) fail through ductile deformation processes, which typically initiate at much lower pore-water pressures but result in a less rapid slope failure. Although liquefaction was not observed, excess pore-water pressures can be generated during rapid shearing, indicating that liquefaction could occur after a landslide has initiated in conditions where excess pore-water pressures are unable to dissipate away from the shear zone. These results provide new insights into the types of failure that may be anticipated from different fill slopes, the hazards they may pose and potential mitigation measures that could be implemented.</p>

Mechanisms of rainfall-induced landsliding in Wellington fill slopes

<p>Recent landslides from Wellington fill slopes have occurred as potentially hazardous, mobile debris flow-slides with long runouts during heavy rainstorms. Globally, catastrophic landslides from fill slopes are well documented, and in many instances their rapid failure and long runout suggests that their shear zones may be subject to liquefaction. Various generations of fill slopes throughout Wellington, and urban New Zealand, have been constructed using different practices and at variable scales. Despite this, very few laboratory based studies to determine how different fill slopes may perform during rainstorms have been attempted, as conventional laboratory tests do not adequately simulate the failure conditions in the slope. This study uses a novel, dynamic back-pressured shear box to conduct rapid shear and specialist pore pressure inflation tests in order to replicate rainfall induced failure conditions in fill slopes with different consolidation histories and particle size characteristics. During each test, excess pore-water pressures and deformation were monitored until failure in order to determine the failure mechanisms operating. This study demonstrates that the failure mechanisms in fill slopes are strongly influenced by the consolidation history and particle size characteristics of the shear zone materials. In over-consolidated and fine grained (< 0.4 mm) fills where cohesion is present, brittle failure was observed. In these materials, failures occur more rapidly but require much higher pore-water pressures to initiate. Conversely, normally-consolidated fill slopes constructed from coarser material (0.4 - 2 mm) fail through ductile deformation processes, which typically initiate at much lower pore-water pressures but result in a less rapid slope failure. Although liquefaction was not observed, excess pore-water pressures can be generated during rapid shearing, indicating that liquefaction could occur after a landslide has initiated in conditions where excess pore-water pressures are unable to dissipate away from the shear zone. These results provide new insights into the types of failure that may be anticipated from different fill slopes, the hazards they may pose and potential mitigation measures that could be implemented.</p>

Molecular Dynamics Simulation of the Effects of Methane Hydrate Phase Transition on Mechanical Properties of Deep-Sea Methane Hydrate-Bearing Soil

In this paper, the methane hydrate phase transition process in deep-sea methane hydrate-bearing soil under heating and compression was simulated by the molecular dynamics method. The evolution of deep-sea methane hydrate-bearing soil’s microstructure, system energy, intermolecular interaction energy, and radial distribution function during heating and compression was investigated. The micromechanism of the influence of the methane hydrate phase transition on the mechanical properties of deep-sea methane hydrate-bearing soil was analyzed. The results demonstrated that the methane hydrate dissociation starts from both sides to the middle and the void spaces between the soil particles had nearly no change during the heating process. For the compression process, the methane hydrate on both sides and the middle dissociated at the same time, and the void spaces became smaller. The methane hydrate phase transition on the effects of mechanical properties of the deep-sea methane hydrate-bearing soil is mainly caused by three aspects. (1) the dissociation of methane hydrate incurs the decrease of methane hydrate saturation. The free water and methane molecules generated cannot migrate in time and thus lead to the increase of excess pore water press and excess pore gas press. (2) The dissipated energy causes the decrease of the effective stress between the soil particles. (3) Due to the methane hydrate decomposition, the free water molecules increase, which reduces the friction of soil particles.