AbstractUnderground structures located in liquefiable soil deposits are susceptible to floatation following a major earthquake event. Such failure phenomenon generally occurs when the soil liquefies ...and loses its shear resistance against the uplift force from the buoyancy of the underground structure. Numerical modeling accompanied with centrifuge experiments with shallow circular structures has been carried out to investigate the floatation failure at different buried depths of the structure. The influence of the magnitude of input sinusoidal earthquake shaking was also studied. Both numerical and experimental results showed matching uplift response of the structures and acceleration and pore-pressure measurements in the liquefied soil deposit. A higher uplift displacement of the structure was observed for shallower buried depth, thereby indicating the influence of overlying soil weight against floatation. Results also showed that the structures commenced floatation in the presence of high excess pore pressure, but they ceased when the earthquake shaking stopped. The higher rate of uplift in stronger earthquake shaking further substantiates the dependency of the uplift to the shaking amplitude. A constant rate of uplift of the structure was attained after the soil liquefied, hence postulating a possible limit to shear modulus degradation of the surrounding soil caused by soil-structure interaction. This is inferred by the lower excess pore-pressure generation near the structure. The displacement of liquefied soil around the displaced structure was also confirmed to resemble a global circular flow mechanism from the crown of the structure to its invert as observed in displacement vector plots obtained from numerical analysis and particle image velocimetry (PIV) in centrifuge tests. Further numerical analysis on the performance of buried sewer pipelines in Urayasu City, Chiba Prefecture following the 2011 Great East Japan Earthquake indicated high damage susceptibility of rigid pipelines in the liquefiable soil deposit. These consistencies with field observations clearly demonstrate and pave the prospects of applying numerical and/or experimental analyses for geotechnical problems associated with the floatation of underground structures in liquefiable soils.
AbstractPartially saturated soils associated with the presence of occluded air bubbles are commonly encountered in seismically active zones. Yet, there is a lack of deep understanding of the ...mechanisms of settlement that partially saturated soils suffer during seismic events. Consequently, a reliable seismic design approach for use in engineering practice is lacking. The aim of this paper is to show that seismically induced settlement of a level deposit of partially saturated soil can be estimated by summing the settlement related to excess pore pressure (EPP) generation/dissipation and increased soil mass compressibility in the presence of air bubbles, which are calculated separately. Predictions of the proposed effective stress-based methodology are found to be in good agreement with geotechnical centrifuge measurements. It is shown that the variation of degree of saturation is of significance. The proposed methodology utilizes the degree of saturation as a parameter and can satisfactorily predict the experimentally observed settlement.
Passage of municipal waste induced greenhouse gases such as carbon di oxide (CO
2
) and methane in landfill covers, majorly depends on the state of unsaturation and compaction of soil biochar ...composite (SBC). The unsaturated state of SBC can be identified by measuring soil suction and volumetric water content (VWC) that can affect the air permeability of landfill covers. To design the landfill covers, it is required to propose a model which can forecast the air permeability up to a certain degree of accuracy. The aim of this study is to investigate the effect of soil suction and moisture content on gas permeability for different biochar application percentages at high degree of compaction and develop an artificial neural network (ANN) based model to predict the gas permeability and obtain optimized value of soil suction and moisture content for extreme gas passage. In this study, results represent that presence of biochar can decrease the gas permeability significantly. For 5% and 10% biochar application percentages decrement in gas permeability is around 50% and 65% with respect to bare soil. Developed ANN model shows that in the presence of biochar, gas permeability of SBC is more sensitive to VWC than soil suction i.e. a small change in VWC can change the gas permeability, significantly. Optimization analysis also shows that addition of biochar can increase the optimized VWC for biochar amended soils which can help to design most effective soil cover to provide required nutrients and water for vegetation growth.
Retaining walls and other waterfront structures were seen to suffer severe damage due to soil liquefaction in previous earthquakes. As part of the LEAP project, cantilever retaining walls with loose, ...saturated backfill were tested at various centrifuge centres participating in this endeavour. The toe of the retaining wall penetrated about 0.5 m into the dense sand layer underlying the loose sand layer. Retaining walls with different ratios of the retained height h over the penetration depth d were tested. As part of the LEAP project, additional testing was carried out at Cambridge to consider the effect of the wall size on its deformation following liquefaction. It will be shown that a larger wall will suffer more rotation and wall top displacement than a smaller wall with the same h/d ratio. This can have implications for numerical modelling in terms of how well the constitutive models capture the suppressed soil dilatancy at higher confining pressures.
•As part of LEAP, three dynamic centrifuge tests were conducted on a retaining wall in liquefiable soil.•The effect of embedment ratios and sizes on the dynamic behaviour of retaining walls and backfill soil is investigated.•Implications are provided for the validation of numerical models based on centrifuge tests conducted at different g-levels.
A series of dynamic centrifuge experiments was conducted on model pile foundations embedded in a two-layered soil profile consisting of soft-clay layer underlain by dense sand. These experiments were ...specifically designed to investigate the individual effect of kinematic and inertial loads on a single pile and a 3 × 1 row pile group during model earthquakes. It was observed that the ratio of free-field soil natural frequency to the natural frequency of structure might not govern the phase relationship between the kinematic and inertial loads for pile foundations as reported in some previous research. The phase relationship obtained in this study agrees well with the conventional phase variation between the force and displacement of a viscously damped simple oscillator subjected to a harmonic force. Further, as expected, the pile accelerations and bending moments can be smaller when the kinematic and inertial loads act against each other compared to the case when they act together on the pile foundations. This study also revealed that the peak kinematic pile bending moment will be at the interface of soil layers for both single pile and pile group. However, in the presence of both kinematic and inertial loads, the peak pile bending moment can occur either at the shallower depths or at the interface of soil layers depending on the pile cap rotational constraint.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Several techniques have been developed in order to mitigate damage to buildings during and after liquefaction events. Benefits of using vertical drains have been verified by analysing their ...performance in the soil and evaluating their effectiveness in dissipation of excess pore pressures generated by the earthquake. However, the effect of drains in the soil below structures requires further investigation. In this paper, a dynamic centrifuge test series was carried out to evaluate the performance of a vertical drains arrangement below shallow foundations. High permeable rubble brick was used as coarse material inside the drains to provide positive results not only from a geotechnical point of view but also from an environmental and sustainable perspective. The behaviour of drains was analysed when they are located under shallow foundations of a building, in terms of the excess pore pressures generated during the earthquake and subsequent post-seismic dissipation, the foundation settlement and its dynamic response.
Induced partial saturation is an innovative soil improvement technique intended to mitigate earthquake-induced liquefaction. Historical records indicate that successive earthquakes may occur in high ...seismic areas. Therefore, a comprehensive understanding of the response of partially saturated soils to sequential ground motions is of great significance for the rational design and execution of this method in engineering practice. In this study, a series of dynamic centrifuge experiments were conducted to investigate the impacts of sequential ground motions on the behavior of partially saturated soils beneath shallow foundations. Two different shallow foundation models with a bearing pressure of 135 kPa and 50 kPa were examined. Three seismic simulations, in order of increasing amplitudes, were sequentially applied to loosely-packed partially saturated sand models prepared with air injection technique. The assessment of the test results indicated that shallow foundations resting on saturated models of loose sand did suffer excessive settlements with each event, producing a large embedment of the foundation. However, much smaller settlements were recorded for partially saturated ground, and the level of the foundation embedment remained limited in this case. The deformation vector fields also indicated that different displacement mechanisms were observed for each successive event.
•1-g shaking table tests were conducted on model tunnels in liquefiable ground.•Uplift failure was observed for a tunnel located in unimproved liquefiable ground.•Backfill replacement with ...non-liquefiable fill was used to remediate the floatation.•Backfill replacement geometry strongly influenced the effectiveness of remediation.
Earthquake-induced liquefaction of soils can result in floatation failure of lightweight buried structures such as tunnels, with the potential for economic and human loss. Ground improvement around a tunnel is one approach to preventing this mode of failure. Four 1-g shaking table tests have been conducted including a reference test with a tunnel in unimproved liquefiable ground, and three tests with different geometries of coarse-grained granular backfill replacing areas of the liquefiable layer around the tunnel. It was observed that ground improvement performed below the tunnel was most effective at reducing uplift during earthquake loading. This was attributed to the reduction of excess pore water pressure (EPP) acting on the tunnel invert and the restriction of the flow of liquefied soil around the tunnel needed to facilitate the uplift. In contrast, ground improvement above the tunnel showed very little benefit in reducing uplift compared to the reference test. The ground improvement above the tunnel was unable to retain sufficient shear strength to restrain the buoyancy of the tunnel and, once uplift had been initiated, was unable to restrict the flow of liquefied soil that would have restricted the uplift to a small value. This suggests that, for shallow tunnels, restricting the flow of the liquefied soil and alleviating EPP acting on the invert is more important than preserving shear strength above the tunnel when designing remediation schemes against uplift.
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