Numerical model of soft ground improvement by vertical drain combined with vacuum preloading

The rapid development of high-speed transportation infrastructure such as highway and high-speed railway has resulted in the advancement of soft soil improvement techniques. Vacuum preloading combined with vertical drains has been proved to be an effective method in the treatment of soft foundation. A three-dimensional numerical analysis of the coupled methods was presented, in which the smear zone and the well resistance were taken into account. The variations of the basic soil parameters including the permeability coefficient and the coefficient of volume compressibility were considered in the numerical model. The result of the numerical model was then compared to the measured value. The results indicate that the decrease of coefficient of volume compressibility accelerates the consolidation of the soil while the influence of hydraulic conductivity is insignificant. A cube drain presents the closest result to the real situation compared to the other equivalent methods of prefabricated vertical drain (PVD). The case study indicates that the numerical model with variation of soil parameters is closer to the measured value than the numerical model without variation of soil parameters.

Performance assessment of prefabricated vertical drains in mitigating soil reliquefaction subjected to repeated seismic events using shaking table experiments

The use of prefabricated vertical drains(PVD)in liquefiable deposits is gaining attention due to enhanced drainage.However,investigations on PVD in mitigating re-liquefaction during repeated shaking events are not available.This study performed a series of shaking table experiments on untreated and PVD-treated specimens prepared with 40%and 60%relative density.Repeated sinusoidal loading was applied with an incremental peak acceleration of 0.1g,0.2g,0.3g,and 0.4g,at 5 Hz shaking frequency with 40 s duration.The performance of treated ground was evaluated based on the generation and dissipation of excess pore water pressure(EPWP),induced sand densification,subsidence,and cyclic stress ratio.In addition,the strain accumulated in fresh and exhumed PVD was investigated using geotextile tensile testing apparatus aided with digital image correlation.No evidence of pore pressure was reported up to 0.2g peak acceleration for 40%and 60%relative density specimens.The continuous occurrence of soil densification and drainage medium restrained and delayed the generation of EPWP and expedited the dissipation process.This study demonstrates PVD can mitigate re-liquefaction,without suffering from deterioration,when subjected to medium to high intense repeated shaking events.

The mechanism and effects of subgrade fluidisation under ballasted railway tracks

The rapid growth in railway infrastructure and the construction of high-speed heavy-haul rail network,especially on ground that is basically unsuitable,poses challenges for geotechnical engineers because a large part of the money invested in the development of railway lines is often spent on track maintenance.In fact around the world,the mud pumping of subgrade fines is one of the common reasons why track performance deteriorates and track stability is hindered.This article presents a series of laboratory tests to examine following aspects of mud pumping:(1)the mechanisms of subgrade fluidisation under undrained condition,(2)the effects of mud pumping on the engineering characteristics of ballast,and(3)the use of vertical drains to stabilize subgrade under cyclic loads.The undrained cyclic triaxial testing on vulnerable soft subgrade was performed by varying the cyclic stress ratio(CSR)from 0.2 to 1.0 and the loading frequency f from 1.0 to 5.0 Hz.It is seen from the test results that for a specimen compacted at an initial dry density of 1790 kg/m3,the top portion of the specimen fluidises at CSR=0.5,irrespective of the applied loading frequency.Under cyclic railway loading,the internal redistribution of water at the top of the subgrade layer softens the soil and also reduces its stiffness.In response to these problems,this paper explains how the inclusion of vertical drains in soft subgrade will help to prevent mud pumping by alleviating the build-up of excess pore pressures under moving train loads.

Measured Performance and Analysis of the Residual Settlement of a PVD-Improved Marine Soft Ground

Prefabricated vertical drains(PVDs)are commonly used to shorten the drainage path for consolidation as part of the improvement of marine soft ground.Many studies that focus on the primary consolidation settlement of PVD-improved soft ground have been conducted;however,residual settlement has been scarcely investigated.Residual settlement is the net effect of secondary compression and the remaining primary consolidation and generally occurs while the facilities are operating.In this study,residual settlement was investigated using the measured field settlement data obtained from the surface settlement plate and multilayer settlement gauges.This study determined that PVD still has some effect on residual settlement and can reduce the settlement times.Residual settlement is only related to the PVD-improved soil layer and only occurs significantly in the middle zone of that layer over a few months.The middle zone may be related to the time delay of excess pore water pressure dissipation.This study concluded that the remaining primary consolidation in the PVD-improved soil layer is the primary cause of residual settlement,whereas secondary compression in the PVD-improved soil layer is only a minor cause.

Consolidation behavior of Tianjin dredged clay using two air-booster vacuum preloading methods

This paper presents model tests(macro aspect)and microstructure tests(micro aspect)for investigating the consolidation behavior of Tianjin dredged clay using the prefabricated vertical drain air-booster vacuum preloading(PAVP)and tube air-booster vacuum preloading(TAVP)methods.The mechanism of air-booster vacuum preloading(AVP)using a spring-like system is explained.The main difference between these two methods is the air-boosting equipment.A new anticlogging air-booster prefabricated vertical drain(PVD)is used in the PAVP technique and a self-designed air-booster tube is used in the TAVP technique.In the model tests,a comparison of the variables that are monitored during reinforcement(vacuum pressure,surface settlement,water discharge,and pore-water pressure)and after reinforcement(water content,dry density,and vane shear strength)is conducted.The results indicate that the consolidation behavior of Tianjin dredged clay using the PAVP method is better than that using the TAVP method.PAVP more efficiently mitigates the issue of water-draining PVD clogging and significantly accelerates drainage consolidation.In addition,in the microstructure tests,a comparison of the variables that are monitored after reinforcement(via scanning electron microscopy(SEM)and mercury intrusion porosimetry(MIP))is conducted,and the results further explain the model test results.

SEEPAGE ANALYSIS OF LANDFILL FOUNDATIONS IN SHANGHAI LAOGANG LANDFILL PHASE IV

A complex seepage flow field with unknown free surface was formed in the Shanghai Laogang Municipal Solid Waste （MSW） Landfill by the perimeter cut-off wall around the landfill site and the Prefabricated Vertical Drains （PVD） in the foundation of landfill. First, the equivalent vertical permeability was presented based on the drainage mechanism of PVD and the related calculation method was studied to macroscopically reflect the impact of PVD on the seepage flow status of the landfill foundation. With the influence of the cut-off wall and the PVD on the infiltration of groundwater, the total inflow was classified into two parts： the lateral inflow through the cut-off wall outside the landfill, and the upward infiltration through the PVD from the foundation of the landfill. Seepage analysis was conducted using the finite element method according to the actual scenario of Laogang Landfill, and the lateral infiltration and the upward infiltration were calculated, respectively. The results show that the total inflow into the landfill area was mostly supplied by the upward infiltration through the PVD and the perimeter cut-off wall was significantly effective to depressurize the seepage pressure and to reduce the flow rate.