6 research outputs found
Elasto-viscoplastic modelling of ground-improvements via vacuum-assisted prefabricated vertical drains with time-dependent boundary conditions
Full text linkIn this thesis, time-dependent boundary conditions are introduced to a creep based elasto-viscoplastic (EVP) model, which can be used to predict soft soil deformations with vacuum consolidation. Use of vacuum-assisted prefabricated vertical drains (PVDs) is a relatively new method and is getting popularity in ground-improvement projects due to its ability to consolidate deep-buried soft clay layers in a comparatively short period. Simple approximations have been reported to idealise vacuum consolidation equivalent to surcharge preloading in previous research. Conversely, in this thesis, it has been shown that considering vacuum consolidation as a time-dependent boundary condition unfolds a large number of possibilities to accurately represent ground improvements with vacuum-assisted PVDs. Combined with a creep based EVP model, it is illustrated that the accuracy of both short and long-term settlement and excess pore pressure (EPP) predictions can be improved. Finite element analyses (FEA) of several case histories and laboratory experimental data have been used in this thesis to illustrate the improvements made in the predictability of the soft soil behaviour. Both axisymmetric and plane strain (PS) FEA are carried out for a period over three years for the Ballina test embankment vacuum applied section. Improvements made in the proposed method are highlighted against the field data and previous FEA attempts. In PS conditions, the implications of unit cell width on the FEA results such as settlements, EPP and lateral deformations are demonstrated which would serve as a guide for predicting the performance in similar scenarios. Also, time-dependent boundary conditions are successfully used in this thesis to capture the removal and re-application of vacuum to model practical scenarios such as vacuum pump breakdowns and recoveries. Validations have been carried out against published laboratory results and with a case history from Singapore. Simple yet effective methods to avoid numerical instabilities in simulating vacuum removal and re-application are proposed and improvements achieved with these solutions are illustrated. Later in the thesis, implications of vacuum distributions with depth of PVDs are discussed. Subsequently, a convenient method to model complex yet practically realistic vacuum distributions encountered in PVDs is presented and validated
Predicting Deformations in Vacuum Assisted Ground Improvements Using an Elasto- Viscoplastic Numerical Model
Full text linkVacuum consolidation can be used to accelerate the soil consolidation in
ground improvement projects. Capped Prefabricated vertical drains (CPVDs) is an improved method where vacuum is applied to each PVD separately. This is particularly useful if the area is inundated or have a high permeable sand layer or seam. Vacuum consolidation in an actual project is much challenging to model and predict the performance. This is due to the switching on and off of the vacuum pump, accidental failures of the pump etc. in the field and they need to be incorporated in the analysis. In this paper an elasto-viscoplastic (EVP) model, capable to simulate
such instances, is presented and is validated against a field case reported from a land
reclamation project in Singapore
Creep based viscoplastic numerical modelling of soil deformations in vacuum application and removal
Full text linkWith recent research on test embankment case studies and laboratory experiments, it is evident that vacuum consolidation can be effectively used to reduce long-term settlements in soft soil by taking the soil to an overconsolidated state with vacuum application and subsequent removal. Difference in settlements in vacuum consolidation and difference in swelling characteristics after removal of vacuum is a challenge in numerical modelling which needs careful attention. In this paper, a fully coupled Biot type elasto-viscoplastic (EVP) model is proposed to predict both excess pore pressures and settlements more accurately upon application and removal of vacuum. In addition, the effect of vacuum removal on secondary compression behaviour is also discussed
Numerical modelling of vacuum suction distribution and its effects in ground improvement with PVD vacuum consolidation
Full text linkPrefabricated vertical drains (PVDs) with vacuum suction have helped to shorten the con- solidation time significantly in ground improvement projects. This method is claimed to be more effective and economical in consolidating deep soft clay layers. However, recent research has shown that the vacu- um suction applied to PVD near the ground surface may not penetrate the full depth (i.e. along the full length) of the PVD and the actual effects of the vacuum suction on the consolidation of the clay are not clearly understood. In this paper, an innovative approach is presented to model the vacuum suction distri- bution along the PVD using Finite Element (FE) modelling. Complex vacuum distributions, close to ellip- tical in shape is modelled and validated against field performance monitoring data. Moreover, the effects of this vacuum distribution on the deformational behaviour of the soft clay are also discussed
Plane strain viscoplastic modelling in vacuum consolidation
Full text linkThe mechanism involved in ground improvements with vacuum assisted prefabricated vertical
drains (PVDs) for road embankments is essentially 3-dimensional (3-D) but could reasonably well approximated
as axisymmetric. In this context, axisymmetric unit cell modelling generally provides a very good representation
for Finite Element (FE) modelling. However, it carries significant limitations in terms of the scope
of the analysis. Hence, to get an understanding of the overall deformational behaviour of the foundation soil
being improved, a full scale Plane Strain (PS) FE model would be necessary. In this paper, conversion of axisymmetric
unit cell to an equivalent PS model is carried in the context of a vacuum consolidation project.
Foundation soft soil is modelled using an elastic-viscoplastic (EVP) model which accounts for the time dependent
behaviour of soft clay. Results of this PS conversion is compared with the axisymmetric FE solution. Stability
of the embankment is also analysed using maximum lateral displacements
