Analysing ground deformation data to predict characteristics of smear zone induced by vertical drain installation for soft soil improvement

University of Technology, Sydney. Faculty of Engineering and Information Technology.The use of prefabricated vertical drain (PVD) assisted preloading has been recognised over the last two decades as a very efficient method of ground improvement for sites with deposits of deep soft soil. One of the major parameters influencing the PVD assisted consolidation process, and consequently the required preloading time, is the formation of a smear zone around the vertical drains, and the corresponding soil properties. In this research a systematic procedure integrated with a developed numerical code is proposed to accurately back calculate the properties of the smear zone based on the consolidation data collected in the laboratory and in the field. Furthermore, an expanded back calculation method is developed to determine the minimum required degree of consolidation and corresponding time after the construction of the trial embankment that would result in accurately predicted smear zone characteristics. The explicit finite difference program FLAC 2D was used to develop the numerical code, simulate the laboratory testing and PVD assisted preloading case histories. Furthermore a comprehensive parametric study was conducted to investigate the effect of smear zone properties variations on the preloading process, and back calculated characteristics of the smear zone. A large and fully instrumented Rowe cell apparatus was used to investigate the effect of the smear zone on the consolidation process and verify the developed numerical code. The Rowe cell was filled with the intact zone, smear zone, and vertical drain materials to evaluate the permeability and extent ratios of kh/ks=4 and rs/rm=3, respectively. The back calculation procedure was used to conduct the parametric study and predict the properties of the smear zone. According to the results, the predicted properties of the smear zone were similar to the properties of the applied soil, proving that the proposed back calculation procedure integrated with the developed numerical simulation can successfully predict these properties. The developed numerical code was used to simulate five PVD assisted preloading case studies, including four trial embankments and a large scale consolidometer, while the back calculation procedure was used to conduct a parametric study to determine the extent and permeability of the smear zone. According to the results, integration of the back calculation procedure in the numerical code can be used as a reliable tool to make an accurate prediction of the smear zone characteristics in PVD and vacuum assisted preloading projects. The developed method in this research can be considered as a practical, accurate and cost effective tool, due to its capability in precise estimation of the extent and permeability of the smear zone in the early stages of constructing the trial embankment. In this study, the proposed systematic back calculation procedure was extended to determine the minimum degree of consolidation (i.e. the minimum waiting time after constructing the trial embankment), and accurately predict the properties of the smear zone. The numerical results of the simulated case studies were used to conduct the analyses. Accordingly, it is found that the extent and permeability of the smear zone can be predicted very well with the proposed calculation procedure when at least 33% of predicted final settlement has been reached (i.e. 33% of the degree of consolidation)

Uncertainties of Smear Zone Characteristics in the Design of Preloading with Prefabricated Vertical Drains

Installing prefabricated vertical drains using mandrels induces disturbance of the soil surrounding the drain, resulting in a smear zone with the reduced permeability. The required time for pore pressure dissipation in preloading design is strongly associated with the smear zone characteristics. In this study, the effects of smear zone properties on preloading time are numerically investigated. Parametric study is conducted to find out the range of smear zone parameters significantly influencing the consolidation period. It is observed that the characteristics of smear zone namely size and permeability have a substantial impact on the preloading design to achieve certain soil strength and stiffness satisfying both bearing capacity and settlement design criteria

Numerical analysis to quantify the influence of smear zone characteristics on preloading design in soft clay

In this paper, the effects of uncertainties of smear zone characteristics induced by installation of prefabricated vertical drains on the preloading design are numerically investigated. FLAC 2D finite difference software with additional developed subroutines has been employed to conduct the numerical simulations. The finite difference analyses have been verified using a case study. Furthermore, a comprehensive parametric study is conducted to investigate the influence of smear zone permeability and extent on the model predictions. Results of this study indicate that the assumptive properties for smear zone characteristics may result in inaccurate predictions of ground deformations and pore water pressures. This may lead to early removal of the surcharge in the construction process causing excessive post construction settlement. It is recommended to practising engineers to use results of trial preloading to back calculate the required smear zone characteristics in the early stages of embankment construction to optimize the design

Analyzing consolidation data to predict smear zone characteristics induced by vertical drain installation for soft soil improvement

In this paper, the effects of variability of smear zone characteristics induced by installation of prefabricated vertical drains on the preloading design are investigated employing analytical and numerical approaches. Conventional radial consolidation theory has been adopted to conduct analytical parametric studies considering variations of smear zone permeability and extent. FLAC 2D finite difference software has been employed to conduct the numerical simulations. The finite difference analyses have been verified using three case studies including two embankments and a large-scale laboratory consolidometer with a central geosynthetic vertical drain. A comprehensive numerical parametric study is conducted to investigate the influence of smear zone permeability and extent on the model predictions. Furthermore, the construction of the trial embankment is recommended as a reliable solution to estimate accurate smear zone properties and minimise the post construction settlement. A back-calculation procedure is employed to determine the minimum required waiting time after construction of the trial embankment to predict the smear zone characteristics precisely. Results of this study indicate that the accurate smear zone permeability and extent can be back-calculated when 30% degree of consolidation is obtained after construction of the trial embankment. © 2014 Techno-Press, Ltd

Experimental and numerical investigations to evaluate two-dimensional modeling of vertical drain-assisted preloading

© 2015 American Society of Civil Engineers. In this study, the efficiency of proposed formulations for plane-strain modeling of vertical drain-assisted consolidation was evaluated. For this aim, the vertical drain-assisted preloading process was experimentally simulated using a fully instrumented large-scale Rowe cell. Nine pore-water-pressure transducers were installed in various locations to measure the changes in pore-water pressure during the test. Two pressure/volume controllers were connected to an infinite-volume controller to provide continuous water flow. Soft clays with predefined properties were used to form the intact and smear zones. A numerical code was developed by using the finite-difference program FLAC 2D to simulate the consolidation test. A numerical study was conducted to evaluate the efficiency of the proposed solutions for converting the axisymmetric state to a plane-strain condition and was subsequently compared with corresponding numerical analysis. From the results, it is observed that some of the proposed methods resulted in more accurate predictions of settlement and changes of pore-water pressure in the early stages of the consolidation process, whereas other proposed methods performed more accurately in the later stages of consolidation. Thus, three-dimensional modeling with actual soil-permeability properties to simulate the time-dependent behavior of soft soil improved with vertical drains is recommended

Numerical Back Analysis of Smear Zone Properties for Vertical Drain Assisted Preloading in Soft Soils

Installation of prefabricated vertical drains (PVDs) using a mandrel causes disturbance of the clay surrounding the drain, resulting in a smear zone of reduced permeability, which adversely affects consolidation process. There are two important parameters to characterize the smear effects, namely, the smear zone diameter and the permeability ratio. In this study, FLAC finites difference software has been employed to investigate the effects of smear zone characteristics on required time for preloading. The results of a fully instrumented trial embankment in Sunshine Motorway, Queensland, have been used to verify the model, and parametric studies have been conducted on the predicted ground settlement and pore water pressure. According to the results, changes in smear zone parameters can significantly affect the consolidation period. It is observed that the characteristics of smear zone namely size and permeability have a substantial impact on the preloading design to achieve a certain soil strength and stiffness satisfying both bearing capacity and settlement design criteria

Evaluating proposed solutions for equivalent plane strain modeling of PVD assisted preloading

In this study, a numerical code has been developed using free lossless audio codec (FLAC) 2-D to model the prefabricated vertical drain (PVD) assisted preloading process considering the smear zone, and evaluate the efficiency of the proposed equations for the conversion of permeability coefficient from axisymmetric state to plane-strain condition. A laboratory PVD assisted preloading test has been conducted employing a fully instrumented large Rowe cell to verify the developed numerical code. The results of the numerical plane-strain and axisymmetric simulations have been compared using four methods of permeability conversion from axisymmetric to plane-strain condition. © ASCE 2014

Trial Embankment Analysis to Predict Smear Zone Characteristics Induced by Prefabricated Vertical Drain Installation

© 2014, Springer International Publishing Switzerland. In this study, FLAC finite difference software has been adopted to simulate the performance of the ground improved using prefabricated vertical drains assisted preloading, considering smear zone characteristics. The numerical code has been applied to predict smear zone properties employing a back calculation procedure using the results of several case studies. The construction of a trial embankment is proposed as a reliable method to predict the smear zone characteristics. The proposed back calculation method is applied to estimate the minimum required degree of consolidation and consequently the minimum required preloading time, resulting in a reliable estimation of the smear zone permeability and extent. Three preloading case studies considering both conventional preloading and vacuum assisted preloading have been simulated to verify the numerical code and to conduct the parametric study using the back calculation procedure. According to the results, the properties of the smear zone can be back-calculated reliably, when at least 33 % degree of consolidation due to trial embankment construction is achieved

Impact of Initial In-Situ Stress Field on Soil Response During Cavity Expansion Using Discrete Element Simulation

Discrete element method (DEM) is gaining its popularity in investigating many complicated geotechnical related problems due to its discontinuous nature in simulating granular materials. Particularly when simulating the processes involving large deformation and displacement of soil (e.g. pile penetration), DEM demonstrates distinct advantages over other numerical solutions that may confront convergence problems. Despite the facts that DEM analysis has been conducted to study the mechanism of the cavity expansion, there is a very limited number of investigations conducted to study the effects of the initial stress field on the soil response. Hence, in this study, a three-dimensional numerical analysis has been conducted using PFC3D to investigate the soil response under different initial stress conditions during cavity expansion. A large-scale model containing an adequate number of particles has been constructed to simulate the soil medium, in which, microscopic contact properties were calibrated against existing experimental data to mimic the realistic behaviour of a sandy soil. To examine the effects of the initial in-situ stresses, several cylindrical cavities were created and expanded gradually from an initial radius to a final radius, while stress and strain variations were monitored during the entire simulation. It should be noted that the internal cylinder boundary was loaded using a constant strain rate, while the outer boundary was controlled through a servo mechanism to maintain a constant external pressure adopting appropriate subroutines. The results obtained confirmed that the initial stress conditions have significant effects on the soil response during cavity expansion