Plane-strain modeling of smear effects associated with vertical drains

The present study provides a methodology to include the smear effect of vertical drains in a two-dimensional (2D) plane-strain finite-element model, employing the modified Cam-clay theory. The analysis is conducted by converting the radius of the smear zone and its permeability (axisymmetric) into equivalent plane strain parameters. The introduction of smear effects improves the accuracy of the numerical model that is tested for a Malaysian soft clay, in this study

Predicted and observed behaviour of soft clay foundations stabilised with vertical drains

A novel plane strain approach is introduced to model the behaviour of embankment foundations on soft clay stabilised with vertical drains, where the classical axisymmetric solutions are converted to an equivalent plane strain model, incorporating the effects of smear and well resistance. This paper describes the behaviour of an embankment stabilised with vertical drains, where a specific case history is selected from Malaysia. The consolidation of soft clay is modelled on the basis of the modified Cam-clay. The settlement behaviour at various stages of embankment loading is analysed using the finite element technique, and the numerical results are compared with field measurements. The behaviour of drains with and without smear is also compared. Inclusion of smear effect in the mathematical model improves the prediction of settlements

Large-Scale, Radial Drainage Consolidometer with Central Drain Facility

A large scale radial draining consolidometer at the University of Wollongong will be used to study the consolidation and drainage properties of saturated or wet materials. A description of the device is given

Laboratory determination of smear zone due to vertical drain installation

This paper is mainly concerned with a laboratory study to investigate the effect of smear due to vertical drain installation. The extent of the smear zone around a vertical drain was studied utilizing a large-scale consolidometer apparatus. The test results reveal that a significant reduction in the horizontal permeability takes place toward a central drain, whereas the vertical permeabilty remains relatively unchanged. The radius of the smear zone was estimated to be a factor of four to five times the radius of the central drain (mandrel), and the measured ratio of horizontal to vertical permeability approached unity at the drain-soil interface. The laboratory measured settlements are subsequently compared with the predictions based on the theory of Hansbo and the finite element method. It is of relevance to note that the inclusion of the correct variation of permeability ratios of the smear zone in the plane strain finite element analysis improves the accuracy of settlement predictions

Soft Clay Stabilisation by Mandrel Driven Geosynthetic Vertical Drains

Geosynthetic (prefabricated) vertical band drains are now considered as one of the most cost effective ground improvement technics in many parts of the world, where construction on soft compressible clays is inevitable. However, smear effects caused by PVD installation (eg. mandrel based), drain clogging, drain kinking and well resistance of long drains retard the excess pore pressure dissipation making these drains often less effective in the field, contrary to expectations. Consequently, the rate of settlement of the stabilised soft clay becomes significantly less than what is expected from ideal drains. This paper addresses comprehensively, the numerical modelling aspects of PVD, and the interpretation of field data taken from several case studies, which elucidate the drain performance under various boundary conditions. Theoretical and finite element modelling details are described based on various research studies, mainly through the authors’ own experience. In particular, the experimental data obtained from large-scale consolidation tests are highlighted and interpreted

Modeling of bolted joint behaviour under constant normal stiffness conditions - laboratory study

The shear behaviour of grouted joints has been studied in the recent past using the conventional direct shear apparatus, where the normal load on the joint plane is kept constant during testing. The Constant Normal Stiffness (CNS) condition is adopted in this study of shear behaviour of bolted joints, as the CNS condition represents a better approximation of the deformation behaviour of grouted joints in underground excavations as compared to the conventional Constant Normal Load (CNL) condition. A series of tests were conducted to study the behaviour of these stabilised joints under constant normal stiffness condition at an initial normal stress ranging from 0.13 MPa to 3.25 MPa, using a large scale CNS testing machine at a controlled strain rate of 0.5 mm/min. An analytical model has been developed to predict the shear resistance of bolted joints using the Fourier Series. The shear resistance of the bolted joint was defined as a function of the applied initial normal stress, the normal stiffness of the surrounding rock mass and the shear stiffness of the bolt. Fourier series was used to formulate the dilation behaviour during shearing, which in effect was utilized to predict the variation of normal stress during test process. Thus, using the analytical model, the complete stress profile for the bolted joint could be predicted at any normal stress and stiffness condition. The laboratory results were found to compare favourably with the predicted stress profiles from the model

Analytical and Numerical Modeling of Soft Soil Stabilized by Prefabricated Vertical Drains Incorporating Vacuum reloading

This paper describes the analytical formulation of a modified consolidation theory incorporating vacuum pressure, and numerical modeling of soft clay stabilized by prefabricated vertical drains, with a linearly distributed ~trapezoidal! vacuum pressure for both axisymmetric and plane strain conditions. The effects of the magnitude and distribution of vacuum pressure on soft clay consolidation are examined through average time-dependent excess pore pressure and consolidation settlement analyses. The plane strain analysis was executed by transforming the actual vertical drains into a system of equivalent parallel drain walls by adjusting the coefficient of permeability of the soil and the applied vacuum pressure. The converted parameters are incorporated in the finite element code ABAQUS, employing the modified Cam-clay theory. Numerical analysis is conducted to study the performance of a full-scale test embankment constructed on soft Bangkok clay. The performance of this selected embankment is predicted on the basis of four different vacuum pressure distributions. The predictions are compared with the available field data. The assumption of distributing the vacuum pressure as a constant over the soil surface and varying it linearly along the drains seems justified in relation to the field data