Buddhima Indraratna

Mathematical and numerical modeling of pore pressure dissipation and deformation of soft clay foundations stabilised with synthetic band drains and sand compaction piles

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:26:12 PST

The influence of surface geometry on the load transfer mechanisms of grouted bolts - A laboratory study

Najdat I. Aziz et al. — Mon, 12 Dec 2011 14:26:04 PST

The surface geometry of a rock bolt plays a very important role in the rock/resin/bolt load transfer mechanism. Very little work has so far been reported on the influence of the bolt surface geometry on the bolt/resin interface failure mechanism. This paper examines the behaviour of bolt surface roughness under constant normal stiffness condition in the laboratory. To study the shear behaviour of the bolt/resin interface a series of shear tests with an initial normal stress between 0.1 to 7.5 MPa has been conducted on the flattened bolt surface of two most popular bolt types currently being used in Australia. Bolts with deeper ribs offered better shear resistance at low initial normal stress conditions whereas bolts with shallow and closer ribs offered better shear resistance at high normal stress conditions. The maximum dilation occurred at a shear displacement of 60 per cent of the rib spacing irrespective of the bolt type and depth of the rib.

Parametric study on the resilient response of ballasted railway track using numerical modelling

Mohamed A. Shahin et al. — Mon, 12 Dec 2011 14:25:56 PST

Ballasted rail track substructure usually consists of graded layers of granular media of ballast and subballast (capping) placed above a compacted subgrade (formation soil). The optimum design of railway track substructure relies on many factors that affect the performance of the track. In this paper, the response of railway track to various factors affecting track performance is investigated by carrying out a parametric study using the finite element analysis (PLAXIS). A plane strain finite element model of railway track section is developed and used for the parametric study. The feasibility of the developed finite element model is examined by comparing its results with measured field test data. The various track factors investigated include the thickness and stiffness of different track layers, shoulder width, and track modulus. The results demonstrate that the subgrade stiffness seems to be the most significant factor affecting track response, thus, the numerical model and experimental data are used to investigate the capability of different stabilization techniques, such as geosynthetics and grout columns, for improving track subgrade behavior.

Behaviour of railway ballast under dynamic loads based on large-scale triaxial testing

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:25:48 PST

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

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:25:40 PST

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.

General strength criterion for geometerials including anisotropic effect

Martin D. Liu et al. — Mon, 12 Dec 2011 14:25:33 PST

The strengths of geomaterials and their variation under different factors are investigated in this paper. First, a general isotropic variation of a strength criterion is proposed for describing the critical state and peak strengths of geomaterials. Second, the proposed criterion is extended to describe the effect of anisotropy on the peak strength. After an analysis of experimental data, the hypothesis is made that the failure of an element of geomaterial generally occurs in a particular plane when the applied shear stress in that plane reaches the shear resistance of the material. Therefore, the variation of the peak strength of anisotropic materials should be described in terms of the stress tensor applied, a vector parameter defining the position of the potential failure plane of the material, and the material properties. A general failure criterion for geomaterials with cross anisotropy is obtained then from the proposed isotropic strength criterion. The proposed criterion is demonstrated to well represent both the isotropic and anisotropic strengths of various geomaterials. Finally, a general anisotropic criterion is introduced.

Modelling of vertical drains in soft clay

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:25:26 PST

Effects of tsunami on coastal ground conditions and appropriate measures for rail track rehabilitation

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:25:18 PST

Modeling the behaviour of sandwick drains installed at a Naval Dockyard

I W Redana et al. — Mon, 12 Dec 2011 14:25:10 PST

Sustainable soil improvement via vacuum preloading

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:25:02 PST

The application of prefabricated vertical drains combined with vacuum and surcharge preloading is considered to be one of the most environmentally friendly ground improvement techniques. The natural atmospheric pressure is used to generate suction via a vacuum pump. In comparison with other ground improvement methods such as deep piling and deep mixing, this method will not only save a substantial cost in road and rail embankment maintenance and construction but will also enhance the speed of construction of coastal highways and rail tracks and their capacity to carry a greater traffic load. Moreover, as drains with vacuum pressure do not involve chemicals, the soil and groundwater chemistry will not change, unlike cement and lime treatment. Vacuum preloading will also reduce the impact on the environment by minimising the greenhouse gas emissions and the need for quarrying natural sand and gravel. The performances of three full-scale test embankments constructed in Thailand, China and Australia are discussed collectively, demonstrating the advantages and environmental benefits of vacuum consolidation in relation to a standard surcharge-only system.

Saturated steady state flow in rough rock fractures using Discrete Element Modeling

Jeffrey Price et al. — Mon, 12 Dec 2011 14:24:55 PST

Behaviour of soft clay foundations with vertical drains based on case history analysis

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:24:48 PST

Experimental investigations into subballast filtrations behaviour under cyclic conditions

Laricar Dominic Ortega Trani et al. — Mon, 12 Dec 2011 14:24:40 PST

In rail track environments the loading system is cyclic unlike the monotonic seepage force that usually occurs in embankment dams. The mechanisms of filtration, interface behaviour and time-dependent changes of the drainage and filtration properties occurring within the filter medium require further research to improve the design guidelines. A novel cyclic process simulation filtration apparatus was designed and commissioned at the University of Wollongong, and a standard testing procedure was established. The test apparatus was designed to simulate heavy haul train operations. Key parameters that influence the change in porosity and pore water pressure within the sub ballast layer under cyclic conditions in rail track environments were identified.

Finite element modeling of vacuum preloading with prefabricated vertical drains

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:24:32 PST

Long-term performance of a permeable reactive barrier in acid sulphate soil terrain

Gyanendra Regmi et al. — Mon, 12 Dec 2011 14:24:24 PST

Deep drainage technique utilised for flood mitigation in low-land coastal areas of Australia during the late 1960s has resulted in the generation of sulphuric acid in soil by the oxidation of pyritic materials. Further degradation of the subsurface environment with widespread contamination of the underlying soil and groundwater presents a major and challenging environmental issue in acid sulphate soil (ASS) terrains. Although several ASS remediation techniques recently implemented in the floodplain of Southeast Australia including operation of gates, tidal buffering and lime injections could significantly control the pyrite oxidation, they could not improve the long-term water quality. More recently, permeable reactive barriers (PRBs) filled with waste concrete aggregates have received considerable attention as an innovative, cost-effective technology for passive in situ clean up of groundwater contamination. However, long-term efficiency of these PRBs for treating acidic groundwater has not been established. This study analyses and evaluates the performance of a field PRB for treating the acidic water over 2.5 years. The pilot-scale alkaline PRB consisting of recycled concrete was installed in October 2006 at a farm of southeast New South Wales for treating ASS-impacted groundwater. Monitoring data of groundwater quality over a 30 month period were assessed to evaluate the long-term performance of the PRB. Higher pH value (~pH 7) of the groundwater immediately downstream of the PRB and higher rates of iron (Fe) and aluminium (Al) removal efficiency (>95%) over this study period indicates that recycled concrete could successfully treat acidic groundwater. However, the overall pH neutralising capacity of the materials within the barrier declined with time from an initial pH 10.2 to pH 7.3. The decline in the performance with time was possibly due to the armouring of the reactive material surface by the mineral precipitates in the form of iron and aluminium hydroxides and oxyhydroxides as indicated by geochemical modelling.

Soft soils improved by prefabricated vertical drains: performance and prediction

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:24:17 PST

The use of prefabricated vertical drains with preloading is now common practice and is proving to be one of the most effective ground improvement techniques known. The factors affecting its performance, such as the smear zone, the drain influence zone, and drain unsaturation, are discussed in this paper. In order to evaluate these effects a large scale consolidation test was conducted and it was found that the proposed Cavity Expansion Theory could be used to predict the characteristics of the smear zone based on the soil properties available. Moreover, the procedure for converting an equivalent 2-D plane strain multi-drain analysis that considers the smear zone and vacuum pressure are also described. The conversion procedure was incorporated into finite element codes using a modified Cam-clay theory. Numerical analysis was conducted to predict excess pore pressure and lateral and vertical displacement. Three case histories are analysed and discussed, including the sites of Muar clay (Malaysia), the Second Bangkok International Airport (Thailand), and the Sandgate railway line (Australia). The predictions were then compared with the available field data, and they include settlement, excess pore pressure, and lateral displacement. The findings verified that smear and well resistance can significantly affect soil consolidation, which means that these aspects must be simulated appropriately to reliably predict consolidation using a selected numerical approach. Further findings verified that smear, drain unsaturation, and vacuum distribution can significantly influence consolidation so they must be modeled appropriately in any numerical analysis to obtain reliable predictions.

The use of particle size distribution by surface area method in predicting the saturated hydraulic conductivity of graded granular soils

L. D O. Trani et al. — Mon, 12 Dec 2011 14:24:10 PST

The hydraulic conductivity (k) of a fully saturated granular material can be predicted by the well-known Kozeny–Carman formula, and its slightly different variations, based on the porosity (n) and effective diameter (d_eff). Most variations of the Kozeny–Carman formula compute the parameter d eff based on a given conventional particle size distribution by mass (PSDm), where the validation would normally be carried out by comparing against laboratory permeability tests conducted on soils having an average coefficient of uniformity (C_u) of about 3. Knowing that the Kozeny–Carman formula was originally developed for uniformly graded materials, inevitable limitations are inherited when it is applied to increasingly graded soils. This study proposes to convert the PSDm into its equivalence in surface area (PSDsa) conforming to the fundamental geometric assumption by which the Kozeny–Carman equation was originally formulated. The estimated d_effbased on this proposed PSDsa method appears implicitly to incorporate the size, shape and angularity of the natural grains which were traditionally represented by the shape coefficient ( α ). The results presented in this paper show that the suggested method is capable of predicting k for fully saturated granular soils with C_u of up 20.

A laboratory study on improvement of railway ballast and formation using geosynthetics

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:24:01 PST

The classical railway track basically consists of a flat framework made up of rails and sleepers, which are supported on ballasted track formation. Ballast particles breakdown and deteriorate progressively under heavy cyclic rail loading. Moreover, excessive consolidation settlement and progressive shear failure may occur in soft track formation under repetitive stresses. In order to rectify these problems, frequent maintenance operations are generally required in ballasted track. In order to minimize the deterioration of track substructure and also to reduce maintenance cost, the use of various types of geosynthetics including recycling of waste ballast have been studied in the laboratory. The prospective use of three types of geosynthetics (i.e. geogrids, geotextiles and geocomposites) in enhancing the performance of fresh and recycled ballast has been examined. The aspects of deformation and degradation of ballast under cyclic loading have been studied using a large-scale prismoidal triaxial rig. The research findings reveal that recycled ballast stabilized with geosynthetic reinforcement has a good potential for resilient track construction and for reducing the cost of track maintenance. The experimental findings may be beneficial to the railway engineers and encourage them to upgrade current tracks based on these innovative techniques.

Use of impedance probe for estimation of porosity changes in saturated granular filters under cyclic loading: calibration and application

Laricar Dominic Ortega Trani et al. — Mon, 12 Dec 2011 14:23:53 PST

Gravimetric and volumetric sampling techniques are reliable for the measurement of porosity in fully saturated granular filters. However, both methods require a significant effort to gather and prepare samples, are time intensive to process, and do not capture real-time changes. Portable impedance probes serve as a valuable alternative to these destructive and laborious sampling methods. These probes measure the dielectric properties of the soil-water mixtures from which the porosity of filters may be inferred. This study demonstrates that generalized calibrations can result in large errors for porosity estimation when using diverse and small-scale filter types. By comparing with gravimetric and volumetric based porosity measurements for saturated granular filter porosity, impedance probes with filter-specific calibration offer the reliability and confidence owing to its reduced error in a quick, nondestructive fashion. This paper also presents the results of a laboratory investigation using an impedance probe to monitor real-time changes in the porosity of saturated granular filters subjected to cyclic train loading.

Management of acid pollution in low-lying coastal soils in NSW, Australia

Buddhima Indraratna et al. — Mon, 12 Dec 2011 14:23:47 PST