Improving geotechnical properties of closed landfills for redevelopment using fly ash and quicklime

University of Technology, Sydney. Faculty of Engineering and Information Technology.Many closed municipal solid waste (MSW) landfills are located near urban areas, even though originally established away from residential or commercial communities. Construction on top of closed landfills is generally a challenging task due to complex behaviour of creep, settlement and weak shear strength of waste materials. There is a high prospective to reuse these sites for redevelopment in spite of potential risk for human health and environment. The deep dynamic compaction technique is a common ground improvement technique due to its relatively economical and easy application for landfill sites. With deep dynamic compaction, large voids reduce and afterward other techniques such as cement, fly ash or lime grouting can further reduce the remaining smaller voids. Numerous studies have been conducted to treat and stabilise different types of problematic soils using fly ash with combination of lime. However, there is no comprehensive research on improvement of physical properties of MSW landfills using chemical admixtures such as fly ash and lime. This study presents the experimental and numerical results of employing fly ash (class F) and quicklime (calcium oxide) in stabilisation of municipal solid wastes. The waste materials, used in this study, were collected from a closed landfill in the south-west of Sydney. The samples were prepared by integrating MSW, with a mixture of fly ash-quicklime with a ratio of 3:1 in percentages of 5, 10, 15 and 20 of fly ash by dry weight of the MSW. An array of experimental tests has been conducted on treated and untreated MSW samples including sieve analysis, Atterberg limits, compaction, permeability, large direct shear, unconfined compressive strength and consolidated-drained triaxial tests. Results of this investigation are evidence for a significant improvement in geotechnical properties of MSW materials, mixed with fly ash and quicklime. It has been found that the chemical stabilisation effectively increases the maximum dry density, the compressive strength, the shear strength parameters, the stiffness and the brittleness index, while decreases the compressibility, the permeability coefficient and the optimum moisture content of the MSW. It has been quantified that by increasing fly ash-quicklime admixtures from 0 to 26.7% (0 to 20% fly ash) the internal friction angle increased from 29° to 39° and the cohesion intercept increased from 11 kPa to 30 kPa. Under an effective confining pressure of 300 kPa, the peak strength, the brittleness index and the Young’s modulus at failure increased from 600 kPa to 1150 kPa, 0.13 to 0.35 and 5.5 MPa to 28 MPa, respectively, by addition of 26.7% fly ash-quicklime admixture. The coefficient of permeability for untreated specimen was 6.2×10-8 m/s and it was reduced to 3.2×10-8 m/s for specimens mixed with 26% fly ash-quicklime (under average confining pressure of 250 kPa). The compression and the secondary compression indices decreased from 0.33 to 0.23 and 0.052 to 0.033, respectively. Moreover, increasing the curing time enhanced the unconfined compressive strength, the friction angle, the cohesion and the preconsolidation pressure of the treated specimens, whereas no change in the permeability coefficient, the primary compression index and the secondary compression index were observed. The findings of this study may facilitate the calculations of the bearing capacity and settlement as well as the slope stability analysis of chemically treated closed landfill sites. A finite element program, PLAXIS version 9, has been used to evaluate the settlement of the untreated and chemically treated landfill layers for 10 and 20 years after applying surcharge loads such as the traffic load. The effects of depth of stabilisation and the fly ash-quicklime content on vertical and horizontal displacements of the model have been investigated. Treated and untreated MSW parameters, used for the model, have been obtained from the results of the extensive laboratory program performed in this study. The numerical results indicated that treatment of MSW with fly ash-quicklime reduced the vertical displacement of the model under traffic load at the midpoint below the embankment. This reduction is more pronounced with higher fly ash-quicklime contents and deeper improvement of layers. For depths of 3m, 6m, 9m, 12m and 24m of the landfill improved with 26.7% fly ash-quicklime, the vertical settlements at the centreline of the embankment, 10 years after applying traffic load, were reduced by 20%, 32%, 40%, 46% and 58%, respectively. Horizontal displacements of the landfill model also significantly reduced in sections below the toe of the embankment, under traffic load. The reduction in horizontal displacements is more pronounced with improvement into deeper layers

Optimising the pattern of semi-rigid columns to improve performance of rail tracks overlying soft soil formation

With Australia facing a rapid increase in population in the next 30 years, the government is being proactive in handling the forecasted growth. The release of 2010 Metropolitan Transport Plan by the New South Wales (NSW) Government shows that the State of NSW will see an increase in commuter travel by rail. The NSW rail system is one of the most complex networks in the world and due to population growth, the network will require further expansion with construction of new railway lines partly on weak and marginal ground and will also require more frequent train running on existing lines. This study seeks to identify the effectiveness of semi-rigid inclusion ground improvement techniques particularly stone columns and deep soil mixing in controlling settlement of soft soils when placed under the dead loads of the rail structure and the large live loads of freight trains. The employed numerical study assesses the relationship between the column position in the track cross section and the overall settlement of the ballasted rail formation. The numerical results show that the overall settlement of the track reduces significantly with the use of columns close to the centre of the track and not just under the rail. In addition, application of one layer of geogrids between sub-ballast and sub-grade assists to reduce the maximum settlement of track decreasing the future maintenance costs

Research-based computer games to train civil engineering students to be lifelong learners

In spite of vast efforts to adopt available information technology in higher education teaching and learning, the truth is that most of university students and academic staff make only limited use of communication technology. Selwyne [1] concluded that there is a growing need for the education community to account for the distinct ?digital disconnect? between the enthusiastic rhetoric and rather more mundane reality of university information and communication technology use. Recent advances in computer science and multimedia as well as optimistic effects of multifaceted modes of education on student learning, have encouraged teachers to look at adopting the new technology to improve students? learning experience. Chang et al. [2] have suggested that digital games can be powerful informal learning environments encouraging active and critical learning, supplementing traditional teaching methods. It is well accepted that well designed discipline based computer games can help with student learning process and experience in higher education. In this study, a computer game called ?Back to Bedrock? has been developed for soil Behaviour subject at undergraduate level and students? learning process has been monitored and evaluated. It was aimed to help Civil Engineering students with information collection methods, creative thinking, problem solving, and lifelong learning abilities, through a research-based computer game. The results of this project indicate that implementing innovative methods such as computer game based assignments can provide enjoyable competitive and cooperative learning environment enhancing students? learning motivation, and critical thinking abilities, improving the overall performance of students in the subject

General and Technical Considerations for Implementing High Speed Rail Systems in Australia

Australia has a number of medium speed rail services such as the Prospector, which runs from East Perth to Kalgoorlie, at speeds of up to 160 km/hr. Speeds as high as 210 km/hr have been reached by the tilt train from Brisbane to Rockhampton. Although there are a few medium speed rail systems in Australia, there is not a passenger rail transport with the high transit speeds seen in other countries. This paper presents the feasibility of implementing high speed rail systems in Australia by looking at the main elements that a high speed train is composed of. This paper also reviews the performance of high speed rail systems around the world and the factors contributed to their success made them successful. The main objective of this study is to look at how the solutions from overseas and how the technical requirements particularly the geotechnical aspects of tracks for a high speed rail system can be applied in Australian existing and new tracks. Australia has its own unique demographic, geographic and economic characteristics and the aim is to identify where there are overlaps between Australiaâs characteristics and countries with high speed rail systems. High speed rail transport might not necessarily be one the best solutions for the transportation at present in Australia, but it can be what a nation needs to succeed in its future transportation system

Numerical Analysis of Geosynthetic Reinforced Soil Wall as Bridge Abutment

This paper presents the finite element analysis of a geosynthetic reinforced soil wall as a bridge abutment built in Tehran, and the predictions are compared with the available field measurements. This abutment is analysed using both Limit Equilibrium Method (LEM) for stability analysis and Finite Element Method (FEM) for deformation analysis. Two dimensional plane strain finite element model is adopted for the simulation. Polyvinyl Alcohol (PVA) geogrid with high tensile moduli and low creep characteristics has been adopted in this project to limit the deformation of the bridge abutment. In this model, the backfill soil and geogrids simulated adopting Mohr-Coulomb model, and the elasto-plastic material model that only works in tension, respectively. Bridge abutments can be stabilised by including geosynthetic layers with high tensile moduli satisfying both stability and deformation criteria reducing the construction cost and time, post construction deformations, and future maintenance cost

Mechanical characteristics of soft clay treated with fibre and cement

In this study, the influence of three types of fibre - polypropylene, recycled carpet and steel - on the mechanical properties of cement-treated clay is investigated. Cement-treated clay specimens were prepared with cement contents of 5%, 10% and 15% by weight of dry soil, and cured for 14 days. To investigate and understand the influence of different fibre types and contents, three different percentages of fibre content were adopted. The results of unconfined compression tests on 90 cylindrical samples of cement-treated clay with varied cement and fibre contents are analysed to discern the relationships between these parameters and the key mechanical properties, including unconfined compressive strength and stiffness of treated soil. Furthermore, indirect tension test results of a further 90 treated soil samples have been used to determine the influence of fibre and cement content on the tensile strength of the treated soil. The fibre reinforcement increases the peak compressive strength. The addition of fibres increases the residual strength and changes the brittle behaviour of the cement-treated clay to that of a more ductile material. The tensile strength of the cement-treated clay is increased by adding carpet and steel fibres, but small quantities of polypropylene fibres do not influence the tensile strength. © 2012 Thomas Telford Ltd

Conceptual development and numerical modelling of vegetation induced suction and implications on rail track stabilisation

The effects of tree roots on soil suction and ground settlement are investigated. This paper highlights the inter-related parameters contributing to the development of a conceptual evapo-transpiration and root water uptake equilibrium model that is then incorporated in a comprehensive numerical model. The developed numerical model based on the finite element analysis (ABAQUS) considers fully coupled flow-deformation behaviour of soil. Field measurements obtained by the authors from a field site in western Victoria and from past literature are used to validate the model. The predicted results show acceptable agreement with the field data in spite of the assumptions made for simplifying the effects of soil heterogeneity and anisotropy. The numerical analysis proves that the proposed root water uptake model can reliably predict the region of maximum matric suction away from the tree axis. The paper also compares the natural favourable effect of tree roots with the stabilising mechanisms of geosynthetic vertical drains subjected to vacuum pressure. Although this analogy is only justified for shallow vertical drains, the comparison still emphasises the obvious economical advantages of native vegetation

Parametric Study On Behavior Of Reinforced Soil Walls With Combined Horizontal And Vertical Geosynthetics

The reinforced soil system employing geogrids, as a cost effective reinforcement technique, has come to play an important role in a variety of civil and geotechnical engineering applications. In regular reinforced soil wal1s, the reinforcements are usually laid horizontally in the soil. In this study, the behaviour of reinforced soil retaining walls with combined horizontal and vertical reinforcements are investigated experimentally as well as numerically. The results, indicating the effects of vertical reinforcement inclusion, are compared to conventional reinforcing types under static and dynamic loads. The performance of retaining walls employing vertical reinforcement in conjunction with horizontal reinforcement is convincing from the results of the shake table tests conducted by the authors. In this paper, PLAXIS, well-known geotechnical software, is used for conducting a series of pararoetric studies on behaviour of reinforced soil walls under construction and subject to earthquake loading, incorporating the vertical reinforcement. The vertical reinforcement layout and its strength are among the major variables of the investigation. The geometry of the model, soil properties and reinforcement characteristics have been kept identical in all different cases selected for parametric studies. The performance of the wall is presented for the facing deformation and crest surface settlement, lateral earth pressure, tensile force in the reinforcement layers and acceleration amplification. The vertical ctefOlIDation, horizontal deflection, reinforcement force and earth pressure develop drastically under earthquake loading compared to the end of construction. The results show that these variables are considerably reduced when incorporating the vertical reinforcement in the system. In addition, the findings suggest better performance and higher structural safety for reinforced soil walls, when employing this proposed orthogonally horizontal-vertical geosynthetics

Mechanical Model to Analyse Multilayer Geosynthetic Reinforced Granular Layer in Column Supported Embankments

© 2016 The Authors. Published by Elsevier B.V. The objective of this paper is to develop a mechanical model to predict the behaviour of a multilayer geosynthetic reinforced granular fill soft soil system improved with controlled modulus columns beneath the embankment. Deformation of geosynthetics embedded granular layer due to bending and shear is considered in this study. Therefore, geosynthetic reinforced granular fill has been idealised as a reinforced Timoshenko beam while the columns and the soft soil have been idealised as a layer of linear springs with varied stiffness. Plane strain conditions are considered for the loading and reinforced foundation soil system. Tension developed in the geosynthetics, rotation and settlements of the improved soft ground are predicted using the proposed model. This study shows the effects of multilayer geosynthetics on the settlement response of the granular layer. A notable reduction of the settlement has been observed as a result of the using multilayer weaker geosynthetic reinforcement system when compare to one stronger geosynthetics layer. It is also observed that the top reinforcement layer is subjected to maximum mobilised tension at the column edge whereas bottom reinforcement layer is more effective in controlling the deflection in the middle of two columns