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Engineering design and earthworks aspects related to basaltic clays in Victoria
This paper discusses the geotechnical engineering design aspects related to reactive basaltic clays in Victoria. The issues associated with earthworks, such as the effect of placement moisture content, strength of the clays and testing of the earthworks are also discussed. Possible measures to reduce the future shrink and swell movements, including lime stabilisation are discussed. The required amount of lime, the effect of addition of lime on plasticity, California Bearing Ratio and permeability are presented and discussed. Durability of lime stabilisation is also briefly discussed. Although the results and discussions presented are specific for basaltic clays in Victoria, the issues and concepts discussed are applicable for reactive clays in general.
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Stability of excavations in unsaturated fissured clay
The problems of the stability of vertical or near vertical excavations in unsaturated fissured clay are discussed. A consideration of the stability of vertical excavations in non-fissured clay, in clay with tension cracks, and by considering the effects of lateral stress relief on excavation in clays without pre-existing fissures, gives greater excavation depths than would be regarded as acceptable in clays. The overall stability of the soil mass containing fissures is governed by the residual shear strength along the joints, particularly with polished slickensides. A common failure mechanism in vertical and near vertical excavations occurs when a slickenside intersects a vertical shrinkage crack. Using the residual shear strength parameters and a planar failure mechanism, the stability of vertical and steeply sided excavations, either supported or unsupported, in fissured clay is discussed. Three case examples are outlined, and recommendations for the successful construction of temporary batters in unsaturated fissured clay are given.
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Coal extraction and ground movement at Huntly East Coal Mine, New Zealand
This paper provides a novel and representative method for characterising coal extraction layouts. This method was used to support an investigation of the correlation between the extraction layout and the ground movement in Huntly East coal mine, New Zealand. A 250 m borehole inclinometer was installed to monitor the strata movement that occurred as underground mining approached the borehole, by measuring the movement of the casing within the borehole. The inclinometer measurement was undertaken once a month on average. In order to analyse the correlation between coal extraction and ground movement, multiple extraction areas for each month were organised into one area. In order to determine the magnitude and location of the extraction in terms of its induced ground movement, the delay time of subsidence was first defined by data analysis. The correlation between the coal extraction and the ground subsidence had been established. Finally, the correlations between the nearest extraction edge and the lateral movement were represented by three nonlinear regression equations. The three equations have been validated by numerical modelling; this is not discussed in the paper. Lateral movement has been analysed and characterised in this study, but vertical subsidence has only been identified to exist and was not discussed in detail due to the limited dataset available.
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Monitoring Slope Instability During Reinstatement of State Highway 11 at Lemon’s Hill(35˚S) Northland, New Zealand
Slope failures within weathered rock are characteristic of road cuttings in the humid subtropics, where weathering profiles can extend 10s of meters into the subsurface. Typically, the high rainfall intensities provided by the passing of tropical cyclones, can generate widespread slopes failures along road transport corridors, requiring engineering responses such as hazard assessment and road reinstatement.
Here, we report on some engineering geological aspects of the 2018 slope failure and subsequent slope monitoring response along State Highway 11 (SH11) on the southern side of Lemon’s Hill (35°S), in Northland, New Zealand. In much of New Zealand, landslides along transport corridors are often initiated by seismic activity, but in the subtropical north, >500 km west of the active plate boundary, rainfall is the key landslide trigger. The 13 February 2018 landslide at Lemon’s Hill adjacent to SH11 followed prolonged rainfall from the passing of Tropical Cyclone Fehi. SH11 is a popular tourist route to the Bay of Islands region of Northland, and the landslide occurred as an ‘overslip’, shallow (2 m deep) translational failure, within completely (CW) to highly weathered (HW) greywacke.
The response included the construction of engineered batters, resulting in six months of traffic disruptions. The engineering geological investigation of the site including characterisation of the materials, and identification of probable failure modes. Monitoring via multi-temporal Unmanned Aerial Vehicle (UAV) photogrammetry and AccuMM GPS nodes installed on the slopes was also part of the engineering response, while recently available LiDAR provides future opportunities for an enhanced engineering geological understanding of slopes instability in the area.
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Index properties and the engineering behaviour of Bringelly Shale
This paper is concerned with the engineering behaviour of Bringelly Shale and how this can be assessed based on laboratory index tests that are widely used for argillaceous rocks. Comparison will be made with data from Ashfield shale to indicate the differences between these two members of the Wianamatta Group. It is shown that Bringelly shale contains reactive clay minerals, absent in Ashfield Shale and, as a result, the shale is more sensitive to changes in environmental conditions. Bringelly Shale is only weakly cemented and its strength and stiffness are lower than Ashfield Shale. Both shales have similar unconfined compressive strengths, typically between 10 MPa and 50 MPa, but in Bringelly Shale a large component of this strength appears to be derived from pore water suctions. When Bringelly Shale is placed in water it disintegrates. The paper concludes with some implications of the data for construction in Bringelly Shale.
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Equilibrium moisture content of volumetrically active clay earthworks in Queensland
The water changes of volumetrically active clays result in movement of the overlying pavements and in a change in the subgrade strength. This adverse effect results in damage to roads and buildings, with over one-third of Queensland covered with such clays.
This paper discusses the equilibrium moisture content (EMC) operating range in southeast Queensland, and the philosophy behind the procedures for assessment and design on reactive clay earthworks. Two important considerations for wet environments with highly reactive clays are 1) the EMC is wet of the Optimum Moisture Content (OMC), and 2) the long-term density is below the Maximum Dry Density (MDD). If this placement condition is not targeted, then movements can be expected in the early years. This may result in damage to overlying structures irrespective of the design subgrade strength adopted. Targeting the OMC and MDD in such cases is building in future long term movement.
This EMC condition must be considered together with construction issues.
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Factor of safety in AS4678: Earth retaining structures
Factor of safety is used to provide safety margin over the theoretical design capacity to allow for uncertainties in loading, material strength and design process. Design of earth retaining structures has traditionally been based on the overall factor of safety method. However, the current Australian Standard for Earth Retaining Structures, AS4678-2002, is based on partial factors of safety method. In this paper, cantilever retaining walls and embedded sheet pile walls have been designed based on the recommendations of AS4678-2002 to examine the overall factor of safety inherent in the standard. Various wall heights and soil parameters are used in the designs. The overall factor of safety is then back-calculated for each wall based on its designed dimensions. The results of analysis are presented in the form of the overall factor of safety associated with the dimension of the walls and soil properties. The overall factor of safety of walls in cohesionless soils varies between 1.7 and 2.3; shorter walls have higher factor of safety. However, when the backfill soil has some cohesion, the overall factor of safety is generally higher than 2 and becomes more than 5 for soil cohesion greater than 30 kPa. For embedded sheet pile walls in cohesionless soils, the factor of safety remains constant for one particular type of soil, regardless of the height of the wall. The results of analyses of these walls in cohesionless soils also show that the factor of safety increases slightly as the friction angle of the soil increases. For the walls embedded in cohesive soils, the overall factor of safety is higher compared to those in cohesionless soils and this behavior is consistent with the one observed in cantilever retaining walls.
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Inorganic transport through composite geosynthetics and compacted clay liners under geomembranes with multiple defects
Geosynthetic clay liners (GCLs) and compacted clay liners (CCLs) are widely used in waste containment systems – usually in conjunction with a high-density polyethylene geomembrane – for the protection of groundwater from contamination. Defects in geomembranes have been shown to have detrimental impacts on their performances. These systems, including defects, have been studied mostly through the lens of hydraulic leakage. Previous contaminant migration studies of liner systems have assumed single rather than multiple defects or have simulated organic, rather than inorganic transport when multiple defects are present. Unlike organic chemicals, inorganic contaminants do not biologically decay and have extremely small coefficients of diffusion through the intact parts of the geomembrane. These differences create different transport regimes, with contaminant levels likely to take longer to build up and dissipate in the aquifer. This paper simulates the transport of inorganic contaminants in systems containing CCLs or GCLs, under a geomembrane with multiple defects. Specifically, we aim to a) assess the extent to which leakage rates are good predictors of concentrations of inorganic contaminants in the aquifer and b) quantify the relative effect of various design and field parameters on the degree to which defects in the geomembranes reduce the performance of these systems.
Two-dimensional models of liner systems with multiple defects are simulated with the finite-element based Soil Pollution Analysis System (SPAS) for the transport of chloride and cadmium in geosynthetic and compacted composite clay liners. The coupled, steady-state seepage equations and time-dependent reactive diffusion advection equations are solved in two-dimensional space in order to compute seepage velocities and chemical concentrations in the system, including the underlying aquifer. A finite mass boundary condition is applied at the top of the system, representing a finite intake of contaminants in the waste. Parametric analyses are conducted to characterise the relationship between, on the one hand, various design and field parameters (intake of contaminant, thickness of primary liner, frequency and size of defects, hydraulic conductivities of clay) and, on the other hand, leakage rates and maximum concentrations of contaminant in the aquifer.
We find that defects in the geomembrane lead to significant increases in maximum concentrations of inorganic contaminants in the aquifer. However, these maxima are predicted to occur a few hundred years after the closure of the landfill, i.e. beyond the usual regulatory limit of the design. Design/field parameters with the strongest effect on maximum contaminant levels in the aquifer are the hydraulic conductivities of the primary liner (CCL or GCL), the frequency of defects and, in the case of the CCL, the thickness of the primary liner. Finally, we find that leakage rates are sometimes poor indicators of the effects of design parameters on chemical concentrations in groundwater. The maximum specific discharge rate under defects can have an important effect on concentrations as well, though it is not usually taken directly into account, nor is it easily measurable.
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In situ and laboratory testing of soft clays
Current Australian practice for sampling, laboratory testing, and in situ testing of soft clays falls short of world’s best practices. Consequences include risks of increased cost and time during construction, as well as geotechnical solutions that are more elaborate than necessary. Some limitations in current practice are identified and alternative methods are proposed that provide higher data quality at similar cost to current methods and improved understanding of the geomaterials under study, thus optimizing the selection of geotechnical solutions.
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Design Of High Strength Geotextiles For Basal Reinforcement Within Embankments
High Strength Geotextiles (HSG’s) are commonly used as basal reinforcement within embankments and structures founded upon weak ground. The polymeric composition of these products can bring high magnitudes of tensile strength to the system to prevent against slope instabilities and bearing type failures.
HSG’s are “passive” forms of reinforcement, whereby activation of its capacity occurs whence destabilising forces are applied, causing the reinforcement to undergo tensile strain. The polymeric materials in HSG’s undergo creep, i.e. deformation under sustained application of constant tensile loads. The combination of straindependent and time-dependent behaviours in HSG’s are a complex mix of material characteristics, which make them complicated to design with. The complexity of their behaviour has resulted in greatly varying design methods adopted in industry. The source of difference is often linked to how creep is addressed in the selection of HSG, and how its strain-dependent behaviour is accounted for. This paper discusses the design methodologies referred to in BS8006 (Code of Practice for Strengthened/Reinforced Soils and other Fills) – which follows a limit state design process to ascertain the design loads to be carried by HSG’s and methods on how to assess the design strength of proprietary products. The authors generally support the methodologies adopted in BS8006 with suggestions on limiting criteria and how these can be assessed.