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Impacts of longwall mining and coal seam gas extraction on groundwater regimes in the Sydney Basin: Part 1 – Theory
The mathematics steady state and transient downwards Darcian flow are given for full or limited recharge and saturated homogenous ground, layered ground, and for unsaturated flow. Data are presented from a physical model that supports the theoretical analyses.
A hypothesis is presented for unsaturated hydraulic conductivity in the Triassic rocks of the Sydney Basin. The theoretical analyses coupled with the important inferences from unsaturated hydraulic conductivity provide valuable aids to understanding possible impacts of depressurisation due to underground coal mining and coal seam gas extraction in the Sydney Basin.
It is acknowledged that flow through jointed rock masses is very complex, and there are limits to the applicability of the equations of flow through porous media. However, as with elastic theory in geomechanics analyses, it is considered that the rigor gained from Darcian flow analysis assists greatly in avoiding flawed thought processes in hydrogeology.
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Performance of a 7.5m high post and panel wall under a 120t piling rig load
Melbourne Metro Rail Infrastructure Alliance (RIA) project is currently under construction as part of the larger Melbourne Metro Rail Tunnel Project in Melbourne. The works at the Eastern entrance to the Metro Tunnel include construction of a cut & cover tunnel and decline structure which will form the entrance to the metro tunnel when completed in 2025. Restricted site access including proximity to residential properties and operational live rail necessitated a purpose built temporary retaining structure as a working platform to support a 120 tonne piling rig for the installation of 18 m long CFA piles for the cut & cover tunnel structure. A post and panel wall comprising 900 diameter CFA piles with 310UC steel sections was considered a feasible solution for constraint site conditions. The maximum height of the wall was 7.5 m including a piling platform to support a piling rig track pressure of 345 kPa operating behind the wall. Ground conditions comprised cemented sandy soils of Brighton Group underlain by Fyansford Formation and high ground water level.
This paper discusses geotechnical characterisation of the site, design development, challenges and site constraints, monitoring of the wall during construction and performance of the retaining wall during the operation of the piling rig from a design and construction perspective.
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A study of reactive soil influence on small diameter pipe failures in Melbourne
Melbourne’s water reticulation system experiences about 4000 pipe breakages or bursts in each year, causing difficulties to both water utilities and water users. The majority of these failures are in old cast iron pipes that can be up to about 100 years old. For reticulation or small diameter cast iron pipes (diameter less than 300mm), the failures occur around the pipe circumference (known as broken back failures) mainly due to pipe bending. It is well established that the seasonal ground movement in reactive soil zones in Melbourne has a notable impact on pipe bending and resultant failures. The present study examines in detail the failure process of small diameter pipes that are affected by reactive soils. Finite element models of unsaturated soil-pipe interaction are used to simulate the response of the pipe to reactive ground movements that are governed by soil moisture variations. Locations where high pipe stresses due to ground movements are likely to occur are identified as stress “hotspots” to determine the potential for pipe failure. The concept of these stress hotspots is verified or corroborated by collecting field information from recent pipe failures in Western suburbs of Melbourne where highly/extremely reactive soils are commonly present. The results of this study are presented in the form of a simplified analytical method to estimate pipe stresses based on the soil moisture changes at the pipe level. With this development, mechanistic failure models are developed that could be implemented in a GIS platform for failure prediction and visualisation.
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The Effect Of Geosynthetic Soil Reinforcement Design Standards On Costs For Earth-retaining Structures: A Qualitative Approach
The use of soil reinforcement in civil engineering applications is not new and in fact, this technology has existed for over 2000 years in the form of natural fibres used as soil inclusions in the Great Wall of China. Today, modern technology and science have developed soil reinforcement solutions that are consistent and mass reproducible. High tenacity polymers, with high molecular weight usually in the form of polyester or high density polyethylene are the most common materials used to achieve superior creep performance vital in earthretaining structures. Design standards in Australia and around the world are available to provide design guidelines in the specification and design of earth-retaining structures using geosynthetics. These include AS4678:2002 and BS8006:2010 as the official design codes. However, Government road authorities have published their own design guidelines such as the NSW Roads & Maritime Services R57 document and the Qld Main Roads MRS11.06 document to name a few. Different applications and loading conditions require different polymer types to get the maximum design benefits such as creep performance from polyester (PET) and high density polyethylene (HDPE), and installation damage resistance from polypropylene (PP) and HDPE. However, other installation and commercial factors may influence the preferred polymer type. This relates to issues such as; the ease of handling, cutting and unrolling of the product and of course even if it satisfies the design criteria, the cost of the product will ultimately determine if the solution is feasible. For permanent earthretaining structures, the two most common polymers used for soil reinforcement are polyester and HDPE. This paper will review two design codes adopted in Australia and how they impact on the feasibility (cost) of constructing a reinforced soil structure using geogrids.
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Spacial variability of the soil at the Ballina National Field Test Facility
This paper investigates the soil properties, stratigraphy and spatial variability of the soils at the National Field Test Facility in Ballina based on extensive CPTU tests. The soil profile in this site consists of an alluvial crust over a relatively weaker layer of clay and underlain with a layer of sand and Pleistocene age stiff clay. The measured cone penetration resistance, sleeve friction and pore pressure for 26 CPTUs are presented along with the deployment of the CPTUs. The spatial variability in both vertical and horizontal direction of each layer of soils is explored based on the CPTU tests. An exponential autocorrelation function is found to best fit the autocorrelation coefficients. The scale of fluctuation in the vertical direction is 0.04 m in the alluvial crust layer, which is much smaller than that in the underlying clay layer, 0.15 m. The reason is that the clay was deposited under lower energy conditions compared to the more granular crust layers. The horizontal scale of fluctuation is 9.21 m in the alluvial crust layer and 4.92 m in the clay layer.
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Seismic Response And Dynamic Deformation Analysis Of Shur River Dam
Seismic load has induced extensive deformations or caused failure in many dams during past earthquakes. As such, it is considered to be the governing load for the design of dams in the regions of moderate to high seismicity by dam designers. This paper discusses the methods and results of the seismic deformation analyses of an 84 m high water retaining rockfill dam with an asphaltic concrete core, located in a region of high seismicity in Iran. The purpose of the dam is to store raw and processed water for use in a copper production complex. The maximum crest settlement of the dam under three earthquake loadings was evaluated using simplified procedures and numerical dynamic analysis. Plane strain dynamic deformation analysis of the dam has been performed using the commercial software FLAC and its built-in Mohr-Coulomb elastic-plastic model. The main objective of these analyses has been to identify the earthquake induced movements that could lead to uncontrolled release of water. The predicted deformations are compared with the results of simplified analyses and the observed displacements in other embankment dams during past earthquakes. The relative movement of the asphaltic core with respect to the rockfill material on either side of the core was also studied.
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Investigation of continuing loss estimation for medium sized tropical catchments
The rainfall based design flood estimation techniques are commonly adopted in hydrological design and require a number of inputs inCLuding information on loss characteristics. A conceptual loss model known as the ‘initial losscontinuing loss model’ is widely used in Australia. The initial loss (IL) occurs at the beginning of the rainfall event, prior to the commencement of surface runoff and the continuing loss (CL) is the average rate of loss throughout the remainder of the storm. The currently recommended design loss values in Australian Rainfall and Runoff (ARR) for Queensland have some basic limitations. This paper investigated how more accurate CL values can be estimated and derived for medium sized tropical Queensland catchments using long term rainfall and stream-flow data. A base-flow separation method was used, which was characterised by a single parameter (α). This paper shows that the continuing loss estimates are not sensitive to α. Importantly, the median continuing loss values derived for three Queensland catchments are found to be much smaller than the present recommended design continuing loss values documented in the ARR (I. E. Aust., 1998), which are used by design engineers The derived value has important implications to improve the design flood estimate. The sensitivity analysis between derived and existing recommended CL values shows that the derived CL value produces higher flood peak than the existing recommended CL value in ARR (I. E. Aust., 1998).