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Building Dewatering In The Botany Sands And The Aquifer Interference Policy
The Department of Primary Industries (DPI) Water is responsible for the management of groundwater across NSW. The management of groundwater extraction impacts including those associated with dewatering for building developments which predominate in Sydney within the Botany Sands Groundwater Source is one of the primary roles of the agency. There are an increasing number of residential and commercial developments that need to accommodate construction below the water table for basements and other functions. These have to be built in a way that mitigates adverse impacts on other building developments or other groundwater users, sensitive ecosystems or structures. Parts of Sydney’s development are remarkably high-density and construction must be well planned to prevent such effects as unintentional groundwater mounding, obstruction of natural flows by the completed building, and potential basement inundation caused by periodic elevated water levels.
In order to demonstrate sustainable development principles, the Aquifer Interference Policy (AIP) requires the proponent to account for all groundwater take during both construction and occupation. This may include the proponent taking measures to avoid or prevent the take of groundwater where possible and include mitigation or avoidance strategies to reduce take of water. The proponent should demonstrate that adequate arrangements will be in place to ensure the minimal impact considerations of the AIP are met. The information needed for assessments of the groundwater impacts of proposed developments under the AIP requires a comprehensive evaluation of the hydrogeological environment. This information is required to allow DPI Water to balance the management of the groundwater resource with facilitating sustainable development.
In some instances, the use of secant piled cut-off walls to enclose an entire site may not be the optimum solution without other engineering measures because the walls effectively work like dams to groundwater flow. Alternate methods of construction might be more appropriate and facilitate flows both beneath and around structures. The dewatering considerations are not limited to individual properties as there are significant interferences from major infrastructure projects, as well as other nearby concurrent building developments, which need to be accommodated at the same time. This paper discusses requirements for proponents and their consultants that relate to the assessment of dewatering proposals in Sydney’s Botany Sands Groundwater Source, which is part of the Greater Metropolitan Region Water Sharing Plan area.
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Geotechnical risk assessment and management for maintenance of water conveyance tunnels in South Eastern Australia
In SE Australia it is estimated that there are more than 500 km of large diameter rock tunnels constructed over a period of more than 50 years for water supply, hydro-electricity, sewage and other purposes. Owners are required to implement an inspection and maintenance program for these critical assets to ensure ongoing reliability of performance and in particular to prevent tunnel blockages from rockfalls. Major cost and time goes into the planning of a tunnel outage for inspection and repairs. Thus it is critical that the frequency between outages is determined on the basis of sound engineering and commercial principles. A critical aspect is the assessment of geotechnical risk at the planning stages for tunnel outages. A Quantitative Risk Assessment (QRA) is considered the most defensible method for prediction of risk of collapse to guide the timing for inspection and repair work.
This paper presents a straight forward QRA method adopted recently by the author on a range of tunnel projects in SE Australia during the planning stage to determine the annual probability of collapse and key geotechnical issues required to be considered during repairs. It also discusses procedures adopted to manage geotechnical risk during an outage with due regard to the range of likely treatment methods that may be required and safe access for personnel.
This risk assessment approach applied in recent years in Australia for dams and slopes can also be adopted for tunnels. This method represents a powerful geotechnical tool as it attempts to quantify the annual probability of failure in numerical terms. It also forces the practitioner to look closely at the range of failure mechanisms possible and their individual contribution to the combined probability of failure. The method may also be applied for the preliminary design for new tunnel projects.
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2020 Young Geotechnical Professionals’ Night
Lavinia Lamipeti, Timothy Kelly, Janarthan Kumarakuruparan and Yun Bai
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Recent advances in the use of prefabricated vertical drains in soft soils
A system of prefabricated vertical drains with surcharge load to accelerate consolidation by shortening the drainage path is one of the most popular methods of soft ground improvement. An analytical solution is proposed based on radial (lateral) soil permeability while considering variations in vacuum pressure. The predicted smear zone and effects of drain unsaturation are compared with laboratory data obtained from large-scale radial consolidation tests. When a higher load is required to meet the desired rate of settlement and the cost of raising a surcharge embankment is also significant, the application of vacuum pressure with a reduced surcharge load can be used. In this method, the vacuum creates a suction head that increases the effective stress. Analytical and numerical analyses were conducted for several case histories using the equivalent plane strain solution for Darcian and non-Darcian flows. The effectiveness of vertical drains on cyclic loading was also investigated based on a laboratory study. . This paper shows that vertical drains can dissipate the built up excess pore pressure under repeated loading and that short drains can be sufficient in certain cases rather than driving the drains to cover the entire depth of soft clay deposits. The research findings verify that the effects of soil disturbance and vacuum pressure can affect soil consolidation considerably, which means that these aspects need to be modelled correctly in any numerical approaches.
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Numerical analysis of geosynthetics and engineering fill in performance of reconditioned ballasted track
Over the past few decades, geosynthetics have been used extensively during track reconditioning to improve soil stability as they offer many advantages including cost effectiveness, ease of installation and minimal earthworks. Among the wide range of products in the market, geogrid remains the most commonly used geosynthetics for soil reinforcement. The aims of this paper are to investigate the effect of varying subgrade properties on track performance and to examine the effectiveness of geogrids and engineering fill for track reconditioning purposes. In the current study, numerical analyses were conducted using engineering software OptumG2, a finite element program for geotechnical stability and deformation analysis. The results of the parametric study indicated that geogrid inclusion within track substructure has considerable effect on settlement reduction and, in particular, increases the bearing capacity of railway track. The results also suggested that increase in axial stiffness of geogrids has minimal impact on track deformation. The most effective and practical location for geogrid reinforcement was achieved at interface between ballast and capping layers irrespective of the subgrade strength and stiffness. Sensitivity analyses showed that both total settlement and the bearing capacity of the railway track were most affected by the changes in the friction angle of subgrade, compared with cohesion and elastic modulus of subgrade, with or without geogrid reinforcement. The findings concluded that proper design of geogrid reinforcement can eliminate the need for or significantly reduce the thickness of engineering fill for ground improvement purposes.
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Laminated rock beam design for tunnel support
The design and construction of semi flat-roofed tunnels, i.e. with a high arch radius to roof span ratio, using a voussoir beam analogy has been proven successful over time. In spite of such a success, the linear arch theory or voussoir beam analogy has always been subjected to a certain level of scepticism due to some of its perceived limitations. Some of the concerns are related to appropriate design methods for the design of rock bolting of multiple beds/laminations in cases where single laminations are deemed unstable upon excavation or while addressing some adverse conditions. This paper investigates the applicability of an analytical solution of the voussoir beam theory for the design of rock bolts in laminated rock beams which has been confirmed with numerical analysis using DEM (Distinct Element Method) analysis. The proposed analysis method can be easily implemented in a spreadsheet to provide rapid assessments though it is considered only one part of the design process with other potential instability mechanisms assessed using other analysis methods.
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A Case Study Of Deep Excavation And Shoring Design For Sydney Metro Northwest
This paper provides an overview of the Sydney Metro Northwest (formerly the North West Rail Link) project and the underground station excavation retention design and construction works, including the key requirements set out in the scope of works and technical criteria (SWTC). Based on the assessment of the geological conditions a soldier piled wall shoring system was adopted for all five new underground stations and one of the two services facility shafts for ease and speed of construction. During the Castle Hill Station excavation a new planar wedge instability mechanism was considered to be credible based on the additional geological data, with the original three-dimensional block instability being no longer suitable. This led to redesign of the south wall stabilisation works based on the updated geological model and input parameters. The instrumentation and monitoring plan was also adjusted to ensure the required additional support provided would be adequate for the safety of the station box excavation. The monitored lateral movements at the capping beam and at the inclinometers were within the trigger values, indicating that the retention system constructed was robust.
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Managing mine subsidence along railways and highway pavements in the Southern Coalfield
There is a long history of underground coal mining in the Illawarra, dating back to the mid-1800s. In recent times, coal mines have worked closely with the owners and operators of both private and public infrastructure to mine directly beneath infrastructure whilst maintaining their safe operation. An overview is presented of recent methods that have been used to successfully manage potential mine subsidence impacts on railway and highway infrastructure, whose presence in the past necessitated sterilisation of coal resources. Some of these methods represent “world first” technology and have applications beyond the field of mine subsidence management.
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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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2019 Sydney Symposium
Innovations in Geotechnical Construction and Design
Harry Poulos and David Puller