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Geotechnical Issues In Construction Of A Large Diameter Raise-Bore Shaft
Major issues were encountered during drilling of a large diameter raise-bored shaft, constructed as part of an underground mine ventilation network. The raise-bored shaft was developed through generally competent, high strength metamorphosed basalt in a high stress environment at depths of +1500 m below ground. Back-reaming was delayed due to blocky ground conditions, leading to ten cutter changes and associated increased costs.
This paper presents details of a geotechnical review of this shaft construction. Analysis of acoustic televiewer (ATV) structural data and seismic data was used to establish the presence of a thin shear causing localized blocky ground as the source of the difficult drilling conditions. Analysis of seismic data collected during drilling is presented as a means to predict difficult back-remaining conditions.
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A Case Study: Trench Stabilization Using Cutter Soil
The Wynyard Edge Alliance (Auckland Council, the Crown represented by Ministry of Business, Innovation and Employment, Downer, McConnell Dowell, Beca, Tonkin and Taylor) was formed to design and construct the infrastructure to support the 36th America’s Cup event in Auckland. In addition to the core infrastructure required for the event, a wider programme of works is underway on the waterfront. One of these projects is the extension of the Daldy Street stormwater pipeline – the Daldy Street Outfall Extension (DSOE) which is a Healthy Waters project (a department of Auckland Council). The DSOE scope of works was included as part of the WEA scope of works in 2019. The DSOE works involves the extension of the pipeline from its current outfall position underneath North Wharf, to a new outflow position at the end of Wynyard Point. To complete this, 510m of High-Density Polyethylene (HDPE) pipe is to be placed beneath Brigham Street.
Cutter Soil Mixing (CSM) was adopted as the preferred ground improvement methodology to stabilise the site to enable the trench excavation. This paper intends to report the quality and performance of the panels produced by the Cutter Soil Mix method, as well as to compare different sampling methods, including Wet Sampling, Double Tube Sampling and core sampling from boreholes. It will also discuss the interlocking observed between two adjacent panels and discussions around the practicality of the CSM method in similar site conditions.
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Case study: Compaction of coal ash fill by impact rolling as a method of subgrade improvement beneath a heavy duty pavement
An intermodal logistics centre for bulk distribution of shipping containers by both road and rail is currently under construction at a site near Enfield in SW Sydney. The 70 ha project site was used as a rail maintenance facility until the early 1990’s, when it was largely dismantled. A container terminal yard will occupy approximately one-third of the project site, which will require construction of a heavy duty pavement to support the operational loads. The design of the terminal area is based on construction of a uniform earthworks platform, requiring both cut and fill to attain design levels, to act as a stable subgrade for the pavement layers. Previous studies revealed the presence of poorly compacted fills across the site that vary significantly both in composition and thickness. Of particular interest was the presence of a significant volume of coal ash fill, which was the thickest fill deposit across a large part of the site. The coal ash was found to be poorly compacted, but laboratory testing indicated it could achieve high strength and CBR index values when compacted to a dense state. The coal ash could thus be treated in place and incorporated into the subgrade earthworks or it could be mined and re-used as general fill in other parts of the site. A programme of field compaction trials was implemented, using conventional vibratory rollers and also impact rollers to determine the optimum compaction method for the coal ash and other fill materials. The field trials showed that compaction of the coal ash significantly reduced its void ratio by approximately 30% with corresponding settlement on compaction. The outcome of these field trials enabled the contractor to develop a ground treatment methodology tailored to the fill material type, which also conformed to the design specification for the subgrade layers.
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Ground Improvement Sustainable by Nature
The construction industry faces significant challenges today due to its high energy consumption and the resulting elevated carbon dioxide emissions. Efforts are underway through global initiatives and evolving frameworks to establish comprehensive environmental and sustainability policies, aiming to guide the construction sector towards a more sustainable development pathway. Geotechnical engineering within the realm of infrastructure is expected to play a critical role in ensuring the safety and stability of superstructures within a sustainable framework, both in the present and the foreseeable future. This article seeks to explore the potential for sustainability in infrastructures through ground improvement techniques. It sheds light on ongoing global research and development efforts aimed at promoting sustainability. The article briefly introduces a case study involving the rigid inclusions type Controlled Modulus Columns (CMC), emphasizing ongoing research and development focused on the reutilization of CMCs under wind turbine foundations during the repowering phase. The repowering phase becomes imperative when onshore wind turbine foundations require replacement after reaching their typical operational lifetime of 25 years, as stipulated by international standards and guidelines.
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Soil nails and rock bolts: construction challenges
Soil nails and rock bolts are a cost effective method for long term stabilisation of slopes or near vertical excavations where staged, top-down construction is advantageous compared to other traditional methods of slope retention. This method of ground support can provide near-instant active or passive support against ground instability. This paper presents the challenges encountered on a recent Melbourne infrastructure project during the installation of soil nails and rock bolts, and the solutions adopted to overcome them. In these case studies, passive soil nails and post-tensioned rock bolts were installed in residual clay soils, weathered siltstone rock, and basalt rock across two sites in Melbourne as part of a transport infrastructure project. The challenges encountered throughout the construction process, in particular the variability of ground conditions and construction timeframes shows the need for supervising engineers to be flexible in the development and implementation of the designs. Where designs have been modified additional checks should be completed to ensure the design intent is achieved. Sacrificial testing required as part of the design was not undertaken to failure of the grout to soil bond so the reported results only provide an indication of the mobilised bond stress. Occasionally this was because the test load required to cause failure would exceed the tendon yield stress, however typically the nominated test loads were not high enough for pull out failure. Whilst the testing confirms adequate bond stress for project-specific requirements, the results may not provide information on the ultimate geotechnical performance of the materials being tested. -
Soft soil improvement induced by tree root suction
Bioengineering aspects of native vegetation are currently being evolved to improve soil stiffness, slope stabilisation, and erosion control. Tree roots provide three stabilising functions: (a) reinforcement of the soil, (b) dissipation of excess pore pressures and (c) establishing sufficient matric suction to increase the shear strength. The effects of vegetation on soil matric suction, shrinkage and ground settlement are discussed in this paper. A mathematical model for the rate of root water uptake that considers ground conditions, type of vegetation and climatic parameters has been developed. A conical shape is considered to represent the geometry of the tree root zone. Based on this proposed model, the distribution of the moisture and the matric suction profile adjacent to the tree are numerically analysed. Field measurements taken from literature published previously are compared with the authors’ numerical model. The predicted results, calculated based on soil, plant, and atmospheric parameters contained in the numerical model, compared favourably with the measured results, justifying the assumptions upon which the model has been developed. The findings of this study indicate that due to significant changes in soil moisture content induced by tree roots, the shear strength of the soil will be enhanced. It is desirable to consider the influence zone of tree roots and the improved soil properties in modern geotechnical designs, benefiting from native vegetation.
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Temperature effects on volume change of bentonite sand liner
Bentonite-sand (B:S) mixture is used as soil liner to protect seepage against contaminant migration. Experiments were conducted to study the volume change of bentonite-sand mix due to high temperature effects. Bentonite-sand (B:S) mix with dry weight ratio of 10% to 90% (10:90) was used as an ideal mixture in this experimental program. Volume change of bentonite-sand mix in normally consolidated (NC) stress condition was investigated at high temperatures (45˚ and 75˚C). Volume changes were measured by measuring the void ratio change at high temperatures using a modified consolidation cell. Experimental results show that volume change was more affected by high temperature and the initial hours of heating play the most significant role. It was found that pore water pressure change due to temperature change is affected by high temperature, which in turn causes the void ratio change. Such studies could be helpful to incorporate design modifications to improve the liner behaviour.
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Geotechnical Challenges for Development in the Hunter Region (Part 2)
This issue continues the theme of the March edition and contains a further six papers which were presented at a mini- symposium organised by the Newcastle Chapter of the Australian Geomechanics Society in May 2005.
These papers, together with those published in the March 2005 edition, present a comprehensive insight into the many and particular issues facing the geotechnical community in the Hunter region. The papers in this edition include discussions on landfill covers and waste water management, monitoring of expansive soils and numerical modelling of slabs on expansive soils, a case study of a major road project, and a gentle reminder that historical data often provides very useful information which should not be forgotten.
The symposium organising committee did an outstanding job in encouraging the authors to produce papers which covered a range of topics of most interest to the geotechnical community, arranging for peer review of the papers, contacting sponsors for the symposium and the journal, and running the mini-symposium.