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Explaining Geotechnical Risks
Tim Chapman
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Reconnaissance Report on Geotechnical Damage Caused by Tropical Cyclone Jasper in December 2023
This paper is focussed on the Department of Transport and Main Roads (TMR) initial geotechnical response to damages caused by Tropical Cyclone Jasper (TC Jasper) to road infrastructure and batter slopes along the state-controlled road network in TMR’s Far North Queensland (FNQ) District of Queensland, Australia.
TC Jasper delivered extreme rainfalls over six days between 13 and 18 December 2023 in FNQ District. The highest accumulated total being 2,096mm in the area of Port Douglas and Mossman north of Cairns. Within areas of the TC Jasper path, rare to extremely rare rainfall occurrences resulted in severe flooding and geotechnical damage and closure of key state-controlled roads in FNQ. The roads closed included all four range crossings providing access to the Northern Tablelands. Communities were isolated, and tourism and regional transport severely impacted.
This paper provides a report on the geotechnical damage caused by TC Jasper to seven key state-controlled roads in TMR’s FNQ District, and outcomes from the event reconnaissance approach used by TMR’s Engineering and Technology Geotechnical Section in collaboration with TMR FNQ District office and RoadTek teams to re-open most of the closed roads and secure significantly damaged road sections within four days of the cyclone passing. Ongoing recovery efforts are progressively re-opening remaining roads.
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Case Studies For Piled Rafts On Clay
Piled rafts have proved to be an effective way of improving the overall performance of a foundation. In the case of structures built on clay, a raft foundation alone may result in excessive settlement. A piled raft foundation is a solution in which the piles provide localised support in heavily loaded regions or areas where deflection is excessive and results in reducing the overall and differential settlements as well as tilting of the foundation. Case histories for piled-rafts foundation on clay will be presented in this paper. The foundations have been studied using a finite layer method for the analysis of a horizontal layered soil and a finite element method for the analysis of the raft and piles. The analysis takes into account the complex soil-structure interactions which govern the behaviour of the piled raft. Case studies of piled rafts on clay will be examined and the results will be compared with the in situ measurements and other numerical results. Comparisons are made in terms of load distribution among piles and the overall and differential settlements.
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Effect of permeant liquid on the swell volume and permeability of geosynthetic clay liners
Nowadays geosynthetic clay liners (GCLs) are frequently used as hydraulic barriers from leachate in liner for waste containment facilities. This research aims to study (1) the effect of cation valence by using monovalent (LiCl, NaCl and KCl), divalent (CaCl2, MgCl2 and CuCl2) and trivalent (FeCl3) salt solutions, (2) the effect of concentration of CaCl2 and (3) effect of pH on swelling volume and permeability coefficient. The consolidation test, the indirect method, was used to find the permeability coefficient. At similar concentrations the swell volume was larger with monovalent cation solutions than with divalent and trivalent cation solutions. With various concentrations, swell volumes decreased with the increasing of concentration for all solutions. GCLs permeated with solutions containing divalent or trivalent cations had a higher permeability coefficient than GCLs permeated with monovalent or distilled water. The permeability coefficient of GCLs decreased with the increasing of concentration. Moreover, it was found that pH only influenced the swell volume and permeability coefficient when pH was very low (pH=2) or very high (pH=12).
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Underpinning the Concord Road bridge under traffic – Westconnex M4 East Project
WestConnex is the largest transport infrastructure project in Australia. It is part of the Australian Federal Government and New South Wales Government’s efforts to ease congestion on Sydney’s roads by widening existing motorways and constructing new tunnels and bridges. M4 East is the section of WestConnex which extends from Haberfield to Homebush and includes the new Concord Road Interchange (currently under construction). The Concord Road Interchange is a complex junction of new bridges, cut-and-cover tunnel portals, retaining walls, widening and altering the alignment of the existing M4 Motorway lanes. Part of the works involves altering the alignment of the eastbound and westbound lanes under the existing Concord Road Bridge. To facilitate these works, the bridge needs to be underpinned with permanent support. The existing bridge abutments are founded on piles which are to be supported on a rock ledge permanently supported by rock bolts and prestressed ground anchors. Further, it is a requirement of the project that the Concord Road Bridge remain open to traffic for the duration of the works. Tight deflection criteria were imposed due to structural requirements at expansion joints. Additionally, the existing Concord Road Bridge was designed and constructed in the 1980s and there was limited information on the ground conditions and as-built founding levels of the bridge abutment piles. These factors, in addition to the requirements for working with low headroom under the bridge, were some of the key challenges during the detailed design and construction support of the works. This paper focuses on the methodology that was adopted to address these challenges during the design and construction phases of the project.
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Rock Mass Parameters For The Brisbane CBD
Rock mass parameters are presented for the typical range of rock conditions encountered in the Brisbane CBD and surrounding area. Rock mass units are classified based on lithology, weathering, intact rock strength and degree of disturbance. The rock mass parameters are based on the Author’s combined experience from Brisbane infrastructure projects including the M7 Clem Jones Tunnel, Airport Link and Cross River Rail. The parameters may be useful for design and construction of future ground engineering projects in Brisbane.
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Simplified analysis of laterally loaded posts
Simplified equations for the strength and deflection of laterally loaded vertical posts embedded in soil foundations, using the modulus of subgrade reaction approach, are presented for simple post embedment and for posts with enlarged footings. Equations describing the elastic behaviour of similarly loaded embedded posts are also detailed. The aim of the paper is to provide a consolidated document containing information for the preliminary design of embedded posts prior to detailed geotechnical investigations and analysis or for the design of such posts where detailed investigations and analysis cannot be justified. The information provided in this paper is not generally available in basic soil mechanics textbooks.
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Analysis of installation forces for helical piles in clay
Installation forces play a central role in the design and performance of helical piles, especially since the installation torque is often used as an indicator of the pile’s ultimate capacity. This paper presents an analytical model for predicting the installation torque for single-helix piles in clay. As an extension of a recent study by the authors, the proposed model considers not only the forces occurring on the helical plates but also the shear stresses generated along the shaft, both of which impact the installation forces. The model yields a straightforward expression that relates installation torque to the undrained shear strength of the soil, embedment depth, helix diameter and pitch, shaft diameter, crowd (axial) force, and adhesion coefficient along the shaft. The influence of these factors on the installation torque, as well as the “capacity-to-torque ratio” used to infer capacity from the installation, is assessed through a sensitivity analysis. Some level of validation is provided through a comparison with empirical capacity-to-torque ratios, and the sensitivity analysis reveals factors that are neglected in empirical models but nevertheless have a significant influence.
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Subsoil Pavement Drains: Panel Drains v Round Pipe Comparison
Liaison with Australian state road authorities in recent years has been the impetus for reviewing current accepted drainage practice, with tested, technically advanced, cost effective products now readily available in Australia. Premature pavement failure commonly caused by slow drainage response has benefited from design methods incorporating geo-composite panel drains, giving improved drainage response times, high flow infiltration, efficient filtration and economical installation.
Pipe stiffness test results highlight the difference in service performance to show that geo-composite panel drains are superior in structural capacity whilst their rigidity compliments efficient installation.
Comparison of factors affecting inflow capacity and outflow discharge identify the optimum flow performance of panel drains. Inflow considerations can be shown to be more relevant criteria than design for discharge capacity. Geotextile filtration performance is an integral part of geo-composite drainage systems with research highlighting the correct assessment of EOS being critical to efficient long term drainage sustainability.
Type of backfill used, permeability rates for in situ soil, pavement materials and drainage backfill play an integral part in overall design. Design practice should consider location of the system, permeability rates for in situ materials and geotextile filtration along with drainage inflow and discharge capacity.