AGS Young Geotechnical Professionals’ Evening Seminar and the 12th Dr Baden Clegg Award

Various

This annual event gives a platform for our younger (under 35) geo‐professionals to present their work, gain feedback from experienced colleagues and become involved in AGS activities. Please support them by coming along to what should be a varied, interesting and exciting evening.

There will be three presentations, each 15 minutes long, with 5 minutes for questions after each talk. The Dr Baden Clegg Award will be presented to the best contribution.

Prizes

FIRST PLACE: Trophy, $500 + a copy of the Australian Geomechanics Society Journal Collection on DVD
RUNNERS UP: $100 + a copy of the Australian Geomechanics Society Journal Collection on DVD

East Port Hedland Fill Study – Geotechnical Investigation and Settlement Prediction

Dennis Osti ‐ Geotechnical Engineer, GHD

ABSTRACT

This presentation will cover the challenges presented and overcome in the completion of the Port Hedland Fill Study. GHD were engaged by LandCorp to conduct a geotechnical investigation for the residential and commercial development of a new suburb east of the Port Hedland town site. The site was approximately 450 hectares and predominately situated on intertidal and supratidal flats. Since the majority of the site is subjected to tidal flooding, major fill earthworks (up to 5m) were anticipated to achieve design grade levels.

The field investigation comprised test pits and CPTs, including dissipation and dilatometer testing. The primary challenge on site was traversing over areas with poor trafficability conditions which presented the risk of bogging. This risk was circumvented by investigating ahead by foot and using bogmats where appropriate however, some areas were completely avoided due to very poor trafficability conditions (i.e. not safe to walk across).

One of the greatest challenges of the project was estimating the magnitude and rate of consolidation settlements, and providing recommendations for reducing post‐construction settlements. Due to the variability of the subsurface ground conditions, it was difficult to calculate total consolidation settlements. In order to simplify the calculations, we opted to define all the layers (i.e. recent mud unit, mangrove mud unit, Red Beds) and assign suitable geotechnical parameters for each layer. Representative parameters were inferred from oedometer results and adjusted according to the dilatometer readings, correlation with horizontal coefficients of consolidation interpreted from dissipation testing and typical values for Port Hedland muds published by Foulsham.

The magnitude of creep settlement became the most critical issue in the end, where pre‐loading became obsolete
(although not practical to begin with). This was overcome by evaluating pre‐surcharging loading to the point where pre‐ surcharge consolidation settlement would negate the remaining design consolidation settlement and the majority of creep settlement.

ABOUT THE PRESENTER

Dennis is currently employed as a Geotechnical Engineer for GHD for the previous 18 months. Dennis graduated with a Bachelor of Civil and Construction Engineering (Hons) from Curtin University in 2011. His recent experiences include geotechnical investigations within the Pilbara, Gascoyne, Mid West and Perth metropolitan area.

Critical State-Based Analysis of Tailings Dam Liquefaction – Prior to Deposition

David Reid ‐ PhD Student, UWA / Tailings Engineer, Golder Associates

ABSTRACT

Flow liquefaction, whether statically or cyclically induced, is a risk for any tailings facility where the deposited material is not completely contained within a non‐liquefiable structure. In Western Australia, this condition generally exists in upstream‐raised and “steepened beach” style tailings facilities. While these deposition methods suffer increased risk of flow liquefaction, they are often the most cost effective, depending on local site conditions and other variables. Therefore, it is incumbent on tailings engineers, in the interests of their clients, to assess the feasibility and safety of these facilities using practical, state of the art techniques based on sound mechanics. This assessment should first be conducted in the design stage, and updated regularly during operation and closure.

The liquefaction susceptibility of an existing tailings facility, where penetration testing is practicable, can be assessed via a number of analysis frameworks. However, such tools are less readily available (and generally accepted) in the initial design stage when only a small sample of tailings is available for testing in the laboratory. A critical review of available liquefaction analysis techniques in the design stage will be provided, with particular reference to liquefaction flow histories that indicate the importance that shear‐localisation plays in flow liquefaction. An argument against the use of cyclic testing for some tailings types and site locations will be provided. The importance of post‐liquefaction strengths will be emphasised, and an analysis technique based on critical state soil mechanics to estimate these strengths will be outlined. Considerations related to local climactic conditions when using such methods will be discussed. Examples from a number of projects and tailing types will be presented as support for this technique, while also allowing discussion of the inherent uncertainties involved. Importantly, the ability of this analysis technique to include recognition of shear localisation in a rational way will be emphasised.

ABOUT THE PRESENTER

David is currently studying as a PhD student in the School of Civil and Resource Engineering at UWA, and also works part time as a tailings engineer at Golder Associates in Perth. He has been in Western Australia working as a tailings engineer for 8 years since arriving from Canada. David obtained a BEng in Civil Engineering from Queen’s University in Canada in 2004, and a MEngSc in Geotechnical Engineering from UNSW in 2010. He obtained chartered status as a professional engineer in 2012.

An Engineering Geological Approach to Assessing Karst Risk – A Case Study from the Pilbara

Matthew Temlett – Engineering Geologist, 4DGeotechnics Pty Ltd

ABSTRACT

Potential karst geohazard was identified at a proposed mine plant site in the Pilbara region by the presence of carbonate rocks, specifically calcrete and dolomite. The hazard was initially investigated by the desktop study of satellite images, observation of a sterilisation drilling programme and field mapping, during which a 100 m wide suffosion sinkhole was identified. Subsequently, a trial study targeted known cavities with a variety of geophysical techniques. Cored boreholes were undertaken at geophysical anomalies and plant infrastructure locations. These studies identified 45 cavities commonly ranging between 1 m to 2.5 m thickness, mostly occurring near the base of the calcrete unit and occasionally within the top of the unit, underlying ferruginous gravels. Additionally, a cavity of 14 m thickness was identified at approximately 90 m depth within the dolomite unit.

A GIS‐based three‐dimensional model was developed from the borehole logs. This included sub‐surface planes that
modelled the upper‐ and lower‐most extents of calcrete and uppermost extent of dolomite, generated from 147 borehole logs. The sub‐surface planes from existing groundwater levels and projected future groundwater levels were also added from previous studies. Additionally, a sub‐surface plane was generated that connected the uppermost locations of known cavities. A notional safe roof thickness of 5.5 m was established by entering data from the drilling programmes, including Rock Mass Rating (RMR) values, and known cavity sizes and depths, into a published nomogram (Waltham and Fookes, 2003).

A risk map was generated for the plant location by intersecting the plane expressing the uppermost surface of the calcrete with the plane expressing known cavities. Areas were then ranked by the number of safe roof thicknesses of calcrete above the cavity plane. A second risk map was generated that expressed areas of accelerated cavity development due to anticipated groundwater drawdown during mine operations.

ABOUT THE PRESENTER

Matthew is an Engineering Geologist working for 4DGeotechnics Pty Ltd based in Perth. Before moving to Australia, Matthew worked for over 2 years at a geotechnical consultancy/contractor in the UK. Matthew moved to Australia 2 years ago and has worked on a variety of iron ore expansion projects around WA for proposed heavy haul rail alignments and mine plant sites. Matthew graduated from Cardiff University (UK) with a degree in Exploration and Resource Geology and a masters in Applied Environmental Geology.

Engineers Australia members participating in AGS technical sessions can record attendance on their personal CPD logs. Members should refer to Engineers Australia CPD policy for details on CPD types, requirements and auditing guidelines.