Western Australia Chapter

Young Geotechnical Engineers’ Evening Seminar &
The 16th Dr Baden Clegg Award – 2017

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.

Pizzas and drinks will be provided after the presentations.

This year’s speakers and their presentations are as follows:

Michael Cocjin

Centre for Offshore Foundation Systems, UWA

A look into the whole-life geotechnical response of seabed infrastructure

Ester Tseng

Australian Road Research Board ARRB

A Discussion on the Cover Requirements over Expansive Soils in Pavement Design

Raffaele Ragni

Centre for Offshore Foundation Systems, UWA

Effects of consolidation on the performance of jack-up platform foundations

Abstracts for the presentation are below.

MICHAEL COCJIN

Centre for Offshore Foundation Systems, UWA

A look into the whole-life geotechnical response of seabed infrastructure

Abstract

Seabed infrastructure, such as pipelines and mudmats supporting offshore subsea field developments, experiences periodic operational loading due to production start-up and shutdown cycles. Consequently, the strength of the seabed around the infrastructure evolves throughout the life cycle of a field. In particular, seabed strength increases during periods of field operation when the infrastructure remains static under its operative weight because of the dissipation of pore pressure leading to reconsolidation of the soil.

For long timescales encountered offshore, the ‘whole life’ strength changes can be tapped in the early design stages to potentially provide improved outcomes. However, positive impacts of an evolving seabed on the long-term soil-structure interaction and whole life system behaviour are often underemphasised, if not unheard of, in practice. This is perhaps due to the current lack of supporting knowledge-based evidence.

Recently, whole life increase in seabed strength was shown to provide design efficiencies in the case of subsea mudmats supporting a pipeline. By accommodating such benefits, in-line mudmats can be designed to translate tolerably back and forth across the seabed in response to pipeline expansion and contraction, subject to other criteria such as ensuring that the associated settlements do not cause unacceptable secondary loads. Mobile mudmats can be smaller than conventional foundations, leading to a reduction in installation costs.

This presentation aims to demonstrate design benefits of long-term gains in seabed strength on two fronts: first, on the breakout capacity of a model pipeline; and second, on the whole life sliding capacity of a model mudmat supporting a pipeline submitted to typical field loading conditions in a geotechnical centrifuge. Laboratory observations through in situ soil characterisation showing the increasing strength of soil following cycles of remoulding and reconsolidation will also be highlighted.

ESTER TSENG

Australian Road Research Board ARRB

A Discussion on the Cover Requirements over Expansive Soils in Pavement Design

Abstract

The design of pavements in areas of expansive soils requires careful consideration as shrink-swell movements can result in increased pavement roughness and cracking. One of the common strategies to minimise the detrimental effects of expansive soils in pavements is the use of a non-reactive material cover over the expansive subgrades. The Austroads pavement design guide does not provide guidance regarding how to determine the thickness of this cover. The Main Roads WA pavement design supplement provides a table with minimum cover requirements over reactive material based on CBR swell. The requirements do not vary depending on climate, thickness of the expansive material layer or soil suction variations.

The presentation will focus on utilising the Australian Standard on Residential Slabs and Footings (AS2870) methodology to determine the required thickness of cover over expansive soils taking into consideration climate and the thickness of expansive material, as well as other potentially varying parameters such as changes in suction at the soil surface. A sensitivity analysis using the AS2870 methodology has been conducted in addition to investigating possible correlations between shrink-swell index and CBR swell. Required cover thicknesses obtained following the proposed methodology will be compared to Main Roads WA and other state road agencies requirements. Calculated vertical movements will be compared to the vertical movements measured for the Karratha-Tom Price Road from 2008 to 2015. Finally, a new suggested design chart to determine required cover over reactive soils as a function of climate zone and subgrade CBR swell will be presented.

The aim of the presentation is to engage WA practitioners and seek feedback on how the methodology can be improved and calibrated based on past local experience.

RAFFAELE RAGNI

Centre for Offshore Foundation Systems, UWA

Effects of consolidation on the performance of jack-up platform foundations

Abstract

Jack-up platforms are widely deployed in the offshore industry for a variety of purposes, making them one of the most popular types of mobile offshore structure.
The configuration is usually a hull, three retractable trusswork legs and circular foundations located at the end of each leg. The entire structure relies on the bearing capacity provided by the circular foundations, named spudcans, which are penetrated for a significant distance into the soil during the installation procedure.

When jack-ups are installed in fine-grained to intermediate soils, the response of the soil is generally considered undrained. Significant build-up of excess pore pressures can be thus expected during spudcan penetration, and when the structure is loaded by a combination of waves, currents and wind. However, the consolidation generated by dissipation of such excess pore pressures can take place rather quickly. This can occur at different stages and scales: from short pauses in penetration to long periods of consolidation post-installation. The consequences of this dissipation can be either beneficial or detrimental for the stability of the jack-up. Beneficially, this means having a stiffer, stronger and reliable soil around the footing. On the other hand, the improvement generated by consolidation could be only temporary and localised, hence not reliable and potentially responsible for triggering catastrophic accidents. In either case, a site-specific assessment is required to evaluate benefits and risks, and guarantee safety throughout the lifetime of the structure.

In this case, the problem was studied with an innovative combination of experimental and numerical techniques. On one hand, cutting-edge centrifuge experiments reproduced the conditions encountered in the field; on the other hand, a wide numerical parametric study analysed the effects of critical parameters. Key-aspects of the problem were revealed, along with the formulation of a predictive framework and recommendations for the improvement of current guidelines.

Venue location

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.