With the continuation of Western Australia’s resources boom, the construction of specialised mining infrastructure in remote locations provides interesting and unique challenges to geotechnical engineers. Large ore-processing structures, such as screening plants and crushing facilities, are designed to vibrate as part of their routine operations. These vibrations force the footing to undergo small-strain dynamic loading that can potentially result in large differential settlements or bearing capacity failures if not founded on suitable material.

A site investigation comprising geotechnical drilling and geophysical MASW testing was performed for a series of dynamically loaded ore-processing structures for a potential mine site in the Pilbara region of Western Australia. The results of the investigation were used to assess the dynamic shear modulus of the in situ soil at each structure location.

To assist the structural and mechanical design of these dynamically loaded structures, an effective dynamic shear modulus value was required to simulate the soil-structure interaction. This effective dynamic shear modulus was calculated using a weighted average of the stress distribution below the footing and by finite layer elastic analysis. For instances where the dynamic shear modulus of the in situ soil was insufficient for design purposes, an assessment of the required depth of ground improvement was also performed as part of the analysis.

Stability and strength analyses in geotechnical problems can be carried out in terms of either effective stresses or total stresses. Given fundamental knowledge of soil mechanics principles and clear understandings of numerical modelling and its limitations, the numerical simulations should result in consistent outcomes from both approaches. Undrained excavations were modelled in ABAQUS, a software application for finite element analysis. The extended Modified Cam Clay was used to model the soil behaviour in the Effective Stress Analysis (ESA) and the Tresca model was used in the Total Stress Analysis (TSA).

For the comparison between ESA and TSA to be valid, it is critical for both analyses to represent identical soil conditions and characteristics. Therefore, the fundamental part of the analytical procedures was to derive the values of total stress parameters from the effective stress parameters and numerical outputs from ESA. In order to confirm the rigorous match of soil conditions between ESA and TSA, initial stress distributions and the initial values of K0 were compared.

The shape of yield surface in ESA was modified to overcome the differences in the yield surface for ESA and TSA. The values of su were also adjusted to reflect the shear strength in the plane strain problem. While those modifications improved results, most of the numerical outputs showed inconsistencies between ESA and TSA. By comparing the maximum values of forces and moments of structural elements, neither method produced results that are consistently greater than the other method throughout all excavation scenarios. It was justified that the differences in the structural forces and moments were mainly due to the differences in the passive stress on the retaining wall between ESA and TSA. The observations from stress paths at those passive soil elements revealed that the passive soil for all ESA did not reach the critical failure state, and for TSA, the soil reached the failure for cantilever problems, but not for propped excavations.

Geotechnical investigations have recently been completed for a mining project in West Africa.

The proposed new iron ore mine is located some 200km inland and will be linked to a new port by approximately 220km of railway.

The presentation provides a summary of the project and discusses the geotechnical investigations that were conducted at the port and along the rail corridor. It also summarises the difficult ground conditions that were encountered, particularly at the port where superficial deposits were found to be underlain by a thick sequence of highly variable marine, estuarine and fluvial sediments.

Some of the obstacles faced during the investigations included challenging site and weather conditions, a lack of available equipment and skilled labour, poor existing infrastructure and a tight project schedule. The talk will discuss how these challenges were overcome without compromising the quality of data obtained or significantly impacting upon the project programme.

It will also reveal how a large amount of effort was spent ensuring that the local community were agreeable with the investigations proceeding in their backyards (literally).

Finally, a number of valuable lessons learnt during the course of the project are presented – with the objective of assisting the planning and execution of future investigations in similar parts of the world.

A riser collector offshore steel gravity base structure (GBS) was installed early this year for a gas development project. The foundation is a square annulus (external dimensions 54x54m, internal dimension 34x34m) with 5m deep internal and external skirts forming 12 compartments to allow for suction installation. Arup (including the author) has been involved in the structural and geotechnical detailed design of the GBS since 2007 and have been providing technical advice throughout the construction and installation phase. In 2009, after completion of detailed design, a jack-up rig servicing the adjacent drilling platform was accidentally installed within the GBS designated site. The jack-up left footprints of three holes in the seabed each with a diameter of about 20m and a depth of about 3m. The client commissioned an additional scope of work for Arup to assess the impact of the spudcan footprints on the GBS foundation and to consider remedial measures.

The spudcan impact investigation was conducted over a period of one year. This included a preliminary assessment phase where a literature study, a centrifuge test, 3D and 2D finite element analyses for stability and settlement, and DNV based installation analyses were conducted. It was found that the GBS could no longer be installed in its proposed location and required reorientation to avoid intersection of the spudcan holes. A geophysical survey, site investigation and further geotechnical analyses were then conducted to assess the foundation at its new location.

Due to time constraints and lack of data, the geotechnical analyses were simplified to assess (i) squashing of heave mounds created by the spudcan which intersect portions the foundation, (ii) additional skin friction on the skirts from the heave mounds, (iii) stability issues due to weakened soil zones, decrease in overburden and lateral resistance, and (iv) differential settlement on the now uneven seabed. Final results indicated that the structure could be safely installed and would be stable over its design life.

This presentation includes an overview of the above events, the findings of the subsequent assessments and concludes with a successful platform installation.