This presentation will discuss the methodology developed for characterising waste dump material strength in the non-homogenous and highly mixed mine waste environment of the BHP Western Australian Iron Ore (WAIO) asset. The method uses drill core logging data, stratigraphic interpretations and field samples applied to a suitable geotechnical linear regression and probabilistic strength analysis to account for natural variation and anisotropy in the mine waste. The project has been recorded such that methods described can be replicated for future validation and reconciliation.  

Mine waste disposal is an intrinsically important aspect of the mine lifecycle; from productive movement and hauling strategy, to long term geomorphic profile and final landform. As such, the designation of a mine waste dump footprint requires coordinated planning and approvals across stakeholder teams including mine planning, geotechnical engineers, production teams and closure specialists. Whilst the dump design is often underpinned by criteria such as proximity to critical infrastructure, mine plan integration and closure requirements, it is the geotechnical stability that will dictate the serviceability and safety of the waste dump. 

Where multiple haulage circuits may exist, the mine waste origin and destination may be decoupled making it difficult to determine the material type once it reaches the dump location. The waste dump is generally highly mixed, variable and composition of several lithological types contributing to the overall strength. This project informs of a practical method to plan for material type by categorising waste dump strength using the geologic stratigraphical interpretations, geotechnical lab testing, geotechnical stability and appropriate Design Acceptance Criteria.

The project included a four phase study: field sampling, geotechnical lab testing and drillhole database extraction, statistical strength derivation and stability modelling. Field sampling included bulk bag and intact rock samples of blasted mine waste to test for geotechnical strength parameters that would later be used to validate the theoretical strength curves generated. It also involved using photogrammetry to determine automated particle-size distributions. The drill hole database step involved deriving Field Estimated Strength (FES), basic friction angle and bulk density from the large geotechnical core logging and lab testing database across the Brockman and Marra Mamba formations, and applying statistical analysis to determine characteristic values by stratigraphic unit.  Strength derivation involved using the Barton-Kjaernsli Shear Strength of Rockfill curve (Barton & Kjaernsli, 1981) to determine central and lower case strength parameters for each unit. The equivalent Mohr-Coulomb strength curves from the field samples were also compared to the shear strength rockfill approximations for validation. The stability analysis involved applying a Design Acceptance Criteria (DAC) for two-dimensional limit equilibrium modelling to determine maximum waste dump tip head heights. 

The results demonstrated the high variation in characteristic strength between commonly mined stratigraphic units that should be carefully planned for and managed. It is suggested that diligent use of the procedure will be a means of providing continuous improvement for input parameters and emphasises the importance of high confidence geological data inputs for geotechnical stability.

The offshore wind industry has, and is expected to continue to grow rapidly as the world seeks secure, renewable sources of energy. Monopiles are currently the dominant foundation type used in this industry. The geometry and loading experienced by these foundations differs from historical offshore structures used by the oil and gas industry, about which design methods and standards have been developed. When applied to monopiles, the widely used p-y method and current design standards tend to be conservative in design for ultimate capacity, and do not adequately capture cyclic behaviour. 

The PICASO (Pile Cyclic Analysis: Oxford and Ørsted) project is a collaboration between Oxford University and Ørsted which aims to address this shortcoming by developing a new design method for cyclic lateral loading. This builds on the success of the PISA (Pile Soil Analysis) project which developed a new design method for monopiles under monotonic lateral loading. 

Medium scale field testing campaigns at two sites are central to the PICASO project, providing information against which to validate the proposed design methodology. The information presented herein specifically relates to the clay testing site, located at Cowden, UK. A thorough characterisation of the site, where ground conditions comprise over-consolidated glacial till, was performed using historical information, and a site investigation and laboratory testing program conducted as part of the PICASO project. Key results from the site characterisation are included for context to the subsequent research. 

A series of 11 tubular steel test piles were installed across the site using impact and vibratory driving methods. The test pile diameters ranged from 1.22 to 2.5 m and had consistent embedded length to diameter ratio (L/D) of 3 to reflect the expected future geometry of monopiles for offshore wind turbine foundations. This presentation includes an overview of the instrumentation and methodology employed for both the pile installation and load testing phases of work, highlighting select interesting results.  

High frequency strain measurements using optical fibre Bragg grating (FBG) sensors over the embedded length of the piles, in addition to conventional pile driving analyser (PDA) sensors, provided a unique opportunity for installation analysis. The successful application of a novel stress wave analysis approach is summarised and the results, in terms of Soil Resistance to Driving (SRD), are compared to the resistance predicted using the widely adopted Alm and Hamre (2002) empirical method.

The methodologies applied to interpret above and below ground pile response during cyclic lateral loading are described, notably Timoshenko beam theory with least-squares optimisation for uni-directional tests and a matrix transformation approach for multi-directional tests. The presentation culminates with discussion of initial insights into pile behaviour for cyclic lateral loading of increasing severity from a pile subjected to 1-way sinusoidal loading at different cyclic magnitudes.

Subsea infrastructure such as seabed pipelines, cables, and shallow foundations are founded either directly on the seabed or shallowly buried. Interface friction affects their geotechnical design. Accurate characterisation of near-surface soil layers requires test methods and devices capable of measuring the frictional properties of the soil-structure interface at very low effective stresses of the order of 1-10 kPa. However, conventional laboratory testing equipment is not well adapted to these applications. 

To address this challenge, the ring penetrometer, a type of shallow penetrometer, has been developed at the University of Western Australia. This device enables interface friction measurement at stress levels as low as 1 kPa. It is actuated by vertical and rotational movement with control of the applied normal stress (and stress history), rate and sequence of shearing movements, and interface material and roughness. Its flat geometry allows for straightforward interpretation and provides a direct measure of interface friction, comparable to interface shear box tests – the current de-facto standard for interface friction measurement. 

This presentation will showcase recent applications of the ring penetrometer across a range of soils, including kaolin clay, coarse and fine-grained silica sands, and offshore calcareous silt and sand from the North-West Shelf of Western Australia. In clay and silt, strain rate-dependency of interface friction is explored, considering the effects of strain-softening, consolidation, and shearing rate. In sands, key findings include the observation that drained residual friction is independent of normal stress level for ring penetrometer and centrifuge sled tests, unlike in shear box tests. A strong correlation is identified between the mobilised friction coefficient and the rate of device settlement, with maximum friction mobilised only after device settlement ceases, consistent with a bearing-sliding mechanism. This behaviour has direct implications for the axial friction on on-bottom cables and pipelines, particularly when their weight exceeds 20% of the soil’s bearing capacity. 

The presentation will provide compelling evidence of the capabilities of ring penetrometer testing and the robustness of the associated interpretation methods across a range of soil types, offering new insights into interface behaviour and encouraging wider adoption in geotechnical practice.

The Yanchep Rail Extension (YRE) is one of Perth’s most ambitious public transport program of works. It will provide a strategic incremental extension to Perth’s integrated public transport system by extending the Joondalup line to Yanchep. One of the design elements used for the retaining walls at YRE are permanent cantilever contiguous piled walls founded in the Tamala limestone Formation, which is the common geology unit encountered in the YRE alignment.

Design and construction of permanent cantilever piled walls in Tamala limestone Formation can present unique challenges due to its geological complexity, and while there were numerous permanent cantilevered piled walls used in YRE, with thousands of piles installed by multiple piling rigs and companies during peak construction, the deepest of these piled walls was a maximum 12.3 m piled wall located between Howden Parade and Romeo Road (termed as RWE-203A), which followed the contours of an existing sand dune.

This presentation will explore the geotechnical aspects of the deepest piled wall in the YRE project, which includes an overview of the adopted design basis for the numerical modelling of the retaining wall and how it compares to the findings from the additional geotechnical investigation (consisting of geophysics and pressuremeter testing), construction supervision, ongoing wall monitoring, and back-analysis of the numerical model based on the available monitoring data. The findings from this presentation are expected to contribute to a more informed approach to the retaining wall design of permanent cantilevered piled walls in the Tamala limestone Formation.