Depressurisation and slope design

The Southern Ridge Cutback 3 (STR3) at the Tom Price mine site will be the highest and steepest slope in Rio Tinto Iron Ore's Pilbara operation. Initial geotechnical assessment of the STR3 western slope using two-dimensional limit equilibrium methods recommended a substantial flattening of the design. This would have resulted in the deferral of 3.2 Mt of high grade ore. Given the good performance of the preceding STR2 cutback, it was considered that the two-dimensional (2D) analysis results were not representative of the expected stability and were overly conservative. Structures constraining the dominant mode of instability strike oblique to the slope. This aspect and the effects of 3D lateral confinement are not considered by 2D analysis. In order to address this, a 3D modelling project was initiated with development of a 3D model by Itasca Australia Pty Ltd. The 3D model method utilises both 3DEC™ (Itasca 2013a) and FLAC3D™ (Itasca 2012) software to develop a constitutive model that takes into account the dominant bedding anisotropy within the slope. As this was the first such model developed for Rio Tinto Iron Ore (RTIO), an external review board was appointed to provide technical guidance during the project. Sensitivities were carried out to address questions regarding in situ stress regime, pervasive joint orientations relative to bedding, potential for large scale wedges, and ore friability. 3D modelling results were favourable and indicated that the existing design exceeded stability acceptance criteria. In addition, further optimisation of the slope was possible and would realise an additional 1 Mt of high grade recovery. In order to achieve this, revision of the slope design configuration was required. Work supporting this included assessment of the viability of 90 degree batter face angles and a rerouted haulage design. Batter-berm configurations and placement of wide geotechnical berms were tested for inherent stability and rockfall risk management effectiveness. Overall stability of the revised slope design was confirmed by the Itasca modelling. Some areas of potential local instability were identified and have been addressed by detailed design changes. This project demonstrates the potential value add that can be realised by 3D analysis, when compared with traditional 2D methods. Due to the high value of the STR3 ore as a blending material, this slope is being mined at a relatively high strip-ratio when compared with other RTIO Pilbara pits. This emphasises the potential impact of the protect plan and optimisation outcomes.

Slope Stability 2013, Brisbane, Australia 513 D.S. Lucas Mining One Consultants, Australia P.J.H. de Graaf Rio Tinto Iron Ore, Australia Rio Tinto Iron Ore's (RTIO) Tom Price open cut mine in the Pilbara region of Western Australia commenced in 1966 and had a cumulative production to mid 2012 estimated at over 800 Mt. The South East Prongs (SEP) pit at Tom Price hosts one of the mine's prime sources of high grade and low impurity hematite ore. An integrated mine planning and geotechnical design approach was required to evaluate and optimise late stage mining design options within large scale structural geological (adverse bedding strength anisotropy and fault) and hydrogeological controls. The SEP pit is structurally complex. The orebody is hosted within a doubly-plunging syncline bounded by the low-permeability shale, with predominant east-west striking faults confining the mineralisation to within the central part of the syncline. Multiple deformation events have resulted in significant folding and additional faulting. The complex structural geology, with bedding dipping into the pit void, and numerous faults, present a challenging geotechnical environment to design and implement robust mine design to maximise late stage mining ore recovery. By mid 2010, the SEP pit had been mined to between 600 and 640 RL. Mining ceased after accelerated movement was detected in the north wall. Additional geotechnical investigations were undertaken in late 2010 and 2011. This augmented information obtained from previous investigations. Stratigraphic units and faults were defined by mapping and geophysical logging, but areas of uncertainty remained, particularly in some fault locations, which could not be improved by further drilling. The planned final 375 m high pit shell is to extend 30 to 70 metres deeper than the current mining levels, and about 100 m below the regional water table. A modified pit design was required to fit within a constrained region of the pit, to minimise the likelihood of fault controlled instability impacting the main access ramp on the west wall, and to account for dewatering and depressurisation requirements. The project geotechnical team, consisting of RTIO geotechnical personnel and their geotechnical consultants (Mining One) worked closely with RTIO's mine design team to develop workable risk assessed options which enabled an optimum design to be adopted, with a detailed hazard and risk management plan for implementation during mining. The final design was optimised to allow access by a large equipment fleet for more efficient mining, while deferring key geotechnical risks to a later stage of mining to provide greater reliability of ore supply for the short-term mining schedule and opportunity to improve design reliability as mining proceeds.

The widening of the Tuen Mun Highway involved demanding aspects of site investigation, design and construction due to the combination of existing steep cut slopes, an extensive weak zone and heavy adjacent traffic. The application, for the first time in Hong Kong, of high energy rockfall protection fences and novel tie-back retaining wall design arrangements contributed to the successful completion of one of the most difficult highway realignment and widening programmes ever undertaken in the Territory. A fatality during a previous attempt to widen the highway led to legal Mediation proceedings, the outcome of which included stringent safety requirements at all stages (investigation through to operation). Consequently the selected design team included specialist rock engineering consultants sub contracted to the main consultant. This paper concentrates on aspects of the geotechnical design solutions developed to improve the stability of the existing rock cuts adjacent to the highway, and outlines the various workable rock-engineering solutions developed to achieve the difficult task of safely widening the carriageways.

Construction of a novel retaining wall and implementation, for the first time in Hong Kong, of high energy rock fall protection fences contributed to the successful completion of one of the most difficult highway realignment and widening programs ever undertaken in the Territory. The Tuen Mun Highway is one of the major arterial roads linking Hong Kong to China and provides a busy commuter link between the New Territories settlements and Hong Kong Island. This highway has seen some of the region's worst traffic congestion, and in order to alleviate it, a highway realignment and widening program was selected as the most appropriate solution for increasing capacity. Initial works on the widening were initiated in 1994. However, in August 1995, a rock fall occurred resulting in the loss of life of a driver using the highway. Following this incident, the Contractor claimed the contracted design for the Tai Lam Section was inconstructable and a subsequent mediation of the claim found in favour of the Contractor. This paper concentrates on certain aspects of the geotechnical design solutions developed to achieve constructability of this challenging highway widening. Some discussion is also presented of the design phase site investigation work, and of the key geotechnical issues that arose during construction.

Previous studies have indicated that a 40 m thick, at approximately 40 m depth, layer of Tertiary clay, will present significant challenges relating to slope and waste dump design as well as operational challenges (trafficability and handleability). Due to uncertainty over the confidence of parameters derived from triple tube sampling and testing, conservative design parameters were adopted for the previous studies. It was recognised that significant upside could be realised through improved material characterisation techniques. The main objectives of the investigation and testing program were to: 1) characterise the physical properties and variability of the clay; and 2) obtain design parameters for the slopes for mining of the pit. This was carried out with conventional drilling, sampling and logging procedures. Hyperspectral scanning of diamond core was undertaken, which provided useful information on the vertical distribution of clay species through the detrital stratigraphy. Following review of the historic sample test results, it was hypothesised that sampling and testing methods had contributed to pre-softening of samples. Subsequent testing focused on Shelby tube and in situ testing; both resulted in significant improvement in measured shear strength parameters over triple tube sampling. A self-boring pressuremeter was used in two locations to carry out in situ testing of the clay at various depths. The results of the pressuremeter testing have indicated that the permeability of the clay is significantly lower than assumed in previous studies. An assessment of the effect of the structure of the clay on the stability of the slopes will be an objective of further studies. Key outcomes so far have emphasised the significant impact of sampling method, preservation, timely testing on derived material properties, and recognition of the limitations of various techniques in characterising heavily over consolidated clays. The material characterisation has also confirmed the critical importance of water management in mining in relation to preserving clay strength. This work has enabled detailed planning to be undertaken for studies covering trafficability, handleabilty, waste dump and slope performance; with field trials scheduled in the mine plan. Previous studies have indicated that in the area of the proposed mining there is a layer of clay, the upper tertiary lacustrine clay (UTLC), approximately 40 m thick, at approximately 40 m depth, through which the mine excavation will penetrate.

Dewatering at the South East Prongs (SEP) pit has lowered in-pit water levels approximately 81 metres below the pre-mining water table. As a result a steep hydraulic gradient has developed behind the pit slope as the more permeable ore material has been dewatered and water levels (pore pressures) remain elevated in the less permeable waste rock materials. The SEP geological setting is complex, with more than 7 tectonic structural events identified. The main orebody is located within a large syncline, disrupted by numerous faults and dykes. Consequently, the hydrogeological model is complex and compartmentalised. Geotechnical design assessments have determined that without depressurisation the required Factor of Safety (FoS) to satisfy design acceptance criteria cannot be achieved. Historically, generic depressurisation designs have been utilised, and have been successful in achieving stable slope objectives. However, until recently limited monitoring of pore pressures was undertaken to optimise these designs. To address this, a total of 82 horizontal depressurisation holes were completed from 2009 to 2010 and the resultant vibrating wire piezometer (VWP) data analysed to characterise the mechanisms controlling slope depressurisation. Four distinct response trends were identified. The Observational Method, as defined by Peck (1969) has been used to design and optimise the depressurisation drilling programme for the final pit design, and to update the slope design recommendations for the remaining life of mine. Practical application of the Observational Method (supported by comprehensive instrumentation monitoring arrays) aims to save costs and time whilst maintaining an acceptable FoS by optimising depressurisation drilling requirements.