The Susten Pass road in central Switzerland was constructed from 1938 to 1946 and follows the topographic feature caused by the northern boundary of the intrusive body, known as the Aar massif, in the central Alps. The road has since become a major tourist attraction, but the tunnels no longer fulfilled requirements for traffic space and safety. During winter the road was covered with ice from inflowing water, and the tunnel lining had deteriorated due to freeze-thaw effects. The true ground conditions did not reveal themselves prior to the reconstruction of the tunnels and caused various problems, once in the tunnel and near the portals. The shear zones were all of different character, but are related to the multiphase tectonic sequence of the zone during formation of the Alps.
1 GENERAL DESCRIPTION
The Susten Pass is located in central Switzerland and runs parallel to the main structure of the Alps. It links Innertkirchen in the Bernese Oberland to Wassen on the Gotthard road which is the main northsouth link through the Alps (Fig. 1). The 36 km long road over the Susten Pass (elevation 2,224 m) was built from 1938 to 1946 and was initially considered a strategic road within the Swiss defence planning. After World War II the road became a major tourist attraction as it passes through an area of particular natural beauty (Zschokke 1997) and is part of practical and popular round trips over several Alpine passes. The 14 km long section, which is open to traffic all year round, begins at the western base of the Susten Pass road at Innertkirchen (elevation 622 m) and reaches the village of Gadmen with a population of a few hundred inhabitants at 1,200 m altitude.
Linking resource modeling and geomechanical numerical modeling tools is arguably a step towards a more integrated mine design process. This paper presents a methodology that aims to integrate tridimensional data, modeled through resource modeling tools, into a tridimensional geomechanical modeling code. Numerical experiments are then conducted on the created model. The objective of the experiments is to establish the impact of internal fracturing and fracture degradation on slope stability.
Nowadays, numerical analyses are performed on a routine basis to study the stability of rock slopes. On the other hand, resource modeling and mine optimization software tools are used everyday to establish the geological resources, ultimate pit and mining sequence for an open pit mine. This paper presents, through a case study, the use of resource modeling tools to define the problem geometrical characteristics within a numerical modeling code. It also presents a series of numerical experiments aimed at establishing the impact of internal fracturing and fracture degradation on the stability of the slope.
2 RESOURCE MODELLING
2.1 Resource modeling tools
Resource modeling is at the very core of today’s mining operations. In this case study, the rockmass was modeled using one of the most popular resource modeling and mine planning tools in the mining industry, Surpac Vision, Surpac Minex Group (2006). Ore reserve estimation relies on the analysis of rock samples obtained through diamond drilling. Block modeling is used to represent the spatial distribution of ore grades. Ore grades are interpolated at those blocks, based on geostatistical methods. The size of the selected block is usually dictated by the diamond drilling pattern and the mining bench height. Each block is assigned various properties such as ore grades, level of contaminants, geology and rock properties.
The oil and gas industry is exploring reservoirs under increasingly difficult geological conditions. Accessing these reservoirs requires drilling through unconsolidated formations, faulted rocks, rubble zones and salt structures. The formations not only have highly overpressured pore pressure and abnormal insitu stresses, but also have very low fracture gradients. With extremely narrow drilling mud window, drilling engineers experience a high frequency of wellbore stability problems during well construction, causing substantial non-productive time in drilling operations. To keep wellbore from shear failure and tensile failure in this narrow mud weight window, wellbore strengthening is crucially important. This paper presents a new method to strengthen wellbore by reducing both rock shear and tensile failures. This novel method is based on the premise that building an impermeable boundary condition at wellbore wall can avoid drilling mud invasion into pore spaces of the formation. This reduces pore pressure increase near the wellbore, and also reduces effective tangential stress and increases effective radial stress. Poroelastic solution with permeable and impermeable boundary conditions is developed to analyze pore pressure and drilling fluid interaction and wellbore stresses changes induced by drilling. Numerical study of this poroelastic solution shows that for the same mud weight the wellbore is stable for the impermeable mud condition; however, it is unstable for the permeable case.
A considerable number of oil and gas reservoirs in the world are located in unconsolidated sands or in the naturally fractured formations. These complicated formations usually have not only highly overpressured pore pressure and abnormal in-situ stresses, but also very low fracture gradients. Consequently, this causes an extremely narrow safe drilling mud window (Figure 1), and drilling engineers experience a high frequency of wellbore stability problems during well construction, inducing substantial non-productive time in drilling operations.
Neubauer, M.C. (Australian School of Petroleum, University of Adelaide) | Hillis, R.R. (Australian School of Petroleum, University of Adelaide) | Reynolds, S.D. (Australian School of Petroleum, University of Adelaide) | King, R.C. (Australian School of Petroleum, University of Adelaide)
Knowledge of the contemporary stress field is vital to the petroleum industry for assessing trap integrity and establishing drilling directions and mud weights to optimise wellbore stability. High quality image log data from 63 petroleum wells in the Carnarvon Basin were analysed for borehole breakouts and drilling- induced tensile fractures to ascertain the orientation of the contemporary horizontal stresses. Orientations were consistent across the basin, with a regional mean maximum horizontal stress orientation of approximately 105°N. Over 80% of the mean borehole breakout and/or drilling-induced tensile fracture orientations of each well, showed mean stress orientations within 15° of the 105°N mean orientation. Stress orientations were also consistent in the vicinity of faults, contrary to previous interpretations from caliper logs, where faults in the area locally perturb the stress field. A preliminary investigation into the magnitude of the contemporary stress field of the region suggests the Carnarvon Basin is in a strike-slip faulting environment, implying that the most stable drilling direction is horizontal. The majority of faults in the Carnarvon Basin are steeply dipping and strike north-south and northeast-southwest. These fault orientations are not at risk of reactivation in the regional stress field. Results from this study are included in the Australian Stress Map database, which is becoming increasingly recognised for its vital importance to petroleum professionals as a source of contemporary stress information throughout the Australian continent.
The key driver of this study was to investigate the occurrence of anomalous northeast-southwest maximum horizontal stress (SH) orientations derived from caliper logs (Mildren 1997) in the otherwise consistently oriented east-west regional stress field. The extent to which faults perturb the stress field is particularly significant to the pre-drill prediction of stress orientations.
In this paper a numerical fracture model based on the displacement discontinuity boundary element method and a finite difference method is presented for solving the problem of coupled rock deformation, fluid transport and interface slip associated with hydraulic fracture propagation across frictional interfaces. The rocks on both sides of the interface are assumed to be impermeable, isotropic, and elastic materials. A Newtonian fluid is injected at a constant rate into the hydraulic fracture. The propagating fracture may induce a tensile stress in excess of the rock strength on the intact side of the interface. In addition, if the interface fails in shear, frictional sliding can also induce tensile stress that may result in fracture initiation in the intact rock. A simple criterion based on the critical tensile strength of the intact rock is used to predict whether a new fracture can initiate at any position along the interface. In the model, only one new fracture is permitted to be introduced. The crossing interaction gives rise to step-over or straight fracture paths depending on the rock strength. The newly-created fracture can be propagated further by fluid entering and pressurizing it, which finally provides the hydraulic fracture a way to cross or escape the interface. Numerical results are presented as a function of rock strength for the time-dependent variations of fluid pressure, crack opening and fluid lag, fracture paths with and without offsets on the interface. Special attention is devoted to the problem of fluid flow associated with different opening profiles and fluid loss from the main fracture into secondary interface fractures during fracture crossing.
A propagating hydraulic fracture often intersects and interacts with existing frictional interfaces in layered sedimentary rocks.
Diederichs, M.S. (Geo-Engineering Centre at Queen's University and Royal Military College) | Lato, M. (Department of Geological Sciences and Geological Engineering, Queen's University) | Quinn, P. (Department of Geological Sciences and Geological Engineering, Queen's University) | Hammah, R. (Rocscience Inc.)
This paper is intended to illustrate the applicability of Shear Strength Reduction (SSR) as a general technique for obtaining Factor of Safety estimates for slopes in variable geology, progressive or locally brittle yield behaviour and with ground-structure interaction. Comparisons are made with Limit Equilibrium solutions (LEM). Implications of the assumptions required to ensure this correlation are discussed including uniform stiffness, rigid-plastic behaviour, instantaneous interaction between geological units, and instantaneous generation of support loads. The paper uses Finite Element Modeling (FEM) as a vehicle for demonstration although Finite Difference solutions are equally valid. General applicability of the method is demonstrated and the limitations explored using Discrete Element simulation of multi-block slope failure.
The Shear Strength Reduction (SSR) technique (Matsui & San 1992, Dawson et al 1999, Griffiths & Lane 1999, Cala & Flisiak 2003, Hammah et al. 2005a, 2006) enables finite element (or finite difference) techniques to be used to calculate factors of safety for slopes, providing an alternative to limit equilibrium calculations and a potentially more reliable analysis of slopes with heterogeneous stiffness, strain-softening and passive structure-ground interaction. The methodology is general and can be applied to other non-linear problems such as multiblock discrete element simulations. Geotechnical engineers primarily conduct slope design based on calculated factor of safety values. Limit Equilibrium (LEM) techniques that compare resisting forces to driving forces (or moments) are ideally suited to the generation of a nominal safety factor (Krahn 2003). Instantaneous slide surface mobilization and consideration of stresses and forces independent of pre-failure movement are inherent in these techniques, and may result in inadequate representation of the system’s actual stability state. Non-linear modeling using the Shear Strength Reduction (SSR) technique can also used to determine factors of safety.
Seismicity and rockburst damage is a common hazard in deep underground mining operations all over the world. Most standard ground support schemes are not or only limited able to cope with dynamic loading caused by rockbursts. High-tensile steel wire mesh has proven its performance and suitability for the application in rockfall protection systems where the load is very similar to rockbursts. Since the boundary conditions in mining are different to rockfall barriers, quasi-static mesh tests with ground support boundary conditions were carried out in collaboration with the Western Australian School of Mines (WASM). With these results, it was possible to calibrate a numerical model based on the finite element software FARO. This software was especially designed, calibrated and validated by the Swiss Federal Research Institute WSL for highly flexible rockfall protection systems. After the calibration of the software, it is possible to simulate dynamic impacts into different ground support setups. The energy absorption capacity, the maximal deflection, the anchor forces and the failure mode for a case study are presented in this paper. It is planned to validate these results with dynamic tests at the dynamic testing facility of the Western Australian School of Mines (WASM).
The primary ground support in underground mining operations without rockburst hazard mainly consists of weld mesh panels and friction bolts. By going deeper and deeper, the mines experience increasing seismicity and according rockburst damage. In order to cope with this hazard, especially designed rock bolts with better energy absorption and elongation capacity were introduced. However, in the area of surface support, the only strategy at the moment is to use thick, fibre or weld mesh reinforced shotcrete which tends to be expensive.
SSR (Slope Stability Rating) classification system is a new rating system which has been proposed in Iran to study the stability of fractured rock slopes of non-structurally controlled failures. In this system, the stability can be evaluated by means of slope design charts. In this study the system is reviewed using 46 slope cases from Iran and Australia. Using the above mentioned design charts, the recommended stable angle for each slope was compared with the current slope conditions. This comparison showed that, in the design charts, increasing of stable excavation angle by slope height reduction is partly large in the cases that the height of slope is less than 100 meters. In addition the design curves overestimate stable design angle especially for flatter slopes. As a result of this work, the modified SSR system rock slope design charts for maximum excavation angle (FS=1.0) and also for some conservative excavation angles (FS=1.2, 1.3, 1.5) are presented.
Estimates of rock slope stability are required by the civil and mining engineering industry for a wide variety of projects. . In these projects, it is necessary to make use of classification systems to estimate the stable angle of a required or existing slope. Many rock mass classification systems have been developed over 100 years since first attempt were made to formalize an empirical approach to tunnel design. Some of classification systems like RMR (Bieniawski 1973) and Q (Barton & Lunde 1974) have gained broad acceptance in the civil and mining industry, while others, such as those suggested by Terzaghi (1946) and Palmstrom (1996) are specific to underground openings. All of these rock mass classification systems have been applied successfully in tunneling and underground mining, but most of them have limitations and shortcomings, when it comes to rock slope problems.
Geological storage of CO2 in disused oil and gas reservoirs is one of the potential techniques to reduce CO2 emissions into the atmosphere because of the economic benefits that incremental oil recovery can bring in a tight energy market. However, the acceptance of this new paradigm will require a perception of geological storage as a safe and environmentally sound practice. Therefore, it is necessary to make CO2 storage predictable to avoid any negative impacts to the environment or society and implement a carbon emissions market. In the short-term, or injection stages, the main trapping element is a competent caprock, and its performance is a vital component of the risk assessment of any CO2 storage project. Geomechanics plays a key role in the performance assessment of the caprock and the reservoir as the hydraulic integrity of this system must be ensured both during the exploitation and production stages (pre-CO2 injection), and during CO2 injection in any CO2-EOR storage project. Phase One of the IEA Weyburn CO2 Monitoring and Storage Project offered an unique opportunity to conduct a geomechanical performance assessment of a caprock system overlying a large scale CO2-EOR storage project.
The IEA GHG (International Energy Agency, Greenhouse Gas R&D Programme) Weyburn CO2 Monitoring and Storage Project (Weyburn Project), coordinated through the Petroleum Technology Research Centre in Regina, Saskatchewan, was initiated to study the potential for geological storage of CO2 in a depleting oil field in southeastern Saskatchewan and to investigate methods of monitoring the movement of CO2 in the subsurface to: 1) enhance the effectiveness of the miscible flood; 2) determine the potential of the reservoir to serve as a vessel for long-term (ca. 5,000 years) storage of the anthropogenic CO2; and 3) to determine the economic feasibility of longterm storage.
“Hazard” in this context is a relative index measuring risk of ground failure. We have developed a 3D GIS-based application that accepts a wide range of real-time data inputs to compute and display a user-defined hazard index on a mine model or other geotechnical model. It is applicable to underground and open-pit mines, tunnels, dams, slopes, and general seismic risk evaluation. In principle it can be used to estimate risk of ground failure of any type. We present a case study from an underground mine in which the system is used to create a relative hazard index as a function of direct observational reporting, microseismicity, excavation geometry, stress, geological structure, and rock type. It responds in real time to data updates and presents a full 3D visualization of hazard index with respect to all aspects of the mine geometry.
This work is an extension and adaptation of previously successful work in multi-disciplinary, 3D integrated data interpretation in mineral exploration applications (McGaughey 2006). The previous work has resulted in significant exploration success, including new orebody discoveries based on the analysis of historical data. Here we apply a proven conceptual methodology for extracting optimum interpretation from complex, multi-disciplinary, 3D data and apply it to the difficult, general challenge of geotechnical hazard estimation.
2 THE “COMMON EARTH MODEL” Successful geotechnical understanding and monitoring of hazardous sites depends on the ability to evaluate dynamically changing ground conditions quickly and accurately. This depends on an accurate and timely understanding of multiple three-dimensional data sets from which we discern the relative level of geotechnical hazard. Whatever we are able to infer from our data becomes our geotechnical “model” of the site, which is used to justify both short-term and long-term safety and economic decisions.