The impact of rock block size on cable bolt performance has been assessed using the Universal Distinct Element Code (UDEC). The results indicate that the interaction between rock mass and reinforcement changes with variable block volumes and highlights that block size plays a key role when modelling and designing support systems. A simple statistical approach for calculating the forces acting along the length of the bar demonstrated that bolt design should not solely based on the maximum predicted loads that commonly concentrate along sliding discontinuities, because this reaction may not represent the overall bolt behaviour and may have been triggered by unrepresentative fracture patterns. Because fracture frequency is difficult to be controlled in UDEC due to interacting joint sets, a simple method for controlling the input block size is suggested to transform borehole or scanline survey data into more realistic block volumes.
The choice of appropriate techniques to evaluate the response of the structural elements used as rock reinforcement in mining and civil engineering projects is both a critical decision and a very subjective matter. In many cases, a simple empirical relationship, a theoretical expression or even the designer’s practical experience may be adequate while, in other cases, sophisticated numerical modelling, in-situ and laboratory testing, validation and redesign may be required to arrive at an effective and economical support solution.
The most common types of reinforcement used to restrict deformation and improve the self-supporting capacity of blocky rock masses are rock bolts, cables and ground anchor bars. Their behaviour is typically assessed based on the maximum predicted loads and a decision is taken by considering the maximum resistance the structural element can display in rock mass deformation. When rock joints are considered explicitly in numerical models, the maximum load-displacement concentrations commonly occur along that portion of the reinforcement where shearing and/or opening or closing discontinuities interact with the support system (Itasca 2011). Although, this observation may result in representative rock-support responses, it may lead to misleading evaluations and the utilisation of conservative support solutions. This is because high loads developed along relatively short lengths of the bar due to localised individual joint movements may not represent the general response of the rock-support system. Additionally, unrealistic high loads may have been triggered due to unrepresentative joint geometrical parameters (e.g. spacing, persistence, number of joint sets, etc.). Therefore, the accurate representation of the structural geology is extremely important when modelling rock masses because the size and shape of rock blocks influence the deformation of the disturbed zone around the excavation (Shen & Barton 1997) which in turn controls the rock-support response and design.
Remotely sensed data, such as high-resolution point clouds of Terrestrial Laser Scanning (TLS), and automated rock structure modelling help us efficiently assess cliff rockslide prone areas where direct contact measurement of fracture characteristics is very difficult. Our approach is performed in four main steps which include: 1) collection of true 3D geomorphology and fracture pattern by means of TLS; 2) visualisation of 3D relief using HSV-colour (H for hue, S for saturation, V for lightness value) and measurement of rock structure; 3) computation of the distinctly located blocks in 3D cliff morphology; 4) assessment of rockfall hazards. The work flow has been illustrated using the limestone escarpment Feldkofel in Carinthia.
There are challenges in rock engineering where the exact spatial structure of a fracture system must be established to evaluate the main risks. One of the examples is the assessment of rockfall hazard in the Alps. For a trajectory modelling and the derived design of structural countermeasures a reliable identification of the position-dependent instable blocks in the detaching area is necessary. Practical examples (e.g. Cabernard et al. 2003, Bauer & Neumann 2011, and Liu et al. 2013) show that some of the source areas are cliffs situated directly a few hundred meters above densely populated areas. How can we get the rock structure data in such a cliff? How can we identify the distinctly located removable blocks there? The answers are primarily relating to an integrated implementation of remote sensing technologies to capture surface structure data, on which the real fracture pattern embedding in 3D morphology should be processed.
Figure 1-A shows the stepped limestone escarpment Feldkofel located in Carinthia, Austria. Figure 1-B is a detailed picture of the top middle area, which is located 540 m above the village road and has a dimension of 80 m (L) × 30 m (W) × 50 m (H). At this site frequent rockfall incidents have been reported since many years. We select this site here to illustrate the application of our methods.
Such cliff rockfall prone areas are innumerable in Western Austria, Northern Italy and Switzerland. Our experiences show that to evaluate the rockfall risk the following engineering geological and geomorphological data are indispensable: the orientation, location and 3D spatial area extent of block-forming fractures; the genetic type and surface roughness of the involving fractures for a reasonable estimation of friction; and the location-dependent geometry of natural slope surface.
Lisjak, Andrea (Geomechanica Inc.) | Tatone, Bryan S. A. (Geomechanica Inc.) | Mahabadi, Omid K. (Geomechanica Inc.) | Grasselli, Giovanni (University of Toronto) | Vietor, Tim (National Cooperative for the Disposal of Radioactive Waste (NAGRA)) | Marschall, Paul (National Cooperative for the Disposal of Radioactive Waste (NAGRA))
This study aims at strengthening the understanding of the mechanical sealing process of the excavation damaged zone (EDZ) in Opalinus Clay, an indurated claystone currently being assessed as the host rock for a deep geological repository in Switzerland. To achieve this goal, hybrid finite-discrete element method (FDEM) simulations are applied to the HG-A experiment, an in-situ test carried out at the Mont Terri underground rock laboratory to investigate the hydro-mechanical response of a backfilled and sealed microtunnel. The mechanical re-compaction of the EDZ is analyzed by accounting for an increase of swelling pressure from the bentonite backfill onto the rock. Simulation results indicate an overall reduction of the total fracture area around the excavation as a function of the applied pressure, with locations of ineffective sealing associated with self-propping of fractures.
In the field of underground nuclear waste disposal, the excavation damaged zone (EDZ) is defined as a zone with hydro-mechanical and geochemical modifications inducing significant changes in flow and transport properties of the rock mass. A sound understanding of the processes involved in the EDZ formation and temporal evolution is necessary to increase the confidence in performance and safety assessment calculations of deep geological repositories. In the short-term, the EDZ is typically associated with an increase of flow permeability of one or more orders of magnitude. In the long-term, the EDZ will experience complex, time-dependent thermo-hydro-mechanical-chemical processes due to the interaction between the rock mass, buffer materials and heat-producing waste. Experimental data from laboratory and in-situ testing clearly show that sealing mechanisms occur in argillaceous rocks, including Opalinus Clay, leading to a reduction in the effective hydraulic conductivity of the EDZ with time (Bock et al. 2010). In this study, the mechanical re-compaction of the EDZ in response to radial stress acting on the excavation walls caused by swelling of the saturated bentonite buffer was numerically investigated. With its explicit consideration of fracturing processes, a hybrid finite-discrete element (FDEM or FEMDEM) simulation approach was applied to the HG-A experiment. The HG-A in-situ experiment was carried out at the Mont Terri underground rock laboratory (URL) to investigate the hydro-mechanical response of a backfilled and sealed microtunnel (Marschall et al. 2006).
The City Rail Loop project is a planned urban railway line for commuter trains under the Helsinki city-centre. The triple tunnels run under the very densely-built city-center peninsula. Three underground stations will be built along the new looped railway. The central downtown station will be constructed under extensive existing underground infrastructure. There are several parking caverns, service and sewage tunnels and an existing metro station in the immediate vicinity of the planned underground downtown railway station. The station will be connected directly into several existing underground facilities including the adjacent metro station. During the construction these existing facilities should be able to operate normally. In addition, major brittle fault zones intersect the planned station. These pose interesting challenges in the project that are dealt with through rock mechanical analysis. The rock mechanical models were used to guide, and advise the engineering design and layout planning of the station caverns.
1 Introduction and Background
The City Rail Loop project, (named ‘Pisara’), is a planned urban railway line for commuter trains under the Helsinki city-centre (Figure 1). The railway is roughly 8 kilometers in length, and is a loop-shaped (Pisara = ‘droplet’ in Finnish). The plan calls for two rail tunnels and an adjoining service tunnel that runs parallel to the rail tunnels. In total, roughly 25 kilometers of drill and blast tunnels will be excavated. This triple tunnel system runs under the very densely-built city-centre peninsula, which is surrounded by the Baltic Sea on three sides. The tunnel will pass under the sea at a narrow strait on the city’s east side. The detailed planning of the railway line is scheduled for 2013-2016.
Three stations will be built along the new looped railway. The downtown station (connects to the Central Railway Station) will be constructed under the extensive existing underground infrastructure. There are several parking caverns, service and sewage tunnels and an existing metro station in the immediate vicinity of the planned underground downtown Pisara railway station. The station will be connected directly into several existing underground facilities like the adjacent metro station cavern. During the construction these existing facilities should be able to operate normally.
The complex station design and the close proximity to, and interconnection with existing underground and above ground spaces pose interesting challenges with respect to the rock mechanics and the engineering design. This paper presents and discusses the rock mechanical analyses that was done to guide, advise and support the engineering design and layout planning of the downtown station caverns and the related access tunnels.
Kumar, Tanmay (Indian Institute of Technology (Banaras Hindu University)) | Garg, Prasoon (Indian Institute of Technology (Banaras Hindu University)) | Rai, Rajesh (Indian Institute of Technology (Banaras Hindu University)) | Shrivastva, B.K. (Indian Institute of Technology (Banaras Hindu University))
The stability of mine waste dump is of vital importance from economic and safety point of view. The internal dumps are composed of a mixture of fragmented rocks and loose soil. Their characteristic is comparable to heavily discontinuous solid mass. The present paper deals with the stability analysis of an internal dragline dump of an opencast coal mine by discrete element method. The discontinuum modelling considerer the discrete nature of the geo-materials found in external overburden dump. The discrete element method uses a circular disk to represent the granular solid mass and their interactions are described by the Newton’s third law of motion. The location of crack has been determine and found in the base of the coal rib. The stable slope angle has been determined for present case and width of coal has been varied to understand their effect on stability of dump slope.
The country’s coal production has increased nearly from 431 MT in 2006-07 to 606 MT in 2013-14 (Chikkatur & Sagar 2015 and Thakkar 2014) (an increase of 38.5%). Still, India is importing about 163 MT of coal to fill the country’s total demand-supply gap (including coking coal). Total annual coal production from open cast mine is nearly 80%. The production of coal can be enhanced from the existing projects by deployment of mega size Heavy Earth Moving Machineries (HEMMs) as well as to extract the coal seams at higher stripping ratios. However, higher stripping ratios lead to problem of overburden dumps and management, which have posed challenge and leads to danger in terms of safety. The overburden material is either dumped in the pit (internal dump) or dumped outside the pit (external dump). Figure 1 shows a typical layout of internal dumping by dragline as well as a heightened dump using the shovel dumper combination. The overburden is removed with the help of dragline and dumped inside the pit creating an internal dump of a certain height. A coal rib is left between the dragline dump and mine benches.
The stability of internal dump is an important aspect of an opencast coal mining. As the production of open cast mining increases, the amount of overburden increases, larger dump height or high slope angle is opted due to limited dumping space in the mine. This will increase chances of dump slope failure. The coal rib is left to prevent dump material from flowing into working area (Figure 1). A number of cases have been reported when the major dump slides and failures have caused substantial damage and interruption on the production circuit (DGMS circular, 2007).
The wear of the bits on a continuous mining machine is a problem that affects mine operators around the world. The fundamental problem (wear) still causes increase in the downtime of the equipment. One aspect, which has been surprisingly overlooked, is the heat developed on the bits during cutting that causes the wear of the bit. Numerical models of the Automated Rock and Coal Cutting Simulator (ARCCS) were developed using the ABAQUS. Two bit designs K-1M and K-2M were analyzed numerically and it was found that the effect of heat developed on the bit not only depends on the friction but also on the bit geometry. Forces developed during the rotary cutting showed that the sharper and the longer bit tips are more effective compared the regular conical bit tips.
Longwall and room-and-pillar methods of mining are the most popular method of extraction of the coal in the United States. These methods use versatile continuous mining machines such as the continuous miners and the shearers for the development and the production from the mine. Bits mounted on the continuous miner and shearer drums are the primary elements that are responsible for coal extraction. Additionally, both the continuous miner and the shearer are affected severely due to wear of bits.
Prominently the worn bits will result in:
1. The downtime of the continuous miner.
2. An inefficient cutting of coal.
3. An increase in dust near the work area.
4. Frictional spark and ignition of gas.
5. An overall economy of the operator.
Clay- and marlstone are low permeable rocks suitable to host radioactive waste. However, their strong layering might cause stability problems, at least when oriented at an unfavourable angle to σ1. We carried out compressive strength tests on these rocks to constrain this “weak bedding plane” effect. We tested plugs whose long axes (parallel to σ1) were oriented with varying angles to bedding, to find out at which angle between lamination and σ1 these lithologies are weakest and strongest, respectively. We observed weak bedding planes in claystone, but not in marlstone. The reason for this is the respective lack or presence of calcite. The presence of calcite in marlstone cements this rock much stronger than this is the case in claystone. Consequently, weak bedding planes occur favorably in fine grained purely clastic rocks and not in chemical-clastic rocks.
Clay-, silt- and marlstone are considered to be suitable host rocks for a radioactive waste disposal site (RWDS) due to their low permeability. However, they are also highly anisotropic rocks because of their strong lamination. Bedding planes may act as planes of weakness, in particular if oriented at an unfavourable angle to the principle compressive stress σ1. We carried out numerous tests on clay-, silt- and marlstone of differing origin to quantify this “weak bedding plane effect” and to find out if there is a general pattern that can be applied to all these fine grained, low permeable and relatively weak rocks. Namely we investigated the Jurassic Opalinus Clay and Effingen Formation from Switzerland, as well as a Dogger siltstone (Dogger ß and γ) from the North German Basin and – for comparison reasons – also a Dogger ß sandstone from the same area. We did compressive strength tests on cylindrical plugs whose long axes (simultaneously the direction of axial loading σ1) were oriented at various angles to bedding (see also Figure 1), to find out at which angle between lamination and σ1 an RWDS host rock is weakest and strongest, respectively.
This paper briefly describes an integrated approach to mine ground water simulation and prediction using an in-house finite element code called COSFLOW developed in CSIRO of Australia. COSFLOW uses a Cosserat continuum theory for the efficient description of load deformation behavior of layered coal measure rocks. Both coupled mechanical deformation and fluid flow model and uncoupled fluid flow model have been used to assess the impact of mining on regional groundwater resources. The coupled model is used to estimate the mining induced permeability and porosity changes in the rock mass and provided inputs to the regional scale fluid flow only model. Comparisons of numerical predictions with mine measurements demonstrate the suitability of such an approach in accurately predicting coalmine water inflow and impact on water resources. First, the model is calibrated using existing extensive mine water inflow and piezometer measurements and then used to make predictions for future longwall panels.
Reliable prediction of rock mass deformation, mine stability, mine water inflow and mine gas emission is not only essential for improving mine safety and reduction of coal production costs, but also important for the assessment of environmental impact of mining. This prediction requires the accurate simulation of complex, highly nonlinear and irreversible mining induced processes including the mechanics of rock deformation and fracture and the consequent water flow and gas desorption and flow.
This paper presents an integrated approach to mine groundwater simulation and prediction using an in-house finite element code called COSFLOW developed in CSIRO of Australia. COSFLOW incorporates (a) Cosserat continuum theory (Cosserat & Cosserat 1909) in its formulation for describing the load deformation of layered coal measure rocks and (2) two phase double porosity fluid flow formulation that can be coupled with either the mechanical model or run separately in isolation. A full description of the COSFLOW code is presented in Adhikary and Guo (2002) and the references cited there.
Both coupled mechanical deformation and fluid flow model and uncoupled fluid flow model have been used to assess the impact of mining on regional groundwater resources. First smaller scale three-dimensional coupled mechanical models were run to estimate the change in permeability and porosity in the strata surrounding the longwall mines and then this information was input into a single-phase regional scale groundwater flow model. The coupled model is fully described in Adhikary & Wilkins (2012) and will not be discussed here.
Durability is one of the key functional properties of natural stones used in architecture and construction. This property is often extrapolated from index physical properties, based on the results of empirical durability tests and/or from the in situ long-term experience. In this recent study, we tested how the data obtained from common rock mechanical tests can be employed as durability estimators. Along with standard rock mechanical parameters (strength, modulus of elasticity), the stress-strain curves were evaluated to obtain crack closure, crack initiation, and crack damage thresholds of several varieties of sandstones commonly used in architectural and sculptural projects. The modulus of resilience and modulus of toughness, computed from the respective parts of the stress-strain curves, proved to have a significant correlation between resistance to weathering in the accelerated laboratory tests and/or to the known field performances of the tested stone varieties.
Durability is the key functional property of natural stones used in construction and for the manufacturing of architectural elements and/or sculptures. Durability is often viewed as the resistance against various weathering processes (Frohnsdorff & Masters 1980 and Lewry & Crewdson 1994); however, in a broader sense, it covers technical serviceability. Durability is not a fundamental property, it can more likely be considered to be the long-term manifestation of repeated interactions between natural stone and the surrounding environment (Soronis 1992). The time period, during which the stone can be claimed durable, is equal to its macro- and microstructural stability, which secures fulfilments of its structural and aesthetic functions (Sims 1991 and Andrew 2002).
Assessment of durability is generally possible through one of five major approaches: (1) practical experience, (2) accelerated laboratory durability test, (3) complex environmental testing, (4) exposure site testing, and (5) extrapolation from other physical properties (Prikryl 2013 and references therein). Each of these approaches has certain advantages and drawbacks. Although practical experience provides the most reliable results, it is highly impractical in terms of sufficient amounts of experimental materials (extensive sampling is often prohibited from heritage structures), time, and knowledge of all past weathering processes that have resulted in the observed decay pattern(s). Accelerated laboratory tests are commonly used for durability assessment because rapid standardized test procedures allow for comparisons between various stone types and/or weathering actions (e.g., freezing/thawing, wetting/drying, salt crystallization, etc.). However, several drawbacks influence their real explanatory power: the size and geometry of test specimens are far from the dimensions and geometries of the real artefacts or structural elements. Consequently, the intensity of damage might not be absolutely comparable. Oversimplification of the weathering process presents another negative factor of accelerated tests, which generally simulate only single weathering action; while during real in situ weathering, numerous actions interact. Generally speaking, most of the accelerated durability tests and post-test evaluation of variable physical properties probably rely on a partly erroneous paradigm.
Date, Kensuke (Kajima Technical Research Institute Singapore) | Narita, Nozomu (Kajima Corporation) | Sato, Hidefumi (Hokkaido Regional Development Bureau) | Saito, Hiroki (Hokkaido Regional Development Bureau) | Kashima, Tatsunori (Hokkaido Regional Development Bureau)
The KITANOMINE tunnel (2928 m, Japan) is a long bored tunnel, which is notable for embracing a 550 m long watertight section. The north part of the tunnel is surrounded by squeezing mudstone, which became heavily multiple fractured and then clay-converted as the tunnel advanced. The deterioration was so severe that the tunnel convergence and the crown settlement reached a non-negligible level. According to this, scrutinizing the responses of tunnel supports to every tunnel excavation, the specification of the tunnel supports has been repeatedly upgraded. A variety of tunnel support patterns were consequently applied: earlier enclosure with supports; profile change (to be more circular); large side-pile installation. They successfully contributed to reducing the ground deformation. This paper presents the modifications of the tunnel support pattern according to the deterioration of the ground conditions and the following reductions in ground deformation around the tunnel.
The Asahikawa-Tokachi Road is planned as a 120 km long regional highway linking Asahikawa City with Shimukappu Village. Connecting the highway with the Hokkaido Expressway and Doto Expressway, it will contribute to helping make a broad transportation network and vitalize inter-regional linkages between people and goods in Hokkaido. The Furano road (8.3 km), a part of the Asahikawa-Tokachi Road, was antecedently launched in 2002 because Furano city had suffered from chronic traffic congestion due to the transportation of agricultural products and the combined traffic from tourists and local residents. The Kitanomine Tunnel (tentative name) is a 2928 m long bored tunnel, a part of the Furano Road, the excavation of which started in 2009. The locations of Asahikawa-Tokachi Road, Furano road and the Kitanomine tunnel are shown in Figure 1.
The tunnel construction site and the surrounding area is rich in water resources and famous for natural scenic attractions. Accordingly, a lot of tourists have visited the Furano city from all over the world. Thus, in order to reduce the environment impact on the natural resources, a watertight section was allocated so that the groundwater level around the tunnel would be quickly recovered after the tunnel lining. The cross section area is 74.9 m2 for the non-watertight section, while 98.5 m2 for the watertight section. The excavated tunnel diameter is 12.6 m, 13.0 m in the same order.