Unstable Rock Research Articles

Анализ и обоснование выбора составов для ограничения водопритоков при заканчивании скважин

The aim of the work is to increase efficiency of well completion under conditions of water inflow by use of viscoelastic gas-liquid mixtures to shut-off permeable formations. At present, there is an increase in rates of drilling of wells in abnormal conditions, such as abnormal formation pressure (both low and high), unstable rocks, rocks of high hardness, permafrost etc. The quality of well construction in such conditions influences subsequent development and operation of the field greatly. Aquifer isolation is an extremely important issue due to the fact that from them water breaks through in production wells which has a significant impact on quality of fluid produced. The main solution for water breakthrough challenge is formation isolation, which is performed by use of various plugging material. At present, there are many mixtures that limit water inflows such as fast-setting plugging materials, gel-cement mixtures, polymeric swelling nets, latexes, synthetic resins, viscoelastic mixtures, materials for selective isolation etc. Under conditions of abnormally low reservoir pressures, it is important to consider density when selecting drilling fluids and plugging materials. Therefore, it is recommended to use screens based on viscoelastic three-phase stabilized gas-liquid mixtures for temporary blockage of permeable aquifer. With use of such mixtures liquid penetration flow rate is significantly reduced compared to other viscoelastic systems, which makes it possible to increase reliability of temporary isolation of aquifers. The paper presents an analysis of domestic and foreign experience of blocking permeable formations by different mixtures. Three-phase blocking mixtures are studied and requirements for them are generalized.

Simple Correlations between Rock Abrasion and Other Significant Rock Properties for Rock Mass and Intact Quartzite

Rock abrasion plays a significant role in geotechnical design, tunneling operations and the safety of foundations from scour. It is imperative to determine such properties of uniaxial compression strength (UCS), rock quality designation (RQD) and hardness for rock engineering to help determine the amount of scour at foundation locations in order to prevent structural collapse, wear on drilling tools and help predict unstable rock conditions. Current practice for estimating maximum rock abrasion is based on the Los Angeles abrasion test; however, more research is needed to provide a more accurate and compatible method for all subsurface materials used in mining and civil engineering projects. This report will provide simple correlations relating abrasion resistance to RQD, UCS, Geological Strength Index (GSI) and Rock Mass Rating (RMR) and shear strength of metamorphic rock (Quartzite). Methods, results, recommendations and conclusions are presented. The paper also introduces recommendations for future rock abrasion techniques and discusses the use of these correlations exhibiting strong relationships between the mentioned rock properties.

Deformation features and failure mechanism of steep rock slope under the mining activities and rainfall

Underground mining activities and rainfall have potential important influence on the initiation and reactivation of the slope deformations, especially on the steep rock slope. In this paper, using the discrete element method (UDEC), numerical simulation was carried out to investigate deformation features and the failure mechanism of the steep rock slope under mining activities and rainfall. A steep rock slope numerical model was created based on a case study at the Wulong area in Chongqing city, China. Mechanical parameters of the rock mass have been determined by situ measurements and laboratory measurements. A preliminary site monitoring system has been realized, aiming at getting structure movements and stresses of unstable rock masses at the most significant discontinuities. According to the numerical model calibrated based on the monitoring data, four types of operation conditions are designed to reveal the effect of mining excavation and extreme rainfall on the deformation of the steep rock slope.

Rock Deformation Behavior Near Excavations Under the Influence of High Tectonic Stress in Coal Seam V-12, “Severnaya” Mine, JSC “Urgalugol”

The article is devoted to determination of roof bolt parameters in coal mine excavations when there is rather high tectonic stress. The work was complicated by the presence of considerable number of fractures in the adjacent rock of seam V-12 in “Severnaya” mine (JSC “Urgalugol”) as well as variable stress field, low roof rock strength and heterogeneity, considerable rock shift during excavations development and maintenance. The article presents the results of horizontal and vertical stress measurements in the rock mass, physical and mechanical properties, roof rock structure and fracturing. According to the results of boreholes study by means of borehole endoscope the authors fully confirmed the forecasted parameters of areas with roof rock discontinuity, the boundaries of unstable rock. The article presents the measurement results of roof rock shift during development face advance. Based on the research, recommendations on excavations development and proper support using were given under conditions of seam V-12 in “Severnaya” mine (JSC “Urgalugol”).

Исследование возможности применения конвейерного транспорта в сложных горно-геологических условиях глубоких высокопроизводительных карьеров

The conveyor transport will remain as one of the most perspective types of transport for high-performance deep quarries in the near future. This is due to its main advantages, such as high efficiency, relatively low cost of transportation of the rock mass, large-angle lifting the rock mass, and others. The modern inclined conveyers could convey the rock mass with different physical and mechanical properties from soft to roach rock with designed rake angle up to 90 degrees. From the perspective of open-pit mining and conveyance of the mined rock on the surface, conveyors are established from 16 degrees to the angle equaled to gradient of slope of spoil bank. At the same time, the placement of conveyor is implemented on spoil bank in a steep trench or foot pieces, mounted on the safety berms of rock and half-rock solids. However, such conditions conveyors accommodation is not suitable for complex geological conditions when the quarry side stacked unstable rocks. The paper presents the study on evaluating the possibility of establishing conveyor lift in complex mining and geological conditions of quarry.

Cut slope stability assessment along ghat road section of Kolli hills, India

In the present study, cut slope stability assessment along ghat road section of Kolli hills was carried out by using various geotechnical parameters of rock and soil slope sections and structural kinematics of major discontinuities is presented. The rock slope (RS) stability assessment was carried out using Rock Mass Rating basic (RMRbasic) and Slope Mass Rating (SMR) classification systems. The type of failure and their Factor of Safety (FOS) for individual RS was calculated using Hoek and Bray method. In the case of soil slopes (SS), the FOS was calculated using Circular Failure Chart (CFC) and Limit Equilibrium (LE) methods. The input data for the slope stability analyses were collected through extensive field work followed by stereonet plotting and laboratory test. There are six rock slope sections, and five soil slope sections were taken into consideration for the cut slope stability analyses. The area depicts class II (RS-1, 2, & 6) and class III (RS-3, 4, & 5) of RMR classes. The SMR result depicts for RS-1, RS-2, and RS-6 are 64.40, 60.02, and 60.70, respectively, and falls in class II stable condition. The SMR values of RS-3 and RS-5 were 44.33 and 57, respectively, and come under the class III partially stable condition. The RS-4 with SMR value of 17.33 falls under the class I completely unstable condition. The FOS of planar failure case indicates that RS-3 (FOS = 0.22) is more unstable, while all other sections are having greater than 1 FOS. The calculated FOS values using CFC method reveals that the FOS is very close to 1 for all the SS sections that fall under completely saturated condition which indicates that these slope sections may fail during heavy rainfall. In LE method, the sections SS-3 and SS-4 are unsafe under partially and completely saturated (natural slope) condition. In average slope condition, all the SS sections are unsafe under partially or completely saturated conditions. The facets 2, 3, 4, and 5 required mitigation measures, to improve the stability of slopes. Site-specific mitigation measures were suggested for partially or completely unstable rock and soil cut slopes.

Analysis of Unstable Rock-Mass Stability Based on Limit Equilibrium Method and Strength Reduction Method

With the slope unstable rock masses of a stope in Longsi mine, Jiaozuo City, China as the target, we computed and analyzed the stability of unstable rock masses using a limit equilibrium method (LEM) and a discrete element strength reduction method (SRM). Results show that the unstable rock masses are currently stable. Under the external actions of natural weathering, rainfall and earthquake, unstable rock mass 1 was manifested as a shear slip failure mode, and its stability was controlled jointly by bedding-plane and posterior-margin steep inclined joints. In comparison, unstable rock mass 2 was manifested as a tensile-crack toppling failure mode, and its stability was controlled by the perforation of posterior-margin joints. From the results of the 2 methods we find the safety factor determined from SRM is larger, but not significantly, than that from LEM, and SRM can simulate the progressive failure process of unstable rock masses. SRM also provides information about forces and deformation (e.g. stress-strain, and displacement) and more efficiently visualizes the parts at the slope that are susceptible to instability, suggesting SRM can be used as a supplementation of LEM.

Quantification of sediment transport by rockfall and rockslide processes on a proglacial rock slope (Kaunertal, Austria)

Rockfalls and rockslides are significant geomorphic processes on high alpine proglacial rock slopes. In this study, we quantitatively evaluate sediment transport on a recently deglaciated rock slope that is directly connected to the shrinking Gepatschferner glacier, by multi-temporal terrestrial and airborne LiDAR observations. According to the LiDAR measurements, sediment transport by gravitational mass movements on this single proglacial rock slope (0.2km2) yielded 159GJ/a geomorphic work. The most significant part (141GJ/a) was released by rockfall while 18GJ/a were performed by the sliding activity of a 1,250,000m3 large unstable rock mass. The results indicate that slope development and talus formation in proglacial environments may be a rapid process despite of being caused by continuous rockfall activity over several millennia.

Optimisation of ground anchor head for non-destructive testing

Ground anchorages and rock bolts are used widely in retaining walls, mines, dry docks, dams and prestressed structures to provide effective support for unstable rock strata. This paper offers a new design for ground anchor heads to maximise the quality of load estimation using the non-destructive ground anchorage integrity testing method. When using this method, the anchor head assembly needs to have a non-linear stiffness characteristic to permit changes in anchorage load to be detected. The initial estimate of the optimal stiffness characteristic is found using a lumped parameter dynamic model of the bolt assembly. Finite-element models of the proposed patented plate designs are then presented along with the corresponding load–displacement diagrams. The optimised bearing plate which was designed in this process was manufactured and tested experimentally, both statically and dynamically as part of a rock bolt assembly. The results from the experiments and the dynamic model are presented, and show good agreement. The resulting frequency shifts with a change of load prove the viability of building in the flexibility required to make non-destructive monitoring of load possible. The proposed design provides excellent load estimation.

Capabilities of Continuous and Discontinuous Modelling of a Complex, Structurally Controlled Landslide

In order to assess the stability of a landslide, continuous or discontinuous models have been proposed. Here, we compare the two methods in their capability to provide a reliable hazard assessment. Both models have been applied to long term monitoring data obtained from a landslide located in Passo della Morte (Eastern Italian Alps). The availability of accurate data obtained in a long-term campaign is a pre-requisite to correctly understand the dynamics of the process and to implement a sound numerical model. First, a detailed geological investigation located the different soil layers and rock formations with their distribution along the slope, allowing the identification of the structural controls of the unstable rock mass. Then, landslide long term monitoring data provided information on the type of failure mechanism. Both the continuous and discontinuous numerical solutions describe the kinematics for the landslide and allow to delineate a hazard assessment for the investigated area. The continuous model is better in delineating the development of the deep slip surface while the discontinuous allows to recreate the toppling phenomenon.

Study on Mode I–II hybrid fracture criteria for the stability analysis of sliding overhanging rock

Overhanging rock is widely distributed in mountainous regions of the world and therefore poses a potential geohazard to life and property because of fracturing and, ultimately, failure. The maximum circumferential stress criterion is widely used in fracture mechanics. However, it is inadequate with respect to predicting the failure of sliding overhanging rock that is subject to extension and shear fracturing. In this paper, the maximum shear stress at a crack tip and a fracture method in combination with the Mohr–Coulomb criterion are used to examine rock with mixed Mode I–II fractures. The methods are ultimately used to analyze the stability of sliding unstable rocks. The two fracture criteria have been compared according to the most unfavorable conditions, and the stability of the sliding rock masses are calculated using these new fracture criteria in two case studies. The calculations show that the rock stability calculated by the comprehensive fracture criterion is very close to that calculated by the limit equilibrium method, but in addition, the two methods can also determine the fracture angle. Overall, the maximum shear stress approach and a fracture mechanics criterion based on the Mohr–Coulomb model are more suitable than the limit equilibrium method for evaluating the stability of sliding overhanging rock.

Monitoring and early warning of the 2012 Preonzo catastrophic rockslope failure

In this paper, we describe the investigations and actions taken to reduce risk and prevent casualties from a catastrophic 210,000 m3 rockslope failure, which occurred near the village of Preonzo in the Swiss Alps on May 15, 2012. We describe the geological predisposition and displacement history before and during the accelerated creep stage as well as the development and operation of an efficient early warning system. The failure of May 15, 2012, occurred from a large and retrogressive instability in gneisses and amphibolites with a total volume of about 350,000 m3, which formed an alpine meadow 1250 m above the valley floor. About 140,000 m3 of unstable rock mass remained in place and might collapse partially or completely in the future. The instability showed clearly visible signs of movements along a tension crack since 1989 and accelerated creep with significant hydromechanical forcing since about 2006. Because the active rockslide at Preonzo threatened a large industrial facility and important transport routes located directly at the toe of the slope, an early warning system was installed in 2010. The thresholds for prealarm, general public alarm, and evacuation were derived from crack meter and total station monitoring data covering a period of about 10 years, supplemented with information from past failure events with similar predisposition. These thresholds were successfully applied to evacuate the industrial facility and to close important roads a few days before the catastrophic slope failure of May 15, 2012. The rock slope failure occurred in two events, exposing a compound rupture plane dipping 42° and generating deposits in the midslope portion with a travel angle of 39°. Three hours after the second rockslide, the fresh deposits became reactivated in a devastating debris avalanche that reached the foot of the slope but did not destroy any infrastructure. The final run-out distance of this combined rock collapse–debris avalanche corresponded to the predictions made in the year 2004.

Investigation of the buried structure of the Volturara Irpina Basin (southern Italy) by microtremor and gravimetric data

We investigated the structure of the Volturara Irpina Plain, a small intra-mountain basin in southern Italy. Microtremor Horizontal-to-Vertical Spectral Ratio (HVSR) measurements were performed on a 50-point grid across the plain, and contour maps of the peak frequencies and maximum amplitudes of the HVSR curves were constructed. In the deepest part of the plain, the fundamental resonance frequencies were 0.8 Hz to 0.9 Hz. Assuming that the HVSR peak frequencies interpret the fundamental resonance frequencies, and taking into account the shear-wave velocity–depth relationship derived from down-hole data, a first thickness–frequency relationship for the plain was estimated. The sediment thickness corresponds to the depth from the ground surface of the seismic basement. A power-law thickness–frequency relationship was also derived through nonlinear regression fitting of the borehole data that intercepted the bedrock. Finally, the contour map of the depth of the seismic basement was constructed by averaging the thickness estimates from the two different approaches. Gravity measurements were also acquired on a 54-point grid across the plain, most of which coincided with microtremor stations. From these data, the Bouguer gravity anomaly was computed with reference to the 1980 Ellipsoid, and using a density of 2670kg/m3, we propose a three-dimensional interpretation of the buried structure of the plain that is mainly formed by three overlapped layers. The separately obtained microtremor and gravity results were also compared, using constraints from the drillings. Thus, we interpret the first layer as Plio-Pleistocenic sediments mostly composed of clay, silty clay, and gravel. The underlying basement has maximum depth ~500m below ground level (altitude, ~200m a.s.l.), and corresponds to Mesozoic carbonate rocks. These rocks are nonhomogeneous, with an upper, less compact, layer formed by unstable rocks with clay, or fractured rocks with water circulation. This is typical for karst basins, such as Volturara Irpina Plain. The deepest stratum recognized by gravity inversion corresponds to the densest and most compact carbonate rocks that form the surrounding mountains, with average density of 2600kg/m3 to 2700kg/m3. These rocks arise at the edges of the plain, which explains the highest HVSR amplitudes in these zones, which are interpreted as strong impedance contrast at depth. Conversely, we obtained complex HVSR curves when irregular discontinuity surfaces were recognized in the subsoil by the gravity inversion.

A case study of pillar-shaped rock mass failure in the Three Gorges Reservoir Area, China

Pillar-shaped rock masses are widely developed throughout the world with some standing firm for thousands of years. Unstable pillar-shaped rock masses display different failure modes and it is often difficult to determine in advance the specific failure mode of a complex rock mass. For example, the displacement deformation of the Jianchuandong rock mass (JRM) in the Three Gorges Reservoir Area has been about 20 mm over the last 2 years. With a volume of 360 000 m 3 , this unstable pillar-shaped rock mass stands on the bank of the Yangtze River's Wu Gorge, threatening the safety of the waterway and the residents of neighbouring areas. Developing Hungr's quantitative discriminant on the rock collapse and rock slump modes, several cases are used to verify its validity. The failure mode of the JRM is judged to be rock collapse. This judgement matches well with the actual field survey and monitoring data. For pillar-shaped unstable rock masses with a rock collapse failure mode, this paper develops a new slope stability calculation mode by deeming the intact pillar-shaped rock mass to be the load. The result of such a stability calculation shows that the JRM is basically stable at present, but the safety margin is extremely narrow. Prevention work should be carried out as soon as possible for better results at a lower investment before the JRM experiences large-scale deformation and failure.

Multiscale geophysical characterization of an unstable rock mass

Hazard mitigation from rockfalls and landslides is a priority in densely populated areas. A proper characterization of the inner structure of the rock mass is key to the comprehension of the mechanisms enhancing the slope instabilities. To this aim multi-scale geophysical methods can provide a novel and valuable tool for a high-resolution imaging of the internal structure of the rock mass and unique constraints on the physical state of the medium. We present here a cross-hole seismic tomography survey coupled with laboratory ultrasonic velocity measurements and physical properties determination on rock samples to characterize the damaged and potentially unstable granitic cliff of Madonna del Sasso (NW Italy).Results allowed to obtain: i) a lithological interpretation of the velocity field obtained at the site, ii) a systematic correlation of the measured velocities with physical properties (density and porosity) and macroscopic features of the granite (weathering and anisotropy) of the cliff. The multi-scale approach adopted within this study revealed to be crucial for the imaging at depth of the main fractures affecting the cliff (site-scale seismic tests) and for the understanding of the variations in the seismic velocity between altered and intact rock (laboratory-scale tests); similar approaches can be potentially used in further microseismic monitoring studies.

Inversion kinematics at deep-seated gravity slope deformations revealed by trenching techniques

Abstract. We compare data from three deep-seated gravitational slope deformations (DSGSDs) where palaeoseismological techniques were applied in artificial trenches. At all trenches, located in metamorphic rocks of the Italian Alps, there is evidence of extensional deformation given by normal movements along slip planes dipping downhill or uphill, and/or fissures, as expected in gravitational failure. However, we document and illustrate – with the aid of trenching – evidence of reverse movements. The reverse slips occurred mostly along the same planes along which normal slip occurred, and they produced drag folds in unconsolidated Holocene sediments as well as the superimposition of substrate rocks on Holocene sediments. The studied trenches indicate that reverse slip might occur not only at the toe portions of DSGSDs but also in their central-upper portions. When the age relationships between the two deformation kinematics can be determined, they clearly indicate that reverse slips postdate normal ones. Our data suggest that, during the development of long-lived DSGSDs, inversion kinematics may occur in different sectors of the unstable rock mass. The inversion is interpreted as due either to locking of the frontal blocks of a DSGSD or to the relative decrease in the rate of downward movement in the frontal blocks with respect to the rear blocks.

Large Deformation Analysis of a High Steep Slope Relating to the Laxiwa Reservoir, China

The unstable rock slope in the Laxiwa reservoir area of the Yellow River upstream, China, shows the signs of gravitational and water-impounding induced large deformations over an area of 1.15 × 105 m2. Slope movements have been measured daily at more than 560 observation points since 2009, when the reservoir was first impounded. At two of these points, an average daily movement of around 60–80 mm has ever been observed since the beginning of the impounding. Based on the observed deformations and the geology of the site, a fluid–solid coupling model was then adopted to investigate the existing rockslide activity to better understand the mechanism underlying the large deformations. The results from the field observation, kinematic analysis and numerical modeling indicate that the slope instability is dominated by the strong structurally controlled unstable rock mass. Based on an integrated overview of these analyses, a new toppling mode, i.e. the so-called ‘conjugate block’ mode, is proposed to explain the large deformation mechanism of the slope. The conjugate block is formed by a ‘dumping block’ and toppling blocks. The large deformation of the slope is dominated by (1) a toppling component and (2) a subsiding bilinear wedge induced by planar sliding along the deep-seated faults. Following a thorough numerical analysis, it is concluded that small collapses of rock blocks along the slope will be more frequent with the impounding process continuing and the water level fluctuating during the subsequent operation period. Based on a shear strength reduction method and field monitoring, four controlling faults are identified and the instability of the loose structure in the surface layer is analyzed and discussed. The factor of safety against the sliding failure along the deep seated fractures in the slope is 1.72, which reveals that (1) the collapse of the free-standing fractured blocks cannot be ruled out and the volume of the unstable blocks may be greater than 100,000 m3; (2) the collapse of the whole slope, i.e. with the volume being greater than 92 million m3, or a very large collapse involving several million m3, is considered to be of very low likelihood, unless there are extreme conditions, such as earthquakes and exceptionally heavy rain.

Analysis on Fracture Mechanics of Unstable Rock

Unstable rock is a kind of global geological disaster with high frequency. This paper, considering three kinds of combined loads which are gravity, fracture water pressure and seismic force, constructs a unstable rock mechanics model and it uses a fracture mechanics method to deduce the composite stress intensity factor of the type I - II. Based on the maximum circumferential stress theory, this article calculates the theo-retical fracture angle by triangle universal formula.

Blast-induced damage and its impact on structural stability of underground excavations

Analysis of rock wedge stability plays important role in tunneling and hard rock mining. Today there are many procedures for prediction of wedge formation and their stability. These procedures consider preexisting joints in rock mass, and there is no procedure that considers blast-induced cracks and their impact on structural stability of excavations. Using main principles of new rock blasting theory it is possible to assess length and density of radial cracks around blastholes. Herein, one simple excavation situation in hard rock is presented, where only two joint sets were present. This made it impossible for rock wedges to form. After blast-induced crack with limited persistence was added to analysis as result unstable rock wedge was formed. in addition to presented problem it is suggested how mechanical properties of blast-induced cracks could be estimated using Barton-Bandis failure criterion. As final result it was clear that without blast- induced cracks no wedge stability analysis is complete.

Simulation of unstable rock failure under unloading conditions

Rockburst is an unstable rock failure and one of the most hazardous problems in deep hard-rock mines. Before excavation, rocks are loaded under a polyaxial condition. Upon excavation, the rocks at the excavation boundaries are loaded in the tangential direction and unloaded in the radial direction. Understanding rock behaviour under this excavation loading condition is critical for developing measures to control rock failure in underground construction. In this paper, numerical simulation results of unstable rock failure using an explicit finite element tool are presented. Firstly, uniaxial compression tests were simulated to confirm the suitability of the adopted numerical tool for simulating unstable rock failures. Transferred energy ratio (TER) and loading system reaction intensity (LSRI) were proposed as indicators to distinguish between stable and unstable failures. Secondly, unstable rock failures under polyaxial unloading conditions were simulated. The influences of loading system stiffness (LSS), specimen’s height to width ratio (H/W), and intermediate principal stress (σ2) on rock failure were investigated. The simulation results showed that rock failure was more violent when the loading system was softer, the specimen was taller, and the confinement was lower. The modelling approach presented in this paper can be useful for predicting unstable rock failure and estimating released kinetic energy, which is important for designing rock support in deep tunnels to control rockburst damage.