Unstable Rock Mass Research Articles

Time-dependent damage mechanism of rock deterioration under freeze–thaw cycles linked to alpine hazards

The unique natural environment of the alpine terrains along the Tianshan highway has led to the frequent occurrence of hazards. The typical hazards in this area, such as sliding collapse, have obvious time-dependent deformation characteristics, and the extreme climatic change causes rock mass deterioration under freeze–thaw cycles, which serves as an important factor that indirectly triggers hazards, while the relationship between the time-dependent characteristics of hazards and the damage characteristics of rock mass deterioration triggering hazards remains uncertain. A series of microscopic examination and macroscopic mechanical experiments were conducted on quartz sandstone from the Tianshan highway to study the mechanism. The experimental results show that the cyclic freeze–thaw action changed the microstructure of quartz sandstone and weakened its mechanical performance. The weakening effect on the long-term mechanical characteristics is much more remarkable than that on the short-term characteristics, and the failure modes become more complicated after long-term loading. Based on the experimental phenomenon, the mechanism was analyzed, and a new constitutive model was established to reflect the long-term mechanical characteristics of rock with freeze–thaw cycles and predict the deformation of an unstable rock mass during collapse. These results can provide a reference for alpine hazard prevention.

Research and application of an improved internal thrust force measurement system for rock and soil mass based on OFDR

Internal thrust force of unstable rock and soil mass is an essential parameter for prediction of many geological hazard. Currently, fiber bragg grating (FBG) and optical time-domain reflectometer (OTDR) are widely used to measure internal stress of unstable rock and soil mass. However, these methods have disadvantages such as low spatial resolution and the paucity of distributed measurements. This paper develops a quasi-distributed thrust measurement system based on an optical frequency domain reflectometer (OFDR). Firstly, we design an optical fiber stress sensor head using the characteristics of the optical fiber microbending effect. And then, the cubic spline interpolation method is used to compensate for the nonlinear effects of the OFDR. Finally, we implement a laboratory experiment of lateral stress to make error calibration. As a result, the OFDR sensing system achieved a spatial resolution of 20 cm by using a 500 m test fiber, maximum measurement pressure reached 1.059 MPa and relative error is 8.9%. We implemented OFDR in the Chenjiagou landslide located at the Three-Gorge of Chongqing in China. The results showed that this system can accurately locate six fiber stress sensors within the landslide over a range of 0 ∼ 420 m, obtaining the lateral thrusts as well.

Fracture Mechanism of Unstable Rock With Double-Crack Control Discontinuity Subjected to High Earthquake Intensities

Unstable rock in the limestone area has typical characteristics of quasimasonry structure and control discontinuity, where the failure essence is the fracture propagation under loads. The unstable rock control discontinuity in the limestone zone was analyzed, and the geological model for a doublecrack unstable rock mass was obtained according to the theory of geomorphologic evolution. The mechanical model and fracture mechanical model for the complex control discontinuity were constructed. Based on the rock weight, the fissure water pressure and the earthquake load, the formula of the fracture stability coefficient was obtained under the maximum circumferential stress criterion. The fracture stability coefficient expression for the unstable rock is rational according to the verifying case analysis. The fracture stability coefficient corresponding to each intensity decreases with the crack length ratio. Under earthquake intensities Ⅷ and Ⅸ, the unstable rock will collapse; the stability of the unstable rock is liable to the coupling effects of the crack length ratio of the main control discontinuity and the earthquake intensity. The critical crack length ratio decreases with the earthquake intensity. Under earthquake intensity Ⅶ, the theoretical value of the critical crack length ratio is 25.8%, which is slightly less than the actual value of 27.7%, and on the slightly safer side. The fracture stability coefficient expression has good applicability. The research results provide an important theoretical support for the treatment of such unstable rock masses.

Automated rock mass discontinuity set characterisation using amplitude and phase decomposition

Mapping structural discontinuities in subsurface environments is critical for several engineering applications. Most instances require stable rock mass to ensure safety while some require an unstable rock mass, such as in block-cave mining, for gravity induced smooth and continuous production of ore. Structural discontinuities have previously been mapped using manual, semi-automated and automated methods which have limitations in terms of required effort, processing time, definition of parameters and accuracy. Most of the semi-automated and automated approaches employ point cloud data, obtained either through structure from motion photogrammetry or laser scanner, and rely on point normal vectors in some form for discontinuity characterisation. Such methods can lead to inaccurate results in the presence of noise as normal vectors tend to show high variability in a local region. This study proposes a new automated algorithm that uses the spatial distribution of points on discontinuities to capture unique signatures in the form of a sinusoidal wave. Each discontinuity points are distinguished by unique amplitude and phase, achieved using fast Fourier transform (FFT), for characterisation in a 3-dimensional (3D) point cloud. The presented amplitude and phase decomposition (APD) approach requires minimal pre-processing and can be applied directly to a raw point cloud as filtering is inherently included in the signal processing. The method has been evaluated on an underground tunnel dataset, consisting of exposed structural discontinuity planes. The efficacy of the developed approach was tested against open-source semi-automated software (discontinuity set extractor), proprietary semi-automated software (Maptek PointStudio), manual segmentation using virtual compass plugin in open-source software (Cloud Compare), and other automated algorithmic approach based on point normal clustering and region growing. When compared against the manual discontinuity segmentation, the APD approach indicated the least error in estimating mean discontinuity dip angle and dip direction which was ±1.15° and ±1.39 with a dispersion error of ±2.24° and ±1.54°, respectively.

Measurements and modeling of the post-failure micro-deformations and tilts of the Preonzo unstable slope, Alpe di Roscioro, Switzerland

The Alpe di Roscioro, is a site of an unstable rock slope located above the village of Preonzo in southern Switzerland. There have been numerous rock-slope failures with a major event that happened in May 2012. After that event, several seismic stations were set up at the site of the remaining highly fractured and unstable rock mass. The analysis of continuous seismic recordings has shown a high number of permanent micro-deformations and tilts, recorded by a seismometer located on top of the unstable part of the slope. In terms of the ground motion, the directions of these disturbances are parallel to the mean strike of the mapped fracture network, implying a connection to the ongoing deformations of the rock mass. The static deformation field of the fractured rock mass is modeled by finite-difference method (FD). The FD method enables us to apply a reciprocal approach by tilting the affected body of block 1, closest to the stable massif, by measured values and observing the deformations below the seismometer and in the neighboring blocks. The numerical model explains the following features observed in situ: feature 1. The combination of the horizontal displacement and tilt observed approximately at the same measurement point; feature 2. The block 1 center of rotation is shifted towards SSE by ~10 m off its center; feature 3. The sensors placed on the stable massif did not record any micro-deformation and tilt signals above the background noise.In the light of the numerical modeling, the weak coupling between the micro-tilt active block 1 and neighboring media explains the absence of detected events in the reference stations placed on the stable massif. The simplified “shear and bending” force system FD model, indicates that the aftermath of the May 2012 main failure consisted of numerous episodic elementary relaxations of the SSE part of block 1 after the collapse of its NNW compartment, which was previously connected to it.

Application of Discrete-Element Numerical Simulation for Calculating the Stability of Dangerous Rock Mass: A Case Study

Abstract An unstable rock mass is a common geological hazard in water conservancy and hydropower engineering in southwest China. The mechanical properties of a rock mass joint and the internal and ...

Deciphering the dynamics of a heterogeneous sea cliff using ambient vibrations: case study of the Sutta-Rocca overhang (southern Corsica, France)

SUMMARYWe report here an interesting case study of dynamic characterization of a limestone sea cliff area, named the Sutta-Rocca overhang, located near the medieval town of Bonifacio (southern Corsica, France). The site belongs to an UNESCO protected area with an important number of visitors per year. Therefore, it is of particular interest to evaluate the potential rockfall hazard along the cliff. With the objective of evaluating the feasibility of an operational technique to monitor the cliff using ambient vibrations, two seismic stations were installed during six months (from 2016 November to 2017 April) in order to identify and monitor the resonant frequencies of the a priori unstable rock mass. Several techniques of classical seismic monitoring are tested on the site: single-station spectral analysis for identification of frequency peaks related to the unstable mass, relative spectral amplifications between stations and the operational modal analysis of the cliff. In particular, data analysis from a temporary campaign of two days of continuous recordings by a linear array of seismometers perpendicular to the sea cliff highlights the main characteristics of the ground motion: the overall motion of the promontory mainly in the north–south direction (perpendicular to the coast) and bending vertical and parallel to the cliff (east–west direction) at higher frequencies. The strong vertical and east–west seismic amplifications are also confirmed by the analysis of five local seismic events recorded at both semi-permanent stations. Even if ambient vibrations and seismic data analysis allows to decipher the dynamics of the Sutta-Rocca overhang, we conclude that classical seismic monitoring of just one single frequency peak with a limited number of sensors is not recommended for this particular site.

Formation mechanism and risk assessment of unstable rock mass at the Yumenkou tunnel entrance, Shanxi province, China

The Yumenkou tunnel is located on the left bank of the Yellow River on the border between Shanxi and Shaanxi provinces, China. It is connected to the Longmen Yellow River Bridge of the Menghua railway. Many unstable rock masses have developed on the cliff above the tunnel entrance, threatening the construction and operation of the tunnel and the bridge. Based on a high-precision three-dimensional (3D) digital terrain model obtained by unmanned air vehicle (UAV) photogrammetry, information of the structural surface which controls the stability of the rock mass in the study area was obtained by manual identification and extraction. We combined the UAV imaging with field survey data to obtain the spatial distribution, shape, scale, and potential failure modes of the unstable rock masses. A 3D numerical model of the unstable rock mass and surface morphology was constructed using a 3D discrete element method. The 3D path and impact energy of the moving unstable rock mass were analyzed. Based on the lithology, topography, structural surface, and other factors, three types of potential rock failure modes were identified in the study area: sliding, toppling, and falling. Differential weathering and the structure plane and stress-unloading cracks are two main factors affecting the stability of the unstable rock mass. According to the result of 3D discrete element method, once the unstable rock masses fail, about 50% of which might hit the portal of the tunnel, and the maximum rebound height at the portal would be about 1.2 m. To prevent such catastrophic failure, active protective net was recommend at each unstable rock zone, and retaining wall with a height of 5 m behind the portal. The results of this study confirm the effectiveness of a UAV photogrammetry-based method for reliable and cost-effective engineering geological surveying of unstable rock mass in complex terrain.

Assessment of Rockfall Hazards of Moziyan in Hechuan District, Chongqing, China

Moziyan is an area containing unstable rock masses located in Hechuan District, Chongqing, China and has constantly seen group-occurring rockfalls since 1964. The present study aims to investigate the damage of Moziyan rock mass and protect the mining areas and residential areas from rockfall phenomenon. Mohr–Coulomb model in FLAC–3D Program was built to find out the rockfall initiation position. RocFall program has been used to calculate the distribution curves of stopping positions, bounce height, total kinetic energy, and translational velocity of the falling blocks varying with horizontal distance. According to the simulation results, the stability of Moziyan rock mass is controlled by the unloading cracks behind the slope, and the falling blocks have a maximum bounce height and maximum total kinetic energy of 0 m and 2720.9 kJ respectively at the horizontal position of 461.6 m, where proper protection measures should be adopted to minimize the risk of the damage of the Moziyan rock mass. Conclusively, the results of simulation indicate that a barrier of 3 m height and 3000 kJ absorption capacity is enough to resist the falling stones with a maximum speed of 13.2 m/s at the horizontal position of 461.6 m.

A numerical study on mechanical behavior of jointed rock masses after tunnel excavation

The tunnel in jointed rock masses is prone to be overbroken and then gives rise to collapse, yet the failure modes of jointed rock masses are still poorly understood. This paper investigates anisotropic deformations and failure modes of tunnels in jointed rock masses. For this purpose, five series of numerical models are modeled for a severe tunnel overbreak occurring on the Panlongshan tunnel. The ubiquitous joint model is used to consider the shearing and tensioning effects of the bedding and joint structures. The factors affecting anisotropic deformation and failure of rock masses, such as bedding spacing (s), joint dip angles (θ), and joint spacing (d), are discussed. The results demonstrate that for the stratified rock masses (without joint), the maximum settlement around the tunnel is located at the roof, and the separation of “virtual blocks” along the bedding planes is a basic failure mode; for stratified rock masses containing joints, the maximum deformation surrounding the tunnel appears in the joint position, and failure happens through a combined rotation, sliding, and falling mechanism. Our study can also reveal that with the increase of the number of joints, the plastic zone at the joint positions has a developing tendency towards the deep part. The more complex the joint structures in jointed rock masses are, the more unstable rock masses are formed.

Failure mechanism and dynamic processes of rock avalanche occurrence in Chengkun railway, China, on August 14, 2019

Rock avalanches are unpredictable due to their complex geological conditions and dynamics and such features have been regarded as important reasons for property damage and fatalities. The Ganluo rock avalanche of August 14, 2019 in China provides an outstanding case to evaluate the failure mechanism and dynamic process of long-runout rock avalanches. To understand the failure mechanism and dynamics of such events, a combination of in situ investigation, remote sensing data study, and simulation is performed. The result shows that the Ganluo rock avalanche released a rock mass volume of approximately 5.20 × 104 m3 that detached from the slope, and 1.52 × 104 m3 of unstable rock mass is still in the source zone. The analysis indicated that the prerequisite for the rock avalanche was continuous heavy rainfall, which occurred in the preceding months. The simulation shows the maximum sliding velocity is approximately 22 m/s. Additionally, the potential destructive zone of the unstable rock mass is evaluated, which can provide a reference for secondary disaster prediction.

Experimental Study on Mix Proportion Parameter Optimization of Cement Anchoring Material.

To prevent major deformation of surrounding rock and improve the stability of unstable rock mass, this study optimized the ratio of cement anchoring material based on the orthogonal design method and rock mechanics tests. Using the six influencing factors of coal ash content, sodium silicate content, ettringite content, naphthalene sulfonate content, water–cement ratio, and sand–cement ratio, a total of 18 sets of material matching schemes was designed. Compressive strength and pull-out force tests were carried out. The results are as follows: the compressive strength of the test piece between 43 and 55 MPa; the bond stress between the bolt and the anchoring agent range is from 103 to 136 kN; and the adhesion stress between the anchoring agent and the rock masses range is from 76 to 112 kN. The main order and degree of influence of various factors affecting the bond stress and the adhesion stress of the anchoring material were determined by range analysis and analysis of variance. The results showed that the amount of coal ash had a clear controlling effect on the bond stress, and the water–cement ratio, sand–cement ratio, and sodium silicate dosage had a significant influence on the bond stress. Moreover, the amount of sodium silicate had the greatest influence on the adhesion stress, and the water–cement ratio had the second largest influence on the adhesion stress. The amount of naphthalene sulfonate had the least influence on the adhesion stress. The above experimental data and results were subjected to multiple linear regression analyses, and the empirical equations of the mechanical indexes of the test pieces and the proportion parameters of the anchoring materials were obtained to guide the engineering support design. The engineering application showed that the optimized anchorage material could be applied to further improve the safety of the surrounding rock anchoring system.

Prediction of a multi-hazard chain by an integrated numerical simulation approach: the Baige landslide, Jinsha River, China

Successive major landslides during October and November 2018 in Baige village, eastern Tibet, dammed the Jinsha River on two occasions, and the subsequent dam breaches instigated a multi-hazard chain that flooded many towns downstream. Analysis of high-resolution aerial images and field investigations unveiled three potentially unstable rock mass clusters in the source area of the landslides, suggesting possible future failures with potential for river-damming and flooding. In order to evaluate and understand the disaster chain effect linked to the potentially unstable rock mass, we systematically studied the multi-hazard scenarios through an integrated numerical modelling approach. Our model begins with an evaluation of the probability of landslide failure, including runout and river damming, and then addresses the dam breach and resultant flood—hence simulating and visualising an entire disaster chain. The model parameters were calibrated using empirical data from the two Baige landslides. Then, we predict the future cascading hazards via seven scenarios according to all possible combinations of potential rock mass failure. For each scenario, the landslide runouts, dam-breaching, and flooding are numerically simulated with full consideration of uncertainties among the model input parameters. The maximum dam breach flood extent, depth, velocity, and peak arrival time are predicted at sequential sites downstream. As a first attempt to simulate the full spectrum of a landslide-induced multi-hazard chain, our study provides insights and substantiates the value provided by multi-hazard modelling. The integrated approach described here can be applied to similar landslide-induced chains of hazards in other regions.

Drilling and bolting rigs operator support using FGS (Feeder Guiding System) and remote control

Abstract The article presents the assumptions and main functionalities of two operator support systems – the FGS (Feeder Guiding System) and the remote control system of a drilling or bolting rig. Both of these systems greatly facilitate the work of an operator of a self-propelled mining vehicle in the conditions of an underground metal ore mine. The first of the described system allows for real-time control of the current position of the mining boom and facilitates the drilling of blast holes according to a predetermined drilling pattern. The second of the systems – a remote control system – is used especially in conditions of unstable rock mass or in the case of work in particularly difficult operating conditions, where it is advisable to limit the presence of the crew from the face-up to the necessary minimum. The intensity of these vibrations was related to the conditions of the cutting process.

Sinusoidal wave fit indexing of irreversible displacements for crackmeters monitoring of rockfall areas: test at Pietra di Bismantova (Northern Apennines, Italy)

Temperature changes affect crackmeters monitoring on a daily and a seasonal basis. This is due to rock mass thermal dilatancy and to instrumental matters. The consequent widening closing cycles can mask small irreversible displacements that might be precursors of rock failures. Recently, Weber et al. (Cryosphere 11:567–583, 2017) have proposed a linear fit method between temperature and fracture opening in order to compute the irreversibility index as a metrics to rank irreversible displacements. However, such an approach requires temperature sensors coupled to crackmeters. In order to overcome these limits, we propose an alternative method for deriving a normalised Z-score irreversibility index. It is based on sinusoidal wave fit of cracks opening time series only; thus, it does not require temperature monitoring. The methodology has been tested using data recorded by a wireless sensor network installed at La Pietra di Bismantova rock slab composed of 14 crackmeters and thermometers monitoring potentially unstable rock masses. A comparison of results obtained using the method of Weber et al. (Cryosphere 11:567–583, 2017) and the sinusoidal approach shows that the latter is much less sensitive to the duration of the moving window used to derive the irreversibility index, making it a much more flexible tool for indexing irreversible displacements over short time periods. Moreover, as rapid high–magnitude temperature changes can also be the causal factor of irreversible displacements, their statistical relation with peaks of the Z-score irreversibility index has been investigated. Results have shown that, depending on which crack is examined, correlations between irreversibility peaks and antecedent extreme temperature variations are more or less relevant. In conclusion, we believe that the Z-score sinusoidal wave fit irreversibility index (ZSFI) can represent a useful metrics for indexing irreversible displacements in unstable blocks using crackmeters’ datasets affected by temperature cycles at the daily and seasonal scale.

Removing of a large unstable rock mass on the national highway slope with road blocked

Removing of a large unstable rock mass on the national highway slope with road blocked

Failure mechanisms of wind-induced post-seismic rockfall hazard

The 2008 Mw 7.9 Wenchuan earthquake generated a tremendous number of coseismic landslides and induced dynamic crack propagation in the bedrock of slopes. These processes produced loose material that could be entrained in subsequent landslides potentially triggered by prevailing valley wind in the earthquake-affected area. However, almost nothing is currently known about the influence of wind in triggering rockfall hazard. Therefore, we provide insights into the failure process and mechanics of wind-induced rockfall hazard by means of wind tunnel tests using different rock geometries. First, a 1:1-scale failure process simulation is used to model the aerodynamic instability of different geometries of rock blocks, and results reveal that (i) cubic rocks fail via downwind sliding; (ii) cuboid rocks fail via tailwind toppling or crosswind toppling (the latter only occurs when the unstable rock mass is close to the neighboring wall); and (iii) ellipsoid rocks fail via headwind clockwise rolling. The critical wind speed is inversely proportional to the angle of attack, the aspect ratio, and the ellipticity ratio. Second, a mechanical analysis is performed in terms of aerodynamic coefficients based on 6:1- and 4:1-scale wind pressure measurements, which is beneficial for studying the crucial factors involved in the occurrence of different failure modes. In addition, fast and effective methods to estimate the critical wind speed for sliding and tailwind toppling are developed, respectively.

The mechanism of the bottom-crashing rockfall of a massive layered carbonate rock mass at Zengziyan, Chongqing, China

Multiple carbonate cliffs are distributed in the karst mountain areas of southwestern China. The dynamic process and failure mechanism of rockfalls in the cliffs are complicated and undefined, and can be dangerous for settlements. Moreover, it is difficult to identify the potential for such failures before they occur. The rockfall of the Zengziyan cliff at Chongqing, China, is used as an example for studying this type of failure and to determine the developmental characteristics of rockfall, expounding on the evolving process from a cliff to an unstable rock mass through to rockfall. A fracturing–crashing failure mode is proposed, based on the evolutionary characteristics of damage development, compression-induced fracturing, rock mass crashing and overall rockfall. Finite difference numerical simulations were used to analyse and validate the failure mode of the Zengziyan cliff under three simulation conditions. The results of the simulations indicated that strength deterioration at the bottom of the cliff was the root cause of its fracturing–crashing rockfall. The outcomes of this paper could provide a new analytical model and theoretical basis for identifying and analysing similar types of geological disasters in the mountainous areas of southwestern China, and may provide valuable reference information for further mechanical analysis, stability evaluation and engineering prevention of unstable rock masses prone to a fracturing–crashing rockfall.

Experience of application of polyurethane and phenolic resins in roadheading in fractured and heavily disintegrated rock mass

The problem of mining and ground support in an unstable rock mass under stoping is discussed. The analysis of the experience gained in application of phenolic and polyurethane resins to fill domes-like voids and for preventive consolidation and roadheading in disintegrated rock mass is presented. The feasibility to fill domes formed under roof caving with Blockfil phenolic resin in combination with grouted bolting and the use of Blockpur polyurethane resin for consolidation of disintegrated rocks is investigated. The laboratory test results on strength properties and time of polyurethane resin flowability loss are presented. The procedures are developed for eliminating accidents due to dome formation under roof and sidewalls collapse, as well as for the support of weakened roadways by yielding steel arcs. It is emphasized that geotechnical conditions worsen due to dome formations during recovery works in mines.

The Use of Gigapixel Photogrammetry for the Understanding of Landslide Processes in Alpine Terrain

The work in this paper illustrates an experimental application for geosciences by coupling new and low cost photogrammetric techniques: Gigapixel and Structure-from-Motion (SfM). Gigapixel photography is a digital image composed of billions of pixels (≥1000 megapixels) obtained from a conventional Digital single-lens reflex camera (DSLR), whereas the SfM technique obtains three-dimensional (3D) information from two-dimensional (2D) image sequences. The field test was carried out at the Ingelsberg slope (Bad Hofgastein, Austria), which hosts one of the most dangerous landslides in the Salzburg Land. The stereographic analysis carried out on the preliminary 3D model, integrated with Ground Based Synthetic Aperture Radar Interferometry (GBInSAR) data, allowed us to obtain the main fractures and discontinuities of the unstable rock mass.