A large-scale rockslide, developed from a deep-seated toppling failure, occurred in October 2018 following reservoir impoundment in the upper Lancang River, Southwest China. The evolution of this ...slope failure was investigated to reveal the instability mechanism of the rockslide after impoundment. Electron spin resonance (ESR) dating was used to determine the formation period of the toppling failure. Detailed field investigations, including joint measurements and surface, borehole and adit surveys, revealed the surface and subsurface features of the deformed rock masses. The Universal Distinct Element Code (UDEC) was used to simulate the evolutionary process of the slope failure. The results show that the toppling deformation rate accelerated with river downcutting and decreased after the modern valley formed. After reservoir impoundment, the mode of slope deformation transformed from creep toppling to rapid sliding. The sliding progressed upslope due to shear failure at the slope toe. In addition, the results of a series of simulations show that the action of tectonic stress can significantly increase the depth of toppling deformation. The investigation results imply that the evolution of the slope failure included five stages: initial deformation, accelerating deformation, creep deformation, formation of a throughgoing sliding surface and failure. The first three stages were dominated by toppling, whereas sliding dominated the latter two stages. Toppling was induced by river downcutting and promoted by the effects of tectonic stress and gravity. Reservoir impoundment promoted sliding at the slope toe, which induced upslope propagation of sliding above the toppling hinge surface. Once the slip zone coalesced with the shear failure surface at the front of the slope, the main rockslide occurred.
•Summarize the complexity and distinctiveness of the deep-seated toppling failure•Propose the evolution of the rockslide evolved from deep-seated toppling failure•Reveal the failure mechanism of the rockslide induced by reservoir impoundment
Block identification serves as a critical preprocessing step in rock block system mechanical analysis. Presently, research primarily focuses on refining identification algorithms to accommodate ...increasingly complex joint models or enhance computational efficiency, while overlooking the influence of the block system expression derived from identification on subsequent mechanical analysis. Specifically, the precise role of small blocks within a block system during numerical simulation remains uncertain. Consequently, this paper developed a novel two-dimensional block identification method integrating small block treatment. The essence of this treatment lies in eliminating the short edges shared between small blocks and their adjacent blocks. Here we find that small block treatment is crucial for enhancing the efficiency and convergence of mechanical simulation. Through comparisons with the existing treatment methods using three cases in the Universal Distinct Element Code (UDEC), our method demonstrates superior computational efficiency and exhibits the best consistency with the results of the original models in Cases 1 and 2. Moreover, our method is uniquely capable of successfully conducting numerical simulations for Case 3. We anticipate our study to be a starting point for advancing the geometric optimization of block systems and bridging geometric and mechanical analyses in the realm of rock block mechanics analysis.
A series of simplified approaches are evaluated for their effectiveness to estimate the seismic vulnerability of historical masonry towers. First, collapse loads are evaluated on sixteen “idealized” ...benchmark cases with different slenderness and shear area. Both analytical and computational approaches are used, namely the analytical procedure proposed by the Italian Guidelines on the Built Heritage and pushover analyses conducted using the commercial codes UDEC and 3Muri. The sixteen towers are representative cases which can be encountered in practice. Their geometry is idealized into parallelepiped blocks with hollow square cross-sections, thus favoring the utilization of 2D approaches, beneficial to drastically reduce the effort required for repeated computations. In addition, a Monte Carlo MC upper bound limit analysis strategy is proposed, in order to have an insight into the possible failure mechanisms for the different cases investigated. Deliberately is avoided the introduction of any form of irregularity and they are supposed isolated from the neighboring buildings, to obtain results exclusively dependent from geometric features. Among all the possible collapse mechanisms, five of them are selected according to the probability of occurrence based on past earthquake experiences. Five million cloud points of collapse accelerations are obtained by carrying the height, slenderness and shear area of the idealized towers. The approach is very fast and allows identifying different regions where single mechanisms are active. The results are confirmed repeating MC simulations with a triangular FE upper bound limit analysis discretization of the idealized towers. A series of equations are provided in order to assist engineers and practitioners to obtain a preliminary estimation of their expected collapse acceleration. For validation purposes, the results obtained previously with refined full 3D FE models of 25 towers located in the Northern Italy are reported. Satisfactory agreement between the predictions provided by simplified methods and sophisticated analyses are obtained.
This paper presents the application of the hybrid continuum-discrete element method, i.e. Universal Distinct Element Code (UDEC), to simulate soil desiccation shrinkage and cracking. Herein, soil is ...modelled using a mix-mode cohesive fracture model that combines tension, compression and shear material behaviour. The fracture model considers both elastic and inelastic (decomposed to fracture and plastic) displacement, with the norm of the effective inelastic displacement being used to control the fracture behaviour. The applicability and capability of the proposed approach is demonstrated through numerical simulations of laboratory linearly-constrained desiccation test. Good agreements with the laboratory observations have been obtained. The dominant influencing factors on soil desiccation cracking have been assessed, several factors including shear strength and tensile strength of soil-base, and soil sample thickness were identified to have a significant controlling influence on desiccation cracking.
•A mixed-mode cohesive fracture model for laboratory soil desiccation simulation•Correlate the micro-parameter to macro-parameter•Investigate the desiccation cracking influential factors•The cohesive fracture model has potential application to field soil desiccation.
Squeezing failure is a common failure mechanism experienced in underground coal mine roadways due mainly to mining-induced stresses, which are much higher than the strength of rock mass surrounding ...an entry. In this study, numerical simulation was carried out to investigate the mechanisms of roadway squeezing using a novel UDEC Trigon approach. A numerical roadway model was created based on a case study at the Zhangcun coal mine in China. Coal extraction using the longwall mining method was simulated in the model with calculation of the mining-induced stresses. The process of roadway squeezing under severe mining-induced stresses was realistically captured in the model. Deformation phenomena observed in field, including roof sag, wall convexity and failed rock bolts are realistically produced in the UDEC Trigon model.
This study introduces a simplified semi-distinct element algorithm for discontinuous rock slopes under toppling instability assessment based on block theory. The presented algorithm is developed to ...investigate block, flexural and block-flexural types of toppling failures which have been coded in the Python high-level programming language. In order to investigate toppling instabilities for different modes, the three modelling steps, namely, geometrical, behavioural and mechanical simulations were conducted to appoint high-order calculation trending loops. These loops were based on the analytical description of modified second-order reliability method with first-order efficiency to determine the factor of safety values for discontinuous rock slopes. Each type of toppling failure was described based on main assumptions that were modified by geometric qualifications such as the analytical procedure, single block slip, and the Voronoi diagram algorithm for the stability analysis of the three different failure types. According to the stability assessment results of the studied slopes which were verified by the distinct numerical method via UDEC software, it has been clarified that the results obtained from the algorithm and the numerical analysis were in an appropriate trend. The used algorithm is superior in pinpointing the critical sliding zones, safety factor estimation and progressive failure analyses as compared to the numerical analyses. Moreover, the block theory based algorithm has higher processing capability and speed with respect to the numerical method.
A numerical study was performed to develop a framework for estimating the confined strength of rock blocks considering scale effects and in-situ heterogeneity (i.e. intensity of structural ...microdefects and degree of weathering). Grain boundary models using the Voronoi tessellation scheme within UDEC have been used to simulate the results of small (lab) and large (field) scale compression (unconfined and triaxial) and indirect tensile (Brazilian) tests on a series of progressively larger in size and degrading in quality numerical specimens. Accordingly, relationships that link rock block strength with its volume and in-situ condition were developed for the preliminary estimation of scaled Mohr-Coulomb and Hoek-Brown parameters. The results from the scaling analysis generally suggest that cohesion decreases with both increasing scale and degrading sample condition in a manner similar to the scale/condition dependant reduction of uniaxial compression strength (UCS), while the friction angle shows only minor variation with no apparent trend. The measured peak confined strength values were also fitted to the Generalized Hoek-Brown criterion and a new block-scale Geological Strength Index parameter is introduced named micro GSI (mGSI) which was also linked to the scale/condition dependant reduction of UCS. By using the proposed linear and non-linear approaches, once the UCS reduction due to scaling effects is known, the confined strength of rock blocks could be then defined and can be used to carry out preliminary rock engineering calculations and to run discontinuum numerical models in which rock blocks are simulated explicitly.
The stress path during underground excavation is critical to the mechanical response of rock masses, and the excavation effects in front of a tunnel face cannot be ignored. In this study, a realistic ...stress path model was built to capture the actual behaviour of the surrounding rock during the entire excavation process. The key technical steps of the two-dimensional excavation simulations with this method were presented. This approach was applied to characterise the deformation and damage evolution of an experimental tunnel, where the soft rock under high stress experienced severe damage. The research results revealed the evolution of the excavation-induced deformation and damage, which was in good agreement with the results of field investigations and previous research. The cause of support failure was found to be that the growth process of the excavation damage zone (EDZ) was different from the convergence displacement, and the evolution of the EDZ did not receive sufficient attention during the design of the support. Corresponding suggestions for the improvement of support designs were put forward. The proposed method could provide key information for predicting the long-term performance and determining a reasonable design for the support of tunnels exposed to a high risk of destruction.
The Hongshiyan landslide, the largest landslide (volume ~ 12.24 Mm3) triggered by 2014 Mw 6.2 Ludian earthquake, blocked the Niulan river forming a rockslide dam; the failure potential of the dam ...created an elevated risk to population and infrastructure downstream. We provide insight into the failure mechanics of the Hongshiyan landslide using data obtained by means of remote sensing techniques and traditional in-situ surveys. Geological data obtained by these methods was then used for kinematic and numerical analyses. Our study shows that failure of the Hongshiyan landslide involved a high rock slope with an average slope of 55° and aspect of 185° SSW, in which the geological setting consists of an upper strong limestone slab and an underlying weak silty mudstone. The geological setting allowed the landslide to develop, as follows: i) the silty mudstone, due to its poor mechanical properties, was subject to ductile deformation under the compressive loading from the overlying strong limestone slabs; ii) a large number of release fractures in the limestone developed as a consequence of pre-failure progressive deformation in the underlying weak mudstone; iii) the oxidation and solution along steep joint walls in the upper limestone indicates that the joints are pathways that allow water ingress into the mudstone, further promoting the degradation of shear strength in this layer. Distinct element modelling (UDEC) was used to back analyse the failure mechanism and develop the geological model that best reproduced the Hongshiyan failure geometry. Results show that the failure surface consisted of two elements. The upper steeply dipping release surface formed along the slope-parallel steep cross joints defining a jointed, strong limestone slab, and the toe of the failure formed a curved failure surface through the ductile weak silty mudstone, underlying the limestone slab, independent of any discontinuities within it. The pre-failure observations and numerical analysis suggest that the Hongshiyan slope was in a state of marginal static stability prior to the earthquake as a result of pre-failure progressive deformation controlled by the shear strength of the underlying mudstone. Overall, the marginal pre-earthquake stability of the Hongshiyan slope, coupled with the strongest seismic loading, which the Hongshiyan slope has been historically subjected to, are the reasons why this moderate earthquake induced a large-scale catastrophic rock slope failure.
•We study a large-scale deep-seated rockslide triggered by a moderate earthquake.•Through a successful numerical analysis we show the mechanics of co-seismic failure.•The slope had been deforming prior to its catastrophic failure as a result of the earthquake.•Instability was only possible because of a weak element in the slope system.•The key to failure was an upper rigid layer overlying a lower ductile layer.
Equivalent continuum models estimating the deformability of complexly fractured rock masses may incur significant discrepancies. This paper proposes an equivalent discrete fracture network (E-DFN) ...method to proficiently capture the non-continuous characteristics of rock masses while significantly reducing computational cost compared with traditional discrete fracture network (DFN) models. A significance index of individual fractures is defined to quantitatively evaluate their impact on the elastic modulus. Accumulative significance indices are then generated to define E-DFNs containing representative quantities of fractures. Numerical simulations are conducted with E-DFN models using UDEC (universal distinct element code). The relationship between elastic moduli of complexly fractured rocks and models with E-DFNs is established by an extrapolation function obtained through regression analyses based on an assumption of the marginal diminishing effect of modulus reduction. The proposed method is validated by stochastic DFNs with different geometric configurations. The results show that reasonable estimation with small errors is achieved by the extrapolation of density-reduced E-DFN models. The E-DFN method strikes a balance between result reliability and computational intensity.
•A significance index evaluates the fracture contribution to rock deformability.•An E-DFN model extracts most significant fractures from the original DFN model.•An extrapolation function is incorporated to project rock mechanical properties.
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