Left Bank Slope Research Articles

Slope displacement reliability analysis considering rock parameters spatial variability subjected to stochastic mainshock-aftershock earthquake

A new and efficient framework for analyzing the stochastic dynamic response and reliability of the rock slope was constructed based on the generalized probability density evolution method (GPDEM). The left bank slope of Jinping dam was taken as an illustrative example and the effects of mainshock-aftershock sequence randomness, spatial variability of geotechnical materials and coupled uncertainties on the slope dynamic response and reliability were compared and analyzed for the first time. Firstly, a series of mainshock-aftershock sequences and parameter random fields were generated as stochastic factors inputs. Then, the effectiveness of GPDEM was validated by comparing with Monte Carlo Simulation (MCS). The Newmark slip displacement method was combined with GPDEM to analyze the slope dynamic response and reliability subjected to single and coupled uncertainties from a probabilistic viewpoint. Finally, the fragility curves of three risk levels were established based on the reliabilities with different seismic intensities. The result suggests that parameter spatial variability affects the dynamic response and reliability of the slope to some extent, but this effect will be overshadowed by the uncertainty of mainshock-aftershock sequences when it is considered simultaneously. Besides, aftershocks will cause the decrease of slope reliability and the impact of aftershock cannot be overlooked.

Seismic collaborative reliability analysis for a slope considering spatial variability base on optimized subset simulation

Seismic reliability analysis of actual slopes considering the spatial variability of soil materials is crucial. However, for the discretization of large-scale random fields, high-precision finite element analysis and analysis of small failure probability events, the random analysis of slopes under seismic loads is inefficient. To address this situation, this study proposes a collaborative reliability analysis framework based on the modified linear estimation method (MLEM) and optimized subset simulation (OSS). First, the random field of the uncertain parameters of the Jinping-I left bank slope model is efficiently discretized by the MLEM, and a sensitivity analysis is carried out. Then, considering the adoption of different degrees of cross-correlation of the sensitive random parameters, the OSS method is used to perform random finite element analysis on the coarse mesh model. Finally, the fine mesh samples are obtained according to the response conditioning method (RCM). The MLEM is used to ensure the consistency of the two sets of random fields, and the seismic failure probability and reliability index of the slope under different cross-correlation coefficients of uncertain parameters are obtained. The results suggest that the degree of cross-correlation of parameters has a great influence on the seismic reliability of the slope. Considering that the shear strength parameters of geotechnical materials are often negatively correlated, the fine analysis based on a fine model is necessary.

A Fast Prediction Method for Stability Safety and Reliability of Reservoir Bank Rock Slopes Based on Deformation Monitoring Data

The stability of reservoir bank slopes is critical to the engineering operation’s safety. Due to the complexity of geological conditions, the monitoring mode based on deformation monitoring data cannot directly respond to the structural damage stability state, whereas anther mode based on structural calculation is time-consuming and lacks real-time capabilities. To that end, this paper proposes a method for fast prediction of the safety state of reservoir bank rock slope based on the physical significance of time-dependent deformation and rock creep at monitoring points, with the safety coefficient and reliability obtained by numerical calculation as the dependent variables and the slope deformation monitoring sequence as the independent variable, based on full verification of the rationality of numerical calculation. The model can be used to forecast the stability and reliability coefficients of reservoir bank slopes online using deformation data from the field. The application verification of the left bank slope of the Dagangshan arch dam reveals that the average and maximum error of slope stability safety coefficient prediction is within 5% for 90 and 180 days and the average and maximum error of reliability index prediction is within 10%, which meet the engineering requirements and can provide a new way for rapid prediction of slope engineering safety.

A Study on the Freeze–Thaw Damage and Deterioration Mechanism of Slope Rock Mass Based on Model Testing and Numerical Simulation

Engineering rock mass in cold regions has obvious freeze–thaw damage as a result of extreme differences in temperature, rainfall, snow, and other factors, which is one of the main causes of frequent geological disasters. Therefore, it is important to investigate the deterioration mechanism and evolution laws of rock mass freeze–thaw damage. Considering a hydropower station’s left bank slope in a cold region, model testing and numerical testing of slope rock mass failure under freeze–thaw conditions are here carried out by developing a generalized model. The results reveal that heat is transmitted from the outside to the inside of the slope and that the rate of temperature change varies with depth; the frost-heave force causes tensile cracks in the rock mass, with crack propagation taking the form of a circular arc; the presence of an original structural plane influences the propagation direction of the frost-heave crack, whereas the freezing rate of the fissure water influences the amplitude and growth rate of the frost-heave force. Additionally, a novel method of measuring frost-heave force is proposed. The largest frost-heave force caused by water–ice phase change is 19.3 kPa, which is equivalent to 3.17 MPa of the actual slope.

Development and verification of three-dimensional equivalent discrete fracture network modelling based on the finite element method

The discrete fracture network (DFN) method is essential for estimating the mechanical properties of naturally fractured rock masses. A three-dimensional (3D) equivalent DFN model was proposed and thoroughly validated for fractured rock masses. In the equivalent DFN modelling, the Monte Carlo method and 3D rock failure process analysis (RFPA3D) software were used. Additionally, the fracture and its spatial distribution were described explicitly, and the strength and deformability of complex fractured rock masses were investigated. A case study was conducted at the dam site area of Lianghekou Hydropower Station's the left bank slope. The ShapeMetriX3D system was used to acquire the geometric parameters and probability distribution of fractures via digital photogrammetry. Additionally, a series of numerical models were created by combining the proposed equivalent DFN method with the fracture geometry information from the case study. Based on the concept of scale effect and anisotropy, the representative element volume (REV) and mechanical parameters of fractured rock masses were determined. The mechanical parameters of slope rock masses obtained by the proposed DFN modelling and generalized Hoek Brown strength criterion were compared. Furthermore, the applicability and reliability of the proposed model were validated using site data and numerical verifications. The proposed 3D equivalent DFN modelling can analyze the scale effect and anisotropy of mechanical properties of fractured rock masses effectively. Finally, the influence of water head on the hydraulic characteristics of fractured rock masses was analyzed based on the proposed model. Results showed that fractures were the main pathway of water.

Scaling Relationships between van Genuchten Model Parameters and Hydraulic Conductivity

The soil-water characteristic curve (SWCC) is indispensable for predicting the behaviour of groundwater flow in vadose zones. It remains difficult to determine SWCC, particularly for fractured rocks, under field conditions when little or no test data is available. It is generally believed that the van Genuchten model (VG) parameters α and n are related to pore size distribution (PSD) of soil and can be estimated from PSD data. In this study, correlations are proposed for predicting the VG parameters α and n from hydraulic conductivity K. The VG parameters were obtained by curve-fitting the experimental data of 993 soil samples collected from UNSODA database and published papers. The hydraulic conductivity K of the soil samples varies in six orders of magnitude from 1.30 × 10 −9 m/s to 1.22 × 10 −3 m/s. The statistical distribution of the VG parameters α and n is, respectively, examined using hypothesis test and Anderson-Darling test, showing that α follows a normal distribution after a Box-Cox transformation, and n follows a three-parameter lognormal distribution. The mean and standard deviation of α can be best estimated from K by and σ α = 1.4K 0.13, and the parameter n can be similarly estimated by and σ n = 6.6K 0.18. These correlations are also validated against site-scale test data at Yucca Mountain and published data on fractured rocks and applied to estimate the VG parameters of fractured rocks in the left bank slope at the site of Changbo Hydropower Project. The compiled data and the proposed equations are useful for estimating a narrower range of VG parameters from the more easily-obtained K values at site scale or in case of unavailable test data, and these estimates can be used as input for inverse modelling and simulation of unsaturated flow in porous or fractured media.

Bearing capacity behaviour of prestressed anchor cable under slope blasting excavation

The stability of high steep rock slopes directly influences the success or failure of whole engineering construction processes. Slope excavation and reinforcement with prestressed anchor cable can effectively improve the stability of the slope. However, the blasting stress waves have a great impact on the anchor cables that have been installed or are under construction. In this paper, a new simulating method of prestressed anchor cables is proposed for high steep rock slopes in consideration of the construction process and the reinforcement mechanism. And a simple simulating method of slope blasting excavation load is developed. The bearing capacity of prestressed anchor cables under slope blasting excavation is analysed via an ideal generalized slope model based on the left bank slope of the Jinping I hydropower station. The results show that the degree by which the distance from the blasting zone, the damping ratio, the maximum single-shot dose and the grade of the prestressed anchor cable impact the bearing capacity behaviours of the prestressed anchor cable decrease over time. Anchor cables are most prone to vibration failure within 5 m from the outer anchor head. The increase in tensile force is the largest on the anchor head at the slope surface. The impact of the maximum single-shot dose and damping ratio on the small-tonnage anchor cable is greater than that on the large-tonnage anchor cable. The anti-explosion performance of the large-tonnage anchor cable is better than that of the small-tonnage anchor cable. The foundations and experience needed to optimize the parameters of blasting excavation and anchor cable design for similar high-slope projects in China or other regions are presented.

Stability Monitoring and Analysis of High and Steep Slope of a Hydropower Station

Slope deformation and failure are major challenges for hydropower station engineering. Taking the left bank slope at the exit of the flood discharge tunnel of the Wudongde Hydropower Station in China as an example, the deformation mechanism of the high-steep rock slope was studied. The height of the slope is about 200 m, which is affected by slope excavation, rainfall, and atomized rain. The results of multipoint displacement meters, surface deformation monitoring, and anchoring stress meters showed that the deformation and deformation rate of the slope have increased dramatically. Through the comprehensive analysis of the slope, it is found that the overall lithology of the slope is poor, the excavation disturbance causes the redistribution of the stress in the slope, and the excavation surface is relatively steep, which provides space for the deformation of the slope rock mass. Unloading relaxation leads to a large number of new fissures in the slope rock mass. These new fissures and fault fracture zones provide convenient conditions for rainfall and atomized rain infiltration. The rainwater infiltrated along the slope surface, formed the seepage field in the slope body, and weakened the rock-soil mass parameters. Meanwhile, saturated runoff is formed on the slope, causing large deformation of the slope rock mass. However, the migration of water in the slope has a time effect, and its influence on the stability of the slope also has a time effect. It is difficult for traditional monitoring methods to monitor the resulting changes in internal sliding force of the slope. Therefore, a remote monitoring and early warning system for landslide anchor cable force was introduced to monitor the slope stability changes caused by the impact of water flow and rainfall infiltration, which provided a reasonable and scientific reference for subsequent slope construction.

A micro-mechanics-based elastoplastic friction-damage model for brittle rocks and its application in deformation analysis of the left bank slope of Jinping I hydropower station

This study develops a micro-mechanics-based elastoplastic damage model within the framework of irreversible thermodynamics. In the model, damage is related to growth of micro-cracks, while plastic deformation is induced by frictional sliding along those cracks. The damage criterion and functions of thermodynamic force are used to describe the damage evolution of rocks and determine whether they are damaged. Time-dependent deformation of rock is also taken into account by considering subcritical growth of micro-cracks. For engineering application, the proposed model is implemented in the standard finite element code Abaqus as a user-defined material model (UMAT) by using a specific local integration algorithm. The accuracy of the proposed model is assessed by comparing numerical results with experimental data in conventional triaxial compression tests and triaxial creep tests. The proposed model is then used for simulating the displacement field, damage and plastic zones of the left bank high slope. The predicted results are in a good consistency with field monitored data.

Stability Analysis of the Left Bank Slope of Baihetan Hydropower Station Based on the MF‐DFA Method

Based on the left bank slope of Baihetan hydropower station in Southwestern China, a high‐precision microseismic monitoring system was established. An early warning model of surrounding rock mass deformation and failure based on MF‐DFA was proposed. The results showed that the multifractal characteristics of the microseismic and blasting waveform time series in the left bank slope were obvious, and the multifractal spectrum width of the blasting waveform is much larger than that of microseismic waveform. Before the slope cracks increased, the multifractal time‐varying response characteristics of microseismic waveform showed strong regularity, which could be regarded as a precursor of surrounding rock mass deformation. Before the deformation and failure of surrounding rock mass, the multifractal spectrum width Δα showed an increasing trend while the multifractal spectrum of microseismic waveforms Δf(α) presented a decreasing trend, which can be regarded as a precursor of surrounding rock mass deformation; when deformation and failure occurred, Δα showed a decreasing trend and Δf(α) showed an increasing trend, which can be regarded as a deformation failure period; after the occurrence of deformation and failure, both Δα and Δf(α) showed a steady trend, and Δf(α) would approach to the zero line, which can be regarded as a stable period.

Deformation Analysis of Large-Scale Rock Slopes Considering the Effect of Microseismic Events

To research the macroscopic deformation of rock microseismic damage, a high-precision microseismic monitoring system was established on the left bank slope of the Baihetan hydropower station in Southwestern China. Based on the microseismic monitoring and field deformation data, the seismic source radius was applied to characterize the rock fracture scale. Numerical simulations introduced the rock micro-fracture information into the three-dimensional numerical model of the left bank slope and established the damage constitutive model. The unloading deformation process of the left bank abutment rock mass is described by numerical calculations. The feedback analysis method considering the effect of microseismic damage is preliminary exploratory research, which provides a new idea for the stability analysis of similar high rock slopes.

Focal mechanism determination for microseismic events and its application to the left bank slope of the Baihetan hydropower station in China

Determination of the focal mechanism of microseismic (MS) events occurring in rock engineering plays an important role in hazard evaluation and forecasting. A complete moment tensor inversion strategy is proposed to analyse MS events recorded at the left bank slope of the Baihetan hydropower station, southwest of China. A coordinate system rotation procedure is adopted to construct a reliable 1-D velocity model for the Green’s function calculation. The layered velocities are accurately determined by inversion using the recorded blasts and geological survey results. To reduce the influence of noise contamination, only the waveforms in the frequency range of 60–150 Hz are analysed. Before moment tensor inversion, a relocation procedure using an eikonal solver is performed to accurately locate seismic sources. A robust global optimization routine is applied to maximize the objective function, thereby determining the focal mechanism. The proposed method is tested using a hypothetical seismic source in the sensor network used for monitoring MS events in the rock slope. The synthetic test shows that the method is robust and can provide accurate solutions when subject to location error, noise contamination and velocity model disturbances. The analysis results of the 12 MS events show that the moment tensors characterizing MS events present significant non-double-couple components and that the majority of the MS events have a strike orientation in accordance with that of the main staggered zones.

Stability assessment of the left bank slope of the Baihetan Hydropower Station, Southwest China

Stability assessment of the left bank slope of the Baihetan Hydropower Station, Southwest China

Stability analysis of complex rock slopes reinforced with prestressed anchor cables and anti-shear cavities

To evaluate the stability of reinforced complex rock slopes, an evaluation method for slope stability is proposed in this paper. In the proposed method, the force equilibrium of the potential slide mass is analyzed by treating the traction on the potential failure surface as an external force. The potential sliding direction is assumed to be the opposite direction of the resultant shear force on the potential failure surface, and the factor of safety is defined as the ratio of the available resisting force to the actual mobilized force. Through classic cases, the proposed method successfully predicts the stability of a slope critical to failure, and is compared with the traditional methods. Finally, the proposed method is used for a stability analysis of the left bank slope of the Jinping first stage hydropower station, considering two main reinforcing measures, prestressed anchor cables, and anti-shear cavities. Results indicate that the effects of the anti-shear cavities are more positively significant than the effects of the prestressed anchor cables. The safety factor is remarkably increased by the anti-shear cavities, which validates the importance of anti-shear cavities on slope stability.

Large-scale Shaking Table Model Test Research on Bai He Tan Hydropower Station Left Bank Slope

The large-scale shaking table model test for a large engineered slope was conducted in this paper to reveal the slope seismic failure mechanism and analyze the seismic motion magnification effect along the slope height, in which dynamic stability of the slope could be evaluated. The model design, seismic motion input method, test program and measuring point arrangements are introduced. The model slope dynamic properties are obtained. The test results indicated that the tested slope is stable under design earthquake, and there are large safety margin.

Comprehensive evaluation of the stability of the left-bank slope at the Baihetan hydropower station in southwest China

The stability of the left-bank slope is a crucial geological engineering problem at the Baihetan hydropower station, China. Due to continuous excavations on the rock slope, different regions of the surrounding rock mass undergo varying degrees of unloading deformation. It is important to assess the stability of the rock slope from a macroscopic viewpoint by investigating its deformation characteristics and mechanisms. Therefore, in this work, microseismic (MS) monitoring was first employed to detect the progressive rock mass damage in the rock slope subjected to excavation, including the initiation, propagation, coalescence, and interaction of rock microfractures. Numerical modeling was subsequently performed to understand the deformation and failure mechanism of the rock slope. Moreover, traditional surveying approaches (i.e., multiple-point extensometers and inclinometers) and field observations were also used to analyze the deformation and failure characteristics of the rock slope. The MS monitoring results showed that spatiotemporal regularities in the evolution of seismic source locations were indicators of deformation failure and potential sliding surfaces. MS event clustering can be used to delineate activated pre-existing geological structures (i.e., LS331 and LS337). The simulation results show that the deformation and failure characteristics of the rock slope are mainly controlled by pre-existing weak structural planes (i.e., the intraformational faulted zones LS3319, LS331, and LS337 and fault F17). These results agree well with the results of geological data and conventional monitoring data. Our study reveals that an integrated approach combining MS monitoring, numerical modeling, traditional surveying, and field observations leads to a better understanding of the behavior of the rock slope under the influence of excavation as well as greater control of the working faces, ensuring safety under complex geological and excavation conditions.

Safety monitoring and stability analysis of left bank high slope at Jinping-I hydropower station

At the Jinping-I hydropower station the excavation height on the slope of the left bank is about 530 m. The stability of the excavated slope is very important because of the complicated geological conditions. To analyse the process of deformation evolution of the left abutment slope during construction, a comprehensive monitoring system was employed, which combined surface deformation observations, multi-point extensometers and graphite rod extensometers. This paper describes this monitoring system and analyses the deformation development with slope excavation. We also established a 3D numerical model of the slope and simulated the whole process of excavation using the finite-difference method. Results from both monitoring and numerical simulation show that the deformation depth in the left bank slope of the Jingping-I hydropower station is over 150 m owing to large-scale excavation unloading, far greater than that of general engineering slopes. The 3D limit equilibrium method and the strength reduction method are applied to evaluate the stability of the excavated slope. The calculated factors of safety indicate that the left abutment slope is stable and safe as a whole, which is in agreement with the deformation monitoring results.

Microseismic Monitoring of the Left Bank Slope at the Baihetan Hydropower Station, China

Microseismic Monitoring of the Left Bank Slope at the Baihetan Hydropower Station, China

Microseismicity and its time–frequency characteristics of the left bank slope at the Jinping first-stage hydropower station during reservoir impoundment

The instability risk associated with the left bank slope of the Jinping first-stage hydropower station in south-western China may increase as a result of the initial impoundment of the reservoir and the raising and lowering of the reservoir level. To improve our understanding of the rock slope stability and the real-time behaviour of the composite system of rock slope and dam, an existing microseismic (MS) monitoring system installed on May 2009 was improved in early 2014. This investigation represents the first time such a technique has been used to evaluate the stability of a high rock slope in China. The safety and maintenance of the rock slope during the reservoir impoundment were investigated through the development of fast and accurate real-time event location technique aimed at assessing the evolution and migration of the seismic activity and through the development of capabilities for forecasting the rock slope stability. Numerous MS events in the rock slope have been recorded using the MS monitoring system. Zones of damaged rock mass were identified based on a spatio-temporal analysis of the seismic activity during the impoundment period. Moreover, a waveform decomposition method based on the S transform was proposed for processing the seismic waveforms during the construction and impoundment periods of the left bank slope, and the time–frequency characteristics of seismic events during these two periods were thus compared. The understanding of the relationships between the time–frequency parameters variations in the seismic signals and the rock mass damage can provide a basis for evaluating the stability of similar rock slopes.

Cracking, stability and slope reinforcement analysis relating to the Jinping dam based on a geomechanical model test

The aim of this study is to examine cracking and instability of the high and steep left bank slope with weak rock mass structures, and effectiveness of the reinforcements designed for the slope at the Jinping hydroelectricity power station, southwestern China. A new geomechanical model testing method is first proposed for evaluating slope safety factors. In the proposed geomechanical model test, the slope is constructed on a flat testing bed, which can be rotated by hydraulic lifts. By increasing the rotation angle of the testing bed, the forces tending to induce the sliding of the slope are increased, which may cause crack initiation and propagation in the slope and result in rock mass slippage. Thus, through the proposed geomechanical model test, the slope failure mechanism, progressive failure process and final failure pattern can be studied. Moreover, the stability safety factors can be evaluated according to displacement data monitored by sensors installed in the geomechanical model. The geomechanical model test of the Jinping left-bank slope reveals that the bending and toppling cracks occur simultaneously in the unreinforced zone of the slope together with strong relaxation and tension cracks. It is found that the factor controlling the rock mass failure and instability is the structurally weak rock mass and the dominant failure mode is the integral catastrophic instability mode, in which the slope energy is totally dissipated and the slope destabilizes at a limit state. The reinforcement installed in the large area above the elevation of the dam-slope abutment and the unloading action due to the excavation of rocks lying above the dam platform have effectively improved the anti-slide safety factor of the slope, and thus increased its inherent safety factor. On the basis of the geomechanical model testing results and their comparisons with field monitoring results, it is concluded that the installed rock bolts and long anchor reinforcement measures are very effective in keeping the cracks closed in the rock mass and maintaining the slope stability.