Lian-heng Zhao

Reliability back analysis of shear strength parameters of landslide with three-dimensional upper bound limit analysis theory

It is essential to determine the shear strength parameters c and φ on the sliding surface for stability evaluation and engineering design of a landslide. In this study, a new parameter back analysis method is proposed by combining the 2D/3D upper bound method of limit analysis and reliability theory to accurately determine the shear strength parameters for a 3D slope with a single failure surface. The proposed reliability back analysis method overcomes the shortcomings of the traditional deterministic analysis method of slope stability that cannot take into account the randomness and uncertainty of geotechnical parameters. Based on the reliability theory, two methods were studied: first-order reliability method (implemented by spreadsheet and Matlab, called spreadsheet method and constrained optimization method, respectively, in this paper) and Monte Carlo simulation. The optimized values of c and φ were obtained by establishing only one balance equation with the consideration of the pore water pressure or other complex conditions, which can solve the problem of the back analysis of strength parameters for a single 3D sliding surface condition. The correlation research showed that the negative correlation between c and φ greatly affected the back analysis results, and the reliability index values were conservative without considering such a negative correlation. A case study for the back analysis of shear strength parameters is conducted based on a practical landslide model with a broken line slip surface slope in Zhuquedong village, Luxi town, Xiangxi County, Hunan Province, China, and a suggestion for the selection of landslide cross section is presented. The results show that the back analysis results determined by the reliability theory coincide well with the survey and experimental results. The proposed method is found to be more accurate and effective in determining the values of shear parameters than that of the traditional deterministic inversion method.

Coupled hydro-mechanical response of saturated asphalt pavement under moving traffic load

Based on the Boits theory, the governing equation was established to account for the response of moisture pavement. The analytical solutions were obtained through the expansion of Fourier series. Furthermore, the effects of parameters (i.e. hydraulic conductivity, traffic load velocity, drainage boundary and solid modulus) on dynamic response were investigated in terms of water-induced damage of pavement. Compared with the dry–elastic pavement, the negative normal stress in saturated asphalt pavement is concentrated beneath the traffic load, which may be a reason for a damage phenomenon in asphalt pavement. Hydraulic conductivity anisotropy plays a significant role in influencing the physical fields. Between vertical and horizontal hydraulic conductivity, the physical field almost depends on vertical hydraulic conductivity rather than horizontal hydraulic conductivity which just affects the horizontal pore-water velocity obviously. Moreover, the drained boundary evidently influences the seepage field of surface course with high permeability.

Stability design charts for homogeneous slopes under typical conditions based on the double shear strength reduction technique

Based on a brief review of the existing shear strength reduction (SSR) techniques, the objective function of the comprehensive safety factor for simple homogeneous slopes is established by combining the double SSR technique (considered the shortest pathway of the strength reduction) with the upper bound limit analysis theorem, leading to a strict upper bound on the safety factor. Combining nonlinear sequential quadratic programming (SQP) with the random walk method, the value of the comprehensive safety factor can be optimized, avoiding the trap of a local minimum. Compared with classical examples, the present method is a conservative and effective method for slope stability evaluation. A set of design charts for homogeneous slopes under simple and typical conditions, such as surcharge load, pore water pressure, and seismic loading are produced by the analysis of substantial data, which can eliminate the necessity for iterations when calculating the safety factor. These stability charts are presented in a convenient manner to determine the comprehensive safety factors and corresponding failure patterns under different typical conditions, which might be preferred by engineers for performing the preliminary evaluations of slope safety. Several examples are used to illustrate the application of these stability charts under different conditions.

Development mechanism for the landslide at Xinlu Village, Chongqing, China

Due to the continuous and intense rainfall from June 26 to 28, 2016, Xinlu Village in Ganshui Town, Qijiang District, Chongqing, experienced a unitary-slip landslide at approximately UTC+8 19:30 on June 28. This landslide disrupted the Chuan-Qian railway and damaged four residential buildings. To analyze and rehabilitate the landslide, the engineering geology, hydrological conditions, and deformation instability mechanism of this landslide were investigated and comprehensively analyzed based on an in situ survey, geophysical drilling, and a laboratory quick-shearing test. The results show that the landslide is a typical gradual progressive landslide.

Distinct Element Modelling of a Landslide Triggered by the 5.12 Wenchuan Earthquake: A Case Study

The Donghekou landslide, which was triggered by the 2008 Wenchuan earthquake, was estimated to have a volume of 24 million cubic meters and resulted in numerous properties and lives lost. The landslide is considered a typical rapid long run-out earthquake-induced event, but the kinematic processes are not well understood. The main objectives of this study were to numerically model the landslide progression and to reproduce the post-failure configuration. We first built a physical model of the slope based on the topography and geology of the source area from field investigations. The corrected baseline and filtered actual ground motions were then used as the volume force acting on the base block, and the kinematic process and mechanics of the throwing phenomenon were modelled using a 2-D discrete element code. We used the non-linear Barton–Bandis criterion to accurately simulate the behaviour of joints. The size effect of the enormous landslide was also considered. The results of the simulations agreed well with those obtained from post-earthquake field investigations. A sensitivity analysis of several related parameters that may control the dynamic movements of the Donghekou landslide was discussed in detail. The results show that: the seismic force and the residual friction angle were main factors that affect the run-out of landslides.

Limit equilibrium method (LEM) of slope stability and calculation of comprehensive factor of safety with double strength-reduction technique

When the slope is in critical limit equilibrium (LE) state, the strength parameters have different contribution to each other on maintaining slope stability. That is to say that the strength parameters are not simultaneously reduced. Hence, the LE stress method is established to analyze the slope stability by employing the double strengthreduction (DSR) technique in this work. For calculation model of slope stability under the DSR technique, the general nonlinear Mohr–Coulomb (M–C) criterion is used to describe the shear failure of slope. Meanwhile, the average and polar diameter methods via the DSR technique are both adopted to calculate the comprehensive factor of safety (FOS) of slope. To extend the application of the polar diameter method, the original method is improved in the proposed method. After comparison and analysis on some slope examples, the proposed method’s feasibility is verified. Thereafter, the stability charts of slope suitable for engineering application are drawn. Moreover, the studies show that: (1) the average method yields similar results as that of the polardiameter method; (2) compared with the traditional uniform strength-reduction (USR) technique, the slope stability obtained using the DSR technique tends to be more unsafe; and (3) for a slope in the critical LE state, the strength parameter φ, i.e., internal friction angle, has greater contribution on the slope stability than the strength parameters c, i.e., cohesion.

Study on Anti-Slide Stability of Rock Slope along River

Based on the assumption that the hydrostatic pressure is maximum at half the total groundwater level, which improved the groundwater pressure distribution in the slope body, the safety factor expression for anti-slide stability of the rock slope along the river were deduced using the limit equilibrium theory. This deduction included various influencing factors, such as slope-top surcharge, seismic load, anchor load, river pressure on the slope surface, groundwater pressure, and scouring effect. The relationship diagrams between the safety factor and the influencing factors were constructed. RESULTS show that the new assumption, which improved the hydrostatic pressure distribution, is reasonable. Hydrostatic pressure is an important influencing factor to the slope anti-slide stability. The higher river water level and the lower groundwater level are beneficial to improve the safety factor of the slope anti-slide stability. Whether the outflow joint blocking has significant effect on the slope stability against sliding, the outflow joint blocking has adverse effect on anti-slide stability of the rock slope along the river in the general case. The anti-slide stability safety factor of the outflow joint blocked is larger than that of the outflow joint unblocked only when Hr > Hw. Language: en

Discussion on Superposition Empirical Formula of Ultimate Bearing Capacity Based on the Upper Bound Limit Analysis Theory

Traditional Terzaghi foundation bearing capacity superposition empirical formula dose not consider the nonlinear effect of the soil unit weight on the ultimate bearing capacity and its failure mode, which is only a kind of approximate method. Therefore, error occurs when unit weight is taken into account. The relatively complete two dimensional conversion velocity field based on slip-line field and the basic method and principle of corresponding failure mechanism were introduced in this paper. Compared with the existing upper bound failure mechanism, this failure mechanism adds a one-dimensional optimization freedom degree, which can reflect the real failure mode, and thus obtain better upper bound solutions. The results indicate that the traditional superposition empirical formula stress on safty, and the error caused by traditional superposition method will increase with the increase of the internal friction angle; the failure range derived from non-superposition method is between the failure range determined by using superposition method to solve the analysis coefficient Nγ and Nc (or Nq), which reflects the influence of soil unit weight on the failure mechanism.

Upper Bound Limit Analysis-Based Determination of the Influence of a Non-Associated Flow Rule on the Stability of Shallow Tunnels

AbstractOn the basis of a non-associated flow rule and the upper bound theorem for limit analysis, the expression of surrounding rock pressure on shallow tunnels was derived by constructing a simple failure pattern. Strength reduction technique was applied to study the stability of shallow tunnels under certain levels of surrounding rock pressure; this analysis was based on the internal consumption and external energy conservation principle. The optimized upper bound solutions of surrounding rock pressure and the safety factor for shallow tunnels were obtained by nonlinear optimization methods. Results show that the dilatancy of geomaterials and the lateral pressure coefficient of surrounding rock significantly affect the surrounding rock pressure and stability of shallow tunnels. Furthermore, the lateral pressure coefficient is an important factor in relation to the stability of such tunnels.

Study on Transfer Behavior of Negative Friction of Single Pile in Two-Layer Soil

This paper aims at calculating the transfer behavior of negative friction of single pile in two-layer soil in pile foundation. Based on the traditional load transfer function method, a model for calculating the transfer behavior of negative friction on a single pile in a two-layer soil profile is presented in this paper. In this model, the settlement of soil is assumed to follow bi-linear distribution; the transfer function is linear elastic and full plastic; the pile toe resistance is assumed to vary linearly with movement. By solving the model equations, the analytic solution of relative displacement and axial force of pile have been obtained. Moreover, a case history from literature is used to validate the proposed theoretical model. The prediction of negative friction from the theoretical model agrees well with the measured value from the case history.