Weidong Lei

Transmission ratio (Tn) in the radian direction normal to joints in 2-D compressional wave propagation in rock masses

Abstract The transmission ratio along the radian direction normal to the joints was studied in the Universal Distinct Element Code (UDEC). The variation of the transmission ratio with the ratio of joint spacing to wavelength was generalized into a general curve, which was determined by two critical points. The relationship between the two critical points and the affecting factors, quantity of joints and the normalized normal stiffness of joints, were obtained. A prediction model of the transmission ratio in the radian direction normal to the joints was proposed. The proposed model was applied to a field explosion test. The estimated values of the peak particle velocity from the prediction model were compared with the field records. The comparisons showed that the prediction model of the transmission ratio in the direction normal to the joints in the process of 2-D compressional wave propagation through multiple parallel joints is reliable.

Anisotropic shear strength characteristics of a tailings sand

The deposition process of tailings in a tailings dam may lead to the anisotropy of mechanical properties of tailings. To investigate the anisotropic shear strength characteristics of a tailings sand, samples deposited under gravity were sheared in a modified direct shear apparatus along different orientations of the shear planes relative to the bedding plane. The experimental results reveal that the anisotropy has a substantial effect on the peak shear strength of the tailings sand, leading to a variation in peak friction angle of 10°. It is found that the effect of anisotropy on the peak shear strength of the tailings sand is mainly attributed to the effect of anisotropy on soil dilatancy. On the other hand, the effect of anisotropy on the residual friction angle of the tailing sand is relatively smaller, likely due to the partial erasure of anisotropy at large shear deformation. The degree of anisotropy is reduced by an increase in normal stress. To achieve more accurate stability analysis of tailings dams, it is suggested to take into account the shear strength anisotropy of tailings soils.

Corner treatment by assigning dual tractions to every node for elastodynamics in TD-BEM

In this study, a corner treatment on the boundary for time domain boundary element method (TD-BEM) by assigning dual tractions to every node is proposed. The proposed corner treatment on boundary mimics the real mechanical situation on the physical boundary for elastodynamic problem, and the computation accuracy is good. Taking the rectangular boundary as an example, the numerical implementation of the proposed corner treatment on boundary by assigning dual tractions to every node is demonstrated. Two verification examples, one-dimensional rod in finite medium and two-dimensional circular cavity in infinite medium under dynamic loads are conducted, in which the results from the TD-BEM, based on the proposed corner treatment on boundary, are compared with the results from the analytical solutions. Good agreement between the analytical results and the TD-BEM results, based on the proposed corner treatment on boundary for TD-BEM, is achieved for the two verification examples.

Influences of Loading Mode and Construction Speed on Position of Phreatic Line of a Tailings Dam

The location of phreatic line of a tailings dam is of significant importance on the stability of the tailings dam. In this study, extensive numerical analysis in ABAQUS have been performed to study the influences of loading mode and construction speed on position of the phreatic line of tailings dam in the process of construction from the existing elevation of 59 m to the final elevation of 100 m. The results show that the location of the phreatic line changes very little at the lower part of the dam; by contrast, the location of phreatic line dramatically changes at the upper part of the dam. At a certain construction speed, loading mode has negligible influence on the position of the phreatic line during the duration of one rising round for the tailings dam. The construction speed has more significant influences on the position of phreatic line in the middle term in the service duration of the tailings dam than in the later period.

Improved Watershed Segmentation Method in Rock Fragmentation Analysis on Digital Photos

Reasonable distribution of the rock fragmentation can reduce the overall cost in a mining industry. Digital image processing technique can be used to determine the rock fragmentation distribution function. This paper proposes an improved watershed segmentation method in digital image processing technique. The seven key steps in the new method are presented. Meanwhile, the detailed rock fragmentation analysis is performed on an example digital photo of fragmented rock pile taken from a practical underground mine, to further explain the mechanism of the new method. It is found that the new method can overcome the phenomenon of over-segmentation and over-lapping in digital photos, and exactly detect boundaries for every individual target in the photo.

Response of Soil Suction to Heavy Rainfalls in a Tailings Dam

Rainfall is a significant factor leading to failure of tailings dams. The impact of rainfall on the instability of dams is mainly reflected in the variation of negative pore-water pressure (i.e. matric suction) during rainfall infiltration. However, there is a lack of study on the effects of rainfall on suction in tailings dams. In this study, the response of suction to artificial heavy rainfalls in a tailings dam was investigated. The effects of rainfall intensity and surface vegetation conditions on the response of suction were studied. It is found that suctions at a certain depth in the tailing dam were kept constant until the wetting front reached this depth. Once suctions were altered, the values dropped rapidly. The magnitude of suction change generally decreased with depth. Rainfall infiltration mainly occurred above the depth of 40 to 80 cm when subjected to rainstorm and heavy rainstorms. Larger rainfall intensity leads to shorter response time and to larger depth affected by rainfall, implying that the tailings dam is more susceptible to shallow landslide failure under larger rainfall intensity. The existing vegetation increases infiltrability significantly and then produces an adverse effect on the stability of the tailings dam. On the other hand, it is observed that the presence of vegetation greatly prevented surface erodibility and then decreases the possibility of debris flow.

Strength Anisotropy of a Tailings Sand and its Effect on Stability of a Tailing Dam

In current engineering practice, the effect of strength anisotropy on stability analysis of tailings dam is often ignored. This paper presents direct shear test results on a tailing sand with different angles between shear direction and bedding plane. It is found that shear strength of this soil is significantly directional dependent. The variations in peak friction angle are about 11o, which is approximately 28.3% of the minimum value. The effect of strength anisotropy on stability of a tailings dam is also investigated. The analysis considering anisotropic strength shows a smaller factor of safety and shallower most critical slip surface as compared with isotropic strength model that is used in current engineering practice. To achieve more accurate stability analysis of tailings dams, it is suggested to take into account shear strength anisotropy of tailings soils.

A Geometric Correction Method in Survey of Geometry Properties of Discontinuities in Rock Masses

A new geometric correction method in processing close range digital photograph is presented. All the critical steps involved in the geometric correction method are briefed and discussed, including determination of Control Points (CP) on the exposure of the rock mass, projection of the 3-D CPs on the exposure of the rock mass to the projection plane, mathematically solving the transformation matrix, geometrically correcting and resampling the digital photograph. As an illustrative example, the whole process of application of the method for a specific digital photograph taken from an outcrop of rock mass is also presented. It is found that the new geometric correction method is effective and more convincing.

Evaluation of Stability of Tailings Dam Based on Evolutionary Artificial Neural Network

The stability of tailings dam is a multifactor-influenced and highly nonlinear problem. In this paper, an Evolutionary Artificial Neural Network (EANN) model was proposed to deal with this problem. In the proposed model, 9 influence factors are adopted as input variables, which are divided into three main categories, i.e., topography, geology and external influence factors. The Genetic Algorithm is used to optimize initial weight distribution of a Back Propagation network. The factor of safety predicted by the EANN model has an error less than 3% as compared with that value calculated by Bishop’s method. This justifies that the EANN is an effective method for assessing the stability of tailings dam. From saliency analysis, it is found that the gradient of tailings dam, friction angle of tailings and the surface flow are the predominant influence factors to the factor of safety.