Experimental Study of Seepage Properties of Broken Sandstone Under Different Porosities

In coal mining the water flow in broken rock is a very common phenomenon. Study of seepage properties of broken rock is one of the basic subjects required in order to understand the stability of rock surrounding roadways, preventing disasters such as water inrush and gas outbursts and developing underground resources. So far, quantitative studies on the nonlinear seepage properties of broken sandstone under different porosities are not extensive in the research literature. In this article, by means of an electro-hydraulic servo-controlled test system (MTS815.02) and a patent seepage device, the seepage properties under different conditions of porosity were tested on broken sandstone of five different grain sizes. Based on the loading method of controlling the axial compression displacement and steady permeating method, we obtained curves of the relation of pore pressure with time, as well as the relation curves between the pore pressure gradient for steady seepage and velocity. Furthermore, we calculated the permeability k and non-Darcy coefficient β corresponding to different porosities by fitting these curves with the binomial expression. This study indicates that: (1) the seepage properties of broken sandstone are closely related to grain size, load levels, and porosity structure; (2) the permeability k decreases, while the coefficient β increases with a decrease in porosity φ, but both the k − φ and the β − φ curves show some local fluctuations; (3) the permeability k of the broken sandstone has a magnitude of 10⁻¹⁴–10⁻¹² m², while the coefficient β ranges from 10¹⁰ to 10¹² m⁻¹. The results obtained provide some information for further study of the nonlinear seepage behavior of broken rock theoretically.     

Multi-filter analysis of infrared images from the excavation experiment in horizontally stratified rocks

This paper presents a morphological study of the low-resolution and noisy thermogram obtained in the laboratory excavation experiment on the large-scale geological model of man-made horizontally stratified rocks. The developed new image processing algorithm consists of image subtraction, median filter, low-pass alternating-morphological filter, and multi-scale morphological enhancement filter. These filters were used sequentially to process the detected infrared images. In comparison with the old algorithm for thermogram process, the excavation border with changed stress and changed brightness can be distinguished more clearly in the new thermogram sequences. The new algorithm is possible to be applied in processing other low-resolution and noisy thermogram, and the analysis results in this paper are helpful for understanding the pure elastic, the stationary frictional effect and the dynamic frictional effect at the excavation processes.     

An experimental study on seepage behavior of sandstone material with different gas pressures

The seepage evolution characteristic of brittle rock materials is very significant for the stability and safety of rock engineering. In this research, a series of conventional triaxial compression and gas seepage tests were carried out on sandstone specimens with a rock mechanics servo-controlled testing system. Based on the experimental results, the relationship between permeability and deformation is firstly analyzed in detail. The results show that the permeability–axial strain curve can be divided into the following five phases: the phase of micro-defects closure, the phase of linear elastic deformation, the phase of nonlinear deformation, the phase of post-peak stress softening and the phase of residual strength. The seepage evolution characteristic is also closely correlated with the volumetric deformation according to the relationship between permeability and volumetric strain. It is found that the gas seepage pressure has a great effect on the permeability evolution, i.e. permeability coefficients increase with increasing gas seepage pressures. Finally, the influence of gas seepage pressures on the failure behavior of brittle sandstone specimens is discussed.     

Review of RGB photoelasticity

Automatic methods of photoelasticity have had a significant progress with the development of automatic acquisition and image processing methods. This article concerns RGB photoelasticity, which allows the determination of the photoelastic retardation using, usually, a single acquisition of the isochromatic fringes in white light by a colour camera. In particular, the article presents an overview of the main characteristics of RGB photoelasticity that is influence of the quarter-wave plate error, number of acquisitions, type of light source, determination of low and high fringe orders, methods for searching the retardation, scanning procedures, calibration on a material different from that under test, combined use of the RGB and phase shifting methods. A short section on the applications of RGB photoelasticity completes the article.     

On the (N+1)-dimensional local fractional reduced differential transform method and its applications

In this paper, we generalize the (N+1)-dimensional local fractional reduced differential transform method (LFRDTM) within the local fractional derivative sense. First, we show some new properties, lemmas, theorems and corollariesfor the (N+1)-dimensional LFRDTM. Second, these new properties, lemmas and theorems can be proved immediately after. Thirdly, we used two examples to state that this approach is efficient and simple to find numerical solutions to higher dimensional local fractional partial differential equations. Finally, we can be seen that this work can be looked as an extension of the prior work.     

Uniaxial experimental study of the acoustic emission and deformation behavior of composite rock based on 3D digital image correlation (DIC)

In this paper, uniaxial compression tests were carried out on a series of composite rock specimens with different dip angles, which were made from two types of rock-like material with different strength. The acoustic emission technique was used to monitor the acoustic signal characteristics of composite rock specimens during the entire loading process. At the same time, an optical non-contact 3 D digital image correlation technique was used to study the evolution of axial strain field and the maximal strain field before and after the peak strength at different stress levels during the loading process. The effect of bedding plane inclination on the deformation and strength during uniaxial loading was analyzed. The methods of solving the elastic constants of hard and weak rock were described. The damage evolution process, deformation and failure mechanism, and failure mode during uniaxial loading were fully determined. The experimental results show that the θ = 0?–45?specimens had obvious plastic deformation during loading, and the brittleness of the θ = 60?–90?specimens gradually increased during the loading process. When the anisotropic angle θincreased from 0?to 90?, the peak strength, peak strain,and apparent elastic modulus all decreased initially and then increased. The failure mode of the composite rock specimen during uniaxial loading can be divided into three categories:tensile fracture across the discontinuities(θ = 0?–30?), slid-ing failure along the discontinuities(θ = 45?–75?), and tensile-split along the discontinuities(θ = 90?). The axial strain of the weak and hard rock layers in the composite rock specimen during the loading process was significantly different from that of the θ = 0?–45?specimens and was almost the same as that of the θ = 60?–90?specimens. As for the strain localization highlighted in the maximum principal strain field, the θ = 0?–30?specimens appeared in the rock matrix approximately parallel to the loading direction,while in the θ = 45?–90?specimens it appeared at the hard and weak rock layer interface.     

三维非稳定渗流自由面边界积分项的精确数值计算

利用固定网格法分析三维非稳定渗流问题时,将要面对两项积分难题:以自由面及单元表面为边界的空间积分及以自由面为边界的曲面积分。针对常用的任意8结点6平面三维普通单元,提出采用坐标变换及等参变换技术求取空间积分项的精确数值解;至于曲面积分项,建议改用单元非饱和区部分表面作为积分边界,经过坐标变换及等参变换处理积分边界后,利用高斯数值积分可求出曲面积分项的精确数值解。通过一个普通单元及一项均质半无限边界堤坝的实例分析,表明此方法的精确性和稳定性良好。     

Nonlocal microplane model with strain-softening yield limits

The paper deals with the problem of nonlocal generalization of constitutive models such as microplane model M4 for concrete, in which the yield limits, called stress–strain boundaries, are softening functions of the total strain. Such constitutive models call for a different nonlocal generalization than those for continuum damage mechanics, in which the total strain is reversible, or for plasticity, in which there is no memory of the initial state. In the proposed nonlocal formulation, the softening yield limit is a function of the spatially averaged nonlocal strains rather than the local strains, while the elastic strains are local. It is demonstrated analytically as well numerically that, with the proposed nonlocal model, the tensile stress across the strain localization band at very large strain does soften to zero and the cracking band retains a finite width even at very large tensile strain across the band only if one adopts an “over-nonlocal” generalization of the type proposed by Vermeer and Brinkgreve [In: Chambon, R., Desrues, J., Vardoulakis, I. (Eds.), Localisation and Bifurcation Theory for Soils and Rocks, Balkema, Rotterdam, 1994, p. 89] (and also used by Planas et al. [Basic issue of nonlocal models: uniaxial modeling, Tecnical Report 96-jp03, Departamento de Ciencia de Materiales, Universidad Politecnica de Madrid, Madrid, Spain, 1996], and by Strömberg and Ristinmaa [Comput. Meth. Appl. Mech. Eng. 136 (1996) 127]). Numerical finite element studies document the avoidance of spurious mesh sensitivity and mesh orientation bias, and demonstrate objectivity and size effect.     

振动对充填裂隙渗透率影响的实验研究

为了研究振动和充填泥沙的共同作用对裂隙岩石渗透率的影响,通过对裂隙试件在充填和振动后条件下的渗流实验,得到了频率和振幅与充填裂隙岩石渗透率的关系曲线。结果表明,充填泥沙后的相对渗透率低于充填前,最小降幅为10.19%,最大降幅为59.74%。在相同的振幅下,试件的相对渗透率随频率的增加都基本呈下降趋势;振动后的渗透率低于振动前的渗透率;充填试件在较大频率振动下,渗透率减小的更为显著。在相同的频率下,充填试件振动后的渗透率基本上都减小了,最小降幅为8.90%,最大降幅为51.23%;而未充填试件振动后的渗透率与振幅存在一定关系;相对渗透率与振幅的变化之间存在拐点,相对渗透率并没有随着振幅的增加而一直减小。