Numerical evaluation of strength and deformability of fractured rocks

Knowledge of the strength and deformability of fractured rocks is important for design, construction and stability evaluation of slopes, foundations and underground excavations in civil and mining engineering. However, laboratory tests of intact rock samples cannot provide information about the strength and deformation behaviors of fractured rock masses that include many fractures of varying sizes, orientations and locations. On the other hand, large-scale in situ tests of fractured rock masses are economically costly and often not practical in reality at present. Therefore, numerical modeling becomes necessary. Numerical predicting using discrete element methods(DEM) is a suitable approach for such modeling because of their advantages of explicit representations of both fractures system geometry and their constitutive behaviors of fractures, besides that of intact rock matrix. In this study, to generically determine the compressive strength of fractured rock masses, a series of numerical experiments were performed on two-dimensional discrete fracture network models based on the realistic geometrical and mechanical data of fracture systems from feld mapping. We used the UDEC code and a numerical servo-controlled program for controlling the progressive compressive loading process to avoid sudden violent failure of the models. The two loading conditions applied are similar to the standard laboratory testing for intact rock samples in order to check possible differences caused by such loading conditions. Numerical results show that the strength of fractured rocks increases with the increasing confning pressure, and that deformation behavior of fractured rocks follows elasto-plastic model with a trend of strain hardening. The stresses and strains obtained from these numerical experiments were used to ft the well-known Mohr-Coulomb(MC) and Hoek-Brown(H-B) failure criteria, represented by equivalent material properties defning these two criteria. The results show that both criteria can provide fair estimates of the compressive strengths for all tested numerical models. Parameters of the elastic deformability of fractured models during elastic deformation stages were also evaluated, and represented as equivalent Young’s modulus and Poisson’s ratio as functions of lateral confning pressure. It is the frst time that such systematic numerical predicting for strength of fractured rocks was performed considering different loading conditions, with important fndings for different behaviors of fractured rock masses, compared with testing intact rock samples under similar loading conditions.

Value Addition to Waste Material Supported by Removal of Available Phosphate from Simulated Brackish Water—A Low Cost Approach

Phosphorus is one of the major nutrients that have been identified as a limited resource that would end up earlier than predicted at the rate of current consumption. Therefore, attempts to recover phosphorus from waste and its subsequent use are a concern of current researchers. Nevertheless, recovery of nutrients from wastewater is cumbersome because nutrients such as phosphates () and nitrates () prefer to remain in aqueous phase rather than being adsorbed on solid matrixes. Investigation of adsorption of available - P from simulated brackish water, on granulated solid waste material, prepared by crushed autoclaved aerated concrete (CAAC), and subsequent use of the material as phosphate fertilizer would be the focus of this research. Treatment of nutrient-rich brackish water is important because such water is discharged in huge volume at the time of harvesting of shrimp aquaculture ponds. Experiments conducted in simulated brackish water confirmed non-linear adsorption association with changing distribution coefficient (KD) which attributed the maximum removal of about 98% - P from 100 mgdm-3solution at its value of 40. The non-linear adsorption supported by both the Langumuir and the Freundlich isotherm models simultaneously satisfied monolayer adsorption and multilayer adsorption depicted by the regression coefficients of greater than .99 by the linearized forms of the isotherm models. Moreover, promising phosphate uptakes characteristics are exhibited by the adsorbent at the process of repetitive adsorption which resulted in 12 g/kg uptake of phosphate at 81% efficiency. The adsorbent seems to be used as a slow-released phosphorus fertilizer at the end of its life as an adsorbent.

Anisotropy of strength and deformability of fractured rocks

Anisotropy of the strength and deformation behaviors of fractured rock masses is a crucial issue for design and stability assessments of rock engineering structures, due mainly to the non-uniform and non- regular geometries of the fracture systems. However, no adequate efforts have been made to study this issue due to the current practical impossibility of laboratory tests with samples of large volumes con- taining many fractures, and the difficulty for controlling reliable initial and boundary conditions for large-scale in situ tests. Therefore, a reliable numerical predicting approach for evaluating anisotropy of fractured rock masses is needed. The objective of this study is to systematically investigate anisotropy of strength and deformability of fractured rocks, which has not been conducted in the past, using a nu- merical modeling method. A series of realistic two-dimensional （2D） discrete fracture network （DFN） models were established based on site investigation data, which were then loaded in different directions, using the code UDEC of discrete element method （DEM）, with changing confining pressures. Numerical results show that strength envelopes and elastic deformability parameters of tested numerical models are significantly anisotropic, and vary with changing axial loading and confining pressures. The results indicate that for design and safety assessments of rock engineering projects, the directional variations of strength and deformability of the fractured rock mass concerned must be treated properly with respect to the directions of in situ stresses. Traditional practice for simply positioning axial orientation of tunnels in association with principal stress directions only may not be adequate for safety requirements. Outstanding issues of the present study and su^zestions for future study are also oresented.

Analysis and calculation of thermal conductivity of rock in deep strata

The thermal conductivity of rock is an important parameter for the deep mine and the geothermal development. It is often not possible to measure the thermal conductivity of the rocks present in the deep strata, and the usual approach is to calculate thermal conductivity including mineralogy and porosity. The compositions of core samples from the MID01 borehole in the Bjorko area were determined, and the mineral composition was classified. The calculation of the thermal conductivity of rock in the borehole was carried out, and the main factors for the thermal conductivity of rock were analyzed. The results show that the calculated thermal conductivity of rock is reliable and useful for the design and calculation of geothermal development in the Bjorko area.

Evaluation of hydrodynamic dispersion parameters in fractured rocks

A numerical procedure to determine the equivalent hydrodynamic dispersion coefficients and Péclet number（Pe） of a fractured rock is presented using random walk particle tracking method.The geometrical effects of fracture system on hydrodynamic dispersion are studied.The results obtained from the proposed method agree well with those of empirical models,which are the scale-dependent hydrodynamic dispersion coefficients in an asymptotic or exponential form.A variance case is added to investigate the influence of longitudinal hydrodynamic dispersion in individual fractures on the macro-hydrodynamic dispersion at the fracture network scale,and its influence is demonstrated with a verification example.In addition,we investigate the influences of directional flow and stress conditions on the behavior of hydrodynamic dispersion in fracture networks.The results show that the magnitudes of the hydrodynamic dispersion coefficients are relatively smaller when the flow direction is parallel to the dip directions of fracture sets.Compressive stresses significantly reduce hydrodynamic dispersion.However,the remaining questions are：（1） whether the deformed fracture network under high stress conditions may make the scale-dependent hydrodynamic dispersion coefficients have asymptotic or exponential forms,and（2） what the conditions for existence of a well-defined equivalent hydrodynamic dispersion tensor are.They need to be further investigated.

Numerical modeling of coupled thermo-mechanical response of a rock pillar

Understanding the rock mass response to excavation and thermal loading and improving the capability of the numerical models for simulating the progressive failure process of brittle rocks are important for safety assessment and optimization design of nuclear waste repositories.The international cooperative DECOVALEX-2011 project provides a platform for development,validation and comparison of numerical models,in which the sp pillar stability experiment（APSE） was selected as the modeling target for Task B.This paper presents the modeling results of Wuhan University（WHU） team for stages 1 and 2 of Task B by using a coupled thermo-mechanical model within the framework of continuum mechanics.The rock mass response to excavation is modeled with linear elastic,elastoplastic and brittle-plastic models,while the response to heating is modeled with a coupled thermo-elastic model.The capabilities and limitations of the model for representation of the thermo-mechanical responses of the rock pillar are discussed by comparing the modeling results with experimental observations.The results may provide a helpful reference for the stability and safety assessment of the hard granite host rock in China＇s Beishan preselected area for high-level radioactive waste disposal.

黄土高原流域土地利用/覆被动态演变及驱动力分析

该文采用土地资源数量变化模型和土地利用/覆被状态指数,研究了甘肃天水罗玉沟流域近20多年来土地利用/覆被的演变及驱动机制。研究结果表明:1986-2004年间,流域内坡耕地急剧减少、梯田迅速增加,林地和果园面积稳步提升。从土地利用类型转移变化看,梯田的转入速度略小于转出速度,为双向高速转换下的平衡状态;而灌木林转入速度小于转出速度,有规模减少的趋势。该地区土地利用类型在前期(1986-1995年)变化程度比后期(1995-2004年)更为剧烈。除特殊的地形地貌和气候因素外,政策、人口增长和经济发展等因素共同驱动土地利用类型的演变,特别实施天然林保护工程和退耕还林工程后,该地区土地利用状况有所好转。

Water,Salt and Heat Influences on Carbon and Nitrogen Dynamics in Seasonally Frozen Soils in Hetao Irrigation District,Inner Mongolia,China

To investigate carbon(C)and nitrogen(N)dynamics in seasonally frozen soils under saline and shallow groundwater supply conditions,in-situ lysimeter experiments with different groundwater table depths(WTD=1.8 and 2.2 m)were conducted in Inner Mongolia,China during the wintertime of 2012-2013.Changes in soil organic C and total N in multiple layers during various periods,as well as their relationships with soil water,salt,and heat dynamics were analyzed.Accumulation of soil organic C and total N during freezing periods was strongly related to water and salt accumulation under temperature and water potential gradients.Water and salt showed direct influences on soil C and N dynamics by transporting them to upper layer and changing soil microbial activity.Salt accumulation in the upper layer during freezing and thawing of soil affected microbial activity by lowering osmotic potential,resulting in lower C/N ratio.Nitrogen in soil tended to be more mobile with water during freezing and thawing than organic C,and the groundwater table also served as a water source for consecutive upward transport of dissolved N and C.The changes in C and N in the upper 10 cm soil layer served as a good sign for identification of water and salt influences on soil microbial activity during freezing/thawing.