Random field characterization of uniaxial compressive strength and elastic modulus for intact rocks

Rock properties exhibit spatial variabilities due to complex geological processes such as sedimentation,metamorphism, weathering, and tectogenesis. Although recognized as an important factor controlling the safety of geotechnical structures in rock engineering, the spatial variability of rock properties is rarely quantified. Hence, this study characterizes the autocorrelation structures and scales of fluctuation of two important parameters of intact rocks, i.e. uniaxial compressive strength(UCS) and elastic modulus(EM).UCS and EM data for sedimentary and igneous rocks are collected. The autocorrelation structures are selected using a Bayesian model class selection approach and the scales of fluctuation for these two parameters are estimated using a Bayesian updating method. The results show that the autocorrelation structures for UCS and EM could be best described by a single exponential autocorrelation function. The scales of fluctuation for UCS and EM respectively range from 0.3 m to 8.0 m and from 0.3 m to 8.4 m.These results serve as guidelines for selecting proper autocorrelation functions and autocorrelation distances for rock properties in reliability analyses and could also be used as prior information for quantifying the spatial variability of rock properties in a Bayesian framework.     

Effect of vegetation cover and sediment type on 3D subsurface structure and shear strength in saltmarshes

The vulnerability of saltmarshes to lateral erosion at their margin depends on the local biogeomorphological properties of the substrate. In particular, the 3D architecture of pore and root systems is expected to influence shear strength, with repercussions for the wider-scale stability of saltmarshes. We apply X-ray computed microtomography (μCT) to visualize and quantify subsurface structures in two UK saltmarshes at Tillingham Farm, Essex (silt/clay rich substrate) and Warton Sands (sand-rich substrate), with four types of ground cover: bare ground, Spartina spp, Salicornia spp and Puccinellia spp. We extracted μCT structural parameters that characterize pore and root morphologies at each station, and compared them with field measurements of shear strength using a principal component analysis and correlation tests. The 3D volumes show that species-dependent variations in root structures, plant colonization events and bioturbation activity control the morphology of macropores, while sediment cohesivity determines the structural stability and persistence of these pore structures over time, even after the vegetation has died. Areas of high porosity and high mean pore thickness were correlated to lower values of shear strength, especially at Tillingham Farm, where well-connected vertical systems of macropores were associated with current or previous colonization by Spartina spp. However, while well-connected systems of macropores may lower the local deformation threshold of the sediment, they also encourage drainage, promote vegetation growth and reduce the marsh vulnerability to hydrodynamic forces. The highest values of shear strength at both sites were found under Puccinellia spp, and were associated with a high density of mesh-like root structures that bind the sediment and resist deformation. Future studies of marsh stability should ideally consider time series of vegetation cover, especially in silt/clay-dominated saltmarshes, in order to consider the potential effect of preserved buried networks of macropores on water circulation, marsh functioning and cliff-face erosion.     

2017年西藏米林MS6.9地震前地磁垂直强度极化异常特征

基于中国大陆65 个地磁台站秒采样观测资料,利用地磁垂直强度极化方法提取了2017年西藏米林M_S6.9 地震震中及附近区域超低频电磁异常信号并分析其时空演化特征。结果显示:米林M_S6.9 地震前在青藏高原出现了大范围的地磁垂直强度极化高值异常,异常过程共持续9天;高值异常开始于2017 年10 月30 日,高值持续3 天之后出现短时间下降,随即转折上升,再次出现高值异常并持续了4 天,在此过程中各高值台站的时序曲线呈现出单峰或双峰的形态。空间分布图显示高值异常在中国大陆西部区域反复出现,尤其是在青藏高原巴颜喀拉地块与羌塘地块的交界处。异常呈“出现—扩大—收缩—消失—扩大—消失”的演化过程,10月31日异常面积达到最大值。异常过程结束后10天发生了西藏米林M_S6.9地震,震中距离10 月31 日异常阈值线5 km。综合分析认为,此次异常与米林地震具有较强的时空相关性,是可靠的地震电磁前兆异常。      

Optimum lateral load pattern for seismic design of elastic shear‐buildings incorporating soil–structure interaction effects

Recently, several new optimum loading patterns have been proposed by researchers for fixed‐base systems while their adequacy for soil–structure systems has not been evaluated yet. Through intensive dynamic analyses of multistory shear‐building models with soil–structure interaction subjected to a group of 21 artificial earthquakes adjusted to soft soil design spectrum, the adequacy of these optimum patterns is investigated. It is concluded that using these patterns the structures generally achieve near optimum performance in some range of periods. However, their efficiency reduces as soil flexibility increases especially when soil–structure interaction effects are significant. In the present paper, using the uniform distribution of damage over the height of structures, as the criterion, an optimization algorithm for seismic design of elastic soil–structure systems is developed. The effects of fundamental period, number of stories, earthquake excitation, soil flexibility, building aspect ratio, damping ratio and damping model on optimum distribution pattern are investigated. On the basis of 30,240 optimum load patterns derived from numerical simulations and nonlinear statistical regression analyses, a new lateral load pattern for elastic soil–structure systems is proposed. It is a function of the fundamental period of the structure, soil flexibility and structural slenderness ratio. It is shown that the seismic performance of such a structure is superior to those designed by code‐compliant or recently proposed patterns by researchers for fixed‐base structures. Using the proposed load pattern in this study, the designed structures experience up to 40% less structural weight as compared with the code‐compliant or optimum patterns developed based on fixed‐base structures. Copyright © 2012 John Wiley & Sons, Ltd.     

Strength characteristics of compacted pond ash

Strength properties of compacted ash layers depend to a great extent on the moulding conditions. This paper focuses on the effects of compaction energy and degree of saturation on strength characteristics of compacted pond ash. The pond ash sample, collected from the ash pond of Rourkela Steel Plant (RSP), was subjected to compactive energies varying from 357 kJ/m³ to 3488 kJ/m³. The optimum moisture content and maximum dry densities corresponding to different compactive energies were determined by conventional compaction tests. The shear strength parameters, unconfined compressive strengths (UCS) and California bearing ratio (CBR) values of specimens compacted to different dry densities and moisture content were assessed and reported. The effects of compaction energy and degree of saturation on shear strength parameters i.e. unit cohesion (c_u) and angle of internal friction (Ф) values and also the UCS values are evaluated and presented herein. The results indicate that the dry density and strength of the compacted pond ash can be suitably modified by controlling the compactive energy and moulding moisture content. The strength achieved in the present study is comparable to the good quality, similar graded conventional earth materials. Hence, it may be safely concluded that pond ash can replace the natural earth materials in geotechnical constructions.     

Rock slope stability assessment using finite element based modelling – examples from the Indian Himalayas

Numerical modelling of rock slides is a versatile approach to understand the failure mechanism and the dynamics of rock slopes. Finite element slope stability analysis of three rock slopes in Garhwal Himalaya, India has been carried out using a two dimensional plane strain approach. Two different modelling techniques have been attempted for this study. Firstly, the slope is represented as a continuum in which the effect of discontinuities is considered by reducing the properties and strength of intact rock to those of rock mass. The equivalent Mohr-Coulomb shear strength parameters of generalised Hoek-Brown (GHB) criterion and modified Mohr-Coulomb (MMC) criterion has been used for this continuum approach. Secondly, a combined continuum-interface numerical method has been attempted in which the discontinuities are represented as interface elements in between the rock walls. Two different joint shear strength models such as Barton-Bandis and Patton’s model are used for the interface elements. Shear strength reduction (SSR) analysis has been carried out using a finite element formulation provided in the PHASE². For blocky or very blocky rock mass structure combined continuum-interface model is found to be the most suitable one, as this model is capable of simulating the actual field scenario.     

A two-grid search scheme for large-scale 3-D finite element analyses of slope stability

It is well known that the trial process for seeking the safety factor in the shear strength reduction finite element method (SSRFEM) is quite expensive, particularly for large 3-D slope stability analyses. The search algorithm for the safety factor is crucial to the entire solution process for the shear strength reduction finite element method, but few studies have attempted to exploit it. Among search algorithms, the commonly used bracketing and bisection search has not been fully optimised. Consequently, to improve the search scheme for the safety factor associated with the shear strength reduction finite element method, two strategies are suggested. First, a generalised bisection search algorithm is proposed to reduce the possibility of encountering non-convergence from a statistical point of view. To further improve the efficiency, a new two-grid scheme, characterised by a coarse mesh search and followed by a fine mesh search, is developed. Based on the drained or undrained analyses of the 3-D slope examples, the new search algorithm can markedly outperform the commonly used bisection search algorithms based on a single finite element mesh.     

分散性土的抗剪强度特性试验研究

分散性土是一种具有特殊工程性质的土,其典型的性质为抗渗性能差,耐冲蚀能力低,在纯水中具有很高的分散性。目前,人们对于分散性土的研究主要集中在鉴定、抗渗和改良等方面,而对其抗剪强度特性的研究很少。采用三轴CU试验研究不同初始条件下分散性土的抗剪强度特性。结果表明:分散性土的应力-应变曲线在小围压、低含水率和高干密度的状态下呈应变软化型,在围压较大、含水率较高的情况下呈弱硬化型或硬化型;黏聚力和内摩擦角均随含水率的增大而减小,并且干密度越大受水分影响越强烈;黏聚力随干密度的增大而增大,在低含水条件下,黏聚力的增幅较大,而在高含水条件下,黏聚力的增幅则较小;内摩擦角随干密度的增大有少量增大。     

城乡生态质量对经济快速增长区建设用地扩张的响应——以合肥为例北大核心CSCD

21世纪以来,中国经济增长促进了城市化进程的快速发展,也在一定程度上带来一系列环境问题。本文以合肥为例,选用2002、2010、2020年的Landsat系列影像,利用改进的遥感生态指数(MRSEI),并引入城市扩张强度、城市重心转移模型等,探讨了城乡生态变化对建设用地扩张的响应。结果表明:2002—2020年合肥市主城区与周边乡镇生态环境质量总体呈恶化趋势,其中周边乡镇MRSEI均值由2002年的0.747下降至2020年的0.495,降幅为33.7%,主城区MRSEI均值由2002年的0.669下降至2020年的0.491,降幅为28.73%;周边乡镇生态质量下降幅度高于主城区,建设用地扩张强度大于主城区,生态破损更为严重。生态环境恶化区域主要由主城区向周边乡镇放射性分布,生态环境变好的区域主要集中在老城区周围,而城市扩张的区域与MRSEI评价结果显示的生态环境恶化区域基本重合,生态环境变化与城市建设用地扩张强度呈明显的负相关关系。     

CPT-Based estimation of undrained shear strength of fine-grained soils in the Huanghe River Delta

The Huanghe River（Yellow River） Delta has a wide distribution of fine-grained soils. Fluvial alluviation, erosion,and wave loads affect the shoal area, resulting complex physical and mechanical properties to sensitive fine-grained soil located at the river-sea boundary. The cone penetration test（CPT） is a convenient and effective in situ testing method which can accurately identify various soil parameters. Studies on undrained shear strength only roughly determine the fine content（FC） without ma...