Experimental research on mechanical characteristics and strength criterion of rock of triaxial unloading tests under different stress pathsEI北大核心CSCD

By carrying out triaxial unloading test under different stress paths on rock, the complete stress-strain curves, deformation characteristics and its strength criterion are studied. The test results show that the relation between lateral strain and confining pressure firstly is linear, and then is nonlinear in the total stage of unloading confining pressure. And its growth rate is about 3 to 5 times the growth rate of the axial strain. It shows obvious lateral dilatancy, and the degree of dilatancy is related to the unloading path. According to the area that is enclosed by confining pressure and volumetric strain curve, it can be seen that the greater the confining pressure is before unloading, the more the energy releases. Deformation modulus decreases gradually with the unloading confining pressure, and increases with the initial confining pressure; the relation between deformation modulus and confining pressure shows negative exponential distribution overall. At the same unloading paths, the reduction of the deformation modulus increases with the increase of initial confining pressure. The Poisson's ratio rises up continually, and the variation law with the confining pressure can be well described by nonlinear. Failure characteristics of rock are mainly shear failure. The strength characteristics of rock can be well characterized by using the Mohr strength criterion of power function. These conclusions provide reliable theoretical reference and guiding significance for the deep mining of the underground metal mine.     

中国县域生态资产损益的影响机制及优化提升路径

生态资源资产化、生态资产资本化是实现“两山”转化的一个重要途径,开展生态资产及其损益核算有助于生态系统的有效保护、科学管理和可持续利用。本文在借鉴国内外生态资产相关研究的基础上,构建了县域尺度生态资产及其损益核算技术体系,分析了1990—2018年中国县域生态资产的时空变化特征,明晰了造成生态资产损益的影响因素及作用机制,并针对具有不同生态资源类型、损益状况、驱动要素特征的县域提出了差异化的优化提升路径。结果表明：① 以农田、森林生态资源为主的县域数分别约占县域总数的45%和37%;1990—2018年中国县域生态存量资产的质量整体呈波动中上升趋势,而生态流量资产的变化趋势差异显著,近70%县域水源涵养量减少。② 生态流量资产价值呈现损与益态势的县域数分别约占44%和37%,空间分布呈现与“胡焕庸线”一致的分割特征,即西北广大地区县域明显增益,而东部和南部县域有所减损。③ 1990—2018年中国超过70%县域生态资产变化受气候变化与人类活动共同驱动,人类活动对生态资产增益县域的平均作用率约80%,而气候变化对生态资产减损县域的平均作用率约为60%。④ 综合分析,可将全国县域分为适应、减缓气候变化和生态资源修复、保护、管理5种类型,采用差异化的生态资产优化提升路径。     

Nutrient export via overland flow from a cultivated field of an Ultisol in southern China

Understanding the influence of complex interactions among hydrological factors, soil characteristics and biogeochemical functions on nutrient dynamics in overland flow is important for efficiently managing agricultural nonpoint pollution. Experiments were conducted to assess nutrient export from Ultisol soils in the Sunjia catchment, Jiangxi province, southern China, between 2003 and 2005. Four plots were divided into two groups: two peanut plots and two agroforestry (peanut intercropped with citrus) plots. During the study period, we collected water samples for chemical analyses after each rainfall event that generated overland flow to assess nutrient export dynamics. The concentrations of potassium (K) and nitrate‐N (NO₃^––N) in overland flow were higher during the wetting season (winter and early spring). This reflects the solubility of K and NO₃^––N, the accumulation of NO₃^––N during the dry season and an increase in desorption processes and mixing with pre‐event water caused by prolonged contact with soil in areas with long‐duration, low‐intensity rainfall. In contrast, concentrations of total nitrogen (TN) and total phosphorus (TP) were higher during the wet season (late March to early July) and during the dry season (mid‐July to the end of September or early October). This was due to the interaction between specific hydrological regimes, the properties of the Ultisol and particulate transport processes. Variations in nutrient concentrations during storm events further identified that event water was the dominant source of total nitrogen and total phosphorus, and pre‐event water was the dominant source of NO₃^––N. In addition, the results obtained for the different land uses suggest that agroforestry practices reduce nutrient loss via overland flow. Copyright © 2012 John Wiley & Sons, Ltd.     

A virtual experiment technique on the elementary behaviour of granular materials with discrete element method

Multi‐scale investigations aided by the discrete element method (DEM) play a vital role for current state‐of‐the‐art research on the elementary behaviour of granular materials. Similar to laboratory tests, there are three important aspects to be considered carefully, which are the proper stress/strain definition and measurement, the application of target loading paths and the designed experiment setup, to be addressed in the present paper. Considering the volume sensitive characteristics of granular materials, in the proposed technique, the deformation of the tested specimen is controlled and measured by deformation gradient tensor involving both the undeformed configuration and the current configuration. Definitions of Biot strain and Cauchy stress are adopted. The expressions of them in terms of contact forces and particle displacements, respectively, are derived. The boundary of the tested specimen consists of rigid massless planar units. It is suggested that the representative element uses a convex polyhedral (polygonal) shape to minimize possible boundary arching effects. General loading paths are described by directly specifying the changes in the stress/strain invariants or directions. Loading can be applied in the strain‐controlled mode by specifying the translations and rotations of the boundary units, or in the stress‐controlled mode by using a servo‐control mechanism, or in the combination of the two methods to realize mixed boundary conditions. Taking the simulation results as the natural consequences originated from a complex system, virtual experiments provide particle‐scale information database to conduct multi‐scale investigations for better understanding in granular material behaviours and possible development of the constitutive theories provided the qualitative similarity between the simulation results from virtual experiments and observations on real material behaviour. Copyright © 2011 John Wiley & Sons, Ltd.     

Fire‐induced changes in soil water repellency increased fingered flow and runoff rates following the 2004 Huelva wildfire

Water repellency (WR) from fire‐affected soils can affect infiltration processes and increase runoff rates. We investigated the effects of fire‐induced changes in soil WR and the related soil hydrological response after one of the largest wildfires in Spain in recent years. The vertical distribution of WR in soil profiles was studied under oak and pine forests and the wetting pattern was analysed after rainfall simulations (85 mm h^?1 during 60 min). After burning, the persistence of WR in soils under oaks increased in the upper 0–5 cm of soil in comparison with pre‐fire WR, but no significant changes were observed under pines. After a fire, WR was stronger and the thickness of the water‐repellent layer increased in soils under pines in the upper 0–16 cm of soil. The hydrophobic layer was thinner under oaks, where no strong to extremely water‐repellent samples were observed below 12 (in burnt soils) and 8 cm (in unburnt soils). Uniform wetting was observed through soil depth in burnt and unburnt soils under oaks, as a consequence of the prevailing matrix flux infiltration. Water was mostly stored in the upper few centimetres and soil became rapidly saturated, favouring a continuous rise in the runoff rate during the experiments. Moisture profiles under pines showed a heterogeneous wetting pattern, with highly irregular wetting fronts, as a result of wettable and water‐repellent three‐dimensional soil patches. In this case, runoff rates on burnt plots increased in relation to unburnt plots, but runoff generation reached a steady state after 25–30 min of simulated rainfall at an intensity of 85 mm h^?1. Rainfall water infiltrated over a small part of the ponded area, where the vertical pressure of the water column overcame the WR. Copyright © 2010 John Wiley & Sons, Ltd.     

A tracer‐based assessment of hydrological pathways at different spatial scales in a mesoscale Scottish catchment

Geochemically based hydrograph separation techniques were used in a preliminary assessment to infer how runoff processes change with landscape characteristics and spatial scale (1–233 km²) within a mesoscale catchment in upland Scotland. A two‐component end‐member mixing analysis (EMMA) used Gran alkalinity as an assumed conservative tracer. Analysis indicated that, at all scales investigated, acidic overland flow and shallow subsurface storm flows from the peaty soils covering the catchment headwaters dominated storm runoff generation. The estimated groundwater contribution to annual runoff varied from 30% in the smallest (ca 1 km²) peat‐dominated headwater catchment with limited groundwater storage, to >60% in larger catchments (>30 km²) with greater coverage of more freely draining soils and more extensive aquifers in alluvium and other drift. This simple approach offers a useful, integrated conceptualization of the hydrological functioning in a mesoscale catchment, which can be tested and further refined by focused modelling and process‐based research. However, even as it stands, the simple conceptualization of system behaviour will have significant utility as a tool for communicating hydrological issues in a range of planning and management decisions. Copyright © 2002 John Wiley & Sons, Ltd.     

Metasomatic albitites and related biotite-rich schists from a low-pressure polymetamorphic terrane, Snake Creek Anticline, Mount Isa Inlier, north-eastern Australia: microstructures and P–T–d paths

Rocks of the Snake Creek Anticline are mainly pelitic schists, psammitic schists and quartzites that were metamorphosed during multiple high‐T/low‐P events extending from D1 to D5, with the metamorphic peak occurring late to post‐D3. Albitites are widespread, but are concentrated in five areas. They are typically fine‐ to medium‐grained, and consist of albite, with or without combinations of quartz, biotite, staurolite, cordierite, garnet, andalusite, sillimanite, kyanite, gedrite and tourmaline. From the presence or absence of albite inclusions in porphyroblasts, the albitites are interpreted as forming early in the D3 event as a result of infiltration of external fluids. Psammitic schists and quartzites were preferentially altered, but pelitic schists were also albitized in localities where the alteration was more extreme, with the replacement of muscovite total and the replacement of quartz and biotite variable. Structural controls on albitization include fracturing and syn‐D3 shear zones in fold hinges. Biotite schists with abundant porphyroblasts (combinations of staurolite, garnet, andalusite and cordierite) occur adjacent to albitites, and it is argued that they formed by the addition of Fe and Mg sourced from the albitites. In several albitite‐rich areas, cordierite grew early in D3 and was partly or entirely replaced during or after D3 by combinations of biotite, andalusite, tourmaline, staurolite and sillimanite. A postulated P–T–d path involved an increase in pressure (with or without a decrease in temperature) subsequent to early D3 albitization, followed by an increase in temperature up to the metamorphic peak (late D3 to early D4. The metamorphism was contemporary in part with the emplacement of the Williams Batholith (c. 1550–1500 Ma), which probably supplied the Na‐rich fluids.     

Crust-mantle relationships in the French Variscan chain: the example of the Southern Monts du Lyonnais unit (eastern French Massif Central)

Garnet lherzolite from the Lyonnais area (eastern French Massif Central) occurs as several lenses elongated within the regional foliation of garnet-biotite-sillimanite gneisses. Within the peridotites a mylonitic foliation can be observed which clearly is oblique to the regional foliation of the surrounding gneisses. Petrological and thermobarometric studies emphasize a tectonometamorphic re-equilibration for both crustal and mantle rocks characterized by a prograde metamorphic stage followed by retrograde evolution. During the burial stage, interpreted as lithospheric subduction, the peridotites underwent their mylonitic deformation, under high-pressure conditions (23–30 kbar). In contrast, the paragneisses have suffered their deformation during the retromorphic evolution under mesozonal conditions (6–8 kbar, 700°C). Our thermobarometric investigations allow us to interpret the granulitic/ultramafic association from the Monts du Lyonnais area as a lithospheric section buried into a Palaeozoic subduction zone, laminated during continental collision and uplifted by erosion processes.     

Modelling mechanical behaviour of limestone under reservoir conditions

High porosity and low permeability limestone has presented pore collapse. As fluid is withdrawn from these reservoirs, the effective stresses acting on the rock increase. If the strength of the rock is overcome, pore collapse may occur, leading to irreversible compaction of porous media with permeability and porosity reduction. It impacts on fluid withdrawal. Most of reservoirs have been discovered in weak formations, which are susceptible to this phenomenon. This work presents a study on the mechanical behaviour of a porous limestone from a reservoir located in Campos Basin, offshore Brazil. An experimental program was undergone in order to define its elastic plastic behaviour. The tests reproduced the loading path conditions expected in a reservoir under production. Parameters of the cap model were fitted to these tests and numerical simulations were run. The numerical simulations presented a good agreement with the experimental tests. Copyright © 2006 John Wiley & Sons, Ltd.