The challenge of numerically implementing numerous force–resultant models in the stability analysis of long on-bottom pipelines

Plasticity models written directly in terms of force and displacement are finding increased popularity in describing the behaviour of offshore foundations. However, when applied to the on-bottom stability analysis of pipelines under random storm loading, hundreds, if not thousands, of models must be incorporated with structural elements in a three-dimensional analysis and simultaneously solved. This necessitates extremely efficient and accurate numerical integration techniques. This paper scrutinizes constitutive integration algorithms within both explicit and implicit frameworks, and applies them to a two-surface mixed hardening model that describes the force–displacement behaviour of a pipe partially embedded in sand. Detailed solutions are provided, allowing implementation of the pipe–soil model into any standard structural finite-element program. Through use of practical examples, the efficiency and accuracy of the solution methods are assessed.     

Metamorphic evolution of medium‐temperature ultra‐high pressure (MT‐UHP) eclogites from the South Dabie orogen,Central China: an insight from phase equilibria modelling

Medium‐temperature ultrahigh pressure (MT‐UHP) eclogites from the south Dabie orogen, as represented by samples from the Jinheqiao, Shuanghe and Bixiling areas, consist of garnet, omphacite, phengite, epidote, hornblendic amphibole, quartz/coesite and rutile with or without kyanite and talc. Garnet is mostly anhedral and unzoned, but a few porphyroblasts are weakly zoned with core–mantle increasing grossular (X_gr) and decreasing pyrope (X_py) contents. Garnet compositions are closely correlated with the bulk compositions. For instance, the X_py and X_gr contents are positively correlated with the bulk MgO and CaO contents. Phengite is occasionally zoned with core–rim deceasing Si content, and phengite grains as inclusions in garnet show higher Si than in the matrix, suggesting differently resetting during post‐peak stages. The maximum Si contents are mostly 3.60–3.63 p.f.u. for the three areas. Pseudosections calculated using THERMOCALC suggest that the MT‐UHP eclogites should have a peak assemblage of garnet + omphacite + lawsonite + phengite + coesite in most rocks of higher MgO content. In this assemblage, the X_py in garnet mostly depends on bulk compositions, whereas the X_gr in garnet and the Si contents in phengite regularly increase, respectively, as temperature and as pressure rise, and thus, can provide robust thermobarometric constraints. Using the X_gr and Si isopleths in pseudosections, the peak P–T conditions were estimated to be 40 kbar/730 °C for the Jinheqiao, 41 kbar/726 °C for the Shuanghe, and 37–52 kbar and 700–830 °C for the Bixiling eclogites. Some eclogites with higher FeO are predicted to have a peak assemblage of garnet + omphacite + coesite ± phengite without lawsonite, where the garnet and phengite compositions highly depend on bulk compositions and generally cannot give available thermobarometric constraints. Decompression of the eclogites with lawsonite in the peak stage is inferred to be accompanied with cooling and involves two stages: an early‐stage decompression is dominated by lawsonite dehydration, resulting in increase in the mode of anhydrous minerals, or further eclogitization, and formation of epidote porphyroblasts and kyanite‐bearing quartz veins in eclogite. As lawsonite dehydration can facilitate evolution of assemblages under fluid‐present conditions, it is difficult to recover real peak P–T conditions for UHP eclogites with lawsonite. This may be a reason why the P–T conditions estimated for eclogites using thermobarometers are mostly lower than those estimated for the coherent ultramafic rocks, and lower than those suggested from the inclusion assemblages in zircon from marble. A late‐stage decompression is dominated by formation of hornblendic amphibole and plagioclase with fluid infiltration. The lawsonite‐absent MT‐UHP eclogites have only experienced a decompression metamorphism corresponding to the later stage and generally lack the epidote overprinting.     

Groundwater response to tidal fluctuations in a leaky confined coastal aquifer with a finite length

The study on the hydraulic properties of coastal aquifers has significant implications both in hydrological sciences and environmental engineering. Although many analytical solutions are available, most of them are based on the same basic assumption that assumes aquifers extend landward semi‐infinitely, which does not necessarily reflect the reality. In this study, the general solutions for a leaky confined coastal aquifer have been developed that consider both finitely landward constant‐head and no‐flow boundaries. The newly developed solutions were then used to examine theoretically the joint effects of leakage and aquifer length on hydraulic head fluctuations within the leaky confined aquifer, and the validity of using the simplified solution, which assumes the aquifer is semi‐infinite. The results illustrated that the use of the simplified solution may cause significant errors, depending on joint effects of leakage and aquifer length. A dimensionless characteristic parameter was then proposed as an index for judging the applicability of the simplified solution. In addition, practical application of the general solution for the constant‐head inland boundary was used to characterize the hydraulic properties of a leaky confined aquifer using the data collected from a field site at the Seine River estuary, France, and the versatility of the general solution was further justified.     

Scenarios of land cover in China

A method for surface modeling of land cover change (SMLC) is developed on the basis of establishing transition probability matrixes between land cover types and HLZ types. SMLC is used to simulate land cover scenarios of China for the years 2039, 2069 and 2099, for which HLZ scenarios are first simulated in terms of HadCM3 climatic scenarios that are downscaled in zonal model of spatial climate change in China. This paper also analyzes spatial distribution of land cover types, area change and mean center shift of each land cover type, ecotope diversity, and patch connectivity under the land cover scenarios. The results show that cultivated land would decrease and woodland would expand greatly with climatic change, which coincides with consequences expected by implementation of Grain-for-Green policy. Nival area would shrink, and desertification area would expand at a comparatively slow rate in future 100 years. Climate change would generally cause less ecotope diversity and more patch connectivity. Ecosystems in China would have a pattern of beneficial cycle after efficient ecological conservation and restoration. However, if human activities would exceed regulation capacity of ecosystems themselves, the ecosystems in China might deteriorate more seriously.     

A finite element model for simulating surface run‐off and unsaturated seepage flow in the shallow subsurface

This work introduces water–air two‐phase flow into integrated surface–subsurface flow by simulating rainfall infiltration and run‐off production on a soil slope with the finite element method. The numerical model is formulated by partial differential equations for hydrostatic shallow flow and water–air two‐phase flow in the shallow subsurface. Finite element computing formats and solution strategies are presented to obtain a numerical solution for the coupled model. An unsaturated seepage flow process is first simulated by water–air two‐phase flow under the atmospheric pressure boundary condition to obtain the rainfall infiltration rate. Then, the rainfall infiltration rate is used as an input parameter to solve the surface run‐off equations and determine the value of the surface run‐off depth. In the next iteration, the pressure boundary condition of unsaturated seepage flow is adjusted by the surface run‐off depth. The coupling process is achieved by updating the rainfall infiltration rate and surface run‐off depth sequentially until the convergence criteria are reached in a time step. A well‐conducted surface run‐off experiment and traditional surface–subsurface model are used to validate the new model. Comparisons with the traditional surface–subsurface model show that the initiation time of surface run‐off calculated by the proposed model is earlier and that the water depth is larger, thus providing values that are closer to the experimental results.     

Observation of Luzon Strait transport in summer 2007

A field experiment was conducted across the Luzon Strait in July 2007, and a total of 51 profiles covering variables of horizontal velocity, temperature, salinity, and pressure were collected at 11 stations. Using this observation, the volume transport through the Luzon Strait, its differences between July 2007 and October 2005, and the distribution of subtidal flow and geostrophic flow have been investigated. The net transport has a two-layer vertical structure, which is eastward both in the upper layer (<26 kg m^?3 σ₀), and in the intermediate layer (26–27.3 kg m^?3 σ₀), while it is westward in the deeper layer (>27.3 kg m^?3 σ₀), with respective values of 3.0, 4.0, and ?1.5 Sv. The net transport is eastward, and estimated to be 5.5 Sv. The distribution of the subtidal flow in the intermediate layer shows that a westward flow exists in the northern part, countered by an eastward flow existing in the southern part of the strait. This distribution is in direct contrast to the previous results obtained in October 2005, in which a westward flow occurs in the south countered by an eastward flow in the north in the intermediate layer. This suggests that the flow pattern varies greatly from October 2005 to July 2007 not only in the upper layer but also in the intermediate layer. The deep layer, on the other hand, shows few changes between the two observation periods.     

仿生自补偿一体式高胎体孕镶金刚石取心钻头研究

针对孕镶金刚石钻头齿间相互独立,在坚硬地层中钻进极易产生打滑和单齿受力太大导致钻头损坏的难题,引入仿生爪趾结构原理和自补偿理念,研制了仿生自补偿一体式高胎体孕镶金刚石取心钻头。钻头的一体式切削齿设计,主要在单齿的齿单元间采用加强筋方式强化单齿强度,单齿与单齿间设置齿间加强筋加强,使得钻头相邻齿单元、相邻单齿间相互连接形成一个整体,在钻进过程中,钻头齿的工作唇面会产生多个超过岩石极限破坏强度但远低于钻头胎体强度的应力集中区域,不容易出现少数几个接近或超过钻头胎体强度的应力集中区域的情况。如此,钻头的整体性更好,受力均匀,切削岩石的效率更高,寿命更长,尤其对于坚硬地层,防打滑效果更好。经现场试验表明：仿生自补偿一体式高胎体孕镶金刚石取心钻头较常规钻头具有更高的碎岩效率和更长的使用寿命。     

砂卵石地层非开挖顶管施工地表沉降控制研究

砂卵石地层具有地层结构松散、透水性高、粘聚力小、自稳能力差等特点,往往为工程施工带来许多难题。为了更好地研究砂卵石地层非开挖顶管施工时地表的沉降规律及相应的控制措施,以成都市华阳、万安、正兴片区雨污水管网W31-W32顶管段为研究对象,采用工法比较、数值模拟与现场实测相结合的方法,提出了适用于砂卵石地层地表沉降的控制措施。研究结果表明：在砂卵石地层中,采用袖阀管注浆加固管周土体与高压旋喷桩加固洞门的措施是合理有效的,加固后地表沉降量相较于未加固时降低了48.1%,洞门处的地表沉降量降低了51.1%,满足了地表沉降控制要求,验证了地层加固措施的效果,可为后续类似的非开挖顶管施工提供借鉴。