Constitutive model for gas hydrate-bearing soils considering different types of hydrate morphology and prediction of strength-band

The present study proposes a new elasto-plastic constitutive model that considers different types of hydrates in pore spaces. Many triaxial compression tests on both methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have been carried out over the last few decades. It has been revealed that methane hydrate-bearing soils and carbon dioxide hydrate-bearing soils have different strength and dilatancy properties even though they have the same hydrate contents. The reason for this might be due to the different types of hydrate morphology. In this study, therefore, the effect of the hydrate morphology on the mechanical response of gas-hydrate-bearing sediments is investigated through a model analysis by taking into account the different hardening rules corresponding to each type of hydrate morphology. In order to evaluate the capability of the proposed model, it is applied to the results of past triaxial compression tests on both methane hydrate-containing and carbon dioxide hydrate-containing sand specimens. The model is found to successfully reproduce the different stress–strain relations and dilatancy behaviors, by only giving consideration to the different morphology distributions and not changing the fitting parameters. The model is then used to predict a possible range in which the maximum deviator stress can move for various hydrate morphology ratios; the range is defined as the strength-band. The predicted curve of the maximum deviator stress obtained by the constitutive model matches the empirical equations obtained from past experiments. It supports the fact that the hydrate morphology ratio changes with the total hydrate saturation. These findings will contribute to a better understanding of the relation between the microscopic structures and macro-mechanical behaviors of gas-hydrate-bearing sediments.     

云南富宁良子老寨钛铁矿成矿规律和矿床成因

云南富宁地区钛铁矿是我国重要的钛铁矿矿床,成矿条件优越。本文对良子老寨矿区残坡积风化壳型钛铁砂矿床的地质特征、矿体、矿石特征进行了总结和研究,探讨了矿床的矿床成因和成矿规律。海西—印支期偏碱性基性钛辉辉长辉绿岩体为该区钛铁矿的形成提供了物质来源。富宁地区温热潮湿多雨气候有利于风化作用形成,岩体出露地表后,岩体中的钛矿物经风化剥离、水解作用、次生富集、原地残留或近距离搬运沉积,形成了残坡积风化壳型钛铁砂矿床。陡坡、临近河流区域矿体厚度薄,风化作用弱部位品位低;山脊缓坡、河谷阶地、地形低凹地段矿体厚度较大、品位高。本文研究成果能够指导滇东南地区残坡积物风化壳内钛矿的找矿工作。     

高含水率有机质河道底泥压缩性状试验研究

河道底泥的压缩性状是影响河道底泥环保疏浚设计及处置的一个关键因素,采集了4条河道的底泥,针对4种城市河道底泥和过氧化氢处理的底泥开展了一系列物理和压缩试验,研究河道底泥的压缩性状,探讨有机质含量对河道底泥压缩性状的影响规律。试验结果表明,河道底泥的有机质含量变化范围大,河道底泥的压缩性状与底泥的物理性状密切相关,与无机质土类似,初始孔隙比和液限孔隙比是河道底泥压缩性状的两个关键控制因素。有机质对底泥压缩性状的影响可以归结于液限孔隙比的改变,有机质含量主要影响河道底泥的液限和比重,进而导致河道底泥重塑屈服应力σ_(yr)~′和压缩性状发生改变。有机质含量越高,液限孔隙比越大,压缩指数越大,呈现出的压缩性越高。     

The influence of pH on phosphate release from lake sediments

pH is believed to be an important regulating factor for the release of phosphate from freshwater sediments. Usually, this is demonstrated in laboratory experiments using intact sediment cores and pH regulation of the overlying water by means of NaOH additions. This technique and a pH regulation technique by means of CO₂-stripping were compared. The first method resulted in a 10-fold increase in release rate at pH 9.5 compared to the rate at pH 8.3. The stripping technique yielded only a doubling of the release rate at pH 9.3. Measurements of pH profiles in the sediment in experiments performed at pH 8.3 and 9.0 proved that the enhanced phosphate release when using NaOH for pH regulation is mainly an artifact caused by an increased alkalinity in the water column and the upper sediment layers, resulting from the NaOH additions.     

老挝塞公省达克金矿区水系沉积物地球化学特征及找矿预测

达克金矿区位于老挝长山—岘港成矿带南缘,区域地质工作程度较低,为明确研究区找矿方向,首次对该地区开展了1∶50000水系沉积物地球化学测量,共采集836件样品,分析测试19种元素,通过相关性分析和因子分析将元素异常分成4类组合：F1（Cu-Cr-V-Ti）、F2（W-Mo-Bi）、F3（Au）、F4（Sn）。圈定综合异常17个,其中DJ5、DJ12、DJ10、DJ14、DJ6异常套合好、强度高、规模大、找矿潜力大。综合区内矿床地质特征及水系沉积物地球化学特征,圈定了岩浆热液型钨锡铋钼矿、铜铬镍钒钛矿及浅成低温热液型金矿3处找矿靶区,指明了下一步找矿方向,也为老挝南部地区开展此类工作积累了宝贵经验。     

Nitrogen gas supersaturation in the recent sediments of lake erie and two polluted harbors

Nitrogen gas distributions in sediment pore water were determined for cores collected in Lake Erie and two nearby harbors. Concentrations of N₂ gas ranged from 11.9 to 37.0 ml l⁻¹ and from 8.9 to 58.3 ml l⁻¹ for open lake and polluted harbor sediments, respectively. Maximum concentrations in the harbor sediments were as high as 3.5 times N₂ saturation calculated for the overlying water. Indirect diffusive flux estimates for nitrogen gas ranged from 20 to 32% of the particulate nitrogen sedimentation rate in Lake Erie. At one location, the amount of nitrogen gas lost by diffusion was calculated to be greater than the nitrogen deposited to the sediments. Nitrogen gas production and diffusive loss from surficial sediments probably represents a major pathway for nitrogen removal from Lake Erie.     

A five-year study on the heavy-metal pollution of Guanabara Bay sediments (Rio de Janeiro, Brazil) and evaluation of the metal bioavailability by means of geochemical speciation

Surface sediments of the Guanabara Bay (Rio de Janeiro, Brazil) were analyzed by a sequential extraction procedure for Cd, Cr, Cu, Pb, Zn, Mn and Fe, determining their distribution among five geochemical phases and in the nitric acid extractable phase. Bioavailable phases and non-bioavailable phases have been determined in six transects in the bay to define the significant level of pollution due to sediment metal contamination. A multiple correlation showed limited responsibility of Mn and Fe oxides and humic acids in the adsorption process, which allowed discrimination among the different processes and suggested the strong influence of the hydrogen sulfide present in the highly reduced bay bottom environment. The authors suggest the need to avoid disturbing bottom sediment by dredging or by artificial bottom aeration which could result in a rapid worsening of the environment due to the accelerated formation of more soluble oxygenated metal compounds making the toxic metals much more available to the benthic fauna and to the bay biota in general.     

Use of slow filtration columns to assess oxygen respiration, consumption of dissolved organic carbon, nitrogen transformations, and microbial parameters in hyporheic sediments

Biogeochemical processes mediated by microorganisms in river sediments (hyporheic sediments) play a key role in river metabolism. Because biogeochemical reactions in the hyporheic zone are often limited to the top few decimetres of sediments below the water-sediment interface, slow filtration columns were used in the present study to quantify biogeochemical processes (uptakes of O2, DOC, and nitrate) and the associated microbial compartment (biomass, respiratory activity, and hydrolytic activity) at a centimetre scale in heterogeneous (gravel and sand) sediments. The results indicated that slow filtration columns recreated properly the aerobic-anaerobic gradient classically observed in the hyporheic zone. O2 and NO3- consumptions (256 +/- 13 microg of O2 per hour and 14.6 +/- 6.1 microg of N-NO3- per hour) measured in columns were in the range of values measured in different river sediments. Slow filtration columns also reproduced the high heterogeneity of the hyporheic zone with the presence of anaerobic pockets in sediments where denitrification and fermentation processes occurred. The respiratory and hydrolytic activities of bacteria were strongly linked with the O2 consumption in the experimental system, highlighting the dominance of aerobic processes in our river sediments. In comparison with these activities, the bacterial biomass (protein content) integrated both aerobic and anaerobic processes and could be used as a global microbial indicator in our system. Finally, slow filtration columns are an appropriate tool to quantify in situ rates of biogeochemical processes and to determine the relationship between the microbial compartment and the physico-chemical environment in coarse river sediments.     

重金属污染河道底泥固化/稳定化研究进展

固化/稳定化技术是处理重金属污染底泥的相当有效的方法,其机理主要是通过沉淀、吸附和络合作用将重金属污染物转化为更加稳定的形态。文章在总结国内外固化/稳定化技术的基础上,简单介绍了固化/稳定化的评估方法。按固化/稳定化药剂的不同,讨论分析了水泥固化、磷酸盐稳定化、无机物混合稳定化和有机物稳定化四种方法,通过对研究成果的总结,探讨了重金属污染河道底泥固化/稳定化处理技术的发展趋势。     

An improved specimen preparation method for marine shallow gas-bearing sand sediments and its validations

Instabilities of shallow gas-charged seabed are potential geological hazards in ocean engineering.In practice,the conventional field sampling techniques failed to obtain undisturbed gas-bearing sediments from the seabed for laboratory mechanical testing because of sensitive gas exsolution and escape from sediments.However,preparation of representative remoulded gas-charged specimens is a challenging issue,because it is rather difficult to quantitatively control the gas content and obtain uniform distribution of gas bubbles within the specimen.Given the above problems,this work proposes a reliable approach to reconstitute the high-saturation specimen of gas-charged sediments in the laboratory by an improved multifunction integrated triaxial apparatus(MITA).This apparatus is developed based on an advanced stress path triaxial system by introducing a temperature-controlled system and a wavemonitoring system.The temperature-controlled system is used to accurately mimic the in situ environments of sediments in the seabed.The wave-monitoring system is utilized to identify exsolution point of free gas and examine the disturbance of gas to specimens during gas exsolution.The detailed procedure of gassy specimen preparation is introduced.Then,the quality of prepared specimens using our improved apparatus is validated by the high-resolution micro-X-ray computed tomography(μCT)scanning test,from which bubble occurrence and size distribution within the gassy sand specimen can be obtained;and preliminary mechanical tests on gassy sand specimens with various initial saturation degrees are performed.The proposed specimen preparation procedure succeeds in proving the postulated occurrence state of gas bubbles in coarse-grained sediments and accurately controlling the gas content.