Measurement of small strain shear modulus of clean and natural sands in saturated condition using bender element test

Bender element (BE) tests of saturated sand have increased interest to researchers currently. However, the measurement of small strain modulus from BE tests shows large difference between saturated and dry conditions. In this study, BE tests of a type of clean sand (Fujian sand) and two types of natural sands (Hangzhou sand and Nanjing sand) were performed. For the purposes of comparison, resonant column (RC) test and torsional shear (TS) test were also carried out on the same specimen. The factors that influence the determination of the travel time of shear wave in BE tests are discussed and a reliable method for the determination of the shear-wave velocity is obtained. It is found that the shear-wave velocities V_s of saturated Fujian sand (clean sand) and Hangzhou sand (natural sand) obtained from BE tests are 5–10% greater than those obtained from RC and TS tests. However, the V_s of saturated Nanjing sand (natural sand) obtained from BE, RC and TS tests show good agreement with a maximum difference of about 3%. Sands with various fines contents were also tested in an attempt to explain the differences between the two saturated natural sands. Biot׳s theory accounting for the dispersion of shear wave was employed to interpret the results of BE tests. The results indicate that the fines content of natural sand plays an important effect on the hydraulic conductivity, which affects the relative motion between soil particles and fluid when a high frequency shear wave propagates in the specimen. Based on this, a method for the determination of small strain shear modulus in BE test was proposed for both saturated clean sands and natural sands.     

Violet stimulated luminescence as an alternative for dating complex colluvial sediments in the Atacama Desert

Luminescence dating has become a key tool in studies of the Quaternary. The typically stable luminescence response of quartz grains and the absence of a significant internal dose, make quartz minerals the preferred dosimeter for monitoring the burial dose in sediments. Unfortunately, the reliability of conventional OSL (optically stimulated luminescence) dating, based on blue stimulation, can be compromised when the luminescence decay is not dominated by a rapidly decaying and stable part of the luminescence signal (i.e. the fast component). On the other hand, standard methods in luminescence dating are limited to ages of a few hundred kiloyears. In this study, violet stimulated luminescence (VSL) has been used as a means to overcome both problems, applied to a series of colluvial deposits in the Atacama Desert, Chile. Quartz from this region, characterized by poor blue-OSL response, showed a reproducible and stable VSL signal capable of recovering given doses up to ∼500 Gy and a saturation dose twice as high as conventional OSL. The VSL response from these samples has been studied in detail and the estimated ages have been compared with an already established chronology for the same site, based on IRSL of potassium feldspar single grains. Results agree for the dose range of the profile studied, ∼100–250 Gy, equivalent to ages of 29–79 ka confirming the suitability of VSL for dating sedimentary quartz with unreliable blue-OSL response and to extend the age range of conventional OSL dating.     

基于流固耦合统一计算框架的三维大规模海域场地地震反应分析?以东京湾为例

海域场地地震响应分析是确定海洋工程结构抗震设计地震动输入的重要环节。然而,针对海水、饱和土、基岩之间的流固耦合分析,目前一般通过对3种介质方程进行离散,然后整体求解或分区耦合求解的方式进行,过程复杂而低效。因此,大规模海域场地地震反应分析仍是一个挑战性问题。本文基于流固耦合统一计算框架求解海域近场波动问题,采用透射边界模拟无限域,通过将海水和基岩视为孔隙率分别等于1和0的广义饱和多孔介质,使得海水、饱和土、基岩之间的相互耦合可在统一计算框架中实现,避免不同介质求解器之间的数据交换。采用集中质量显式有限元并行计算,不同进程之间采用MPI进行数据交换,提高计算效率;采用逐元技术,按单元类别存储单元刚度,大大节省了内存,便于大规模计算。通过自编程,输入界面高程数据和材料参数,实现建模-自由场-三维地震动模拟全流程自动化。以东京湾为例,使用该方法和程序在超级计算机上模拟SV波垂直入射时的地震响应,证实了该方法用于三维大规模海域地震波场模拟的高效性和可行性。     

Ovalization of steel energy pipelines buried in saturated sands during ground deformations

Cross-sectional ovalization of buried steel pipes subjected to bending moment induced by end displacements is discussed. A three dimensional finite element analysis was conducted employing Abaqus/CAE. The pipe was simulated using 3D shell elements while the saturated sand soil medium was simulated by employing discrete nonlinear springs along the pipeline. The effects of normalized burial depth (H/D), diameter to wall thickness ratio (D/t), sand density and level of the internal pressure on the ovalization are investigated, and resulting ovalization distribution with respect to bending moment at critical sections is presented. The results of this study enable simple one dimensional finite element models to consider geometrical cross-sectional nonlinearities in the analysis of buried pipelines.     

饱和粗砂、粉土内甲烷水合物形成与分解过程中的水分迁移规律

用可探测土壤水分变化的pF-meter探头组装了新型水合物研究设备,并对两种饱和介质内甲烷水合物形成与分解过程中的水分迁移规律进行研究,以期望能为进一步探索多孔介质内甲烷水合物形成与分解机制提供参考.通过实验发现相同条件下,不同饱和介质内生成的甲烷水合物形态存在较大差异,且形成过程中的水分迁移规律也不同;水合物分解过程...     

Advances in Air Sparging Technology of Saturated Zone

In situ air sparging involves injecting atmospheric air, under pressure, into the saturated zone to remove those volatile and semi-volatile organic groundwater contaminants and to promote their biodegradation by increasing subsurface oxygen concentrations. Due to the advantages of low cost, high efficiency and in situ constructability, groundwater Air Sparging (AS) technology has been quickly developed in the world recently. Based on the explanation of its remediation principle, literature review is done on the research advancement of air sparging technology mainly from three aspects. First, various methods for determination of the zone of influence and visualization techniques of air flow forms during air sparging are summarized. Then the influence of environmental geological conditions and construction technology parameters on the remediation effect of air sparging is systematically analyzed. Thereafter, two main types of air sparging theoretical models including lumped-parameter model and multiphase fluid flow model are discussed respectively in detail. Finally, based on the problems and difficulties existing in present research and engineering practice, several future tasks such as the enhancement remediation techniques in complex geological sites, microscopic intrinsic mechanisms, and establishment of related design and construction standards which require to be done are briefly analyzed.     

Laboratory and pilot studies on reclamation of a salt-affected alluvial soil

Soil column experiments showed that a surficial sodic soil is efficiently reclaimed using freshwater, after the addition of saturated gypsum solution. Gypsum application in the field was beneficial in terms of maintaining high soil permeability, increased water infiltration and neutral pH after a rainfall event. In the present paper, two different reclamation techniques for the plough layer of a sandy loam sodic soil were tested in laboratory columns, 25 cm long and 10 cm in diameter; the first using freshwater alone and the second using a saturated gypsum solution. The dynamics of salt removal were studied by continuous analysis of the water drained from the bottom of the columns. When freshwater was used, sodium presented the lower removal rate and adversely affected soil permeability. When gypsum solution was used, calcium was present in the flushing solution and the effect of sodium dominance on clay dispersion and soil clogging was limited. The results presented in this study are of practical importance with respect to the reclamation of sodic soils found in the coastal area of the east Nestos Delta, Greece, where freshwater is limited, due to seawater intrusion, and saline groundwater is used for irrigation.     

Dynamic response and pore pressure model of the saturated soft clay around the tunnel under vibration loading of Shanghai subway

This paper studies the saturated soft clay around the tunnel between Jingansi Station and Jiangsu Station of Shanghai subway line No.2. The continuous dynamic monitoring is conducted by means of embedded earth pressure piezometers and pore piezometers around the tunnel. The response frequency and stress amplitude of the saturated soft clay are studied with the distance from the tunnel due to the subway vibration loading. A formula is proposed for the attenuation of the dynamic pressure response in the soil. Then, based on the continuous field monitored data, and using the laboratory GDS (Global Digital System) test apparatus, the developing law of the pore water pressure of the saturated soft clay around tunnel is explored with the distance. By the GDS test, the model of the increasing pore water pressure of the saturated soft clay is put forward under the vibration loading. It is also amended by field monitored data. Finally the model is obtained. The result offers a valuable reference to the design, construction and the safe operation of the subway tunnel.     

Evaluation of hydraulic conductivity through particle shape and packing density characteristics of sand–silt mixtures

Abstract

This study aims to evaluate the relationship between saturated hydraulic conductivity with particle shape and packing density characteristics of silty sand soils. The article presents a series of hydraulics tests performed on three kinds of sand with different particles shapes (Chlef rounded sand, Fontainebleau sub-rounded sand and Hostun sub-angular sand) mixed with low plastic rounded Chlef silt in the range of 0–30% fines content. The sand–silt mixture samples were tested in the constant-head permeability device at a loose relative density (D_r = 18%) and a constant room temperature (T?=?20?°C). The obtained results indicate that the measured saturated hydraulic conductivity (K_s) correlates very well with the fines content (F_c), packing density in terms of [maximum void ratio “e_max,” minimum void ratio “e_min,” predicted maximum void ratio “e_max”_pr and predicted minimum void ratio “e_min”_pr] and particle shape characteristics ratios in terms of roundness ratio (R_r = R_hs/R_mixture) and sphericity ratio (S_r = S_hs/S_mixture) of the silty sand materials under consideration. Moreover, the analysis of the available data show a noticeable success in exploring the prediction of the saturated hydraulic conductivity (K_s) based on the particle shape and packing density characteristics (R_r, S_r, e_max, and e_min) of the studied sand–silt mixture samples.     

Streamflow generation in a headwater basin on the precambrian shield

Current conceptual runoff models hypothesize that stormflow generation on the Canadian Shield is a combination of subsurface stormflow and saturation overland flow. This concept was tested during spring runoff in a small (3.3 ha) headwater basin using: (1) isotopic and chemical hydrograph separation and (2) field mapping and direct tracing of saturated areas. Isotopic and chemical hydrograph separation indicated three runoff components: (1) pre-melt subsurface flow; (2) subsurface flow of new (event) water; and (3) direct precipitation on to saturated areas (DPS). During early thaw-freeze cycles, their relative contributions to total flow remained constant (65 per cent, 30 per cent, and 5 per cent respectively). It is hypothesized that lateral flow along the bedrock/mineral soil interface, possibly through macropores, supplied large volumes of subsurface flow (of both old and new water) rapidly to the stream channel. Much higher contributions of DPS were observed during an intensive rain-on-snow event (15 per cent of total flow). Mapping and direct tracing of saturated areas using lithium bromide, suggested that saturated area size was positively correlated to stream discharge but its response lagged behind that of discharge. These observations suggest that the runoff mechanisms, and hence the sources of stream flow, will vary depending on storm characteristics.