Recolonization and recovery dynamics of the macrozoobenthos after sand extraction in relict sand bottoms of the Northern Adriatic Sea     

Simonini R  Ansaloni I  Bonini P  Grandi V  Graziosi F  Iotti M  Massamba-N'siala G  Mauri M  Montanari G  Preti M  De Nigris N  Prevedelli D 《Marine environmental research》2007,64(5):574-589

The long-term effects of sand extraction on macrozoobenthic communities were investigated in an offshore area in the Northern Adriatic Sea characterised by relict sands formed during the last Adriatic post-glacial transgression. Surveys were carried out before, during and 1, 6, 12, 18, 24 and 30 months after extraction at three impacted and seven reference stations. The operations did not influence the physical characteristics of the sediment, but they caused almost complete defaunation at dredged sites. Univariate and multivariate analyses highlighted that the macrozoobenthic community responses to the dredging operations were (1) a rapid initial recolonisation phase by the dominant taxa present before dredging, which took place 6-12 months after sand extraction; (2) a slower recovery phase, that ended 30 months after the operations, when the composition and structure of the communities were similar in the dredged and reference areas. This pattern of recolonisation-recovery fits well with the commonly encountered scenario where the substratum merely remains unchanged after marine aggregate extraction.  相似文献   

钙质砂中模型桩的试验研究     

江浩  汪稔  吕颖慧  孟庆山 《岩土力学》2010,31(3):780-784

根据钙质砂中桩基工程的现状,针对取自南沙群岛永暑礁的钙质砂,设计一个室内模型试验装置来研究钙质砂中钢管桩的承载和变形性能以及影响因素,并进行了石英砂中的对比试验。试验结果表明,钢管桩在钙质砂和石英砂中的表现有着显著差异。钙质砂中钢管桩承载能力很低,仅为石英砂的66%～70%,钙质砂中桩身轴力衰减速率缓慢,桩侧摩阻力远远小于石英砂的,仅为石英砂的20%～27%,并具有深度效应,开口钢管桩和闭口钢管桩的桩侧摩阻力相差不大。同时表明,钙质砂中桩侧摩阻力对相对密度的变化没有石英砂敏感,受相对密度影响很小。由颗粒破碎引起的桩周水平有效应力的大幅降低是造成钙质砂中钢管桩桩侧摩阻力低的主要原因。  相似文献   

Découverte de structures périglaciaires (sand-wedges et composite-wedges) sur le site de stockage de déchets radioactifs de l'Aube (France)     

Pierre Antoine  Albert Marchiol  Marcel Brocandel  Yves Gros 《Comptes Rendus Geoscience》2005,337(16):444

Since 1990, a dense network of subvertical cracks has been observed at the upper part of the Albian sands of the site of radioactive waste storage operated by ANDRA. New studies carried out in 1999 during enlargement of the site of Soulaines and in 2003 on the nearby TFA site highlighted an especially well-developed network of periglacial features that are mainly of the sand-wedge type (subvertical stratified infill). These observations demonstrate the periglacial origin of these cracks, which are mainly superimposed on a dense network of former tectonic extension joints, and that rapidly disappear at the top of the underlying clays. In addition, these observations currently represent the only example of a large sand-wedge network developed during the Last Glacial in northern France. To cite this article: P. Antoine et al., C. R. Geoscience 337 (2005).  相似文献   

改装G-2型钻机用于藏北砂钻施工     

谢亮 《四川地质学报》2006,26(3):182-184

目前国内不生产专用砂钻机,通过对G-2型工程钻机进行适当改造,加大柴油机的功率,加装副卷扬,去掉动力头,设计加工专用的套管夹持器和组合吊锤,实现了机械拧卸套管,大大减轻了劳动强度,提高了该型设备对藏北高原恶劣自然条件的适应能力。  相似文献   

Stone columns as liquefaction countermeasure in non-plastic silty soils   总被引：1，自引：0，他引：1  

K. Adalier  A. Elgamal  J. Meneses  J. I. Baez 《Soil Dynamics and Earthquake Engineering》2003,23(7):571-584

In many cases densification with vibro-stone columns cannot be obtained in non-plastic silty soils. Shear stress re-distribution concepts [1] have been previously proposed as means to assess stone columns as a liquefaction countermeasure in such non-plastic silty soils. In this study, centrifuge testing is conducted to assess the performance of this liquefaction countermeasure. Attention is focused on exploring the overall site stiffening effects due to the stone column placement rather than the drainage effects. The response of a saturated silt stratum is analyzed under base dynamic excitation conditions. In a series of four separate model tests, this stratum is studied first without, then with stone columns, as a free-field situation, and with a surface foundation surcharge. The underlying mechanism and effectiveness of the stone columns are discussed based on the recorded dynamic responses. Effect of the installed columns on excess pore pressures and deformations is analyzed and compared. The test results demonstrate that stone columns can be an effective technique in the remediation of liquefaction induced settlement of non-plastic silty deposits particularly under shallow foundations, or vertical effective stresses larger than about 45 kPa (1000 psf) in free field conditions.  相似文献   

Breaching in fine sands and the generation of sustained turbidity currents in submarine canyons   总被引：6，自引：0，他引：6  

Dick R. Mastbergen  Jan H. Van Den Berg† 《Sedimentology》2003,50(4):625-637

Abstract Natural, moderately loosely packed sands can only erode from the surface of the bed after an increase in pore volume. Because of this shear dilatancy, negative pore pressures are generated in the bed. In cases of low permeability, these negative pressures are released relatively slowly, which retards the maximum rate of erosion. This effect is incorporated in a new, analytically derived, pick‐up function that can explain the observation of gradual retrogressive failure of very steep subaqueous slopes, sometimes more than 5 m high, in fine non‐cohesive sands. This process, termed ‘breaching’ in the field of sediment dredging, may produce large failures in sand bars or river banks. The analytical function that describes the breaching process in fine sand is incorporated in a one‐dimensional, steady‐state numerical model of turbidity currents describing the spatial development of flow. This model is applied to simulate a large ‘flushing’ event in Scripps Submarine Canyon, Pacific coast of California. Breach retrogradation and the successive evolution in time of the resulting turbidity current in the canyon are predicted in a sequence of discrete steps. Predicted velocities are compared with values measured during a flushing event. Implications for the interpretation of deep‐water massive sands are discussed.  相似文献   

Liquefaction-induced ground deformations at Hotel Sapanca during Kocaeli (Izmit),Turkey earthquake     

K. Onder Cetin  T. Leslie Youd  Raymond B. Seed  Jonathan D. Bray  Rodolfo Sancio  W. Lettis  M. Tolga Yilmaz  H. Turan Durgunoglu 《Soil Dynamics and Earthquake Engineering》2002,22(9-12):1083-1092

This paper presents a study of liquefaction-induced ground deformations at the Hotel Sapanca site during the 1999 Kocaeli (Izmit)—Turkey earthquake. The paper discusses: (1) observed ground deformations and displacements after the earthquake, (2) the results of field investigations by means of borings and in situ index tests including standard penetration tests (SPT), static cone penetration tests (CPT) and piezocone (CPTU) tests, (3) analyses of observed ground settlements and lateral deformations by a suite of methods, and (4) comparisons of observed and calculated ground movements.  相似文献   

PASSIVE PRESSURE DETERMINATION BY METHOD OF SLICES     

Jyant Kumar  Kanakapura S. Subba Rao 《国际地质力学数值与分析法杂志》1997,21(5):337-345

A method of slices satisfying all the conditions of statical equilibrium has been developed to deal with the problem of determination of passive earth pressure over a retaining wall in sand. A method similar to that of Morgenstern and Price, which was used to solve the stability of slopes, has been followed. The earth pressure coefficients with the proposed methodology have been computed for a vertical retaining wall for both positive and negative wall friction angle. Also examined is the variation of the interslice shear force between the retaining wall and the Rankine Passive boundary. Due to complete satisfaction of the equilibrium conditions, the method generates exactly the same earth pressure coefficients as computed by using Terzaghi's overall limit equilibrium approach. © 1997 by John Wiley & Sons, Ltd.  相似文献   

苏州阳澄湖地区淤泥质粘土工程地质特性探讨     

郑军  阎长虹  夏良斌  梁清雨  郭军辉  王吉刚 《工程地质学报》2006,14(5):592-596

苏州阳澄湖周围的沼积湖积平原存在一种性质较为特殊的淤泥质粘土,其亲水性粘土矿物含量和液塑限指标均较高,但在天然状态下虽具一定的胀缩性却未达到膨胀土的标准,可是经过扰动压实后,随着干密度的增大和含水量的减少却表现出较强的胀缩性,即转变为人工膨胀土[1]。本文通过苏州阳澄湖东北方向某一级公路附近3个取土场土样的试验分析,探讨了此地区这种淤泥质粘土的特殊性,为以后的工程施工提供了科学依据。  相似文献   

Fundamental yet simplified model for liquefaction instability     

Chandra S. Desai  Innjoon Park  Changming Shao 《国际地质力学数值与分析法杂志》1998,22(9):721-748

A fundamental procedure is proposed for the identification of liquefaction in saturated soils based on the instability in the material's microstructure. The disturbed state concept (DSC) provides a unified constitutive model for the characterization of entire stress–strain behaviour under cyclic loading, and the values of disturbance at threshold states in the deforming microstructure provides the basis for the identification of liquefaction. The procedure is verified with respect to laboratory behaviour of two sands, saturated Ottawa and Reid Bedford. A mathematical analysis of the DSC constitutive matrix is also performed. Procedures for the application of the DSC for simplified analysis and design, and in finite element procedures are presented. It is believed that the proposed model can provide a fundamental yet simplified procedure for liquefaction analysis, and as a result, it is considered to be an improvement over the available empirical and energy-based procedures. © 1998 John Wiley & Sons, Ltd.  相似文献