An Analytical Solution for the Run-Out of Submarine Debris Flows

Submarine debris flows have a significant impact on offshore and coastal facilities. The unique characteristics of submarine debris flows involve large mass movements and long travel distances over very gentle slopes. To improve our insight and knowledge of the basic mechanism behind submarine debris flows, an analytical model was derived for the mobility of submarine debris flows. This model takes into account the mass change of debris flows induced by deposition, stagnation pressure, and the topography of the depositional area. One case study on the Palos Verdes debris flow proves its ability to predict the run-out distance of a submarine debris flow to a reasonable level of accuracy. On the gentle slopes, the submarine debris flow progressively loses mass due to deposition, which in turn influences the flow velocity. In addition, the results show that the slope angle and spreading angle of the debris depositional zone play important roles in the sliding process.     

荔湾3-1气田潜在海底滑坡滑动距离预测及动态特性分析

A large number of submarine landslides with different scales have been identified in the canyon area of the submarine pipeline route of Liwan 3-1 gas field. There is still much chance that submarine sl...     

海底滑坡对海洋单桩冲击力试验研究

为了研究海底滑坡对海洋单桩的冲击力大小,首先通过调整高岭土、粉砂的不同含量,得到不同流变特性、不同密度的碎屑流,采用Herschel-Bulkley模型和幂率模型对流体流变性质进行描述;随后利用自制海底滑坡模型槽,模拟碎屑流在不同流速和黏度下对模型桩的冲击;并结合流体力学理论,建立阻力系数与非牛顿流体雷诺数之间关系表达式。试验数据表明:碎屑流黏度和流速是影响海底滑坡冲击力的主要因素,海底滑坡冲击力随着泥浆黏度和流速的增加而增大。同时,考虑碎屑流剪切稀释特性,得到管桩阻力系数随雷诺数变化的拟合公式,为海洋桩基础设计提供参考。     

Initial results and progress of the Mississippi delta sediment pore water pressure experiment

Abstract

This report describes the instrumentation, initial results, and progress of an experiment designed to measure and monitor submarine sediment pore water and hydrostatic pressures in a selected area of the Mississippi Delta. The experiment also is intended to monitor significant pressure perturbations during active storm periods. Initial analysis of the data revealed excess pore water pressures in the silty clay sediment at selected depths below the mudline. Continuous monitoring of the pore water and hydrostatic pressures was expected to reveal important information regarding sediment pore water pressure variations as a function of the geological processes active in the Mississippi Delta.     

Numerical simulation of water content dependent undrained shear strength of clays

Abstract

A nonlinear mathematical model for estimating the water content dependent undrained shear strength of clayey soils was developed. Three types of clay mixtures (kaolinite, bentonite, and kaolinite-bentonite) were considered. The shear strength of the given soil samples was determined via torvane tests. Experimental results were compared with three numerical results: (i) the analytical function fit, (ii) modeling without the water content effect, and (iii) modeling with the water content effect using the Mohr-Coulomb (M-C) model. There was good agreement among the experimental, analytical, and numerical results with and without the water content effect in the fully softening zone. However, there was a large difference between the numerical results obtained from the developed model with and without the water content effect in the flow zone with a high liquidity index, because the shear strength may decrease significantly to low value in the case of an abrupt increase of the water content. The greatest advantage of the developed model is that it can simulate the reduction of the shear strength and shear band development under the high water content condition, which may trigger a large mobile mass movement.     

The nature and engineering behavior of carbonate soils at Bombay High,India

Abstract

This paper presents the results of a laboratory investigation undertaken to study the nature of two submarine carbonate soils from Bombay High off the west coast of India, as well as to study the shear and plasticity behavior of their sand and silt‐clay fractions, respectively. Scanning electron micrographs reveal that the carbonate content in both soils is comprised primarily of nonskeletal particles of various types. X‐ray diffraction and infrared absorption analyses indicate that in one soil the carbonate fraction consists of calcite and aragonite minerals, whereas in the other soil dolomite is also present. The non‐carbonate fraction of both soils is comprised primarily of quartz and feldspar, and also some clay minerals. The nature of the carbonate fraction of the two soils indicates that they were formed by different depositional processes.

During drained triaxial shear the nonskeletal sand grains of both soils exhibit a lower degree of crushing when compared with that of the skeletal carbonate sands, and thus appear to be stronger foundation material.

Although the carbonate contents of the silt‐clay fractions of the two soils are similar, they exhibit markedly different plasticity characteristics . This is probably because of the microlevel cementation produced by carbonate material in one soil.

This study leads one to the conclusion that carbonate content alone should not be treated as a parameter which controls the engineering behavior of submarine soils; the nature and form of carbonate material must also be identified.     

Probabilistic analysis for wave‐induced submarine landslides

Abstract

When waves propagate over the ocean floor, they induce a change of hydrodynamic pressure, positive under the crest and negative under the trough. These pressure changes may cause shear failure in soft sediments and lead to submarine landslides. This paper presents a general analytical procedure for evaluating the probability of wave‐induced failure in offshore clay sediments. Both the wave and the un‐drained shear strength of clay sediments are considered random. Numerical results of some analyses are also presented.     

Submarine debris flow impact on pipelines — Part II: Numerical analysis

Computational Fluid Dynamics (CFD) numerical analysis was employed to analyze the situations tested experimentally, as described in Part I. The methodology and results of the CFD analyses are discussed and compared with the observations made from the experiments. The numerical model performed satisfactorily with regard to obtaining the impact forces exerted on the model pipe as well as simulating the hydroplaning phenomenon and estimating slurry flow heights. The experimental results were combined with the results of the CFD analyses to develop a practical method to compute the drag force caused by a submarine debris flow impact on a pipeline. The CFD analyses provided some insight to the separated region characterization, but the attempt to analyze the vortex shedding phenomenon as observed in the experiments was unsuccessful. Additional studies are required for better understanding of both the separated region characteristics and vortex shedding.     

基于流变仪测试海底浅表层软黏土不排水强度研究

鉴于海底浅表层软黏土强度测试精细化程度不足的现状,引入流体测试中的流变仪,对青岛海域海底浅表层软黏土开展多组原状和重塑试样的不排水剪切强度试验,通过对比静力触探和微型十字板测试结果,验证了流变仪测试方法的有效性。基于流变仪试验结果,揭示了海底软黏土原状和重塑状态下不排水剪切破坏模式,探讨了海底软黏土不排水剪切强度和灵敏度随埋深及液性指数的发展演变趋势,评价了软黏土的结构性特征。最后,引入含水率与液限之比对海底浅表层软黏土重塑不排水剪切强度进行了归一化分析,为近海海洋开发活动提供技术支撑。     

Scarifying and fingering surfaces of plunging jets

The local surface deformation resulting from the oblique impact of a columnar water jet has been computed, using a three-dimensional large eddy simulation, as a model of the overturning jet of a breaking wave. The emergence of the secondary jet from the front face of the initial jet has been examined and the organisation of the vortices within the jet characterised. As the secondary jet emerges, the vorticity field becomes unstable under the action of the strong shear beneath the jet surface and pairs of longitudinal counter-rotating vortices stretched along the direction of the jet projection are formed. The presence of these longitudinal vortex pairs creates convergent surface flows, resulting in the formation of longitudinal scars on the rear face of the projecting jet. Following significant growth of the scars on both its upper and lower surfaces, the jet decouples into fingers. The lateral widths of the longitudinal vortices provide a minimum measure of the finger size. A horizontal Froude number Fr_h, representing a measure of strength of horizontal shear in a gravity-dominated impacting flow is defined, which characterises the organisation of the longitudinal vortices occurring in the shear flow, and the resultant formation of scars and fingers. For higher Fr_h, stronger longitudinal vortices and deeper scars are formed at longer lateral intervals, enhancing the fingering process during the splashing event. Fundamental features of material transport in the vicinity of the surface of jets (e.g. gas transfer across a sea surface) are related to the entrainment of surface fluid by the longitudinal vortices, and is thus also characterised by Fr_h.     

Strength evaluation of marine clay stabilized by cementitious binder

Abstract

Evaluation of the strength of cement-treated clay with a broad range of mix ratios and curing periods was conducted using unconfined compression tests (UCTs). The influence of cement content, total water content, and curing period on the unconfined compressive strength of cemented clay are investigated. It is found that, at constant total water content, higher cement content results in higher unconfined compressive strength, while the total water content has an opposite effect. A power function can be used to correlate the unconfined compressive strength with the cement content or the total water content. For a fixed mix ratio, the unconfined compressive strength of cement-stabilized clay increases with the curing period, the effect of which can be characterized by a semi-log formula. Also, a strength-prediction model that considers both mix ratios and curing periods for cement-admixed marine clay is developed and validated; the model can capture the effect of clay type by considering the plastic index of untreated soils. It is also proved that the proposed framework for strength development is also applicable for other cement types.     

Analysis of the experimental compressibility of deep water marine sediments from the Gulf of Guinea

This study strives to analyze the compressibility of deep water marine sediments originating from the Gulf of Guinea. Our approach is multi-scaled and, on the macroscopic level, based on two equations which link the overburden effective stress with the liquidity index, the Biarez & Favre equation on the one hand, and the Burland equation on the other hand (after calculation of the ICL expression as a function of the I_L parameter). On the local level, the evolution of the microstructures is analysed and compared with GoG remoulded clay. Two additional observation techniques have been used, the scanning electron microscope and the mercury intrusion porosimetry.     

On the use of Atterberg limits on marine soils

Abstract

On the basis of the Atterberg indexes of various sediments collected in the Mediterranean Sea, we select two major parameters indicative of the mechanical behavior of marine soils: the sedimentary facies (i.e., mechanism of deposition expressed by silt content) and the clay mineral association characterized by smectite content in the fraction finer than 2 μm. Activity and real thixotropic ability appear to be particularly important parameters in marine engineering; better values are obtained by a double test operation in distilled and marine water.     

Oceanography and marine biology, 10: Harold Barnes (editor). George Allen and Unwin,London, 1972, 565 pp., £10.50

A bathymetric survey of the Gilliss Seamount, in the northwest Atlantic Basin, using a multi-beam sonar array system reveals an extremely complex morphologic character of this feature. A new chart provides the most detailed topographic presentation of an Atlantic seamount published to date and highlights the similarity of the Gilliss Seamount with terrestrial strata-volcanoes. Bottom photographs and samples reveal pillow-Iava formation. Seismic profiles show that the volcanic basement is irregularly covered by acoustically transparent deposits that are as much as 668 m thick. Volcanic debris and sediments locally are displaced down the flanks of the seamount. Bottom photographs and cores indicate that the transparent layer has accumulated slowly by deposition from suspensate-rich (mostly clay and planktonic foraminifera) water masses that flow around the mid to lower sectors of this submarine volcano. Bottom-current activity also modifies the abyssal plain turbidite-hemipelagic sequence surrounding the seamount.     

Centrifuge Study on the Submarine Mudflows

Submarine mudflow is one of the most serious geohazards. A new Mini-drum Centrifuge apparatus was developed for use. A series of experiments were performed to investigate the flow behaviour of submarine mud, based on the following aspects: (i) side view; (ii) change in basal pore pressure; (iii) change in water content and (iv) change in the Reynolds number. In this series of centrifuge tests in the subaqueous environment, the submarine mudflow transformed from a highly concentrated mudflow to a turbulent current at a water content of 113% to 121%. Accordingly, the Reynolds number Re, as the threshold of turbulent flow was about 1,150–2,250. Based on the water content measured before and after each test, the water entrainment during the flowing process was minimal for laminar flows, but large for turbulent flows. In addition, the evolution of basal pore pressure was found to be closely related to the regime of submarine mudflow: laminar or turbulent.     

水土分离生态综合治理泥石流方法探讨——以青岛市崂山区河东村泥石流治理为例

2005年9月12日,青岛市崂山区河东村发生泥石流地质灾害,其周边大面积山坡仍存在潜在泥石流隐患,有再次发生泥石流的可能。通过对河东村泥石流"致灾因子"和"主灾因子"分析,得出该区发生泥石流的短历时雨强指数KH为1550.8。泥石流产生的物源条件和水源条件是缺一不可的,是水土共同作用的结果,为避免河东村再次发生泥石流,将水土分离是至关重要的。根据现场实际情况,综合考虑各方面因素,摸索出适合河东村泥石流水土分离生态综合治理方法,即地表、地下水土分离—生物生态治理—拦挡工程相结合方法,该方法在投资少、不影响风景区景观基础上,根本治理河东村泥石流发生。     

Coincident application of a mass balance of radium and a hydrodynamic model for the seasonal quantification of groundwater flux into the Venice Lagoon, Italy

A mass balance of the naturally occurring short-lived radium isotopes (^223,224Ra) in the Venice Lagoon was conducted by an integrated approach combining the directly estimated individual Ra contributions and hydrodynamic model results. Hydrodynamic data allows for the calculation of the Ra mass balance in sub-sections of the Venice Lagoon (boxes), which are characterized by physically homogeneous properties, instead of investigating the entire lagoon. Utilizing this method, both the seasonal and the spatial variability of the submarine groundwater discharge in the Venice Lagoon have been estimated. Between 14–83 × 10⁹ L d^− 1 of water were calculated to flow across the sediment–sea interface, corresponding to 5–28 times the mean annual river input. The submarine groundwater discharge estimates were correlated with the residence time calculation to better understand spatial and seasonal variation.     

Secondary Compression Features of Jiangsu Soft Marine Clay

The results of one-dimensional compression tests conducted on undisturbed specimens of Jiangsu soft marine clay is presented. Because of its high in situ void ratios and natural water content, Jiangsu soft marine clay displays high values of both the virgin compression index, C_c, and the secondary compression coefficient, C_α. The laboratory data indicates that the value of the ratio C_α/C_c for Jiangsu soft marine clay is constant. However, neither C_α nor C_c are constant: they both depend upon the natural water content (or void ratio) and thus are also dependent on the deformation (or compression) of Jiangsu soft marine clay. Settlement analyses show that the secondary settlement of Jiangsu soft marine clay is a significant component of the field settlement. The concept of a constant value for C_α/C_c is used to predict the secondary settlement of a surcharged embankment founded upon Jiangsu soft marine clay. The predictions are in agreement with the limited post-construction field measurements of the embankment settlement.     

An improved analytical approach for simulating the lateral kinematic distress of deepwater offshore pipelines

Offshore pipelines are critical infrastructures and any possible damage may have devastating financial and environmental consequences. Earthquake-related geohazards (such as strong ground motion, active seismic faults, submarine landslides and debris flows) consist crucial threats that an offshore pipeline has to overcome. The main aim of the current study is to examine analytically a seabed-laid offshore pipeline subjected to a lateral kinematic distress due to a submarine landslide or a debris flow. Extra emphasis is given on the impact of pipe-soil interaction on the pipe response, by the realistic representation of the soil resistance via a tri-linear model. Firstly, the proposed analytical model is validated with a numerical model utilizing the finite-element method. Subsequently, various combinations of soil parameters and loading conditions that affect the examined problem are investigated with realistic input data taken from the offshore section of the high-pressure natural-gas pipeline TAP (Trans Adriatic Pipeline) in the Adriatic Sea. Finally, useful conclusions are drawn regarding the applicability and the efficiency of the proposed approach.