Cnoidal water wave induced seepage in a permeable seabed with a defined thickness

Extreme waves can induce seepage in a seabed and cause problems to marine structures in coastal regions. In this study, the seepage under cnoidal waves was studied using the transient seepage equation. An analytical solution is presented for the pore pressure in a seabed of defined thickness. Parametric studies were carried out to examine the influence of air content in the pore water, and of the soil hydraulic conductivity on the seepage. It has been shown that the air content and the soil hydraulic conductivity can affect the pore pressure response significantly. An increase in the air content or a decrease in the soil hydraulic conductivity will increase the magnitude of the pore pressure gradient and results in the pore pressure varying sharply. The liquefaction potential of a seabed under cnoidal waves is discussed. Consequently, comparative studies are carried out to show that the soil shear modulus and Poisson constant can influence the difference between the transient seepage equation and Biot's equation, and the transient seepage equation is a limit of Biot's equation.     

Numerical analysis of surcharge preloading consolidation of layered soils via distributed sand blankets

Abstract

Surcharge preloading consolidation of soft soils often implements a layer of fully arranged aggregate materials. The volume of drained water is abundant at the early stage of consolidation, but it reduces at middle and later stages, during which the fully arranged sand blanket will be a waste. In this investigation, a concept of distributed sand blankets is proposed to save aggregate materials. A series of finite element analyses have been performed on layered soils with distributed sand blankets. A mixed type of drainage boundary is assigned to a representative model, where a half sand blanket is perfectly pervious and a half width of soil among sand blankets is impervious. From parametric study, it has been found that a pave ratio between sand blankets and the total soil width can be selected in a range of 40%–60%, which will save aggregates by approximately 50% but cause an increase of consolidation time by less than 10%. For a fixed pave ratio, more evenly spaced sand strips with smaller width should be employed to optimize the design. The effectiveness of distributed sand blankets is not influenced by the anisotropy of hydraulic conductivity, elastic modulus, Poisson’s ratio, and thickness in multiple soil layers.     

A coupled CFD-DEM simulation of upward seepage flow in coarse sands

Abstract

The suction anchor becomes more popular for offshore oil and gas industry in deeper water. For suction anchor–soil interaction, the prediction of hydraulic conductivity of porous materials is a long-standing problem in offshore engineering. To investigate the hydraulic characteristics, an upward seepage flow through saturated sands is considered in this study. A numerical approach, which is able to describe the fluid–particle interaction at particle scale, has been employed to analyse fluid flow in sands. This approach is constructed by adopting a coupled discrete element method and computational fluid dynamic approach (CFD-DEM numerical model). The coupled CFD-DEM approach is first benchmarked by a classic geomechanics problem where analytical solutions are available, and then employed to investigate the characteristics of upward seepage flow in coarse sand columns. Through numerical modelling, the predicted relation between hydraulic gradient and flow velocity is obtained and it is compared with the classical analytical correlation. The effect of several bulk and micromechanical parameters including packing porosity, particle size combination and inter-particle rolling resistance on the flow characteristics is numerically examined. The results show that the particle polydispersity and packing porosity have significant effect on the hydraulic conductivity in the seepage flow. The introduction of inter-particle rolling resistance can change initial packing structure of particle assembly in some extent rather than the hydraulic conductivity from the particle shape effect perspective. A further development of numerical model, in which the effect of non-spherical particles on the seepage flow, will be carried out later.     

An Approach to Evaluate Hydraulic Conductivity of Soil Based on CPTU Test

This article presents a new approach to estimate hydraulic conductivity of soil from cone penetration test with pore water pressure measurement (referred to as CPTU hereafter). The proposed approach is based on the test result of the spherical cavity expansion of the soil at the tip of a pile. During the piezocone penetration, the flow shape of pore water around the tip of the cone is assumed to be a spherical crown and induced excess pore water is assumed to dissipate from the crown surface. Based on this assumption, a bi-linear relation between the piezocone sounding metric (which is the product of the pore water pressure ratio B_q and the tip resistance Q_t) and the hydraulic conductivity index K_D is derived to estimate the hydraulic conductivity of the soil layer. The derived approach expands the applicable range of existing approaches in the literature. It is demonstrated that the proposed approach can cover the entire tip angles of the cone and the modified equation can fit the CPTU test data well.     

Surcharge preloading consolidation of reclaimed land with distributed sand caps

Abstract

For land reclamation using the conventional surcharge preloading method, a sand cap layer is often fully placed at the ground surface of ultra-soft compressible estuary or marine clays. A novel strategy of distributed sand caps is proposed to save cost associated with sand materials. At an early stage of consolidation, the drainage capacity of distributed sand caps is less than the drainage demand, whereas at a later stage, the capacity exceeds the demand. Hence, the overall drainage capacity of distributed sand caps is equivalent to the drainage demand. A numerical model is developed to study the effectiveness of the proposed technique, and calculations are compared against theoretical solutions. A parametric study is conducted to optimize design parameters. It has been found that distributed sand caps should be uniformly placed with a size of 3% of the total width and a spacing of 1 times the size, such that sand materials can be saved by up to 50% without compromising the consolidation efficiency (an increase of consolidation time by less than 5%). The use of distributed sand caps will be effective regardless of soil properties, including hydraulic conductivity, elastic modulus and Poisson’s ratio.     

Analyses of Seepage Problems in a Subsea Tunnel Considering Effects of Grouting and Lining Structure

Analytical solutions for water inflow and water pressure based on steady-state water inflow into a circular subsea tunnel in a semi-infinite aquifer are deduced depending on the rock permeability, drainage at the lining, and grouting parameters. Based on the parameters of the Qingdao Jiaozhou Bay subsea tunnel, the differences between the proposed solutions and three previous methods are analyzed. Comparisons of the results of different methods verify the validity of the proposed analytical solutions and manifest the superiorities of the proposed solutions in reflecting the effects of grouting and lining structure on the seepage problems in a subsea tunnel. Discussion of the lining structure is also made and we conclude that “block-based and limited drainage” should be the design principle of lining structures. Finally, the method of identifying the most rational and economical grouting parameters is presented.     

Geotechnical sealing material for coastal disposal facility for soils and wastes contaminated by radioactive cesium

Safe disposal of wastes produced due to the process of decontamination in and around Fukushima No. 1 nuclear power plant is an urgent requirement. The purpose of this study was to develop a sealing material which can be used as an engineered barrier for a final disposal facility for the soils and wastes contaminated by radioactive cesium. The analyses conducted based on 4-m-thick sealing layer revealed that the hydraulic conductivity of the sealing material needs to be kept below 5.0?×?10^?10?m/s to avoid the seepage of contaminants below the environmentally safe limits. Sealing material was developed using marine clay–bentonite mixture and the engineering characteristics were examined. The results of laboratory experiments showed that, with the addition of bentonite, the hydraulic conductivity equal to or less than 5.0?×?10^?10?m/s was achieved when the effective consolidation stress is equal to or more than 27?kPa. From the tests for adsorption properties for cesium, it was found that the sealing material showed the significant capacity of adsorption for cesium in seawater. It was concluded that the construction of waste disposal facility on the sandy seafloor is feasible using the sealing layer proposed in this study.     

Geologic control of natural marine hydrocarbon seep emissions, Coal Oil Point seep field, California

High-resolution sonar surveys, and a detailed subsurface model constructed from 3D seismic and well data allowed investigation of the relationship between the subsurface geology and gas-phase (methane) seepage for the Coal Oil Point (COP) seep field, one of the world’s largest and best-studied marine oil and gas seep fields, located over a producing hydrocarbon reservoir near Santa Barbara, California. In general, the relationship between terrestrial gas seepage, migration pathways, and hydrocarbon reservoirs has been difficult to assess, in part because the detection and mapping of gas seepage is problematic. For marine seepage, sonar surveys are an effective tool for mapping seep gas bubbles, and thus spatial distributions. Seepage in the COP seep field occurs in an east–west-trending zone about 3–4 km offshore, and in another zone about 1–2 km from shore. The farthest offshore seeps are mostly located near the crest of a major fold, and also along the trend of major faults. Significantly, because faults observed to cut the fold do not account for all the observed seepage, seepage must occur through fracture and joint systems that are difficult to detect, including intersecting faults and fault damage zones. Inshore seeps are concentrated within the hanging wall of a major reverse fault. The subsurface model lacks the resolution to identify specific structural sources in that area. Although to first order the spatial distribution of seeps generally is related to the major structures, other factors must also control their distribution. The region is known to be critically stressed, which would enhance hydraulic conductivity of favorably oriented faults, joints, and bedding planes. We propose that this process explains much of the remaining spatial distribution.     

The structure and dynamics of the Bottom Boundary Layer in shallow sea areas without tidal influence: an experimental approach

This report describes extensive investigations of the near bottom layer of the Western Baltic (Mecklenburg Bight, Darss Sill and Arkona Basin) which were conducted over a 5 year period to determine the typical structure, vertical thickness, vertical turbulence structure, and spatial and temporal variability of this water mass with regard to the area's particular hydrographic conditions. Series of vertical profiles were obtained using the microstructure profiler MSS86, which is capable of measuring high resolution profiles of temperature, conductivity, current shear, light attenuation and pressure down to the seafloor. The near bottom current structure was simultaneously measured with conventional current metres at fixed depths. A typical vertical density structure of the near bottom layer was found. At all investigation sites the Bottom Boundary Layer was separated from the overlying water mass by a well pronounced thermohaline pycnocline. A homogeneous water layer was situated above the bottom with a mean thickness of 2.2 m and typical variation between 0.5 and 3.5 m. The thickness of both the homogeneous layer and of the near bottom layer vary considerably. It is suggested that horizontal advection is responsible for these fluctuations in thickness. The variation in thickness of the Homogeneous Layer is independent of the local mean current velocity, wind speed and energy dissipation rate. Over periods of about 2 days the thickness of the Homogeneous Layer is determined by the average wind speed. The Bottom Boundary Layer shows its own characteristic dynamic, which is largely decoupled from that of the remaining water body. A logarithmic layer was generally not resolved by the current measurements. From dissipation rate measurements, the wall layer was determined to be 0.9 m thick. There was no significant correlation between the dissipation rate and the local wind speed, or between the dissipation rate and local mean current u₁₀₀. This means that any simple parameterisation relating u₁₀₀ or friction velocity to the locally produced turbulence and consequently to the resuspension of sediment is probably not applicable to shallow sea areas with properties like the Western Baltic. The investigation of sediment concentration in the BBL illustrates the importance of local effects combined with advection. The sediment stratified layer covers only the bottom most 50 cm.     

End wall effects in experiments on water intrusion along the interface between two homogeneous layers

End wall effect puts an inherent limitation on tank experiments, especially when the problems in a stratified fluid are dealt with. During experiments involving horizontal intrusion along the interface between two homogeneous layers, a curious phenomenon was found, i.e., the tip of the intruding water wedge continues to extend for a short time after supply is stopped, but then it begins to retreat in the cases of relatively high Reynolds numbers. The cause of this retreat of the wedge was investigated and was shown to be attributable to the initial disturbance generated near the mouth of the feeder at the start of water supply which propagates along the interface layer and reflects at the end of the tank as a bulge of the interface layer. The retreat of the intruding wedge would not occur in a sufficiently long tank, and so the cause of the retreat can be considered as one kind of end wall effect in a tank.     

水槽造流系统水力计算与泵机选择

提供一种完整的用于试验水槽造流系统的水力计算和泵机选择方法 ,可用于其他类似工程设计。     

Nutrient chemistry and hydrology of interstitial water in brackish tidal marshes of Chesapeake Bay

Nutrient concentrations in interstitial water were measured throughout the year in two brackish tidal marshes differing in elevation and vegetation. At all sites, sulfate to chloride ratios were lowest during the fall. In contrast, dissolved ammonia, phosphate, organic nitrogen, and organic phosphorus concentrations did not vary seasonally but differed among sample sites. These nutrients were generally enriched in interstitial water relative to tidal water and those that were most enriched declined in concentration with increasing proximity to creeks. In the low elevation marsh, flow of interstitial water towards creek banks was traced with Rhodamine WT dye. Consequent seepage of interstitial water into the creek of the low marsh was estimated from continuous monitoring of water table heights and from measurements of hydraulic conductivity. The estimated seepage could account for a portion, probably less than half, of the tidal export of dissolved nutrients from the low marsh.     

栉孔扇贝海上中间培育阶段提高保苗率的试验研究

栉孔扇贝Chlamys farreri (Jones et preston)的人工育苗研究,在我国已有十余年的历史。室内培育阶段已有一套较为完整可行的培育方法。但至今,对如何提高稚贝出池后海上中间培育阶段的保苗率,众家纷纭,尚无成熟经验,保苗率很低,以致影响栉孔扇贝的苗种生产。为了探索提高扇贝稚贝出池后在海上中间培育阶段的保苗率的措施,我们于1983年采用正交试验法,对海上中间培育阶段     

Mechanism test on inhomogeneity of dredged fill during vacuum preloading consolidation

Abstract

First, this article presents a simulation experiment of hydraulic reclamation, and then a vacuum preloading (VP) test using the sedimentary soil obtained by the first experiment. In the VP test, the distribution and variation of different physico-mechanical parameters before and after the treatment were tested. According to the test results, the concept “non-Terzaghi soil” is proposed to explain the inhomogeneity and its mechanism rendered by “seepage separation.” And then a staged VP (SVP) drainage consolidation method has been introduced to improve the inhomogeneity and seepage curtain phenomena around prefabricated vertical drains (PVDs) during the consolidation. The test results demonstrate that the clogging problem around PVDs has been prevented and the consolidation efficiency has been promoted after the SVP test. It has been noticed that the cumulative drainage volume and the settlement displacement of SVP test were 27% and 24%, respectively, greater than that of VP test, and the soil tends to be more homogeneous. Moreover, it has been shown that the inhomogeneity degree of the permeability coefficient, unit weight, void ratio, water content, cohesion, internal friction angle, compression modulus, and the soil surface settlement in slurry after the SVP test were 3.10, 1.02, 1.03, 1.09, 2.30, 1.92, 1.19, and 1.02, whereas that after VP test was 397.27, 1.07, 1.40, 1.40, 4.74, 3.00, 1.76, and 1.22. Finally, the mechanism of SVP method has been discussed.     

基于FLUENT的波浪溢流水动力数值模拟

波浪溢流现象使得海堤受到了越浪和溢流的联合作用,复杂的水动力过程会引起海堤后坡产生严重的侵蚀破坏。基于FLUENT软件建立了二维数值波浪溢流水槽模型,该模型运用UDF速度边界造波法分析在不同超高条件下海堤后坡流量和水流厚度的水力学特性。结果表明数值模拟结果与前人物理模型试验结果吻合,该模型可以真实地模拟出海堤波浪溢流现象。在此基础上进一步研究了波浪溢流中越浪和溢流在不同相对超高条件下的主导性作用,而后建立了十分准确的波浪溢流海堤后坡稳定水流厚度计算公式。     

Influence of Bottom Stress on the Two-layer Flow Induced by Gravity Currents in Estuaries

Baroclinic circulation in highly stratified and partially stratified estuaries is characterised by a two-layer flow: a bottom salt- water inflow and a surface brackish-water outflow. Tidal period variation of the thicknesses of a two-layer flow is observed to be associated with mixing, bottom stress and hydraulic characteristics of superposed tidal and gravity currents. Here, both analytical two-layer hydraulic equations with weak friction and a numerical model including a turbulence closure were utilised to understand the mechanism of the layer tendency within a two-layer flow under different barotropic flow conditions. It has been found that in the weak bottom friction case, a gravity current has two critical solutions at the layer thickness equal to 0·5Hand 0·292H. The layer thickness towards a particular critical solution is dependent on the sign of the bottom stress, i.e. when the bottom stress is opposite (favor) to the bottom gravity current, its layer thickness converges to 0·5H(0·292H). In the case of strong bottom stress and mixing opposing the gravity current, the solutions of the gravity current layer thickness at 0·5Hand 0·292Hwill not be valid. Both mixing and vorticity produced by bottom stress erode the halocline, and produce a high velocity core in the mid-depth, which leads to the thickness of a bottom gravity current greater than 0·5H. These internal hydraulic tendency and mixing processes, varying with time-dependent barotropic tidal current forcing, determine the tidal period variation of the gravity current structure.     

Lancelot—a seabed piezometric probe for geotechnical studies

A seabed instrument called Lancelot has been developed to measure pore pressure characteristics within potentially unstable marine sediment deposits, in any water depth. An estimate of the coefficient of consolidation can be obtained from the sediment pore pressure dissipation response that occurs following penetration into the seabed. This data can be used to calculate the coefficient of hydraulic conductivity, which is useful in analyzing the seepage velocity of pore fluids moving within the sediment mass. Pore-water pressures in excess of the normal hydrostatic profile are detected and used in static and dynamic liquefaction analyses, with a view toward understanding the origin of unusual seabed features such as pockmarks, as well as short-term instability caused by wave loading. An example of static liquefaction due to seepage stress is given for sites in and adjacent to the well-known 1929 Grand Banks debris flow, along with an example of incomplete liquefaction caused by dynamic wave loading within Miramichi Inner Harbor, New Brunswick. The adoption of in situ measurement techniques is shown to produce data of a quality not normally obtainable from recovered core samples, due to pressure relief and sampling disturbance effects.     

Laboratory modeling of siphon drainage combined with surcharge loading consolidation for soft ground treatment

Conventional drainage consolidation methods cause significant energy consumption and environmental issues. In this paper, a method combining siphon drainage and surcharge loading is proposed to drain water from soft soil with vertically installed prefabricated vertical drains (PVDs) and a siphon tube. To investigate the availability and effectiveness of this method, a laboratory physical modeling test was conducted to investigate the drainage and consolidation behavior. The laboratory modeling test results of this method were compared with the calculated results of the ideal sand-drained ground consolidation method to clarify the advantages and mechanism of this method. Comparison results show that the pore pressure and settlement in the proposed method developed faster than the calculation results of ideal sand-drained consolidation theory. About 10?m thickness of unsaturated zone can be formed by siphon drainage which produce a surcharge loading effect on the soil below the flow profile. Drainage is a very slow process in soft soil, and siphon drainage can work continually. Siphon drainage combined with surcharge loading is potentially a good alternative to drain water from soft clay economically and environmentally.     

吕宋海峡深层水体体积输运的诊断分析

使用WOD09, WOA05, WOA01和SODA四种水文数据,利用水力学理论对吕宋海峡的深层水体体积输运进行了估计。结果表明：由于忽略了摩擦和地形等因素的影响,水力学理论得到的输运结果是深层体积输运的上限。不同数据以及吕宋海峡东西两侧选取的不同区域,都对水力学理论的体积输运估计存在显著影响。SODA数据在吕宋海峡附近的深层海区存在偏差,得到体积输运估计明显偏大,因而不适用于水力学理论进行吕宋海峡深层水体体积输运的估计。为对于南海的环流、混合及生态环境存在重要影响的吕宋海峡深层水体体积输运进行合理估计,对利用水力学理论估计深层水体体积输运的研究方法做了进一步探讨。     

A pair of Langmuir cells in a laboratory tank (I) wind-only experiment

Three velocity components of subsurface flow, observed in a rectangular tank under the action of a constant wind speed, are measured systematically at mesh points distributed uniformly over a vertical cross-section of the tank. Measurements are carried out for two cases: 1) reference wind speedU _r =7.5 m/s and fetchF=10 m; and 2)U _r =10 m/s andF=25 m. A pair of Langmuir cells is observed for both cases; downwelling zones are found along both of the sidewalls and an upwelling zone in the centre of the tank. Near the water surface, the vertical momentum flux is dominated by the Reynolds stress resulting from small-scale turbulence, while over the entire cross-section except near the surface, the Reynolds stress due to the Langmuir cells dominates the vertical momentum flux. As the result of the occurrence of this Langmuir cells, the vertical momentum flux, which consists of both mean advection and small-scale turbulence, is markedly inhomogeneous in the spanwise direction; for example, the largest vertical flux of the order of the wind stress is observed in the downwelling zone near one sidewall, while at the centre of the tank, the vertical momentum flux occupies only 30% of the wind stress. This indicates that a pair of Langmuir cells plays more important role than small-scale turbulence in the mixing process in a greater part of the wind-wave tank.Address after April 1, 1992: Department of Civil Engineering, Hiroshima Institute of Technology, Miyake 2-1-1, Saeki-ku, Hiroshima 731-51, Japan.