孔压探杆贯入及潮汐作用下超孔压响应规律研究

Excess pore water pressure is an important parameter that can be used to analyze certain physical characteristics of sediment. In this paper, the excess pore water pressure of subseafloor sediment and its variation with tidal movement was measured following the installation of a wharf in Qingdao, China by using a fiber Bragg grating(FBG) piezometer. The results indicated that this FBG piezometer is effective in the field. The measured variation of excess pore water pressure after installation is largely explained by the dissipation of excess pore water pressure. The dissipation rate can be used to estimate the horizontal consolidation coefficient, which ranged from1.3×10~(–6) m~2/s to 8.1×10~(–6) m~2/s. The measured values during tidal phases are associated with the variability of tidal pressure on the seafloor and can be used to estimate the compressibility and the permeability of the sediment during tidal movement. The volume compression coefficient estimated from tidal oscillation was approximately2.0×10~(–11) Pa~(–1), which was consistent with the data from the laboratory test. The findings of this paper can provide useful information for in situ investigations of subseafloor sediment.     

Excess pore pressure in Mississippi delta front sediments: Initial report

Abstract

In September 1975, a differential piezometer probe was successfully implanted in the soft seafloor sediments of Block 28, South Pass, Mississippi Delta. The probe sensor is located approximately 6.4 m below the mudline in a water depth of 19 m, and has essentially continuously monitored excess pore pressure (the difference between sediment pore pressure and hydrostatic pressure at that depth) since installation. Excess pore pressure will be monitored until March 1976, when the probe will be recovered.

Immediately after deployment, an excess pore pressure of 54 kPa was recorded. An ambient excess pore pressure of approximately 32 kPa remained after dissipation of that developed during probe installation. Because of the possible presence of gas in the sediments in this area, it is not known with certainty whether the measured excess pressure is pore water pressure, pore gas pressure, or some combination of the two. An excess pore pressure of about 32 ±4 kPa was monitored during Hurricane Eloise and subsequent storms. The exact magnitude and time distribution of these pressure fluctuations is presently being evaluated.     

Discussion of “formation mechanism of large pockmarks in the subaqueous Yellow River Delta”

Abstract

The excess pore pressure accumulation is a key factor when estimating the formation mechanism of large pockmarks, as it determines the liquefaction potential of marine sediments due to water waves. The governing equations for excess pore pressure may have different forms for various types of sediments and then shall reflect the cyclic plasticity of the soil. For water waves propagating over a porous seabed, the liquefaction area induced by waves is generally progressive, which indicates that the liquefaction area will move forward following the wave train. Therefore, the excess pore pressure accumulation can be used to explain the occurrence of the large pockmarks, but the dimension of the pockmark may be related to the heterogeneity of sediment or the wave properties affected by the topography in the subaqueous Yellow River Delta.     

Model test of stratum failure and pore pressure variation induced by THF hydrate dissociation

ABSTRACT

A model test program for studying soil stratum failure and pore pressure variation during tetrahydrofuran (THF) hydrate dissociation considering the effects of heating and drainage conditions is presented in this paper. The temperature and pore pressures are recorded during heating. Test results show that the THF hydrate would dissociate to be liquid and then gas when heating. Once pore pressure generated by the flow of released gas/water exceeded the strength of over layer, the layered fractures and soil-gas/water mixture outburst would occur. The high heating temperature and low permeability of over layer both cause excess pore pressure generation and more serious stratum failures.     

Parametric study for underwater blast-induced pipeline response embedded in marine sediments

Abstract

Blast response of submerged pipelines has been a research focus in recent years. In this article, a three-dimensional numerical model is established to investigate dynamic response of pipelines due to underwater explosion. The u–p approximation is integrated into finite element method (FEM) to simulate pore water effect in the seabed. Numerical continuity between hydraulic pressure in the flow field and pore pressure in the marine sediment is guaranteed to realize the blast response of submerged pipelines in ocean environment. Both fluid–structure interaction (FSI) and pipeline–seabed interaction (PSI) have been considered in the proposed model simultaneously. A comprehensive parametric study is carried out after validation of the present model with test data from underground explosion and underwater explosion, respectively. The effect of embedment depth, TNT equivalent, stand-off distance, pipeline diameter, and pipeline thickness to blast response of the submerged pipelines is investigated based on numerical results. Variation of deformation patterns and stress distribution of the pipeline with various installation and structure parameters has been illustrated and discussed to facilitate engineering practice.     

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.     

Formation mechanism of large pockmarks in the subaqueous Yellow River Delta

Abstract

We have identified large pockmarks in an area of approximately 0.3?km² in the subaqueous Yellow River Delta in the Chengdao Sea. Gas eruption channels not been identified in the sediment layers in this area, and the formation mechanism of these large pockmarks remains unknown. To study the formation mechanism of these large pockmarks, we constructed a layered silty sediment model composed of appropriate geological materials. Then, we calculated the stress, displacement, and excess pore pressure in the layered silty sediment from the surface to a depth of 10?m using the Biot theory. A comparative analysis of the calculated results and the data measured in the field was then performed. Based on these results, we established a new formation mechanism for the large pockmarks. With the occurrence of storm waves, two extreme areas of displacement and excess pore pressure appeared in the layered silty sediment. These extreme values increased quickly in the seabed during the continuous action of storm waves. When the excess pore pressure surpassed the effective stress, the top silty layer instantly liquefied and then reconsolidated. Then, when the pore pressure of the interface position exceeded the effective stress produced by the overlying sediment, the sediments experienced “sand boil” damage. With the repeated action of strong waves, the boundary of the pockmark continued to expand, forming a large and stable pockmark. This work is of great value for further understanding and mitigating marine geologic hazards, such as coastal erosion, silt deposition, and unstable sediment, in the subaqueous Yellow River Delta.     

黄河口埕岛海域粉土波致孔压现场监测

波浪会对海床产生反复的作用力,由此引起的土体颗粒间孔隙水压力变化是造成土体液化的主要原因。使用自行研发的孔压监测设备,对黄河口埕岛海域易液化区海底孔压进行了长时间、高精度的观测,并对孔隙水压力、波高以及潮位间的关系进行分析。监测结果显示,本次监测条件下波浪最大作用深度介于0.5～1.5 m之间,超过该作用深度后孔压无明显变化。土体内部孔隙水压力的变化主要由潮位和波高决定,潮位的作用可使孔压缓慢平滑的变化且对超孔压无影响;波高的作用可使孔压快速、剧烈地振荡并导致超孔压的出现。     

The SEASWAB experiment

Abstract

SEASWAB is one element of the Delta Project of the U.S. Geological Survey, a cooperative effort with several universities and other governmental agencies to investigate the processes that cause marine‐sediment instability. The basic purpose of the SEASWAB experiment was to obtain field measurements of sediment motion and pore‐pressure variations in soft sediment affected by wave‐pressure perturbations. This article serves as an introduction to the six papers that follow and that together make up a report on the results of SEASWAB.     

光纤温深传感器在物理海洋的应用

光纤传感器技术已经在土木工程、航空航天、石油石化等方面得到应用,其温度、压力、流速传感日趋成熟,且具有本征绝缘、易于复用、远距离传输等优势,在物理海洋领域有较大的潜在应用。在先导专项的支持下,课题组完成了光纤温深传感器及其系统在海洋牧场遥测方面的实践应用,是光纤温度链在海洋养殖方面的应用创新,为现代海洋牧场的研制提供了技术支撑作用;课题组研制了船载拖曳温深链系统,并在北黄海和南海海域进行了温度跃层的快速高效海试应用,实时、连续、原位获取了水下温度剖面情况,该领域温度测量的首次试验应用,为物理海洋的科学研究提供了一种全新的快速、高时空密度的测量手段,将会推动物理海洋的科学研究和海洋仪器设备研发水平,具有一定的先导作用。     

波浪作用下海床孔压累积过程离散元数值模拟

海床土层在波浪的循环荷载作用下会逐渐累积孔压,降低土层的稳定性,并威胁海上工程。为了研究孔隙水压力的累积机制,本文提出离散元孔隙密度流方法,并改进研发离散元分析软件MatDEM,实现了海床沉积物孔压的累积过程模拟。基于现场试验装置及土体力学参数建立离散元模型,通过对比试验和数值模拟结果发现:对海床沉积物施加波浪荷载后,表层土体中产生较高孔压,并逐渐向深层传递;在循环波浪荷载作用下,土颗粒间孔压累积范围逐渐增加;当孔压累积时间足够长时,土层中孔压收敛于所施加最大荷载与最小荷载的平均值,此时若孔压达到初始有效应力,土体将发生液化,内部土颗粒成为再悬浮沉积物;在周期性波浪荷载作用下,土颗粒液化悬浮后发生移动,浅层颗粒位移量大,土体整体表现为圆弧形移动。     

内孤立波浅化破碎过程斜坡沉积物孔压响应特征实验分析

观测资料显示内孤立波沿斜坡浅化过程对海底沉积物的作用犹如一台水中吸尘器,在破碎转换阶段达到最强,甚至会触发一系列地质活动,引发地质灾害。为界定此过程中沉积物的动力响应特征和影响因素,在大型重力式分层流水槽中模拟不同振幅内孤立波和不同类型沉积物斜坡连续作用过程,利用孔隙水压力采集系统实时记录孔隙水压力变化,对比分析不同水动力、坡度、沉积物类型情况下沉积物中超孔压变化特征。分析结果表明,内孤立波破碎过程,破波位置海床表层波压力和不同深度超孔隙水压力都存在相似的"U"型负压力变化过程;破碎波经过位置沉积物表现为和表面波压力正相关的孔压响应特征。破碎点沉积物中超孔压幅值随深度减小,约在6%波长深度位置减少到坡面压力的50%。超孔压幅值和内孤立波振幅、沉积物类型和斜坡度密切相关,坡度由0.071变化到0.160时,波压力幅值可增大至1.6倍。内孤立波振幅变化不影响不同类型海床土动力响应规律,只与超孔隙水压力值大小有关,内孤立波对海床的动力作用可认为弹性作用。     

Improvement of estuarine marine clays for coastal reclamation using vacuum-applied consolidation method

Consolidation occurs in estuarine marine clays for coastal reclamation by dissipation of the excess pore pressure, which is induced by increasing the total overburden stress during conventional mechanical surcharging. The excess pore pressure can be decreased usually by the use of several construction methods such as sand drain and paper drain. Besides the drain methods, vacuum can also be used in the soil mass to consolidate the estuarine marine clays by decreasing the pore pressure as well as increasing the effective stress.The study on vacuum consolidation is devoted so far mainly for laboratory model tests or numerical analysis in Korea. Recently, an instrumentation system was applied to manage the vacuum-applied consolidation on a field, in which a sewage disposal plant was constructed. While vacuum was applied, the behaviors of estuarine marine clays such as the settlement, lateral deformation and pore water pressure have been investigated precisely. The behavior of estuarine marine clays during vacuum-applied consolidation shows some difference from the behavior of estuarine marine clays in the case of conventional preloading. A principal difference is that the lateral deformation corresponding to settlement is smaller than before vacuum application even though the surcharge height has been increased.     

Experiment and analysis of the formation,expansion and dissipation of gasbag in fine sediments based on pore water pressure survey

Deep-seated gas in seabed sediments migrates upwards from effect of external factors, which easily accumulates to form gasbags at interface of shallow coarse-fine sediments. Real-time monitoring of this process is important to predict disaster. However, there is still a lack of effective monitoring methods, so we attempt to apply multi-points pore water pressure monitoring technology when simulating forming and dissipation of gasbags in sediments through laboratory experiment. This study focuses...     

Evaluation of liquefaction potential of marine sandy soil with piles considering nonlinear seismic soil–pile interaction; A simple predictive model

Abstract

An elastoplastic, dynamic, finite-difference method was applied to study the effects of nonlinear seismic soil–pile interaction on the liquefaction potential of marine sand with piles. The developed model was well validated using the centrifuge test. The results showed that acceleration, bending moment, and excess pore water pressure complied well with centrifuge test results. The effect of different affecting parameters on liquefaction potential was investigated using parametric study. Using a sensitivity analysis, the pile embedment parameter was shown to be the most influential parameter. Finally, applying the evolutionary polynomial regression technique, a new model for predicting the liquefaction potential was presented.     

The physical model with temperature and pressure controlled for measuring acoustic velocity of marine sediments

Abstract

The superficial marine sediment is an important boundary of ocean acoustic propagation. So, the acoustic property of seafloor surface is always research hotspot. The acoustic property of sediment is affected by temperature and pressure which is not considered by conventional lab acoustic measurement. A new type of system, called “Small-scale Geo-acoustic Physical Model Pilot System” (SGPMP) has been developed. The system measures geo-acoustic property of sediment under specific temperature, pressure and frequency conditions which can be controlled conveniently and accurately. The components, structure, measurement principle, error analysis and application example of this system are introduced in this article. As a laboratory platform, the system makes it convenient for us to study the relationship between the temperature, pressure, frequency and acoustic properties of marine sediment.     

Spatial and temporal variation process of seabed dynamic response induced by the internal solitary wave

Internal solitary wave(ISW) is often accompanied by huge energy transport, which will change the pore water pressure in the seabed. Based on the two-dimensional Biot consolidation theory, the excess pore water pressure in seabed was simulated, and the spatiotemporal distribution characteristics of excess pore water pressure was studied. As the parameters of both ISW and seabed can affect the excess pore water pressure, the distribution of pore water pressure showed both dissipation and phase lag...     

Stress History Estimation Method of Underconsolidated Soil by Partial Piezocone Dissipation Tests

The paper discusses a new method to put forward to determine the initial pore pressure by extrapolating the last segment of measured pore pressure versus the inverse square root of time scale through incomplete pore pressure dissipation test. For underconsolidated soil, the estimated initial pore pressure is greater than the hydrostatic pore pressure. With the calculation of the initial pore pressure, the status of the consolidation of underconsolidated soil can be evaluated by calculating the apparent degree of consolidation which is defined as excess pore pressure generated by piezocone penetration divided by the difference between the total pore pressure measured by piezocone and in situ hydrostatic pore pressure. The apparent degree of consolidation is less than one as the soil is underconsolidated. The Northern Expressway Connection project of Chongqi Bridge is introduced as an example of practical application. In this case, the studied area is slightly underconsolidated, which is consistent with the results of the laboratory oedometer tests. Finally, compared with overconsolidation ratio (OCR) values from the oedometer tests, a new formula to estimate the OCR of underconsolidated soil using the apparent degree of consolidation was presented. It indicates that the OCR of underconsolidated soil can be proposed directly from partial piezocone dissipation tests.     

循环荷载下粉土液化流动特性拖球试验研究

基于流体力学中的Stokes黏滞阻力理论,以振动台试验为基础,开发了一套测量液化过程中粉土流变特性的拖球试验装置。在铺有粉土海床的模型箱内埋设光滑小球,通过测量小球水平运动过程中所受阻力值的大小,计算粉土液化的表观动力黏度,分析粉土液化过程中的表观动力黏度与超孔压比之间的关系,以及液化后表观黏度与应变率的变化规律。试验结果表明,振动台试验下,孔隙压力表现为迅速上升,粉土迅速达到液化状态;振动过程对海床固结影响较大;粉土海床在未达到完全液化状态时（ru<1）,表观黏度随超孔压比增大而减小,在液化状态下（ru=1）,剪应力随应变率增大而减小,粉土呈现出剪切稀化的特点,为典型的非牛顿流体特征。     

Laboratory Test on Volumetric Characteristics of Saturated Sea Sand Under Cyclic Loading

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