南中国海神狐海域天然气水合物上覆地层不排水抗剪强度预测

浅层沉积物不排水抗剪强度（Su）是深水作业的关键参数之一。为了获取南海神狐海域首次海域天然气水合物试采区W18-19框体的基本工程地质特征,试采工程准备阶段开展了原位孔压静力触探测试（CPTU）及大量的室内实验。本文将主要基于CPTU计算不排水抗剪强度的基本模型,采用微型十字板、电动十字板、袖珍贯入仪及不固结不排水三轴实验,确定该区域不排水抗剪强度的基本模式,并提出适用于南海神狐钙质黏土层的不排水抗剪强度纵向分布规律计算模型,对该区域水合物上覆层的不排水抗剪强度进行预测。 结果表明,基于总锥端阻力、有效锥端阻力、超孔隙压力的模型系数分为13.8、4.2、14.4。综合考虑地层压实效应和含气情况,本文提出的分段函数预测模型与室内结果的一致性较好,可用于工程设计阶段进行工区不排水抗剪强度纵向分布规律的预测。另外,基于有效锥端阻力的不排水抗剪强度经验模型适应于浅层极软-较硬压实的钙质粘土层,基于超孔隙压力的不排水抗剪强度模型适用于较硬-坚硬的不含气层,而基于总锥端阻力的不排水抗剪强度计算模型则适用于坚硬含气的钙质黏土层。本文提出的分段函数模型有效的提高了经验模型在南海神狐水合物赋存区的适用性,计算结果可为工程安全评价提供支撑。     

利用落锥试验确定深水软黏土的不排水抗剪强度

不排水抗剪强度是黏性土的重要力学指标,在边坡稳定性分析、地基和桩基承载力的计算中都有广泛的应用。在国内海洋勘察工程中,通常软黏土不排水抗剪强度的确定方法有不固结不排水三轴压缩试验、手动十字板试验、电动十字板试验和无侧限压缩试验等。中国南海北部陆陂深水海域,海底浅层沉积物多为饱和软土,这种软土土质比较均一,具有高含水量、高液限和塑性指数、容重小、抗剪强度低等特点,从而使这种沉积物与常规意义上的饱和软土特性相比可能存在差异,利用落锥试验方法确定其黏性土不排水抗剪强度,具有很强的适用性。本文介绍了落锥试验的试验方法和试验关键技术点,通过该深水海域两个孔位的部分土质资料对比,说明落锥试验确定的黏性土不排水抗剪强度与海上工程设计、分析中使用的不排水抗剪强度具有很好的相关关系。     

神狐海域W18/19站位天然气水合物上覆层不排水抗剪强度预测

天然气水合物上覆层不排水抗剪强度是水合物试采过程中导管喷射设计、地层沉降和井壁稳定性分析的关键参数。孔压静力触探(CPTU)为地层强度参数纵向分布规律研究提供了有效途径。本文基于南海北部神狐海域W18/19站位水合物上覆层的CPTU测试结果和室内实验数据,建立了该区经验锥头系数的确定方法,并基于确定的经验锥头系数求解上覆层不排水抗剪强度。结果表明:W18/19站位水合物上覆层由上至下为强度逐渐增大的钙质黏土,基于总锥尖阻力、有效锥尖阻力、超孔隙压力求解钙质黏土不排水抗剪强度的经验锥头系数分别为13.8、4.2、14.4。水合物上覆层不排水抗剪强度随着深度的增加逐渐增大。基于CPTU建立的不排水抗剪强度计算方法能反映钙质黏土不排水抗剪强度的纵向分布规律。     

深海能源土含气储层力学特性三轴试验研究

为研究深海能源土在负压开采过程中含气储层的力学特性,基于含气土赋存理论,提出一种能够控制含气量及气泡大小的制样方法,通过GDS标准应力路径三轴试验系统,开展深海能源土含气储层的固结排水试验研究,分析深海能源土在不同黏土含量及不同含气量下的力学响应规律。研究结果表明：围压变化对深海能源土含气储层的抗剪强度峰值大小影响显著,围压越大抗剪强度峰值越高;黏土含量是决定应力应变曲线变化趋势的关键影响因素,黏土含量越高试样抗剪强度越低,试样抵抗应变软化效应的能力越强;含气土比饱和土体承载能力更低,且承载能力随含气量的增大呈衰减趋势;黏土含量和含气量是深海能源土含气储层抗剪强度指标的重要影响因素,黏土含量、含气量越高,土体自身的总抗剪强度值越低。     

海洋粉质粘土在波浪荷载作用后的不排水抗剪强度衰化特性

通过对南海重塑粉质粘土土样的大量动三轴试验结果分析,得到此种土在波浪荷载作用后不捧水抗剪强度衰化同动载作用引起的动应变幅及平均累积孔压之间的相互关系和预估公式;并通过与超固结土样的静三轴剪切试验结果的比较,发现动、静三轴两种试验结果具有很好的吻合关系。建议可用超固结土样的静三轴剪切试验同时结合部分动三轴试验来预估土样在波浪荷载作用后不排水抗剪强度衰化与平均累积孔压之间的关系。     

电阻率法监测黄河口海床沉积物固结过程现场试验研究

为了研究黄河口海床沉积物固结过程中电阻率同工程力学性质指标的对应关系,探索海床土体固结过程的新型原位监测技术,本文在黄河刁口流路三角洲叶瓣潮坪上,现场取土配置黄河口快速沉积形成的流体状沉积物和观测研究粉质土海床的固结过程。利用静力触探、十字板剪切试验、孔隙水压力监测等原位土工测试手段,实时测定固结过程中海床土强度变化和孔隙水压力消散过程;同时通过埋置自行研制的环形电极探杆,实时测定海床土固结过程中的电阻率变化。通过对比分析海床土电阻率与工程力学性质指标的同步测定数据发现:黄河口饱和粉土的电阻率与微型贯入试验测得的土体贯入强度,静力触探试验测得的比贯入阻力,十字板剪切试验测得的不排水抗剪强度(峰值强度、残余强度)均呈乘幂关系,且相关性良好;海床沉积物在固结过程中的电阻率与孔隙水压力呈负线性相关性。     

利用单剪SHANSEP试验方法评价海洋粘性土强度

利用单剪SHANSEP试验方法测定了南海某石油平台场址两种粘性土层的不排水剪切强度。通过对比单剪SHANSEP试验结果、原位静力触探(CPT)试验结果和三轴不固结不排水(UU)试验结果,表明通过单剪SHANSEP试验确定的粘土不排水强度与现场CPT解释结果更趋于一致,同时也表明了SHANSEP方法适合于中国南海粘性土强度的评价。因此利用单剪SHANSEP试验能够克服土样扰动对粘土强度的影响,更加客观评价粘性土的强度,为工程设计提供可靠的数据。     

海上原位十字板剪切试验方法介绍

在海洋工程地质勘察中,海上原位十字板剪切试验能够获得海底原位土体的剪切强度,其测试成果已成功应用于工程实践,本文主要从原位十字板剪切的试验原理、设备种类、测试机理、进行剪切试验所具备的若干技术要求、影响剪切试验的几个因素、测试数据进行室内处理的原理及工程实际应用等方面做简要的介绍,最后总结得出进行原位十字板剪切试验时若严格按照试验的技术要求进行试验,并掌握影响试验结果的的若干因素,得到的黏性土的不排水抗剪强度值,可应用于实际工程设计并提供可靠的岩土设计参数。     

基于API规范的海上大直径钢管桩静压载试验分析

大直径钢管桩在海洋工程建设中的使用越来越广泛。海上大直径钢管桩试桩周期长、难度大,相关测试数据及研究成果较为少见。本文基于美国石油行业协会API规范,分析了某海上风电大直径钢管桩静压载试验数据,研究了桩侧阻力与桩端阻力随压荷载变化关系。结果表明,对于本文分析的海上大直径钢管桩,其土塞状态一般为不完全闭塞,计算抗压极限承载力时应计入桩管内侧摩阻力;桩侧摩阻力随压荷载增大而逐渐发挥,相同荷载作用下,无黏性土地层侧摩阻力发挥程度较黏性土高;桩端土体破坏分为弹性变形与塑性变形两个阶段,两阶段端阻发挥程度分别可达约50%和80%,当桩端产生0.05d位移时,桩体达到极限平衡状态。     

软黏土海床条件下的管道侧向失稳预测模型

针对软黏土海床条件,基于被动土压力理论建立了海底管道侧向失稳预测模型。利用准静态平衡分析,该模型将极限侧向土阻力分解为被动土压力和滑动土阻力两个分量。验证分析表明,模型预测结果与文献[3]实验结果吻合较好。参量研究发现,在稳态海流作用下,随着软黏土不排水抗剪强度增大,维持管道在位稳定所需的最小埋深呈幂指数减小趋势;同时,极限侧向土阻力中的滑动土阻力分量呈非线性增大,相应的被动土压力分量则逐渐降低。     

Field Evaluation of Undrained Shear Strength from Piezocone Penetration Tests in Soft Marine Clay

The use of the piezocone penetration test (CPTU) in a geotechnical site investigation offers direct field measurement on stratigraphy and soil behavior. Compared with some traditional investigation methods, such as drilling, sampling and field inspecting method or laboratory test procedures, CPTU can greatly accelerate the field work and hereby reduce corresponding operation cost. The undrained shear strength is a key parameter in estimation of the stability of natural slopes and deformation of embankments in soft clays. This paper provides the measurements of in situ CPTU, field vane testing and laboratory undrained triaxial testing of Lianyungang marine clay in Jiangsu province of China. Based on the literature review of previous interpretation methods, this paper presents a comparison of field vane testing measurements to CPTU interpretation results. The undrained shear strength values from both the field vane tests and cone penetration resistances are lowest at the mid-depths of the marine clay layers, and the excess pore water pressures are highest at the mid-depths of the marine clay layers, indicating that the marine clay layer is underconsolidated.     

静力触探技术在东海陆架工程勘察中的应用研究

通过对东海大陆架海底浅表层土样的静力触探参数进行统计分析,得到该区域细砂、粘土、淤泥质土和淤泥的静力触探统计值,建立了研究海区基于静力触探参数的土类判别图,可用于快速判定土的类别.根据静力触探参数,并应用经验公式估算了海底粘性土的抗剪强度和灵敏度,对估算公式经验系数选择进行了深入研究,提出在研究海区应用经验公式进行粘性...     

Laboratory and Field Observations for Golden Horn Marine Clay

Istanbul, the largest city in Turkey and one of the major metropolitan areas in the world, cleaned one of its environmentally polluted areas—Golden Horn—by dredging 5 million m³ of the bottom sediments and pumping the resulting sludge to a storage area behind a dam built at an abandoned rock quarry site in Alibey district. The reclamation of the land that formed over the storage area of Golden Horn dredged material is socially and economically very desirable. In this paper, results from experimental studies that are focused on determining the shear strength behavior of the dredge material and undisturbed soil are presented. Slurry consolidometer test, large model tests and small model tests are used to consolidate the dredged soil samples from Halic to simulate the natural consolidation behavior of these soils. Shear strength parameters are determined by laboratory vane tests; unconfined compression tests; undrained-unconsolidated (UU) and consolidated-undrained (CU) triaxial tests on samples that are obtained through in situ undisturbed samples and laboratory model tank and slurry consolidation. Moreover, the effects of fly ash and lime additives on the undrained shear strength were determined by mixing the materials with the dredged clay from Golden Horn during the model experiments conducted in the laboratory. Based on these findings, equations are proposed that govern the relationships between undrained shear strength and water content value.     

Estimation of undrained shear strength for saturated clay using shear wave velocity

Undrained shear strength is a fundamental parameter for estimating the stability of soft soils. This study explores the relationship between undrained shear strength, void ratio, and shear wave velocity for saturated and normally consolidated clay specimens. The undrained shear strength void ratio-shear wave velocity relationship was correlated to empirically determined parameters of selected marine clay specimens. To verify the proposed relationship between undrained shear strength and shear wave velocity, in situ flat dilatometer tests were used for determining the undrained shear strength, and downhole tests were used to assess the shear wave velocity on a natural soil deposit at various depths. The undrained shear strength estimated from the in situ shear wave velocities was compared to the undrained shear strength obtained in the field. The results show that the inferred undrained shear strength yield similar values and follow the same trends as the in situ undrained shear strength data. This method using shear wave velocity can help to nondestructively estimate the undrained shear strength of soft soils in the field and be used in both on-shore and off-shore geotechnical engineering projects.     

Estimation of Undrained Bearing Capacity for Offshore Soft Foundations with Cyclic Load

The degradation strength of soils under cyclic loading is studied and a method for deter-mining the cyclic degradation strength with cyclic triaxial tests is given in the paper.Furthermore,a dum-my static method for estimating the undrained bearing capacity for offshore soft foundation under waveloads is developed.It can consider the effect of the difference of cyclic stress for different parts of the foun-dation on both the degradation strength of the foundation soil and the bearing capacity so that the esti-mated result can better reflect the real condition of foundation under cyclic loading.The method can be ap-plied to plane and space problem.     

Behavior of Loose Silty Sand of Chlef River: Effect of Low Plastic Fine Contents and Other Parameters

This article presents a laboratory study of static behavior of silty-sand soils. The objective of this laboratory investigation is to study the effect of initial confining pressures and fines content on the undrained shear strength (known as liquefaction resistance) response, pore pressure, and hydraulic conductivity of sand–silt mixtures. The triaxial tests were conducted on reconstituted saturated silty-sand samples at initial relative density D_r = 15% with fines content ranging from 0 to 50%. All the samples were subjected to a range of initial confining pressures (50, 100, and 200 kPa). The obtained results indicate that the presence of low plastic fines in sand–silt mixture leads to a more compressible soil fabric, and consequently to a significant loss in the soil resistance to liquefaction. The evaluation of the data indicates that the undrained shear strength can be correlated to fines content (F_c), inter-granular void ratio (e_g), and excess of pore pressure (Δu). The undrained shear strength decreases with the decrease of saturated hydraulic conductivity and the increase of fines content for all confining pressures under consideration. There is a relatively high degree of correlation between the peak shear strength (q_peak) and the logarithm of the saturated hydraulic conductivity (k_sat) for all confining pressures.     

黄河口粉质土沉积物侵蚀性动态变化试验研究

通过一系列室内试验,研究了黄河口粉质类沉积物在循环动荷载作用后侵蚀性的变化.在试验中,采用真空预压固结装置制备重塑土样,采用液压伺服土动三轴试验机实现了不同幅值及不同振次的动荷载作用条件,并通过动三轴静态试验及贯入强度测试,测定了动荷载作用后土样强度的变化.通过水槽冲刷试验测试了不同条件的动荷载作用后,土样抗冲刷能力的变化,并进行了临界剪切应力的计算.试验结果表明:对于黄河口粉质类沉积物,动荷载作用后其强度及抗侵蚀性均降低;沉积物强度指标(不排水剪切强度、残余强度、贯入阻力)、侵蚀性指标(起动流速、临界剪切应力、冲刷率)与动荷载幅值、动荷载振次均呈现出较好的线性相关性.     

Shear strength of soil berm during lateral buckling of subsea pipelines

The soil resistance developed during temperature- and pressure-induced large lateral movements of shallowly embedded subsea flowlines is an important input parameter for the structural design process. A major source of uncertainty in calculation of the soil resistance is the undrained shear strength of the soil berm produced as the flowline moves across the seabed, which is affected by the level of remoulding. To investigate the effect of pipeline embedment and displacement amplitude on the shear strength of the berm, a set of centrifuge model tests was conducted on kaolin clay, involving laterally moving pipelines with constant embedments in the range 5%–35% of the pipe diameter. Back-analysis of the test results, using finite element limit analysis, showed that the shear strength of the soil berm is a function of pipe displacement amplitude, pipe embedment, and soil sensitivity. On the basis of these results, we propose that the overall berm undrained shear strength may be determined as a convolution of the shear strengths of its constituent soil elements. Finally, a formula is presented for calculating the shear strength of soil elements within the soil berm, and this is used to back-analyse the overall soil berm resistance from the model tests.