海上静力触探(CPT)测试技术的发展现状和应用

静力触探(CPT)是一种速度快,数据连续、再现性好、操作省力等优点的原位测试方法。随着海洋开发的迅速发展,CPT测试技术在国内外海洋工程领域的使用越来越普遍。因此,了解国外海上CPT设备和应用情况,探讨应用中的问题,对CPT测试技术在我国海上工程领域的应用具有重要意义。文章描述了国外海上静力触探(CPT)测试技术的发展现状,阐明了主要的海床CPT(Seabed CPT)设备和井下CPT(Downhole CPT)设备的特点,并对不同CPT设备在海洋工程领域的应用作了分析,这对海上CPT测试技术在我国的应用和发展具有一定的参考价值和指导作用。     

粉土中吸力桶沉贯阻力与土塞失稳机理研究

针对粉土中吸力桶在吸力沉贯中的两个关键性问题——沉贯阻力和内部土体稳定性,在自行研制的试验平台上进行了一系列吸力桶沉贯模型试验。试验结果表明,粉土中吸力沉贯时,Andersen所提供的基于CPT锥尖强度的阻力计算公式能较好预测阻力发展趋势,但计算结果偏小;而修正的承载力公式预测结果在沉贯前期与实测值较吻合,沉贯后期预测偏大。吸力沉贯前期,贯入阻力随深度稳步增长,直到贯入深度达到某个临界值后,阻力增长缓慢甚至保持不变。桶内粉土在吸力作用下的失稳机理与黏土或砂土不同,在渗透力作用下表现为从土层表面开始的自上而下管涌或渗流侵蚀。沉贯后期,端部土体在较高水头差下发生失稳并向桶内流动,造成了桶内土体密度降低,端阻力与内壁摩阻力降低,总贯入阻力主要由外壁摩阻力提供。并结合有限元模拟对上述实验现象的内在机理进行了解释。     

Downhole CPT海底基座液压自动调平系统研究

井下式(Downhole)海底CPT贯入设备中的海底基座需要为探头和探杆竖直贯入海底土体时产生的地层阻力提供支撑反力,然而当海底基座坐落于倾斜的海床面时会对触探数据的真实性造成影响,严重时甚至造成作业事故。文中针对前期方案中球铰被动式调平装置存在的问题,提出了球铰液压式自动调平方案,从机械结构、液压原理、控制策略等方面进行了详细论述,可为今后海底CPT系统的进一步完善和海底钻探取样设备的研制提供借鉴。     

深海浅层沉积物强度贯入式原位测试装置研制

自由下落式CPT测试技术(FF-CPT)是一种重要的海上沉积物强度原位测试方法。本文在FF-CPT室内试验原型样机基础上,通过进一步改进,研制了深海浅层沉积物强度贯入式原位测试装置。FF-CPT系统主要由数据采集单元、贯入探杆、配重、耐压控制舱及释放器等组成,设备总长度约6m,贯入深度约5m,极限工作水深1 500m。FF-CPT采用自容方式进行数据采集与存储,可同时测量深海表层沉积物的侧摩阻力、锥尖阻力、孔隙水压力等参数。目前已将该FF-CPT原位测试装置初步应用于渤海、南海等海域的表层沉积物强度测量,FF-CPT工作状态良好。期望本研究可为国内海上FF-CPT自主研制与进一步工程应用提供经验与借鉴。     

探头尺寸对FFP确定海底沉积物性质影响的原位试验研究

自落式贯入仪(Free-Falling Penetrometer, FFP)是一种新型海底沉积物原位测量设备,用于海底沉积物工程性质原位调查。本文基于自主研发的FFP设备在黄海海域开展现场贯入试验,分析探头尺寸对判断沉积物性质的影响。研究表明,在不同沉积物中探头尺寸对贯入深度、贯入阻力和沉积物类型判断有不同影响。在细粒沉积物中探头尺寸对贯入深度、贯入阻力影响较小;在根据贯入阻力变化划定分层时,小直径探头能反应贯入阻力更为细小的变化。在粗粒沉积物中探头尺寸影响更明显,大尺寸探头使贯入阻力变大,贯入深度减小。根据标准化深度和硬度因子可区分粗粒沉积物和细粒沉积物,在粗粒沉积物中探头尺寸增大,硬度因子减小,有利于细化沉积物类型。     

砂质沉积物强度动力贯入室内试验研究

自由下落式CPT测试技术(FF-CPT)是新兴的海上沉积物强度测试方法。本文构建了海上FF-CPT原型样机,并以海洋干砂质沉积物为研究对象,分析了FF-CPT的贯入特征、速度相关性及其影响因素。结果表明,装置触底速度与最终贯入沉积物的深度受装置释放高度影响,释放高度越大,则触底速度与贯入沉积物深度越大。贯入过程的阻力分布规律是影响速度变化的主要因素。FF-CPT初始贯入阶段,沉积物贯入阻力随深度线性增加,但增幅不明显,这导致贯入速度变化不大,几乎可视作匀速,该段行程超过总贯入深度的2/3。后段行程贯入阻力骤增,导致贯入速度迅速降低至0。对于干砂质沉积物,FF-CPT贯入阻力与贯入速度不存在正相关关系。FF-CPT贯入阻力的速度相关性与装置质量、沉积物密实度关系不大。     

深水井口导管管土界面摩擦退化试验研究

深水井口导管在黏土中的贯入阻力与导管土体界面的黏性摩擦系数密切相关。在导管安装过程中,管土界面会产生大位移剪切,使得导管-黏土界面剪切力产生退化。本文针对传统剪切试验装置小位移剪切的限制,无法正确描述井口导管贯入时导管-黏土界面大位移剪切时的退化机理,采用GDS多功能界面剪切试验仪在恒应力条件下进行大位移单调剪切试验,研究法向应力对导管管土界面摩擦退化机理的影响规律。试验结果表明,大位移单调剪切时,界面剪应力随剪切位移先增大后减小,最终趋于稳定,界面表现出剪应力退化现象,随着法向应力的增大界面峰值应力与残余应力均增大;土样法向位移随着剪切位移、法向应力的增加而增大最终趋于稳定。最后在对试验数据分析的基础上,基于剪应力随无量纲位移的退化规律,提出了界面剪应力随累积位移退化的计算方法。并结合算例对比分析了井口导管在贯入80 m深度处,由于界面摩擦退化效应导致导管贯入阻力降低了28.48%。     

自由落体式球形贯入仪在硬-软黏土中的贯入响应

自由落体贯入仪（free falling penetrometer,简称FFP）通过自由下落贯入土层中,由于其高效便捷的特性,越来越广泛地被应用于海底浅层土体原位勘察。采用耦合欧拉-拉格朗日（Coupled Eulerian Lagrangian,简称CEL）方法,进行了球形FFP在硬-软双层黏土中贯入的大变形有限元分析,考虑了土体的应变率和应变软化效应。与离心机试验、现场试验和数值模拟结果进行了对比,验证了模型的可靠性。分析了成层土对贯入过程的影响,发现FFP在贯入过程中上部硬黏土层形成空腔,且底部会伴随着土塞。通过广泛的变参数分析结果,拟合了球形FFP最终贯入深度与总能量之间的归一化表达式,建立了土体不排水抗剪强度、FFP直径和贯入速度与FFP在硬-软黏土层中最终贯入深度的关系。     

波致海底缓倾角无限坡滑动稳定性计算分析探讨

波浪作用下海底无限坡滑动稳定性计算的极限平衡法中,忽略了坡体水平向应力状态的影响,为此,针对波浪作用下海底缓倾角无限边坡的特点,提出直接基于滑动面处土体应力状态的滑动稳定性计算方法(简称应力状态法),并分析了其适用范围。对具体算例的分析表明,应力状态法计算得出的安全系数大于极限平衡法的安全系数,且随着滑动面深度的增加、土体泊松比以及边坡坡角的增大,两种计算方法得出的安全系数的差异会逐渐增大;对于波浪作用下的海底缓倾角无限边坡,在失稳时极可能沿具有一定厚度的滑动带而不是单一的滑动面而滑动,且波致最大剪应力所在的深度,常常不是斜坡体最易失稳滑移的深度。     

基于Downhole工艺的海底静力触探及其设备研制

论述基于Downhole CPT的海底静力触探工艺和实施方案,并在此基础上研制了适用于水深10~100 m,触探最大深度120 m的海底静力触探设备。对设备中的管内锥探总成、海底机座以及推进速度控制策略等核心问题进行了探讨。码头与海底试验中,触探数据结果与钻探取样基本符合,验证了工艺方案与触探设备达到了预期的工艺要求。     

用静力触探参数确定地基土水平地基抗力系数的比例系数m值--以天津地区为例

承受水平荷载作用的桩基，规范中常采用m法进行桩基水平承载力的计算，地基土水平地基抗力系数的比例系数m值在规范中根据地基土的状态、类别以表格给出。在地基勘察中，现在广泛采用静力触探试验。直接利用静力触探数据给出比例系数m值。将使桩基设计所用参数更加直接准确。本文利用天津地区地层大量静力触探资料与地基土状态数据，利用统计分析回归方法，总结出地基土的液性指数IL与静力触探参数锥尖阻力qc及摩阻比Rf间的关系式，针对天津的地层土体，给出利用静力触探资料查用m值的表格，为桩基的设计计算提供资料。     

Direct CPT and CPTu methods for determining bearing capacity of helical piles

Helical piles are structural deep foundation elements, which can be categorized as torque-driven piles without any limitations to implement in marine situations. Different methods are used to predict the axial capacity of helical piles, such as static analysis, but have some limitation for this type of piles on marine conditions. In situ testing methods as supplement of static analysis have been rarely used for helical piles. In geotechnical engineering practice, the most common in situ tests particularly applicable for coastal or offshore site investigation are cone penetration test (CPT) and piezocone penetration test (CPTu). The CPT is simple, repeatable, and prepares the continuous records of soil layers. In this paper, a data bank has been compiled by collecting the results of static pile load tests on thirty-seven helical piles in ten different sites including CPT or CPTu data. Axial capacities of thirty-seven helical piles in different sites were predicted by direct CPT methods and static analysis. Accuracy estimation of ten direct CPT methods to predict the axial capacity of helical piles was investigated in this study. Comparisons have been made among predicted values and measured capacity from the pile load tests. Results indicated that the recently developed methods such as NGI-05 (2005), ICP-05 (2005), and UWA-05 (2005) predicted axial capacity of helical piles more accurately than the other methods such as Meyerhof (1983), Schmertmann (1978), Dutch (1979), LCPC (1982), or Unicone (1997). However, more investigations are required to establish better correlation between CPT data and axial capacity of helical piles.     

Behavior of soil plugs in open‐ended model piles driven into sands

Calibration chamber tests were conducted on open‐ended model piles driven into dried siliceous sands with different soil conditions in order to clarify the effect of soil conditions on load transfer mechanism in the soil plug. The model pile used in the test series was devised so that the bearing capacity of an open‐ended pile could be measured as three components: outside shaft resistance, plug resistance, and tip resistance. Under the assumption that the unit shaft resistance due to pile‐soil plug interaction varies linearly near the pile tip, the plug resistance was estimated. The plug capacity, which was defined as the plug resistance at ultimate condition, is mainly dependent on the ambient lateral pressure and relative density. The length of wedged plug that transfers the load decreases with the decrease of relative density, but it is independent of the ambient pressure and penetration depth. Under several assumptions, the value of earth pressure coefficient in the soil plug can be calculated. It gradually reduces with increase in the longitudinal distance from the pile tip. At the bottom of the soil plug, it tends to decrease with increase in the penetration depth and relative density, and to increase with the increase of ambient pressure. This may be attributed to (1) the decrease of friction angle as a result of increase in the effective vertical stress, (2) the difference in the dilation degree of the soil plug during driving with ambient pressures, and (3) the difference in compaction degree of soil plug during driving with relative densities. Based on the test results, an empirical equation was suggested to compute the earth pressure coefficient to be used in the calculation of plug capacity using one‐dimensional analysis, and it produces proper plug capacities for all soil conditions.     

SPT-CPT Correlation and Its Application for Liquefaction Evaluation in China

The standard penetration test (SPT) and cone penetration test (CPT) are the two most commonly used in-situ tests. In this paper, SPT-CPT correlation is investigated using statistical and regression method based on a database of in-situ tests. Two correlation equations of the SPT N-value and the CPT cone tip resistance q_c are proposed. The equations are applied for the liquefaction potential evaluation. Three effect factors including the soil type, mean particle size, and fine content are investigated for SPT-CPT correlation. The simple correlation can be used for the preliminary site investigation with least data and information. The comprehensive one can give more accurate result with detailed data. These two correlation equations are useful for different purposes with different accuracies. The liquefaction evaluations using the proposed correlation method are compared with those from the Chinese code method. It is shown that the proposed correlation can be helpful for site investigation and geotechnical design in practice.     

Reliability-based assessment of drilled displacement piles bearing capacity using CPT records

Drilled displacement piles (DDPs) are known as an alternative to conventional foundations in coastal areas, given the elimination of environmental impacts and difficulties caused by installation process of driven piles and more consistency with environment. Despite increasing employment of these piles, the extent of research works does not yet suffice the requisites to reach a routine design. This paper aims to analyze six cone penetration test (CPT)-based methods of determining the bearing capacity of DDP. The statistical and reliability-based approaches were used in two parts of assessing performance of the methods with respect to soil–pile characteristics followed by evaluating reliability of the prediction outcome. A database is compiled including 65 DDP load tests with adjacent CPT profiles. Performance of the methods are analyzed. Finally, a reliability parameter, i.e., confidence interval, is introduced to demonstrate a more realistic insight into the evaluations by expressing performance of the methods in terms of a range for possible average values of the predictions ratios, rather than simply an arithmetic mean. The study reveals that the commonly used CPT-based methods which have not been specifically developed for DDP show great potential for design. The results indicate that the investigated methods can have promising performance if some modifications are applied.     

天津沿海地层静力触探参数与土体物理力学性质对比研究

近年来,在天津沿海地区的工程勘察中,积累了大量的静力触探参数资料与地基土层的物理力学参数资料。利用计算机的EXCEL数据处理软件,将各地基土层的地基土物理力学参数统计平均,同时将相应土层的静力触探参数统计平均,然后对平均后的数据进行统计回归,得到静力触探参数(锥尖阻力qc与侧摩阻力fs)与地基土层物理力学参数(天然含水量ω、天然密度ρ、孔隙比e、压缩系数a及压缩模量Es)间的统计回归方程。在天津沿海地区进行工程勘察时,利用所得方程可从静力触探参数(或土体物理力学参数)方便地得到土体物理力学参数(或静力触探参数),进而可以节约外业勘察或室内试验的工作量。     

CPT-Based estimation of undrained shear strength of fine-grained soils in the Huanghe River Delta

The Huanghe River（Yellow River） Delta has a wide distribution of fine-grained soils. Fluvial alluviation, erosion,and wave loads affect the shoal area, resulting complex physical and mechanical properties to sensitive fine-grained soil located at the river-sea boundary. The cone penetration test（CPT） is a convenient and effective in situ testing method which can accurately identify various soil parameters. Studies on undrained shear strength only roughly determine the fine content（FC） without ma...     

海床式静力触探在黄河口海底土工程特性研究中的应用

海床式静力触探设备工作效率高,能够实现从垂向上对地层的连续探测,数据分辨率较好。通过对黄河口海域3个静力触探孔和1个地质钻孔的分析,将研究区20m以浅地层自上而下划分为粉土、粉质黏土、粉土夹粉砂或粉砂、粉质黏土、粉土五层,静力触探曲线对粉土层中的硬壳层和粉质黏土层中的贝壳夹层等特殊地质现象有很好的反映。对粉质黏土的静力触探参数与物理力学参数进行相关性分析,得到适合该区域的回归方程。采用基于CPT数据的液化判别法,对研究区表层粉土液化可能性进行评判,通过Seed简化法计算的周期应力比(CSR)与Olsen法计算的周期阻力比(CRR)的大小比较,得出当抗震设防烈度为7度时,研究区表层粉土不会液化,当抗震设防烈度为8度时,粉土层将发生液化,计算最大液化深度3.7m。