珠江口海洋软土不排水抗剪强度及循环弱化特性试验研究

为了获得广东珠江口海洋软土的不排水抗剪强度,分别开展了不固结不排水剪切和固结不排水剪切三轴试验、无侧限抗压试验、现场取土离心机正常固结模型地基的T-bar静力贯入试验以及现场十字板剪切试验和静力触探试验（CPT）,并采用土力学经验公式进行计算,确定了珠江口海洋软土不排水抗剪强度沿深度的分布规律,并综合评价了各种方法的有效性。为了获得该区域软土的循环弱化规律,分别开展了循环动三轴试验、现场取土离心机正常固结模型地基的T-bar循环贯入试验和现场单桩水平循环加载试验,揭示了软土循环动模量比与循环次数之间的双对数线性关系,获得了3种试验条件下珠江口海洋软土的循环弱化因子。其研究结果可为该区域海洋建筑物设计提供直接依据,也可为其他类型工程提供借鉴。     

Effect of rod friction on vane shear tests in very soft organic harbour mud

Very soft organic harbour mud is increasingly used as a filling and construction material in harbour construction and reorganization. The undrained shear strength of such soft sediments is the critical geotechnical soil parameter with regard to any specific construction design. Field and laboratory vane shear testing is a standard method to quickly determine this important parameter. So far, the effect of rod friction on vane shear tests in very soft organic soils is unclear. In this study we present results from laboratory experiments on harbour mud from a construction site in northern Germany. Relations among vane and rod geometry, penetration depth, water content, rod friction and undrained shear strength are derived. Based on these relations the influence of rod friction on vane shear test results is investigated. The results indicate that field and laboratory vane shear test measurements may be significantly influenced by rod friction. Methods are proposed to correct for the rod influence, which is shown to increase with rising water contents.     

In-situ testing of Singapore marine clay at Changi

The Changi East Reclamation Project in the Republic of Singapore comprises of the ground improvement of marine clay with the installation of prefabricated vertical drains and subsequent surcharge placement. Prior to the commencement of land reclamation works, a series of in-situ tests were conducted in marine conditions with the use of various in-situ testing equipment. The In-Situ Testing Site was located in the Northern area of the project where the thickest compressible marine clay layers existed. The in-situ tests carried out were with the field vane, piezocone, flat dilatometer, self-boring pressuremeter and BAT permeameter. In-situ tests were conducted to determine the undrained shear strength and overconsolidation ratio of the marine clay. In-situ dissipation tests provide a means of evaluating the in-situ coefficient of horizontal consolidation and horizontal hydraulic conductivity of soft soil and were used to estimate these properties of Singapore marine clay at Changi.     

Ground Investigations for Changi East Reclamation Projects

Geotechnical ground investigation is important in all construction works particularly, land reclamation projects. The Changi East reclamation project is a mega project that involves creation of land space as large as 3,000 hectares in multiple phases between 1991 and 2005. Ground investigation plays a critical role in the Changi project which involved substantial amount of ground improvement works after reclamation by sand filling. One important aspect of the investigation was to evaluate the geotechnical characteristics of the underlying compressible soils beneath the original seabed for the selection of suitable methods of ground improvement. The other important aspect of the investigation was to characterize and verify the compactness of the reclaimed sand fill. Many types of in situ tests were extensively used in the project. The field vane shear test provides correlations between the undrained shear strength and depth for the upper and the lower marine clay strata in the Northern part and the Southern Part of the reclaimed site. Combined with laboratory undrained shear strength tests, the field vane strength profiles allow the derivation of the undrained shear strength over effective stress ratios for the upper and the lower marine clays. In addition, results from three other in situ tests, the piezocone cone penetration test, the flat dilatometer test, and the self boring pressuremeter test provided useful verification of these correlations. A field-performance based method of assessing degree of consolidation in the underlying clay was developed by combining field monitoring, laboratory testing and conventional as well as specialized in situ testing. An effective use of in situ testing methods for assessment, the outcome of ground improvement and for fill quality control in the densification of granular soils is illustrated with field observation data collected at the project site.     

上海地区土体抗剪强度与静力触探比贯入阻力相关关系研究

基于上海地区19个工程共201组数据,通过黏土、粉质黏土和粉砂在上海地区土层中的分布,比贯入阻力Ps,室内固结快剪试验黏聚力cu和摩擦角?u,孔隙比e,塑性指数Ip沿土层深度的变化,分析得到3种土类的抗剪强度与比贯入阻力的相关关系,并总结了国内软土地区土体抗剪强度与静力触探比贯入阻力之间的相关关系。本文得到黏土的相关关系与以往的经验公式相近,同时给出了原位土层粉质黏土和粉砂的相关关系。区别不同土类使得相关关系可用性提高,在工程应用中更加合理。根据本文相关关系,通过静力触探比贯入阻力可以得到不同土层的抗剪强度值,进而应用于基坑稳定计算。本文研究的结果可以应用于海相沉积土的勘察和工程设计工作。     

深海浅表层沉积物不排水剪切强度的多探头原位测试系统及评价方法研究

海洋工程建设已步入深海,但与之匹配的浅表层沉积物不排水剪切强度原位测试技术尚不成熟。为此,本文开发了可用于评估深海浅表层沉积物土力学性质的多探头原位测试系统,包括锥形触探仪、球形贯入仪和十字板剪切仪,可实现对深海沉积物土力学性质的快速、准确和智能化评价。进一步,通过CEL大变形数值模拟和足尺土工模型试验确定锥形触探仪的锥尖阻力系数,基于离心模型试验给出球形贯入仪的贯入阻力系数,从而完善深海浅表层沉积物不排水剪切强度的评价方法;在此基础上,探讨3种特定仪器各自适合的强度测试区间。结果表明:对于深海浅表层沉积物不排水剪切强度的评估,锥形触探仪和球形贯入仪的阻力系数建议分别取值为9.5和11.1。     

Empirical correlations of soil parameters based on Cone Penetration Tests (CPT) for Greek soils

This study presents the results of Cone Penetration Tests (CPT) and estimated correlations with the results of other in-situ tests (standard penetration test, cross-hole, seismocone) and laboratory tests (classification and undrained shear strength tests). These tests cover nearly all types of penetrable soils in Greece and, together with the formulation of empirical correlations, complement and improve on previous studies due to the significant number of CPT measurements.     

An analytical study of the effect of penetration rate on piezocone tests in clay

Finite element cavity expansion analysis investigating the effect of penetration rate on piezocone tests in clay is presented. A coupled analysis was performed, in which the rate of cavity expansion was linked to the penetration rate of the cone and the cone angle, using the assumption that the deformation was wholly radial, and took place only between the cone tip and the cone shoulder. The soil was modelled using modified cam clay with two sets of parameters and varying values of overconsolidation ratio (OCR). The influence of penetration rate on the stress and pore pressure distributions was examined. For slower penetration rates, the excess pore pressure at the cone shoulder is lower since consolidation is permitted coincident with penetration. The radial profiles of post‐penetration voids ratio demonstrate that partially drained penetration is permitted by volume change in the near field, in addition to radial movement in the far field. The radial distribution of excess pore pressure after slow penetration differs from the undrained case, with a relatively low radial gradient existing at the cone face. As a result, the dissipation curves after slow penetration lag behind those following fast penetration. The cone velocity is made dimensionless by normalizing with the coefficient of consolidation and the cone diameter. ‘Backbone’ curves of normalized velocity against normalized tip resistance and excess pore pressure capturing the transition from undrained to drained penetration are derived. The normalized pore pressure backbone curve is unique, whilst the normalized tip resistance shows a small dependency on OCR. These backbone penetration curves are compared with centrifuge model piezocone tests conducted at varying rates, and subsequent dissipation tests. The numerical and experimental results suggest that the value of consolidation coefficient operative during the dissipation phase is 2–4 times higher than the virgin compression value due to changes in the operative soil stiffness, as demonstrated from the stress paths of individual soil elements. The use of multi‐rate penetration tests to deduce values of consolidation coefficient is discussed, in light of these differences. Copyright © 2005 John Wiley & Sons, Ltd.     

Characterization of thick varved-clayey-silt deposits along the Delaware River by field and laboratory tests

The existence of thick compressible varved clayey silt deposits along the Delaware River presents one major obstacle for design and construction of structures or industrial facilities in this region. To understand the characteristics of this thick weak soil layer and then develop reliable soil parameters for future earthwork design and construction, a comprehensive soil testing program was carried out. The program consisted of both in situ tests [e.g., standard penetration test borings, piezocone penetration test (CPT) soundings, Marchetti flat dilatometer test soundings, seismic shear wave test soundings, CPT pore pressure dissipation tests, and field vane shear tests] and laboratory tests (e.g., soil index tests, consolidation tests, triaxial tests, and chemical tests). Additionally, a field surcharge test program with and without installation of wick drains was carried out to investigate the compressibility of the thick clayey silt deposit and the efficiency of wick drains to help accelerate consolidation. The field and laboratory test results showed that the varved clayey silt deposit was normally consolidated to slightly overconsolidated and moderately to highly compressible. Its physical properties were intermediate between those of Savannah muddy clay and Shanghai soft clay, while much closer to that of Shanghai soft clay. Due to its lamination structure in the horizontal direction, the horizontal consolidation coefficient of the varved clayey silts was significantly greater than that in the vertical direction. The measured undrained shear strength of the varved clay silt almost increased linearly with depth. The installation of wick drains in the thick clayey silt deposits accelerated primary consolidation, but showed no effect on the secondary consolidation under constant loading. The findings obtained from this study will be beneficial for future design and construction of earthworks along the Delaware River.     

长春硬塑状态老黏性土地基承载力

为提供长春硬塑状态老黏性土可靠的地基承载力依据,探讨适宜的确定地基承载力的方法,采用载荷试验、静力触探试验原位测试和室内土工试验等方法对长春老黏性土进行了测试研究,获取了地基承载力特征值（306~425 kPa）、静力触探锥尖阻力（1.9~3.3 MPa）、天然孔隙比(0.63~0.72)和液性指数(0.02~0.25)等试验数据。通过对试验数据的统计分析,得出地基承载力特征值与静力触探锥尖阻力线性相关,并与天然孔隙比和液性指数线性相关的结论。据此,提出了根据静力触探锥尖阻力计算长春老黏性土地基承载力特征值的经验公式,总结出根据室内土工试验指标确定长春老黏性土地基承载力特征值的数据表。     

真空轻型井点地基处理试验研究

为分析利用真空井点进行沿海软土地基处理的可行性,在天津汉沽区南部沿渤海区域某电厂建筑场地,布置了利用真空轻型井点抽水的现场地基固结试验,为分析其固结试验效果,试验过程中进行了地下水位、孔隙水压力的现场监测,试验前后安排了现场检测试验。抽水固结试验结束后,经试验前后的静力触探试验、十字板剪切试验及室内土工试验对比分析,证明加固效果显著。抽水试验开始后较短的时间（75 d内）即可使加固的地基达到固结稳定,比贯入阻力提高200～300 kPa,十字板试验抗剪强度平均提高30～50 kPa,地基承载力可达120 kPa。试验说明,采用真空轻型井点处理沿海浅部软土地基是可行的。     

排水粉喷桩加固软土地基桩周土体强度变化规律

排水粉喷桩(2D工法)是同时采用粉喷桩和塑料排水板加固软土地基的一种新工法。在某高速公路中进行了排水粉喷桩加固软土地基的现场试验段施工,分别采用静力触探试验、十字板剪切试验等方法测定加固前及其加固后不同龄期的粉喷桩桩周土强度,得到了施工后桩间土强度变化的规律。测试结果表明采用粉喷桩联合排水板法加固后,桩周土强度有一个先降低后提高的过程,28天龄期时恢复到原状土强度,随着龄期的增长桩周土强度可以进一步增加。最后对桩周土强度的变化机理进行了初步探讨。     

南海迭层土物理力学特性的研究

我国海域普遍分布有迭层土,由于结构的特殊性,对于其性质的准确把握,直接影响到海洋工程诸如打桩、钻井船插深等问题预测的准确性,关系到海洋工程建设的安全。现场进行孔压静力触探CPTU试验以及室内试验,分析我国南海海域迭层土的物理、力学性质的特点,对迭层土抗剪强度值的确定进行探讨。CPTU试验数据显示,迭层土的物理力学性质与单一土层具有较明显的差异;由于迭层土中砂层的存在,室内三轴不排水剪切试验（UU试验）确定的不排水强度明显偏低,是CPTU确定的不排水抗剪强度的下限值。CPTU的结果可以较好地反映了迭层土的特点,根据CPTU试验结果得到的摩阻比变化范围较大,在0～8之间远大于一般的砂土或黏土的变化范围,摩阻比与修正端阻之间的关系可用双曲线近似表示。建议进行静力分析时,迭层土不排水抗剪强度参数选取应参考CPTU试验结果。     

考虑扰动的饱和软黏土圆柱孔扩张弹塑性解

根据原位十字板试验扰动导致饱和软黏土不排水强度降低的事实,应用圆柱形孔扩张理论,假设饱和软黏土在塑性阶段满足Tresca屈服条件,提出了一种基于饱和软黏土灵敏度的扰动度D且是塑性区半径的对数函数。考虑在塑性区内不排水强度su是扰动度D的函数,得到了考虑扰动的圆柱形孔扩张的弹塑性解答。     

上海地区扁铲侧胀试验与其他原位试验结果的相关性分析

由于扁铲侧胀试验（DMT）在国内应用时间不长,如何合理地解释和应用DMT试验成果尚待进一步探讨。与其他原位试验结果进行对比分析是研究方法之一。介绍了多元回归在岩土工程相关分析中的应用。根据上海地区多个工程的扁铲侧胀试验、十字板剪切试验、标贯试验和静力触探试验的数据,通过选元分析和多元回归,得出扁铲侧胀试验与其它原位试验的相关关系,可供勘察设计相关人员参考使用。     

双层高岭黏土中沉桩特性模型试验

为了揭示黏土中静压沉桩的宏观力学特性,采用西澳大学的梁式离心机压桩系统,通过在双层高岭黏土中进行一系列1g重力场情况下的T-bar触探测试试验、静压沉桩试验、桩拉伸试验和压缩试验,对比分析了T-bar分别在均质软质黏土和双层黏土中随贯入深度变化测得的不排水剪切强度的特性;研究了桩体在贯入过程中沉桩阻力、桩端阻力以及桩侧摩阻力的发展规律;并对桩身径向总应力在沉桩过程中的变化规律进行了探讨;此外,还揭示了不同桩型沉桩终压力与桩承载力之间的关系。结果表明：在黏土中沉桩时,桩侧摩阻力在整个沉桩阻力中发挥主要的作用;对于同一土层而言,当桩超过土层分界面以下2D（D为桩径）范围后,桩端阻力并不受相邻土层性质的影响;随着压桩后时间的推移,桩侧摩阻力对承载力的时效影响较桩端阻力大。     

扁铲侧胀试验在软土地基评价中的应用研究

在回顾分析国内外利用扁铲侧胀试验评价地基土性参数基础上,介绍了应用扁铲侧胀试验（DMT）和其它试验手段进行高速公路软土地基综合分析评价所取得的一些成果。根据扁铲侧胀试验结果估算的不排水抗剪强度与十字板试验结果基本一致,并能够反映土的强度随深度增长的规律,也分析了造成扁铲侧胀试验结果与室内试验结果之间差异的因素,研究结果可为扁铲侧胀试验在我国岩土工程领域的应用提供参考。     

球型全流触探仪的机理研究及工程应用综述

近年来,随着全球海洋开发战略的逐步开展,越来越多的工程建设和试验研究将在海洋中进行,为了掌握海底的土体环境,静力触探（CPT）技术在国内外海洋工程领域中的运用变得更加重要。本文根据国内外大量文献中相关海洋原位测试技术的报道,对海洋静力触探的国内外发展历程进行总结,重点阐述和研究了一种适用于海底淤泥质土的球型全流触探仪。分析结果表明,球型全流触探仪在海洋岩土工程勘察中具有很好的应用前景,且集测量数据精确可靠、试验方法多样化、理论解析丰富全面等优点于一身;利用球型全流触探仪能够有效避免较大的实测数据修正,贯入机理清晰严谨,开展循环贯入试验可以评价土的重塑特性,变速率贯入试验在评价应变速率和现场土体强度以及测试中土体固结状态之间的关系上具有优势;结合现场试验,对比分析传统CPT和球型全流触探仪的试验结果,并考虑海底软土的性质,摸索出一种可停靠球型探头新形式。     

现代多功能孔压静力触探及在软土地区测试要点

简要分析了我国CPT技术与国际CPT技术的差距,介绍了现代多功能CPTU测试系统、应用于CPT的传感器技术的发展、现代多功能CPTU技术的优势及SCPTU、RCPTU、VisCPT技术的新应用。同时,分析了软土地区多功能CPTU测试技术要点,包括测试计划的制定、孔压系统的饱和与标定、测试精度的保证、负值的分析及探头的保养等问题。     

密度孔压静力触探的测试理论和工程应用综述

孔压静力触探试验(CPTU)是目前使用最为广泛的原位测试方法之一,但CPTU不能原位测量土体最基本的指标——密度。密度孔压静力触探试验(DCPTU)是把核子密度仪结合到孔压静力触探系统中,贯入过程中可以同时测量锥尖阻力、侧壁摩阻力、孔隙水压力和密度的一种新型原位测试方法。本文首先介绍了DCPTU的试验装置和密度测试理论;然后通过与室内试验结果对比评价了利用DCPTU测量砂土、黏土、填土地层以及海底淤泥的密度的可靠性,分析了非均质地层中DCPTU测量密度与真实密度的区别、并提出了真实密度剖面的推求方法;然后综述了DCPTU在判别软弱夹层、评价砂土的液化势、检验地基处理效果、检测块状填土中的空隙、调查浮泥层的厚度和密度分布等方面的工程应用;最后展望了DCPTU在工程勘察中的发展趋势和应用前景。