基于电阻率孔压静力触探的浅层气可靠度识别

地下浅层气的存在会对地铁项目的勘察、施工以及后期正常运营构成安全隐患。采用基于地质统计学和可靠性分析的概率方法,对杭州某地铁站场地的电阻率静力触探测试数据进行分析。通过回归分析移除电阻率数据的趋势项,以变差函数为工具,描述电阻率残差在竖向和水平向上的各向异性,采用克里格插值对未采样点参数值进行最优线性无偏估计,采用一阶可靠度方法获取基于电阻率的地下浅层气分布三维概率图。分析表明：电阻率残差具有显著的各向异性,在竖直和水平方向的自相关距离分别为5.1 m和55.6 m。在考虑不同土体电阻率背景值不同的前提下,采用一阶可靠度方法建立了浅层气三维概率分布图。该场地竖直方向上浅层气分布于25 m附近及30～35 m深度区间内,水平方向上则集中分布于RCPTU1点位附近。可基于该结果提出针对性的防治对策,为布设排气孔、浅层气监测等治理措施提供依据。     

Numerical analysis of the miniature piezocone penetration tests (PCPT) in cohesive soils

An analytical model to simulate the penetration of the piezocone penetrometer in cohesive soils is presented here. The elasto-plastic coupled field equations of the saturated cohesive soils (given by Voyiadjis and Abu-Farsakh) is used in this analysis. The numerical simulation of the piezocone penetration is implemented into a finite element program. The analytical model is used to analyze the miniature piezocone penetration tests (PCPT) conducted at LSU calibration chambers. Simulation of the piezocone penetration is done for two cases. In the first case, the soil–penetrometer interface friction is neglected, while in the second case, the soil–penetrometer interface friction is taken into consideration. The constraint approach is used to model the soil–piezocone interface friction in which the Mohr–Coulomb frictional model is used to define the sliding potential. Analysis is done for three different soil specimens with different stress histories. The results of the numerical simulations are compared with the experimental measurements of the miniature piezocone penetration tests (PCPT) in cohesive soil specimens conducted in LSU calibration chambers. The resulting excess pore pressure distribution and its dissipation using the numerical model are compared with some available prediction methods. © 1998 John Wiley & Sons, Ltd.     

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.     

三氯乙烯污染土壤的复电阻率特征和频谱参数研究

复电阻率法(complex resistivity, CR)由于其对孔隙水化学性质、固-液界面和流体含量的敏感性已成为具有良好前景的污染物识别工具。本研究选取三氯乙烯(trichloroethylene, TCE)作为典型有机污染物,利用频谱激电仪对TCE污染土壤进行了复电阻率测量,讨论孔隙水盐度、水饱和度和土壤类型对污染土壤的复电阻率特征和频谱参数影响。试验结果表明:污染土样的复电阻率均随水饱和度的降低和孔隙水盐度的减小而增大;土壤黏土含量影响孔隙水连续性,当孔隙水出现不连续状态时,实部和虚部出现大幅度变化。总充电率M随着孔隙水盐度的增加而增加,随水饱和度的降低先减后增。黏土颗粒重排导致的比表面积减小使得总充电率M减小,双电层极化加强使总充电率M升高;平均弛豫时间常数随孔隙水盐度的升高而降低,二者呈对数相关,随水饱和度变化的主要原因为可极化孔隙尺寸的变化。本研究探讨了地球物理参数与水文地质参数间的关系,为SIP方法在实际有机污染场地调查与评估提供了理论依据。     

温度对污染土电阻率影响的试验研究

应用二相电极法研制了低频交流电阻测试仪,对南京工业污染土进行了电阻率随温度变化的试验测定。结果表明,随着温度的升高,污染土电阻率降低,电阻率随温度的变化呈良好的对数关系。结合目前应用较多的温度校正Keller和Frischnecht法和Campbell法,提出了最理想校正温度法,并推导出了相应的计算公式。试验证明,对于污染土,最理想校正温度法比Keller和Frischnecht法和Campbell法效果更好,拟合精度更高。     

基于孔压静力触探力学分层的土体边界识别方法研究

对于岩土工程设计来说,准确识别均质土层的边界是极其重要的。孔压静力触探(CPTU)测试数据常用于土分类、土层边界识别和岩土工程参数评价等方面。基于CPTU测试数据,利用统计分析方法进行力学分层,可以有效地识别均质土层边界。采用组内相关系数法和修正的Bartlett检验,对CPTU测试数据进行统计分析,生成相应的组内相关系数指标RI和Bartlett统计量。结合R_I和修正的Bartlett统计量临界值,检验土体的均质性,确定均质土层的边界,从而进行力学分层。利用3个场地的CPTU数据进行力学分层,并将其结果与土性分层结果进行对比。结果表明,力学分层结果不仅能分辨出全部主要的土性分层界面,而且还显示更多潜在的次级力学分层界面。     

水泥-粉煤灰固化/稳定重金属污染土的电阻率特性试验研究

固化/稳定法修复重金属污染土固化机制的研究方法多基于力学测试和化学分析,因而提出无损、快捷方便且经济有效的物理评价方法是目前亟待解决的关键问题。通过系统的室内试验,利用电阻率法研究了水泥-粉煤灰固化重金属污染土的微观作用机制,并建立了基于电阻率的固化铅污染土无侧限抗压强度预测模型。试验结果表明,随着养护龄期的增加,试样电阻率、孔隙水电阻率、结构因子、形状因子均显著增大,而各向异性系数则随之减小。而增加重金属离子浓度,电阻率、孔隙水电阻率以及平均形状因子明显减小,各向异性系数A则逐渐增大。扫描电镜分析结果与电阻率参数变化规律吻合,进一步验证了电阻率法揭示水泥-粉煤灰固化重金属污染土微观作用机制的可行性。此外,电阻率与无侧限抗压强度之间存在良好的线性关系,可作为评价固化土体力学性能的指标。     

Numerical modeling and neural networks to identify model parameters from piezocone tests: II. Multi‐parameter identification from piezocone data

This paper completes the study presented in the accompanying paper, and demonstrates a numerical algorithm for parameter prediction from the piezocone test (CPTU) data. This part deals with a development of neural network (NN) models which are able to map multi‐variable input data onto typical geotechnical characteristics and constitutive parameters of the modified Cam clay model, which has been applied in this study. The identification procedure is designed for the coupled hydro‐mechanical boundary value problem in normally‐and lightly overconsolidated clayey soils including partially drained conditions that may appear during cone penetration. The NN models are trained with pseudo‐experimental measurements derived with the aid of the numerical model of the piezocone test, presented in the accompanying paper. Different input configurations containing CPTU measurements and some complementary data are studied with respect to the accuracy of predicted parameter values. Finally, the performance of the developed NN predictors is tested with field CPTU data which are derived from three well‐documented characterization sites, and the obtained predictions are compared with benchmark laboratory results. Copyright © 2011 John Wiley & Sons, Ltd.     

孔压触探测试中探头饱和问题的探讨

孔压触探（ＣＰＴＵ）作为一种新型土体原位测试技术,日益引起人们的重视和推广,但ＣＰＴＵ测试中的一些技术环节尚需进一步探索和规范。本文对严重影响ＣＰＴＵ测试结果精度的探头饱和问题进行了探讨,指出探头饱和的重要性、一般方法、注意事项及检验措施。     

一种孔压静力触探数据采集系统

从实现孔压静力触探多种测量功能和自动测量与数据记录的要求出发,开发了一套孔压静力触探数据采集系统,并完成了TCY-1型测试仪以及PC机配套软件的设计与调试;介绍了其组成单元、工作原理与流程。系统具有测量精度高、数据容量大、体积小和功耗低等特点。应用表明,系统操作简便、配置灵活、工作效率高,可以广泛应用于工程勘测领域。     

Numerical study of partially drained penetration and pore pressure dissipation in piezocone test

The piezocone penetration test (CPTU) is commonly used as a fast and economical tool to identify soil profile and to estimate relevant material properties in soils ranging from fine to coarse-grained. Moreover, in the case of fine-grained soils (clays and silts), the consolidation coefficient and the permeability can be estimated through the dissipation test. Undrained conditions are commonly assumed for the interpretation of CPTU in fine-grained soils, but in soils such as silts, penetration may occur in partially drained conditions. This aspect is often neglected in data interpretation thus leading to an inaccurate estimate of soil properties. This paper investigates numerically the effect of partial drainage during penetration on the measured tip resistance and the subsequent pore pressure dissipation response contributing to a more accurate interpretation of field data. A realistic simulation of the cone penetration is achieved with the two-phase Material Point Method, modelling the soil response with the modified Cam-Clay model. The approach takes into account large soil deformations induced by the advancing cone, soil–water, and soil–structure interactions, as well as nonlinear soil behavior.     

黏土结构性对孔压静力触探结果的影响分析

针对目前黏土结构性对孔压静力触探结果的影响及其机理尚不是很清楚的问题,首先在ABAQUS软件中对结构性剑桥模型进行了程序实现,然后建立了孔压静力触探贯入结构性黏土的数值模型,通过数值计算分析了黏土结构性对孔压静力触探结果的影响。数值计算结果表明：（1）黏土的结构性会导致锥尖周围土体的偏应力增大,由于锥尖附近土体的结构性损伤,最大偏应力出现在距锥尖一定距离的右下方;（2）表征结构性强度的参数——初始结构屈服应力$ p{\text{'}}_{ {\rm{yi}}} $越大,锥尖阻力qc越大,而表征结构性损伤速度的参数——结构性对流动法则影响参数ω越大,qc值越小;（3）$p\text{'}_{ {\rm{yi}}} $值越大,贯入停止时的超静孔隙水压力u越大,而ω值越大,贯入停止时的u值越小;（4）经验圆锥系数Nkt随$p\text{'}_{ {\rm{yi}}} $增大明显减小,随ω的增大略有减小。因此,黏土的结构性强度及其损伤会明显影响孔压静力触探的测量结果,基于锥尖阻力qc估算结构性黏土的不排水抗剪强度su时应采用考虑结构性影响的经验圆锥系数Nkt,否则将会低估结构性黏土的su。研究结果可为孔压静力触探结果的分析和应用提供参考。     

电阻率法评价膨胀土改良的物化过程

掺石灰、粉煤灰是工程中通常采用的改良膨胀土的方法。土电阻率是土的基本物理指标之一,其变化可反映土的其他物理性质指标的变化。通过掺灰改良膨胀土不同养护龄期下的电阻率测试以及膨胀量、膨胀力及无侧限抗压强度等试验研究,探讨了掺灰改良膨胀土养护过程中的物理化学反应过程。根据养护过程中的电阻率随龄期的变化规律,可将改良膨胀土的物理化学反应过程划分为瞬时反应阶段、主体反应阶段、残余反应阶段和稳定阶段4个不同阶段。针对改良膨胀土质量控制和评价体系中存在的不足,提出了基于电阻率指标的改性膨胀土的质量评价方法,通过试验证实了该方法的有效性和实用性。     

Applicability of piezocone and dilatometer to characterize the soils of the Venice Lagoon

The effectiveness of two geotechnical investigation tools—the piezocone and the dilatometer—to characterize the soils forming the shallowest deposits of the upper quaternary basin of the Venice lagoon soil is examined in this study. For this purpose, the results of a comprehensive site and laboratory investigation carried out recently over a small lagoon area—the Malamocco Test Site—are used to evaluate the applicability of the most widely used charts or correlative equations to characterize soil profile and estimate the main geotechnical properties of these soils, when applied to the interpretation of CPTU and DMT results. The particular interest of this site—apart from its unquestionable historical relevance—is the presence, apparently without any regular pattern in depth and site, of a predominantly silty fraction combined with clay and/or sand, thus forming an erratic interbedding of various types of sediments. This case represents therefore the opposite condition of that which has been normally utilized in the past to calibrate the two devices, namely the presence of particularly homogeneous natural deposits or artificially sedimented homogeneous layers of sand or clay. The Malamocco Test Site may therefore be considered as test benchmark for the applicability of the two devices to characterize highly heterogeneous silty deposits.     

Prediction of pile shaft resistance using cone penetration tests (CPTs)

Accurately predicting pile shaft resistance when designing pile foundations is necessary for ensuring appropriate structural and serviceability performance. The scope of this research includes four main components: (I) compiling shaft resistance datasets obtained from the published literature; (II) developing two artificial neural network (ANN) and non-linear multi regression models for predicting pile shaft resistance using cone penetration test (CPT) results; (III) investigating the influence of input parameters on the resulting shaft friction and their degrees of importance; and (IV) assessing the relative accuracies of the presented models using a number of traditional methods. It is quantitatively demonstrated that the ANN and non-linear multiple regression models proposed in the current study out perform the traditional methods and can be used by engineers to accurately predict pile shaft resistance.     

Simulations of realistic granular soils in oedometer tests using physics engine

Discrete element method (DEM) has become a preeminent numerical tool for investigating the mechanical behavior of granular soils. However, traditional DEM uses sphere clusters to approximate realistic particles, which is computationally demanding when simulating many particles. This paper demonstrates the potential of using a physics engine technique to simulate realistic particles. The physics engines are originally developed for video games for simulating physical and mechanical processes that occur in the real world to produce realistic game experiences. The simulation accuracy and efficiency of physics engines have been significantly improved in the last two decades allowing them to be used as a scientific tool in many disciplines. This paper introduces modeling methodologies of physics engine including realistic particle representation and the contact model. Then, oedometer tests are simulated using realistic particles scanned by X-ray computed tomography (X-ray CT). The simulation results agree well with experimental results. This paper demonstrates that physics engines can output contact parameters for geotechnical analysis and force chains for visualization.     

基于地质统计学方法的孔压静力触探锥尖阻力固有空间变异性研究

地质统计学是用于模拟土体固有空间变异性的方法之一,以变差函数为工具,采用Kriging插值提供未采样点处土工参数值的最优线性无偏估计。将地质统计学方法应用于宿-新（宿迁至新沂）高速公路某试验段内孔压静力触探（piezocone penetration test,CPTU）锥尖阻力qt空间变异性研究中,采用回归分析移除数据中的趋势项,从而获得具有弱平稳性的残差数据。指数型理论变差函数能够准确描述试验段内土体的连续空间变异性特征。根据估计结果,试验段内锥尖阻力qt残差的变程具有显著各向异性,在水平方向和竖直方向分别为4.05 m和1.2 m。采用普通Kriging插值结合趋势分析,绘制了qt在试验段的空间分布图和平面投影图,用于指导工程实践。结果表明,普通Kriging插值的估计结果能够与试验段内实测资料形成较好的对比,仅仅在部分极值变化和远离采样点的位置处估计值可靠性会降低。     

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。