室内试验公路交通荷载参数的确定

如何确定公路交通荷载是探究路基土体动力特性首先必须要解决的问题。本文对各类公路荷载的确定方法进行了比较分析,重点总结分析了静荷载模型、移动荷载模型(移动点荷载、移动线荷载及移动简谐荷载)、冲击荷载模型、随机振动荷载模型及耦合振动模型。在此基础上,对研究公路荷载作用下路基材料动力特性的试验过程中的主要荷载参数取值给出了大概的范围:荷载频率f=1.0Hz,静载基值p0=20k N,动载pd=20~30k N。     

重复荷载作用下支盘桩受力特性模型试验研究

对一个双盘模型支盘桩在非饱和粉土中进行了5次循环加载-卸载试验。试验证明:支盘桩在小于其极限承载力约0.75倍的重复荷载作用下变形增加很小,工作性能十分稳定;支盘桩与普通等直径桩相比,其承载力和抗变形能力十分优越;支盘桩的荷载传递机理十分复杂,盘附近土体对桩周的摩擦阻力在不同荷载作用下有时为正有时为负;当两盘的间距小于2D(D为盘直径)时,桩周的摩擦力对桩的承载力贡献很小,在每次重复荷载作用下,盘间土体都会经历加压和卸压的过程,卸载后土体会建立新的平衡和物理性质工作状态。研究表明,支盘桩应用于桥梁等承受重复荷载的结构是可行的。     

考虑时效性的静压桩荷载–沉降关系预测方法

考虑天然饱和黏土地层的原位力学特性,采用圆孔扩张模型考虑沉桩效应,结合太沙基一维径向固结理论推导了桩周土再固结过程中土体强度和剪切模量的解析解。在此基础上,根据桩基加载过程中桩侧土体的剪切变形特性,采用指数函数型荷载传递曲线分别建立了静压桩的桩侧和桩端荷载传递模型,提出了考虑时效性的静压桩荷载–沉降关系理论预测方法。通过现场试验对本文解答进行验证,研究了沉桩结束后静压桩荷载–沉降特性随时间的变化规律,分析了静压桩沉桩后不同历时的荷载传递特性。研究结果表明,沉桩结束后静压桩承载特性的变化主要是由于桩侧承载特性的提高;特定休止期后的静载试验结果与静压桩真实承载特性存在一定差异。因此,实际工程中应根据桩周土体力学特性的改变结合静载试验合理确定静压桩的承载特性。     

地铁列车移动荷载作用下地基土的动应力响应

为了通过室内试验研究交通荷载引起的主应力轴旋转对土体循环特性的影响,以单个移动荷载和两个相邻转向架列车荷载为例,分析了土体中各项应力的时程曲线和主应力轴旋转的过程。     

交通荷载作用下黄河三角洲低液限粉土地基累积沉降规律研究

低液限粉土在黄河三角洲分布非常广泛,属液化土,在交通荷载反复作用下地基会产生很大的累积沉降,但目前对粉土地基累积沉降规律研究很少。为此,研发一套适于研究地基累积沉降的落锤式交通荷载模拟装置,通过改变锤的落距等参数能够实现对不同车载的模拟。用该装置对黄河三角洲低液限粉土地基累积沉降进行现场模拟,同时基于Chai-Miura土体累积变形模型对累积沉降进行数值计算与分析,并与试验结果进行比较。现场试验结果表明:累积沉降和超静孔隙水压力基本上均随作用次数增大先快速增大,然后逐渐趋于稳定,而且均随交通荷载的增大而增大;随着加载次数增大,地基会出现翻浆冒泥现象;加载完毕后沉降和孔压均在初期变化较快,随后逐渐变缓。累积沉降的计算结果与试验结果总的来说符合较好,但在交通荷载作用初期,计算结果偏大,这是因为Chai-Miura模型是由不排水试验得到的,而实际地基在车载作用过程中始终处于部分排水状态。     

CFG桩竖向荷载作用下水平位移和弯矩的试验研究

通过室内模型桩试验 ,分析了竖向荷载对CFG桩的水平位移、桩身弯矩的影响。接着 ,运用三维有限元的方法 ,考虑了土体的弹塑性 (Prandtl-Reuss模型 ) ,以及在桩土界面处设置接触单元来考虑桩土的滑移和开裂 ,对模型桩进行了模拟分析。然后 ,通过现场原形实验 ,对分析结果进行验证。结果显示轴向荷载的存在对CFG桩产生附加弯矩和水平位移 ,但在试验荷载范围内影响很小。最后得出有一定参考价值的结论     

黄河三角洲细粒土微振液化分析

在黄河口潮坪选择典型研究区,利用现场原位对其循环荷载试验,通过实时孔压监测、静力触探试验、十字板剪切试验及原状样土工试验对比,研究了在循环荷载作用下黄河三角洲地区饱和细粒土的动力响应过程、土体比贯入阻力、峰值强度、残余强度和灵敏度的变化以及孔隙水压力和液化发生层的变化对黄河口粉质土液化过程的影响。研究结果表明:循环荷载作用使土体对能量吸收特性的变化、孔压的响应与振动导致的土体结构性变化有密切的关系;循环荷载作用导致黄河三角洲地区粉质土液化主要发生在硬壳层;随着荷载循环次数增加,液化发生层的深度向深部扩展;循环荷载作用使土体的物性指标变化趋势因深度的不同而具明显的差异性。     

真空荷载作用下劈裂效果分析

采用自行研制的模型试验装置进行不同喷气压力和有无真空作用下土体的气压劈裂室内模型试验,研究真空荷载对土体劈裂效果的影响。试验结果表明,气压劈裂使土体产生裂隙,裂隙集中于喷气口深度处,裂隙使得超静孔隙水压力呈三阶段规律,即孔压骤升并产生裂隙阶段、停止喷气后孔压迅速消散阶段和缓慢消散阶段。对比不同喷气压力和有无真空作用下气压劈裂试验结果发现,超静孔压峰值随着喷气压力的增大而增大,真空荷载使得超静孔压峰值变小;裂隙排水时间随着喷气压力增大而增大,为20~30 min,裂隙排水约占超静孔压峰值的70%,真空荷载加快超静孔压消散速率;抽真空过程中喷气劈裂应采用低压短时脉冲方式。     

落锤冲击气囊施加均布动荷载的试验方法

在结构构件上施加均布冲击(爆炸)荷载一直是动力试验领域的难题。提出了一种落锤冲击气囊施加均布动荷载的试验装置,和一个双自由度弹簧阻尼冲击加载简化分析模型;给出了该模型的基本力学方程、边界条件和求解方法,并进行了有限元验证;建立了所提出冲击加载试验装置的精细化有限元模型,分析了构件上各受荷区域的荷载均布情况,验证了所提出装置的可行性。基于所建立的简化分析模型,讨论了刚度、荷载作用时间和阻尼比等关键参数,对构件上动荷载的影响,并给出了基本规律。计算结果表明,在选定合适的参数后,落锤冲击气囊试验装置能够较好地模拟构件上的均布动荷载,为实验室中进行爆炸冲击试验提供了一种可能。     

竖向受荷混凝土桩-地基土模型试验研究

桩基础广泛应用于建设工程项目中,桩基础承载力和地基土体沉降是影响建设工程项目质量的重要因素。通过自主设计加载装置和量测系统,进行竖向荷载下室内单桩静荷载模型试验,分析桩顶沉降、桩身应变、桩身轴力、桩侧摩阻力以及桩周土压力,研究桩体荷载分担和地基土体沉降机理。试验发现,存在一个地基土沉降盆,范围内的土体沉降较大,易对地基承载力和稳定性产生影响;存在一个有效桩长,当桩基长度超过此值时,增大桩长无法增大桩基极限承载力。试验结论对桩基设计提供参考,旨在保证桩基和地基土的承载作用。     

Development of a new loading test apparatus for microfocus X-ray CT and its application to the investigation of soil behavior surrounding driven open-section piles

In recent years, X-ray CT scanners with task-specific modifications, such as microfocus and/or a synchrotron, have been widely implemented in a variety of fields, including medical and industrial fields. Since microfocus X-ray CT systems enable the visualization of whole samples, and also yield a relatively high resolution of the region of interest in the sample, they are commonly utilized in the fields of soil mechanics and geotechnical engineering. This paper describes a novel loading test apparatus that is specifically designed for microfocus X-ray CT systems capable of performing loading tests to investigate soil behavior during the installation of driven open-section piles. The loading tests were designed to take the micro-level spatial resolution of microfocus X-ray CT systems into account. Digital image correlation (DIC) is subsequently implemented to analyze the obtained CT images, making it possible for measurements to be taken of the displacement fields in the ground following installation of the model pile. Finally, the versatility of this novel test apparatus is proof of its high potential for application in different types of loading tests.     

数字图像伪彩色增强方法在岩土CT图像分析中的应用

直观岩土CT图像分析是最基本的CT分析方法，而高分辨率的岩土CT图像是这一分析方法的关键。为了提高岩土CT图像的分辨率，根据人眼的视觉特点，结合计算机数字图像颜色模式和图像格式，采用灰度级．彩色变换方法，对灰度CT图像进行伪彩色增强，显著地提高了CT图像的分辨率。     

Strip loading test theory and equipment development

One of the important issues for geotechnical engineers is the characterization of soil properties such as cohesion and internal friction angle by means of soil testing. A new experimental method of soil characterization based on the surface displacement of strip loaded soils is proposed. The theory to relate the soil deformation/displacement to soil strength properties is presented and compared with a series of conventional soil characterization techniques with direct shear tests. The proposed/developed strip loading tests provide reasonably accurate results compared with traditional direct shear tests. The new strip loading physical simulation and testing devices are helpful for understanding soil strength concepts and also provide an effective bridge connecting with engineering mechanics and foundation engineering courses instructions wherein derivation of bearing capacity theory equations is based on the same Mohr-Coulomb soil strength parameters. The advantages, limitations, and use of the strip loading modeling/testing technique in engineering education and further more in depth researches are discussed in the concluding remarks part.     

Engineering geological approach to the evaluation of seismic risk in metropolitan regions: case study of Patras,Greece

The city of Patras in western Greece has experienced several destructive earthquakes during the past centuries. The engineering geological conditions of the area have been analyzed and described using a relational geotechnical database management system including geotechnical and geological information obtained from exploratory boreholes, cone penetration tests and cross hole seismic prospecting. The foundation soils of the city were grouped into individual geotechnical units and several engineering geological maps and geotechnical cross sections were compiled. Criteria are proposed for the approximate evaluation of the effect of local soil conditions on ground motion, especially on the distribution of amplification features.      

Influence of density on the post-suffusion behavior of gap-graded sand with fines

Suffusion erosion, characterized as the selective detachment and transportation of finer particles by seepage flow, is hazardous to the stability and serviceability of geotechnical structures. The removal of finer particles deteriorates the structure and fabric of the soil, leading to the degradation of its mechanical properties. Studies into the effects of suffusion on mechanical behavior have so far produced disparate results depending on the pre-erosion relative density of the specimens tested. To investigate this issue, small cyclic and monotonic loading tests were performed on intact and eroded gap-graded silty sand specimens in three dispersed density states, using a triaxial cell modified for the purpose of erosion. The variation of Young's modulus showed an inverse relationship with the pre-erosion density of the specimen, as the small strain stiffness decreased in the dense cases and increased in the loose cases. Conversely, Poisson's ratio increased in value as suffusion progressed regardless of the initial density of the specimens. In the contractive phase of monotonic loading, the densification of the coarse soil skeleton by the downward seepage flow resulted in a decrease in contractiveness and an increase in secant stiffness. In the dilative phase, the increase in porosity by the erosion of finer particles reduced the dilatancy and peak strength of the specimens. The results suggest that the pre-suffusion density determines the primary locus of affected mechanical behavior in triaxial compression, which shifts from the contractive phase to the dilative phase with the increase in pre-suffusion density. The critical state strength, inferred using stress–dilatancy theory, was largely unaffected by the erosion of finer particles. As the results indicate, pre-erosion density may be of practical significance in assessing the vulnerability to deterioration and collapse of geotechnical formations and structures subjected to suffusion erosion.     

EVALUATION OF PARTICLE CRUSHING IN SOILS USING X-RAY CT DATA

The objective of this paper is to evaluate particle crushing in the soil using the results of X-ray CT scanning without any destruction and sampling. First of all, the properties of X-ray CT data are discussed using its statistical consideration. Then, a method for evaluating particle crushing is proposed. This method is implemented by scanning simple granular materials using aluminum rods and soil. Then, a series of model pile loading tests are conducted for the materials of non-crushable and crushable soils. Based on the results of CT scanning for the model grounds, the effectiveness of the proposed method for evaluating particle crushing using X-ray CT data is confirmed. Finally, one of the effective usages of X-ray CT data in geotechnical engineering is convinced.     

高能级强夯处理碎石土地基的应用研究

通过碎石土回填地基采用高能级强夯技术的工程实例,得到了大量的静载试验、动力触探等现场实测数据,比较完整地反映了12000kN.m高能级强夯的加固效果,分析比较了碎石填土地基夯前和夯后的土工性能指标、地基承载力及变形模量。研究表明,深度12m以内各土层的地基承载力均具有较大幅度的提高,可为其它工程高能级强夯技术的施工、检...     

Utilisation of transparent synthetic soil surrogates in geotechnical physical models: A review

Efforts to obtain non-intrusive measurement of deformations and spatial flow within soil mass prior to the advent of transparent soils have perceptible limitations. The transparent soil is a two-phase medium composed of both the synthetic aggregate and fluid components of identical refractive indices aiming at attaining transparency of the resulting soil. The transparency facilitates real life visualisation of soil continuum in physical models. When applied in conjunction with advanced photogrammetry and image processing techniques, transparent soils enable the quantification of the spatial deformation, displacement and multi-phase flow in physical model tests. Transparent synthetic soils have been successfully employed in geotechnical model tests as soil surrogates based on the testing results of their geotechnical properties which replicate those of natural soils. This paper presents a review on transparent synthetic soils and their numerous applications in geotechnical physical models. The properties of the aggregate materials are outlined and the features of the various transparent clays and sands available in the literature are described. The merits of transparent soil are highlighted and the need to amplify its application in geotechnical physical model researches is emphasised. This paper will serve as a concise compendium on the subject of transparent soils for future researchers in this field.     

Visualization for Engineering Property of In-Situ Light Weight Soils with Air Foams

The objective of this paper is to evaluate both physical and mechanical properties of in-situ light weight soil with air foams using an industrial X-ray CT scanner. The specimen used in this study was in-situ SGM (Super Geo-Material) which was a light weight soil created by mixing dredged slurry with cement and air foam. This was sampled at the trial construction site of new Kumamoto seaport, Japan. Two different materials of air orientation which were surface active agent and protein were examined at the site. First of all, the effect of different kinds of air foams on the physical property of in-situ light weight soil was discussed using the results of CT scanning, in which the distributions of the density and air foams in the soil were taken into consideration. Furthermore, as a mechanical property, a series of unconfined compression tests was conducted for the in-situ specimen, and the density evolution was also investigated by scanning of the specimen during unconfined compression. Here, the effects of both distribution and size of air foams were also clarified. Finally, the effectiveness of the industrial X-ray CT scanner in geotechnical engineering was confirmed based on all these results.     

土层锚杆预应力损失原因分析与补偿对策研究

阐述了预应力锚杆的加固机理，对土层锚杆的预应力损失原因进行分析。基于工程实践和土工试验，提出了相应的预应力损失补偿对策，并深入剖析了其中的理论依据。