Travel time observation using numerical simulation of bender element testing in time and frequency domain

Due to poor understanding of the output signal, the reliability of the travel time estimated by bender element tests has been their most serious problem since their introduction to the soil testing. Recent establishment of a concept interpreting the bender element apparatus as a linear system enables us to calculate and probe the output signals more theoretically. In this study, first, the concept is reviewed to understand the linearity of the whole system, which is composed of some fundamental subsystems, reflecting the responses of the soil specimen, and the source/receiver bender elements including their peripherals. For modeling the response of the whole system, simplified models for the subsystems are employed and combined, with some updates to describe the interaction among the subsystems, by sharing some parameters throughout the models. With a set of hypothetical, but expected parameter values, the responses of each subsystem and the combined whole system are simulated in the time and frequency domains, and the resulting travel times are thoroughly examined. The validity of this model is demonstrated by comparing the system responses between the simulation and the experimental data previously conducted in the author’s laboratory. A potential that removing the response of the bender element subsystems from the whole response could dramatically improve the accuracy of travel time estimation in actual tests is concluded.     

Monitoring lime and cement improvement using spectral induced polarization and bender element techniques

Spectral induced polarization(SIP)and bender element(BE)techniques show a high sensitivity to particle size,particle distribution and content of generated hydration products,which essentially govern the efficiency of ground improvement.In this context,both SIP and BE were integrated on a column setup to monitor the processes of lime and cement stabilization.A 5 mmol/L Na2CO3 solution was injected into the sand-lime mixture to produce CaCO3 precipitation,while deionized water was injected into the sandcement mixture to induce the hydration of cement.The average diameters of the precipitated particles or clusters were calculated from the relaxation time,which was a significant parameter of SIP signals,via the Schwarz equation.Two pairs of BE were used to demonstrate the heterogeneity of the product precipitation,which was probably caused by the location of the inflow and outflow on the SIP-BE column.SIP and BE showed the capability of nondestructively monitoring the spatiotemporal chemical evolution processes,which could be helpful for engineering applications.     

Laboratory measurements of small strain properties of dry sands by bender element

The S-wave and P-wave velocities in dry sands are simultaneously measured by a single pair of bender elements (BE) incorporated into a standard resonant column (RC) apparatus with a torsional shear (TS) function. The small strain properties, including shear modulus G₀, constrained modulus M₀ and Poisson's ratio υ, are determined for specimens at various densities and confining pressures. The results show that the G₀ values obtained from the BE tests agree well with those from RC and TS tests, indicating the reliability of the signal interpretation and the testing system. Furthermore, the G₀ data of the present test series is in good agreement with the data from the literature. The results also show that G₀ increases faster than M₀ as the soil density and the confining pressure increase. In terms of Poisson's ratio, it decreases with an increasing soil density and confining pressure and generally lies in the range of 0.18–0.32 for the tested sands. Empirical equations are established to approximately estimate Poisson's ratio from the measured G₀ or M₀ values.     

Monitoring the strength properties of electrokinetically treated soil by bender elements to determine the treatment period

Bender elements have been used as a non-destructive soil investigation technique by many researchers and have proven to be effective in predicting the shear strength of various soils. In this paper, electrokinetic treatment tests were performed with embedded bender elements to monitor the increase in the shear strength of a soft sandy clay during the treatment. The bender element system, designed and assembled for this study, was integrated into the electrokinetic treatment process in order to quell a common uncertainty associated with this form of soil improvement technique, namely: when is the treatment completed? The cross-correlation and first-peak arrival times were used to measure the shear wave velocity of a clayey soil under the treatment of electrokinetics using bender elements. To determine shear wave velocity before and during treatment, a variety of shear wave tests were performed every hour of treatment using frequencies ranging from 100 Hz to 2500 Hz via the use of bender element. The results show that monitoring the soil improvement during the treatment by bender elements can shorten the treatment time by 43% and reduce the energy consumption, which is a major expenditure in an electrokinetic treatment process, by 33% while consistently improving the shear strength and the load capacity by approximately 200% and 300%, respectively.     

干砂最大剪切模量的共振柱与弯曲元试验

对德国 4 种干砂试样进行了共振柱与弯曲元对比试验,旨在分析弯曲元法测定砂土最大剪切模量时存在问题和解决方法。研究表明：时域初达法判定的剪切波传播时间较其他方法具有更好的稳定性;弯曲元试验测定最大剪切模量输入电压脉冲频率的减小而减小,衰减程度因砂土类型而异,该影响随围压的增大而减弱;对比分析表明,弯曲元与共振柱试验测定的最大剪切模量具有良好的线性关系,对柏林砂和不伦瑞克砂,存在一个临界最大剪切模量,小于该临界值时,弯曲元测值大于共振柱测值,而大于该临界值时,前者小于后者,两者差值随土样刚度增大而增大。对比研究指出,弯曲元试验尽可能采用合适高频脉冲电压作为激发电压,实践中应事先与共振柱试验进行对比。     

弯曲元试验高精度测试土样剪切波速方法

在HX-100型多功能三轴仪上开发了压电陶瓷弯曲元剪切波速测试系统,研究了高精度测试不同种类和刚度土样剪切波速的方法以及剪切波速弥散性问题,并将弯曲元测试结果与DrnevichLong-Tor型共振柱试验结果进行了对比。研究结果表明,对不同种类和刚度的土样,通过选用合适的激发波形和频率就可以消除接收波形的近场效应和过冲现象,高精度地确定土样剪切波速和极小应变剪切模量;在所使用的激发频率范围内,剪切波速不具弥散性。     

干砂饱和砂小应变剪切模量共振柱弯曲元对比试验研究

通过对2种洁净的干砂与饱和砂进行弯曲元与共振柱对比试验研究,得出了可靠的弯曲元剪切波传播时间确定方法,并对饱和砂土弯曲元与共振柱试验结果的偏差进行解释。不同频率激发信号对弯曲元剪切波速传播时间的研究证实,剪切波速随激发频率的增大而增大,采用激发频率为10kHz的初达波法与共振柱试验得到的结果具有较好的一致性。饱和砂土弯曲元与共振柱试验剪切波速结果相差6%～10%,可通过饱和砂土中剪切波速的弥散性对其进行解释。大部分洁净砂在弯曲元试验中激发频率一般大于特征频率,均需考虑剪切波速的弥散性。而对于天然砂土或粉土一般具有较高的特征频率,其弯曲元试验中一般不考虑剪切波速的弥散性。为洁净砂和天然砂土饱和土样的弯曲元测试提供有效方法。     

砂土剪切模量测定的弯曲元、共振柱和循环扭剪试验

利用弯曲元、共振柱和循环扭剪试验联合测定了不同围压和密实度条件下干砂的小应变剪切模量,研究了弯曲元试验中输出信号的特性和各种信号分析方法确定剪切波传播时间的可靠性,并对比了不同试验的结果。研究结果表明,在共振柱试验时,需要考虑系统刚度对试验结果的影响,特别当试样刚度较大时。弯曲元输出信号中最先到达的是近场效应,其初始极化方向跟剪切波相反。近场效应随着输入频率的增加有一定程度的减小,但不会消失。实际试验中建议输入频率不断增加,直至输出信号比较稳定。值得注意的是,剪切初达波的振幅可能远小于后续振动,因此容易造成初达波的误判。时域初达波法能比较可靠准确地确定剪切波的传播时间,波峰法、交互相关法和交互功率法结果均随输入频率有较大波动,不能可靠地确定剪切波的传播时间。对比试验表明,弯曲元、共振柱和循环扭剪试验确定小应变剪切模量对细砂和粗砂均吻合良好,初步表明砂土粒径对不同试验方法结果的一致性没有影响。     

Mindlin理论的八节点等参元

按照传统有限元方法,以位移为基本未知量,将经典弹性力学理论中的八节点等参元推广到考虑偶应力的情况,分析了考虑偶应力的小孔应力集中问题,算例表明该单元计算精度好,能够得到比较理想的数值结果。     

有限单元法概述及其基本概念的分析

通过对有限单元法的概述,对有限单元法的分析过程以及基本概念进行了剖析,并作了简要说明,以使工程设计人员能更好的掌握有限单元法,更加高效安全的解决实际问题。     

信息理解的义元场结构及可信性

主要讨论与专业理解子系统相对应的信源伴随义元集,建立了其语义源、模糊语义源及灰色语义源的模型．并进一步得到了零阶可信性函数,使计算各阶理解推理信度成为可能     

浸水路堤稳定性有限元分析

通过有限元模拟计算了水位升降变化对浸水路堤变形和稳定的影响,计算分析表明,水位升降对浸水路堤变形和稳定性的影响,主要是路堤外静水压力和路堤内渗流的破坏性作用,得出了在浸水路堤工程实践中,控制路堤外水位、阻止路堤内渗流作用有利于浸水路堤的稳定的结论。     

两种常见的梁柱单元理论对比与实例分析研究

介绍了基于力形函数和位移形函数的梁柱单元理论，将两种单元理论进行了对比分析，并编制有限元程序选取混凝土结构进行了非线性分析，通过实例的计算结果可见，基于力形函数的梁柱单元更具优势，且计算精度更高。     

碾压混凝土诱导缝等效强度试验的有限元分析

通过对试验中不同类型的轴拉试件进行数值模拟,对试验所得数据与模拟结果进行对比分析,证明了模型建立和参数选择的合理性.根据诱导缝周围应变发展情况,结合应力分布云图,判断诱导缝在荷载作用下的破坏过程,提出了切实有效诱导缝布置方式.     

A second degree approximation for the calculation of the transfer function coefficients for heat conduction through walls

Calculation of the conduction transfer function coefficients using a state space representation requires the transient governing differential equation to be discretized in space by the use of finite difference or finite element methods, in order to obtain a set of first order differential equations. The use of FEM to discretize the media gives an additional advantage due to it is possible to use a higher order approximation of the dependent variable, which gives us a better accuracy with less elements. In this paper, the transient heat flow problem is tackled using a quadratic finite element. The variational formulation for the governing differential equation is developed, the Ritz approximation to construct the finite element formulation is used and the approximation functions are presented using a normalized local coordinate system for elements with three equally spaced nodes for the one-dimensional problem. The 2D transient problem is presented using a rectangular 8 node element. Results with 1, 2 and 3 three-node elements are compared with the ASHRAE conduction transfer functions for the 3, 5, 6, 8 and 32 wall groups and a 2D-example is given.     

用反函数求解等参数单元应变矩阵

针对传统方法在求解等参数单元应变矩阵[B]时的复杂性,介绍了一种显式方法,整个求解过程不需用到隐函数的导数和逆矩阵,并以四结点等参元为例介绍了整个求解过程,并引入了符合数学习惯的Jacobi矩阵表达式,为求解等参数单元应变矩阵提供了参考依据.     

Correlation between sub-micron surface roughness of iron oxide encrustations and trace element concentrations

Iron oxide encrustations are formed on black slate surfaces during oxidative weathering of iron sulfide and phosphate bearing, organic matter-rich slates. Synchronously, trace elements are released during ongoing weathering. Laser ablation ICP-MS analyses of a weathered and encrusted slate showed that major portions of the V, Cu, As, Mo, Pb, Th, and U reside in the encrustation.Recently a potential relationship between several micrometer to 500 nm surface topography roughness of such encrustations and its uranium concentration was shown. Based on laser scanning microscopy measurements, the present study shows that this interrelation must be expanded to small submicron-sized half-pores with diameters between 100 nm and 500 nm. We demonstrate that the relationship is not limited to topography variations of a single encrustation in the hand-specimen scale. Surface topography and geochemical analyses of iron oxide encrustations from several locations but from the same geochemical environment and with similar weathering history showed that the concentrations of U, P, Cu, and Zn correlate inversely with the surface roughness parameter F. This parameter represents the total surface area and is - in this case - a proxy for the root-mean square surface roughness Rq.This study substantiates the environmental importance that micrometer- to submicrometer topography variations of fluid-rock interfaces govern the trapping of trace elements.     

Calculation of prestressed anchor segment by 3D infinite element

Based on 1D infinite element theory, the coordinate transformation and shape function of 3D point-radiation 4-node infinite elements were derived. They were coupled with 8-node finite elements to compute the compressive deformation of the prestressed anchor segment. The results indicate that when the prestressed force acts on the anchor segment, the stresses and displacements in the rock around the anchor segment are concentrated in the zone center with the anchor axis and are subjected to exponential decay. Therefore, the stresses and the displacement spindles are formed. The calculation results of the infinite element are close to the theoretical results. __________ Translated from Journal of Huazhong University of Science and Technology (Nature Science Edition), 2007, 35(9): 28 30 [译自: 华中科技大学学报自然科学版]     

外包钢法加固混凝土构件的设计和施工

通过对加大混凝土构件截面法与粘钢加固法的分析对比，说明了采用粘钢加固法的优越性，并阐述了粘钢加固法的运用范围及详细的计算方法，最后介绍了其构造要求和施工工艺，实践证明该法的施工效果良好。     

应用有限元和离散元法分析边坡稳定性的对比

利用有限元和离散元两种方法对同一连续性岩质边坡的稳定性进行分析,从计算原理、计算有若干节理的岩质边坡两方面对有限元与离散元计算结果进行了比较,得出相关结论,从而更好地指导边坡稳定性计算。