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。     

砂土小应变动力特性弯曲–伸缩元联合测试试验研究

近年来弯曲元在小应变动力特性测试中得到了广泛应用,但对弯曲–伸缩元的联合测试研究较少,国内对弯曲–伸缩元联合测试的研究几近空白。采用弯曲–伸缩元对福建砂进行了剪切波和压缩波联合测试,通过更宽区段激发频率和不同分析方法对剪切波速和压缩波速的确定进行了系统分析,得到了可靠的剪切波和压缩波传播时间确定方法。采用10~20 k Hz峰值法和互相关法可较方便准确地确定剪切波传播时间;压缩波初始到达较易判断,激发频率对压缩波速影响较小。利用弯曲–伸缩元联合测试得到剪切模量G_0和侧限模量M_0,试验结果表明围压和孔隙比对G_0的影响程度均较对M_0的影响程度大。通过剪切波速和压缩波速可计算得到泊松比,随密实度和围压的增大泊松比均以线性规律减小。为弯曲–伸缩元的进一步应用提供了有效的方法。     

基于弯曲元的剪切波扁铲侧胀技术可行性试验研究

扁铲侧胀技术由于其对土体扰动小,试验简单,可以真实地反映土体的实际状态,已被广泛用于各种工程的岩土勘察与测试中。本文提出了将侧装式弯曲元应用于扁铲侧胀技术的设想,并对其可行性进行了探讨。通过搭建侧装式弯曲元测试系统,分别对三种土类在不同上覆压力和含水率条件下土体的剪切波速进行了测试,可以看出,土样的剪切波速与上覆压力基本符合线性关系,且土中细砂含量越高,其剪切波速越大。在相同上覆压力条件下,砂土的剪切波速随着含水率的增大呈现先增大后减小的趋势,当含水率为6.8%时,砂土的剪切波速达到极值。试验结果表明,基于弯曲元的剪切波扁铲侧胀技术是可行的,在常规扁铲侧胀测试的基础上,可以实现对不同深度土体相关物理力学参数的原位测试。     

超重力离心模型试验土体弹性波速监测与表征#br#

压电陶瓷传感器在岩土体弹性波速测试方面具有独特的优势,已在常重力工况下得到了较多的应用。文章针对离心机模型试验的超重力环境对弹性波速测试所提出的严苛要求,研发压电弯曲元和压电压缩元传感器及相应的弹性波速测试技术,主要包括传感器微型化和高频响性能设计,电磁屏蔽和防水接地技术,接收信号的快速叠加和带通滤波处理方法,以及时域初达波法确定接收信号到达时间等。在60g离心加速度下开展一组水平黄土场地的超重力模型试验,重现降雨入渗过程中模型场地土体的软化过程,并利用所研发的弯曲元和压缩元实时监测土体的剪切波速和压缩波速。结合TDR技术监测到的土体密实度,获得土体剪切模量、杨氏模量和泊松比等弹性参数随着土体软化的变化过程,实现超重力环境下模型土体弹性参数的实时监测与表征。     

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。     

超重力离心模型试验土体弹性波速监测与表征

压电陶瓷传感器在岩土体弹性波速测试方面具有独特的优势,已在常重力工况下得到了较多的应用。文章针对离心机模型试验的超重力环境对弹性波速测试所提出的严苛要求,研发压电弯曲元和压电压缩元传感器及相应的弹性波速测试技术,主要包括传感器微型化和高频响性能设计,电磁屏蔽和防水接地技术,接收信号的快速叠加和带通滤波处理方法,以及时域初达波法确定接收信号到达时间等。在60g离心加速度下开展一组水平黄土场地的超重力模型试验,重现降雨入渗过程中模型场地土体的软化过程,并利用所研发的弯曲元和压缩元实时监测土体的剪切波速和压缩波速。结合TDR技术监测到的土体密实度,获得土体剪切模量、杨氏模量和泊松比等弹性参数随着土体软化的变化过程,实现超重力环境下模型土体弹性参数的实时监测与表征。     

离散元模拟中颗粒材料剪切波速的剪切振动确定方法

砂土等颗粒材料的剪切波速是反映其工程性质的重要参数,离散元模拟中实现剪切波速实时测试对研究砂土材料力学性质变化的细观机理具有重要意义。本文基于软件 PFC^3D ,在试样模型中设置激发源和接收源,通过对激发源施加产生剪切振动的速度脉冲、在接收源监测扰动信号,实现了剪切波在颗粒材料中的传播模拟和信号监测,提出了利用激发和接收信号的互相关系数确定剪切波传播时间和评价剪切波传播质量的方法。系统分析了激发频率、激发幅值、激发源和接收源尺寸、阻尼等因素对剪切波速测定的影响,给出了合理的参数取值范围。研究结果得到了均匀颗粒试样理论解的验证。     

两种土样剪切波速测试方法的对比试验研究

扭剪波传感器是一种刚刚兴起的土样剪切波速测试装置。将扭剪波测试方法与弯曲单元测试方法通过试验进行了对比，对比结果表明：无论对于粘性土还是砂性土，弯曲单元方法测试结果比扭剪型剪切波速传感器测试结果偏低20％左右。然后用理论剪切波速已知的有机玻璃棒对扭剪型剪切波速传感器的测试结果进行了验证，发现该方法测试结果与理论值相差仅1．2％。这表明用扭剪波测试土样的剪切波速是一种更加可靠的方法。     

动态三轴试验确定砂土抗液化强度

结合压电陶瓷弯曲元动测技术 ,对杭州砂进行控制剪切波速的动三轴试验 ,验证了砂土抗液化强度与弹性剪切模量间的相关性。试验结果表明 ,土体抗液化强度与其剪切模量的平方成线性关系 ,这使得结合剪切波速测试的动三轴试验研究土体液化更为简单、有效。根据研究实例 ,提出了进行液化判别的一般步骤 ,与常规方法比较后说明该方法的合理性和优越性     

卸荷应力路径下饱和粉土静力特性的试验研究

选取杭州城东地铁5号线隧道周边的典型饱和粉土进行不同卸荷应力路径下的静力三轴试验，研究在临近基坑施工情况下地铁隧道周边影响范围内该类土体静力特性的变化情况，并通过加装压电陶瓷弯曲元装置进行弯曲元试验，从而探寻不同卸荷应力路径对土体抗剪强度和变形的影响程度。研究结果表明：土体的抗剪强度指标会随着轴向及侧向卸荷强度的增大而呈现不同幅度的减小，在轴向卸荷应力路径下饱和粉土各静力特性指标变化幅度小于侧向卸荷应力路径下的变化幅度，但在卸荷再加荷应力路径下，轴向应力路径与侧向应力路径的变化对饱和粉土的抗剪强度的影响并不明显。同时，土体抗剪强度与剪切波速之间可以用模型拟合来建立关系，在实际的卸荷工程中对准确获取土体力学参数有实际意义。     

上海黏性土的初始剪切模量试验研究

以上海天然沉积黏性土②~⑥层土为研究对象,利用共振柱仪器实现了各土层在不同固结压力下的弯曲元和共振柱试验,得到了各层土体小应变水平下的初始剪切模量。试验结果表明,利用弯曲元试验时域初达波法和时域峰值法能得到与现场单孔波速原位测试结果相一致的剪切波速;共振柱试验得到的土体小应变初始剪切模量与弯曲元试验结果基本一致。分析表明,在所试验的各层土中,第③层淤泥质粉质黏土和第④层淤泥质黏土的剪切模量相对较小,第⑥层土的剪切模量最大;第②、⑤层土的剪切模量接近,且大于③、④层,小于第⑥层土,这与土样的实际天然状态是相吻合的。利用Rampello公式可较好地拟合上海黏性土的初始剪切模量,所得的拟合曲线可用来确定各土层在不同深度处的初始剪切模量。     

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.     

Determination of Shear Wave Velocity by Bender Elements using Variable-Path Length Method

For determining shear wave velocity by bender elements, several types of travel time methods have been proposed and are being used commonly. Although the methods have been established over a period of time, there are still many aspects that need to be improved. For example, the ambiguity of arrival time due to near-field effect and the distance measurement for soft sample during testing installation. This study seeks to propose an alternative method which could minimise such ambiguities. In this regard, a method called variable-path length method is proposed, which uses a continuous sinusoidal wave instead of a pulse wave, is applied to a series of tests on reconstituted Kasaoka clay in a modified triaxial apparatus. Based on the results, it is verified that the proposed method can measure shear wave velocity without the ambiguity of arrival time and independent of initial distance between transmitter and receiver. In addition, the proposed method has an advantage to determine phase velocity of shear wave at arbitrary frequency which is useful to study the frequency dependence of geomaterials.     

A Method for Received Waveform Reconstruction Based on Bender Element Test Using Frequency-swept Signal

An experimental technique for reconstructing the received wave of bender element tests based on linear system theory is shown in this paper. In order to identify the frequency response of testing apparatus, bender element tests using frequency-swept signals are performed. Received waves for one-period sine pulse transmitting with various frequencies are calculated and compared with observed waves in several kinds of soil samples and testing apparatuses. The linearity of the testing system is also confirmed by coherence function. It is shown that calculated data are less affected by random noise and show good agreement with observed data in regards to not only waveform itself but also resulting shear wave velocity. It is mentioned that this technique can provide simulations for arbitrary transmitted waveform with high signal/noise ratio after laboratory tests are performed. It is also mentioned that this technique is advantageous when sufficient amplitude of received wave cannot be ensured or shear wave velocity must be verified, for example, due to uncertainty of arrival time by near-field effect.     

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。     

砂土液化势剪切波速简化判别法的改进

结合压电陶瓷弯曲元波速测试技术开展了饱和标准砂的不排水循环三轴试验,并根据试验结果改进了以往提出的利用剪切波速进行砂土液化势判别的简化方法。这种方法本质上基于砂土抗液化剪切强度与弹性剪切模量之间良好的相关性。用改进后的方法全面评价了26次地震、70多个液化场地的液化势,并与国内外其他液化势简化判别法的判别结果作了比较。分析结果表明,改进后的简化方法的评价结果与现场震害调查数据更趋一致。最后,通过一个实例分析]示了利用该改进方法进行土层液化势判别的一般步骤。该改进方法仍有待于深入研究,尤其是对密实砂土场地在强震下的液化评价,需要进一步的试验和现场数据加以佐证。     

Evaluation of erosion resistance capacity on compacted weathered granite soil using non-destructive tests

This paper deals with the evaluation of the erodibility of compacted weathered granite soil through non-destructive tests. A rotating erosion cylinder test (RECT) was employed to evaluate the erosion resistance capacity of weathered granite soil with various relative densities and pre-consolidation pressures. In addition, non-destructive tests, i.e., electrical resistivity and bender element tests, were conducted to investigate a correlation between erosion resistance characteristic and non-destructive property. The results indicated that the critical shear stress increased with an increase in relative density and pre-consolidation pressure, and the threshold shear stress increased with an increase in relative density, while its increase was independent of an increase in pre-consolidation pressure. Thus, the relative density of weathered granite soil has a more significant effect on erosion resistance than its pre-consolidation pressure. The electrical resistivity and shear wave velocity increased with increases in relative density and pre-consolidation pressure, and these results had a good correlation with the porosity of weathered granite soils. The empirical equations for estimating the critical shear stress and the threshold shear stress of weathered granite soils, based on the relationship between the values of erosion resistance capacity normalized by those of non-destructive tests and porosity, were proposed. It is expected that the proposed correlation might be useful in determining the erosion resistance capacity of compacted weathered granite soil by using non-destructive tests.