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。

Combining bender element,resonant column and cyclic torsional shear tests to determine small strain shear modulus of sand

Bender element (BE), resonant column (RC) and cyclic torsional shear (TS) tests are performed to measure the small strain shear modulus of dry sands. The characteristics of the received signals in BE tests are investigated, and different interpretation methods are used to determine the travel time of shear waves. The results indicate that it is important to account for the compliance of the testing system in RC tests, especially when the specimen is stiff. In BE tests, the near field component arrives first and its initial polarization is opposite to that of the input signal. The near flied decreases as the input frequency increases, but it always exists. In practice, it is recommended that the input frequency should increase continuously until the output signal becomes stable. Note that the amplitude of the first arrival of the shear waves may be much smaller than that of the succeeding ones. The travel time can be reliably determined by the start-start method in the time domain, however, it significantly depends on the input frequency by the peak-peak, the cross correlation and the cross power methods. The comparison indicates that the BE, RC and TS tests can obtain consistent results on clean sand, and the consistency does not depend on the particle size.