Study on improvement of accuracy in evaluating cyclic deformation properties of geomaterials by torsional shear tests on hollow cylindrical specimens

在实践中常用的基于等效线弹性模型的地震反应分析中，需要准确地评估岩土材料的循环变形特性。由于水平地面在地震中受到作用于水平-垂直平面上的循环剪切应力，因此在实验室模拟这种应力条件时，对空心圆柱形试样进行扭转剪切试验比对固体圆柱形试样进行常规三轴试验更合适。鉴于此，为了提高循环变形性能评价的准确性，通过对空心圆柱形试件进行一系列三轴和扭剪试验，研究了干燥致密土在极小应变水平下的准弹性变形特性。采用新研制的针式局部变形传感器进行局部应变测量。在试验中，通过在扭转方向上施加小的卸载/重新加载循环来测量外部的小应变剪切模量，发现顶盖和基座的端部约束对小应变剪切模量的影响是显著的。在其他通过改变密度和土壤级配进行的试验中也观察到类似的趋势。考虑主应力轴与材料轴的旋转，提出了一种新的准弹性模型来模拟砂土准弹性变形特性的固有各向异性和应力诱导各向异性。实验结果与模拟结果吻合较好。在模拟中，发现固有各向异性对土优拉砂准弹性变形特性的影响很小。

In seismic response analyses based on equivalent linear elastic modeling that are frequently employed in practice, cyclic deformation properties of geomaterials need to be evaluated accurately. Since level ground is subjected to cyclic shear stress acting on horizontal-vertical planes during earthquakes, it is more appropriate to use torsional shear tests on hollow cylindrical specimens than to use conventional triaxial tests on solid cylindrical specimens in simulating such stress conditions in the laboratory.In view of the above, in order to improve the accuracy of evaluating cyclic deformation properties, the quasi-elastic deformation properties of dry, dense Toyoura sand at very small strain level were investigated by conducting a series of triaxial and torsional shear tests on hollow cylindrical specimens. Strains were measured locally by a newly developed pin-typed local deformation transducer.In the tests, effect of end-restraint at the top cap and pedestal on small strain shear modulus that was measured externally by applying small unload/reload cycles in the torsional direction was found to be significant. Similar tendency was observed in the other tests conducted by changing the density and the soil gradation.A new hypo-elastic model was proposed to simulate the inherent and stress-induced anisotropies of quasi-elastic deformation properties of sand, considering rotation of principal stress axes from material axes. Good agreements between experimental and simulation data were observed. In the simulation, effect of inherent anisotropy on the quasi-elastic deformation properties of Toyoura sand was found to be small.