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

在实际中常用的基于等效线弹性模型的地震反应分析中，需要准确地评价岩土材料的循环变形特性。由于水平地基在地震作用下会受到水平-竖直面上的循环剪应力，在实验室模拟这种应力状态时，采用空心圆柱体试件的扭转剪切试验比采用实心圆柱体试件的常规三轴试验更合适。鉴于此，为了提高评价循环变形特性的准确性，通过对空心圆柱体试件进行一系列三轴和扭转剪切试验，研究了干燥、致密的Toyoua砂土在极小应变水平下的准弹性变形特性。用一种新研制的销钉式局部变形传感器进行了局部应变测量，在试验中，顶盖和底座的端部约束对在扭转方向上施加小卸载/重载循环外部测量的小应变剪切模数的影响是显著的。通过改变密度和土级配进行的其他试验中也观察到了类似的趋势。提出了一种新的亚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的各向异性，该模型考虑了主应力轴相对于材料轴的旋转。实验数据与模拟数据吻合较好。在模拟中，固有各向异性对Toyoua砂的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.