Zero gravity tests on deformation characteristics of sand under low effective stress

近年来，人们努力将基于性能的设计原理引入到工程实践中，岩土地震工程也不例外。本研究涉及减少地面残余变形的液化缓解技术的评价。对于河堤、港口码头和地下管道的缓解技术来说，情况尤其如此。研究人员一直在努力建立一个有效应力极低的液化砂土变形模型，但遇到的问题是，在极低有效应力下的室内剪切试验导致砂样自重坍塌和砂粒自重引起的应力不均匀。另一方面，振动模型试验不能准确地反映砂土的应力-应变状态，尽管很容易形成完全液化状态。为了克服这些问题，本研究进行了剪切TE…失重环境下的STS较多。三轴剪切装置是专门设计和制造的，以适应利用深竖轴的零重力测试设备。由于剪切设备在逃逸下降时，零重力持续时间只有几秒钟，因此需要特殊的机器控制才能进行满意的测试。第一，控制操作要符合自由墙的启动要求小心。其次，研制一种用于三轴压缩试验的小型加载机需要努力。尺寸必须很小，才能装在有限大小的自由落体太空舱中。最困难的问题是细落过程中饱和水砂土的正剪胀所产生的有效应力的消除。最后一个问题通过使用进行排水剪切的干燥试件最终被克服。试验表明，在恒定应力下，应变以恒定的速度增加，这意味着砂子的粘性。极低应力下的摩擦角接近70度。因此，粘度系数约为400kPaSec。这些数据被用于评估地下板桩和护堤等堤防措施的缓解效果。较少

In the recent times, much effort has been made to introduce into practice the principle of performance-based design, and geotechnical earthquake engineering is not an exception. The present study concerns evaluation of liquefaction mitigation technologies by which residual deformation of ground is reduced. This is particularly the case of mitigation technologies for a river dike, harbor quay wall, and buried pipelines. The investigators have been doing efforts to develop a deformation model for liquefied sand m which effective stress is extremely low Encountered problems are, for example, that laboratory shear tests with extremely low effective stress resulted in self-weight collapse of sand specimens and the stress nonuniformity due to self weight of sand particles. On the other band, shaking model tests could not precisely reveal stress-strain states of sand, although the state of full liquefaction was easily developed. To overcome these problems, the present study conducted shear te … More sts under zero-gravity environment. A triaxial shear device was specially designed and fabricated so that it would suit a zero-gravity testing facility which made use of a deep vertical shaft Special machine control was needed for satisfactory tests because the duration time of zero gravity was merely a few second during flee fall of the shear device.There were three difficulties in running experiments. First; controlling the operation in line with the commencement of free wall required care. Second, development of a small loader for triaxial compression test needed efforts. The size had to be small to be cased in a limited size of a free-fall capsule. The most difficult problem was the removal of effective stress developed by positive dilatancy of water-saturated sand during fine fall. This last problem was overcome finally by employing dry specimens for which drained shear was conducted. Tests showed that strain increased at a constant rate under constant stress, implying viscous nature of sand. The friction angle under the extremely low stress was nearly 70 degrees. Consequently the viscosity coefficient was about 400 kPaSec. This data was employed in assessment of the mitigative performance of measures for river dike such as underground sheet piles and berms. Less