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

近年来，人们努力将性能化设计原则引入实践，岩土抗震工程也不例外。本研究的重点是评估减少地面残余变形的液化缓解技术。这尤其适用于河堤、港口码头墙和埋地管道的减缓技术。研究人员一直在努力建立有效应力极低的液化砂的变形模型，遇到的问题是，在有效应力极低的实验室剪切试验中，砂样自重坍塌，砂粒自重导致应力不均匀。另一方面，振动模型试验不能精确地揭示砂土的应力-应变状态，但容易形成砂土的完全液化状态。为了克服这些问题，本研究在零重力环境下进行了剪切试验。三轴剪切装置是专门设计和制造的，使其适合于利用深立井的零重力试验设施，因为剪切装置的零重力持续时间仅为几秒钟，因此需要特殊的机器控制来进行满意的试验。进行实验有三个困难。第一个;控制操作符合自由墙开始所需的护理。其次，研制小型三轴压缩试验装载机需要努力。尺寸必须很小，以便装入尺寸有限的自由落体胶囊中。最困难的问题是去除细落时饱和水砂的正剪胀所产生的有效应力。最后一个问题，最终克服了采用干试件排水剪切进行。试验表明，在恒定应力下，应变以恒定速率增加，表明砂土具有粘性。极低应力下的摩擦角接近70度。因此，黏度系数约为400 kPaSec。这些数据被用于评估地下板桩和护堤等河堤措施的缓解效果。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