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An Investigation on Dynamic Stability of Silty Soils.

粉质土动力稳定性研究。

基本信息

批准号：
02402041
负责人：
KIMURA Tsutomu
金额：
$ 7.74万
依托单位：
Tokyo Institute of Technology
依托单位国家：
日本
项目类别：
Grant-in-Aid for General Scientific Research (A)
财政年份：
1990
资助国家：
日本
起止时间：
1990 至 1991
项目状态：
已结题

项目摘要

In order to investigate the dynamic behavior of soils with extremely low plasticity (Ip=2,5 ; silty soils), a series of cyclic triaxial tests was conducted on these soils and two others with Ip equal to 10 and 30. Extremely low plasticity soil speci13EA\ : mens were prepared by freezing in a specially designed mold since it is not possible to trim specimens from preconsolidated chunk of these soils. The cyclic triaxial tests were carried out on both normally and over consolidated soils with varying i13EA\ : nitial void ratio, initial shear stress (consolidated isotropically or anisotropically) and cyclic stress ratio. Sinusoidal waves with the frequency ranging between 0.01Hz and 1Hz were applied to the specimens. Conclusions derived from these tests a13EA\ : re as follows.1.For a higher stress ratio, a lower frequency and a smaller plasticity index, during cyclic loading, a higher pore pressure is generated and the reduction in stiffness is more marked.2.For normally consolidated spec … More imen, cyclic loading causes the soil to build up a higher pore pressure and as a result a larger reduction of the stiffness than for over-consolidated specimen.3.There is a marked difference in the dynamic response of low plasticity soils between specimens with and without initial shear stress. When the initial shear stress is small, liquefaction was observed and the stiffness is almost equal to 0 during c13EA\ : yclic loading. Otherwise, large irreversible plastic axial strain is induced by cyclic loading without total loss of stiffness.4.If the same initial shear stress is applied to soil specimen, there is a unique relationship between the irreversible axial strain and the excess pore pressure caused by cyclic loading.Dynamic centrifuge tests were also carried out using a rigid box in which a model caisson sits on a soil layr so as to study the dynamic response of these soils as a soil mass. Two types of input waves were applied to the soil mass, namely a sinuso13EA\ : idal and random wave simulating the El Centro record. In particular, for extremely low plasticity soil, dynamic centrifuge tests using overconsolidated soil were conducted to investigate the effect of the initial void ratio on the dynamic response. Conclusions derivel from these tests are as follows.1.When a structure sitting on a soil layr is subjected to an actual earthquake, a larger settlement will result for soils with a smaller plasticity index.2.In case of soils with extremely low plasticity, it was observed that the rigidity of the ground was reduced during the early stage of shaking and as a result, the acceleration amplitudes of the model casson was much smaller.3.In this study, the settlement of the model caisson induced by the shaking is slightly larger for over-consolidated soil layr than for normally consolidated one. For this reason, the observed amplification of the model caisson's acceleration is gr13EA\ : eater for the overconsolidated case. Thus the overconsolidated soil layr was subjected to a higher cyclic stress because of its higher stiffness. This implies that the stability of soil layrs consisting of soil with higher stiffness is not always better, when the soil-structure interaction is taken into account.4.The ratio of the observed acceleration spectrum of caisson to the input acceleration spectrum is larger at the dominant frequency but smaller in the high fequency region for the higher plasticity soil layr.Using a laminar box, dynamic centrifuge model tests were also carried out to compare with the test results obtained using a rigid box under the same conditions. The ovserved settlement of the caisson is smaller for the laminar box than for the stron13EA\ : g box. Since in the rigid box, shear deformations are restricted by the two side plates, the input waves tend to be transmitted to the model caisson with less energy dissipated, and therefore imposing a more severe vibration on the caisson. However, 13EA\ : the general trend that larger settlement can be induced by shaking for soils with lower plasticity is confirmed in the test results using a laminar box. Less

为了研究极低塑性土(Ip=2，5；粉质土)的动力特性，对这些土和另外两个Ip分别为10和30的土进行了一系列的循环三轴试验。由于不可能从预先固化的土块中修剪试件，因此通过在特殊设计的模具中冷冻来制备极低塑性土料。对初始孔隙比、初始剪应力(各向同性固结或各向异性固结)和循环应力比不同的正常和超固结土进行了循环三轴试验。对试件施加频率在0.01~1 Hz之间的正弦波。试验结果表明：1.应力比越大，频率越低，塑性指数越小，在循环荷载作用下，孔压越大，刚度降低越明显；2.对于正常固结的规范，…与超固结试件相比，循环荷载会使土体产生更大的孔压，从而使土体的刚度降低更大。3.有初始剪应力和无初始剪应力的低塑性土样的动力响应有显著差异。当初始剪切力较小时，在循环加载过程中，土体发生液化，其刚度几乎为0。在相同的初始剪切力作用下，不可逆轴向应变与循环荷载引起的超孔压之间存在着独特的关系。为了研究土体作为土体的动力响应，还进行了刚性箱体的动力离心机试验。对土体施加了两种类型的输入波，即模拟El Centro记录的正弦波和随机波。特别是对于塑性极低的土体，采用超固结土进行了动力离心机试验，研究了初始孔隙比对动力响应的影响。这些试验的结论如下：1.当土层上的结构受到实际地震作用时，塑性指数较小的土体将产生较大的沉降量。2.对于塑性极低的土体，在振动的早期阶段，地基的刚度降低，因此Casson模型的加速度幅值较小。3.在本研究中，超固结土层的模型沉箱在地震作用下的沉降量略大于正常固结土层。因此，对于超固结情况，模型沉箱加速度的观测放大系数较大。因此，超固结土层由于其较高的刚度而受到较高的循环应力。当考虑土-结构相互作用时，由较高刚度的土层组成的土层的稳定性并不总是较好。4.对于塑性较高的土层，沉箱的观测加速度谱与输入加速度谱的比值在主频处较大，而在高频区较小。层流箱的沉箱沉降量小于强度为13EA的沉箱沉降量。在刚性箱体中，由于剪切变形受两侧边板的限制，输入的波易传递到耗能较小的模型沉箱上，从而对沉箱产生较大的振动。然而，层板箱试验结果证实了塑性较低的土体在振动作用下产生较大沉降量的总体趋势。较少