An Investigation on Dynamic Stability of Silty Soils.

为了研究极低塑性土（Ip=2、5；粉质土）的动力特性，对极低塑性土和Ip= 10、30的土进行了一系列循环三轴试验。极低塑性土壤样品是通过在特殊设计的模具中冷冻制备的，因为不可能从这些土壤的预固结块中修剪样品。采用不同的初始孔隙比、初始剪应力（各向同性或各向异性固结）和循环应力比，对正常固结土和超固结土进行了循环三轴试验。对试件施加频率为0.01Hz ~ 1Hz的正弦波。从这些试验中得出的结论如下：当应力比越高、频率越低、塑性指数越小时，循环加载过程中产生的孔隙压力越大，刚度降低越明显。通常情况下，与超固结试验相比，循环荷载会使土体产生更高的孔隙压力，从而导致较大的刚度降低。低塑性土的动力响应在有初始剪应力和无初始剪应力的情况下存在显著差异。当初始剪应力较小时，c13EA\：循环加载过程中出现液化现象，刚度基本为0。反之，循环加载会产生较大的不可逆塑性轴向应变，但刚度不会完全丧失。在相同初始剪应力作用下，不可逆轴向应变与循环加载引起的超孔隙压力之间存在独特的关系。采用刚性箱，将模型沉箱置于土层上，进行动态离心试验，研究土体的动力响应。将两种类型的输入波应用于土体，即模拟El Centro记录的正弦波和随机波。特别针对极低塑性土，采用超固结土进行动力离心试验，研究初始孔隙比对动力响应的影响。从这些试验中得出的结论如下。当位于土层上的结构受到实际地震作用时，塑性指数越小的土体沉降越大。2 .对于塑性极低的土体，在振动初期，地基刚度降低，导致模型卡森的加速度幅值小得多。在本研究中，超固结土层的模型沉箱在振动作用下的沉降量略大于正常固结土层。因此，在超固结情况下，观察到的模型沉箱加速度放大为gr13EA\: eater。因此，超固结土层由于具有较高的刚度而承受较高的循环应力。这表明，当考虑土-结构相互作用时，由刚度较高的土组成的土层的稳定性并不总是更好。在高塑性土层中，沉箱实测加速度谱与输入加速度谱的比值在主频率处较大，在高频处较小。采用层流箱进行了动态离心机模型试验，并与相同条件下采用刚性箱的试验结果进行了比较。层流箱的沉箱观测沉降量小于强13ea \: g箱。由于在刚性箱中，剪切变形受到两侧板的限制，因此输入波倾向于以较少的能量耗散传递到模型沉箱，从而对沉箱施加更大的振动。然而，用层流箱进行的试验结果证实了振动对塑性较低的土体产生较大沉降的总体趋势。少

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