Cause Investigation of Abnormal Settlement Phenomenon in Pleistocene Clay Layers by Experimental Examination of Clay Structural and Mineralogical Composition Changes

本文采用应变路径控制三轴仪对更新世粘土的k_0固结特性进行了研究。介绍了该试验与常规试验相比的优点。采用弯折单元系统测量剪切模量，间接观察颗粒结构的变化。此外，用扫描电镜直接观察了颗粒的结构。从这项工作中可以得出以下结论。应变路径控制试验的优点表现在：试样的应变和超孔隙压力可以自由控制，即使在大应变域中也能保持均匀性；可以精确控制土体的排水模式和k_0固结模式，缩短试验时间。应变路径控制的k_0固结试验结果表明更新世粘土具有非常典型的固结行为；孔隙率在预固结应力后迅速减小。这种行为在重构的高岭土样品中并不明显。K_0值减小到预固结应力，之后迅速增大，并趋于稳定。这样的结果似乎是由于粘土在预固结应力后发生的结构变化。在k_0固结试验中，剪切模量G逐渐增大至100 (MPa)，之后基本保持不变。G中这一变化点的孔隙比与预固结应力后迅速变化的孔隙比完全一致。这种变化的主要原因G可以认为是更新世粘土在预固结应力作用后发生了明显的结构变化。利用扫描电镜（SEM）观察了更新世粘土的微观结构。在测试后的样品中观察到大量的硅藻被破坏，尽管在测试前的样品中观察到完整的硅藻。可以认为，在预固结应力作用后孔隙率迅速降低的原因之一是硅藻的骨架被破坏。在本研究中，未观察到粘土颗粒连接结构的变化，这可能是在预固结应力附近孔隙比迅速减小的另一个原因。少

In this research, the K_0-consolidation characteristics of Pleistocene clay were examined using the strain-path controlled triaxial apparatus. The advantages of this test are described when compared with the conventional tests. Shear modulus was measured using bender element system to observe the change of particle structures indirectly. In addition, the particle structures were observed directly using the scanning electron microscope. The following concluding remarks can be obtained from this work.1. The advantages of the strain-path controlled test were shown in the following : Strain and excess pore pressure in a specimen can control freely and can keep homogenous even in the large strain domain ; the drained mode and the K_0-consolidation mode of clays can be controlled exactly and their testing times can be shortened.2. The result of this strain-path controlled K_0-consolidation test showed the very characteristic consolidation behavior of Pleistocene clay ; the void ratio decreas … More ed rapidly just after the pre-consolidation stress. This behavior is not apparent in a reconstituted kaolin clay sample. The K_0 value decreased to the pre-consolidation stress, then increased rapidly after that, and became a steady value. It seems that such results were due to structural change occurring in the clay just after the pre-consolidation stress.3. The shear modulus G during the K_0-consolidation test gradually increased to 100 (MPa), then it almost kept constant after that. The void ratio of this changing point in G precisely agrees with the one changed rapidly just after the pre-consolidation stress. The main cause of this change G can be recognized to be structural changes of Pleistocene clay significantly just after the pre-consolidation stress.4. The microscopic structure of Pleistocene clay was observed using the scanning electron microscope (SEM). A large number of diatoms destroyed were observed in the sample taken after a test although intact diatoms were observed in the one before a test. It can be considered that one of the causes for the rapid reduction of the void ratio just after the pre-consolidation stress is the frame of diatoms destroyed. In this research, the change of link structures of clay particles was not observed, which was considered as another cause of the rapid reduction of void ratio near the pre-consolidation stress. Less