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The secondary consolidation degree of soil layer is closely related to the plasticity of cohesive soil, and increases with the increase of clay plasticity index. For ultra-dense soil, it is clay washed in historical period. This clay is consolidated and aged under effective covering stress. Due to terrain change or flowing water scouring, the existing soil layer is thinned by removing the overburden, or large glaciers cross the ground, and the effective overburden stress p 1 is reduced to the existing overburden stress p0. If the consolidation test of this kind of soil is carried out in the laboratory, from e~log
On the p curve, we can see that the turning point pc of the curve is not equal to p 1, but greater than p 1, which is mainly because the influence of over-compaction does not change with depth, that is, p 1? The additional load of p0 will not cause significant volume change of the clay. In addition, the secondary consolidation effect and reserve strength during the load p 1 increase with depth, which, like the load p 1, comprehensively affect the early consolidation pressure. The void ratio of over-compacted soil is smaller than that of normal compacted soil, but when the structural strength disappears, the soil will expand under the same load and transform into normal compacted soil. Unloading is an important condition for the formation of ultra-dense soil. In engineering practice, foundation pit excavation and surface surcharge preloading can make the foundation into ultra-dense state. It should be pointed out that the cyclic load of soil layer, such as the continuous fluctuation of groundwater level, will also have the same impact as ultra-dense soil. In addition, due to environmental factors such as climate change, over-compacted soil will also be formed. When the clay evaporates, the soil is over-compacted due to capillary pressure. Generally speaking, the over-compaction characteristics of topsoil are mainly caused by this reason.