Evolution of pore-size distribution of intact loess and remolded loess due to consolidation

Abstract

It is widely acknowledged that the mechanical behavior of a soil is significantly influenced by the soil microstructure; and the microstructure can evolve as a result of any mechanical, hydraulic, chemical or thermal change taken place on soil sample. The present study aimed to investigate the microstructures of intact loess and remolded loess and to explore the evolution of the microstructure and PSD (pore-size distribution) due to consolidation for both intact and remolded loess. A loess from the Loess Plateau of China was used as test material. A series of intact loess specimens were consolidated to various vertical pressures in oedometer cells. The same loess was remolded at the optimum water content state to various compaction degrees using static compaction method. It is equivalent to consolidate the remolded loess with the least compaction degree under various stress levels. The microstructures of intact loess specimens after consolidation tests and remolded loess specimens are characterized using the SEM (scanning electron microscope) and MIP (mercury intrusion porosimetry) techniques. The micrographs and PSDs under various consolidation pressures (or compaction energies) were compared to investigate the evolution of the microstructure and PSD induced by mechanical loading for each kind of soil. The microstructure of intact loess is more homogeneous and is less dependent on consolidation stress than remolded loess. In both soils, the pores with entrance diameter smaller than 6 μm are almost not influenced by consolidation. In intact loess, inter-aggregate pores with entrance diameter greater than 6 μm are compressed randomly due to loading. However, in remolded loess, the pores are compressed until all larger pores have been compressed. The cumulative pore volume curve of remolded loess of any compaction degree can be divided into two segments, large-pore segment (6–50 μm) and small-pore segment (< 6 μm). The large-pore segment is simplified into a horizontal line and a straight line which slope is independent on the compactness. Remolded loess has very different microstructure and PSD from intact loess even though they may have the same GSD (grain-size distribution), mineralogical composition, and some other physical properties. The microstructural evolution induced by mechanical loading also varies in both kinds of soils. Based upon the measured PSDs, a method is proposed for predicting the PSD of remolded loess of any compaction degree using a reference PSD.