The changing pore size distribution of swelling and shrinking soil revealed by nuclear magnetic resonance relaxometry

Determining soil pore size distribution is difficult and time-consuming using traditional methods. Additionally, for swelling and shrinking soil, the specific volume of soil changes with soil moisture conditions. Nuclear magnetic resonance (NMR) relaxometry allows observation of pore size distribution changes during the process of dehydration and soil shrinkage. Naturally structured soil cores of a Vertisol with characteristic swelling and shrinking behavior were collected from the Huang-Huai-Hai Plain of China. The samples were saturated with water and dehydrated gradually at room temperature and relaxometry tests conducted at different moisture contents as they air-dried. Then, the soil cores were oven-dried at 105 °C and saturated with n-octane, which prevented clay swelling. The pore size distribution of soil cores at the end of the shrinking process was determined through the description of n-octane-filled pores. The shrinkage characteristic curves were determined as naturally structured soil cores were air-dried from moisture content at full expansion to constant volume, indicating the relationship between the volume of bulk soil and moisture content. The transverse relaxation time (T 2) distribution deduced from NMR relaxometry gives a good description of the size distribution of pores filled with protons (contained in water or n-octane). The T 2 distribution curves of soil cores at saturation were trimodal, due to the presence of interlayer, interparticular, and interaggregate pores. Combined with the shrinkage characteristic curves, it was deduced that the structural pores were evacuated during the structural shrinkage period. The normal and residual shrinkage was accompanied by the narrowing and closure of the interlayer spaces. During the residual shrinkage period, the frame structure of the soil particles prevented further shrinkage of the bulk soil. The shrinkage process was accompanied by the closure of interlayer spaces and the formation of large cracks between aggregates. 1H NMR relaxometry was especially suited to studying the changing pore size distribution of swelling and shrinking soils. When the soil cores began shrinking, almost all remaining water was retained in interlayer spaces. The volume change of the interlayer space was the main cause of swelling and shrinking. The swelling limit could be estimated from the T 2 distributions of soils at full expansion.