Index properties of a highly weathered old alluvium

An experimental programme was developed to investigate extensively the variation in the index properties of a highly weathered old alluvium in San Juan, Puerto Rico. Both Atterberg limits and particle size distribution were measured on disturbed samples subjected to increasing degrees of remoulding and different drying conditions. Results revealed that the soil exhibits variable index properties with different sample preparation techniques, owing to the presence of silt-sized aggregates that contain inter-cemented clay platelets consisting of paralleloriented kaolinite and smectite particles. Remoulding mechanically disintegrates the soil aggregates, causing large increases in the clay fraction and plasticity index. In addition to mechanical disintegration induced by shrinking, drying is accompanied with both physico-chemical and chemical reactions, which can either decrease or increase a soil's plasticity. For this soil, the influence of air-drying differs from that of oven-drying. The results are interpreted by linking the index properties to the soil's mineralogy and microstructure and by considering the five forms of water existing in the soil. This investigation demonstrates the importance of the knowledge of mineralogy and microstructure in understanding a soil's engineering properties, and provides rough guidance on sample preparation techniques for measuring index properties of residual soils used in practice.