Soil structure of a highly weathered old alluvium

The old alluvium in San Juan, Puerto Rico, is an early Pleistocene deposit that has undergone substantial post-depositional weathering in a tropical climate. Preliminary geotechnical laboratory testing found that the soil exhibits some unusual index and consolidation properties compared with stereotypical sedimentary soils. For instance, both the swelling index and the coefficient of consolidation are greatly affected by the maximum past stress applied to the soil. In order to understand its unusual behaviour, a variety of experiments were performed to characterise its composition and microstructure. Direct observations of the microstructure used environmental scanning electron microscopy, and indirect examinations included slaking, cation exchange capacity, selective chemical dissolution, particle size analyses and Atterberg limits. The results clearly indicate that the weathered deposit possesses a complex microstructure. The intact material consists of a stiff network of aggregates interconnected by Fe oxides. These relatively large and stiff aggregates actually comprise groups of clay platelets, which in turn contain kaolinite and smectite particles, arranged in a parallel configuration, that are coated and connected by Fe oxides. Based on the results, a conceptual microstructure model was developed that links the unusual macroscopic engineering properties to the particle-level phenomena.