Laboratory characterization of a highly weathered old alluvium in San Juan, Puerto Rico

Abstract

The old alluvium underlying much of metropolitan San Juan was formed in early Pleistocene and has undergone substantial post-depositional weathering in the tropical climate of Puerto Rico, resulting in a special combination of soil mineralogy and structure, with very unusual engineering properties. The soil mineralogy was determined both qualitatively and quantitatively by a series of analytical techniques, consisting of X-ray diffraction, thermal analysis, X-ray fluorescence, and chemical analyses including cation exchange capacity (CEC), soil pH, and selective chemical dissolutions (SCD). Results show that the old alluvium contains: (1) two most weathering resistant primary minerals: quartz and orthoclase; (2) kaolinite and smectites as major clay minerals; and (3) Fe-oxides (goethite and hematite) as special fine-grained minerals, which give the soil distinct red, brown, and yellow coloration. The subsequent quantitative analysis yields high accuracy results, such that the identified mineral phases account for 94-95% of the bulk material. Characterization of soil microstructure also used a variety of techniques including environmental scanning electron microscope (ESEM), slaking tests, CEC, and SCD of Fe-oxides. The results reveal an aggregate structure comprising groups of clay platelets, which each consist of clay particles associated with face-to-face contact. Cementation and aggregation agents are positively identified by SCD as Fe-oxides, which form coatings over clay platelets and aggregates, and bridge bonding between aggregates. These results were confirmed by slake tests in water and glycerol.

(cont.) Index properties vary due to the microstructure. Particle size distribution and Atterberg limits are affected by remolding energy and drying conditions, resulting in difficulties for soil classification. The combination of mineralogy and structure, cause the consolidation behavior to differ from conventional sedimentary soil behavior in the following aspects: (1) the coefficient of consolidation decreases by four orders of magnitude as the sample is compressed to 300ksc; (2) the swelling strains increase significantly with maximum past consolidation pressure; (3) the intact soil exhibits an exceptionally high yield stress ([sigma][subscript]y [approximately equal to] 8ksc); and (4) vertical consolidation strains can be completely recovered upon unloading when samples are pre-loaded above the yield stress. Triaxial compression and extension shear tests on intact samples suggest that the intact shear strength can be described by a conventional Mohr-Coulomb criterion with an isotropic cohesive strength component. The current conceptual models of microstructure offer a framework for developing realistic constitutive models to describe the complex mechanical behavior of this complex residual soil.