SITE CHARACTERISTICS OF A WEATHERED OLD ALLUVIUM IN SAN JUAN, PUERTO RICO

摘要

The old alluvium underlying the metropolitan San Juan area of Puerto Rico was formed in early Pleistocene and has undergone substantial post-depositional weathering in the tropical climate, resulting in a unique combination of mineralogy, appearance, and macrostructure. This paper presents the site characteristics of this special geomaterial - a highly weathered old alluvium. As a result of tropical weathering, the fraction of clay minerals decreases with depth, while primary weathering resistant minerals have greater proportions in depth. It is the finely divided Fe-oxides that give this material a red to brown color. The macrostructure is affected by both deposition and post-depositional weathering. Stratification is formed through initial deposition, while weathering tends to reduce such large-scale heterogeneity by converting coarse grains into clays. However, localized weathering along cracks and root paths introduces small-scale non-uniformity - white veins into the site and causes additional difficulties in how to model this material and interpret its properties.