Consolidation properties and structural alteration of Old Alluvium

We present experimental observations and a conceptual model for understanding the compression and swelling characteristics of Old Alluvium (OA) from San Juan, Puerto Rico. Prior studies have classified OA as a transported, in situ weathered tropical soil whose intact macrostructure comprises a cemented, pseudo-silt with a mixture of quartz grains and aggregated clay particles. The aggregates include mixtures of kaolinite and smectite coated by Fe-oxides. The Fe-oxides also act as cementing agents between the particles. This study describes results from a series of high-pressure (up to 63 MPa) incremental consolidation tests. Breakdown of the clay aggregates and cemented structure is observed through changes in the compression properties, significant swelling during unloading, and an extraordinary reduction (i.e., by three orders of magnitude) in the coefficient of consolidation. These experimental observations are explained by a combination of mechanical processes, comminution and breakdown of cementing bonds, and physicochemical changes linking pore fluid in the intra- and inter-aggregate pore space. These processes alter the fundamental particle size distribution and macro-porosity of the soil and activate the swelling potential of the smectites concealed by the Fe-oxides coating in the intact material. The experimental observations provide the basis for the formulation of a constitutive model to describe macroscopic compression and swelling behavior of Old Alluvium and offer a framework to understand the response of piedmont transported residual soils found elsewhere.