Residual soils have evolved from the in situ weathering of geomaterials; therefore, a typical cross-section shows finer particles closer to the surface where weathering processes are most intense. However, there are residual soils that exhibit finer grain size at depth; this is often the case with volcanic ash soils. These soils have formed from the in situ weathering of wind-transported volcanic ash emanated during different volcanic eruptions and environment conditions. Water-mediated weathering has facilitated the dissolution and leaching of some compounds and led to the re-precipitation of new minerals such as allophane, imogolite and halloysite. In the process of diagenesis from volcanic ash into volcanic ash soil, the sediment has experienced a large increase in void ratio and important fabric changes. The purpose of this study is to gain further insight into the formation of Colombian volcanic ash soils and ensuing physical properties in view of their mechanical response in geotechnical applications. The experimental program explores chemical, thermal, electrical and mechanical properties, and their interpretation in terms of structure, composition and particle-fluid interaction. Experimental results and field observations suggest the need for reanalyzing the application of standard geotechnical characterization and design approaches when volcanic ash soils are involved.