Mineralbestand, Gefüge und bodenmechanisches Verhalten tropisch verwitterter Vulkanite

The investigation of tropically weathered andesites from Costa Rica and of basalts from the Vogelsberg area, subject to tropical-subtropical weathering during the younger tertiary shows changes in mineral composition with different degrees of weathering (tab. 1, 3).A sequence of Smectite/Allophane ➛ Halloysite/Metahalloysite ➛ Kaolinite Gibbsite was found which might be modified especially during the early phase. The weathering process results in decreasing base Saturation and an increase of sorbed Aluminium-ions and crystalline Al-Oxides (tab. 1, 3). The relative enrichment of iron oxides causes the change to a brick-like colour in the gibbsite stage or even earlier.The change in mineral composition is accompanied by the development of a different rock texture. The jointed rock bodies can no longer be distinguished in the kaolinite stage of weathering or might only be left as relic structures. Even in the smectite/allophane stage where the texture seems to be intact the strength decreases dramatically.According to the appearance of rock bodies the outcrops in Costa Rica must be assigned to weathering stages W 2 and W 3, the two outcrops in the Vogelsberg to W 3 — W 5 and W 2 (fig. 6). W 2 and W 3 representing the smectite/allophane and halloysite/metahalloysite stage respectively, W 4 the kaolinite and W 5 the gibbsite stage.In all weathering stages the rock bodies or ground mass is characterized by an aggregate structure. Even though the material consists mostly of clay minerals it decomposes usually to a sandy-silty system. This behavior is a result of the Al-bonding and the positive Charge of Fe- and Al-Oxides and allophane caused by the low pH-values. The aggregates (fig. 3) as cellular-spongy structures show a high water binding capacity, which results in high liquid and plastic limits and a low plasticity index (fig. 4, 7); The aggregate structure and the stability of the aggregates cause relatively high friction angles. For the Costa Rica samples they are between 30° and 40° (tab. 2). The Vogelsberg samples show lower values but higher cohesion and commonly higher clay mineral content as well (tab. 4).The material is exposed to extremely high precipitation in Costa Rica. Street embankments usually show very steep slopes which prevents immediate infiltration. Landslides commonly occur where water is seeping into the embankments through flow Channels.