Allophane clays are derived primarily from the in situ weathering of volcanic ash, although they may be derived from other volcanic material. This parent material may be either basic or acidic in nature. It appears that the primary condition for Allophane formation is that the parent material be of non-crystalline (or poorly ordered structure) composition. Volcanic ash meets this criteria; it is formed by the rapid cooling of relatively fine-grained pyroclastic material, the cooling process being too rapid for the formation of well ordered crystalline structures.

In addition to the above requirement regarding non-crystalline parent material, it appears that the weathering environment must be well drained, with water seeping vertically downward through the ash deposit. High temperatures also appear to favor or accelerate the formation of Allophane clays. Generally the Allophane content of such clays in Indonesia is substantially higher than in similar clays in the cooler, temperate climate of New Zealand (Wesley, 1990).

The weathering process is essentially one of the chemical conversion and leaching out of silica by seeping pore water. A requirement for the weathering process to progress from Allophane to halloysite and on to kaolinite (and the sesquioxides) appears to be a warm wet climate of the kind found in the wet tropics.

From a geotechnical point of view a very good approach to the understanding to how this material behaves is done until now. Rao (1995) gave us an approach to the shear strength of this Allophane clay, the same as Meyer (2005); some others made contributions on the evaluation of volcanic soil properties (Nanzyo). In addition, Charles W. Ng (2002) worked on the behavior of compaction on Allophane clays, while So (1998) worked on the correlation between the Allophane content and the index properties, just to mention a few.

Recently Meyer (2005) made the first approach to the understanding of the dynamic properties of this material, but until now no more additional material has been written in this topic.

Based on what we know about this material until now, this paper will show the results from a study on inherent anisotropy on undrained condition, since information in this topic is very limited; Moreover, this paper is a contribution towards filling this gap.