Sensitivity to remoulding of some volcanic ash soils in New Zealand

Abstract

Sensitivity to remoulding of some volcanic ash materials in New Zealand has been investigated. The subsoils studied were derived from airfall andesitic ashes containing a high proportion of allophane or from hydrothermally altered ashes containing mainly halloysite. These materials were characterised by large losses of strength on remoulding. Sensitivity values vary from 5 to 55 but do not directly relate to soil mineralogy or classification characteristics of the materials. Sensitivity is, however, associated with high undisturbed strength rather than with low remoulded strength. The study shows that only a small component of sensitivity may be attributed to thixotropy. Scanning electron micrographs of the most sensitive material showed imogolite fibrous linkages of allophane particles that were destroyed on remoulding and did not reform after long periods of resting. It is suggested that the irreversible breakdown of electrostatic and physical bonds, existing between clay minerals, largely explains sensitivity in volcanic ash soils.