Geotechnical classification of ignimbrite and prediction of engineering behaviour from simple index tests

Ignimbrites are typically weak rocks of low density and high porosity, and undergo considerable softening on saturation. Extensive systems of open, continuous, vertical joints occur in many ignimbrites, while others are effectively non-jointed. Large changes in strength and jointing may occur within a single profile. Two broad categories of ignimbrite are recognised: durable ignimbrites with Id2 ≥ 90 %, and non-durable ignimbrites with Id2 ≤ 30 %. Durable ignimbrites behave as weak rocks for which the rock mass characteristics exert the primary influence upon slope stability and engineering behaviour. Non-durable ignimbrites are typically non-jointed and are the weakest materials; they are classified as soft rocks, and the intact rock characteristics are the dominant control on their behaviour. Second-cycle slake durability index and effective porosity together allow classification of ignimbrites and prediction of likely material characteristics.