Engineering geological behaviour of volcanic formations. References to instability phenomena and town planning in Lesvos Island, Greece

The volcanic formations have varying behaviour between rock and soil depending on their geological origin, weathering degree and fracturing degree. The engineering behaviour and weathering classification of these formations is briefly reviewed. Slope stability problems in these rocks have either the form of soil type instabilities (mainly circular failures and creep phenomena) or rock instabilities such as rock sliding on discontinuities and rockfalls. These slope instabilities pose significant problems to human activities.A case study referring to slope stability problems in different volcanic formations is given from the area of Petra municipality in Lesvos Island, Greece. The main volcanic formations in the study area are rhyodacitic and latiandesitic lavas, having rock behaviour and perlites, silicated lavas and pyroclastics, which have a soil like behaviour. Additionally, profiles of totally weathered andesites resulting to clay rich soils are frequently encountered. The mechanism of the anticipated instability phenomena are presented and analyzed.Finally, a simple vulnerability map for instabilities was prepared, which can assist in town planning. Based on the field observations, the potential for occurrence of instability phenomena is related to the following factors: a) Slope aspect and b) geological nature of volcanic rocks (weathering degree and permeability).

A Critical Review of Section 8 (BS 5930)—Soil and Rock Description

BS 5930 proposes fundamental changes to current practice in the description of soils and rocks. The paper reviews these changes and suggests alternative proposals which provide a workable reconciliation between the aims of the new Code and current practical soil and rock description. The main changes made by the Code and the authors' recommendations are:The distinction between fine soil and coarse soil solely on grading is rejected in favour of description according to engineering behaviour.The distinction between silt and clay solely on the basis of the A-line is supplemented with additional terms for borderline cases.Simplification and extension of terminology for description of secondary constituents of mixed soils.Rejection of proposed weathering classification for rocks as inapplicable, in favour of current practice.Clarification of rock nomenclature to conform with geological conventions.The changes to current practice made by BS 5930 are considered to be a retrograde step and could lead to confusion and errors. The paper also considers the process of sample description and attempts to clarify the definition of terms employed in fracture logging of rock core.The paper calls for a revision and reissue of Section 8 of the new Code.