The influence of alteration on the engineering properties of dolerites: The examples from the Pindos and Vourinos ophiolites (northern Greece)

The influence of alteration on the physico-mechanical properties of dolerites from the Pindos and Vourinos ophiolites, northern Greece, is investigated. Quantitative petrographic analysis shows that the samples display various percentages of secondary minerals. Two new micropetrographic indices are calculated from the results of petrographic modal analysis for the engineering assessment of dolerites: the micropetrographic strength index Ips and the replacement index Irep. The samples are also tested to determine water absorption, apparent density, total porosity, sand equivalent, uniaxial compressive strength, Los Angeles abrasion value, aggregate impact value, aggregate abrasion value and polished stone value. The influence of alteration on the rock quality and especially the relationships between the proposed micropetrographic indices and the engineering parameters are determined using regression analysis and the derived equations were verified by the t-test and the F-test. The relationships between the Ips and Irep indices and the engineering properties show that alteration products generally result in mechanically weaker rocks. However, the results of the physico-mechanical tests indicate that dolerites retain their strength and durability due to the low proportions of soft minerals and microcracks, along with the preservation of their igneous textures. The polishing resistance is the only engineering parameter that is positively affected by alteration processes, as expressed by the Ips index. Particularly, the replacement of the primary mafic minerals by secondary minerals, expressed by the Irep index, can enhance the polishing resistance of dolerites when used in the wearing surface of a road. The results also indicate strong positive correlations between the chlorite content and the water absorption and total porosity values.