On the variations of geo-engineering properties of dunites and diorites related to weathering

Abstract

Geo-engineering properties of rocks, including discontinuity properties, physical and mechanical properties and crack propagation under axial loads, are affected by weathering. All of these rock properties have an impact on various scales on engineering applications. Therefore, weathering-related variations on rocks and rock masses are of great importance. In this study, dunites and diorites are compared from the view of weathering-related geo-engineering property variations both on field and on laboratory scales. The studies were initiated in the field by measuring the discontinuity properties, including spacing, aperture fill material, roughness and Schmidt hammer rebound tests. Mineralogical and petrographic investigations were performed on thin sections gathered from different weathering grades. The quantitative petrographic studies were concentrated on the micropetrographic index and the quantification of microcracks. At this stage of the study, a new method to quantify microcracks was proposed. Basic physical and mechanical properties (e.g., specific gravity, dry and saturated unit weights, water absorption, effective porosity, P-wave velocity, uniaxial compressive strength and slake-durability index test) were also investigated on samples of all of the weathering grades where the sample preparation was available. The data obtained from the field studies suggest that dunite discontinuity spacing decreases from 577 to 1 cm and ranges from 487 to 1 cm for diorite. Schmidt hammer rebound test values (SHV) in the field were only obtained for fresh, slightly weathered and moderately weathered grades. SHV values range from 54 to 41 for dunite and 63 to 32 for diorite. The quantitative microcrack data obtained during the petrographic studies indicated that microcracks for diorite increase with increasing weathering, but a decrease was observed for dunite, which was related to the formation of serpentine minerals with weathering. Furthermore, with increasing weathering, the changes in the physical properties of dunite specimens exhibit a wider range than in the diorite specimens. Unconfined compressive test results for the rocks showed similar decreasing trends with weathering, but the dunite specimen started to slake by the highly weathered stage, where the diorite exhibited a considerable slaking by the completely weathered stage. This research concludes that the micropetrographic index and quantitative microfracture data provide useful input to recognize the weathering grades of studied rocks. It is also emphasized that serpentinization is the distinct process for dunite weathering and has a high impact on the weathering process, influencing physical and mechanical properties considerably. However, the increase of micro- and macrocracks is the dominant distinctive feature for diorite weathering.