Petrographic features influencing basic geotechnical parameters of carbonate soft rocks from Apulia (southern Italy)

A multidisciplinary petrographic and geotechnical approach, aimed at understanding the conditioning by rock-fabric on the mechanical properties of carbonate soft rocks, is shown in this paper. The analysed rocks extensively crop out in the Apulia region and can be referred to three geological units: Pietra leccese (Late Burdigalian-Early Messinian), Calcarenite di Gravina (Middle Pliocene-Early Pleistocene) and Salentino Supersynthem (Midde-Late Pleistocene). 69 specimens of carbonate soft rocks, belonging to all these units, were selected from different sectors of Apulia region and characterized according to petrographic, physical and mechanical properties. Petrographic data allowed us to group all specimens in the following six classes: i) wackestone; ii) mud-dominated packstone; iii) packstone; iv) grainstone; v) floatstone; vi) rudstone. Differently, uniaxial compression strength (UCS) values classify the most part of specimens in â very low and low strengthâ , revealing a dominant weak resistance and high deformability. Particular attention has been given to the relationships between petrographic features and physical and mechanical parameters of the Apulian soft rocks by means of statistical tests based on the univariate analysis of variance (one-way Anova) and on the principal component analysis (PCA). At first, statistical analysis revealed that the rock geotechnical values are strictly dependent from the geological units. Particularly, the samples of Pietra leccese show significantly higher means of UCS, Young's modulus (Et), and dry unit weight than samples of the Calcarenite di Gravina Fm. Conversely, samples of this latter unit have significantly higher means of porosity, imbibition coefficient and permeability coefficient than those ones of the Pietra leccese fm. Moreover, statistical analysis was applied to estimate if geotechnical values differ according to the petrographic classes of the Calcarenite di Gravina Fm and to verify the covariance between the geotechnical parameters of this unit. The following results have been remarked: i) floatstone and rudstone have significantly higher means of UCS and dry unit weight, and lower means of porosity than finer-grained classes; ii) mud-dominated packstone has significantly lower means of UCS, dry unit weight, Et and permeability coefficient and higher means of porosity and imbibition coefficient than other petrographic classes; iii) grain-dominated, matrix-free, rudstone and grainstone have significantly higher values of permeability coefficient than mud-dominated classes; iv) the occurrence of carbonate lithoclasts and of different dominant couples of sparry cement in pore spaces, are two of the most important controlling factors of the rock strength, and can also explains much of the intraformational variability observed in geotechnical data. Total porosity has not a clear correlation with permeability. This means that the contribution of isolated pore spaces to the total porosity is relevant in Apulian soft rocks, as also suggested by the relatively low values of permeability compared to the relatively high percentages of total porosity. A multidisciplinary characterization of carbonate soft rocks based on petrographic analysis and geotechnical determination of physical and mechanical properties is here recommended for future studies since this approach can give useful information to understand the peculiar behaviour of carbonate soft rocks and to drive the subsequent geotechnical investigations.