Geotechnical characterization of the Macigno Fm. debris by dynamic penetration tests in the Serchio River basin (Tuscany, Italy)

Due to its structural-tectonic features, geographic- morphological configuration, and climatic conditions, the Serchio River basin is frequently interested by seismical events and floods. Therefore, since the Eighties several studies and surveys aimed at the reduction of seismical and hydrogeological risk were carried out. The landslide hazard evaluation is of primary importance for the territorial planning (DALLAN et alii, 1991; D’AMATO AVANZI et alii, 1999). Moreover, previous studies showed that the highest number of landslides in this basin occurs where argillitic rocks and sandstones (Macigno Fm.) crop out, or they involve debris materials (CANCELLI et alii, 2002; D’AMATO AVANZI et alii, 1999, 2002, 2004). The main purpose of this study is to provide a contribution to the study of the mechanical strength properties of the debris covers of the Macigno Fm. (sandstone interbedded by phyllite), which can be involved in instability phenomena, mainly by soil slip-debris flow (Fig. 1) during intense rainfall (D’AMATO AVANZI et alii, 2002, 2004). These slope materials exhibit a great textural variability, either vertical or lateral; however, they are commonly composed by sandy-pebbly composition, with minor silty-clayey fraction. The knowledge of physic-mechanical properties of these materials is very important in order to comprehend the conditions that trigger landslides in case of both rainy and seismical events. On the other hand, the Macigno Fm. is particularly widespread in the Serchio River basin. The activity was carried out by research and elaboration of the raw data obtained by means of penetration tests (DPT) (medium and super-heavy penetrometers), standard penetration tests (SPT), in order to determine the typical values of the internal friction angle f p (°) and the relative density Dr (%). In fact, due to the low costs and time of analysis, the penetration test is one of the most used techniques in order to obtain an approximate reconstruction of the underground model and the soil parameters. The results of the dynamic penetration tests were afterwards related to the mechanical strength properties of the debris covers by means of empirical methods used for granular materials. In particular, the relative density was determined using the correlation proposed by SKEMPTON (1986), while the SCHMERTMANN (1978) relation was used in order to obtain the internal friction. Figure 2 shows the trend of debris internal friction angle at different depths. These values are typical of the sediments studied, in particular fine sand, medium sand with gravel and sand and gravel. These data were compared with those obtained by means of direct shear tests, reaching a good agreement (Fig.3). In conclusion, this study contributes in determining the typical values of the friction angle and the relative density of the Macigno Fm. cover materials by means of commonly used and money saving indirect methods.