Behaviour of granitic residual soils assessed by SCPTu and other in-situ tests

Residual soils resulting from weathering processes cannot be well modelled by the classical theories of Soils Mechanics, creating several difficulties in the interpretation of in-situ test results and in the consequent geotechnical design. In fact, the presence of a cementation structure generates an extra strength component materialized by the presence of a cohesive intercept in the Mohr-Coulomb failure envelope. Moreover, when suction is present the interpretation of in-situ data becomes even more complicated, since it increases this cohesive intercept mixing both (cementation structure and suction) in one final result. In granular soils, as it is the case of granitic residual soils, three components of strength have to be considered, namely the shear resistance angle, cohesion and suction, which only can be solved if more than one field or laboratory measurement is obtained. As a consequence, Piezocone (CPTu), Marchetti Dilatometer (DMT) and Pressuremeter (PMT) tests may be considered appropriate to characterize this type of soils, while dynamic tests such as Standard Penetration (SPT) or Dynamic Probing Super-Heavy (DPSH) tests cannot be effective in this determination. On the other hand, cementation structure also affects deeply the stiffness behaviour, deviating from typical response of transported soils represented by Classical Soil Mechanics. In the last two decades a big effort has been made in Portugal to study and characterize this kind of residual massifs in granitic environments (quite common in the North and Centre of Portugal), namely through DMT and CPTu data, materialized by several research frameworks and its consequent publications. In this paper, a methodology to evaluate the strength of these residual soils by means of SCPTu tests is presented and discussed, leading to a set of new correlations that allows for the evaluation of both cohesive and frictional contributions, hardly possible with the available interpretation models. The work was developed on the Polytechnic Institute of Guarda (IPG) experimental site, which was also previously used in the calibration of SDMT tests for the characterization of these granitic soils.