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Geotechnical characterization of natural soils by in situ testing

Fig. 1. The CPT/CPTU truck of the DICAM Geotechnical Engineering Laboratory (Image by M.F. García Martínez).
Fig. 2. CPTU log profiles in Venetian lagoon sand and silt mixtures (Figure by L. Tonni).
Fig. 3. Preliminary assessment of drainage conditions in CPTU (Figure by L. Tonni)

In situ testing plays a crucial role in the subsoil geotechnical characterization. Over the last decades cone penetration testing, with or without pore pressure measurements (CPTU/CPT), has become the most widely used in-situ testing technique for stratigraphic profiling and site characterization. The amount of knowledge so far gained on the interpretation of CPT/CPTU in sands and clays, in terms of fundamental mechanics of cone penetration, soil classification charts and both theoretical and empirical correlations for the estimate of soil parameters, is undoubtedly wide.

Since early ‘90s, the DICAM Geotechnical Engineering Laboratory has been equipped with a Delft Geotechnics piezocone device, installed on a suitably arranged and fully dedicated independent lorry, fitted for continuous and automated data acquisition. Significant efforts have been put on the refinement of testing procedures as well as on the improvement of data interpretation.

Our recent research activity in this field has been mainly focused on the CPTU-based characterization of silts and other intermediate soils, such as clayey sands, silty sands or silty sands with clay, having very scattered grain size distributions and thus falling in the so-called intermediate permeability range (i.e. 10-5 to 10-8 m/s).

These non-standard geomaterials are still relatively poorly understood and the applicability to such sediments of the existing interpretation approaches, typically developed for CPT/CPTU in standard sands and clays, has come into question. Indeed, cone penetration test response in similar deposits is, in all probability, affected by partial drainage effects and the preliminary identification of the drainage degree is a key step in order to avoid misinterpretation of field measurements and, thus, invalid estimates of soil parameters.

In order to analyse the effect of partial drainage on measurements and to identify probable consolidation patterns during cone penetration, in last years a large number of variable rate CPTU tests have been carried. Different intermediate soil deposits have been selected as test site, with the aim of gathering a wide, coherent experience in rate effects on cone penetration response to be used for the identification of the transition point from undrained to partially drained and drained testing conditions.

Most of the relevant experimental data in intermediate soils have been collected in the context of the extensive piezocone testing campaign performed at the Treporti Test Site (Venice, Italy), within a long-lasting research project funded by the Italian Ministry of Research and carried out in cooperation with the Universities of Padova and L’Aquila. The project aimed at better understanding the stress-strain-time response of the heterogeneous, predominantly silty sediments of the Venetian lagoon basin.

The large amount of available piezocone data, obtained from both standard and variable rate tests, has been interpreted with reference to a number of geotechnical issues, among which it is worth mentioning the evaluation of compressibility characteristics of silts and silt mixtures. These sediments have been found not to follow the framework published for other soils, hence new empirical correlations have been developed to obtain reliable estimates of soil parameters in both primary and secondary compression domains.

Current research is also focusing on the numerical modelling of CPTU in order to help in better identifying probable consolidation patterns during cone penetration.

The final purpose of the research activity in this field is essentially to provide some practical guidance on both testing procedures and data interpretation in intermediate sediments for the selection of appropriate values of soil parameters in geotechnical design.

Main publications

García Martínez, M.F., Tonni, L., Gottardi, G. and Rocchi, I., Influence of penetration rate on CPTU measurements in saturated silty soils, Proceedings of the 5th Int. Conf. on Geotechnical and Geophysical site characterization, Gold Coast (Australia), 2016, in print.

García Martínez, M.F., Tonni, L. and Gottardi, G., On the interpretation of piezocone tests in natural silt and sand mixtures, Proceedings of the 1st IMEKO TC-4 International Workshop on Metrology for Geotechnics, 17-18 March 2016, Benevento (Italy), pp. 63-68.

García Martínez M.F., Tonni L. and Romagnoli, M., Variable rate CPTU in liquefaction-prone silty sand deposits of the Emilia-Romagna region (Italy), Proceedings 3rd International Symposium on Cone Penetration Testing, CPT'14, 13-14 May 2014, Las Vegas (Nevada), pp. 737-744.

Tonni, L., García Martínez, M.F., Simonini, P. and Gottardi, G., Piezocone-based prediction of secondary compression settlements of coastal defence structures on natural silt mixtures, Ocean Engineering, 116, 2016, pp. 101-116.

Tonni, L. and Simonini, P., Shear wave velocity as function of cone penetration test measurements in sand and silt mixtures, Engineering Geology, 163, 2013, pp. 55-67.

Tonni, L. and Simonini, P. (2013). Evaluation of secondary compression of sands and silts from CPTU, Geomechanics and Geoengineering, 8, No. 3, 2013, pp. 141-154.

Tonni, L. and Gottardi, G., Analysis and interpretation of piezocone data on the silty soils of the Venetian lagoon (Treporti Test Site), Canadian Geotechnical Journal, 48, No. 4, 2011, pp.616- 633.

Tonni, L. and Gottardi, G., Interpretation of piezocone tests in Venetian silty soils and the issue of partial drainage, ASCE Geotechnical Special Publication (GSP) No. 205, 2010, pp. 367-374.

Tonni, L. and Gottardi, G., Partial drainage effects in the interpretation of piezocone tests in Venetian silty soils, Proceedings 17th ICSMGE, Alexandria (Egypt), 2009, Vol. 2, pp. 1004-1007.

Research projects

PRIN 2000. A geotechnical model of the soil for safeguarding Venice and its lagoon. Research project funded by the Italian Ministry of Research..