Characterization of Partial Drainage during Medusa Flat Dilatometer Testing

The flat dilatometer (DMT) is a widely recognized in situ test for site characterization and parameter identification. The penetration procedure is stopped periodically for total stress measurements (A, B, and C pressures) along a circular, expandable membrane, situated at one side of the blade. Changes in pore water pressure are not measured during test execution and are assumed to remain constant for data interpretation. The enhanced Medusa DMT equipment used in this study enables a fully automated test execution and requires no manual membrane expansion by the operator. Changes in pore water pressure can be studied by measuring the A pressure automatically over time at a defined membrane expansion. In the present study, Medusa DMT were executed at three test sites (Rhesi and Lokalbahn Salzburg in Austria; Fucino in Italy) to investigate, on the one hand, partial drainage effects in different soil types using variable membrane expansion rates and repeated A readings and on the other hand, to study the influence of partial drainage on existing correlations (used for parameter identification). The present results indicate that partial dissipation of generated excess pore water pressures occurs in a wide range of soils during test execution. The degree of dissipation can additionally be influenced by the soil heterogeneity, especially in silty soils. Soil parameters and intermediate parameters determined based on A readings are less influenced by partial drainage effects compared with parameters, derived from the difference of corrected B and A readings. The influence of partial drainage on the latter correlations increases in fine-grained soils, characterized by small differences in B and A pressure. Therefore, pressure readings should be corrected about partial drainage effects based on an enhanced testing procedure, considering repeated A readings (DMT-RA).