Effectiveness of an advanced fully automated in-situ test for the characterization of very soft soils

¹ Studio Prof. Marchetti, Rome, Italy
² Federal University of Rio de Janeiro, Rio de Janeiro, Brazil

^* Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Abstract

The Sarapuí II site, located on a very soft clay deposit in Guanabara Bay, Rio de Janeiro, is one of the most extensively documented geotechnical research sites in Brazil. Since 1985, it has been investigated using also with all the versions of the Marchetti Flat Dilatometer (DMT), including the conventional pneumatic and the recently developed fully automated Medusa DMT. As reported by Januzzi et al. (2015), the pneumatic DMT produced acceptable repeatability in readings p₀ and p₁, but showed significant variability in the differential pressure (Δp = p₁ − p₀), resulting in considerable scatter for all derived parameters. This variability stems from the extremely low Δp values (~10 kPa) and limitations inherent to the pneumatic system, such as operator-dependent inflation/deflation rates and pressure differences along the pneumatic cable. Testing in such soft geomaterials requires advanced instrumentation capable of precise pressure control and accurate measurement at extremely low stress levels. The Medusa DMT addresses these issues by generating and measuring pressure directly at depth, ensuring a fully automated and repeatable membrane expansion rate. This innovation substantially improves the accuracy and precision of p₀, p₁, and also of Δp readings in very soft soils. Comparisons of key geotechnical parameters—such as undrained shear strength (sᵤ) and overconsolidation ratio (OCR)—interpreted using standard DMT correlations with high-quality laboratory and other in situ test results demonstrate excellent agreement. These findings confirm that the Medusa DMT is highly effective to evaluate correctly geotechnical parameters and with negligible scatter very soft clays like the ones at Sarapuí II.