Shear strength evaluation of soft dredged sediments
Type:
Presented during:
CEDA Dredging Days 2007 - The Day After we Stop Dredging, Rotterdam
Authors:
Meriggi R, Del Fabbro M and Blasone E, University of Udine, Italy
Abstract: Prevision of the undrained shear strength of dredged sediments during the consolidation phase is an important consideration during the design of confined disposal facilities because there may be severe instability phenomena during the application of external actions, even of low intensity, such as the placement of dry soil covers or vehicle transit. However, when the dredged sediment consistency is still fluid, it is impossible to collect undisturbed specimens for triaxial compression tests, or direct shear tests, to directly evaluate their shear strength.
This paper presents a method to overcome this difficulty and indirectly evaluate undrained shear strength for dredged sediments as a function of their water content by means of relationships with the results of a simple and fast laboratory test, the slump test. The analysis has been performed on sediments collected during dredging operations in Friuli Venezia Giulia (Italy), measuring their main geotechnical characteristics and forming mixtures with different water contents. On every sediment mixture, vane and slump tests have been performed in the laboratory for different ageing times, and the results interpreted in relation to their common variable, W/WL.
The results highlight an increment of the peak shear strength with time; however this increment decreases with the normalised water content, while the residual shear strength is not influenced by previous stress history. Relative to the analyzed water content, of between 30% and 75%, the values of undrained peak shear strength versus normalised slump height are well interpolated by a regression power function. Also in residual conditions, the undrained shear strength follows a decreasing trend of the power type, with values of about one third those of peak ones. The results may be utilized directly to evaluate consistency and indirectly, using the proposed relationships, to evaluate undrained shear strength.
Keywords: consistency, shear strength, laboratory tests, water content.