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A new correlation is proposed between the residual shear strength and the methylene blue value (‘value of blue’; VB) for weathered clay soils on argillaceous bedrock and on alluvial soils in the Oltrepo Pavese area (Northern Italy), which can be considered as geologically representative of a large part of the Italian Apennines. Shallow landslides occur periodically in these soils as a result of high-intensity rainfall events. A number of soils were tested. Trench pits were used for sampling and for the soil profile description (lithology, structure, grade of weathering, thickness). Field surveys were integrated with some standard geotechnical laboratory tests. The methylene blue dye adsorption (VB) was determined in accordance with the French AFNOR standards. The residual strength friction angle was measured with direct shear tests; the procedure employed for the measure involved inserting the soil at the liquid limit in the direct shear ring, applying consolidation in stages and then shearing (Kanji method). The applicability of some existing correlations between the residual friction angle and index properties were tested. The correlations are characterized by a large amount of scatter in the data and tend to overestimate the residual friction angle. The VB test is easy and rapid to perform and appears to be a good indicator of the residual strength friction angle for the tested soils.

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