Effect of soil structuring on stiffness evaluated by triaxial and seismic flat dilatometer tests

Granitic residual soils are considered structured soils. The bonding between the particles and the fabric inherited by the original rock play an important role in controlling the mechanical behaviour. These materials considered as "cemented" soils are often seen as non-textbook materials, since they do not fit into the usual behaviour of transported soils in the light of classical Soil Mechanics theories. To study the stiffness of the granitic residual soils located in Guarda (northeast of Portugal) several SDMT tests carried out on structured soils (natural granitic residual soils and artificial cemented soils) and triaxial tests (carry out with internal instrumentation), performed on samples collected at the same depth, at similar confining stresses, to those involved in the SDMT tests. The results show that in the stiffness decay curve the value of the stiffness modulus (E-DMT or G(DMT)) increase with the increasing level of cementation. In addition, the strain levels involved in the calculation of the stiffness modulus (E-DMT or G(DMT)) decreases with the increase of the cementation level, which can have important consequences for practical purposes of geotechnical design.