Non-linear stiffness characterisation – a practical framework

Limit state design codes such as Eurocode 7 require good estimates of displacement to be made for serviceability checks. It is known that the stiffness of overconsolidated soils and weak rocks is highly non-linear and this needs to be taken into account for reliable displacements to be calculated. This paper outlines a relatively simple means of characterising the undrained and drained non-linear stiffness of a wide range of soils and rocks. A series of practical applications are described where the calculated stiffness non-linearity is compared with high-quality field and laboratory test data, including back-analysis of full-scale structures. The non-linear stiffness function requires just four inputs: shear modulus at small strain; shear strength; failure strain; and elastic threshold strain. Applications include: checking advanced test data and calibration of non-linear constitutive models; derivation of field stiffness, including variations with depth; non-linear stiffness curves; and assessing the potential effects of stiffness anisotropy.