A Constitutive Model for Overconsolidated Clay based on the Hardening State Parameter

Natural deposits are often characterized by a given degree of over-consolidation resulting from a wide range of geological processes and human activities (erosion, melting of glaciers, cyclic loading, water table variations, etc). Their presence in highly populated urban areas may have an important effect on the geotechnical design of structures like foundations, tunnels and excavations. From a modelling standpoint, although the formulation of the Original and Modified Cam-Clay model (MCC) explains the fundamental mechanisms of normally consolidated clays, the applicability of these constitutive theories to simulate the behaviour of Over-Consolidated Clays (OCC) show a limited model performance due to:

• The existence of a large elastic region.
• A quick transition from the elastic to plastic regime.
• An inadequate prediction of the peak stress and dilatancy on the dry side (i.e., the so-called supercritical region).

Compared with normally consolidated clays, over-consolidated clays are characterized by a lower void ratio, a higher strength stress and exhibit a stress dilatancy in combination with strain softening failure during the postpeak regime (Yao et al., 2008 ).

To fill this gap, several formulations have been proposed in the past to better capture the salient features of an OCC deposit (Pender, 1978; Hueckel et al., 1992; Whittle, 1993; Mita et al., 2004; Yao et al. 2009; Gao et al., 2017; Chen and Yang, 2017; Sternik, 2017). Recently, a model using a state parameter as a fundamental variable to characterize the consolidation process of a clay has been implemented by Jockovic and Vukicevic (2017) to mimic the behavior of OCC, thus showing the applicability of this approach for this class of materials. Specifically, the framework of critical state mechanics has been combined within a bounding surface approach through the definition of a state parameter.

The PLAXIS implementation of the HArdening State Parameter (HASP) model (Jockovic and Vukicevic, 2017) is presented to show the capability of these constitutive equations in simulating laboratory experiments on OCCs. To improve the model performance, the Small-Strain Overlay model (Benz, 2007) is integrated in the HASP framework to better capture the non-linear degradation of the shear properties which are crucial to evaluate the deformability of geotechnical structures. After the material calibration, some practical problems solved with the HASP model and PLAXIS 2D/3D code are also presented.

---

With the release of PLAXIS 2D/3D CONNECT Edition V20 Update 1 (January 2020), the PLAXIS User Defined Soil models are delivered with the PLAXIS installation. When enabling the Geotechnical SELECT Entitlement [GSE] when starting the PLAXIS application, this model becomes available in the Parameters tab when selecting "User-defined" for the Material model.