The influence of initially unsaturated clay on the macro-stability of river dikes

Abstract

In the Netherlands the design condition for a dike is surmised to occur during winter where it is assumed that the dike is fully saturated. Preliminary results by Van Duinen (2020) indicate that suction remains present, albeit ever decreasing, during the winter. This means that it would be possible for a dike to dry out during the summer period such that it becomes unsaturated, and that any effects may not have fully disappeared during a normative event.

This problem is two-fold, as (i) the strength of an unsaturated soil is higher compared to a saturated soil, and (ii) it is unknown how the strength associated with the initially unsaturated zone can be modelled in a macro-stability calculation as it varies temporally. This report investigates the strength of the initially unsaturated zone in clay river dikes in the Eastern part of the Netherlands and it explores the opportunities which the better understanding of the strength may present for the factor of safety in a macro-stability calculation

The feasibility and applicability of the SSCC as formulated by Lu and Likos (2006) for conventional geotechnical laboratory tests is investigated. A literature review bundles international knowledge on unsaturated soil behavior which is relevant in the context of dike design. Silty clay samples from the dike trajectory of Ravenstein-Lith were investigated using the SSCC. It can be successfully applied to K0-CAU triaxial test when determining strength parameters using strain-compatibility. Compared to its conventional variant, the saturation stage must be skipped. Samples are air-dried under laboratory conditions to a prescribed volumetric water content which can then be linked to an amount of suction using the SWCC and a comparison can be made with the closed form of the SSCC (Lu et al., 2010).

The SSCC cannot be applied to DSS tests successfully. In DSS tests both slip and a diagonal failure plane was observed. This research was able to pinpoint at which shear strain slip would occur, and hence when tests are valid. DS tests were used to confirm the validity of DSS tests using CCSM.

A case study performed on a representative cross-section found a positive effect on the factor of safety due the additional strength related to the initially unsaturated zone. The FoS increased by at least 1.7% in the most conservative case and at least 5.0% in a scenario supported by field measurements. The current limited number of field measurements restricts a more precise determination. It is advised to perform more of these such that the initially unsaturated zone in a clay river dike can be modelled effectively and reliably with a high degree of certainty.

To conclude, with the application of the SSCC concept, the additional strength produced by considering the initially unsaturated zone in clay river dikes in the Netherlands leads to an opportunity where the dike can be designed more efficiently by reducing conservatism. Thus, taking the initially unsaturated zone in a dike into account can be considered as a valuable contribution to the toolbox of Dutch geotechnical engineers.