Some experiences from the introduction of wet deep mixing in Scandinavia

The dry deep mixing (DDM) method has been used extensively – and exclusively – in the Nordic countries since its development in the 1970s. Despite being a versatile method, it is mainly limited to sensitive clays with low shear strengths and the method has limitations due to considerable verticality deviations. However, increasing ground improvement needs for stiffer clays, improved homogeneity, and better verticality at greater depths has led to the introduction of the wet deep mixing method (WDM) in both Sweden and Norway. This paper summarizes some experiences from three projects where WDM recently have been employed both onshore and offshore to increase stability and reduce settlements. Two Swedish projects, one large scale commercial and one trial test, were performed in clays with low shear strengths and high water contents for stability purposes offshore for land reclamation. The Norwegian project involved improvement of a low water content clay for settlement reduction of a building foundation. Results from strength verification using wet grab and core sampling are presented and are compared to experiences from DDM and preceding laboratory tests. The paper furthermore discusses some practical issues around execution and strength verification in the three projects. Overall, it is concluded that WDM in many cases is suitable also for Scandinavian clays and can therefore replace or supplement DDM. However, the lack of experience still calls for certain conservatism. Trial columns are recommended until there is more field experience from WDM in Scandinavian clays.