Experience of Consolidation Process from Test Areas with and without Vertical Drains

Two different methods of how to analyse the consolidation process arising due to loading of soft clay deposits are presented, one of which is the classical approach based on validity and the other on non-validity of Darcy’s law at small hydraulic gradients. The two methods of analysis are compared with results of full-scale loading tests on areas without vertical drains and areas provided with vertical drains. Two test areas in Sweden without vertical drains are presented, which have been followed-up by pore pressure and settlement observations for half a century. The results obtained in these test areas is of particular interest since the observed delay in excess pore pressure dissipation in relation to what would be expected according to Terzaghi’s one-dimensional consolidation theory depends on non-Darcian flow and not on secondary consolidation yields better agreement with the pore pressure observations than the theory based on the generally assumed effect of creep. Well-instrumented test areas provided with vertical sand drains or pre-fabricated band-shaped drains, established in Sweden, Thailand and Italy, are also presented. In one of the cases described, surface loading was replaced by the vacuum method. Although the consolidation characteristics of the soil in the test areas and the contract project are quite different, a better correspondence between practice and theory based on non-Darcian flow is obtained in all the studied cases than with theory based on Darcian flow. Finally, a study made in connection with the extension of the Stockholm-Arlanda project with band drain installation the heavy loading condition, causing high hydraulic gradients, results in an acceptable correlation between theory and practice also by assuming Darcian flow.