Degree Name

Doctor of Philosophy


School of Civil, Mining and Environmental Engineering


Vertical drains accelerate consolidation and as such they are a very effective and popular ground improvement method. When vacuum preloading is applied with vertical drains, consolidation increases even more and the stability of an embankment is enhanced due to the inward lateral movement exerted by vacuum preloading. Previous analytical models developed to predict consolidation when vacuum preloading is used with vertical drains assumed average compressibility and permeability values within the applied stress range. Even though the smear effects were incorporated into the solution by considering a reduced but constant permeability inside the smear zone, the actual variation of permeability was ignored in the vacuum preloading models developed. More importantly, compressibility of the in-situ clay structure due to the installation of a mandrel driven drain was often ignored as well. The aim of this study is to develop an analytical solution for vacuum preloading which accurately represents the variations in compressibility and permeability in actual ground conditions as a result of drain installations.

The disturbed zone created by drain installation can be characterised by using the extent of the smear zone and the ratio of the horizontal coefficient of permeability in the undisturbed zone and in the smear zone. These parameters were obtained using laboratory experiments performed on large-scale tests of samples of remoulded clay. Laboratory tests performed on samples extracted around an actual drain installed in field conditions indicated that soil was subjected to more smear under field conditions than was previously anticipated. Furthermore, this study revealed that the compressibility of soil was also adversely affected by the changes to the soil structure, and therefore it was deemed imperative to capture these changes in the analytical models developed for radial consolidation with vacuum pressure.

A novel mathematical model was developed to incorporate soil destructuring due to drain installation and the associated changes in compressibility as the soil is improved using vertical drains and vacuum preloading. A more realistic distribution of permeability was assumed within the smear zone and the variation in permeability with the void ratio was also considered in the analysis. The predictions of average excess pore water pressure, degree of consolidation and resultant settlement, and the consolidation responses obtained using this analytical model were compared with other existing models. The importance of this model is illustrated via the case study simulations of two embankments in Australia and China that were stabilised with vertical drains; the proposed model gave more accurate predictions than the previous models. To model more realistic soil behaviour, variations in soil compressibility and permeability were incorporated into the latest edition of the PLAXIS finite element package which enabled the application of vacuum pressure, and very good agreement was observed between simulated results and the field data.

Laboratory tests were conducted on reconstituted and in-situ soil samples obtained from a soft clay site at Ballina, using the newly designed consolidation apparatus that can enable radial consolidation with vacuum pressure. These experiment results enabled the empirical relationship between the vacuum surcharge ratio and lateral strain to be postulated, and they can also be used as a design tool in initial embankment planning.