Numerical modelling of the improvements to primary and creep settlements offered by granular columns

Analytical vibro-replacement design approaches typically quantify the settlement reduction using a dimensionless settlement improvement factor, defined as the ratio of the settlements without and with treatment. Most approaches do not explicitly consider the improvement to creep settlement. This is unfortunate because the ‘stone column’ technique is being increasingly used to reduce settlement and improve bearing capacity in soft normally consolidated and lightly overconsolidated cohesive soils (in which creep settlement tends to account for a significant proportion of the total settlement). Analytical design approaches typically consider primary settlement only. In this study, two-dimensional axisymmetric analyses have been carried out using PLAXIS 2D to establish the variation of improvement factor with time using different soil models, one of which incorporates creep behaviour. Two different approaches have been used to establish the influence of creep on predicted settlement improvement factors. The first approach is based on a direct comparison of two different soil models (one of which incorporates creep) whereas the second approach is based solely on the model incorporating creep. The settlement improvement factors have been evaluated for different area-replacement ratios, modular ratios and column lengths. The primary settlement improvement factors are in good agreement with some of the more popular analytical design methods while the creep settlement improvement factors are either equivalent or lower (depending on the approach used). The primary settlement improvement factors show a dependence on the modular ratio whereas it appears that the corresponding creep settlement improvement factors are relatively independent of it.