Abstract
The project site in Nardo is underlain by a thick sequence of soft clays. An evaluation of site investigation data alongside a thorough review of the site history has produced a strong geological and geotechnical model. The study site is located at the base of a shallow slope, underlain by Dry Crust, Landslide Deposits, Quick Clay, and Marine Clay. The top of the slope is capped with river deposits.
A combination of laboratory and insitu testing has provided parameters for the back-analysis of a prefabricated vertical drain system using settlement and pore pressure observations for verification.
Use of prefabricated vertical drains was analysed by back calculation of consolidation parameters and inserted into closed-form analytical solutions, before more sophisticated numerical methods were utilised to model the drain system using non-linear parameters and a staged construction sequence.
A soft soil creep model was utilised for the analysis that was able to calculate settlements mostly to within $\pm$ 8mm, equating to an average percentage error of $\pm$ 15$\%$. The calculated pore pressures aligned closely with measured values also although initial excess pore pressures were slightly under-estimated.
A good understanding of the stress history enabled the creep model to be used confidently, although some inaccuracies were introduced when averaging previous overburden pressure along the sloping ground.
It is concluded that the drains provided a good means of accelerating consolidation and providing additional stability to foundation soils. An interesting observation that the drains increased groundwater to a close to hydrostatic condition highlighting that care must be taken when designing a deep drainage system if pore pressures are significantly greater than hydrostatic, as a decrease in stability may result.
