Case study on soft soil improvement using vertical drains—field measurements and numerical studies

This paper presents an interesting case study for a very large ground improvement project in north Egypt using prefabricated vertical drains (PVD) and surcharge. The first 15m of soil at this site is soft silty clay to silty sand over 35m layer of very soft clay. To build a trustable numerical model that can predict the behaviour of the complex situation, a trial area was built, including several types of instrumentations for intensive monitoring of deformations and excess pore water pressures over time. Undisturbed samples were collected all over the soil profile and tested in the lab to determine the soil parameters to be used in the numerical model. An axisymmetric numerical model was built and calibrated using the collected data to predict the soil behaviour and the development of settlements due to the construction works. The analyses were performed using the soft soil model and the soft soil creep model; thus, the numerical investigations allow an evaluation of the long-term effects. The comparisons of the finite element analysis with the measured deformations presented in this paper show a very good match. Moreover, pre and post in situ measurements of standard cone penetration tests with sensors for the measurement of the pore water pressures (CPTu) have been used to calibrate the numerical model. These CPTu tests were used additionally to verify the improvement of the soil shear parameters over the preloading duration. It is shown that a combination of surcharge and vertical drains can be successfully used to improve the soft soil conditions present at the discussed project.