Settlement of a railway embankment on PVD-improved Karakore soft alluvial soil

Abstract

This paper presents a case history of the settlement performance of a railway embankment built on a Prefabricated Vertical Drain (PVD) improved soft alluvial soil of Karakore area, Wollo province, north-east of Ethiopia. The embankment was constructed on Awash - Kombolcha - Haragebaya (AKH) railway project for the purpose of preloading so as to facilitate the consolidation settlement in a short period of time; and it was monitored for more than 750 days. Numerical predictions of Class A and Class C (based on back-calculation procedure) were performed using Plaxis 2D. The soft soil creep (SSC) model was employed for the soft soil layers, while the Mohr-Coulomb (MC) model was used for the drainage and fill layers. The predictions were compared with the field monitoring settlement data. Quite a reasonable agreement was achieved from the finite element modelling of Class C prediction when the prediction was compared with the actual field settlement values. Parametric sensitivity studies were carried out to examine the influence of parameters on the rate and magnitude of settlements. The results of the numerical predictions and parametric studies have been discussed in detail. The soil on the flood plain area of Karakore is found to be very soft to soft alluvial deposit and formed over Tertiary Eocene stiff clay. The variable conditions of the soil formation and deposition highly dictated the deformation and excess pore water pressure behaviour of the soft subsoil sediment. The parametric sensitivity study showed that, following the compression index, the initial void ratio, the hydraulic conductivity, and the OCR highly influenced the settlement responses of PVD-improved soft alluvium soil respectively.