An experimental study of piled embankments incorporating geosynthetic basal reinforcement

Abstract

Basal reinforcement along with individually capped foundation piles is used in cases where both embankment stability and surface settlement control are required. The technique has been utilised to prevent differential settlement between new embankment construction over soft soil and an existing embankment where settlement has ceased. The piled embankment solution is also adopted to prevent differential settlement between an approach embankment constructed over soft soil and the piled foundations of a bridge abutment. The study was conducted to investigate the behaviour of an idealised piled embankment incorporating basal geosynthetic reinforcement. Three-dimensional model tests at self-weight conditions were carried out to evaluate the effect of some of the factors affecting the arching mechanism and the development of surface settlement in piled embankments. The physical model was designed to represent a square grid of individually capped piles centrally located within an embankment. Three different pile cap sizes and four different geosynthetic materials were employed in the experimental study. A movable base supported on hydraulically operated jacks was used to model the soft ground. The use of a movable base permitted the simulation of a worst case scenario in which the soft ground was not involved in the load sharing mechanism. The experimental results indicated the existence of two modes of behaviour pertaining to a shallow and deep mechanism. The piled embankment geometry represented by a combination of height of fill, pile cap size and spacing was found to govern the mode of behaviour. The arching mechanism in the fill was found to be mobilised at a relatively small reinforcement deflection which supported the adoption of the two step approach utilised in designing the basal reinforcement. The circular and parabolic arc geometries were found to be adequate in describing the deflected shape of the reinforcement. The use a modified flexible cable formulation to describe the loaddeflection response of the reinforcement was found to be in good agreement with the experimental results. In addition, the validity of a number of current methods and recommendations relating to the design of piled embankments was addressed. A numerical study was undertaken using FLAC, a plane strain finite difference based programme. The calculated and measured results were compared to assess the suitability of modelling piled embankment behaviour using the finite difference programme. A parametric study was conducted to investigate the role of the basal reinforcement in the load transfer mechanism and in the prevention of surface settlement. The embankment geometry was identified as the significant factor influencing the reduction in differential settlement. A surface settlement mechanism was established based on results of the parametric study.