Performance of embankment supported by piles with different lengths in soft clay

Abstract

Geogrid-reinforced and pile-supported embankment (GRPS) systems have been widely used in high-speed railway projects due to their economy. Most of the piled embankments are constructed on end bearing pile, however, at large depths of foundation soil, floating pile is more economical and technically feasible. The settlement of floating piles involves complex soil-structure interaction which is related with the pile lengths. The research aims at investigating the soil-structure interaction mechanisms under floating pile foundation with different pile length. There are mainly two approaches adopted in the research, i.e., centrifuge modelling and finite element analysis. To study the pile and soil response, the geometry in the Taizhou railway project was adopted in the centrifuge test. Centrifuge test was carried out in saturated clay to explore the long-term response of embankment supported by different pile lengths. Aluminium tubes are used to model the piles and the progress of embankment construction was simulated by adding water in the rubber bag on clay surface. Strain gauges are used to record the axial load of piles for enhance the understanding of pile-soil interaction under the embankment. Finite element analysis is used to conducted the back analysis and parametric study. The equivalent entity method is modified and the equation was validated with the centrifuge result based on assumptions. It is verified from the centrifuge test that the axial force along the pile shaft keeps increase during the long-term monitoring. The settlement of pile head in the end of loading is about 42% of the settlement in the long-term performance. In the end of the test, 25m short pile settles 12% more than 33m long pile. The axial load on the pile head increases in the long-term performance due to the higher geogrid tensile force. More skin friction is mobilized along the pile shaft, and the neutral plane in the end of test located at a distance of about 40% pile length from the pile head. The floating pile can release the effective stress of clay in the reinforced area, but the effective stress increase on the surface of the substratum. For the final ground settlement, the calculation of skin fiction was developed based on the design guideline which is close to the measured result. To consider the nonlinearity of compress modulus, the results from Equivalent method with a correction factor of 1.2 was adopted to match the measured settlement. The revised empirical method can have a better prediction on ground settlement of piled embankment. The effects of geogrid reinforcement on stress concentration ratio are significant (about 8 times). However, the influences of pile length/spacing and soil depth are limited (less than 35%). Moreover, considering a geogrid reinforcement with an optimum stiffness (4000kN/m), changing pile length/spacing can have a better performance in the transition section between the bridge part and road section.

Date of Award	2020
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology