No abstract

A programme of 20 centrifuge tests was carried out on piled embankments without reinforcement to assess the influence of geometric parameters on surface settlement and on the load transfer mechanism. The three main geometric parameters studied were pile spacing, embankment height and area ratio, which were varied in ranges commonly adopted in practice. The measurements of the forces on the piles allowed assessment of the load transfer mechanisms, and thus the efficiency, which were shown to increase with embankment height and area ratio. Subsoil stress between piles was also assessed and compared with literature data. The centrifuge model test results for stress on the subsoil at the point of ‘maximum arching’ presented good agreement with the method proposed by Hewlett and Randolph in 1988. Measurements of the surface settlements allowed evaluation of the critical height at which surface differential settlements did not occur. The values of critical height obtained herein are in good agreement with recent literature recommendations.

Rigid piles are used to reinforce soft soil base and increase embankment stability. This technique is improved by placing one or more geosynthetic reinforcement (GR) layers inside or at the base of the embankment. A series of 33 small scale models has been tested using a geotechnical centrifuge. Soft soil settlement is imposed by the downward displacement of a tray. First, a series of models is prepared to examine how the load transmitted to the pile network increases with the embankment thickness. For a same configuration, two identical models are prepared to test successively two different types of GR (Geosynthetic Reinforcement). Another approach consists in studying how the external surcharges applied on the embankment affect load transfer. The results show that, in comparison with the piled embankment, the load transfer is increased in the case of the Geosynthetic-Reinforced Pile-Supported Embankment (GRPSE) due to the membrane effect. The membrane effect is higher when the GR is stiff and its vertical distance from the pile is reduced. Numerical modelling reveals that, when adding a GR layer, the second GR has an effect only if punching is sufficient. However, the benefits of it could not be highlighted here.

No abstract

The soil reinforcement method, which consists of placing a granular mattress above a set of rigid inclusions, is used to transfer part of the surface load to the piles through arching. The addition of an extensible geosynthetic layer at the base of the mattress increases load transfer. A small-scale centrifuge model, the mobile tray device, is used to simulate the foundation behaviour by modelling soft soil settlement between the inclusions. A parametric study was conducted to examine the influence of the mattress thickness and of the pile spacing. The different test configurations are compared with regard to the stress transmitted to soft soil, to differential settlement and to the efficiency of load transfer. With thicker mattresses and/or closer pile meshing, load transfer increases and surface settlements are reduced. Geosynthetic maximum experimental and analytical deflections are also examined. For the analytical study, conducted using the EBGEO German standard, three different distributions of the vertical stress applied to the geosynthetic layer are compared: uniform, triangular and inverse triangular. All three scenario results fall within the range of the experimental results. The inverse triangular distribution reveals some deflection values that were lower than the uniform one. The triangular distribution gives the highest deflection values.