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Deltares | R&D Highlights 2015 Delta Infrastructure
A solid foundation for
piled embankments
Further reading: Van Eekelen (2015). Basal Reinforced Piled Embankments. PhD thesis Delft UT, downloadable from: http://repository.tudelft.nl suzanne.vaneekelen@deltares.nl
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Eighty percent of the world’s population lives on compressive soft soil. Building in and on such soils is a challenge. The weight of a road construction, for example, causes compression of the soft soil, resulting in a bumpy and unsafe road. One of the solutions is to apply a piled embankment reinforced with geosynthetics. This doctorate research examined the design of the geosynthetic reinforcement.
The study started with a series of experiments in a test set-up with four small piles and a foam cushion in between them. This cushion was saturated with water and wrapped so that it was leak-proof. A tap was installed on this structure. When the tap was opened and pressure applied from above to the saturated foam cushion, the water was squeezed out, simulating the compression of soft soil. The next step was to put the geosynthetic reinforcement on top of the piles and the cushion, and to complete the set-up with the embankment. A water cushion on top allowed for a large top load.
On the basis of experiments in this set-up, an analytical model was developed for the mechanism active in the geosynthetic reinforcement and the soil. This analytical model is based on a set of concentric hemispheres and arches. The load first travels along the 3D hemispheres until the hemispheres intersect and then the load travels further along the 2D arches towards the baseline. The larger the arch, the more load it transports and the more load it exerts on its subsurface.
This Concentric Arches model results in a load distribution on the subsurface that matches the load distribution observed in the experiments. This distribution, however, is very different from the load distribution used in older design methods. The strain in the geosynthetic reinforcement can be calculated on the basis of the load distribution.
The figure shows the improvement obtained with the new Concentric Arches model. The strains measured in twelve field and laboratory projects are shown on the horizontal axis. The left-hand pane shows the corresponding strains calculated with an old method on the vertical axis; the right-hand pane shows the strains calculated with the new Concentric Arches model. The result with the Concentric Arches model is much better than the result with the old method. On average, the old method calculates a strain that is 2.5 times the measured strain; the new model calculates a strain that is 1.1 times the measured strain. In other words, the new Concentric Arches model gives less extreme values and a much better match with the measurements in these twelve projects. The new Concentric Arches model has therefore been adopted in the new Dutch design guideline that will be published in 2016.
Comparison of measurements and calculations
The test set-up The Concentric Arches model
