In the Netherlands, there is a history of living with water. Dikes, both at sea and at rivers, need to protect the country against flooding. The height of the dikes in the Netherlands needs to be increased in the future.
Adjustment is needed because of climate changes, raising sea level and ongoing settlements. If raising of the crest level is required, stability is best increased by widening the dike. However, this is not possible in case of existing buildings at the land side and in case narrowing of the river flow section is not allowed. Within this framework the project INnovations on Stability Improvement enabling Dike Elevations (INSIDE) is started in 2001.
Witteveen+Bos developed a new concept ‘dijkvernageling’, which stands soil nailing, reinforcing dikes by micro piles. This way steeper slopes are possible. The soil nails add to the stability of the dike, but do not take over the full load. The dike keeps functioning the way it did for hundreds of years. The soil nails increase the internal strength of the dike.
Research
Reinforcing embankments by soil nailing is a proven method to stabilise, especially in case of constructing a steeper slope in granular soils. But how does it work in dikes consisting of soft clay?
Dike reinforcement by soil nailing is mainly achieved by the anchorage. However, the shear connection by the soil nails is important for the performance of the soil nail. The research is focussed on the shear connection in clayey soils, as there is little experience on this subject. To investigate the behaviour of soil nail in clayey soils, large scale direct shear tests were executed to explore the strengthening effects with different types of soil nails in clayey soils. The tests were executed in the laboratory by two circular steel structures with a diameter of 0.9 metre and a total height of 1.2 metres. The soil nail was placed in the centre in the cylinder with a rotation possibility at the bottom. The upper ring could move by horizontal loading, while the lower ring was fixed.
Before performing the large scale direct shear tests, analytical and numerical finite element analyses (FEM) were carried out to define the soil nail properties such as diameter and bending stiffness in relation to the soil strength and the soil stiffness. After the tests, postdiction analyses were made to understand the behaviour and to improve the FEM model.
Conclusions
The testprogram improves the understanding of soil nailing in clay and dikes. Also, there is a good match between the test results and the FEM modelling. INSIDE ‘dijkvernageling’ has now been approved by the Dutch committee on design of dikes (ENW). Feasility study and design for actual dike improvement projects are expected for 2008.
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