2 ARCADIS Nederland B.V., P.O. Box 220, 3800 AE Amersfoort, The Netherlands. 3 Van der Meer Consulting, P.O. Box 423, 8440 AK Heerenveen, The ...
LARGE SCALE TESTS OF BOULDER CLAY EROSION AT THE WIERINGERMEER DIKE (IJSSELMEER) Guido Wolters1, Jan-Willem Nieuwenhuis2, Jentsje van der Meer 3 and Mark Klein Breteler 4 A large scale model investigation (scale 1:1) of the erosion resistance of boulder clay in dikes under wave attack was carried out in the Delta Flume of Deltares|Delft Hydraulics. The investigation showed that boulder clay has considerable erosion resistance, indicating that the Wieringermeer dike can withstand extreme storm conditions, at least several hours, after the stone revetment has been damaged. However, depending on the location of the dike section, the local wave conditions for safety assessment and the thickness of the boulder clay layer, the remaining strength is not always large enough to meet the assessment conditions without the additional protection of a toe berm and/or a revetment. The investigation indicates that the erosion resistance of boulder clay could be taken into account in the safety assessment of dikes.
INTRODUCTION
Dutch dikes are typically tested every five years on their safety. The latest assessment showed that a part of the dikes along the IJsselmeer does not meet the safety criteria anymore. The dikes in that particular section have a core of boulder clay and a revetment of natural pitched stone blocks. At the Wieringermeer dike the latter have deteriorated over the last years. The dike is further characterised by a stone berm of varying thickness up to a height of +0.5m NAP (Normal Amsterdam Datum MSL) which lies in front of the dike. The goal of the study was to investigate if the boulder clay core still provides sufficient erosive resistance when the revetment has completely failed without necessity of refurbishment. The core material, boulder clay (in the Netherlands termed keileem), is a material consisting of large fractions of clay, silt, sand and, to a lesser degree, shingle. It has a high density of approximately kl = 2000 kg/m3 and a low permeability. Previous field experience (INFRAM, 2005) indicates that boulder clay can have a considerable erosion resistance even after the revetment has failed. Since previous studies were conducted at varying model scales, they have always been affected by model effects. It was therefore decided to investigate
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Department Coastal Structures & Waves, Deltares | Delft Hydraulics, P.O. Box 177, 2600 MH Delft, The Netherlands ARCADIS Nederland B.V., P.O. Box 220, 3800 AE Amersfoort, The Netherlands 3 Van der Meer Consulting, P.O. Box 423, 8440 AK Heerenveen, The Netherlands 4 Department Coastal Structures & Waves, Deltares | Delft Hydraulics, P.O. Box 177, 2600 MH Delft, The Netherlands 2
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2 the erosion resistance of the dike at a scale of 1:1 in a large scale model investigation conducted in the Delta Flume of Delft Hydraulics. For the erosion tests a section of the Wieringermeer dike was excavated and installed in the Delta Flume (without revetment) where it was exposed to wave conditions typical for the IJsselmeer. BOULDER CLAY (KEILEEM)
Boulder clay is a clay-like material which has been employed in dikes since the 1920s. In contrast to modern dikes was the clay-like material not only used for a thin cover layer, in order to prevent water ingression, but also formed a substantial part of the core material of the dike. Boulder clay consists of varying fractions of clay, silt and sand with pieces of shingle. The high gradation ensures a high density of approximately kl = 2000 kg/m3 and a low water permeability. In its characteristics it can be compared to boulder clay. Recent literature studies (e.g. GeoDelft 2006, INFRAM 2005, LGM 1985) indicate that boulder clay has more erosion resistance than current assessment methods take account of. During the construction phase of the Wieringermeer dike for example the unprotected boulder clay dike was able to withstand flow velocities of 3-3.5 m/s without failure. However, boulder clay seems to be sensitive to any kind of disturbance or dynamic loading, which can undermine its shear (strength) significantly. This is mainly caused by its high sand fraction (in contrast to clay). By compaction and the passing of time lost strength can again be regained. For more information on the material properties of boulder clay in general and the investigated material from the Wieringermeer dike reference is made to Delft Hydraulics (2007), GeoDelft (2006) and Fugro (2005).
Figure 1. Cross section of the Wieringermeer dike (IJsselmeer).
EXCAVATION PROCESS
The large scale tests were conducted at a scale of 1:1 in the Delta Flume of Delft Hydraulics. The installed dike section was directly taken out of the old Wieringermeer dike, next to a section blasted during the second World War but
3 which has no water defensive purpose anymore. The material was transported in blocks of 10 ton and installed in the Delta Flume, see Figures 2 and 3. The tests were conducted without revetment in order to test the erosive strength of the boulder clay material alone. After the tests the excavated section of the Wieringermeer was restored and vegetated.
(a) Boulder clay blocks with supporting framework (b) Lifting of boulder clay blocks Figure 2. Excavation process at the Wieringermeer dike.
Before excavation the top layer of the dike (disturbed gras/root layer of 1m thickness) was removed. 35 boulder clay blocks of 1.8m x 1.3m x 1.6m were carefully cut out of the old dike using rectangular, at the bottom sharpened metal frames (low friction between steel sheets and soil), which were slowly pushed (not vibrated) into the dike after each other, see Figure 2. In this manner the block was incased in sheets of metal. Then the ground around the sheets was excavated, a pointed plate pushed beneath the block and the block lifted by a pallet fork out of its position. Between blocks sufficient distance was kept to ensure that excavation work did not disturb the quality of the next blocks. The above described process guaranteed that the disturbance of the boulder clay material was kept to a minimum during excavation and transport. The maximum deformation of the blocks was about 3-5cm (
