IJkdijk

Abstract: The IJkdijk (Dutch for ‘calibration levee’) is an unique Dutch facility where full scale levees are built and brought to failure with two explicit goals: to increase the knowledge on levee behaviour and to develop and test new sensor technologies for flood early warning systems under field conditions. In 2009 four experiments have been carried out, aiming at failure caused by underseepage erosion (piping). Several innovative sensor technologies have been tested during these experiments. In this article the use of fibre optics is evaluated. To ensure a proper calibration of these experimental measurements, an extensive reference monitoring program has been carried out employing proven technology. The process of seepage erosion resulted in a huge amount of advanced sensor information. The results and predictive values of the measurements are analysed, compared and evaluated. INTRODUCTION The IJkdijk (pronounced as ‘Ikedike’ and Dutch for ‘calibration levee’) is an initiative where knowledge on levees and sensor technology comes together. As part of the IJkdijk program, in Booneschans, the Netherlands, levees are built at full scale and brought to failure with two explicit goals: to increase the knowledge on levee behaviour and to develop and test new sensor technologies for flood early warning systems under field conditions. This should increase both the quality of the levee inspection process and the safety assessment of levees. The final goal is to develop a toolbox to respond to flood threats timely with appropriate measures. The IJkdijk project functions as an open innovation platform for testing sensor techniques and to improve dike technology, providing benchmarking to all contributors. By conducting experiments in a controlled environment and under pre-determined conditions, knowledge about the failure mechanisms of levees can be improved and the value of new technologies can be demonstrated. At present, more than forty companies and institutions from five different countries cooperate in this initiative. The full-scale piping tests were performed at the location of the IJkdijk in the Northeast of the Netherlands. Two large basins were created (size 30x15m), each filled with a different type of sand, having a d50 of 150 m and 210 m respectively. These sands are denoted as ‘fine sand’ and ‘coarse sand’ in this paper. A clay levee with a height of 3.5m and slopes of 1:2 was built on top of the sand by densification of smaller clay lumps. After construction a levee with a 15 m seepage length was obtained, see Figure 1. At the downstream side, a weir has been created to keep the downstream water level at a constant level (approx 0.15m above the sand layer). At the upstream side, the water level could be raised to a level of 3 m. A total of four tests has been carried out: 1. Fine sand, with reference monitoring and little disturbing innovative monitoring techniques; 2. Coarse sand, with reference monitoring techniques only; 3. Fine sand, with reference monitoring techniques only; 4. Coarse sand, with reference monitoring and little and medium disturbing innovative monitoring techniques. In this paper results from the first experiment are presented. Figure 1. Cross-section and longitudinal section of the full-scale tests. The innovative sensor technologies which have been tested during these experiments comprised deformations, vibrations and temperature measurements by glass and plastic fibre optics, dynamic imaging by acoustics, waterleakage from self potential, deformation and well location with infrared cameras, and pore pressure, tilt and temperature from transducers integrated with MEMS technology. In this article the glass fibre optics measurements are evaluated against visual observations and pore pressure measurements from ‘traditional’ vibrating wire piezometers, laid in a dense grid at the interface of the levee and the sand layer. PIPING Piping, the process of retrograde erosion in sandy layers underneath clay levees, is considered one of the most dominant failure mechanisms of levees in the Netherlands (Havinga and Kok, 2005). The process, visualised in Figure 2, starts with heave and cracking of the soft soil top layer at the land side of the levee, caused by high water pressures which are easily transferred through the permeable sand layer. The cracks in the top soft soil layer allow for seepage. In case the water level difference between river and land side (the hydraulic head) is large enough, sand grains may be transported along with the water flow, thereby creating a pipe underneath the levee. Continued erosion may finally lead to instability and failure of the levee. Several models and empirical relations are available to predict the occurrence of retrograde piping, of which the most well-known are the empirical rule of Bligh (1910) and the model of Sellmeijer (1988), of which the latter describes the process in most detail. However, in a recently performed safety assessment of Dutch levees using the model of Sellmeijer, a discrepancy emerged between calculation results and the opinion of levee managers. Scepticism existed on whether piping would actually result in failure of the levee and the validity of the model was questioned. A large research program was started to validate and possibly improve the model. This program and its results are more extensively described in Van Beek & Knoeff (2009) and Van Beek et al. (2010). a) Heave e) Progressive erosion b) Seepage f) Instability of the levee c) Pipe-formation g) Breakthrough