|Research projects_21||[email protected]||Wear-resistant materials in hydraulic Engineering|
Hydroabrasion includes all types of wear on wetted surfaces caused by water and sediments contained therein. It presents an ubiquitous and not yet satisfactorily solved problem which causes in hydraulic Engineering worldwide high maintenance costs. Particularly affected by hydroabrasion are hydraulic strucutres, which are exposed to high flow velocities and increased sediment transport. Next to dams, weir systems and construction diversions, also sediment diversion tunnels are affected. They contribute to a sustainable management of storage power plants by allowing sediment rich inflows to bypass the dam and in this way avoid or slow down the progressing silting-up of the reservoir. In order to be able to economically operate plants affected by hydroabrasion, not only the operation modus, but also the choice of materials for the internal lining plays an important role. However, there are still no generally accepted recommendations or guidelines. In many cases different linings are installed, refurbished and again substituted according to the “trial and error” principle, while a satisfactory solution in this context has not yet been found. To close this knowledge gap, the laboratory of hydraulics at the ETH Zürich (VAW) initiated a corresponding research project. The aim is to determine and optimize wear-resistant materials on the basis of field tests in the sediment diversion tunnel Solis as well as in the diversion tunnel Pfaffensprung, and to determine the situational most economical lining solution in consideration of the whole lifecycle costs.
The research is performed within the framework of a PhD program at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) at the Swiss Federal Institute of Technology in Zurich (ETHZ).
|Research projects_30||[email protected]||Hydraulic performance of stepped spillway aerators and related downstream flow features|
In order to prevent cavitation damages on spillways, bottom chute aerators can be installed. They have been extensively studied on smooth spillways. However, in the past decades stepped spillways have become widespread and research has shown that for high specific discharges they might be affected actually more by the cavitation process than smooth spillways. As consequence, and besides issues of energy dissipation, the specific discharge of stepped spillways is usually limited to lower values than smooth spillways. Until today there is no guideline for the design of such aerators.
Therefore, the hydraulic performance of stepped spillway aerators and related downstream flow features is now addressed by a PhD work performed at the Laboratory of Hydraulic Construction, LCH at the École polytechnique fédérale de Lausanne. The main objective of this research is to reduce this knowledge gap by in-depth studies of an aerator and its effects on the flow by means of a physical model. The model involves a 0.5 m wide channel, up to 8 m long, which can be supplied with a discharge of 240 l/s. Altogether six parameters, that is spillway slope, step height, approach flow depth, approach flow Froude number, deflector slope, and deflector height are varied in order to study their influence on the global air entrainment and the local air concentrations in the flow downstream of the aerator. With the results of this research, it is expected to establish general design recommendations for stepped spillway aerators.
|Research projects_33||[email protected]||High-Performance Concrete in Tunnels - The spalling sensitivity in case of fire|
Despite the rather good knowledge about the mechanical behaviour of High-Performance Concrete (HPC) at high temperatures, further studies related to spalling (to which HPC is very sensitive) are still required. Spalling presents a complex process, which is especially for tunnels of particular concern. It is a result of a combination of mechanical concrete decay with temperature, pore-pressure build-up due to water vaporization, and stress induced by both thermal gradients and external loads. How these different aspects influence each other is not yet completely understood.
In collaboration with the CTG-Italcementi Group, the Politecnico Milan launched therefore a research project with the purpose to investigate the interaction between pore-pressure and stress triggering explosive spalling. This is done by subjecting concrete slabs (800x800x100 mm) to a standard fire on one side. At the same time a membrane compressive stress of 10 MPa is applied and kept constant by means 8 hydraulic jacks as shown in the figure above. The membrane load helps to limit the onset of tensile stress, which might lead to cracking and to the consequent release of vapour. A steel frame restrains the hydraulic jacks, while the whole loading system is located on a small horizontal furnace. During the test, the development of pressures and temperatures is continuously monitored at 6 different depths, and transversal displacements are measured via LVDTs.
With this research project, which is co-financed by the foundation Lombardi Engineering, it is expected to obtain a better insight into the actual spalling process and its causes.
|Research projects_37||[email protected]||Hydro-Power Control Valve - Numerical study to develop an energy recovery control valve|
In hydraulic engineering, control valves are used to control various parameters, such as the pressure and flow rate in a pipeline system through the dissipation of flow energy. Within the framework of a research project launched at the Department of Civil and Environmental Engineering (D.I.C.A.) at the Politecnico di Milano, the performance of the so-called Green Valve, a control valve designed to use energy from the flow control process, is numerically evaluated.
The Green Valve concept is based on the idea that it is possible to recover part of the dissipated energy and to re-use it. But it is not only this positive environmental aspect in general that makes the Green Valve an interesting device, but also the possibility to produce off-grid energy that may be useful for many applications, such as for feeding monitoring and measuring tools or the valve actuator.
By means of a computational fluid dynamic approach this research allows to extend the fields of applications and to improve the performances of the device. It is aimed to fully understand the potential uses of the Green Valve involving CFD methods and to evaluate the environmental impact of this valve in terms of control performances and energy production.
|Research projects_61||[email protected]||Research Project for Soil Compactness Testing, SCT|
Liquefaction of alluvial soils presents one of the major risks for civil engineering projects and has caused already extensive damages due to foundation failures. Its determination is essential for the evaluation of the feasibility and the design of foundations in seismic areas.
The liquefaction potential depends mainly on the grain size distribution and the relative density. While first mentioned can be easily obtained, the relative density is especially difficult to determine since it requires an undisturbed sample. Therefore, usually indirect methods, such as SPT, are used. In 2007, partners from industry (Smartec), economy (Lombardi) and university (SUPSI) joined in a research project to improve the evaluation of liquefaction risk.
They developed an antenna to measure the average dielectric constant of saturated soil between two boreholes at various depths. The whole system consists of two mobile antennas lowered in adjacent casings and a portable main-unit. From the measurements, the water content can be easily derived and stored with a user-friendly software included in the system. Together with the soil granulometry, the water content allows then to determine the void ratio. From this, the relative density, thus the critical parameter for liquefaction risk can be derived. The application is however not limited to liquefaction, but gives relevant information on the soil compactness.
|Research projects_68||[email protected]||Large Deformation Simulation of Tunnel Collapses in Weak Grounds|
Tunnel stability problems have received a lot of attention since the early work of Peck who de-scribed the shape of the greenfield settlement. More recently, closed-form solutions have been proposed and permit the determination of the shape and magnitude of settlement. However, these are mostly based on elastic or perfect-plastic theories. Recent developments in numerical methods now permit large deformation simulations of tunnels. Adaptive Lagrangian-Eulerian (ALE) FEM has been used to model tunnel-induced ground deformations but suffers from its high computational cost as well as numerical issues related to the destruction and reconstruction of elements (remesh-ing). Discrete element method (DEM) simulations and smoothed particle hydrodynamics (SPH) simulations have also been used but these simulations do not include a support pressure and, hence, did not allow establishing the ground reaction curve. The material point method (MPM) is well suited for large deformation simulations. Formulated in the weak form and based on continuum mechanics, it permits the use of well-established models which facilitates its use as a design tool for practitioners for which more information is available in Fern et al. (2019)
This project aimed to investigate and validate the use of MPM for tunnelling applications by simu-lating centrifuge tests carried out by Potts (1976) and Mair (1979) for which a thorough geotechnical characterization of the soils are given. This permitted predicting the model parameters and, hence, the validation focussed on the use of the method itself.
The first series of simulations were two-dimensional (see figure below) and replicated the centrifuge tests with a plane strain configuration. The results showed that MPM was able to predict the correct s-shape ground reaction curve, the maximum settlement and the correct stress-strain distribution in the soil block.
The second series of simulations were three-dimensional and showed that MPM was able to predict the complex flow mechanism as shown in the animation below, although these simulations were computationally expensive.
© E.J. Fern 2019
Mair, R.J., 1979. Centrifugal Modelling of Tunnel Construction in Soft Clay. PhD dissertation. Uni-versity of Cambridge.
Fern, E.J. et al., 2019. The Material Point Method for Geotechnical Engineering - a Practical Guide, London: CRC Press.
Potts, D.M., 1976. Behaviour of lined and unlined tunnels in sand. PhD dissertation. University of Cambridge.