Turbiditetsmätning vid övervakning av fyllningsdammar
Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
Internal erosion, which causes 1/3 of all dam failures and damage to embankment dams, is formed when the natural water leakage through the dam brings eroded material. Sinkholes on the dam crest are formed when the soil gradually falls down where a soil deficit develops due to erosion. When these holes on the surface are detected, the erosion has been going on for a long time. There are reported damages to embankment dams which have been preceded by muddy water downstream from the dam. Monitoring the turbidity in the leakage water can provide a good picture of the status of the dam regarding internal erosion. The dam safety industry shares the same positive view regarding the possibilities with turbidity measurements, but the knowledge is limited. The purpose of this thesis is to develop the understanding within the dam safety industry concerning turbidity measurements and to further improve the usefulness of existing equipment regarding detection of soil particles. This is performed by the use of a literature review, contact with representatives from other industries, and by laboratory experiments. Turbidity is measured by a sensor registering the amount of light emitted from a light source and reflected by particles in a liquid. More particles in the solution infer greater reflection of light, and therefore a higher turbidity. Turbidity is measured in different units but FTU, FNU (International standard) and NTU (American standard) has been accepted as primary standards of measurement. The reference solution Formazin is used to calibrate these measuring units and thereby ensuring the relation FTU=FNU=NTU. Turbidity less than 20 FNU in the present study is difficult to distinguish visually. Turbidity measurement probes which are used for continuous monitoring of embankment dams are installed in monitoring pits collecting leakage water from the dam. Knowing the amount of suspended solids in the leakage flow is of interest from a dam safety point of view, but turbidity can only be correlated to the amount of suspended solids if all particles are of the same size and shape, which is not to expect. This correlation should therefore be avoided when relating to embankment dams. When other users of the technology were contacted, no direct problems or disadvantages with the monitoring method could be identified. In a laboratory setup similar to a typical monitoring pit with a water flow and continuous turbidity measurement, known fractions of a moraine material of the type used as an impervious layer core in embankment dams were added while the turbidity was recorded. Effects due to the placement of the measurement probe and the influence of air bubbles and other factor were also examined. The placement of the measurement probe in the measurement pit, the effects of air bubbles in the water and other external influences, proved a marginal effect on the turbidity in comparison with the impact of the fine soil particles. Generally it can be concluded, based on laboratory tests performed, that the finest particles in the material examined, affects the turbidity the most, and primarily the clay particles. Kaolin clay affected the turbidity the most in the laboratory tests and this is partly due to its small particles, but may also be explained due to the large specific surface of the kaolinite clay mineral. The effect on turbidity measurements caused by the specific surface of the particles should be further investigated as it could have greater impact than the grain size. Particle analysis showed that particles smaller than 0.063 mm, which is likely to affect the turbidity, remained as sediments on the bottom of the barrel in the laboratory tests. For all materials with particle size less than 0.063 mm in the laboratory tests, an exponential relation for the decay of turbidity could be observed, even in tests with different grain size and quantity added. This trend could possibly be used when determining a minimum interval for continuous monitoring. Kaolin follows the same trend, but affects the turbidity more than twice as much as myanite and the moraine material. A mean value of the turbidity is continuously monitored, and for a large embankment dam it is less likely that an internal erosion process can fail to be detected. Nevertheless, there is a possibility that a dangerously high level of turbidity can fail to be detected when using a long monitoring interval. Turbidity probes, method of monitoring, monitoring interval and alarm level should be carefully chosen for the purpose of each project and which particles to expect.
Place, publisher, year, edition, pages
2011. , 121 p.
Life Earth Science
Bio- och geovetenskaper, turbiditet, inre erosion, fyllningsdammar
IdentifiersURN: urn:nbn:se:ltu:diva-51327Local ID: 887925dd-62c5-4c10-8a96-83b980418715OAI: oai:DiVA.org:ltu-51327DiVA: diva2:1024688
Subject / course
Student thesis, at least 30 credits
Civil Engineering, master's level
Validerat; 20110615 (anonymous)2016-10-042016-10-04Bibliographically approved