Effekter av packning av finfilter i fyllningsdammar

Sammanfattning: There are over 10 000 dams in Sweden of various types and sizes. Most of the larger dams were built around 1950-1970 when the Swedish hydropower was expanded. Since then, the knowledge of engineering dam structures has increased and there are new guidelines for how a dam should be built. Energiföretagen (the Swedish energy companies) have since 1997 published guidelines for dam safety, (RIDAS for short in Swedish), which describes how dam safety should be conducted. Since 1997 the knowledge and recommendations have changed. In RIDAS TV Embankment dams from 2020 the recommendations for compacting fine filter has changed from previous versions. In the new version it is recommended that the fine filter should be compacted as little as possible to allow the filter to collapse on itself to slow down erosions. This recommendation brought up a discussion amongst the dam-builders on how loose a filter should actually be compacted. Generally, a loosely compacted soil is less stable while a highly compacted soil is stiffer and can hold cracks open which promotes erosion. The purpose with this master thesis is to improve the understanding of how a low initial compaction degree affects the function of a filter. To answer this question, both a literature study and some laborations have been conducted. The literature study examines erosion, collapsibility, and shear strength for different degrees of compaction. To describe loosely packed soil and densely packed soil, dilation and contrancy is also examined. The main focus of the laborations was studying deformations for natural and crushed filters. The deformations were measured both in a static load test and a dynamic load test between two possible effective pressures in a theoretical dam. Crushing from compacting the filter was also examined by using modified proctor test with different packing energies. The two materials used in the study is a natural filter from Burvattnet and a crushed filter from Kiruna. The degree of compaction on the dam in Burvattnet was also determined during a field study for comparison with the published recommendations in RIDAS. From the laborations with dynamic load no change in grain size distribution was found. But it clearly showed that a varying load can increase the deformations with a high factor. By adding a dynamic load, the deformation increased from 50% to 400% if compared to only a static load, where the crushed filter had a slightly higher difference than a natural filter. The largest difference observed between crushed and natural filter was the degradation and change in particle size distribution that happened when the filters was compacted. From the work done some pro´s and con´s have been identified with loosely packed filters. With a low compaction degree the filter will be better at hindering the propagation of an erosion by collapsing and healing the cracks that appear. But by lowering the compaction degree the risk of erosion increases as a loosely packed filter is less good at filtering. Another effect that a low compaction has on the filter is that its inherent shear strength is lower compared with densely compacted filters, and this can lead to a slide of the embankment. A loosely compacted filter also suffers from larger subsidence that can create cracks if the surrounding soil is much stiffer. To be able to decide what rate of compaction that is most beneficial, more studies are warranted to be able to assess the risk of the dam degrading as a result of a change in compaction of the filter. By analysing the found pro´s and con´s effects on the probability on a dam failure the most beneficial compaction rate can be found.