Feasibility study for stabilising the Plavinas dam, Latvia

Abstract

The stabilisation of the Plavinas dam, Latvia was the subject of this final thesis. The objective was to investigate the weaknesses of the Plavinas dam and to present possible stabilising treatments. This includes a rough estimate on costs and construction methods. Plavinas hydroelectric power station is the most important dam in Latvia, supplying 30% of the countrys power. The Plavinas blocks the runoff of the Daugava River, resulting in the formation of a reservoir. The power plant is a composite type structure consisting of a concrete power house and a spillway, which is located on top of the power house. The water from the reservoir can either flow through ten generating units located in the concrete power house, which is approximately 200 m long, or flow over the power house, the spillway. The head difference between the reservoir and the tailrace measures 40 m. Several kilometres of hydraulically filled embankment dams extend to both sides of the concrete power house. The dam is founded on glacial till overlying a sandstone layer. Drainage wells were incorporated in the foundation of the power house. This drainage system reduces the large uplift water pressures from the head difference between the reservoir level and the tailrace. This results in a larger effective weight of the structure, which is favourable for the bearing capacity of the structure against sliding and toppling. There are indications that the drainage wells, which can not be replaced under the concrete structure, do not function according to specifications: an increase of uplift water pressures is monitored. This could endanger the stability of the power house. transport of fine soil particles with the groundwater flow in the regional aquifer is occurring towards the drainage galleries near the right embankment as well as the drainage system underneath the structure. settlements of the power house have been observed, which seems to be caused by collapsing seepage channels. The power house structure and the aprons were schematised and the normative loading cases were determined. The stability of the structure was calculated for the following situations: original design assumptions in which the uplift water pressure is reduced by approximately 90% rising water pressures due to continuously less effective original drainage system complete failure of drainage wells without compensating treatments different treatments, increasing the stability of the power house in case the original drainage fails entirely, either by adding more weight to compensate for the larger uplift water pressure or by reducing the uplift pressure by an extension of the seepage path or new drains. The different stabilising treatments were analysed in a multi criteria analysis taking into account amongst others the problem elimination, durability and feasibility of execution. The alternative of new drains in the downstream apron close to the power house provided the best alternative.