The lower area of the reserve spillway – water dropping sump

Work by methods of engineering seismic and electrical tomography performed on inaccessible for the drilling island site in the central part of the riverbed, drained while stopping the operation of the Plavinas power station and the lowering level of located below the cascade of reservoir of Kegums HPP. The main objective of the work was defining the contours of the edges of buried pre-glacial valley of Daugava river, cutting the Plavinas sediments of the upper Devonian bedrock dolomites through the construction site of the Plavinas HPP dam. The steep side of the valley, along which occur the landslide and karst processes and the increased rate of filtration of underground waters, are disadvantageous to device of heavy concrete foundations and walls of the water dropping sump.

Section of longitudinal wave velocities along the axis of the emergency spillway through the bed of the duct

To determine the depth of the bedrock dolomite occurrence, longitudinal wave velocity sections calculated according to the method of refracted waves were used. The position of the dolomite surface in the dry part of the channel with a width of about 105 m is obtained from seismic records from the receiving geophone line located on the island and the excitation of oscillations on the shore.

 

Section of longitudinal wave velocities on the island section of the drained river bed

On palaeovalley edge the immersion of the roof of the dolomites more than 6 m is marked.

The section of longitudinal wave velocities along the profile oriented at an angle to the stretch of the valley side

The depth of the dolomite roof in the initial part of the profile reaches 20 m. Most immersed part of the valley filled with water-saturated moraine loam is also displayed on the geoelectrical section, obtained by electrical tomography.

Assessment of physical properties of moraine soils

By failing to achieve at the island area data of the cone penetration test and sampling for laboratory tests to classify types and evaluation of physical properties of morainic deposits, filling the valley, the modified Olsen nomogram was used taking into account statistical dependence and adjusted according to the geophysics and geotechnics on the banks obtained in local conditions.

Section of predicted values of the general deformation module of moraine soils on one of the profiles on the island