Safety and maintenance issues for a long embankment on a karstic foundation under artesian pressure

The Riga run-of-river power plant in Latvia is located 20 km from the capital city of Riga in a rather flat area. The 380 m long powerhouse/spillway complex, designed for a head of 18 m, is flanked by 7.9 km and 7.0 km long homogeneous embankment sections on its right and left side, respectively. These embankment sections are founded on glacial, fluvio-glacial and alluvial deposits of variable thickness (3 to 10 m). They are underlain by a marly stratum which, however is not persistent, followed by a 6 to 12 m thick dolomite layer with solution features. The dam has performed well over a period of operation of more then 30 years. Of particular interest at this dam site is the foundation drainage which is affected by the karstic dolomite layer. The drainage water is collected in a drainage channel which has been rehabilitated in very recent time. It brings the water to the downstream area. The dolomite layer is partly fed by the reservoir creating an artesian condition with respect to the drainage channel. Pressure control is accomplished by relief wells, but still in some sections of the channel water from the dolomite flows upwards into the channel slopes, which can be observed as wet spots on the slope surface or as sand boils along joints in the channel lining. Of concern is also the flow regime in the dolomite layer which may lead to piping and to further dissolution of the rock. Rehabilitation and maintenance concepts for the drainage channel are briefly discussed.