Flooding Speed of Former Lignite Open Pits Regarded as a Determining Factor on the Stability of Final Slopes

Surface mining is the main exploitation method of lignite in Europe. EU lignite production since 2017
is somehow stable at aprox. 380 million tones/year and is a very important source of energy in
Poland, Germany, Greece, Czech Republic, Romania and Bulgaria. Flooding the residual gaps of
former lignite open pits is often applied worldwide, offering opportunities for the use of the lake and
the reclaimed land, as well as many benefits, ranging from the restoration of ecosystems to the
development of the local economy. The flooding process is conditioned by the available quantities of
water from natural and/or artificial sources. Flooding can be done either by natural methods
(underground water inflow plus intake of water from rainfall and superficial drainage), or by natural
and artificial methods (water adductions from superficial collectors). Unlike artificial flooding, natural
flooding has the advantage of reduced costs, but it is a long-lasting process. Depending on the
flooding speed, as a result of rising water level and saturation of rocks from the final slopes,
geotechnical phenomena such as landslides may occur. The paper aims at establishing the
dependence between the rhythm of raising the water level in the lake and the geotechnical behavior
of the rock in the internal dump. Depending on this, there is a need to accelerate the flooding process
through adductions, as the hydrostatic pressure manifested on the final slopes determines the
increase of their stability reserve.