17 LANDSLIDES IN LITHUANIA V. MARCINKEVICIUS 1 , R.B. MIKŠYS 2 , V. MIKULENAS 3 Geological Survey of Lithuania, Lt. 2600 Vilnius, st S. Konarsko 35, Lithuania e-mail: 1 [email protected], 2 [email protected] , 3 [email protected] Slope landsliding is the most important engineering geological process on the territory of Lithuania. There are known historical facts about the slope failures starting with the 16 th century. These are landslides of Gediminas and Bekešas hills in Vilnius. The Seredžius bricked church was washed down by Nemunas River stream, and collapsed into the valley 1829. The principal part of large slope failures of the past are remained unknown. Ca. 85–90 landslides have been studied or described during the last century (1937 and later). It is noteworthy mention, that about 60 % of landslides are fixed in Lithuania jointly. Landslide studies in Lithuania started beginning with the 19 th century. Systematic engineering geological research of slope failures in the city of Kaunas was carried out by Prof. M.Kaveckis since 1937. Engineers geologists from Kaunas University of Technology (F.Norkus, A.Žiedelis, P.Masiulis) performed research of slopes of the largest rivers in Lithuania 1960–1964. Since 1966, specialists from the Department of Hydrogeology and Engineering geology of Vilnius University perform study of landslides. R.Stulpinas, R.B.Mikšys, V.Vaitiekunas are interested with a problem of archaeological monuments - hillforts - slope stability and an application of engineering means to stabilise these historical sites. Recently specialists from the Geological Survey and Institute of Geology are able to apply quantitative or qualitative methods of landslide hazard assessment. Landslides damage both natural and artificial slopes. Natural: due shore abrasion of the Baltic Sea, Kuršiu Marios lagoon and lakes, bank undercutting by artificial water reservoirs, erosion of rivers and ravines, etc. Artificial: slopes in excavations and causeways (roadbeds) of traffic roads, walls of quarries of natural resources, slopes of the largest dumps; slopes of archaeological monuments. The landslide activity is related to location of the largest cities (Vilnius, Kaunas, Alytus and others) in the deep valleys of the largest rivers Nemunas, Neris and others. Due to human economic activity, with a construction on the slopes and their nearest environment, the stability and natural moisture balance of soils are often disturbed. During these works, when the strength properties and strength stress state of soil have changed, landslides could occur on the already urbanized areas. According to economical significance and their danger (by significance for the land-use), landslides could be subdivided into: 1. Urban areas; 2. Traffic roads; 3. Deposits of natural resources; 4. Protected areas and monuments; 5. Dumps of waste materials; 6. Other economically significant territories. Favourable engineering geological conditions for the landslide occurrence (geological structure, hydrogeological, geomorphological,technogenical), generally effecting a whole complex of natural environment, where the slope failures form and develop are as follows: geological structure - slope failures develop in the series of Quaternary sediments, formed of differently sized sand, sandy loam, loam and clay. Landslides develop sometimes on the traffic roads especially due to deformations of peat and silt. The most unstable slopes are composed of stripped clay characterised by liquid plastic or soft plastic consistency, or incompletely densed artificial grounds. Their best examples are landslides of Kuksa, Taurage, Rašnava clay deposits, also Vilkpede in Vilnius and Gandinga hillfort. hydrogeological (and hydrological) –caused by surface and underground water influence conditioning changes in consistency and strength parameters of clayey soils, or by hydrogeodynamical pressure, or when a filtrational stream effects the soil in the basic frontal part of slope. The last is very important on Lithuanian territory, because hydrodynamical forces provoke the unstabillity of slopes in the walls of natural resource deposits. The influence of hydrogeological factors is very often linked to suffosion process. geomorphological – Slopes are unstable in the natural geomorphological conditions due to erosional and abrasional strips, where the basic frontal part of slopes is washed out by water stream, and the steepnes of slopes increases till the stability limit. technogenical – Landslides are happened in Tonribis, Kuksa, Taurage and Rašnava quarries (pit) due to vibration, and on the areas of testing exploitation. They also occur during construction works, when practically used retaining-walls raise the local future changes of hydrogeological conditions to become worse parameters of soil strength. Very frequent causes are an increase of slope stepness by digging-off of their lower part, extra loading of slopes, damages in a water-supply system, removal of vegetation on slopes for building, etc. The landslide research at the time being is provided according to these topics: