The influence of clay minerals on slope stability in landslide areas of the Polish Flysch Zone

Deręgowska A.M.

Institute of Applied Geology, University of Natural Ressources and Life Sciences, Vienna, Austria

aleksandra.deregowska@students.boku.ac.at

Landslides are one of the most common events that cause natural disaster. Major landslides in Poland occur in the southern part of the country in the Polish Flysch (Outer) Carpathian Mountains. Subdivisions along the arcuate mountain range of the Outer and Inner Carpathians are attributed mainly to structural, tectonic and lithologic differences. Till now, 20.000 places of appearing landslides have been counted. This is more than 95% of all landslides in Poland, which represents only 6% of the country.

The presence of slopes with geological and morphological factors and high level of precipitation are the main causes of the landslides. The temporal incidence of mass movements is strongly correlated with climate. Southern Poland received heavy rainfall during the summer of 2010, which led to, continued flooding and reactivation and mobilization of existing and new landslides.

The main reason of the occurrence of the mass movements are mostly expandable clay minerals contained in sedimentary rock. Due to their small grain size and other properties they influence the mechanical behavior of rocks very strongly. Clay minerals are amazing and destructive materials. They are called “quiet murders”.

The enormous scale of the sliding processes that had occurred in the Carpathian Mountains provided an opportunity to study the role of mass movements in landform development in mountains of medium height. The study focused on trying to understand how clay minerals determine the occurrence of landslides.

The Polish Outer Carpathians are dominated by Cretaceous to Tertiary (Paleogene) flysch deposits consisting of sequential layers of shales and sandstones deposited in a deep marine basin during continental mountain building. The flysch is often mantled with weathered material forming slope covers (clays, loams, silty sands and debris) [1] In Poland flysch deposits are divided into three categories: normal flysch (consisting of sandstone and shale beds of approximately equal thickness), shale flysch (shale beds are thicker than sandstone beds) and sandstone flysch. Landslides occur in all flysch deposits type: parallel and perpendicular to existing structures.

Sliding is a common process in the Beskid Wyspowy Mountain range. Slides occupy approximately 3% of the total area of mountain range [2]. The widespread is caused by local geology, which is dominated by flysch-type formations alternating layers of shale and layers of shale and sandstone as well as by presence of shale and debris cover that had evolved via weathering processes. These deposits contain layers with clay minerals having high and very high plasticity.

Generally two locations in the Beskid Wyspowy (the landslides in Kłodne and Łososina river basin) were chosen to show different ways of looking at the problem of slope instability. A very intensive and chaotic building construction in the Carpathian valleys and on the slopes adds to larger and larger damages in the area every year. The most recent activity in Kłodne began at 2 am on June 3, 2010 and continued about 6 hours. The resultant deformation with up to 12 m horizontal displacement at the toe destroyed 16 houses. During the period 1997-2010, a series of extreme rainfall events caused large-scale changes in slope morphology in the Łososina river basin. The most powerful event the rainfall of June 1997- produced 1340 landslides, mudflows and creeps on the slopes of the Łososina basin. This area conducted about 2000 landslides forms covering a combined area of 254,391 m3.

In all above, mentioned locations the relatively fresh material of the landslide head was sampled and investigated. The composition of the bulk samples and the clay fraction, grain size, water content, pH values, electrical conductivity, cation exchange capacity and carbon content were measured and the soil type was described. The mineral composition of the less than two microns of this material was determined by X-Ray diffraction. The method was carried out to detect clay minerals including chlorite and smectite in the soil samples. High amounts of mixed layer minerals, smectite and illite were found. There is a correlation between the occurrence of those minerals and the gliding tendency of observed slopes.

In the Beskid Wyspowy area landslides were activated in the first summer months, which is associated with content of water in soil and high groundwater levels. It is known that increasing the amount of water in soil leads to swelling of clay minerals. The presence of swelling clay minerals in Kłodne and Łososina causes instability of slopes. Differences in levels of slope stability are related to the different amount of expandable clay minerals in sediments and differences in water content in soil that directly influence the behavior of expandable clay minerals. Another factors which have important influence on instability of slopes are tectonics and anthropogenic influence.

The aim of this project is to make an assessment based on the methodological background and case study to explain the mass movements, which cause structural damages. The analysis of the results of the laboratory studies and field surveys should give information about the landslide problems in the Beskid Wyspowy.

References:

1. Wojcik A., Mrozek T., Granoszewski W.( 2006) Lithological conditioning of landslides and climatic changes with examples from the Beskidy Mts., Western Carpathians, Poland: Geografia Fisica e Dinamica Quaternaria, v.29, no2, p.197-209.

2. Bober J.( 1984) Rejony osuwiskowe w polskich Karpatach fliszowych i ich związek z budową geologiczną regionu. Biuletyn Instytutu Geologicznego 340, p. 115-158.