Landslides

    Introduction

Landsliding is a gravitational earth movement. It is caused by geological and climatic conditions as well as relief morphology and human activity. All these factors aid or incurr a slope faliure. The development of a landslide can change the relief and cause new landscape structures to be formed. In populated areas, landslides damage and destroy infrastructure, buildings, telecommunications, pipelines and anything else in their path (Kirchner et.al. 1998).

In July 1997, the intense rainfalls caused many slope movements. The northern and eastern parts of Moravia were particularly affected due to the flysch rocks, which are susceptable to movement after heavy rain. One of the worst affected areas of Moravia was the district of Vsetín, where peoples lives and properties were at considerable risk. Over 200 landslides were activated during the period of heavy rain. Map shows the distribution of landslides in the Vsetín district.  

    The History of landslides in theVsetín District

The problems with landslides has been present for many centuries. In the past however, only well documented cases exist in populated areas such as Prague. The Carpathian Flysch series in and around the Vsetín district was less populated than the west of the country but periods of industrial development caused for some landscape surveying. The first documented landslide in the Vsetín district was in Hoštálková in 1919. It was a large landslide of 750m in length and 200-350m in width. It destroyed six farms and a small lake was formed as the landslide blocked a nearby stream. In the same year there was a large flood, which was probably a main cause that helped trigger the landslide. The recording of all the landslides in the country was made between 1956-1962. With every new landslide activated, the record gets more extensive and it is being peranently updated as part of the Slope Failures Register in Geofund, Prague.

In 1967, a landslide occured near the village of Oznice in the Hostýnské vrchy hills. The landslide was controlled by a synclinal geological structure, although it was 450m in length and 200m wide. An inhabitant of the nearby village of Napasekách found himself by the landslide at the most critical moment. He claimed that the movent of the slide was travelling the same speed as normal walking pace.

Studies of the geology of the area became more in-depth and important when the construction of several dams were proposed. The main dam was situated on the Stanovnice Brook, a tributary of the Vsetínská Bečva river from the Javrníky Mountains, near the village of Karolinka. Also the šance Dam on the Ostravice River in the Moravian-Silesian Beskidy Mountains was at risk from slope faliure.  

Geophysical research at Karolinka revealed a landslide which had an exceptional depth to the slope movements of 70m. Fortunately, no activation of this landslide occurred after the heavy rainfall of 1997. However at the šance dam, some landslide activation occurred. This was probably due to the landslide being permanently in contact with water from the reservoir. Although it was possible to remidiate the movment, these processes and techniques are very expensive and are only applied to the places at the most risk.

    Landslides activated in 1997, Vsetín District

    Geology 

The southern and central parts of the Vsetín District are composed of the Magura Flysch, which is the strata that makes up the White Carpathian Mountains, the Vizovická vrchovina highlands, the Javorniky Mountains, the Hostýnské vrchy hills and the Vsetínské vrchy hills(see map). The northern part of the district consists of the Outer Flysch Belt, which makes up the Moravian-Silesian Beskidy Mountains and the Podbeskydská pahorkatina hills (see map).

The actual flysch rocks consist of alternating layers of claystones and sandstones. These rocks have low permeability, which means that the surface of the rock and the interface with the alternating rocks will get saturated by water very quickly. This will easily cause a slip surface. However, tectonic faults and fractures promote access for the water and create shear zones for the activation of a slope movement.

Other types of material that affect slope movent include; loamy stone, loamy clay and loamy sand sediments.

    Landslide Mechanics  

    1.Slope Stability  

Most slope failures result from instability which may be caused by a number of different factors such as increased pore water pressure, self weight and decreasing shear strength with increased degree of saturation (Tsuchiya, 1999). The ratio of the maximum restraining force along a slip surface to the amount actually required for stability gives the factor of safety for a slope failure (Carter). The factor of safety is a figure used to convey the stability of slope. Parameters used to calculate the factor include; the effective stress, the effective soil strength and the pore pressure ratio.

For analysis, slope failure may be divided into different categories:

1.Plane slide

- surface slide

- subsurface slide

2.Deep seated slide

- wedge failure

- rotational failure

- failure along an irregular surface

All of these different types of slope failure require different analysis to determine the relative factor of safety. The plane slides offer the simplest calculations of the factor of safety. The deep-seated slides require more calculations as they are not as simple as the plane slides. It is necessary to calculate several sets of wedge failures to obtain a minimum factor of safety. This can be done graphically or by means of a computer program (Carter). The rotational slip surface assumes that the slide occurs on a circular arc and is therefore more difficult to determine. More information is needed to calculate the irregular slip surfaces’ factor of safety.

     2.The Factor of Safety

The factor of safety is a figure used to convey the stability of slope. Parameters used to calculate the factor include the effective stress, the effective soil strength and the pore pressure ratio (Carter). The end result is a number, which gauges the slope safety. Numbers from 1.4 to 1.6 are considered safe slopes. Below this and the slope becomes more susceptible to landslides. Draining landslides can increase the slope factor of safety by 0.05, but compact sand and gravel piles can improve the factor by 0.4, aiding the slope stability immensely.

     3.Water and Slope Stability

Studies of the mechanics of slope failure have concentrated on rising groundwater level due to heavy rain. Most examples of slope failures caused by rainfall in Japan have occurred at a depth of 1.5m. Due to this, the confining stress may not be more than 20Kpa. This suggests that the shear strength should be calculated at low confining stress (Tsuchiya, 1999). Experimental evidence was done for different types of soils but mainly, the shear strength decreases sharply at normal stress below 10Kpa.¨

Following these experiments, calculations showed differences between the strength of soils under natural water conditions (1Kpa) and submerged conditions or higher groundwater levels. Reasons for this are considered that the suction between the soil particles decrease proportionately to the amount of water added under submerged conditions. Other studies in tropical regions have proven differences between the rainfall intensity and the duration period. This is concerned with calculating the factor of safety and the variation that occurs between duration and intensity.