Růžďka(map)village  

This landslide is located in the centre of the village of Růžd‘ka of the Vsetín district. It is around 800m in length and 100-200m in width. The landslide damaged five farm buildings (see plates 2 and 3), local roads, gardens and the adjacent forest land.The Růžd‘ka stream, main village road and phone and power cables are also endangered. The landslide is still active especially when large amounts of rainfall occur. The movement varies between a mm per week to a cm per day(see map).

It is deeply founded with a 20m thick layer of sandstones which has been forced out towards the accumulation zone due to the underlying claystones.The accumulation part of the slide has reached up to 30m thickness and this is the area where the majority of the village is situated. The results of this slope movement have been very destructive.

There are many fens, springs, open water tables and wells present at the landslide. Fens can be found at the centre of the root area and at the sides of the accummulation and transport zones. Springs are also present here as well. The yeilds of the springs reaches 80-250 ml/s .

Several of the boreholes intersected the water table, which was found at 3.0m to 35.3m. This indicated a saturation level of the underlying prequaternary rocks (mainly sandstones). The pemeability of these is more than the clays of the quaternary deposits, which have low to zero permeability. This has caused the low angle slope at Růžd’ka to slide significantly due to the saturation and the permeability of the rocks, which have created very high pore pressures and hydrodynamic pressures. This has uplifted sandstone blocks located above sliding planes and caused a lot of damage.

    Geology of Růžd‘ka

The slope is dipping to the east at about 10-12^o. The rocks involved with the slide are of Paleogene age, which are covered by quaternary sediments of deluvial origin. The geology of the slope is very complicated due to the intense weathering that has occurred and also the repeated sliding process of the landslide. This creates a very complex sequence of bedding and so a series of boreholes were drilled to prove the geology of the slope.

    Geological borehole findings at Růžd‘ka

    Boreholes J1 to J6:

1)  quaternary deposits of deluvial origin, thickness (0.1-1.6m) apart from accumulation zone where deposits were 8.5m thick.

2)  quaternary deposits of  clays to sandy clays

3)  prequaternary underlying rocks of the Paleogene are present with weathered interbedded layers of sandstones, claystones and siltstones.

4)  Localised sandstones at depths of 0.7m to 20m. Claystones were also present at depths from 12.9m to 40m.

5)  Completely weathered sandstone with the texture of sandy clay. Fully saturated, uncompacted, and rounded grains at depths of 3-3.2m, 3.7m-3.9m and 5.2-5.7m, proving ideal slide surfaces.

6)  This borehole proved there are deeper sliding surfaces.

Laboratory tests of these borehole results above showed the material in the landslide had a soft to very soft consistency.

    Geophysical Research

The geophysical results showed the existance of a channel, which accomodated the landslide due to stronger blocks of sandstone on either side. The results also revealed minor sliding planes and zones of the different rocks present. A tree shaped tectonic structure was located on the top of the slope. This probably enabled slope saturation from the Palenisko hill, which is made from sandstone. These results also showed the large amount of weathered sandstone together with the alternating beds of claystones.

Conclusions about this data can be made, i.e. the tectonic drainage structure at the top of the slope fed the highly weathered sandstones with the increased rainfall of July 1997. This increased the pore pressure and hydrostatic pressure, which then excerted force on the sandstones and caused the slip between the interface of the alternating beds.

The presence of completly weathered materials, together with the increased water table reduced the slope stability and activated the slide. A channel structure with a narrowing width, (200m at the top, 60-80m at the bottom) reduced the sliding speed. This was favourable for the village but the sliding still contiues today. Currently the landslide moves at around 2-3cm per month.

    Stability analysis

Two sliding surfaces at Růžd‘ka were distinguished according to depth.

1)  Shallow slide surface. Depth of 8m. Mostly sandy clays or clay sands, with a soft to very soft consistency.

2) Deep slide surface, depth from 21-80m. Containing completely weathered and highly saturated beds of prequaternary rocks of sandstone and claystone.

For stability analysis, the GEOSTAR 3.3 software by the GeoTechnca Prague was used. This showed:

- Free choice of slliding surfaces

- Effective stress acceptance

- Saturaton of several beds with different pore pressures

- Automatic search of a slding surface with the lowest stability factor

- Simulation of external factors accepting slope stability

After this analysis, a final result is produced as a stability factor Fmin. When this is applied to unreduced parameters of angle of internal friction and effectve cohesion, Fmin should be between 1.4 and 1.5 (to say that the slope is stable). If the Fmin is between 1.05 and 1.2, the slope is at a stage of slow permanent set. Below Fmin 1.05, the slope can be characterised as unstable.

The parameters for this analysis were based on slope conditions but mainly on underground water tables.The following results were found:

- For the sliding surfaces located at the transporting and accummulation zones

Fmin= 1.1 to 1.23

- For sliding surfaces located at the root area

Fmin= 1.05 to 1.14

The stability factor results between Fmin 1.0 and 1.15 suggested that slope movements are active at different speeds at different sliding levels. To slow down or stop the movements it is necessary to drain the sliding surfaces to regain the shear strength of the slope material.

    Remediation  

The following steps were suggested for the remediation of the Růžd‘ka landslide. Two stages were proposed, drainage being a priority in both of them. 

   First stage (upper part)

- Drainage of slopes by means of four rows of sub-horizontal boreholes and two rows of sandstone piles (see plate 3).

- Establish surface ditches to drain the upper part of the slope and remove the water form the boreholes.

- Large diameter tamped gravel piles in a belt of about 80m, to increase landslide strength.

- Two anchored pile walls 50m long stationed 500m from the root. Pile length 15m and bearing load, 25 Kpa.

- Observaton and monitoring boreholes, to follow levels of underground water in thelandslide and to measure the dip of the slope (inclinometers).

 Second Stage (lower part)

- Compact, large diameter sand and gravel piles at a depth of 10-11m, with 8 rows in a belt across the lower part of the landslide. Diameter of a plug is 1.2m.

- Anchored pile walls, 265m from the heel of the landslide. 15m pile length and a load pressure of 80-100 Kpa.

More geophysical research was suggested to explore the deeper slide surface. This included 2 boreholes, 40m deep and the use of geophysical radar exploration. This would verify the existance of the deeper slide surface and further work could be undertaken to remedy this.

For example:  

- 32 horizontal boreholes to the depth of the deeper sliding plane i.e 35-60m deep

- A belt of 50 compact sand piles, connected to the horizontal dewatering boreholes.

- A network of surveying points and updated monitoring each month.

- Installation of 2 inclination boreholes, 25m deep.

The expected costs for such remediation does not come cheaply. The budget for such measures is as follows:

1^st Stage total                                           approx.   3,530,000 CZK

2^nd Stage total                                          approx. 36,500,000 CZK

Total (inc. VAT)                                         approx. 40,030,000 CZK