Reasons and mechanism for soil sliding processes in the Rebrnice area, Vipava valley, SW Slovenia

A motorway Razdrto–Podnanos is being built across Rebrnice area in Vipava valley. There is limestone scree material deposed on flysch rocks. Limestone scree is unstable and gradually slides downslopes because of slope inclination, underground water and clayey zones. Clayey zones have been created on the contact between scree and flysch rocks. Material movements can be observed on the surface but they were also measured in wells. Limestone scree composition and reasons for landslides were described in this article.


Reasons and mechanism for soil sliding processes in the Rebrnice area, Vipava valley, SW Slovenia Introduction
The Rebrnice area extends over the part of the southwest slope of the Nanos, between Podnanos and Razdrto. To the west, it proceeds to the Vipava Valley, to the southeast, to the Piv{ka kotlina basin. The area is crossed by a regional road. At the moment, a motorway section Razdrto-Podnanos is being built here. The area dealt with in the article stretches from the Nanos limestone walls to the flat land near Lozice, which is a part of the Upper Vipava Valley.
Today's geological structure of the broader area results from early Tertiary thrust tectonics (P l a c e r, 1981). Cretaceous limestone is thrust on the Eocene flysch rocks. The belt of flysch rocks, going past the southern edge of the Nanos into the Vipava Valley, is a part of the Snežnik thrust sheet (P l a c e r, 1981; Janež et al., 1997). The Hru{ica nappe together with the Nanos as a part of it is then thrust into the rocks of the Snežnik thrust sheet. Because of physical weathering of the Upper Cretaceous rudist limestone, a large amount of limestone scree material has been formed and deposited on the underlying flysch rocks.
Recently, a lot of detailed geological investigations of the terrain have been conducted for the purposes of building the motorway. There are many problems concerning the building of some objects, such as digs and viaducts, because of unstable rock deposition. To consolidate them, expensive sustaining constructions have to be built.
With data, obtained from the engineerical-geological mapping, and the ones gathered from the geomechanical wells, an engineerical-geological map has been made. The purpose of mapping was to find out the expansion of scree material and the dip of the flysch rocks as well as to disclose the signs of scree fossil sliding. On the basis of the collected data it is possible to explain the dynamics and the mechanism of sliding processes.

An engineerical-geological map
The Rebrnice area has never been dealt with as an independent unit in the literature separately although it has been included into the treatment of the Nanos and the Vipava Valley by many authors. Beside H acquet (1789), who wrote about a huge amount of scree material in the Vipava Valley, there were many others who followed him later, dealing with the southern edge of the Nanos, taking into account broader area Stur (1858), Stache (1889), Kossmat (1905), Winkler (1924) and Limanovsky (1910. The geological situation of this area was also described while mapping the Basic Geological Map 1:100 000, as well as the Gorica (Buser, 1986) and the Postojna (Bus er et al., 1967, Pleni~ar, 1970 map sheets. Many data were obtained from geological mapping of individual springs and its hinterlands in the High altitude karst area (Jan ež et al., 1997).
The area consists of part of the slope at the motorway section between 4.7 and 6.3 km, in width 2 km. Eocene rocks are developed in a typical flysch. Mainly, there alternate grey shalley marlstone and quarzitic and carbonatic sandstones. On few places outcrop thick layers of calcarenit and calcrudit as well as the layers of greenish grey claystone. The general dip inclines towards northeast, the rocks dip into the slope. With the exception of calcarenit and calcrudit, flysch layers are impermeabil and form underground hidrogeological break for the Nanos karst water (Janež et al., 1997). Cretaceous limestones, of which the Nanos Plateau consists, are thrust on the Eocene rocks. The southwestern edge of the plateau is composed of the thick-layer organogenetic rudist limestone of the Upper Cretaceous, Senonian age (Buser, 1973). Both the thick limestone and the underlying flysch layers dip into the north or northeast. Physical weathering and the Cretaceous limestone disintegration cause the formation of scree material, which is then deposited on the flysch rocks in the form of slope talus. Scree material, which can in some places be formed as breccia, covers approximately 2/3 of the area. From under the limestone walls, the zones of the tongue-shaped scree material, which forms the ridges or ribs, expand towards the valley. The thickness of sediment is difficult to determine on the terrain. As it can be concluded from relief configurations, it exceeds 30 m in some places. The position and contact of both Cretaceous and Eocene rocks and Quaternary slope sediments are shown in Figure 1. In it, the profile A-B can be seen on the engineerical-geological map. It is located near the village Podgri~ and is directed NE-SW. The thickness of scree material is measured in the VK-2 geomechanical well (U m e k, 2000).

Scree material and breccia
Larger part of the Rebrnice area is covered with scree material and breccia. The processes occurring there depend on the contact of the sediments. There are very few outcrops on the terrain, hence a detailed description of scree is taken from geomechanical wells and profiles in larger digs. The wells located on scree material have perforated four main horizons ( fig. 2). The upper horizon is composed of limestone scree, namely limestone fragments and blocks of different size. Scree is sometimes transformed into breccia. Sharp-edged limestone fragments are 5 to 15-centimetre large. There can also appear 0,2 to 2 metre large individual blocks in scree material. Limestone blocks and breccias are partly karstified or cavernous. Scree is usually sandy, silty or locally clayey, ranging from fair brown to grey. The density of usually permeabil scree is determined by clayey and silty grains. Hinterland water is not filtered before the bottom of that horizon. The thickness of the horizon changes significantly; in one of the wells, for instance, measures 45 metres. Downwards, there follows the second horizon, composed of mixed talus of flysch components and limestone. Inside it are found individual larger limestone blocks together with fragments of flysch sandstone. Mixed talus is usually more clayey than pure limestone scree. Water is filtered thro-ugh more permeabil parts of the horizon. In-situ weathered flysch rocks, being partly semi solid rocks and partly disintegrated into clayey scree, form the third horizon. Marlstone and sandstone grains measure up to 5 cm. Also in this horizon is hinterland water filtered through more permeabil parts of it. Scree ranges from dark grey to dark brown. The lowest -fourth -horizon is of solid rock, composed of flysch marlstone, siltstone and fine-grained sandstones. In areas where flysch rocks are not covered with limestone scree and breccia, the upper two horizons are missing. They are replaced by clayey weathered residual of flysch rocks with fine flysch scree.
Hidrogeological characteristics of limestone scree and breccia change locally (Janež et al., 1997). Depending on gradation and the quantity of fine grains it is permeability that changes a lot. More permeabil scree consists of thick fragments of limestone which has fewer clayey and silty grains. In general, scree is of good permeability. Underground water flows into sediments between the more clayey zones, which are usually found either in the lower part of the upper limestone horizon or in the second horizon. This can also occur in numerous geomechanical wells. Therefore, water appears on the contact between scree and flysch. The contact is difficult to determine because it is found in the zone where scree limestone and flysch material are mixed. Underground water flows under scree, thus worsening the already bad geomechanical characteristics of the clayey zones. The flowing can be observed by numerous springs emerging in the lower parts of the scree cover ( fig. 1).

Dynamics and reasons for sliding processes
Due to the position and extension of the sediments on the Rebrnice area, scree material moves in forms of various slope processes, which can be seen from the movements measured by means of inclinometer wells and by some recent landslides. Sliding has already caused the regional road deformations, the cracks on some new motorway sections and the damage on the objects in the village Lozice. The movements in geomechanical-inklinometer wells measure between a few millimetres and a maximum of 15 milimetres monthly. Taken into account are particularly the movements in the K-2, VK-3 and POL-I1 wells, all located on the objects The Polance Dig and the Na Polancah Viaduct (U m e k, 2000, 2004). Approximately 500 metres southeast of Lozice, above the motorway, the Rebrnica landslide activated in spring 2001. Above the larger dig slided around 400,000 m 3 of scree material. A shear plane was formed on the edge between scree material and flysch rocks. There formed up to 3-metre wide scarp high in the slope. The landslide was stopped by the anchor pile wall.
Limestone material, resulting from physical weathering of the Nanos southwestern edge limestone, is deposited on the underlying flysch rocks in the form of slope talus. Due to continuous increase of the quantity of material, slope talus starts to slide on the foundation because of gravity. In the upper part, it slides across the circular shear plane, resulting in numerous flat lands along the Rebrnice limestone walls. Slope talus, which is usually inclined by 45°, reached a horizontal position because of the circular sliding. In the hinterland, scree is formed simultaneously, hence the repeating of the processes. Younger slope talus pushes forward the older ones, which have meanwhile formed into breccias. Thus has the scree material slided downwards to the valley.
Causes for such sliding and minor movements are combine with one another. Firstly, the Rebrnice slope is moderately-inclined with the average slope inclination of 15-20°, which can already cause the sediment to slide because of gravity. More inclined is only upper part of the area, where slope talus is formed. Secondly, on the contact of scree and flysch rocks there appear clayey zones with bad geomechanical characteristics. Shear planes are formed there. Besides, underground water flowing in those zones, adds to the decrease of clay geomechanical solidity. Lastly, with building dips through scree ridges, the slope sediment stability above the motorway section is severely decreased.

Conclusion
Recent movements of large masses of limestone scree material and breccias are only a part of the sliding occurring already in the past. This can be seen from the tongue-shaped scree covers, which is typical of great regional landslides ( fig. 1). Another consequence of fossil landslides is the mixing of limestone fragments and the fragments of flysch rocks in the second horizon, on the contact between limestone scree and flysch. While sliding, weathered flysch rocks mixed with limestone fragments. If limestone scree had been normally deposited on the flysch, there would have been no mixed horizon. Clayey zones, appearing inside this horizon or just above it, enable the filtration of underground water, which also adds to the formation of shear planes. The influence of filtration of underground water on the formation of a shear plane can be seen in the VK-2, VK-3A and POL-I1 inclinometer wells. In the VK-3A well the movement at the depth of 15 metres was observed (Umek, 2004). The zone consists of partly damp clayey limestone scree and flysch with the brown clay in between. In the VK-2 well the movement occured at the depth of 26 metres, in the zone of strongly clayey fine limestone scree. At the depth of 25.8 m the inflow of ground water was observed. At the depth of 26 m, the movement was measured in the POL-I1 well in the zone of clayey limestone scree and flysch with the insertion of reddish brown clay. The sediment is damp in some parts. In the same well, the inflow of hinterland water was observed at the depth of 29.7 m.
The mentioned landslides can be arranged in separate categories. According to the type of sliding material, they belong to soil landslides. They can be grouped under compact regional landslides due to their geological structure and the depth of a shear plane. The emersion of slides on the wellknown shear plane is conditioned by oblong shear plane shape condition. With respect to the slide speed, they are slow-sliding. The shape of landslides is simple. According to the quantity of containable water the scree material belongs to dry material.
Although slow but active sliding in the Rebrnice area affects regional road and a motorway (Razdrto-Vipava), the villages Lozice and Podgri~ are not under threat. This could, however, change in the future in case of catastrophes, such as a sudden quick slide of larger quantities of scree, caused by unfavourable natural conditions (e.g. longlasting rainfalls, earthquakes or a combination of both).
There have been many engineerical-geological investigations carried out in the Rebrnice area to resolve the difficulties in building a motorway Razdrto-Podnanos.
Issues concerning slope processes are also topical. Due to the fact that the processes are directly interconnected with the structural geological elements, a detailed structural geological mapping of the whole slope of the Nanos from Razdrto to Vipava would be appropriate. Thus the data on the exact position of thrust plane, flysch bed position, tectonic rock deformation and the impact of activity thrusting on the formation of landslides would be obtained.