GEOLOGIJA 50/1, 55–63, Ljubljana 2007
Vzroki in mehanizem zemeljskega plazenja na Rebrnicah
v Vipavski dolini
Reasons and mechanism for soil sliding processes in the Rebrnice area, Vipava valley, SW Slovenia
Jernej JEŽ
Geološki zavod Slovenije, Dimičeva 14, SI-1000 Ljubljana e-mail: jernej.jez@geo-zs.si
Ključne besede: Rebrnice, plaz, niš, apnenčev pobočni grušč, Vipavska dolina Key words: Rebrnice, landslide, flysch, limestone scree, Vipava valley, Slovenia
Abstract
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.
Izvle~ek
Trasa hitre ceste Razdrto-Podnanos prečka območje Rebrnic v Vipavski dolini, kjer so apnenčevi pobočni grušči odloženi na flišnih kamninah. Pobočni grušči so zaradi nagnjenosti terena, prisotnosti talne vode in zaglinjenih con, ki se pojavljajo na kontaktu grušča s flišem, nestabilni in postopoma drsijo po podlagi. Premiki se kažejo na površini, potrjujejo pa jih tudi meritve v vrtinah. V prispevku podrobneje opisujem sestavo poboč-nega grušča in navajam vzroke za nastanek plazov.
Uvod
Rebrnice obsegajo del jugozahodnega po-bo~ja Nanosa, med Podnanosom in Razdrtim. Proti zahodu prehajajo v Vipavsko dolino, proti jugovzhodu pa v Piv{ko kotlino. Obmo~je pre~ka regionalna cesta, trenutno pa preko Rebrnic gradijo tudi hitro cesto, odsek Razdrto–Vipava. V prispevku obravnavam pas ozemlja med kilometroma 4,7 in 6,3 omenjene hitre ceste. Pas zajema pobo~je od apnen~evih sten Nanosa do izravnave pri Lozicah, ki pripada zgornjemu delu Vipavske doline.
Dana{nja geolo{ka struktura {ir{ega ob-mo~ja je posledica starej{e terciarne narivne
tektonike (Placer, 1981). Kredni apnenci so narinjeni na eocenske fli{ne kamnine. Pas fli{nih kamnin, ki se ob južnemu robu Nanosa vle~e v Vipavsko dolino, pripada Sne-žni{ki narivni grudi (Placer, 1981; Janež et al., 1997). Na Snežni{ko narivno grudo je narinjen Hru{i{ki pokrov, ki zajema tudi Nanos. Zaradi mehanskega preperevanja in drobljenja zgornjekrednih rudistnih apnencev je nastala velika koli~ina apnen~evega pobo~nega gru{~a, ki se je odložil na spodaj leže~ih fli{nih kamninah.
V okviru gradnje hitre ceste se v zadnjem ~asu izvajajo podrobnej{e geolo{ke raziskave terena. Zaradi neugodne kamninske zgradbe se gradbeniki pri gradnji nekaterih
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objektov, predvsem vkopov in viaduktov, sre~ujejo s {tevilnimi problemi. Za utrditev omenjenih objektov se poslužujejo dragih podpornih konstrukcij.
S pomo~jo podatkov zbranih pri inže-nirsko-geolo{kem kartiranju ter podatkov iz geomehanskih vrtin sem izdelal inženir-sko-geolo{ko karto v merilu 1 : 5000. Namen kartiranja je bil ugotoviti razprostranjenost pobo~nih gru{~ev in vpad fli{nih kamnin ter ugotoviti znake fosilnega plazenja gru{~ev. Na podlagi pridobljenih podatkov lahko pojasnimo dinamiko in mehanizem plazenja.
Inženirsko-geolo{ka karta
Rebrnice kot samostojna enota niso posebej obravnavane nikjer v strokovni literaturi. Kljub temu pa so mnogi avtorji pri opisovanju geologije Nanosa in Vipavske doline zajeli tudi ta del ozemlja. O veliki koli~ini gru{~ev v Vipavski dolini je pisal že Hacquet (1789). Kasneje so pri obravnavi {ir{ega obmo~ja o južnem robu Nanosa pisali Stur (1858), Stache (1889), Kossmat (1905), Winkler (1924) in Limanovsky (1910), v okviru izdelave Osnovne geolo{ke karte v merilu 1 : 100.000, listov Gorica in Postojna pa Buser (1986), Buser in sodelavci (1967) ter Pleni~ar (1970). Veliko podatkov je bilo zbranih tudi pri geolo{kem kartiranju posameznih izvirov in njihovih zaledij na obmo~ju Visokega krasa (Janež et al., 1997).
Obravnavano ozemlje obsega del po-bo~ja ob odseku hitre ceste med 4,7 in 6,3 kilometrom in v {irini 2 km. Eocenske kamnine so razvite v zna~ilnem fli{nem razvoju. V glavnem se menjavajo sivi skrilavi laporovci in kremenovi ter apnen~evi pe-{~enjaki. Na redkih mestih izdanjajo debele plasti kalkarenita ter kalkrudita in plasti zelenkasto sivega glinavca. Generalni vpad fli{nih plasti je proti severovzhodu, plasti vpadajo v pobo~je. Z izjemo kalkarenitov in kalkruditov so fli{ne plasti nepropustne in tvorijo talno hidrogeolo{ko pregrado za kra{ke vode Nanosa (Janež et al., 1997). Na eocenske kamnine so narinjeni kredni apnenci, ki gradijo Nano{ko planoto. Jugozahodni rob planote sestavljajo debeloplast-nati organogeni rudistni apnenci zgornje-kredne, senonijske starosti (Buser, 1973). Debele plasti apnenca vpadajo proti severu oziroma severovzhodu, enako kot spodaj leže~e fli{ne plasti. Z mehanskim prepere-
Jernej Je`
vanjem in razpadanjem krednih apnencev nastaja pobo~ni gru{~, ki se v obliki meli{~ odlaga na fli{nih kamninah. Gru{~i, ki so na nekaterih mestih sprijeti v bre~o, pokrivajo približno 2/3 obravnavanega ozemlja. Izpod apnen~evih sten se obmo~ja gru{~a v jezi-~asti obliki {irijo proti dolini. Gru{~ tvori grebene oziroma rebra. Debelino sedimenta je na terenu težko dolo~iti, iz reliefnih oblik lahko sklepamo, da ponekod presega 30 metrov. Položaj in odnos krednih ter eocenskih kamnin in kvartarnih pobo~nih sedimentov je prikazan na sliki 1.
Na sliki 1 je viden tudi profil A–B preko inženirsko-geolo{ke karte. Profil je lociran v bližini vasi Podgri~ in poteka v smeri NE– SW. Podatek o debelini pobo~nega gru{~a je pridobljen iz geomehanske vrtine VK-2 (Umek, 2000).
Pobo~ni gru{~i in bre~e
Ve~ji del Rebrnic pokrivajo pobo~ni gru-{~i in bre~e. Odnos med sedimenti je glavno gibalo pobo~nih procesov, ki se tu dogajajo. Njihovih izdankov je na obravnavanem terenu malo, zato podrobnej{i opis gru-{~ev povzemam po geomehanskih vrtinah in profilih v ve~jih vkopih. Vrtine locirane na pobo~nem gru{~u so prevrtale {tiri glavne horizonte (sl. 2). Zgornji horizont gradi apnen~ev pobo~ni gru{~. Sestavljajo ga raz-li~no veliki apnen~evi kosi ter skalni bloki. Ponekod je gru{~ sprijet v pobo~no bre~o. Kosi apnenca so ostrorobi, obi~ajno merijo od 5 do 15 cm, lahko pa se v gru{~u pojavljajo tudi ve~je skalne samice, ki merijo od 20 cm do 2 m. Skalni apnen~evi bloki in bre~a so delno zakraseli ali kavernozni. Gru{~ je svetlo rjave do sive barve, obi~ajno pe{~en, meljast ali lokalno zaglinjen. Gostoto gru{~a dolo~ajo zrna glinaste in meljaste frakcije. Obi~ajno je dobro prepusten. Zaledna voda se preceja {ele na dnu tega horizonta. Debelina horizonta se lokalno zelo spreminja, v eni od vrtin njegova debelina zna{a kar 45 metrov. Navzdol sledi drugi horizont, ki ga sestavlja pome{an gru{~ fli-{nih kamnin in apnenca. Tudi znotraj tega horizonta so posamezni ve~ji bloki apnenca, ki se jim pridružujejo kosi fli{nega pe-
Slika 1. Inženirsko-geolo{ka karta dela Rebrnic s profilom
Figure1. Engineerical-geological map of part of Rebrnice area with profile
Vzroki in mehanizem zemeljskega plazenja na Rebrnicah v Vipavski dolini
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Jernej Je`
Slika 2. Skica glavnih horizontov, ki sestavljajo pobo~ni gru{~ Figure 2. Sketch of the main horizons which compose scree
{~enjaka. Pome{an gru{~ je navadno mo~-neje zaglinjen, kot ~ist apnen~ev gru{~. Voda se preceja po prepustnej{ih delih horizonta. V tretji horizont uvr{~am na mestu preperele fli{ne kamnine. Kamnine nastopajo delno kot poltrdna kamnina, delno pa so razpadle v glinast gru{~. Klasti laporov-ca in pe{~enjaka so veliki najve~ do 5 cm. Zaledna voda se tudi tu preceja po prepust-nej{ih delih horizonta. Gru{~ je temno siv do temno rjav. Spodnji, ~etrti horizont, zastopa kompaktna kamnina, ki jo sestavljajo fli{ni laporovci, meljevci in drobnozrnati pe{~enjaki. Na nekaterih obmo~jih, kjer fli{ne kamnine niso pokrite z apnen~evim gru{~em in bre~o, zgornja dva horizonta manjkata. Nadome{~a jih preperina fli{nih kamnin. Sestavlja jo glinena preperina z drobnim gru{~em fli{nih kamnin.
Hidrogeolo{ke lastnosti apnen~evih po-bo~nih gru{~ev in bre~ se lokalno spreminjajo (Janež et al., 1997). V odvisnosti od zrnavosti in koli~ine glinene frakcije se od lokacije do lokacije precej spreminja predvsem prepustnost. Bolje prepustni gru{~i so sestavljeni iz debelih odlomkov apnenca in vsebujejo manj glinenih in meljastih zrn. V splo{nem velja, da so gru{~i dobro prepust-
ni. Talna voda se v sedimentih pretaka po mo~neje zaglinjenih conah, ki se obi-~aj-no pojavljajo v spodnjem delu zgornjega apnen~astega horizonta oziroma znotraj drugega horizonta. Tovrstno pretakanje je bilo mogo~e zaznati tudi v mnogih geomehanskih vrtinah. Voda je potemtakem vezana na kontakt gru{~a s fli{em, ~eprav je tega težko natan~no dolo~iti, ker je ravno v coni, kjer se me{ata gru{~nat apnen~ev in fli{ni material. Pretakanje talne vode pod gru{~em dokazujejo tudi {tevilni izviri, ki se pojavljajo na spodnjih delih gru{~natih pokrovov (sl. 1). Pretakanje vode {e poslab-{a že tako slabe geomehanske lastnosti za-glinjenih con.
Dinamika in vzroki plazenja
Že sam položaj in razprostranjenost po-bo~nih sedimentov na Rebrnicah nam da slutiti, da se gru{~i v obliki razli~nih po-bo~nih procesov premikajo. Poleg tega nam to potrjujejo tudi premiki, ki so bili izmerjeni s pomo~jo inklinometrskih vrtin ter nekateri recentni plazovi. Znaki drsenja so opazni v deformacijah na obstoje~i
Vzroki in mehanizem zemeljskega plazenja na Rebrnicah v Vipavski dolini
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regionalni cesti, v razpokah na nekaterih odsekih nove hitre ceste ter na po{kodbah objektov v vasi Lozice. Meritve premikov v geomehansko-inklinometrskih vrtinah kažejo premike reda velikosti nekaj milimetrov do maksimalno 15 milimetrov mese~-no. Navezujemo se predvsem na premike v vrtinah VK-2, VK-3 in POL-I1, ki so locirane na objektih Vkop Polance in viadukt Na Polancah (Umek, 2000, 2004). Približno 500 m severovzhodno od vasi Lozice, se je spomladi leta 2001 nad traso hitre ceste aktiviral plaz, imenovan Rebrnica. Nad ve~jim vkopom je drselo okrog 400.000 m3 pobo~nega gru{~a. Drsna ploskev se je oblikovala na meji med gru{~em in fli{nimi kamninami. Visoko v pobo~ju je nastal do 3 m {irok odlomni rob. Plaz so uspeli zaustaviti s sidrano pilotno steno.
Apnen~ev material, ki nastaja pri mehanskem preperevanju apnencev jugozahodnega roba Nanosa, se v obliki meli{~ odlaga na spodaj leže~e fli{ne kamnine. Zaradi kontinuiranega pove~evanja koli-~ine materiala za~nejo meli{~a pod vplivom gravitacije drseti po podlagi. V zgornjem delu meli{~a drsijo po krožni drsini, kar dokazujejo {tevilne izravnave oziroma zatrepi, ki se vzdolž Rebrnic pojavljajo pod apnen~evimi stenami. Meli{~e, ki je obi~aj-no nagnjeno pod kotom 45°, je zaradi krožnega drsenja pri{lo v vodoraven položaj. V zaledju gru{~ so~asno na novo nastaja, zato se dogodki ponavljajo. Mlaj{a meli{~a pred seboj potiskajo starej{e pobo~ne gru{~e, ki so se med tem ~asom že lahko povezali v pobo~ne bre~e. Na tak na~in so pobo~ni gru{~i splazeli vse do doline.
Vzroki za opisano plazenje ter manj-{e premike in zdrse se med seboj kombinirajo. Povpre~en nagib pobo~ja na Rebrnicah zna{a med 15 in 20°, kar teren uvr{~a med zmerno nagnjena pobo~ja. Strmeje je nagnjen le zgornji del pobo~ja, kjer nastajajo meli{~a. Zmeren nagib pobo~ja že omogo~a drsenje sedimenta pod vplivom gravitacije. Drugi vzrok so zaglinjene cone, ki se pojavljajo na meji med gru{~em in fli{nimi kamninami. Gline imajo slabe geomehanske lastnosti, znotraj njih se lahko oblikujejo drsne ploskve. Poleg tega se v teh conah pretaka podzemna voda, ki dodatno zmanj{u-je geomehansko trdnost gline. Tretji razlog so gradbeni posegi na trasi hitre ceste. Pri gradnji velikih vkopov skozi gru{~nate grebene se stabilnost pobo~nih sedimentov nad traso ceste mo~no zmanj{a.
Zaklju~ki in razprava
Dana{nji premiki velikih mas apnen~e-vih pobo~nih gru{~ev in bre~ na Rebrni-cah so le del plazenja, ki se je dogajalo že v preteklosti. To potrjuje jezi~asta oblika gru{~natih pokrovov, ki je zna~ilna za regionalne plazove (sl. 1). Me{anje apnen~e-vih kosov in kosov fli{nih kamnin v drugem horizontu, na kontaktu apnen~evega gru{~a s fli{em, je prav tako posledica fosilnega plazenja. So~asno z drsenjem so se me{ale preperele fli{ne kamnine z apnen~e-vimi kosi. V primeru, da bi bil apnen~ev gru{~ normalno odložen na fli{, me{anega horizonta ne bi bilo. Zaglinjene cone, ki se pojavljajo znotraj tega horizonta oziroma neposredno nad njim, omogo~ajo pre-cejanje talne vode. Voda pripomore tudi k nastanku drsne ploskve. Nazoren primer vpliva precejanja talne vode na nastanek drsne ploskve je razviden v inklinometrskih vrtinah VK-2, VK-3A in POL-I1. V vrtini VK-3A so zaznali premik na globini 15 metrov (Umek, 2004). Cono sestavlja delno vlažen zaglinjen gru{~ apnenca in fli-{a z vmesnim slojem rjave gline. V vrtini VK-2 je premik nastal na globini 26 metrov, v coni mo~no zaglinjenega drobnega gru{~a apnenca. Na globini 25,8 m so zaznali dotok podzemne vode. Tudi v vrtini POL-I1 so v coni zaglinjenega gru{~a apnenca in fli{a z vložki rde~erjave gline izmerili premik na globini 26 metrov. Sediment je mestoma vlažen. Dotok vode iz zaledja so v isti vrtini zaznali na globini 29,7 m.
Obravnavane plazove uvr{~amo v posamezne kategorije. Glede na vrsto plaze~ega materiala spadajo med zemljinske plazove. Geolo{ka struktura in globina drsne ploskve jih ume{~ata med kompaktne regionalne plazove. Podolgovata oblika drsne ploskve pogojuje nastanek zdrsov po naprej pogojeni drsini, glede na hitrost zdrsa pa jih uvr-{~amo med po~asno plazenje. Oblika plazov je enostavna. Glede na vsebnost vode spada plazina med suhe plazine.
Po~asno aktivno plazenje na Rebrnicah v tem trenutku ogroža regionalno in hitro cesto Razdrto–Vipava, medtem ko naselji Lo-zice in Podgri~ nista ogroženi. V prihodnosti bi lahko katastrofi~ni dogodki v smislu nenadnega hitrega zdrsa ve~jih koli~in gru{~a ogrozili tudi omenjeni naselji. Katastrofi~ne dogodke bi lahko povzro~ili neugodni naravni pogoji, kot so dolgotrajno deževje in potresi ali kombinacija obojega.
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Inženirsko-geolo{ke raziskave z namenom odpravljati težave pri gradnji hitre ceste Razdrto–Podnanos so v zadnjem ~asu na Rebrnicah pogoste. Aktualna je tudi tema o pobo~nih procesih. Ker so omenjeni procesi neposredno vezani tudi na strukturno geo-lo{ke elemente, bi bilo v prihodnje smiselno natan~no strukturno geolo{ko kartiranje celotnega pobo~ja Nanosa med Razdrtim in Vipavo. S tem bi pridobili podatke o na-tan~nem poteku narivnice, o položaju in legi fli{nih plasti, o tektonski deformiranosti kamnin ter vplivu aktivnega narivanja na nastajanje plazov.
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 (Placer, 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 (Placer, 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.
Jernej Je`
With data, obtained from the engineeri-cal-geological mapping, and the ones gathered from the geomechanical wells, an en-gineerical-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 Hacquet (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 (Buser 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 (Janež 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 quar-zitic 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 impermea-bil 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
Vzroki in mehanizem zemeljskega plazenja na Rebrnicah v Vipavski dolini
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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 geomechani-cal well (Umek, 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 marlsto-ne, 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 geo-mechanical 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 geo-mechanical-inklinometer wells measure between a few millimetres and a maximum of 15 milimetres monthly. Taken into account are particularly the movements in the
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Jernej Je`
VK-2, VK-3 and POL-I1 wells, all located on the objects The Polance Dig and the Na Polancah Viaduct (Umek, 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 m3 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 well-known 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. long-lasting rainfalls, earthquakes or a combination of both).
There have been many engineerical-geo-logical investigations carried out in the Rebrnice area to resolve the difficulties in building  a  motorway  Razdrto–Podnanos.
Vzroki in mehanizem zemeljskega plazenja na Rebrnicah v Vipavski dolini
63
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.
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