GEOLOGIJA 50/1, 55–63, Ljubljana 2007 doi:10.5474/geologija.2007.005

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 Geolo{ki zavod Slovenije, Dimi~eva 14, SI-1000 Ljubljana e-mail: jernej.jez�geo-zs.si

Klju~ne besede: Rebrnice, plaz, fli{, apnen~ev pobo~ni gru{~, 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 pre~ka obmo~je Rebrnic v Vipavski dolini, kjer so apnen~evi pobo~ni gru{~i odloženi na fli{nih kamninah. Pobo~ni gru{~i so zaradi na- gnjenosti terena, prisotnosti talne vode in zaglinjenih con, ki se pojavljajo na kontaktu gru{~a s fli{em, nestabilni in postopoma drsijo po podlagi. Premiki se kažejo na povr{ini, potrjujejo pa jih tudi meritve v vrtinah. V prispevku podrobneje opisujem sestavo pobo~- nega gru{~a in navajam vzroke za nastanek plazov.

Uvod tektonike (Placer, 1981). Kredni apnenci so narinjeni na eocenske fli{ne kamnine. Pas Rebrnice obsegajo del jugozahodnega po- fli{nih kamnin, ki se ob južnemu robu Na- bo~ja Nanosa, med Podnanosom in Razdr- nosa vle~e v Vipavsko dolino, pripada Sne- tim. Proti zahodu prehajajo v Vipavsko do- žni{ki narivni grudi (Placer, 1981; Janež lino, proti jugovzhodu pa v Piv{ko kotlino. et al., 1997). Na Snežni{ko narivno grudo Obmo~je pre~ka regionalna cesta, trenutno je narinjen Hru{i{ki pokrov, ki zajema tudi pa preko Rebrnic gradijo tudi hitro cesto, Nanos. Zaradi mehanskega preperevanja in odsek Razdrto–Vipava. V prispevku obrav- drobljenja zgornjekrednih rudistnih apnen- navam pas ozemlja med kilometroma 4,7 in cev je nastala velika koli~ina apnen~evega 6,3 omenjene hitre ceste. Pas zajema pobo~je pobo~nega gru{~a, ki se je odložil na spodaj od apnen~evih sten Nanosa do izravnave pri leže~ih fli{nih kamninah. Lozicah, ki pripada zgornjemu delu Vipav- V okviru gradnje hitre ceste se v zadnjem ske doline. ~asu izvajajo podrobnej{e geolo{ke razi- Dana{nja geolo{ka struktura {ir{ega ob- skave terena. Zaradi neugodne kamninske mo~ja je posledica starej{e terciarne narivne zgradbe se gradbeniki pri gradnji nekaterih 56 Jernej Je` objektov, predvsem vkopov in viaduktov, vanjem in razpadanjem krednih apnencev sre~ujejo s {tevilnimi problemi. Za utrditev nastaja pobo~ni gru{~, ki se v obliki meli{~ omenjenih objektov se poslužujejo dragih odlaga na fli{nih kamninah. Gru{~i, ki so na podpornih konstrukcij. nekaterih mestih sprijeti v bre~o, pokrivajo S pomo~jo podatkov zbranih pri inže- približno 2/3 obravnavanega ozemlja. Izpod nirsko-geolo{kem kartiranju ter podatkov apnen~evih sten se obmo~ja gru{~a v jezi- iz geomehanskih vrtin sem izdelal inženir- ~asti obliki {irijo proti dolini. Gru{~ tvori sko-geolo{ko karto v merilu 1 : 5000. Namen grebene oziroma rebra. Debelino sedimenta kartiranja je bil ugotoviti razprostranjenost je na terenu težko dolo~iti, iz reliefnih oblik pobo~nih gru{~ev in vpad fli{nih kamnin ter lahko sklepamo, da ponekod presega 30 me- ugotoviti znake fosilnega plazenja gru{~ev. trov. Položaj in odnos krednih ter eocenskih Na podlagi pridobljenih podatkov lahko po- kamnin in kvartarnih pobo~nih sedimentov jasnimo dinamiko in mehanizem plazenja. je prikazan na sliki 1. Na sliki 1 je viden tudi profil A–B preko inženirsko-geolo{ke karte. Profil je lociran v Inženirsko-geolo{ka karta bližini vasi Podgri~ in poteka v smeri NE– SW. Podatek o debelini pobo~nega gru{~a Rebrnice kot samostojna enota niso po- je pridobljen iz geomehanske vrtine VK-2 sebej obravnavane nikjer v strokovni lite- (Umek, 2000). raturi. Kljub temu pa so mnogi avtorji pri opisovanju geologije Nanosa in Vipavske doline zajeli tudi ta del ozemlja. O veliki Pobo~ni gru{~i in bre~e koli~ini gru{~ev v Vipavski dolini je pisal že Hacquet (1789). Kasneje so pri obravnavi Ve~ji del Rebrnic pokrivajo pobo~ni gru- {ir{ega obmo~ja o južnem robu Nanosa pi- {~i in bre~e. Odnos med sedimenti je glavno sali Stur (1858), Stache (1889), Kossmat gibalo pobo~nih procesov, ki se tu dogaja- (1905), Winkler (1924) in Limanovsky jo. Njihovih izdankov je na obravnavanem (1910), v okviru izdelave Osnovne geolo{ke terenu malo, zato podrobnej{i opis gru- karte v merilu 1 : 100.000, listov Gorica in {~ev povzemam po geomehanskih vrtinah Postojna pa Buser (1986), Buser in sode- in profilih v ve~jih vkopih. Vrtine locirane lavci (1967) ter Pleni~ar (1970). Veliko po- na pobo~nem gru{~u so prevrtale {tiri glav- datkov je bilo zbranih tudi pri geolo{kem ne horizonte (sl. 2). Zgornji horizont gradi kartiranju posameznih izvirov in njihovih apnen~ev pobo~ni gru{~. Sestavljajo ga raz- zaledij na obmo~ju Visokega krasa (Janež li~no veliki apnen~evi kosi ter skalni bloki. et al., 1997). Ponekod je gru{~ sprijet v pobo~no bre~o. Obravnavano ozemlje obsega del po- Kosi apnenca so ostrorobi, obi~ajno meri- bo~ja ob odseku hitre ceste med 4,7 in 6,3 jo od 5 do 15 cm, lahko pa se v gru{~u po- kilometrom in v {irini 2 km. Eocenske kam- javljajo tudi ve~je skalne samice, ki merijo nine so razvite v zna~ilnem fli{nem raz- od 20 cm do 2 m. Skalni apnen~evi bloki voju. V glavnem se menjavajo sivi skrilavi in bre~a so delno zakraseli ali kavernozni. laporovci in kremenovi ter apnen~evi pe- Gru{~ je svetlo rjave do sive barve, obi~ajno {~enjaki. Na redkih mestih izdanjajo debele pe{~en, meljast ali lokalno zaglinjen. Gosto- plasti kalkarenita ter kalkrudita in plasti to gru{~a dolo~ajo zrna glinaste in meljaste zelenkasto sivega glinavca. Generalni vpad frakcije. Obi~ajno je dobro prepusten. Za- fli{nih plasti je proti severovzhodu, plasti ledna voda se preceja {ele na dnu tega ho- rizonta. Debelina horizonta se lokalno zelo vpadajo v pobo~je. Z izjemo kalkarenitov spreminja, v eni od vrtin njegova debelina in kalkruditov so fli{ne plasti nepropustne zna{a kar 45 metrov. Navzdol sledi drugi in tvorijo talno hidrogeolo{ko pregrado za horizont, ki ga sestavlja pome{an gru{~ fli- kra{ke vode Nanosa (Janež et al., 1997). {nih kamnin in apnenca. Tudi znotraj tega Na eocenske kamnine so narinjeni kredni horizonta so posamezni ve~ji bloki apnen- apnenci, ki gradijo Nano{ko planoto. Jugo- ca, ki se jim pridružujejo kosi fli{nega pe- zahodni rob planote sestavljajo debeloplast- nati organogeni rudistni apnenci zgornje- kredne, senonijske starosti (Buser, 1973). Slika 1. Inženirsko-geolo{ka karta dela Rebrnic Debele plasti apnenca vpadajo proti severu s profilom oziroma severovzhodu, enako kot spodaj Figure1. Engineerical-geological map of part leže~e fli{ne plasti. Z mehanskim prepere- of Rebrnice area with profile Vzroki in mehanizem zemeljskega plazenja na Rebrnicah v Vipavski dolini 57 58 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~- ni. Talna voda se v sedimentih pretaka po neje zaglinjen, kot ~ist apnen~ev gru{~. mo~neje zaglinjenih conah, ki se obi-~aj- Voda se preceja po prepustnej{ih delih ho- no pojavljajo v spodnjem delu zgornjega rizonta. V tretji horizont uvr{~am na mestu apnen~astega horizonta oziroma znotraj preperele fli{ne kamnine. Kamnine nasto- drugega horizonta. Tovrstno pretakanje je pajo delno kot poltrdna kamnina, delno pa bilo mogo~e zaznati tudi v mnogih geome- so razpadle v glinast gru{~. Klasti laporov- hanskih vrtinah. Voda je potemtakem ve- ca in pe{~enjaka so veliki najve~ do 5 cm. zana na kontakt gru{~a s fli{em, ~eprav je Zaledna voda se tudi tu preceja po prepust- tega težko natan~no dolo~iti, ker je ravno nej{ih delih horizonta. Gru{~ je temno siv v coni, kjer se me{ata gru{~nat apnen~ev in do temno rjav. Spodnji, ~etrti horizont, za- fli{ni material. Pretakanje talne vode pod stopa kompaktna kamnina, ki jo sestavlja- gru{~em dokazujejo tudi {tevilni izviri, ki jo fli{ni laporovci, meljevci in drobnozrnati se pojavljajo na spodnjih delih gru{~natih pe{~enjaki. Na nekaterih obmo~jih, kjer pokrovov (sl. 1). Pretakanje vode {e poslab- fli{ne kamnine niso pokrite z apnen~evim {a že tako slabe geomehanske lastnosti za- gru{~em in bre~o, zgornja dva horizonta glinjenih con. manjkata. Nadome{~a jih preperina fli{nih kamnin. Sestavlja jo glinena preperina z drobnim gru{~em fli{nih kamnin. Dinamika in vzroki plazenja Hidrogeolo{ke lastnosti apnen~evih po- bo~nih gru{~ev in bre~ se lokalno spremi- Že sam položaj in razprostranjenost po- njajo (Janež et al., 1997). V odvisnosti od bo~nih sedimentov na Rebrnicah nam da zrnavosti in koli~ine glinene frakcije se od slutiti, da se gru{~i v obliki razli~nih po- lokacije do lokacije precej spreminja pred- bo~nih procesov premikajo. Poleg tega vsem prepustnost. Bolje prepustni gru{~i so nam to potrjujejo tudi premiki, ki so bili sestavljeni iz debelih odlomkov apnenca in izmerjeni s pomo~jo inklinometrskih vrtin vsebujejo manj glinenih in meljastih zrn. V ter nekateri recentni plazovi. Znaki drse- splo{nem velja, da so gru{~i dobro prepust- nja so opazni v deformacijah na obstoje~i Vzroki in mehanizem zemeljskega plazenja na Rebrnicah v Vipavski dolini 59 regionalni cesti, v razpokah na nekaterih Zaklju~ki in razprava odsekih nove hitre ceste ter na po{kodbah objektov v vasi Lozice. Meritve premikov v Dana{nji premiki velikih mas apnen~e- geomehansko-inklinometrskih vrtinah ka- vih pobo~nih gru{~ev in bre~ na Rebrni- žejo premike reda velikosti nekaj milime- cah so le del plazenja, ki se je dogajalo že trov do maksimalno 15 milimetrov mese~- v preteklosti. To potrjuje jezi~asta oblika no. Navezujemo se predvsem na premike v gru{~natih pokrovov, ki je zna~ilna za re- vrtinah VK-2, VK-3 in POL-I1, ki so loci- gionalne plazove (sl. 1). Me{anje apnen~e- rane na objektih Vkop Polance in viadukt vih kosov in kosov fli{nih kamnin v dru- Na Polancah (Umek, 2000, 2004). Približ- gem horizontu, na kontaktu apnen~evega no 500 m severovzhodno od vasi Lozice, se gru{~a s fli{em, je prav tako posledica fo- je spomladi leta 2001 nad traso hitre ceste silnega plazenja. So~asno z drsenjem so se aktiviral plaz, imenovan Rebrnica. Nad me{ale preperele fli{ne kamnine z apnen~e- ve~jim vkopom je drselo okrog 400.000 m3 vimi kosi. V primeru, da bi bil apnen~ev pobo~nega gru{~a. Drsna ploskev se je ob- gru{~ normalno odložen na fli{, me{anega likovala na meji med gru{~em in fli{nimi horizonta ne bi bilo. Zaglinjene cone, ki kamninami. Visoko v pobo~ju je nastal do se pojavljajo znotraj tega horizonta oziro- 3 m {irok odlomni rob. Plaz so uspeli za- ma neposredno nad njim, omogo~ajo pre- ustaviti s sidrano pilotno steno. cejanje talne vode. Voda pripomore tudi k Apnen~ev material, ki nastaja pri me- nastanku drsne ploskve. Nazoren primer hanskem preperevanju apnencev jugoza- vpliva precejanja talne vode na nastanek hodnega roba Nanosa, se v obliki meli{~ drsne ploskve je razviden v inklinometrskih odlaga na spodaj leže~e fli{ne kamnine. vrtinah VK-2, VK-3A in POL-I1. V vrtini Zaradi kontinuiranega pove~evanja koli- VK-3A so zaznali premik na globini 15 ~ine materiala za~nejo meli{~a pod vpli- metrov (Umek, 2004). Cono sestavlja del- vom gravitacije drseti po podlagi. V zgor- no vlažen zaglinjen gru{~ apnenca in fli- njem delu meli{~a drsijo po krožni drsini, {a z vmesnim slojem rjave gline. V vrtini kar dokazujejo {tevilne izravnave oziroma VK-2 je premik nastal na globini 26 metrov, zatrepi, ki se vzdolž Rebrnic pojavljajo pod v coni mo~no zaglinjenega drobnega gru{~a apnen~evimi stenami. Meli{~e, ki je obi~aj- apnenca. Na globini 25,8 m so zaznali do- no nagnjeno pod kotom 45°, je zaradi krož- tok podzemne vode. Tudi v vrtini POL-I1 so nega drsenja pri{lo v vodoraven položaj. V v coni zaglinjenega gru{~a apnenca in fli{a zaledju gru{~ so~asno na novo nastaja, zato z vložki rde~erjave gline izmerili premik se dogodki ponavljajo. Mlaj{a meli{~a pred na globini 26 metrov. Sediment je mestoma seboj potiskajo starej{e pobo~ne gru{~e, vlažen. Dotok vode iz zaledja so v isti vrtini ki so se med tem ~asom že lahko povezali zaznali na globini 29,7 m. v pobo~ne bre~e. Na tak na~in so pobo~ni Obravnavane plazove uvr{~amo v posa- gru{~i splazeli vse do doline. mezne kategorije. Glede na vrsto plaze~ega Vzroki za opisano plazenje ter manj- materiala spadajo med zemljinske plazove. {e premike in zdrse se med seboj kombini- Geolo{ka struktura in globina drsne ploskve rajo. Povpre~en nagib pobo~ja na Rebrnicah jih ume{~ata med kompaktne regionalne zna{a med 15 in 20°, kar teren uvr{~a med plazove. Podolgovata oblika drsne ploskve zmerno nagnjena pobo~ja. Strmeje je na- pogojuje nastanek zdrsov po naprej pogoje- gnjen le zgornji del pobo~ja, kjer nastajajo ni drsini, glede na hitrost zdrsa pa jih uvr- meli{~a. Zmeren nagib pobo~ja že omogo~a {~amo med po~asno plazenje. Oblika plazov drsenje sedimenta pod vplivom gravitacije. je enostavna. Glede na vsebnost vode spada Drugi vzrok so zaglinjene cone, ki se po- plazina med suhe plazine. javljajo na meji med gru{~em in fli{nimi Po~asno aktivno plazenje na Rebrnicah v kamninami. Gline imajo slabe geomehanske tem trenutku ogroža regionalno in hitro ce- lastnosti, znotraj njih se lahko oblikujejo sto Razdrto–Vipava, medtem ko naselji Lo- drsne ploskve. Poleg tega se v teh conah pre- zice in Podgri~ nista ogroženi. V prihodnosti taka podzemna voda, ki dodatno zmanj{u- bi lahko katastrofi~ni dogodki v smislu ne- je geomehansko trdnost gline. Tretji razlog nadnega hitrega zdrsa ve~jih koli~in gru{~a so gradbeni posegi na trasi hitre ceste. Pri ogrozili tudi omenjeni naselji. Katastrofi~ne gradnji velikih vkopov skozi gru{~nate gre- dogodke bi lahko povzro~ili neugodni na- bene se stabilnost pobo~nih sedimentov nad ravni pogoji, kot so dolgotrajno deževje in traso ceste mo~no zmanj{a. potresi ali kombinacija obojega. 60 Jernej Je`

Inženirsko-geolo{ke raziskave z name- With data, obtained from the engineeri- nom odpravljati težave pri gradnji hitre ce- cal-geological mapping, and the ones gat- ste Razdrto–Podnanos so v zadnjem ~asu na hered from the geomechanical wells, an en- Rebrnicah pogoste. Aktualna je tudi tema o gineerical-geological map has been made. pobo~nih procesih. Ker so omenjeni procesi The purpose of mapping was to find out the neposredno vezani tudi na strukturno geo- expansion of scree material and the dip of lo{ke elemente, bi bilo v prihodnje smisel- the flysch rocks as well as to disclose the no natan~no strukturno geolo{ko kartiranje signs of scree fossil sliding. On the basis of celotnega pobo~ja Nanosa med Razdrtim the collected data it is possible to explain in Vipavo. S tem bi pridobili podatke o na- the dynamics and the mechanism of sliding tan~nem poteku narivnice, o položaju in legi processes. fli{nih plasti, o tektonski deformiranosti kamnin ter vplivu aktivnega narivanja na nastajanje plazov. An engineerical-geological map

The Rebrnice area has never been dealt Reasons and mechanism for soil with as an independent unit in the lite- sliding processes in the Rebrnice area, rature separately although it has been in- Vipava valley, SW Slovenia cluded into the treatment of the Nanos and the Vipava Valley by many authors. Beside Hacquet (1789), who wrote about a huge Introduction amount of scree material in the Vipava Val- ley, there were many others who followed The Rebrnice area extends over the part him later, dealing with the southern edge of the southwest slope of the Nanos, bet- of the Nanos, taking into account broader ween Podnanos and Razdrto. To the west, it area Stur (1858), Stache (1889), Kossmat proceeds to the Vipava Valley, to the southe- (1905), Winkler (1924) and Limanovsky ast, to the Piv{ka kotlina basin. The area is (1910). The geological situation of this area crossed by a regional road. At the moment, was also described while mapping the Ba- a motorway section Razdrto–Podnanos is sic Geological Map 1:100 000, as well as the being built here. The area dealt with in the Gorica (Buser, 1986) and the Postojna (Bu- article stretches from the Nanos limestone ser et al., 1967, Pleni~ar, 1970) map sheets. walls to the flat land near Lozice, which is a Many data were obtained from geological part of the Upper Vipava Valley. mapping of individual springs and its hin- Today’s geological structure of the broader terlands in the High altitude karst area (Ja- area results from early Tertiary thrust tecto- než et al., 1997). nics (Placer, 1981). Cretaceous limestone is The area consists of part of the slope thrust on the Eocene flysch rocks. The belt at the motorway section between 4.7 and of flysch rocks, going past the southern edge 6.3 km, in width 2 km. Eocene rocks are of the Nanos into the Vipava Valley, is a part developed in a typical flysch. Mainly, there of the Snežnik thrust sheet (Placer, 1981; alternate grey shalley marlstone and quar- Janež et al., 1997). The Hru{ica nappe to- zitic and carbonatic sandstones. On few gether with the Nanos as a part of it is then places outcrop thick layers of calcarenit thrust into the rocks of the Snežnik thrust and calcrudit as well as the layers of gre- sheet. Because of physical weathering of the enish grey claystone. The general dip inc- Upper Cretaceous rudist limestone, a large lines towards northeast, the rocks dip into amount of limestone scree material has been the slope. With the exception of calcarenit formed and deposited on the underlying and calcrudit, flysch layers are impermea- flysch rocks. bil and form underground hidrogeological Recently, a lot of detailed geological inve- break for the Nanos karst water (Janež et stigations of the terrain have been conduc- al., 1997). Cretaceous limestones, of which ted for the purposes of building the motor- the Nanos Plateau consists, are thrust on way. There are many problems concerning the Eocene rocks. The southwestern edge of the building of some objects, such as digs the plateau is composed of the thick-layer and viaducts, because of unstable rock de- organogenetic rudist limestone of the Upper position. To consolidate them, expensive su- Cretaceous, Senonian age (Buser, 1973). staining constructions have to be built. Both the thick limestone and the underlying Vzroki in mehanizem zemeljskega plazenja na Rebrnicah v Vipavski dolini 61

flysch layers dip into the north or northeast. ugh more permeabil parts of the horizon. Physical weathering and the Cretaceous li- In-situ weathered flysch rocks, being par- mestone disintegration cause the formation tly semi solid rocks and partly disintegra- of scree material, which is then deposited on ted into clayey scree, form the third horizon. the flysch rocks in the form of slope talus. Marlstone and sandstone grains measure Scree material, which can in some places be up to 5 cm. Also in this horizon is hinter- formed as breccia, covers approximately 2/3 land water filtered through more permeabil of the area. From under the limestone walls, parts of it. Scree ranges from dark grey to the zones of the tongue-shaped scree mate- dark brown. The lowest – fourth – horizon rial, which forms the ridges or ribs, expand is of solid rock, composed of flysch marlsto- towards the valley. The thickness of sediment ne, siltstone and fine-grained sandstones. is difficult to determine on the terrain. As it In areas where flysch rocks are not covered can be concluded from relief configurations, with limestone scree and breccia, the upper it exceeds 30 m in some places. The position two horizons are missing. They are replaced and contact of both Cretaceous and Eocene by clayey weathered residual of flysch rocks rocks and Quaternary slope sediments are with fine flysch scree. shown in Figure 1. In it, the profile A–B can Hidrogeological characteristics of lime- be seen on the engineerical-geological map. stone scree and breccia change locally (Ja- It is located near the village Podgri~ and is než et al., 1997). Depending on gradation directed NE–SW. The thickness of scree ma- and the quantity of fine grains it is perme- terial is measured in the VK-2 geomechani- ability that changes a lot. More permeabil cal well (Umek, 2000). scree consists of thick fragments of limesto- ne which has fewer clayey and silty grains. In general, scree is of good permeability. Scree material and breccia Underground water flows into sediments between the more clayey zones, which are Larger part of the Rebrnice area is covered usually found either in the lower part of the with scree material and breccia. The proces- upper limestone horizon or in the second ho- ses occurring there depend on the contact of rizon. This can also occur in numerous geo- the sediments. There are very few outcrops mechanical wells. Therefore, water appears on the terrain, hence a detailed description on the contact between scree and flysch. The of scree is taken from geomechanical wells contact is difficult to determine because it and profiles in larger digs. The wells loca- is found in the zone where scree limestone ted on scree material have perforated four and flysch material are mixed. Underground main horizons (fig. 2). The upper horizon is water flows under scree, thus worsening the composed of limestone scree, namely lime- already bad geomechanical characteristics stone fragments and blocks of different size. of the clayey zones. The flowing can be ob- Scree is sometimes transformed into breccia. served by numerous springs emerging in the Sharp-edged limestone fragments are 5 to lower parts of the scree cover (fig. 1). 15-centimetre large. There can also appear 0,2 to 2 metre large individual blocks in scree material. Limestone blocks and breccias are Dynamics and reasons for sliding processes partly karstified or cavernous. Scree is usu- ally sandy, silty or locally clayey, ranging Due to the position and extension of the from fair brown to grey. The density of usu- sediments on the Rebrnice area, scree mate- ally permeabil scree is determined by clayey rial moves in forms of various slope proces- and silty grains. Hinterland water is not fil- ses, which can be seen from the movements tered before the bottom of that horizon. The measured by means of inclinometer wells thickness of the horizon changes significan- and by some recent landslides. Sliding has tly; in one of the wells, for instance, measu- already caused the regional road deforma- res 45 metres. Downwards, there follows the tions, the cracks on some new motorway second horizon, composed of mixed talus of sections and the damage on the objects in flysch components and limestone. Inside it the village Lozice. The movements in geo- are found individual larger limestone blocks mechanical-inklinometer wells measure together with fragments of flysch sandsto- between a few millimetres and a maximum ne. Mixed talus is usually more clayey than of 15 milimetres monthly. Taken into acco- pure limestone scree. Water is filtered thro- unt are particularly the movements in the 62 Jernej Je`

VK-2, VK-3 and POL-I1 wells, all loca- limestone fragments and the fragments of ted on the objects The Polance Dig and the flysch rocks in the second horizon, on the Na Polancah Viaduct (Umek, 2000, 2004). contact between limestone scree and flysch. Approximately 500 metres southeast of While sliding, weathered flysch rocks mi- Lozice, above the motorway, the Rebrnica xed with limestone fragments. If limestone landslide activated in spring 2001. Above scree had been normally deposited on the the larger dig slided around 400,000 m3 of flysch, there would have been no mixed ho- scree material. A shear plane was formed on rizon. Clayey zones, appearing inside this the edge between scree material and flysch horizon or just above it, enable the filtrati- rocks. There formed up to 3-metre wide on of underground water, which also adds scarp high in the slope. The landslide was to the formation of shear planes. The influ- stopped by the anchor pile wall. ence of filtration of underground water on Limestone material, resulting from phy- the formation of a shear plane can be seen sical weathering of the Nanos southwestern in the VK-2, VK-3A and POL-I1 inclino- edge limestone, is deposited on the under- meter wells. In the VK-3A well the move- lying flysch rocks in the form of slope talus. ment at the depth of 15 metres was observed Due to continuous increase of the quantity (Umek, 2004). The zone consists of partly of material, slope talus starts to slide on the damp clayey limestone scree and flysch with foundation because of gravity. In the upper the brown clay in between. In the VK-2 well part, it slides across the circular shear pla- the movement occured at the depth of 26 ne, resulting in numerous flat lands along metres, in the zone of strongly clayey fine the Rebrnice limestone walls. Slope talus, limestone scree. At the depth of 25.8 m the which is usually inclined by 45°, reached a inflow of ground water was observed. At the horizontal position because of the circular depth of 26 m, the movement was measured sliding. In the hinterland, scree is formed in the POL-I1 well in the zone of clayey li- simultaneously, hence the repeating of the mestone scree and flysch with the insertion processes. Younger slope talus pushes for- of reddish brown clay. The sediment is damp ward the older ones, which have meanwhile in some parts. In the same well, the inflow of formed into breccias. Thus has the scree ma- hinterland water was observed at the depth terial slided downwards to the valley. of 29.7 m. Causes for such sliding and minor move- The mentioned landslides can be arran- ments are combine with one another. Firstly, ged in separate categories. According to the the Rebrnice slope is moderately-inclined type of sliding material, they belong to soil with the average slope inclination of 15–20°, landslides. They can be grouped under com- which can already cause the sediment to sli- pact regional landslides due to their geo- de because of gravity. More inclined is only logical structure and the depth of a shear upper part of the area, where slope talus is plane. The emersion of slides on the well- formed. Secondly, on the contact of scree and known shear plane is conditioned by oblong flysch rocks there appear clayey zones with shear plane shape condition. With respect to bad geomechanical characteristics. Shear the slide speed, they are slow-sliding. The planes are formed there. Besides, undergro- shape of landslides is simple. According to und water flowing in those zones, adds to the quantity of containable water the scree the decrease of clay geomechanical solidi- material belongs to dry material. ty. Lastly, with building dips through scree Although slow but active sliding in the ridges, the slope sediment stability above Rebrnice area affects regional road and a the motorway section is severely decreased. motorway (Razdrto–Vipava), the villages Lozice and Podgri~ are not under threat. This could, however, change in the future in Conclusion case of catastrophes, such as a sudden quick slide of larger quantities of scree, caused by Recent movements of large masses of li- unfavourable natural conditions (e.g. long- mestone scree material and breccias are only lasting rainfalls, earthquakes or a combina- a part of the sliding occurring already in the tion of both). past. This can be seen from the tongue-sha- There have been many engineerical-geo- ped scree covers, which is typical of great logical investigations carried out in the regional landslides (fig. 1). Another conse- Rebrnice area to resolve the difficulties in quence of fossil landslides is the mixing of building a motorway Razdrto–Podnanos. Vzroki in mehanizem zemeljskega plazenja na Rebrnicah v Vipavski dolini 63

Issues concerning slope processes are also kra{ke podzemne vode Banj{ic, Trnovskega gozda, topical. Due to the fact that the processes Nanosa in Hru{ice, Geologija d.o.o, 1–167, Idrija. Kossmat, F. 1905: Geologische Spezialkarte are directly interconnected with the struc- etc. Blatt: Heidenschaft und Adelsberg 1 : 75.000, tural geological elements, a detailed struc- Geol. R.–A., Wien. tural geological mapping of the whole slope Limanovsky, M. 1910: Wielkie przemieszenie of the Nanos from Razdrto to Vipava would mas skalnych w Dynarydach kolo Postojny. – Raz. Wydz. pryr. Akad. Umiej. Serye 3, 109–171, Kra- be appropriate. Thus the data on the exact kow. position of thrust plane, flysch bed position, Placer, L. 1981: Geolo{ka zgradba jugozahod- tectonic rock deformation and the impact ne Slovenije. – Geologija, 24/1, 27–60, Ljubljana. of activity thrusting on the formation of Pleni~ar, M. 1970: Tolma~ za list Postojna. OGK 1 : 100.000 – Zvezni geol. zavod, 62 p. Beo- landslides would be obtained. grad. Stache, G. 1889: Die Liburnische Stufe und deren Grenzhorizonte. Abh. k.k. geol. 1–170, Literatura – References Wien. Stur, D. 1858: Das Isonzo – Thal von Flitsch Buser, S. 1973: Tolma~ za list Gorica. OGK abwärts bis Görts, die Umgebung von Wippach, 1 : 100.000 – Zvezni geol. zavod, 50 p. Beograd. Adelsberg, Planina und Wochein. – Jahrb. Geol., Buser, S. 1986: Osnovna geolo{ka karta SFRJ 324–366, Wien. 1 : 100.000, list Gorica. – Zvezni geolo{ki zavod, Umek, U. 2000: Geomehanska vrtina VK-2, Beograd. vkop v P290. – GEOT d.d. Tehnolo{ki park Grad- Buser, S., Grad, K. & Pleni~ar, M. 1967: benega in{tituta ZRMK, Ljubljana. Osnovna geolo{ka karta SFRJ 1 : 100.000, list Po- Umek, U. 2004: Geomehanska vrtina VK-3A, stojna. – Zvezni geolo{ki zavod, Beograd. vkop v P290, desno 12 m. – GEOT d.d. Tehnolo{ki Hacquet, B. 1789: Oryctographia Carniolica park Gradbenega in{tituta ZRMK, Ljubljana. oder Physikalishe Erdbeschreibung des Herzogt- Umek, U. 2004: Geomehanska vrtina POL-I1, hums Krain, Istrien und zum Theil der benacht- viadukt Na Polancah. – GEOT d.d. Tehnolo{ki barten Länder, 91 pp., Leipzig. park Gradbenega in{tituta ZRMK, Ljubljana. Janež, J., ^ar, J., Habi~, P. & Podobnik, R. Winkler, A. 1924: Über den Bau der östlichen 1997: Vodno bogastvo Visokega krasa. Ranljivost Südalpen. Mitt. Geol. Ges. 16, Wien.