In order to design the by-pass road of the city of Omiš which will partially he founded on the landslide Mlija and for the safety of foundations of houses, detail subsurface exploration especially engineering-geological surveys, geophysical surveys, boring and laboratory tests of the sliding material were conducted. Obtained results are presented in this paper. Due to numerous damages of roads, walls and houses built in the area of Mlija and Borak, the possibility of construction in the area was discussed and the foundation method proposed. In order to obtain an overview of circumstances, constant monitoring of variations of the water table level and velocity of the moving material has been proposed.Za potrebe projektiranja obilaznice Omiša koja će dijelom biti temeljena na klizištu Mlija te za sigurna temeljenja obiteljskih kuća na toj padini, provedena su detaljna istraživanja, posebice inženjersko-geološko kartiranje, geofizička istraživanja, istraživačko bušenje i laboratorijska ispitivanja materijala koji klizi, U ovom radu prikazani su postignuti rezultati, a zbog brojnih oštećenja na prometnicama, zidovima i stambenim zgradama izvedenim na području Mlija i Borak, raspravljena je mogućnost gradnje na odnosnom terenu s prijedlogom načina temeljenja. Uz to, za dobivanje cjelovite slike stanja, predloženo je stalno praćenje kolebanja razine podzemne vode i brzine kretanja materijala

Duško Barčot

Slobodan Šestanović

English

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Zagreb, l&J UDC 551.244:624.13(497.13) ENGINEERING-GEOLOGICAL CHARACTERISTICS OF THE LANDSLIDE MLIJA AND THEIR IMPACT ON ENGINEERING STRUCTURES (OMIS, SOUTHERN CROATIA) Slobodan SESTANOVIC~) & DuSko BAR~OT') ')~acuky of Civil Engineering University of Split, Maticc hrvafske 15, HR-21000 Split, Croatia 2 )~ iv i l  ~ n ~ i n e c n ' n ~  Instihrte of Croalia, B~tuiness Cenfre Split, Malice hrvatske 15, HR-21000 Splil, Croatia Key*: Landsti* Mlija, ~I~~IIwM - eological characteristics, Flysch, Elunum, ~e iunum,  ~truclwes, 0m8,kuthern Croatia In order to design the by-pass road of the city of OmiS which will artially be founded on the landslide Mlija and for the safety of !oundations of houses, detail subsurface exploration especially engi- neering-geolo 'cal surveys, geophysical surveys, brink and laboratory tests of the sking material were conducted. O b t u e d  results are resented in this paper. Due to numerous damages of roads, walls and go- built in the area of Mlija and Borak, the p i b i l i t y  of mmtmc- tion in the area was discussed and the foundatron method proposed. In order to obtain an overview of circumstances, constant mon~toring of variations of the water table level and velocity of the moving material has been proposed. Introduction Natural slope Mlija is situated southeast from the old centre of OmiS (Fig. 1). Geologicaly speaking, a wider area of the slope has been marked as recent landslide. Due to engineering activities (construction of family houses in the middle part and close to the foot of the slope , a relative stability of the area has been endan- d gere , especially of its middle part. Engineering-geological, geophysical and geome- chanical surveys were conducted during the period 1986-1991 for purposes of the construction of the by- pass road of the city of OmiS. For the most part, the documentation on engineering-geological and geome- chanical surveys can be found in archives of Facul of Civil Engineering of the University of Split and z. ivil Engineering Institute of Croatia - Business Centre Split. The relevant reports for the area of Mlija were made by R o j e - B o n a c c i  e t  a l .  (1986)andSestanoviC & BarEo t (1991 . Information on geophysical surveys were elaborate d by D u j  m iC (1991). Results of engi- neering-geological and geophysical surveys of the OmiS by-pass road, which contain some data on the Mlija landslide were published by K a p e l  j e t a l .  (1994). Since 1991 there were only occasional subsurface explo- rations in order to obtain a geotechnical opinion on the possibility and conditions of foundation for family houses. A recent knowledge on the subject has been presented in this paper. The objective of this paper is to describe geological structure of the slope, geomo hologic and engineering- geological characteristics of %ding upthrow and down- throw beddings and a sliding mechanism and their impact on structures in the area. Basic geological and geomorphologic data Surveyed area consists of Eocene flysch beddings (MarinEik e t  a l . ,  1975, and MarinEiC e t .  a l . ,  1977) eluvial deposits (cohesive soil) formed from the weathering of marl from the base and deluvial deposits (non-cohesive and cohesive soil). K@~i%w rij&: KliziSte Mlija, InZenjerskogeoldke znahjke, FliS, Eluvij, Delwij, Gradevine, Om& JuZna Hrvatska Za potrebe projektiranja obilamice OmiSa koja k dijelom biti temeljena na kli&tu Mlija te za sigurna temeljenja obiteljskih kuCa na toj adini, provedena su detaljna istm%ivanja, bice an je rsko-  geokko kartiranje, geofiitkalstra2ivanja, i s t r g ~ k o  b d h j e i h b  ratorijska ispitivanja materijala koji klizi. U ovom radu prikazani su postignuti rezultati, a zbog brojnih dteknja na prometnicama, zi- dovima i stambenim zgradama izvedenim na podruEju Mlija i Borak, ras ravljena je mo uCnost gradnje na odnosnom terenu s prijedlo om nalnd temellenja.&z to, za dobivanje jelovite slike stanja, predlokno je stalno praknje kolebanja razine podzemne vode i brzine kretanja materijala. 4 '4 0 1 2  3 4 S k m  L : Fig. 1. Geographic situation of the explored location Eocene flysch beddings (E2, 3) consist of marls with thin layers of sandstone (arenites). These sediments are the base rock of the slope Mlija. General direction of layer dip of flysch beddings is north-northeast (toward the slope) and the angle of dip is 30'-SO0. Layer thickness ranges from few millimeters to several meters, but thin layers are predominant. Sporadical flysch layers, espe- cially marl, occurs in the tectonized (crushed) zone. Sandstone and marl alternate almost regularly and the bedding planes characteristic signs for flysch beddings can be observed (impressions, traces of organism move- ment, flow marks). Eluvial sediments (Q,,) are the result of weathering of marl from the base rock mass and consist of yellow- brown clay with small percentage of marl and limestone Rud.-geo1.-naft. zb., Vol. 10, Zagreb, 1998. 90 .$estanoi~i~, S.& Bartot, D.: Landslide Mlija Fig. 3. Thrust OmiSka Dinara above the landslide Fig. 2. Deluvial sediments in the cut of the slope fragments. In some areas beddings are several meters thick. I Deluvial sediments Qd) are the result of the weather- ing of carbonate and h vsch beddings in the hinterland I wfiich were transwrted to a short-distance mostlv bv gravitation (Fig. 2) to the foot of the slope and a m m i -  lated over older beddings. They consist of silty clay of low to intermediate plasticity and compactness with small percentage of angular limestone fragments. According to the results of exploration, thickness of deluvial depos- its in the area of Mlija ranges between 2 and max. 8 m (mostly 2 to 5 m). Slope movements occur in deluvial and partially in eluvial deposits. Investigated area is too small to evaluate global tec- tonic relations. According to the Legend of the general- ised geological map of the sheet OmiS (M a r i n E i C e t a l ,  1977) explored location is situated within tectonic unit Tertiary sinclinore which comprises coastal belt to the front of thrusts Kozjak, Mosor and Biokovo. Thrust OmiSka Dinara is one of the most significant in the wider area of the explored location. A basic type of structural deformation is thrustv structure where Senonian lime- stone beddings lie ovir Eocene flysch. A tectonic activity locally breaks physical-mechanical features of the rock mass. However, general condition of autochtonous beddings of the Mlija slope point at a simple structure without pronounced deformations. Lo- cally more pronounced decom sition and crushing of the flysch sediments is due to P" aulting and thursting of Senonian limestones of OmiSka Dinara over flysch bed- dings (Fig. 3). Morphologically speaking explored location is a slope cascadely shaped by small stone walls. General direction of dip is southwest and the observed dip angle 20°, locally to 35" (Fig. 4). Slope is covered with deluvial sediments (Qd). A rocky steep carbonate massif of OmiSka Dinara, of height 565 m above sea level, rises above the slope. The main geological feature of the explored area is the occurrence of two lithogenetic base complexes that are flysch formations overlain by the Upper Cretaceous Fig. 4. Land5lide Mlija limestone beddings (K: ).Therefore two contrasted and by elevation dissectable natural basis of the slope profile can be distinguished: - flysch and - carbonate complex. The flysch rock mass is heterogeneous in structure and condition as well as physical-mechanical parameters and geotechnical features primarily due to genesis of flysch as specific formation and extremely pronounced tecton- ics. Flysch slope has been formed during the Quaternary; its morphogenetic evolution altered the base rock mass and weathering products were accumulated. Hydrogeology Hydrogeological relations are defined with the per- meable rock mass of the carbonate complex and slightly permeable to impermeable flysch. With regard to engi- neering-geology, a ground water which is retained in the surface area and influences the condition of the soil and the rock mass as well as slope stability is very important. Subsurface ground water is occasionally obtained by wells. Deluvial deposits have intergranular porosity so pre- cipitation percolates faster or slower depending on the clay content to beddings of degraded flysch (eluvium) which are slightly permeable. Precipitation is also dis- persed by occasional runoff as torrents through gullies. At the contact of carbonate and flysch complex there are also periodical springs of low capacity. Ground water percolates to the contact of deluvium and eluvium depending on preferred filtration courses. Rud.-geo1.-naft. zb., Vol. 10, Zagreb, 1998. $ e . ~ t ~ o v i ~ ,  S. & Barcot, D.: Landslide Mlija 91 P R O F I L E  A -  B B L e g e n d  Fig. 5. Engineering-geological map and profilc. Legend 1 -Deluvium, 2 -Eluvium, 3 - Flysch, 4 - Uppcr Cretaceous limcstoncs (Senonian), 5 -Landslide, 6 - Unstablc slope boundary, 7 -Thrust OmiSka Dinara, 8 -Fault, 9 -Profile A-B, 10 -Ground water level (max. and min.), 11 -Boring Ground water level was monitored during three winter moths of 1986 and minimal levels in five boreholes, from -1.80 m to -4.10 m from the ground surface, were meas- ured as shown in Fig. 5. Maximal water levels ranged from -2.70 m to -0.65 m. In a well situated near the upper part of the landslide ground water level of -1.60 from the ground surface was measured. , Generally speaking, the entire area is an aquifer which can be confirmed by numerous wells, percolation of water in building pits in the body of the landslide and a notch close to the foot of the sope as well as periodical wells in the Borak area which is situated east from Mlija. Test boreholes were used to determine the thickness of Quaternary sediments which ranges between 6.0 rn and 7.40 m in the inspected profile. Engineering-geological model of the slope Two basic engineering-geological elements are: Up- per Cretaceous carbonate complex q) and clastic Eo- cene flysch (E2, 3). The former is we1 \ cemented and well petrified rock mass while the latter can be described as poorly petrified marl beddings ()>soft rock<<). Well ce- mented and well petrified but jointed sandstones (arenites) in the form of intercalations can be sporadi- cally observed in marls. Engineering-geological map and characteristic profile obtain by boring is shown in Fig. 5. The surface area is formed from deluvial deposits (I) which is covering de- graded flysch-Eluvium (11) and the base rock mass - flysch (111). There are two boudary planes between mentioned elements. One of them is at the contact of deluvium and eluvium and it should be considered as potential slope failure plane. Another negative factor is the ground water level mainly in the contat zone. Geophysical surveys (D u j mi 6 ,  1991, and K a  p e l  j e t a I. ,  1994) provided the minimum velocity of propa- gation of longitudinal seismic waves in surface beddings as: 660 m/s in deluvium, and up to 1250 m/s in eluvium. Velocities to 2000 m/s were registered in medium to well jointed and weathered flysch while greater velocities were registered in non-degraded flysch (ap rox. 3000 rnh). In areas with more sandstones (arenitesyvelocities greater than 3800 m/s were registered. Laboratory tests ( R o j e - B o n a c c i  e t  a l . ,  1986)wereused to 7btain soil parameters of the slope surface: c=22 kN/m , fric- Rud.-gal.-naft. zb., Vol. 10, Zagreb, 1998. 92 jcrtanovik, S. & Badot, D.: Landslide Mlija LOCATION: MLWA BORING No: 8.3 DEPTH: 2.7-3.0 rn PARTICLE SIZE CURVE 1W 90 n 80 70 fi 60 B 50 - f r rn z 4a !2 Z rn 30 rn P m O M 5 r g@ 10 $82 I ! I ! ! ! !  ! ! 1 1 1 1 1 1  I I I I 1 1 1 1 1 1  I I I I I I ~ I I  I I I 1 1 1 1 1 1  I I I I 8 g J  m L I I C W N  . I - i p  s s %  e d O b C) in O -; g -$ ; ' , - & ; '  ' ' iU*rnrn 5.4 6 J o  g 8 ?a a $j $ 5  Fig. 6. Particle size distribution curve of the deluvium from the boring B-3 1 7 5  tion angle cp =21,3O and unit weighty =20 kN/m3. These Conducted explorations enabled to define engineer- parameters were used for slope stability analyses for ing-geological model of the slope that consists of three maximal and lowered ground water level using personal units: flysch-eluvium-deluvium. computer. Generally speaking the entire slope is an aquifer. Characteristical grain size distribution curve of the Sporadically tensile cracks can be observed. They are not deluvial material from the boring B-3 is shown in Fig. 6 widespread but it is obvious that the slope is unstable due while the subsurface exploration log of the borin B-3 is to slow movements - creeping. There is also a possibility shown in Fig. 7. Standard penetration test ( S ~ T  using of the sudden slope failure especially in places where the cone and cutter shoe obtained results between 13 natural equilibrium was deranged due to cutting into the and 21 blow for the same material shown in grain size slope. As it could have been observed by field visits, all distribution diagram cuttings for purposes of preparation works for the con- According to the s e i s m i M a of the Yugoslavia ~tlllction have caused slope failure of a greater extent. (1987) investigated location is situated in the zone for In the area of Borak the following can be observed: which for the recurrene period of 200 years and prob- defollnations of new stone walls (Fig- 8), creeping of blocks and deluvial breccias (Fig. 9) and cracks in asphalt ability 63% maximum earthquake intensity of 8' of MSC pavement (Fig. The most vivid example of inade- can be evcted while for the recurrence period of quate foundations on the Mlija slope, as can be observed 500 years 9' of MSC Scale can be expected. in Fig. 11 and Fig. 12, is an family house overturned In order to determine seismic impact on engineering during the construction. The structure was situated on structures of the 1st category, it is necessary, according the top of the slope (south from the local road) and had to the >>Regulations on technical ~~~rmat ives  for the shallow foundations in deluvium which was the mass that construction of buildings in seismic areas<< (An o n . , moved. 1981) to determine the impact of local Soil condition. Based on obtained infomation, a conslusion can be With regard to the slope structure, investigated area drawn that the slope is in the state of constant creeping belongs to the third category (poorly compacted and soft due to the hydrodynamic pressure, with sporadical fail- soil, semi solid clays, medium compacted sand etc. with ures of local character while each intervention as the thickness greather than 10.0 m). Due to engineering- slope cutting causes landslide and inadequate founda- geological features of the slope, it is necessary to use tions overturning of houses. intensity of 9' of MSC Scale for structural analyses, and ~ ~ ~ ~ d ~ ~ i ~ ~ ~  be constructed in non-degraded to foundations in the ground of the first category, flysch beddings, with deeper strictly controlled excava- that is non-degraded fl~sch which ~ccurs at the average tion if flysch is found on depths less than 3 m. Pilots depth of 6 to 7 m. should be used when non-degraded flysh is found on Discussion and conclusion greater depths. If intervention and construction costs are to high construction should be avoided. Therefore, any The Mlija slope is situated within tectonic unit Terti- action in this slope should be strictly programmed, and ary sinclinore and is limited with the thurst OmiSka preparatory works designed and undertook before any Dinara, which is elevated and lies over the Eocene flysch other activity. which is the base rock mass of the slope. Final form of Preparatory works should consist of drainage of the slope is a result of subsequent physical-geological ground water and intake surface Water out of the processes in the ground. construction site. Drainage should cover the permeable Rud.-ge.01.-naft. zb., Vol. 10, Zagreb, 1998. ~estamvid, S. & Barcarcot, D.: Landslide Mlija 93 LOCATION : MLIJA BORING No : B-3 DEPTH : 2.70 - 3.0 m. SCALE f 1 : 100 SUBSURFACE EXPLORATION LOG STANDARD PENETRATION TEST GROUND WATER LEVEL Fig. 7. Subsurface exploration log of the boring B-3 Rud.-geo1.-naft. zb., Vol. 10, Zagreb, 1998. 94 $e~tamvif, S.& Bartot, D.: Landslide Mlija F ~ L  icd t : duc to the slopc movemcnt Fip. 8. Crack in thc retaining wall of thc family housc in thc arca of - - Borak Fig. 12. Shallow foundation. in deluviurn mass which moved as a cause of overturning Fig. 9. Crnmnin- 3f blocks and deluvial brcccias i~ ~ 1 ~ ~ ~ .  R o j e - B o n a c c i  e t  a l . ,  1986)determinedthatsur- ace zone is in a state of constant slow creeping because f of the alternation and weakening of shear strength pa- rameters. Cause is the constant process of weathering of marls from flysch due to water activity and deformations of the material because of its movement due to hydrody- namic pressure. According to the results of explorations as well as deformation monitoring of the slope after the cutting, it can be assumed that the shear stress is ap- proaching residual shear strength in the deluvium. Sys- tematical monitoring of the Mlija landslide has not been established. However if OmiS by-pass road is going to be constructed and the construction of family houses will continue, without systematic monitoring of the ground water level variations and velocities of the moving mate- rial, it will not be possible to obtain reliable information for interventions in the ground and safe foundations. Recrii~rd: 1998-05-20 Accepted: 1998-07-07 K a p e l j ,  J . ,  B u l j a n ,  R: & DujmiC D. (1%): InZenjersko- surface area to depth of approx. 4.0 m from the ground rcoldka i gcofiz~Eka ~straiivanja uzduf trase ccstovne zaobi- surface. faznicc grada OmiSa. Zhornik simp. ~Geotehnika prometnih gractcvinacc. SaopCenja 1, 133-145, Novigrad. Impact of possible high embankments On the entire M a r inE i  6 ,  S . ,  Karol i j  a ,  B . ,  M a j c c  n ,  i., BoroviC, I , ,  slope stability should be considered. Stability analyses BrklC, M. ,  Magah,  N., BenEck ,  0. .  M a m u f i t .  P .  Rud.-geol.-naft. zb., Vol. 10, Zagreb, 1998. &Pa vi E i C ,A .(1975): Osnovna geoloSka karta, List OmiS, M. u pcxlruEju Mlija za potrebe idejnog rjeSenja trase zaobilaznice 1:100000. Institut za geoloSka istrdivanja, Zagreb. Omib oko stacionake 4+100. GI-FGZ Split, RN 3910-315528- MarinEiC, S . ,wKoro l i j a ,  B.,  Marnut i f ,  P., MagaS, N. ,  142J85. Arhiv IGH - PC Split. Ma'cen,  Z. ,  BrkiC, M. & BenEek, D. (1977):TumaE Ses tanovi f ,  S. & BarEot, D.  1991 InZenerskogeoloSka za ~ G K  List Omig. Institut za geoloSka istra2ivanja, 51 str., osnova obilaznice Omiza. GI - FG Sp ~ t ,  RN 4 910-2 - 349226- Zagreb. 27/91, svezak A-7. Arhiv IGH - PC Split. Roje-Bonacci ,  T., Dodig,  D., JuriC, B .  & BarEot, D. ********* (1987 : SeizmoloSka karta SFRJ s povratnim periodima. M. (1986): IzvjeStaj o geomehaniEkim istrdnim radovima na klizigtu 1:1000000, k ajednica za seizmologiju, Beograd. 

INŽENJERSKOGEOLOŠKE ZNAČAJKE KLIZIŠTA MLIJA I NJIHOV UTJECAJ NA INŽENJERSKE OBJEKTE (OMIŠ, JUŽNA HRVATSKA)

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