Željko Arbanas

The Croatian–Japanese Joint Research Project on Landslides: Activities and Public Benefits

The main activities of the research groups involved in the Croatian–Japanese joint research project on “Risk Identification and Land-Use Planning for Disaster Mitigation of Landslides and Floods in Croatia” include investigations of recent landslides using landslide monitoring, the establishment and development of early warning systems for landslides and the definition of hazard zones using a methodology for assessing susceptibility and hazards based on local geological and landslide conditions. This project is also designated as on-going IPL project 161. The final objective of the joint research is the development of risk mitigation measures that can be instituted through urban planning. Dissemination and use of the results should ensure significant benefits for the local and regional communities that are directly and indirectly threatened by landslides.

Review of Monitoring Parameters of the Kostanjek Landslide (Zagreb, Croatia)

Since 2011, in the framework of the Croatian-Japanese SATREPS FY2008 Project, scientists have been working on the establishment of the Kostanjek landslide monitoring system in the City of Zagreb (Croatia). External triggers at Kostanjek landslide are measured with rain gauge and accelerometers. Displacements at the surface are measured by GNSS sensors and extensometers, while subsurface displacement is measured by vertical extensometers and inclinometer. Hydrological measurements consist of groundwater level measurements, discharge measurements, chemical and isotope analysis. Monitoring sensors recorded landslide reactivation due to external triggers in the winter period of 2012/2013. During the period from September 2012 to March 2013 the total cumulative precipitation was 793.7 mm and horizontal displacements were in the range of 9–20 cm. The installed monitoring sensor network proved to provide reliable data for the establishment of relations between landslide causal factors and landslide displacement rates aimed at establishing threshold values for early warning system.

The ICL Adriatic-Balkan Network: analysis of current state and planned activities

International Consortium on Landslides (ICL) Adriatic-Balkan Network was established in January 2012 as one of eight regional and thematic ICL networks to promote activities of the International Consortium on Landslides and the International Programme on Landslides. This paper presents the annual report of the ICL Adriatic-Balkan Network for the year 2012. The main activity of the regional network was to complete an overview of publicly available data and sources about landslides in the region, scientific and professional practices related to evaluation and mitigation of landslide hazard, as well as related legislative framework. Recommendations for the discussion and endorsement in the course of the ICL Adriatic-Balkan Network activities will be derived from the analysis of strengths, weaknesses, opportunities, and threats related to landslide issues which are present at the national level in Croatia, Slovenia, and Serbia and at the regional level. Discussion and endorsement of the recommendations are planned as a further activity which will be held in March 2013 in Zagreb (Croatia), during the planned first regional symposium on landslides in the Adriatic-Balkan Region.

A complex landslide in the Rječina Valley: results of monitoring 1998-2010

Results of landslide monitoring undertaken between 1998–2010 on the north-eastern slope of the central part of Rjecina Valley in front of Grohovo village (north-eastern coastal part of Adriatic Sea, Croatia) are presented. This is the largest regional active landslide. The slopes around the Rjecina riverbed are formed in siliciclastic sedimentary rocks with flysch characteristics. The bedrock is mostly covered with unstable slope formations. A limestone rock mass is visible on the cliffs around the top of the river valley. The landslide is complex and retrogressive, with 13 sliding bodies. It occurred in December 1996 by displacement of an initial landslide body where movement had been registered in the 19th century. The limestone mega-blocks and separated rocky towers on the top of the slope have also moved, which is an atypical phenomenon of the flysch slopes in the area of Rijeka. After initial sliding ceased, and major movements subsided, monitoring of benchmark movements from 1998 until 2010 determined further maximum displacements on the upper part of the slope, and minimum movement in the lower part. The area of the Rjecina Valley from the Valici Dam to the Pasac Bridge was selected as a pilot area in the framework of the Croatian-Japanese bilateral joint research project. Monitoring results provided the basis of establishing an early warning system for possible landslide occurrence and estimating the degree of landslide risk.

Weathering Influence on Properties of Siltstones from Istria, Croatia

Slaking and weathering of weak rocks result in slope instability. Siltstones from flysch rock masses are highly susceptible to weathering, which causes rapid changes in the geotechnical properties and durability. This study investigated siltstone samples of different weathering grades from flysch rock masses from the Istria Peninsula, Croatia, and determined the effects of weathering on their engineering properties. Laboratory testing of siltstone samples of different weathering grades was conducted to determine the specific gravity, grain size distribution, Atterberg limits, and uniaxial compressive strength. The standardized slake durability index is not sufficient to classify the durability of weak rock masses such as siltstones. Therefore, the durability of siltstone samples of different weathering grades was quantified from the fragment size distribution after each of five slaking cycles. The tested samples were classified based on the disintegration ratio, and the modified disintegration ratio was used to determine potential long-term degradation of the tested samples. The results indicated that weathering has a significant influence on the plasticity, uniaxial compressive strength, and durability characteristics and thus affects the landslides and erosion processes in siltstones in the flysch zone of the Istria Peninsula.

Study of Landslides in Flysch Deposits of North Istria, Croatia: Landslide Data Collection and Recent Landslide Occurrences

The northeastern part of the Istrian Peninsula in Croatia is built of Palaeogene flysch deposits, in which instability is common and where a large number of landslides, with significant consequences, have been recorded. Study of landslides in flysch deposits of North Istria will be conducted as one of the main activities of ongoing IPL-184 Project. It includes analyses and recognition of sliding mechanisms to use in landslide modelling and to determine landslide susceptibility and hazard in flysch rock mass deposits. Several landslide types, mechanisms and conditions occur in the study area. The majority of studied landslides occurred during the spring and winter. Generally, the landslides are of rotational and translational sliding type, and rarely rock falls and debris flows. Local roads and rarely other structures and facilities suffered major damage as a consequence of the landslides. Landslide inventories, as well as landslide susceptibility maps, have never been carried out in the study area. Today’s knowledge about landslides in this area is based on investigations of individual landslides and partial scientific research. After the project started, some of the activities from all stages of the project have been initiated. The proposed first stage of the project includes supplementation of the existing database, and field investigations of recent and existing landslides in the study area, as well as soil sampling for laboratory testing. Moreover, the methods used for landslide susceptibility and landslide hazard assessment are presented. This paper presents the current state of investigations and research in the initial stage of the IPL-184 Project.

A Landslide Monitoring and Early Warning System Using Integration of GPS, TPS and Conventional Geotechnical Monitoring Methods

An advanced comprehensive monitoring system was designed and used on the Grohovo Landslide in Croatia. Equipment selection was based on scientific requirements and consideration of possible ranges of monitored values and sensors precision. Establishment of an early warning system and defining of alarm thresholds is based on existing knowledge of the landslide behavior as well as collected comprehensive monitoring data. The focus of the early warning system establishment at Grohovo Landslide was on an effective combination of sensors (equipment fusion) with respect to detecting device malfunctions and reducing false alarms in the future. The weakest component in the Grohovo Landslide monitoring system is power supply based on solar devices, field data collecting and the data transmitting from the field PC to the control room at the University of Rijeka. This paper presents the main ideas and advances of the monitoring equipment fusion as well as weaknesses of the applied monitoring system at the Grohovo Landslide.

Rockfall Hazard Analyses and Rockfall Protection along the Adriatic Coast of Croatia

During the last decade, large rockfalls occurred on the steep limestone slopes along the Adriatic Coast of Croatia, causing injury to people and serious damage to buildings and traffic facilities. The rockfalls along the limestone slopes were caused by unfavorable characteristics of the rock mass, weathering in combination with heavy rainfall and artificial influences during highway construction. Rockfall protection projects were conducted to protect human lives and facilities from future rockfalls. The rockfall protection program started with rockfall hazard analyses to identify the potential of rockfalls to occur and the potential consequences. At the locations of hazards where related risks were determined, detailed field investigations were conducted. Based on the indentified characteristics of potentially unstable rock masses, analyses of movement and resulting pathways were conducted. The trajectories, impact energy and the height of bouncing are dependent on slope geometry, slope surface roughness and rockfall block characteristics. Two protection measure approaches were adopted: prevention of rockfalls by removing potentially unstable rock mass or installation of rock mass support systems and suspending running rockfall masses with rockfall protection barriers. In this paper, rockfall hazard determination, rockfall analyses and rockfall protection designs for rockfall protection systems at selected locations on the limestone slopes along the Adriatic coast of Croatia are presented.

Causes of debris flow formation in flysch area of North Istria, Croatia

The middle and northern part of the Istrian Peninsula situated in the northwestern part of the Croatian Adriatic coast were formed in Paleogene flysch rock complex. Flysch zone on the northeastern part of the peninsula border with the mountain chain of Cicarija consists mostly of carbonate rocks. On the southwestern slopes of Cicarija Mountain, where the fault contact between carbonate and flysch rock mass exists, thick bodies of slope deposits are sporadically present. These slope formations were formed by mixtures of coarse-grained limestone fragments and silty-clay as the result of weathering and eroding of flysch bedrock. The described slope deposits are potentially unstable due to unfavourable hydrology conditions, and mass movements are frequent. The phenomena of debris flows, which caused the biggest damages on transport and other constructions, are largely present. In this paper is it presented the causes and triggering mechanisms of debris flow triggering, sporadically occurring in the Krbavcici village area near Buzet city. Potentially unstable soil mass do flysch bedrock erosions ranging to the limestone slopes of the Cicarija Mountain form filling narrow and length ravines. As a rule, the debris flows appear after hard rainy periods, when water content in clayish sediments reaches liquid limit and soft materials in ravines transform in liquid consistency mass. On the slope where these phenomena are occurring, two roads and a railway are present and the debris flows have been causing numerous damages and traffic interruptions. The occurrence of a particular debris flow was described in detail in this paper, along with its repetitive appearances covering the period from winter 1979 until today. The debris flow mass damaged the road embankment and caused their sliding about 300 m down the slope in one night in winter 1979. Debris mass movements triggered in ravine put into danger the bridge constructed over an old unstable zone in winter 2003, but fast intervention measures prevented the possible damage.

Influence of weathering processes on the shear strength of siltstones from a flysch rock mass along the northern Adriatic coast of Croatia

Weathering processes cause significant changes in the engineering properties of rocks. Slope instability in flysch rock formations along the northern Adriatic coast of Croatia is related to the effects of weathering on the shear strength of siltstones from the flysch rock mass. Therefore, changes in geotechnical properties according to weathering grade are of immense importance in relation to instability processes. In this work, we investigated siltstones from flysch rock masses in the study area, and evaluated changes in engineering properties due to weathering. The research began with field observations and determination of the strength of different weathering grades of siltstones in the area. Mineralogical and laboratory studies were subsequently conducted, and mineral content was determined for siltstones of different weathering grades. We also performed a series of drying–wetting cycles to simulate natural conditions of the weathering process involved in the disintegration of the rock material into sand-sized and smaller particles. This weathering process resulted in disintegration of the siltstone rock mass into smaller particles that were not a unique rock block, with the soil-like material consisting of unbound particles of rock. Laboratory tests were also carried out on the soil-like material to determine the specific gravity, grain size distribution, Atterberg limits and residual shear strength for the different weathering grades of siltstones. Based on this research, we determined the changes in engineering properties for different weathering grades. Our results underscore the significant influence of the weathering process on mineral content, cation exchange capacity, liquid limit and residual shear strength, thus affecting slope stability in siltstones in flysch rock masses.