AB STRA CT The Grohovo landslide is the largest active slope movement along the Croatian coast, situated on the north-eastern slope in the central part of the Rječina River Valley (north­eastern coastal part of Adriatic Sea, Croatia). Slopes in this valley are formed of siliciclastic rocks (i.e., fl ysch), while the limestone rock mass is visible on the cliffs around the top of the river valley. The slopes are at the limit of a stable equilibrium state, and slope movement phenomena have been recorded since the 19th century. Samples for laboratory testing were taken from the fl ysch bedrock, weathered zone and slope deposits to provide specimens for determination of their mineralogical, physical and geotechnical properties. Correlation between mine­ ralogical and geotechnical properties and their infl uence on sliding processes are presented here. The clay fraction in samples ranges from 17 % to 38 %. Clay activity of the tested samples is from 0.45 to 0.89, and the residual friction angle varies from 13.0° to 17.7°. These results correspond to the presence of kaolinite and illite groups of clay min­ eral. Both the mineral composition and decrease in strength of fi ne­grained soil materials, due to the increase of pore water pressures, contributes to slope movements. Keywords: Grohovo landslide, fl ysch, clay, geotechnical properties, mineral composition, grain size Geologia Croatica 67/2 127–136 9 Figs. 1 Tab. Zagreb 2014 Geotechnical properties in relation to grain-size and mineral composition: The Grohovo landslide case study (Croatia)  Čedomir Benac1, Maja Oštrić2 and Sanja Dugonjić Jovančević1 1 University of Rijeka, Faculty of Civil Engineering, Radmile Matejčić 3, Rijeka, Croatia 2 Croatian Waters, Department of Rijeka, Đure Šporera 3, Rijeka, Croatia doi: 10.4154/gc.2014.09 The infi ltration of vadose water and increase of inter­particle and inter­aggregate pore water causes swelling of the mont­ mo rillonite and illite groups of clay minerals. These proces­ ses cause the fl ysch rock massto gradually increase in volume and soften while the clay content as well as the clay fraction increases. The mineral composition is a key factor control­ ling the value of residual friction angle fr for soils (SKEMP­ TON, 1985).The shear strength of soil greatly depends on the type of clay minerals present and the quantity of inter­ particle and inter­aggregate pore water (SELBY, 2005). The Palaeogene fl ysch of the Adriatic part of Croatia is characterized by the alternation of fi ne­grained sedimentary rocks including shale, marl, silt and sandstone (MARINČIĆ, 1981).The ratio of each of these fi ne grained rocks within 1. INTRODUCTION Due to their geological complexity, fl ysch formations are dif fi cult to characterize from a geotechnical behaviour point of view. Some attempts at applying rock mass classifi cation systems to these complex rock masses have been carried out (HOEK & MARINOS, 2001). Weathering processes signifi cantly infl uence changes of strength properties of the fl ysch rock mass. In the fi rst stage of weathering, characteristic grey­bluishcolour changes to yellow­brownish. The cause of this change is the oxidation of dispersed pyrite that expands in volume and destructs the original structure of the bedrock. The content of the clay frac­ tion in the weathered zone is increased by the alteration of silicate minerals in clay (ATTEWELL & FARMER, 1979). Geologia CroaticaGeologia Croatica Geologia Croatica 67/2Geologia Croatica 128 this rocky complex, varies greatly even in nearby locations. The flysch rock complex in the Adriatic part has in the past been exposed to stresses of different intensity and direction (KORBAR, 2009). The investigated area is situated on the north-eastern slo pe of the Rječina Valley. This valley is a part of a domi- nant morphostructural unit which strikes in the direction of the Klana – Rječina River Valley – Sušačka Draga Valley – Bakar Bay – Vinodol Valley (Fig. 1). The geological struc- ture could be considered to be a Palaeogene flysch syncline limited by faults, analogous with the tectonic style of the Vinodol Valley (BLAŠKOVIĆ, 1999; BENAC et al., 2009). The unstable phenomenon studied here known as the Grohovo landslide, is the biggest known active landslide on the Adriatic coast and is located in the wider unstable zone with numerous features of dormant historical landslides. The slopes are at the limit of a stable equilibrium state, and sev- eral slope movement phenomena have been recorded since the end of the 19th century (VIVODA et al., 2012). The dy- namics and complexity of the whole phenomenon was pre- sented in several papers (BENAC et al., 1999; BENAC et al., 2002; BENAC et al., 2005; BENAC et al., 2006). Remediation works on landslide locations were never performed due to the huge displacement mass and relatively stable equilibrium state of the lower parts of the Grohovo landslide body. Measurements of benchmark movements and changes of groundwater level were provided periodically, every two to three months from 1998 until 2010. Maximum displacements were determined in the upper part of the slope (BENAC et al., 2011). An advanced comprehensive monitoring system was in- stalled on the Grohovo landslide in 2011. It includes geodetic monitoring with an automatic total station measuring 25 geo­ detic benchmarks, a GPS master unit and nine GPS re ce i vers, as well as geotechnical monitoring equipment including ver- tical inclinometers, long-span extensometers, pore pressure gauges, seismographs and rain gauges (ARBANAS et al., 2012). Yet, earlier papers do not contain analyses of the relationships between the mineral composition and geotech- nical properties of silty clay from colluvial and weathering zone materials. Due to the low strength parameters, a major part of the sliding surface was formed in these materials (BE­ NAC et al., 2005). The aim of this paper is to analyse the relationship be- tween the geotechnical properties of the finegrained materi- als taken from the Grohovo landslide location, with the grain size distribution and mineral composition. Laboratory inves- tigations performed in 2000 and considered in this research (IGH, 2000), were supplemented with new specific minera- logical and geotechnical analyses to obtain more reliable re- sults. Consequently there are no uniform analyses performed on all 22 samples, which has aggravated interpretation of the results. 2. GEOLOGICAL SETTING OF THE STUDY AREA The unstable phenomenon on the north­eastern slope of the Rječina Valley is situated between the Valići Reservoir and the Pašac Bridge. The bottom of the valley is 150 to 200 m above sea level, and the peaks in the north­eastern side reach Figure 1: A simplified geological map of the investigated area according to BENAC et al. (2009): 1 – Upper Cretaceous and Palaeogene carbonate rocks, 2 – Palaeogene siliciclastic rocks-flysch. Figure 2: Geological map of the Grohovo landslide area: 1-Palaeogene limestones, 2-Palaeogene flysch rock mass mostly covered by colluvium, 3-alluvial sediments, 4-position of boreholes. Benac et al.: Geotechnical properties in relation to grain-size and mineral composition: The Grohovo landslide case study (Croatia) Geologia Croatica 129 a height of 412 m.The Cretaceous and Palaeogene limesto­ nes are situated on the top of the slopes, while the Palaeo- gene siliciclastic rocks or flysch are located on the lower slo pes, including the bottom of the valley (Fig. 2). During neo-tectonic and recent tectonic movements the limestone rock mass was repeatedly faulted and fractured. Such tectonic movements and weathering processes enabled the separation of limestone blocks and their gravitational slid­ ing over the flysch bedrock, disintegration of the rock mass, as well as accumulation of talus deposits at the foot of the rock cliffs (BENAC et al., 2006). Siliciclastic or flysch bedrock is characterized by great lithological heterogeneity, because of the frequent vertical and lateral alternation of different lithological sequences. Microscopic petrological analysis of the bedrock has shown the presence of silty marl, laminated silt to silty shale, as well as fine­grained sandstones. Unlike the limestones, the flysch rock mass is more prone to weathering, which results in a clayey weathering zone on the flysch bedrock. Over time, coarse grained fragments of limestone, originating from the rock falls were mixed with clay from the weathered flysch zone forming several metre thick slope deposits (BENAC et al., 2005) (Fig. 3). The Grohovo landslide has the form of a complex land- slide with 13 landslide bodies. According to the WP/WLI Suggested Nomenclature for Landslides (IAEG, 1990), the length of the displaced mass is Ld = 420 m, width is Wd = 200 m, and depth is Dd = 6–20 m. The estimated total vol- ume of the displaced mass is 850.000 m3. The total displace- ment of the landslide to eis more than 20 m in the initial state of slope movement (BENAC et al., 1999). The affected slope has distinctive filtration anisotropy. Groundwater flow in cohesionless talus material is very ra­ pid, in contrast to cohesive talus material, where infiltration and water flow are very slow. Subsurface groundwater can be accumulated locally in clayey to silty slope material and in the weathered bedrock zone. This water originates either from direct infiltration of precipitation, or from the karst aq- uifer on the top and behind the slope. The groundwater level changed less than 10 cm in the upper boreholes (G­5 and G­7), but varied up to several metres in the lower boreholes (G­1, G­2 and G­3) (Fig. 2) (BENAC et al., 2005). 3. METHODS Material samples have been recovered to provide specimens for laboratory testing to obtain data on their mineralogical, physical and geotechnical properties.The 22 representative samples were selected and taken from the flysch bedrock, weathering zone and from slope deposits­ colluvium. From a total of 22 samples, 18 were taken from borehole cores (Fig. 3). The boreholes were drilled during the second phase of field investigation in 1999 (IGH, 2000). The other 4 sam- ples were taken from the surface during 2006 (Fig. 2, 3 and 4, Table 1). Table 1 summarizes the results of all the performed tests. On 12 samples selected from borehole cores, quantitative and semi­quantitative mineralogical analyses were per- formed (No. 1­4, No. 6­9 and No. 13­16). For the purpose of mineralogical analysis, grain size distribution of the fine­ grained fraction (up to 1 mm) was also determined. This ana­ ly sis will be referred to as sedimentological methods of grain size analysis in the following text. In this way, the finer frac- tion percentage is additionally increased. One additional mineralogical analysis was performed on sample from the surface (No. 22). Figure 3: Photo of the core from borehole G-2: boring interval 0.0-4.0 m (photo: Č. Benac, 1999). Borehole location is shown in Fig. 2. Geologia Croatica 67/2Geologia Croatica 130 Standard geotechnical laboratory tests were performed on 13 borehole samples (No. 2­5, 7, 10­12, 14­22) and on 4 surface samples (No. 19­22 in Table 1). Grain size analysis (sieving and hydrometry) were performed on all samples, according to ASTM standard (IGH, 2000). On some of those samples, Atterberg limits, plasticity index and water content were determined. Results of sedimentological and geotech- nical methods grain size distribution analysis are shown in Fig. 5. The clay fraction (CF) refers to the percentage of particles <0.002 mm, as determined by geotechnical grain size analy- sis. Most of the authors recognized that the use of CF as an indicator of platy shaped particles did not give real insight into the soil composition. Measurement of the clay content (CC) proportion in total clay minerals indicates soil cha rac teristics more comprehensively than CF (TIWARI & MARUI, 2005). However, for practical reasons, the use of CF for the descrip- tion of soil behaviour is still more widely used (LUPINI et al., 1981; SKEMPTON, 1985). The CF in Fig. 5B ranges from 17 % (No. 15) to 38 % (No. 14) (Table 1). The sample (No. 20) has been analysed using a scanning electron microscope (Philips XL­30). The minerals were iden­ tified by the habitus of crystallographic shapes and identifi- cation picks in the energy spectrum of x­rays (EDAX). Shear strength tests were also performed on samples from borehole cores: three direct shear tests (No. 10­12) and one ring shear test (No. 3). Additional ring shear tests were performed on surface samples (No. 19­22). Direct shear tests were performed for normal stress of 50, 100 and 200 kPa to determine peak shear strength and ring shear tests for normal stress of 100, 200 and 300 kPa to determine residual shear strength (Table 1). A remoulded specimen was used in ring shear tests in which the first shear surface was formed after consolidation and before shearing. 4. RESULTS Both methods of grain size analysis, (sedimentological (Fig. 5A) and geotechnical (Fig. 5B)) show that the silt and clay fractions prevail in all samples. Consequently, material can Table 1: Summarized results of analyzed samples. Sample No. Borehole /depth (m) Grain size analysis CF W0 (%) WL (%) Wp (%) Ip (%) A= Ip/CF Im Ring shear Direct shear c (kPa) f (°)Sedi- ment. Geo- tech. 1 G-1/ 5.0 m +               1.09         2 G-2/ 1.0 m + + 19           1.30         3 G-2/ 2.5 m + + 27 19.41 42.6 20.53 22.07 0.82 1.23 +   16.67 16.1 4 G-2/ 4.5 m + + 37           1.37         5 G-3/ 2.0 m   + 23                   6 G-3/ 3.0 m +               1.31         7 G-3/ 3.4 m + + 33           1.38         8 G-3/ 7.0 m +               1.02         9 G-3/ 10.3 m +               0.23         10 G-4/ 2.5 m   + 20 14.19 32.22 18 14.22 0.71   + 9.5 23.7 11 G-4/ 4.3 m   + 36 19.2 33.97 17.73 16.24 0.45   + 1.0 26.1 12 G-4/ 5.5 m   + 35 16.32 33.01 17.28 15.73 0.45   + 7.5 25.1 13 G-4/ 14.0 m +               0.68         14 G-5/ 1.0 m + + 31           1.54         15 G-5/ 4.5 m + + 17           0.68         16 G-5/ 8.5 m + + 26           1.14         17 G-6/ 11.0 m   + 32                   18 G-7/ 20.0 m   + 18                   19 G-1/ 0 m   + 28   41.14 23.75 17.4 0.62 +   0 13.0 20 G-2/ 0 m   + 27   37.41 21.72 15.69 0.58 +   0 14.5 21 G-3/ 0 m   + 27   37.38 22.5 14.88 0.55 +   0 17.7 22 G-4/ 0 m + + 19       16.99 0.89 2.14 +   0 15.0 Benac et al.: Geotechnical properties in relation to grain-size and mineral composition: The Grohovo landslide case study (Croatia) Geologia Croatica 131 be considered clayey silt or silty clay. Fig. 5B shows that the average particle size (D50) according to geotechnical me­ thods of analysis ranges from 0.004 to 0.042 mm. Fig. 5A shows a much wider range of D50 according to sedimento- logical methods analysis from 0.0028 to 0.056 mm. Results of Atterberg limit testing and plasticity indices are given in Table 1 and are presented in Fig. 6. Besides showing consistency limits (liquid limit and index of plas- ticity) in Fig. 6, areas of the main clay minerals: kaolinite and illite groups are also shown. The tested materials have low to medium plasticity according to the plasticity index (Ip = 14–22 %), and liquid limit (wl = 32–43 %) respectively. Clay activity is defined as the ratio of plasticity index (Ip) and clay fraction (CF). This simple index gives the in- Figure 4: Schematic borehole cross-sections with locations of the analyzed samples. Figure 5: Results of grain-size analysis: a-sedimentological method, b-geotechnical method. Geologia Croatica 67/2Geologia Croatica 132 sight into the mineral composition of materials (Fig. 6 and 7). Water quantity that can be absorbed within soil particles depends on the quantity and type of clay minerals. The high- est clay activity occurs in the montmorillonite group, then illite and the lowest in the kaolinite group. Active clays pro- vide the most potential for expansion. Activity of the tested samples ranges from A = 0.45 (No. 12) to A = 0.89 (No. 22). Accordingly, samples for non­active clays (A < 0.75), include samples No. 10­12 and No. 19­21, and normally active clays (A = 0.75–1.25), for samples No. 3 and No. 22 (Table 1). X­ray diffraction analysis was performed and the fol- lowing minerals were identified: quartz, calcite, plagioclase, K­feldspar and phyllosilicates (Fig. 8). Quantitative minera- logical analysis detected the presence of the following clay Figure 6: Diagram of Atterberg limits and plasticity indexes (according to GRIM, 1968). Numbers refer to analyzed samples (Table 1). Figure 7: Estimation of expansiveness of clay in analyzed samples (according: BELL, 1993): numbers refer to analyzed samples (Table 1). Benac et al.: Geotechnical properties in relation to grain-size and mineral composition: The Grohovo landslide case study (Croatia) Geologia Croatica 133 minerals: kaolinite, illite, chlorite, mixed­layer clay miner- als, and in some samples vermiculite (not detected in sample No. 9) and smectite (not detected in sample No. 5­9 and No. 14­16) (IGH, 2000). Phyllosilicates in tested samples are prevalent and are re presented by micaceous minerals, kaolinite, vermiculite, smec tite and chlorite groups, and mixed­layer clay minerals. From the mineral composition of the fractions <4 μm, it is clear that the main clay minerals are illite and kaolinite and spo radic ones are vermiculite, smectite and mixed­layer clay mi nerals. Sample No. 22 consists of illite­smectite minerals (Fig. 8). Quartz, calcite and phyllosilicates constitute 86–96 % of the mineral composition (Fig. 8). Quartz, calcite, and feldspar are the commonly observed massive minerals, while the most common types of clay minerals include: kao linite, illite, smectite, halloysite, chlorite and micaceous minerals. Grains of partially dolomitized calcite are observed in sample No. 20 using the scanning electron microscope. Clay minerals from chlorite or chlorite-illite groups have dimen- sions between 5–15 mm. The particles of albite (plagioclase group) have dimensions around 40 mm. Micaceous minerals are visible only sporadically. Individual crystals of quartz have dimensions between 5–10 mm. Skeletal principal micro- structural types prevail in analyzed material (Fig. 9). The index parameter Im (mineralogical index) was in- troduced in Table 1. It is defined as a ratio of the mass frac- tion of phyllosilicates and the sum of quartz and calcite. From among the different indices and parameters, Im was used to define the proportion of platy (clay) to rotund (mas- sive) particles. The value of Im ranges from 0.23 (No. 9) to 2.14 (No. 22). The lowest Im is the result of the smallest frac- Figure 8: Mineral composition of samples (Table 1): a-content of minerals, b- content of massive and fine-grained particles. Figure 9: Electron micrographs of sample No. 20 (position of sample is pre- sented in Fig. 4). Geologia Croatica 67/2Geologia Croatica 134 tion of phyllosilicates (16 %). In contrast, sample No. 22 has the highest value of Im due to the highest fraction of phyllo- silicates (60 %). Results of eight shear strength analyses are given in Ta- ble 1. Peak values determined by direct shear are in the range of 23.7°1.25 for montmorillonite groups (BELL, 1993). There- fore, according to activity, the tested samples are in the group of illite and kaolinite (Fig. 7).The quantity of adsorbed wa- ter on the external surfaces of the clay minerals greatly de- pends on their size and clay fraction content. The interlayer water quantity depends mostly on the quantity and the type of the swelling clay minerals in the soil composition and their exchangeable cations (GRIM, 1968). The prevailing gravitational type of sediment transport, which is usual during the sliding process, has a strong influ- ence on the orientation of fine grained particles (LAMBE & WHITMAN, 1979). According to morphogenesis of slope deposits (BENAC et al., 2005) the preferred orientation of platy particles and laminar microstructural type could not be expected. The results of analysis using scanning electron mi- croscope illustrate the chaotic texture of particles (Fig. 9). Grain­size and mineral composition were correlated to geotechnical properties. Geotechnical properties of fine­ grained materials which prevail in the lower part of the land- slide are mostly unfavourable and often determined by clay minerals (Fig. 8).The values of the peak and residual friction angles fr obtained from direct shear apparatus (average 25°) and ring shear apparatus (average 15°) show a difference of 10°. Parameters of residual strength obtained for borehole sam- ple (No. 3) and surface samples (No. 19­22) were in the same range regarding the residual friction angle (13.0°< fr <17.7°), but they differed greatly with regard to the value of cohesion. The cohesion determined in ring shear testson samples taken from the surface (for a cumulative displacement of 300–350 mm) was c = 0 kPa, while the borehole sample (No. 3) had an unusually high value of cr = 16.7 kPa (Fig. 4, Table 1). Residual cohesion is often assumed to be cr = 0 kPa, es- pecially after the sample is sheared at large displacements (SKEMPTON, 1985). There has been some discussion on the accuracy of this assumption, as cohesion values as large as 9.2 kPa have been observed for residual strength enve- lopes for some soils (TIWARI et al., 2005). Most previous studies indicated a reduction in the re- sidual friction angle (fr) with an increase in the clay fraction (CF) (LUPINI, 1981; TIWARI & MARUI, 2005). Many of these studies tried to correlate the residual friction angle of soils with their index parameters. The effect of particle reo- rientation has an influence only in soils having CF values exceeding 20–25% (SKEMPTON, 1985). According to LU- PINI et al. (1981) three modes of shearing have been identi- fied: turbulent (CF < 25 %), sliding (CF > 50%) and one that is transitional between these two. The mineral composition is a key factor controlling the magnitude of fr for soils that exhibit the sliding shear mode. The angles of residual shear- Benac et al.: Geotechnical properties in relation to grain-size and mineral composition: The Grohovo landslide case study (Croatia) Geologia Croatica 135 ing resistance of the three most commonly occurring clay mineral groups are approximately equal to 15° for kaolinite, 10° for illite and 5° for montmorillonite (SKEMPTON, 1985). Electrostatic bonding has been reported as contribut- ing about 80 % of shear strength for the montmorillonite group, 40–50 % for the illite group and <20 % for the kao- linite group (SELBY, 2005). The obtained residual friction angles (fr) are in the range of the illite and kaolinite groups, but values disperse due to the wide range in clay fraction (CF) composition of samples. Regarding the influence of CF on fr, the data did not show similar trends to those widely reported. Samples with the highest CF (No. 21, CF = 37) had the highest residual fric- tion angle (fr = 17.7°). Sample No. 22 has the lowest CF value (CF = 19) but the highest fraction of phyllosilicates (Table 1; Fig. 8). This can be explained by the existence of micritic material and aggregated particles in soils originat- ing from flysch rock mass, which makes it difficult to give any precise relationship between fr and CF (KALTEZIOTIS, 1993). Soils derived from a rock mass like flysch (marls, mud- stones and shales) may generate platy particles by degrading during shearing (LUPINI et al., 1981). Similarly, bonded co- hesive soils might exhibit higher residual friction angles than those in the laboratory due to bonding and particle aggrega- tion, which are destroyed when tested in a remoulded state in the laboratory. Decalcification during weathering, reduces the calcite content due to the elimination of medium and coarse silt particles and the increase in clay fraction and plas- ticity index (HAWKINS & Mc DONALD, 1992). As a re- sult, lower internal friction angle values were obtained in weathered samples compared to the corresponding values in non­weathered samples. The last large landslide occurred after a longer rainy pe- riod (BENAC et al., 1999), and according to historical notes, sliding in the Rječina Valley often appears after heavy rain- fall (VIVODA et al., 2012). The stability analyses have in- dicated that the high water level influences landslide insta- bility (BENAC et al., 2005). Accordingly there are indications that increased saturation of the fine grained particles influ- ences the strength properties on the potential sliding surface. Similar conclusions have been drawn for other terrains form ed in a Palaeogene flysch rock mass (FIFER BIZJAK & ZUPANČIČ, 2009; DUGONJIĆ JOVANČEVIĆ & AR- BANAS, 2012). Based on past periodic groundwater measurements and benchmark movements, it was not possible to establish any clear correlation between these two parameters (BENAC et al., 2005; BENAC et al., 2011). The establishment of a new monitoring system could provide continuous measurement data (ARBANAS et al., 2012; MIHALIĆ & ARBANAS, 2013; ARBANAS et al., 2014). More precise data could be taken from installed vertical inclinometers, long-span extensom- eters, pore pressure gauges and rain gauges. Therefore it will be possible to investigate the influence of water on the strength parameters of fine grained sediments in the investigated slope in the future. 6. CONCLUSIONS The investigated landside is the biggest known active mass movement in the Adriatic coast and is located in the wider unstable zone with numerous traces of dormant landslides. The slopes are at the limit of a stable equilibrium state, and mass movement phenomena have been recorded since the 19th century. Quartz, calcite, feldspars and phyllosilicates including micaceous and clay minerals, comprise 86 to 96 % of the mineral composition of the analyzed samples taken from flysch bedrock, the weathering zone and colluvium. The clay fraction ranges from 17 % to 38 % in samples. The most common groups of clay minerals are: kaolinite, illite and chlorite. Smectite and vermiculite were found in some sam- ples. Clay activity of the tested samples is from 0.45 to 0.89. This is in the range of low to normally active clays and cor- responds to kaolinite and illite groups. The results of analy- sis using scanning electron microscope presented the chaotic microstucture of particles that corresponds to the morpho- genesis of the investigated slope. The residual friction angle is in the range 13.0°