Specific aspects of engineering-geological models in Croatian karst terrain

The experiences of developing engineering geological models in karst areas for designing and construction purposes prove the necessity of considering at least three basic submodels: sedimentological, structural-tectonic and the weathering one. The research presented here deals with very important and frequently neglected segments in each of the submodels. Therefore, particular attention should be directed to: better understanding of carbonate sediment deposition, determination of environment and diagenetic processes, study of the 3D anisotropy of discontinuity frequency, and differentiation of weathering zones. The given data and examples elaborate and justify such an approach, which enables a more realistic detailed engineering model, more reliable evaluations of the engineering geological/geotechnical parameters and real site conditions

RAZVOJ MODELA ZA PROCJENU POSMIČNE ČVRSTOĆE DISKONTINUITETA U MASIVNIM I OKRŠENIM VAPNENCIMA

In this paper, the problem of estimating the shear strength of discontinuity is presented, which especially occurs in massive and karstified limestones, where discontinuity walls can be extremely rough and irregular, with or without filling material, and for which the current models have proven to be unsatisfactory. A characteristic example of such limestones is the deposit of dimension stone “Kanfanar”, located on the Istrian peninsula in Croatia. For the purpose of developing a model for estimating the shear strength of discontinuity, field research was conducted in which large samples of blocks with natural discontinuities were prepared, as well as samples of filling material in limited conditions, on which detailed laboratory tests of shear strength were performed. Special attention was paid to determining the joint roughness coefficient JRC, the actual contact area between the discontinuity walls, the basic or residual friction angle and the friction angle of the built-in filling material between the discontinuity surfaces. The development of the model for estimating the shear strength of discontinuity was based on Barton’s JRC-JCS empirical model, given the fact that it is one of the most commonly applied models in engineering practice. Based on the results of the tests, a modification of Barton’s JRC-JCS model was made, in such a way that the friction angle of the built-in filling material in the case of discontinuity with a filling was applied instead of the basic or residual friction angle. In addition, for the correct evaluation of the roughness of the discontinuity walls in massive and karstified limestones, it was found that it is necessary to increase the roughness coefficient to values larger than 20, which has been proposed as the maximum so far. Evaluation of the proposed model showed that it is satisfactorily accurate in estimating the shear strength of discontinuity with clay filling material of different states of consistency.U radu je prikazana problematika procjene posmične čvrstoće diskontinuiteta, koja posebice dolazi do izražaja u masivnim i okršenim vapnencima kod kojih stijenke diskontinuiteta mogu biti izrazito hrapave i nepravilne, s materijalnom ispunom ili bez nje, a za koje se pokazalo da postojeći modeli ne mogu dati zadovoljavajuće rezultate. Karakterističan primjer takvih vapnenaca jest ležište arhitektonsko-građevnoga kamena „Kanfanar” koje se nalazi na Istarskome poluotoku u Hrvatskoj. Za potrebe razvoja modela za procjenu posmične čvrstoće diskontinuiteta provedena su terenska istraživanja tijekom kojih su pripremljeni veliki uzorci blokova s prirodnim diskontinuitetima te uzorci materijala ispune u graničnim stanjima, na kojima su zatim provedena detaljna laboratorijska ispitivanja posmičnih čvrstoća. Posebna pozornost posvećena je određivanju koeficijenta hrapavosti pukotina JRC, stvarnoj kontaktnoj površini između stijenki diskontinuiteta, baznomu, odnosno rezidualnomu kutu trenja te kutu trenja ugrađenoga materijala ispune između ploha diskontinuiteta. Razvoj modela za procjenu posmične čvrstoće diskontinuiteta temeljio se na Bartonovu JRC-JCS empirijskome modelu, s obzirom na činjenicu da je to jedan od najčešće primjenjivanih modela u inženjerskoj praksi. Na temelju rezultata provedenih ispitivanja modificiran je Bartonov JRC-JCS model tako da je umjesto baznoga ili rezidualnoga kuta trenja primijenjen kut trenja ugrađenoga materijala ispune za slučaj diskontinuiteta s ispunom. Uz navedeno, za ispravno vrednovanje hrapavosti stijenki diskontinuiteta u masivnim i okršenim vapnencima utvrđena je nužnost povećanja koeficijenta hrapavosti pukotina JRC10 na veće vrijednosti od 20, koliko su do sada bile predložene kao maksimalno moguće. Evaluacija predloženoga modela pokazala je zadovoljavajuću točnost prilikom procjene posmične čvrstoće diskontinuiteta s materijalom ispune različitoga stanja konzistencije

Large gravitational collapse structure on a rocky coast (Kvarner, NE Adriatic Sea)

The studied rock collapse structure is located on the Liburnian coast (Rijeka Bay, channel zone of the NE Adriatic). The relief of the southern part of this coast, with a length of 6.5 km, is a large escarpment with very steep to vertical slopes reaching heights of 100 m above sea level, as a result of tectonic movements along the Kvarner fault zone. These events probably led to a sudden relaxation of the highly fractured rock mass. The progressive expansion occurred at locations where previously favourably oriented faults and fissures had formed a polygonal rock collapse resembling a rock-slide which is the focus of this study. Another aim of this study is to reconstruct and explain the complex morphological evolution of the studied landslide, from the pre-failure deformations, through the failure itself, to post-failure displacements, as well as possible future instabilities. Recent techniques to survey the instability, location and to analyse the evolution of the rupture surface and its dimensions were combined (Unmanned Aerial Vehicle, Side Scan Sonar and Remotely Operated Vehicles). The estimated total volume of displaced rock mass is 950,000 m3. The lower part of the instability phenomenon was submerged during the Holocene sea level rise. Since then, a large part of the displaced rock mass has been in a stable position, with sporadic rock falls. However, given unfavourable orientation and discontinuity characteristics, as well as unfavourable environmental influences, possible instabilities might also be expected in the future

Adapted Schmidt Hardness Testing on Large Rock Samples—Kanfanar-South Quarry Case Study

This paper deals with the possibility of using the Schmidt hardness test, which does not require much preliminary preparation and is easy to perform, in the production of commercial blocks in a quarry. Previous recommendations for Schmidt hardness testing of rock materials were specifically related to tests performed for geomechanical purposes. They also referred mostly to smaller samples, but testing of commercial blocks has some special features, mainly because they are large samples for which practically larger areas must be tested. This paper presents the testing methodology in terms of the number and position of hammer strikes on a commercial block, as well as the application of corrections to the test results in terms of the way the blocks are cut. The tests were conducted on natural stone blocks from the Kanfanar-South quarry, which is characterized by limestone rocks. The test results show that the test methodology can be applied in quarries with similar geological structure and natural stone mining methodology

Adapted Schmidt Hardness Testing on Large Rock Samples—Kanfanar-South Quarry Case Study

This paper deals with the possibility of using the Schmidt hardness test, which does not require much preliminary preparation and is easy to perform, in the production of commercial blocks in a quarry. Previous recommendations for Schmidt hardness testing of rock materials were specifically related to tests performed for geomechanical purposes. They also referred mostly to smaller samples, but testing of commercial blocks has some special features, mainly because they are large samples for which practically larger areas must be tested. This paper presents the testing methodology in terms of the number and position of hammer strikes on a commercial block, as well as the application of corrections to the test results in terms of the way the blocks are cut. The tests were conducted on natural stone blocks from the Kanfanar-South quarry, which is characterized by limestone rocks. The test results show that the test methodology can be applied in quarries with similar geological structure and natural stone mining methodology

Coarse-Clast Storm Deposit and Solitary Boulders on the Island of Mana (NP Kornati, Central Adriatic, Croatia)

There is growing evidence that many large coastal boulder deposits found on the exposed rocky ocean shores were deposited by extreme storm waves rather than by catastrophic tsunamis, as previously thought. In addition, before the first discovery in the northern Adriatic a few years ago, such deposits were not expected in relatively shallow semi-enclosed inland basins. Here we report on a large coastal coarse-clast deposit on the central Adriatic island of Mana, which also contains numerous large storm boulders that weigh up to several tons. Large solitary boulders are also located outside of the deposit, closer to the partly submerged sea cliff and the wave impact. The erosion of the cliff top and displacement of the carbonate bedrock fragments began when the extreme waves inundated the lowermost part of the cliff edge, probably during the late Holocene sea-level rise. The UAS photogrammetry-based fragmentation analysis of the storm deposit and the calculated fractal dimension value indicate that the material was fragmented by multiple high-energy events. A comparison of the available photographs indicates that displacements of the most exposed solitary boulders probably occurred during Vaia, the last extreme storm that hit the Adriatic on the 29th of October 2018. However, the modeled maximum wave height south of Mana during the peak of the storm would be insufficient to move these boulders. Yet local geomorphology probably further influenced the increase in wave height that, in combination with specific geological features, caused displacements of the boulders. There is a shorter fetch affecting Mana Island with respect to the northern Adriatic boulder field in southern Istria. Thus, such an active local erosion of the generally stable eastern Adriatic karstic coast depends on the extreme storms that have a weaker impact in the central than in the northern Adriatic

Coarse-Clast Storm Deposit and Solitary Boulders on the Island of Mana (NP Kornati, Central Adriatic, Croatia)

There is growing evidence that many large coastal boulder deposits found on the exposed rocky ocean shores were deposited by extreme storm waves rather than by catastrophic tsunamis, as previously thought. In addition, before the first discovery in the northern Adriatic a few years ago, such deposits were not expected in relatively shallow semi-enclosed inland basins. Here we report on a large coastal coarse-clast deposit on the central Adriatic island of Mana, which also contains numerous large storm boulders that weigh up to several tons. Large solitary boulders are also located outside of the deposit, closer to the partly submerged sea cliff and the wave impact. The erosion of the cliff top and displacement of the carbonate bedrock fragments began when the extreme waves inundated the lowermost part of the cliff edge, probably during the late Holocene sea-level rise. The UAS photogrammetry-based fragmentation analysis of the storm deposit and the calculated fractal dimension value indicate that the material was fragmented by multiple high-energy events. A comparison of the available photographs indicates that displacements of the most exposed solitary boulders probably occurred during Vaia, the last extreme storm that hit the Adriatic on the 29th of October 2018. However, the modeled maximum wave height south of Mana during the peak of the storm would be insufficient to move these boulders. Yet local geomorphology probably further influenced the increase in wave height that, in combination with specific geological features, caused displacements of the boulders. There is a shorter fetch affecting Mana Island with respect to the northern Adriatic boulder field in southern Istria. Thus, such an active local erosion of the generally stable eastern Adriatic karstic coast depends on the extreme storms that have a weaker impact in the central than in the northern Adriatic

SALTECTA – salt diapirs and active tectonics in the Central Adriatic

Srednji Jadran je predgorje dvaju orogenih sustava – Dinarida na sjeveroistoku i Apenina na jugozapadu. Osim velike debljine naslaga, to područje obilježava i specifična predgorska tektonika te brojne solne strukture. Te halokinetske strukture različito su prikazivane na objavljenim preglednim kartama i ilustracijama geoloških profila, a tek sporadično na interpretiranim seizmičkim profilima. Solni dijapiri čine impozantne potpovršinske strukture koje mjestimice izbijaju na površinu i tako utječu na morfologiju (batimetriju) tog područja. Solne strukture srednjeg Jadrana su znanstveno nedovoljno istražene, a nije istraživana niti uzročno-posljedična veza halokinetike i umjerene seizmičke aktivnosti koja obilježava to područje. Pretpostavlja se da je izdizanje soli vezano uz neotektonsku reaktivaciju mezozojskih rasjeda, od kojih su neki vjerojatno i danas aktivni. Projektom se namjeravaju interpretirati najnoviji 2D seizmički profili odobreni od strane Agencije za ugljikovodike (AZU), reinterpretirati gravimetrijski podatci, definirati glavni rasjedi te 3D modelirati geometrija odabranih solnih struktura. Strukturno-tektonski sklop (re)definirat će se na pučinskim otocima srednjeg Jadrana (slika 1). Uspoređivanjem prostornog rasporeda epicentara i hipocentara zabilježenih potresa s interpretiranim geološkim strukturama, pokušat će se razjasniti povezanost aktivne tektonike sa solnim strukturama. Istraživanjem i datiranjem najmlađih (kvartarnih) naslaga pokušat će se definirati neotektonska aktivnost solnih dijapira. Istraživanjem odabranih markantnih (sub)recentnih erozijskih oblika nastojat će se procijeniti prošla i buduća seizmotektonska aktivnost i seizmogeohazardi