Marine Geotechnical Research in Greece: A Review of the Current Knowledge, Challenges and Prospects

Abstract

Greece is expanding its energy grid system with submarine power and fiber optic cables between the mainland and the Aegean Sea islands. Additionally, pipelines have been installed to support natural gas facilities, and sites are being demarcated for the development of offshore wind parks. The above developments have necessitated extensive geotechnical surveying of the seabed; however, the survey data cannot be accessed for academic inspection or for desktop studies of future developments. This is further hindered by the limited geotechnical information in the Aegean and Ionian Seas. This review examines the existing information concerning the geotechnical behavior of the surficial sedimentary layers, including certain challenges associated with geotechnical sampling and CPTu interpretation. Certain prospects are discussed regarding marine geotechnical research in Greece, with examples from other European countries. The marine geotechnical data in Greece include geotechnical analyses of sediments cores and slope stability estimations, which are commonly associated with the seismic profiling of unstable slope areas. Underlying mechanisms of slope failure have mainly been attributed to the interbedded presence of weak layers (e.g., sapropels, tephra and underconsolidated sediments), the presence of gas and the cyclic loading from earthquake activity. Due to the limited geotechnical information, geological studies have contributed considerably to describing the distributions of gravity-induced events and lithostratigraphy. Within this context, a geological/geotechnical database is suggested where data can be collated and utilized for future studies.

1. Introduction

Greece is expanding and accelerating energy cable interconnections between islands of the Aegean and Ionian Seas and mainland Greece. The purpose of this endeavor is to minimize the energy costs from fossil fuels, as well as to introduce renewable energy sources (RESs) to non-interconnected islands (NIIs) [1,2,3]. The project to expand the electric grid of the country has been undertaken by the Independent Power Transmission Operator (IPTO) with the goal of interconnecting most islands to the mainland grid by 2030 [4]. The advantages from the interconnections regarding energy supply and capacity have been addressed in the past, with a particular emphasis having been placed on the prospect of the islands to develop RESs [5,6,7]. Notable examples are the Attica–Crete (500 kV DC) and Crete–Peloponnese (150 kV AC) links [2]. In the foreseeable future, projects such as the Great Sea Interconnector (GSI—formerly known as EuroAsia) will establish a HVDC link between Greece (Crete Island), Cyprus and Israel, thus expanding the European grid within the southern Aegean Sea. The CREGY interconnection between Egypt and Greece and the GAI (Green Aegean Interconnection) between Greece and Germany through the Adriatic Sea are also considered big and strategically important planned projects, where the vision is of carrying green energy from the renewable energy plants in Egypt and Greece, respectively. The above projects will enhance Greece’s position as an energy focal point in the region.

In terms of RESs, marine areas have been inspected in the Aegean and Ionian Seas for the utilization of offshore wind energy with promising results [8,9,10,11,12]. For this reason, there are ongoing scheduled studies to designate areas for offshore wind park installation. Additionally, uninhabited islands have also been examined in the South Aegean Sea for onshore wind farm installation [13], similar to the case of Agios Georgios in the Saronic Gulf (https://www.terna-energy.com/acivities/wind-energy/aghios-georgios-island/, accessed on 17 July 2024). Moreover, Greece’s anticipated role as an energy hub for southeast Europe is corroborated by a series of projects that are related to offshore natural gas facilities (LNG FSRUs and pipelines). This is exemplified by the expansion of the Revithousa LNG terminal and the prospective construction of a second Floating Storage and Regassification Unit (FSRU) next to the existing Alexandroupoli FSRU, as well as new FSRUs in the west Saronikos Gulf next to Ag. Theodoroi, Thermaikos Gulf (near Thessaloniki) and Pagasitikos Gulf (near Volos). Based on the above offshore developments, extensive marine geotechnical surveys will be needed for (i) the safe laying and burial of projected international power cables, (ii) the demarcation of suitable sites for pile foundations or floating wind turbine anchoring, (iii) the inter-array cables and links between offshore infrastructures and nearby islands or mainland and (iv) the installation of the FSRUs and related pipelines. Moreover, ports and other coastal facilities are expanding to support natural gas and offshore wind farm projects.

Initially, the description and interpretation of the seabed morphology, prevailing sedimentary processes and marine geohazards for sites with offshore infrastructures were dependent on geophysical surveys; however, when considering structure installation, it is imperative that sampling and geotechnical testing are conducted on surficial and deeper sedimentary layers. These are performed for engineering/design purposes, as well as for the validation of geophysical records. In the case of cable/pipeline installations, surveys inspect the upper 3 to 5 m of the seabed for the purpose of burying the infrastructure to protect it from physical processes (submarine landslides, currents, etc.) and/or anthropogenic activities (fishing, dredging, anchoring, etc.) [14,15]. Most of the marine surveys that have been conducted in Greece, primarily for the laying and burial of power and fiber optic cables, have utilized gravity coring for sampling, often with limited penetration and ambiguous results. Only in the last decade have surveys started to use vibro-coring (VC) and in situ cone penetration tests (CPTs), which have been facilitated by a few private energy companies, as well as the Independent Power Transmission Operator (IPTO). The above equipment was utilized for the safe characterization of subsurface layers, which correspond to the physical and mechanical properties of sediments. The usage of the above means has minimized physical and technical restrictions during sampling; therefore, questionable geological strata are now more easily validated and defined. Nonetheless, even after geological layers have been characterized according to their geotechnical properties, uncertainty can still exist. The uncertainty is usually produced from problems associated with the physical conditions of the study area (sea state during sampling, seabed slope and topography), followed by transportation, laboratory handling and geotechnical analyses of sediment cores [16].

Unfortunately, most of the data collected from surveys are not available for academic inspection. This is also an issue for desktop studies that support, in a preliminary stage, surveys for offshore installations (e.g., offshore wind parks), which rely on data accessibility. Moreover, published geotechnical research from universities/institutions in Greece is limited and has not been thoroughly updated to include additional measurements such as longer core samples from VC and/or in situ (CPT) data. Consequently, the present status in geotechnical research, regarding survey data and bibliographic information, prevents researchers from addressing challenges in geotechnical investigation such as seismic stratigraphic validation and precise geotechnical measurements. Furthermore, establishing reliable records of topography and geotechnical conditions in the Aegean and Ionian Seas is a challenge. This is primarily attributed to the irregular bathymetry in both the Aegean and Ionian Seas, which consists of numerous basins that are mainly tectonically formed, followed by high-relief slope environments (e.g., refs. [14,17,18,19,20,21,22,23,24]). In the case of slope stability, mass wasting processes have frequently been documented (e.g., refs. [14,15,25,26,27,28,29,30,31,32,33,34,35]); hence, their presence may compromise seafloor installations.

Considering the increasing number of planned power interconnections and other offshore construction activities as well as the demand for precise information on the geotechnical behavior of hosting sediments, the review aims to examine the current knowledge in marine geotechnical research in the Aegean and Ionian Seas and provide a better picture of the gaps as well as opportunities in marine geotechnical research.

2. Marine Geotechnical Research in Greece and the Supplementary Role of Geological Studies

Marine geotechnical studies have been implemented in various regions in the Aegean Sea (Thermaikos shelf/slope, North Aegean Shelf, Cretan Sea), Ionian Sea (Zakynthos Canyon, Zakynthos/Kefallinia Channel) and Gulf of Corinth; however, they are limited. Their primary focus concerns slope stability and mass gravitative processes, which are examined through a geotechnical analysis of sediment cores and interpretation of seismic profiles [33,36,37,38,39,40,41,42,43,44,45]. Yet, high-resolution seismic profiles crucially contribute to delineating changes in the composition of the seabed and sections where the substrate can shift from a soft/fine-grained texture to coarse (Figure 1).

Sediment cores are typically obtained with gravity coring; hence, sediment description/analysis and seismic validation are mostly constrained at 2 to 3 m. Nonetheless, some studies [33,43,44,46,47,48,49,50] have achieved higher penetration. Deep geotechnical sampling has been implemented for the foundation and design of the Rio–Antirrio bridge in western Greece [51,52]. In terms of in situ measurements (CPT, SPT), applications have been found in onshore coastal sediments in the Gulf of Corinth [53], offshore slope environments in the Cretan Sea/Eastern Mediterranean [44,54], western Greece for the foundation design of the Rio–Antirrio bridge [51,52] and between the islands of Evia and Skiathos in the Aegean Sea [55].

Early geotechnical investigations have been implemented on shelf and slope environments with gravity cores and piston cores [36,46,56,57]. In the case of piston core sampling, Chassefiere and Monaco [46] achieved a 10 m penetration. An extensive geotechnical inspection was conducted by Lykousis and Pechlivanoglou [56], where geotechnical properties (porosity, plasticity, shear strength, carbonate content, organic matter and texture) were recorded at various sites in the Aegean Sea (Alexandroupoli Gulf, Kavala Gulf, South Evia Gulf, Petalion Gulf, Keratsini–Psitalia) using short piston cores. The data are in the form of ranges and hence do not provide detailed descriptions in relation to depth; therefore, they can be regarded as preliminary. Nonetheless, geotechnical variations among regions were attributed to bottom current activity and differences in sedimentation rates.

Over the decades, geological studies have provided information on gravitative processes, sediment properties (texture, carbonate content, mineralogy, etc.) and sedimentation rates [43,58,59,60,61,62,63,64,65]. In addition to sedimentary information, descriptions of eustatic sea-level changes, tectonic structure, stratigraphy (lithostratigraphy, chronostratigraphy, etc.) and oceanographic conditions (met-ocean data) provide the framework for interpreting the geological regime of a site. Important contributions of geological studies include seismic profiling of gravity-induced deposits [27,28,30,32,34,35,39,40,43,45,53,59,60,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89] as well as gas seepage and diapirism [39,40,77,78,79,90,91,92,93]. All these processes/formations must be recorded and inspected since they can compromise offshore infrastructure [14].

In the context of geological investigation, paleoclimate and sedimentological studies have extracted drilled core samples and long piston cores (20 to 30 m), thus offering detailed descriptions of deep sedimentary layers. Examples can be found in the Gulf of Corinth basin area [43,49,94,95,96], North Aegean Shelf [97,98], Thermaikos Shelf [99,100,101] and Cretan Trough [102]. Deeper core sampling from basin environments offers a more comprehensive picture of the depositional history and gravitative processes (e.g., turbidites) [49,94,95,96,102], while drilled samples from shelf environments are mainly examined for the purposes of hydrocarbon exploration [97,101]. Additional core descriptions can be found in paleoclimate reconstructions, albeit from shallower cores [103,104,105,106,107,108,109,110,111,112,113].

In general, geological studies can be used for describing the lithostratigraphy (e.g., sapropels, tephra layers, organic layers, turbiditic layers, etc.), tectonic structure and geohazards of a site. In terms of lithostratigraphy, it is invaluable for recording sedimentary units that exhibit distinct physical features and by extension geotechnical behavior. In the case of sapropels, their presence represents anoxic depositional conditions due to poor water circulation; hence, they contain high organic matter, which can affect the shear strength of sediments [28,29,50,114,115,116,117,118,119]. In a similar way, the presence of interbedded tephra layers in the sedimentary column can decrease the cohesiveness of sediments and therefore their shear strength [28,117]. Tephra (volcanic) sedimentary layers are particularly prominent in the South Aegean Sea, where the Aegean Volcanic Arc is situated [37,118,119,120,121].

Studies that have conducted surficial sampling in shelf environments provide preliminary insights into the distribution of coarse/fine substrates, including sedimentological attributes (mineralogy, carbonate content, etc.). Surficial sedimentological mapping can be found in the North Aegean Shelf [88,122,123,124,125,126,127,128,129,130], Thermaikos Shelf [131,132], Kafireas Strait [133], South Evia Gulf [134], between Sifnos and Kimolos Islands [135] and in the South Aegean Sea [37,136], whilst Poulos [65] compiled information on the distribution and origin of the terrigenous components of the surficial sediments in the Aegean Sea seafloor. In conjunction with oceanographic data, sedimentary distributions can be used as predictors or trends of sediment mobility and erosion [131,133]. This is particularly important regarding cable interconnections, where some segments are left exposed on the seabed due to the inability to bury the infrastructure [130]. Erosional features and bedforms are also examined from seismic reflection profiles and sonar images, which can indicate the effects of local hydrodynamics (e.g., bottom currents) [86,137,138]. Comprehensive maps of sediment distribution as well as sediment thickness and geohazards (active faults, slides, gas, etc.) have been developed by IGME (e.g., ref. [139]) and have provided information for areas such as the North Aegean Shelf, Pagasitikos Gulf and Cyclades near Santorini island.

In the absence of sufficient geotechnical information on sedimentary strata, geological studies provide a preliminary framework in studying the seabed, considering the complex structure of the Aegean and Ionian Seas [21]. In that regard, changes in eustatic sea level control the distribution of lithostratigraphic units such as sapropelic layers or underconsolidated deltaic sediments that have been deposited during low sea-level conditions [29,140]. According to the above, the collation and assessment of geological studies marks the first step in laying the foundations for geotechnical investigation.

3. Geotechnical Properties of Quaternary Sedimentary Layers and Mechanisms of Sediment Instability in the Aegean and Ionian Seas

3.1. Geotechnical Study of Quaternary Sedimentary Layers and Sediment Instabilities

Geotechnical analysis of sediment cores principally encompasses texture, Atterberg limits, bulk density, carbonate content, water content, shear strength and consolidation measurements. Table 1 provides an overview of important geotechnical studies, including some notable geological studies, while Figure 2 presents the geographical location of the main marine geotechnical investigations in the Aegean and Ionian Seas. Undrained shear strength is probably the most challenging and laborious parameter in geotechnical analysis. This is due to disturbance effects inflicted on sediment cores during sampling, transportation and handing in the lab [16,141]. Moreover, different lab tests (e.g., vane shear tests, triaxial tests, direct shear, etc.) can be performed, which may produce variable estimations [55].

In Greece, shear strength measurements have been conducted with a pocket penetrometer [46], field vane test [142], vane shear apparatus [29,31,33,36,37,38,41,42,44,143,144], Bromhead ring shear device [44] and triaxial test [31,41]. In essence, vertical variations in shear strength provide information, in conjunction with other parameters (texture, organic matter, bulk density, Atterberg Limits, etc.), on the presence of weak layers that may be susceptible to failure; therefore, they indicate the predisposition of sediments to gravity-induced flow (e.g., slides, flows, etc.) [29].

Table 1. Sampling and analysis of sediment cores from geotechnical and geological studies in the Aegean and Ionian Seas.

Study	Location	Core Equipment and Penetration	Geotechnical Measurements
[58]	NW Aegean Sea	Gravity cores (1.9–2.4 m)	Grain size, carbonate content, SEM microscopic inspection
[56]	Aegean Sea (multiple locations)	Short piston cores (unspecified)	Grain size, mineralogy, carbonate content, organic matter, porosity, Atterberg limits, wet bulk density, shear strength
[59]	Ionian Sea (Zakynthos Channel)	Gravity cores (~1.5 m)	Grain size, organic carbon, calcium carbonate, clay mineralogy
[60]	NW Aegean Sea (Thermaikos Slope—Sporades Basin)	Gravity cores (~2.5 m)	Grain size, organic carbon, calcium carbonate, clay mineralogy
[57]	North Evia Gulf	Sediment cores (0.85–2.7 m)	Grain size, water content, carbonate content, wet bulk density, Atterberg limits, shear strength
[46]	South Aegean Sea	Piston cores (5–10 m)	Grain size, clay mineralogy, Atterberg limits, water content, wet bulk density, shear strength
[36]	NW Aegean Sea (Thermaikos Shelf/Slope)	Gravity cores (1.5–3 m)	Grain size, carbonate content, water content, Atterberg limits, shear strength
[29]	Multiple sites (Aegean Sea)	Gravity cores	Shear strength
[116]	North Aegean Sea (Thermaikos Gulf, North Aegean Shelf)	Gravity cores (~2 m)	Visual description, organic carbon and radiocarbon dates, examination of sapropelic layers
[48]	Gulf of Corinth	Gravity cores (0.5–1 m), piston cores (7–7.5 m)	Grain size, organic carbon, calcium carbonate
[37]	South Aegean Sea	Gravity cores (0.12–2.76 m)	Grain size, Atterberg limits, water content, density
[80]	Ionian Sea (Corfu–Kefalonia Valley System)	Piston cores (3.5 m)	Grain size, organic carbon, X-ray
[38]	South Aegean Sea (Cretan Sea)	Gravity cores (2–3 m)	Grain size, carbonate content, bulk density, Atterberg limits, water content, shear strength
[41]	North Aegean Shelf/Slope	Gravity cores (~2.5 m), box cores	Bulk density, carbonate content, Atterberg limits, shear strength
[49]	Gulf of Corinth	Piston core (Calypso core—30 m)	Physical properties (core logging): magnetic susceptibility, gamma ray, resistivity, etc.
[50]	North Aegean Sea	Gravity cores (2.1–4.1 m)	Grain size, organic carbon, carbonate content, clay mineralogy, radiocarbon dating
[26]	Ionian Sea (Zakynthos Canyon/Valley)	Gravity cores (~2 m)	Sedimentological description (geotechnical analysis not presented)
[42]	NW Aegean Sea (Thermaikos Shelf and Sporades Basin)	Gravity cores (2–3 m)	Water content, wet density, shear strength
[27,28]	South Aegean Sea	Gravity cores (~2 m)	Grain size, water content, shear strength, Atterberg limits, bulk density
[44]	South Aegean Sea (Cretan Margin)	Gravity cores (4–4.6 m)	Physical properties (MSCL), shear strength
[43]	Gulf of Corinth	Gravity cores (~2.5 m), piston cores (max 30 m)	Gravity cores: physical properties (MSCL), grain size, carbonate content, organic carbon Piston cores: physical properties (MSCL), grain size
[31]	Ionian Sea, western Greece (Gulf of Corinth), NW Aegean Sea (Thermaikos Shelf/Slope)	Gravity core (2–5 m)	Grain size, organic carbon, carbonate content, Atterberg limits, water content, bulk density, overconsolidation ratio (OCR)
[94]	Gulf of Corinth	Calypso piston core (20 m)	Grain size, magnetic susceptibility and radiocarbon dates, examination of earthquake-induced layers (turbidites)
[32,33]	South Aegean Sea (Cretan Margin)	Gravity cores (1.5–4.6 m)	Grain size, shear strength, multi-sensor core logging (bulk density)
[86]	Gulf of Corinth	Gravity cores (1.1–2 m)	Grain size, mineralogy, X-ray diffraction
[138]	South Aegean Sea	Gravity cores (3.5 m)	Radiocarbon dating, visual description
[144]	Southeast Mediterranean Sea (south off Crete near Gavdos Island)	Gravity cores (1–2.4 m)	Multi-sensor core logging (magnetic susceptibility, gamma-ray density, p-wave velocity), grain size, shear strength, organic carbon, X-radiography
[96]	Gulf of Corinth	Drilled core (IODP Expedition 381)	Visual description, grain size, isotope analysis (Th/U)

Table 1. Sampling and analysis of sediment cores from geotechnical and geological studies in the Aegean and Ionian Seas.

Study	Location	Core Equipment and Penetration	Geotechnical Measurements
[58]	NW Aegean Sea	Gravity cores (1.9–2.4 m)	Grain size, carbonate content, SEM microscopic inspection
[56]	Aegean Sea (multiple locations)	Short piston cores (unspecified)	Grain size, mineralogy, carbonate content, organic matter, porosity, Atterberg limits, wet bulk density, shear strength
[59]	Ionian Sea (Zakynthos Channel)	Gravity cores (~1.5 m)	Grain size, organic carbon, calcium carbonate, clay mineralogy
[60]	NW Aegean Sea (Thermaikos Slope—Sporades Basin)	Gravity cores (~2.5 m)	Grain size, organic carbon, calcium carbonate, clay mineralogy
[57]	North Evia Gulf	Sediment cores (0.85–2.7 m)	Grain size, water content, carbonate content, wet bulk density, Atterberg limits, shear strength
[46]	South Aegean Sea	Piston cores (5–10 m)	Grain size, clay mineralogy, Atterberg limits, water content, wet bulk density, shear strength
[36]	NW Aegean Sea (Thermaikos Shelf/Slope)	Gravity cores (1.5–3 m)	Grain size, carbonate content, water content, Atterberg limits, shear strength
[29]	Multiple sites (Aegean Sea)	Gravity cores	Shear strength
[116]	North Aegean Sea (Thermaikos Gulf, North Aegean Shelf)	Gravity cores (~2 m)	Visual description, organic carbon and radiocarbon dates, examination of sapropelic layers
[48]	Gulf of Corinth	Gravity cores (0.5–1 m), piston cores (7–7.5 m)	Grain size, organic carbon, calcium carbonate
[37]	South Aegean Sea	Gravity cores (0.12–2.76 m)	Grain size, Atterberg limits, water content, density
[80]	Ionian Sea (Corfu–Kefalonia Valley System)	Piston cores (3.5 m)	Grain size, organic carbon, X-ray
[38]	South Aegean Sea (Cretan Sea)	Gravity cores (2–3 m)	Grain size, carbonate content, bulk density, Atterberg limits, water content, shear strength
[41]	North Aegean Shelf/Slope	Gravity cores (~2.5 m), box cores	Bulk density, carbonate content, Atterberg limits, shear strength
[49]	Gulf of Corinth	Piston core (Calypso core—30 m)	Physical properties (core logging): magnetic susceptibility, gamma ray, resistivity, etc.
[50]	North Aegean Sea	Gravity cores (2.1–4.1 m)	Grain size, organic carbon, carbonate content, clay mineralogy, radiocarbon dating
[26]	Ionian Sea (Zakynthos Canyon/Valley)	Gravity cores (~2 m)	Sedimentological description (geotechnical analysis not presented)
[42]	NW Aegean Sea (Thermaikos Shelf and Sporades Basin)	Gravity cores (2–3 m)	Water content, wet density, shear strength
[27,28]	South Aegean Sea	Gravity cores (~2 m)	Grain size, water content, shear strength, Atterberg limits, bulk density
[44]	South Aegean Sea (Cretan Margin)	Gravity cores (4–4.6 m)	Physical properties (MSCL), shear strength
[43]	Gulf of Corinth	Gravity cores (~2.5 m), piston cores (max 30 m)	Gravity cores: physical properties (MSCL), grain size, carbonate content, organic carbon Piston cores: physical properties (MSCL), grain size
[31]	Ionian Sea, western Greece (Gulf of Corinth), NW Aegean Sea (Thermaikos Shelf/Slope)	Gravity core (2–5 m)	Grain size, organic carbon, carbonate content, Atterberg limits, water content, bulk density, overconsolidation ratio (OCR)
[94]	Gulf of Corinth	Calypso piston core (20 m)	Grain size, magnetic susceptibility and radiocarbon dates, examination of earthquake-induced layers (turbidites)
[32,33]	South Aegean Sea (Cretan Margin)	Gravity cores (1.5–4.6 m)	Grain size, shear strength, multi-sensor core logging (bulk density)
[86]	Gulf of Corinth	Gravity cores (1.1–2 m)	Grain size, mineralogy, X-ray diffraction
[138]	South Aegean Sea	Gravity cores (3.5 m)	Radiocarbon dating, visual description
[144]	Southeast Mediterranean Sea (south off Crete near Gavdos Island)	Gravity cores (1–2.4 m)	Multi-sensor core logging (magnetic susceptibility, gamma-ray density, p-wave velocity), grain size, shear strength, organic carbon, X-radiography
[96]	Gulf of Corinth	Drilled core (IODP Expedition 381)	Visual description, grain size, isotope analysis (Th/U)

Figure 2. Map of the Aegean and Ionian Seas and locations of studies that have conducted geotechnical analyses on subsurface sediments (e.g., refs. [28,31,33,36,37,38,41,42,44,45,46,47,51,52,53,57,142,144,145]).

Figure 2. Map of the Aegean and Ionian Seas and locations of studies that have conducted geotechnical analyses on subsurface sediments (e.g., refs. [28,31,33,36,37,38,41,42,44,45,46,47,51,52,53,57,142,144,145]).

As mentioned in the previous section, eustatic sea-level changes, seismicity (ground accelerations) and proximity of shelf and slope environments to river outflows have primarily shaped the depositional sequences of the Aegean and Ionian Seas [25,62,100]. Eustatic sea-level changes mainly concern the last glacial period (Wurm) and the unconformities (potential failure planes) that have developed within the sedimentary column [142]. During low sea-level conditions, slope depositional environments experienced rapid sedimentation near rivers, forming underconsolidated sedimentary sequences in the process that were subsequently overlain by hemipelagic sedimentation during higher sea levels. The stacking of underconsolidated sediments between more consolidated sedimentary layers has left their internal composition susceptible to external stresses, primarily from seismic loading, and hence underlying consolidated layers can act as failure planes, initiating slides in the process [29,35,140]. Fluvial sedimentation and its effect on slope stability have been examined extensively in the Gulf of Corinth. The presence of river outflows along with their proximity to the steep slopes of the gulf have induced rapid progradation of deltas, specifically during high sedimentation rates [31,40,48,71,78]. Slope failures can be triggered within interlayered, sandy, liquefiable sedimentary sequences that are bounded by low-permeability clayey layers [53]. Liquefaction can develop under cyclic loading from earthquakes, which increases the pore stresses within sandy–silty layers, thus reducing the shear strength of sediments. This type of failure has been documented offshore for the Eratini and Tolofonas deltas [31,40,53,78,89]. The presence of shallow liquefiable horizons in the delta plains at the southern coast of the Gulf of Corinth, between the Selinountas and Kerynitis deltas, has been attributed as the main factor in the destruction and submergence of ancient Helike (372/373 B.C.), which generated a translational slide caused by an earthquake of 6 to 6.7 on the Richter scale [45]. The above studies illustrate that underlying alluvial depositional sequences may pose considerable geohazards both offshore as well as onshore, with the latter case also having the potential to generate tsunamis [45,53].

Gas seepage has been documented within deltaic sedimentary sequences in the Aegean Sea and western coast of Greece. Their presence is typically indicated on seismic reflection profiles with a characteristic acoustic blanking signal [77]. The Pockmark formation has been documented in western Greece (Gulf of Patras and Gulf of Corinth) and the Ionian Sea (offshore Killini), where gas expulsion has eroded the surficial sedimentary layers due to pore-pressure-induced stresses, forming distinct depressions in the process [26,39,77,79,91]. Gas-charged sediments have been recorded in other areas of the Ionian Sea (Corfu Shelf, Amvrakikos Gulf), western Greece (Katakolon Bay) and the North Aegean Sea (Thermaikos Gulf, North Aegean Shelf) [77,88,90,92,93]. The lateral migration of gas and subsequent concentration at subsurface sedimentary layers or seepage at the seabed are usually aided by the presence of unconformities, fault planes and salt diapirs; however, seismic activity is probably the most important factor in causing gas expulsions, which can disrupt the depositional configuration of the seabed and strength of sediments [26,39,77,79].

In the Zakynthos canyon/valley system, between Zakynthos island and the Killini Peninsula, factors affecting slope stability (gas seepage, salt diapirism, slope steepness, seismicity) are particularly pronounced, leading to frequent mass movement along the bounding slopes. The high seismicity is attributed to its proximity to the subduction zone of the Hellenic Trench [26,68]. In the NE Ionian Sea and extension of the Hellenic Arc, the Corfu–Kefalonia valley also exhibits high-inclination slopes and submarine mass movements, with oceanographic measurements also indicating erosion from currents corresponding to the deep-water formation in the Adriatic Sea [80]. Along the Hellenic Trench, south of Crete, the presence of weak layers predisposes slopes to failure, leading to frequent turbidity and mass flow [72,144,146]. Instabilities are also facilitated by alluvial deposition at the Cretan slope, which further increases the strain on slope sediments. At the northern margin of Crete, the high steepness due to tectonic uplift and seismic activity predisposes weak layers to failure [32,38]. Furthermore, subsidence of Cretan Basins induces over-steepness, thereby enhancing mass flows towards the basins [34]. It is noteworthy to mention that under an extensional tectonic regime, concurrent uplift and subsidence may not only increase slope steepness but also relocate high-inclination sections, which may reactivate mass transport processes. This indicates that gravitative deposits may not always be found at the flank of basins but may be located mid-slope, where they may become reactivated by an external force (e.g., seismic activity) [33,34]. In the North Aegean Sea, sedimentary deformations (creep) have been recorded in the Thermaikos slope (NW Aegean Sea), caused by the progressive downbending of slope sediments, which leads to slumping and sliding [25,67,69]. Sediment instabilities on slopes are also affected by the presence of growth faults within the sedimentary column [36,69].

Lithostratigraphic units that exhibit distinct physical attributes within the sedimentary column can shift the geotechnical behavior of sediments. As addressed in the previous section, the presence of sapropels in slope environments, specifically S1 (~10 to 6 Kyr B.P.), tends to leave overlying sediments susceptible to failure, depending on the external stresses induced on the seabed (seismic activity) along with other factors (gas, diapirism, sedimentation rate, etc.). The high water content and low shear strength and bulk density of this layer produce weak internal zones that can generate mass movements [15,29,36,46,115,116,117].

At the northern margins of the Cretan Trough (Argolikos Basin, Amorgos Basin, Christiana Basin), weak layers (sapropels, tephra, turbidites) are documented in slope environments, with local normal faulting inducing creeps and slumps along the slope [37]. The general northern region of the Cretan Trough and Cyclades Plateau exhibits frequent but not intense seismic activity; however, large seismic events do occur, such as the 1956 event (M_s = 7.5), which produced a large slump and tsunami in the surrounding vicinity [37,147]. In addition, hydrothermal activity and seismicity produced from volcanic activity are also contributing factors in inducing slope instability at island margins [27,28].

Tephra layers in the surficial sedimentary sequences can also shift the geotechnical behavior of sediments by making the sedimentary structure non-cohesive, thus leaving it prone to liquefaction [27,32,46]. Their distribution has been documented and studied primarily in cores from the South Aegean Sea [27,32,38,46]. The percentages of clay and silt are important factors in tephra layers, specifically regarding interparticle friction and granularity [46]. According to the particle shapes recorded within tephra layers, rounded silt-sized particles tend to produce less plastic behavior and a lower water content, while platelet particles increase the plasticity and water content. This illustrates the importance of particle characteristics in shaping the sedimentary fabric of sediments and by extension geotechnical behavior. Furthermore, tephra layers proximate to sapropels tend to exhibit higher water contents and lower bulk densities, while, when intermixed with clayey sediments, they can become more cohesive and thus can exhibit a higher shear strength [46]. Within this context, the geotechnical behavior of sediments may be affected by their proximity to weak layers (e.g., sapropels) within the sedimentary column. This is also corroborated by the occurrence of turbidites within sapropelic layers [37,41,47,115].

The above studies provide information on the distribution of weak zones in slope environments, along with interpretations of the underlying mechanisms of slope failure. The detection of weak layers (sapropels, tephra layers, underconsolidated layers, etc.) within the sedimentary column and seismic profile interpretation of gravity-induced deposits along the seabed can provide indications not only of existing instabilities but also of potential slope failures in the future. For this reason, it is important to provide ranges of shear strength, bulk density, water content and plasticity and, if possible, of other parameters (i.e., overconsolidation ratio, relative density, unit weight, etc.). In terms of lithostratigraphy, the physical attributes of sediments and their effect on geotechnical behavior can be examined and hence provide a link between geological and geotechnical interpretation [46]. Additionally, clay mineralogy (illite, smectite, kaolinite, etc.) also has an important role in shaping the geotechnical behavior of sediments; hence, its measurement in cores and examination of its effects on geotechnical properties is important (i.e., refs. [46,50]). When geological studies are conducted in marine environments, their approaches can enhance geotechnical assessments of the recorded lithostratigraphy. In the context of geohazards, the above studies demonstrate that there are multiple factors leading to slope instability, depending on the region; however, the most important instigating factor is seismic activity. Other reviews on geohazards in the Aegean and Ionian Seas have also highlighted seismic activity as a major factor triggering mass movements [14,25].

3.2. Geotechnical Analysis of Deep Sedimentary Strata and In Situ Testing of Sediments

Long core samples (piston cores, drilled cores) obtained from basins [49,94,95,96,102] and shelves [97,98,100,101] can display the structure and distribution of depositional layers. The description of intercalated turbidites from the Gulf of Corinth basin indicates the frequency of turbiditic events, which are intermittent and occur by hemipelagic sedimentation [49,94,95]. In shelf environments and near the mouths of major rivers such as Evros, drilled core samples have revealed gas present within prograding deltaic sequences, a result of the rapid sedimentation that has occurred in the area [98].

In the context of lithostratigraphic interpretation of deep sedimentary strata, geotechnical analysis is rare in the published literature. The foundation design of the Rio–Antirrio bridge in NW Peloponnese provides a rare case study, not only in terms of a geotechnical study but also for the engineering issues associated with the foundation of the structure. Specifically, the presence of underconsolidated alluvial strata (silty sands to sandy clays) at the designated site posed considerable risks in terms of the bearing capacity of the structure for seismic loading [51,52]. To examine potential failure and deformation near the foundation (gravity caissons), extensive sampling was conducted. This involved the acquisition of borehole samples, piezocone tests (CPTu), standard penetration tests (SPTs), vane tests, dilatometer tests and seismic cone tests, with sampling depths reaching 60 to 100 m below the seafloor. The geotechnical profiles obtained from the analysis showed generally weak-strength behavior, with the undrained shear strength exhibiting decreased values at certain depth intervals due to the presence of cohesionless layers. To support the structure, ground reinforcement was implemented by placing steel tubular pipes in an 8000 m² circular area, along with the backfilling of a gravel layer (2.8 m thick) on top, between the foundation of the caisson and the tubular pipes. The above case study shows the rigorous engineering solutions employed for the safe foundation and settlement of the Rio–Antirrio bridge in a generally unfavorable environment. Similar publications of engineering inspections and solutions for the installation of existing and/or scheduled offshore structures are missing but it is obvious that they might further improve future projects.

An important aspect that was addressed early in the review is the lack of more sophisticated or rather additional information for the interpretation of soil profiles. In the case of in situ tests (CPT), they have been utilized only over the last decade in Greece to further enhance subsurface strata. Figure 3 shows an example of a CPTu profile that registers in situ shear strength, relative density, sediment characterization (based on soil behavior charts), along with laboratory shear strength measurements from a vane test conducted on a vibro-core sample proximate to the CPTu (see Section 4.2). Figure 4 displays a CPTu profile with basic registered parameters (cone resistance, sleeve friction, pore pressure, etc.). Some studies have examined onshore coastal sedimentary layers at the northern coast of the Corinth Gulf [53] and slope sediments at the Cretan Margin [44,54]. On the coast between the Tolofonas and Eratini deltas, cone penetration tests (CPTs) as well as dynamic penetration tests (DPSH) were performed to assess the slope stability and liquefaction potential of onshore sediments [53]; however, no core samples were obtained near the in situ testing locations, and hence, soil interpretation was based on empirical information instead of field data. Conversely, CPT and gravity cores were obtained in proximity at the slope of the Cretan Margin to assess the stability of undisturbed slope sediments and slide deposits in conjunction with seismic profiling [44]. Despite some issues in CPT deployment, the registered cone resistance and shear strength were higher for undisturbed sediments and lower in the slide portion, with Multi-Sensor Core Log (MSCL) data exhibiting a low water content, bulk density and shear velocity for the slide deposits. This allowed for distinguishing the undisturbed seabed section from the landslide, which was initially not discernable from the lithostratigraphic description of the gravity cores. On the other hand, pore pressure measurements were deemed problematic due to the lack of dissipation tests. The above studies show some rare examples of geotechnical investigation and hence can offer insights into the particularities and challenges of marine environments in the Aegean and Ionian Seas (e.g., slope, topography, underconsolidated layers, etc.).

3.3. Slope Stability Estimations

Slope stability estimations have been conducted in the Ionian Sea [47,142], western Greece [45,53,142,143,145], North Aegean Shelf [41], Saronic Gulf [142], Thermaikos slope [31,36,142] and South Aegean Sea [28,33,38,46].

Table 2. Slope stability estimations in marine areas of the Aegean and Ionian Seas, along with slope gradient ranges (or averages), ground acceleration and a general assessment.

Study	Location	Slope Gradient Range (°)	Ground Acceleration (g)	General Assessment
[142]	Ionian Sea (Zakynthos–Kefalonia–Peloponnesus), Gulf of Corinth, NW Aegean Sea (Thermaikos slope), Central Aegean Sea (Saronic Gulf)	4°–7°	0.18–0.28 g	General instability for the Zakynthos–Kefalonia–Peloponnesus and Corinth Gulf slopes. Instabilities in the Saronic Gulf and some sections of the Thermaikos slope.
[36]	NW Aegean Sea (Thermaikos slope)	1°–4°	0.18–0.33 g	Slope stability for 10 m layers and instability at 20 m (theoretical approximations).
[47]	Ionian Sea (Zakynthos continental slope)	8° (average)	0.08–0.30 g	Sufficient ground acceleration for failure is 0.08 g for S1 sapropels and 0.30 g for overlying strata. Presentation of critical distances of epicenters for sufficient failure.
[38]	South Aegean Sea	1°–4°	0.07–0.13 g	Ground accelerations 0.07 to 0.13 g are sufficient to trigger failures.
[145]	Ionian Sea (Paxi and Corfu slopes)	5°–30°	0.43–1.26 g	Paxi slope exhibits instability under dynamic conditions. Corfu slope shows instability in both static and dynamic conditions.
[53]	Gulf of Corinth	2.5°–11°	0.30–0.50 g	Stable conditions of coastal sediments under cyclic loading of a Ms = 6.2 R earthquake. Failure is initiated from earthquake-induced pore pressure of subsurface liquefiable layers.
[41]	North Aegean slope	0.5°–2.9°	0.08–0.26 g	Calculated ground accelerations for failure were less than the predicted ones, indicating high instability.
[28]	South Aegean Sea (Milos slope)	3°–26°	0.05 g	General stability of sediments under static conditions. Instability is found in shallow surficial layers (<50 cm).
[31]	NW Aegean Sea (Thermaikos slope), western Greece (Corinth Gulf, Patraikos Gulf), Ionian Sea (Kyparissiakos Gulf)	0.5°–5°	0.12–0.30 g	Instability of prodelta sediments corresponding to the low section of the High System Tract (HST), in addition to the presence of gas.
[33]	South Aegean Sea (Cretan Margin)	3°–5°	0.37–0.67 g	Critical ground accelerations of 0.37 to 0.67 g for 3° slopes and 0.40 to 0.66 g for 5° slopes.
[45]	Gulf of Corinth	2°–20°	0.30–0.55 g	Stable conditions for coastal sediments under static and cyclic loading. Failure is initiated from earthquake-induced pore pressure increase in subsurface liquefiable layers.

shows the results from slope stability assessments in the Aegean and Ionian Seas. The Infinite Slope Stability represents a simple approach in estimating slope stability; however, other relatively more intricate methods have been utilized to assess the safety factor of slopes, such as the Janbu and Bishop method [53,143]. In the case of Infinite Slope Stability, this assumes a smooth extended homogenized seabed, with important parameters being the ratio of undrained shear strength to overburden pressure, slope angle and peak ground acceleration (seismic activity) [148]. It has been implemented in both Aegean and Ionian Sea slope environments [28,33,36,38,46,47,53,142,145]. To utilize the above approach, it is also necessary to examine the available seismic reflection profiles along the sloping environments as well as assess the lithology with core samples or indirectly with CPT data [53]. The application of the Normalized-Soil Parameter (NSP) is considered a more accurate method to assess soil stability, since it compensates for the disturbance effects that are often inflicted on core samples [149]. The NSP method is usually applicable in smoother slopes; therefore, it has been implemented on prodelta depositional environments of the North Aegean Shelf [41] and other marine sites (Thermaikos slope, Gulf of Corinth and Gulf of Patra) [31]. It is noteworthy to mention that the NSP approach necessitates triaxial testing to simulate conditions under static and cyclic stresses, which is often expensive and laborious; therefore, NSP applications are rare.

In Table 2, some indicative gradient ranges and ground accelerations are provided based on the slope stability calculations. It becomes apparent that local failures can be initiated even at low inclinations with a minimum seismic disturbance. Stability is dependent primarily on the sedimentation rate, seismotectonic activity and the gradient that has developed in slope environments. The high sedimentation rate between Zakynthos, Kefalonia and Peloponnesus has formed thick underconsolidated sequences that are susceptible even at very small gradients [142]. In terms of S1 sapropelic layers in the Zakynthos slope, critical distances from epicenters of 6.5 R and 7 R earthquakes, as well as ground accelerations, were found to be less compared to overlying sedimentary sequences [47]. This further demonstrates the susceptibility of such deposits to seismic activity; however, it should be mentioned that the specific study makes some assumptions for overlying non-sapropelic strata regarding bulk density and shear strength. In the case of limited core penetration, assumptions can be made on the sedimentation conditions (sedimentation rates, composition, etc.) and shear strength behavior of subsurface sedimentary layers using theoretical (linear regression) approximations [36,143]. To further test slope stability, such assumptions can be useful in the absence of sufficient information; however, it is important that adjacent geotechnical information is available, and that seismic interpretation can provide some indications of similar sedimentation processes (e.g., prodelatic sequences). Documentation of active faults is also paramount, including potential vertical offsets that could displace sediments [41,84]. The above information shows that extensive geotechnical sampling and recording of active faults in marine areas can further enhance slope stability estimations. In that respect, the seismic activity and tectonic structure of the Aegean and Ionian Seas present the references for the calculated ground accelerations.

4. Current Challenges in Marine Geotechnical Surveying

4.1. Core Sampling

Commonly, sediment core samples are obtained at certain locations along the seismic survey lines to classify subsurface strata and subsequently ground-truth the seismic reflection character of the seabed. Additionally, in situ tests (CPTu) must also be validated, since they provide indirect information on the composition and geotechnical properties of the seabed. Both core sampling and CPTu testing are utilized to interpret the seismic stratigraphy for the purposes of burial assessments (cables/pipelines) or foundation design of offshore strictures (e.g., OWF). In the former case, the geotechnical properties of subsurface layers must be inspected to support estimations of the burial depth and method of burial. In the case of sediment coring, the type of equipment employed during sampling (gravity coring, vibro-coring, piston coring, etc.) as well as the composition of subsurface sedimentary layers (sandy or muddy texture or mixed composition) will determine the extent of ground-truthing. When considering cable/pipeline surveys, the demanding and/or limited penetration of gravity core sampling constrains lithological interpretations and by extension investigation of deeper “weak” deposits. Also, penetration is usually hindered by the presence of cohesionless sediments (sandy and/or gravelly sediments) as well as cohesive/muddy layers that display increasing consolidation with depth (e.g., stiff to hard clays). This issue becomes especially prominent when depositional environments change considerably, considering the complex and pronounced topography of the Aegean and Ionian Seas. For the above reasons, it is often justifiable to employ vibro-coring so that a deeper penetration can be achieved. Vibro-core penetration in cohesive sediments can reach up to 6 m for cable/pipeline burial and even more; however, the vibratory effects used during sampling can disturb the sediments, and hence, certain geotechnical estimations may not reflect actual conditions (e.g., undrained shear strength, consolidation). Figure 5 exhibits examples of vibro-core samples from a geotechnical survey conducted for the purposes of a power cable installation. It is noteworthy to mention that some level of disturbance, specifically concerning cohesive sediments, is expected such as changes in pore pressure. Furthermore, survey operations can disturb the internal sedimentary fabric within the cores, and thus lab measurements may be affected [16,141].

4.2. CPTu Testing and Soil Property Estimation

An important opportunity in marine geotechnical research in Greece concerns piezocone (CPTu) measurement and its interpretation as well as calibration with nearby core retrieval. Over the last decade, geotechnical surveys in Greece have been systematically employing CPTu data more often since they provide continuous and supplementary information on the subsurface. The records of cone resistance (q_t), sleeve friction (f_s) and pore pressure (u₂) are measured and described in conjunction with core samples to further validate subsurface lithology. An important aspect of this process concerns the estimation of index properties (undrained shear strength, relative density, overconsolidation ratio, etc.). Specifically, CPTu index properties and undrained shear strength are derived from empirical relationships between basic CPT parameters (q_t, f_s, u₂) and lab tests that are conducted on core samples [150]. Based on these relationships, indicative cone factor values (N_kt, N_ke, N_Δu) have been estimated for different sediment types (stiff clays, overconsolidated clays, etc.); however, variations are observed among studies, probably due to differences in laboratory testing (triaxial tests, simple shear, vane shear test, etc.) as well as sample disturbance. In marine environments, it is typical to use the relationship with net cone resistance (N_kt) since it accounts for pore pressure effects.

Due to the variation when estimating cone factor values, it is often appropriate to present indicative ranges for sediment types rather than a single value; nonetheless, Robertson and Cabal [151] have suggested an average N_kt value of 14 for unknown sedimentary deposits. In that respect, direct estimation of undrained shear strength from CPTu can indicate trends of increase or decrease; however, it cannot provide reliable estimations. An average cone factor value or rather any value produced in a study may not reflect the regional conditions at another site; therefore, core sampling and testing will need to be continuously conducted so that correlations can be established [152]. Moreover, proper core sampling and testing becomes even more crucial in terms of obtaining consistent cone factor values for a site. The process of correlating CPTu index parameters with lab tests is also relevant for other properties such as the overconsolidation ratio, unit weight, relative density, etc. The direct application of CPTu index profiles without adequate lab testing can lead to overestimations or underestimations of soil properties; therefore, it is necessary to establish sufficient correlations for in situ readings so that reliable interpretations can be made [152,153]. In the Aegean and Ionian Seas, CPT correlations of index properties have not been published; however, existing geotechnical data from surveys that have conducted CPT tests adjacent to core sampling sites could produce a preliminary dataset of undrained shear strength measurements and indicative N_kt cone factor values.

5. Discussion

This review examined marine geotechnical studies in the Aegean and Ionian Sea. It is evident from Figure 2 that the central and eastern parts of the Aegean Sea lack any geotechnical study that could support information regarding the sediment composition, at least of the topmost seabed layers, or geohazards. Also, the eastern Aegean Sea islands (of the North Aegean and Dodecanese) are at the frontline of future power and communication interconnections and of other potential offshore works (i.e., locations for floating wind farms), highlighting a shortfall of geological information that could aid desktop and front-end engineering design studies. It is notable that due to the generally limited geotechnical information, supplementary interpretations from geological studies, specifically regarding lithostratigraphy, have been necessary to provide a comprehensive picture of the distribution and importance of weak layers for sediment instabilities (Table 1). The term “weak” is a general and ambiguous term; however, it reflects certain distinct attributes in relation to overlying and underlying strata, such as a low shear strength, low bulk density and high water content. The most prominent layers exhibiting weak behavior are S1 sapropels, underconsolidated deltaic layers deposited under rapid sedimentation rates, tephra (intermixed or intercalated) and loosely packed turbidites. Examination of the factors affecting slope stability in the Aegean and Ionian Seas has proven that seismic activity is the most important triggering mechanism (e.g., refs. [14,25,29]). In the context of the factors affecting the strength of slope sediments, gas-charged sediments, steep fault escarpments and liquefaction of coastal, shallow-seated sandy horizons also contribute to the initiation of mass failures. The distribution of gravitative mass events has been documented with seismic profiling mainly in slope and basin environments, but also along coastal areas fed by fluvial/deltaic sediments; however, an important parameter that may facilitate geohazards assessments is the frequency of such processes [154]. This has been demonstrated by the assessment of long core samples, followed by continuous multi-sensor core logging (MSCL) readings, where turbiditic deposition can be distinguished from hemipelagic sedimentation. Another element is the correlation of cable failures with mass movement, or any other process that may compromise seabed infrastructure, which can be used as a proxy for assessing the frequency of submarine landslides. An example of such an archive can be found in the Gulf of Corinth [14,71,155]. Based on the above, direct sediment sampling of weak layers and laboratory measurements could shed light on the detection of potential failure planes along sloping environments and on the mechanisms governing mass movements [156].

In the case of slope stability estimations, the infinite slope stability method is the most common, followed by the normalized soil parameter. The latter method is considered a more sophisticated approach in slope stability estimation; however, since it necessitates triaxial testing, it is generally rare and has been implemented only by a few studies in Greece [31,41]. To assess slope stability, shear strength profiles in slope environments are necessary as well as relatively deeper core sampling.

Regarding shear strength measurements, these are important in identifying weak zones within the sedimentary column as well as for slope stability estimation. In the case of CPTu shear strength estimation, no correlations have been established between CPTs and lab tests of shear strength, or any index property for that matter. Moreover, profiles of basic parameters also necessitate some validation from nearby core specimens, especially for establishing a reliable interpretation of soil behavior charts. This is particularly important for mixed sediments, where the CPT response may be less clear [151]. Based on the above, it should be emphasized that direct CPT index property data should be used carefully when assessing engineering conditions. The CPT can be a very useful tool; however, additional sampling is needed to establish more robust interpretations of soil strata. This also concerns the development of indicative or rather site-specific cone factor ranges, so that a priori in situ knowledge of fine-grained sediments can be established.

Another important aspect regarding shear strength is the type of laboratory test used to assess the strengths of sediments. The pocket penetrometer and the field vane test are simple, cheap and easy to use; however, other tests mentioned may produce relatively more reliable results of shear strength. This, however, is difficult to assess without some level of cross-validation. In the case of the laboratory vane apparatus, it is advised that its usage is strictly limited to soft clays; hence, application on sandy/silty clays may produce unreliable estimations. The handling of specimens also becomes an issue when estimating shear strength. This is particularly relevant for triaxial testing and/or direct shear tests.

6. Prospects and Concluding Remarks

Offshore projects are initially based on well-documented desktop studies, which rely on the availability of public or private geotechnical and geophysical datasets. In Greece, numerous marine surveys have been carried out, principally for submarine power and fiber optic links between islands and the mainland, but also for pipelines or big port construction/extension or other infrastructures, from private companies. Unfortunately, the original or interpreted data from these surveys are under a classified status; hence, they are not available to public universities/institutions for scientific inspection. Moreover, other public data are not accessible since there is no established procedure for officially requesting or even purchasing them. Addressing issues in geotechnical surveying and interpretation can be managed by developing an open-access policy regarding the dissemination of data, which can be controlled by a certain governmental authority, thus allowing for a more systematic recording of geotechnical information. The issues regarding CPTu index validation and ground-truthing can be examined thoroughly through the collation of data, where comparisons can be made on a regional and national level. This becomes even more relevant, or even essential, when considering the planned offshore projects (OWF, cables, pipelines) in the Aegean Sea [3,157].

Examples of open-access data policies can be found in the Netherlands, which has allowed for the development of geodatabases for the Dutch section of the North Sea [158]. The databases provide information on borehole data, seismic reflection profiles (2D and 3D), CPTu tests and core samples. Specifically, a large database (BRO database) has been created, where information from other databases has been collated, harmonized and processed, according to INSPIRE European Directive stipulations [159]. The BRO database has assisted in many offshore operations, such as installation of monopiles, predictions of seabed mobility and seismic integration with borehole data/in situ tests [158,160]. Furthermore, the application of neural networks and machine learning can also be tested with the utilization of integrated data from seismic and CPTu profiles or laboratory measurements, thus producing synthetic CPT records of known sedimentary strata in the process [160]. Additionally, site-specific correlations concerning CPTu index parameters can be examined more thoroughly, such as in the case of undrained shear strength [161,162]. The development of such databases has been implemented for the undrained shear strength [141,161,162,163,164]. Implementing such inventories can offer insights into the variations that are often exhibited in lab measurement and CPTu testing. Moreover, a priori knowledge can be established, thus enhancing future geotechnical investigations. Additional examples can be found in Ireland, where extensive surveying of the continental shelf has been implemented, followed by open-access documentation. Open-access data have been utilized to designate suitable sites for OWF development on the Irish continental shelf, as well as geotechnical characterization of seismic stratigraphy [165,166].

Based on the above examples, marine geotechnical research requires a more concrete approach in terms of how data are acquired, processed and interpreted. From a European perspective, the EMODnet initiative provides a prime example; however, regarding the Aegean and Ionian Seas, geological data are not extensive. In the case of geotechnical interpretation, the collation and accessibility of survey data, including published information on sedimentary strata, can provide the first step in addressing the issues concerning in situ interpretation, ground-truthing and index property estimation. For these reasons, an open-access data repository may prove instrumental, at least in terms of academic initiatives.

This review examined available published information regarding the geotechnical properties of sediments in the Aegean and Ionian Seas. Due to the limited geotechnical research, geological studies were examined for their contributory role in assessing lithostratigraphy (weak layers), gravitative mass processes and factors that trigger sediment instabilities. Yet, geotechnical analysis of cores and slope stability estimations represent important initiatives in addressing regional geohazards. Within that context, the inclusion of CPTu or other in situ measurements (e.g., SPT) can further enhance the interpretation, followed by a more extensive sampling of the seabed. A database consisting of core descriptions and physical and mechanical properties (e.g., index properties, relative density, shear strength, consolidation properties, etc.) can offer insights into the engineering behavior of sediments and establish a priori knowledge of the seabed.

According to the above, to further advance geotechnical research in the Aegean and Ionia Sea regions, it is necessary to complete the following:

(i)

Gather existing data from older public surveys as well as unclassified information from private companies, to construct a marine geological (geotechnical and geophysical) inventory.

(ii)

Prioritize areas where scheduled core sampling should take place to eliminate gaps in knowledge for the existing lithological units on the seabed and detect potential weak layers that could act as failure planes, as well as collate records of lithostratigraphy that may lead to weak behavior (sapropels, tephra layers, underconsolidated layers, etc.).

(iii)

Adopt a systematic protocol of measurements for a minimum set of of parameters (i.e., undrained shear strength (field or laboratory), bulk density, water content, Atterberg limits, grain size) following international standards (ASTM, BS, etc.) for all the cores collected, to assess the geotechnical behavior of subsurface sediments. This is also important in the context of assessing slope stability.

(iv)

Wherever feasible, provide CPTu index correlations of shear strength as well as other properties with lab measurements so that reliable estimations and interpretation of in situ data can be established. In the case of cone factor values (N_kt), indicative ranges can be produced and compared among studies (site-specific) so that consistency can be examined. Within that context, the testing method as well as handling of specimens are important.

(v)

Finally, the development of an open-access data repository may further improve the above actions, similarly to how other countries have developed their own systems to assess the geotechnical conditions of the seabed. In the case of the Aegean and Ionian Seas, the complex geological regime and the current boom in offshore construction activities make this imperative.