Sediment Shear Strength Research Articles

Challenges in the measurement of cross-shore geotechnical characteristics of sandy beach surface sediments

Geotechnical properties of surficial beach sediments affect beach erosion and shoreline changes. This study sets out to measure relative density, moisture content, friction angle, and shear strength of sandy beach surface sediments from dune to swash zone towards the goal of assessing their importance in sandy beach morphodynamics. Methods of sediment sampling, a conductivity based moisture probe, field penetrometers, and a field vane shear were deployed to collect data at the sandy Atlantic-side beach in Duck, North Carolina. The tools used were assessed based on their operability in the beach environment and data quality, and results are discussed in the context of beach morphodynamics. A digital field vane shear provided an efficient and direct method of measuring shear strength, but the difficulty of computing stresses on the failure plane, which is necessary to validate the results, ultimately reduced the usability of this instrument. The results from three penetrometers were compared to a partially-saturated bearing capacity model, where a portable free-fall penetrometer yielded the best fit. However, a modified velocity-dependent strain rate correction factor (K=0.31vi) was required to convert dynamic sediment resistance to a quasi-static resistance for the partially saturated sands. A small-scale digital push in penetrometer also achieved a positive correlation when compared to moisture content, but the small tip diameter (5 mm) coupled with the grain size at the beach (0.35 mm) raised concerns about the ability to derive an accurate measure of strength. It was determined that estimating strength parameter using a partially saturated bearing capacity model was appropriate for water contents less than 25% by volume, or anywhere in the crosshore above the swash to the dune. Relative density and moisture content were found to be closely linked, with partial saturation resulting in samples that featured negative relative densities up to −40%.

A soil mechanics model to determine the onset of wind erosion

Determining the friction threshold velocity (FTV) of wind-induced erosion is crucial to understanding and predicting the morphodynamics and management of foreshores and dunes. However, the FTV is influenced by multiple factors, including particle size, mineralogy, surface roughness, and moisture content. Although existing models account for these parameters, they suffer from limited precision, are not generalized and developed for specific sediment types, and rely on expensive and time-consuming testing procedures. Furthermore, no predictive equations for FTV currently consider the combined effects of bed inclination and moisture common to coastal dunes. This study presents a comprehensive closed-form model for predicting the FTV in the onset of wind erosion for different types of sands. The model considers various geometric and material properties, including cohesion, moisture, soil packing, slope, and grain size distribution. A soil water retention curve (SWRC) is incorporated into the formulation to establish a relationship between water content and sediment shear strength. This simple SWRC approach enables simplified calculations of the onset of wind erosion under various conditions, requiring only a few inexpensive inputs. Extensive parametric wind tunnel experiments were conducted to measure the FTV in combinations of bed slope, sand particle size, moisture, and density. The findings indicate that the combined influence of slope and moisture increases the FTV. Furthermore, compared to the FTV for dry sediments on a horizontal bed, the amplification factor exhibits a nonlinear combination of the effects of inclination and moisture. The proposed FTV predictive model demonstrates adequate agreement with published results when applied to scenarios such as dry sand on a horizontal surface, dry sand on an inclined bed, and moist sand on a horizontal surface.

Free fall penetration test the undrained shear strength of submarine sediments

To perform in-situ testing of deep-sea sediment undrained shear strength, a novel free-fall penetrometer has been developed. This penetrometer is released from above the seabed and penetrates the sediments under gravity, resulting in intense interaction with the sediment. The study conducted free fall penetration tests in the Xisha area of the South China Sea, yielding the following achievements: (I) the establishment of a free-fall penetration governing equation, influenced by penetration attitude, along with the development of a model for undrained shear strength, revealed that at a 5° penetration angle, the sediment support force exceeded 100 N, and the undrained shear strength is approximately 40 kPa. (II) the relationship between dynamic penetration resistance coefficient and penetration velocity was inverted by the data from in situ test and laboratory. An error analysis revealed that inclined penetration and unreasonable penetration resistance coefficient can lead to errors exceeding 40 % in the test results.

Thermo-mechanical behavior of reconstituted deep-water sediments

Understanding the thermo-mechanical behavior of deep-water sediments is essential for the safe design of high-temperature oil/gas pipelines. In this study, a series of temperature-controlled triaxial tests was conducted to investigate the thermo-mechanical behavior of reconstituted deep-water sediments sampled from the South China Sea. The experimental results revealed the thermo-mechanical behavior of sediments was closely related to their thermo-mechanical loading paths and sodium chloride (NaCl) concentrations. The increasing rate of excess pore water pressure decreases with an increase in temperature increment, effective confining pressure, and overconsolidation ratio, regardless of NaCl concentration. For normally consolidated specimens, the undrained shear strength generally decreases as temperature increases. The undrained shear strength of specimens was lower under undrained heating and higher under drained heating than that at 5 °C, respectively. Factors such as thermal softening, thermal consolidation, and thermal-induced excess pore water pressure significantly impacted the undrained shear strength of sediments under isothermal, drained, and undrained heating conditions. Additionally, the undrained shear strength decreases with increasing NaCl concentration. The results highlighted that the critical state line is unique under varying loading paths and NaCl concentrations. Furthermore, the effect of changes in void ratio induced by NaCl solution on the shear strength of sediments was more pronounced than that of the intergranular stress.

Undrained Shear Strength from Field Vane Shear Test (VST) of Soil at Shoreline Area

The effect of soil shear strength at the shoreline area is the focus of this study. Pantai Punggur, Batu Pahat, Johor, has been chosen as the subject location for the soil shear strength investigation. It is located on the west coast of Johor, with latitudes ranging from 1.62° to 1.87° N and longitudes ranging from 102.78° to 103.19° E. The purpose of this study is to use a field vane shear test (VST) to determine the undrained shear strength of shoreline areas. Soil samples were collected at specific locations known as Zones A, B, C, D, and E, and data for sediment shear strength was obtained from field vane shear test (VST) methods. From the undrained shear strength test at 0.5 m depth at high tide (HT) and medium tide (MT) data for December 2021, Zone A had a Cu reading of more than 4.5 kPa, followed by Zones B, C, and D, and lowest at Zone E. Next, in the December 2021 data at the HT level, Zone A has a Cu reading greater than 4.5 kPa at 0.5 m and 1.0 m, while other Zones have readings less than 3.0 kPa. It can be concluded that the depth of a part also influences the data in this study. When the depth increases, the shear strength of the data also increases. This study will serve as a benchmark for future conservation efforts. Based on the findings, more effective control measures for preserving and conserving coastal areas may be developed.

Hybrid engineering incorporating salt marsh terraces into sea wall repair maintains their defence function and creates new habitats

Sea walls are widely-used engineered structures designed to protect low lying land from flooding. Rising sea levels and coastal erosion threatens sea wall integrity and requires maintenance and repair. As an alternative to conventional repairs, a hybrid engineering design incorporating sediment-filled terraces to allow the development of salt marsh has been trialled to protect sea walls in the Colne-Blackwater Estuary complex, Essex. U.K. Over a 10 year period, three different trajectories of development were measured. Salt marsh halophytes colonised 7 out of 15 terraces, with % plant cover ranging between 5% and 170%. Halophyte species richness on terraces ranged from 1 to 8 species (average = 1.7 per 4 m2), compared to existing salt marsh adjacent to the repairs (max. species richness 9, average 6.85), with positive significant relationships between % halophyte cover and halophyte richness and between % halophyte cover and sediment water content (19–51% on terraces). Organic carbon content was significantly lower on the terraces compared to existing salt marsh (7–19 % AFDW on terraces, 17–24% in marsh sediments). A sediment shear strength of 30 kPa was optimal for % plant cover on terraces. The height of the terraces relative to tidal inundation was a key determinant of successful halophyte colonisation. A second trajectory (3 of 15 terraces) resulted in development of macro- and microalgal mats, up to 100 % cover. Both these trajectories resulted in up to 25 cm of vertical sediment accretion. Shell banks formed on 2 terraces. Seven terraces were subject to erosion, associated with desiccation and absence, or loss, of halophyte cover. Eroding terraces showed both surface lowering (20–30 cm) and massive sediment failure (blocks >50 cm deep falling away). After a decade, protection of the sea wall was still provided by 14 of the 15 terraces, with only one having to be rebuilt. Utilising a nature-based approach incorporating sea wall terraces resulted in the formation of three types of linear habitat (salt marsh, microbial mats, shell banks) providing some ecosystem services that would not have existed if a conventional hard-engineered repair had been used.

Identification of the sediment-water interface during field full-flow penetration tests

The undrained shear strength of marine surface sediments is critically important for both the development of marine resources and environmental protection. In situ full-flow penetration tests offer significant advantages for evaluating these surface sediments, particularly those with low strength. Accurately analyzing the strength of surface sediments during in situ full-flow penetration tests relies on correctly identifying the sediment-water interface, also known as the mudline. Unfortunately, there is currently no established method for solving this problem. This study utilizes computational fluid dynamics to conduct a numerical analysis of full-flow penetrometer probes (specifically of ball and T-bar probes) as they penetrate marine surface sediments of varying strengths and densities from different heights. By examining the variation in the dimensionless drag force coefficient of the probes in relation to the dimensionless penetration depth, the impact of the mudline on the drag force trend is systematically analyzed. Additionally, a methodology for determining the mudline location using a mudline distinguishing factor is proposed. This mudline distinguishing factor was tested during a field trial of the in situ ball full-flow penetration device in the Philippine Sea, The proposed methodology for identifying the mudline and analyzing the strength of marine surface sediments has significant value for the field.

Undrained Triaxial Shear Tests on Hydrate-Bearing Fine-Grained Sediments from the Shenhu Area of South China Sea

Changes in undrained shear strength are important to the stability analysis of hydrate reservoirs during natural gas hydrate production. This study proposes a prediction model of undrained shear strength of hydrate-bearing fine-grained sediments based on the critical state theory. Several consolidated undrained triaxial shear tests are conducted on hydrate-bearing fine-grained samples from the Shenhu area of the South China Sea. The effects of effective consolidation stresses and hydrate saturations on the undrained shear strength are investigated. The results show that the undrained shear strength increases linearly with increasing effective consolidation stress. When the hydrate saturation is greater than the effective hydrate saturation, the undrained shear strength significantly increases with increasing hydrate saturation. The undrained shear strength of hydrate-bearing fine-grained sediments is a two-parameter function of effective hydrate saturation and a void ratio. The instability risk of the hydrate reservoir under undrained conditions is greater than that of under-drained or partially drained conditions. Furthermore, low-porosity reservoirs face more shear strength loss from hydrate decomposition yet lower risk than high-porosity ones. These results can improve the understanding of mechanical properties of hydrate-bearing fine-grained sediments under undrained conditions. This study also has implications for the design of marine structures in areas with hydrate-bearing sediment.

Improved predictive model for the strength of fluidized seabed sediments with rate effect characteristics by full-scale spherical penetrometer tests

Sampling fluidized seabed surface sediments with viscous fluid characteristics is highly challenging, making in situ penetration tests essential to evaluate their strength. Unfortunately, an in situ full-flow spherical penetrometer (i.e., ball) is typically used to assess the strength via deep penetration, and research on surface penetration, extensive viscous fluid shear behavior of fluidized surface sediments, and ball-ambient water-surface sediment interaction is lacking. To address this gap, this study utilizes a computational fluid dynamics (CFD) model validated through centrifugal tests to examine the penetration of balls with free and no-slip in fluidized seabed surface sediments with the shear rate effect. This study analyzes the stress characteristics on a ball under complex conditions and proposes three phases of the penetration resistance coefficient of the ball in fluidized surface sediments. Additionally, this study quantifies the effects of various factors, such as initial undrained shear strength, rate effect, ambient water above the ball, and contact relationship between the ball and fluidized seabed surface sediment, on sediment strength evaluation. Finally, this study establishes a three-phase method to evaluate the undrained shear strength of fluidized seabed surface sediments by full-scale spherical penetrometer tests. This method calculates the surface penetration resistance coefficients as functions of the dimensionless penetration depth, providing a vital basis for acquiring the strength of fluidized seabed surface sediments.

Numerical Investigations of Undrained Shear Strength of Sensitive Clay Using Miniature Vane Shear Tests

The laboratory miniature vane shear test (MVST) has been widely used to measure the undrained shear strength of marine sediments in offshore engineering. However, the transfer of the soil sample in tube samplers from the seabed to the laboratory releases the in situ confining stress acting on the soil and will decrease the soil strength. In this research, in order to investigate the effects of confining stress on the undrained shear strength of marine sediments, the Coupled Eulerian-Lagrangian (CEL) approach in ABAQUS is used to model the three-dimensional standard and miniature vane shear tests to estimate the undrained shear strength of sensitive clay with different sensitivities under various stress conditions. Based on the numerical simulation results, a linear strength model that not only considers confining stress effects but also can eliminate the size effects caused by vane blades of MVST is proposed. The proposed model can be used to estimate the undrained shear strength of the sensitive clay under shallow seabed surfaces.

Bearing capacity factors of T-bar from surficial to stable penetration into deep-sea sediments

In-situ full-flow penetrometers (i.e., the ball and T-bar) have shown great advantages and promising applications in testing and evaluating the undrained shear strength of low-strength deep-sea surficial sediments with fluidization characteristics. However, compared with a ball with the same projected area, a T-bar with the advantages of smaller volume and length along the penetration direction still suffers from the lack of better understanding of the surficial penetration mechanism and a method to estimate the bearing capacity factor under the effect of ambient water. In this study, a computational fluid dynamics method, Eulerian–Eulerian two-phase flow modeling, is used to analyze a T-bar with the no slip and free slip wall boundaries penetrating deep-sea surficial sediments defined by the dynamic viscosity of a non-Newtonian fluid with shear thinning behavior under ambient water. The stress characteristics and corresponding mechanism of the entire process (i.e., surficial and deep penetration) of a T-bar penetrating deep-sea sediments from the mudline to stable penetration are revealed from the evolution of the open and trapped water cavities above the T-bar. Finally, a methodology to evaluate the undrained shear strength of deep-sea sediments tested using a T-bar with different interface contact relations is proposed, and the corresponding equations are established, providing a basis for marine engineering geology survey, engineering construction, and hazard assessment.

Salt leaching by freshwater and its impact on seafloor stability: An experimental investigation

Offshore freshened groundwater (OFG) has been documented in many continental margins worldwide. OFG systems are dynamic, expanding and contracting with falling and rise sea-levels. OFG has long been thought to be an important geomorphic agent in continental margins, either via active discharge at the seafloor, which can erode depressions, or the generation of excess pore pressure, which can deform sediments and cause slope failure. It has also been proposed that OFG flow can drive the loss of sediment shear strength via salt leaching, when seawater in pores is replaced by freshwater. Here, we measure changes in the geotechnical properties of seafloor clayey silt due to salt leaching using flushing experiments, and assess the implications of these changes on the stability of siliciclastic continental margins with 2D limit equilibrium modelling. We document a ∼ 50% decrease in undrained cohesive strength of seafloor sediment after flushing, as well as a decrease in its shear strength, bulk density, and moisture content, which is similar to that reported for subaerial quick clays undergoing salt leaching. When applied to a theoretical submarine domain 300 m wide by 100 m high, we estimate that salt leaching can trigger slope failure when the thickness of the flushed layer is >3.5 m or when the slope gradient is >3°. Such conditions are primarily satisfied on the continental slope or the shallow seafloor close to the shoreline. Salt leaching by OFG flow merits consideration as a potential mechanism destablising submarine sedimentary slopes.

Estimation of undrained shear strength in rate-dependent and strain-softening surficial marine clay using ball penetrometer

The spherical penetrometer (known as ball) has been widely used in laboratory tests and in-situ investigation to determine the undrained shear strength of fine-grained sediments in offshore geotechnical engineering. The conventional interpretation of the ball penetration tests suffers from either neglecting the effect of the strain softening and strain rate for steady state penetration or inaccurate accounting for the evolving bearing factor during the shallow failure stage of soil. In this study, the continuous penetration of the ball is simulated using the “Remeshing and Interpolation Technique with Small Strain” (RITSS) with a strain softening and rate-dependent soil model. Three stages from shallow failure mechanism to deep flow-round mechanism are clarified. The strength distribution of ball penetration into softening and rate-dependent soil is also revealed. A parametric study is performed to investigate the effect of strain rate and softening parameters on the failure mechanism and the evolved bearing factor profile. The effects of the dimensionless soil strength sum /γ′D and the frictional condition of the soil-ball interface are explored. Empirical formulas are developed to estimate the critical depths of the ball penetration test. A practical interpretation framework, incorporating the effects of strain rate and softening, is developed to assess the soil undrained shear strength using the ball penetrometer.

Drainage-Improved Sediment Strength Enhances Saltmarsh Seedling Establishment Chance

Identify critical factors driving seedling establishment is essential for saltmarsh restoration. Recent studies highlight drainage can facilitate seedling establishment for different saltmarsh species, yet the underling mechanism remains largely unknown. By a manipulative mesocosm experiment with contrasting sediment drainage treatments, we investigated the impacts of drainage onSpartina anglicaseedlings survival, growth and stability. Surprisingly, neitherS. anglicaseedling survival nor seedling growth were observed to be directly promoted by well drained treatment. However, the critical erosion depth (maximum erosion tolerance) ofS. anglicaseedlings was found to positively correlate with seedling root length. Measurements on the sediment properties revealed that drainage increased the critical shear strength of sediments with lower water content and higher dry bulk density. These findings indicate that, by dewatering, drainage aids to sediment consolidation and critical shear strength, and thereby facilitate seedling establishment in tempering sediment dynamics. The latter is supported by a field study, in which we found that the magnitude of the sediment dynamics on two tidal flats were positively related to water content of sediment. Overall, present study suggests that drainage could be a key factor that determines seedling stability during the critical windows of opportunity for seedling establishment, and that it could be integrated into future designs for saltmarsh restoration to enhance establishment success.

Vegetation interactions with geotechnical properties and erodibility of salt marsh sediments

Salt marshes provide diverse ecosystem services including coastal protection, habitat provision and carbon sequestration. The loss of salt marshes is a global scale phenomenon, of great socio-economic concern due to the substantial benefits that they provide. However, the causes of spatial variability in marsh loss rates are inadequately understood for the purposes of predicting future ecosystem distributions and functions under global environmental change. This study investigated the relationship between the presence of different saltmarsh plants and the mechanical properties of the underlying substrate that relate to its vulnerability to erosion. Relationships between three halophytes (Puccinellia spp., Spartina spp. and Salicornia spp.) and sediment stability were assessed and compared to unvegetated substrates using in-situ and laboratory tests of substrate geotechnical properties and sediment characteristics. Sampling was conducted at two UK sites with contrasting sedimentology, one sand-dominated and one clay-rich. Sediment samples, collected simultaneously with measurements of shear strength, were analysed for moisture content, particle size and organic, carbonate and mineral compositions. These data were then used to explore the contribution of plant type, alongside the sedimentological parameters, to measured shear strength.Shear strength of the sediment varied between and, to a lesser extent, within sites, with the four cover types having a similar effect on shear strength within sites relative to each other. Sediments covered by Puccinellia spp exhibit the highest shear strength, while bare sediments exhibit the lowest. The effect of vegetation type on shear strength was greater in the coarser sediments of Warton Sands. Surface cover type made a significant contribution to exploratory statistical models developed for the prediction of sediment shear strength. The findings support existing recognition that vegetation can enhance sediment shear strengths but extend the insight to reveal differences in this effect that show generality between sedimentological settings. Further, the combination of methods provides insight into the fundamental mechanics by which various measures of sediment stability may be affected by different surface cover types. Cohesion appears to be a more appropriate descriptor of sediment erodibility than shear strength or friction angle and is most greatly enhanced by the presence of a fine, fibrous root system such as that of Puccinellia. A more detailed understanding of the multi-scale mechanisms by which plants confer strength to substrates is needed to better anticipate their impact on sediment erodibility, and therefore salt marsh vulnerability.

Characterizing the microstructural anisotropy of fine-grained phyllosilicate-rich rocks

Abstract. The physical properties of claystones, shales, and slates are highly dependent on the alignment of phyllosilicate minerals. With increasing overburdening, the shape and the crystallographic preferred orientation of these minerals are affected by uniaxial shortening as well as tectonic processes including recrystallization under elevated pressure and temperature conditions. The microstructural anisotropy expressed mainly by the alignment of phyllosilicates significantly predetermines the orientation of fractures, hence the shear strength and stability of clay-rich sediments and rocks. A quantitative analysis of phyllosilicate alignment is therefore essential to evaluate the properties and the mechanical behavior of these rocks. This can be carried out by analyzing the crystallographic preferred orientation (texture). Although texture analysis is a common tool in geosciences, it becomes more difficult in fine-grained rocks owing to for example particle size, heterogeneity, the polyphase composition, and difficulties in sample preparation. Methods such as electron backscatter diffraction, neutron diffraction, or laboratory X-ray diffraction are restricted with respect to preparation artifacts, sampling size and statistics, water content, etc. To overcome these issues, we successfully apply high-energy X-ray diffraction as available at synchrotron research facilities, e.g., at the German Electron Synchrotron Facility (DESY) in Hamburg, Germany, or the European Synchrotron Research Facility (ESRF) in Grenoble, France. In combination with Rietveld refinement we analyze the bulk texture of phyllosilicate-rich rocks. Here we present the results of texture analysis from a wide range of these rocks: Pleistocene poorly consolidated mud (rocks), affected only by sedimentation and burial; more highly consolidated but tectonically largely unaffected Jurassic claystone from the Opalinus Formation of the Swabian Alb; Carboniferous shales from the Harz mountains representing low-grade metamorphic and deformed rocks. Our methodical approach to quantifying the microstructural anisotropy using texture analysis in fine-grained rocks allows for the quantification of physical properties resulting from the alignment of phyllosilicates. Furthermore, it enables the prediction of direction-dependent mechanical strength, which is crucial for the establishment of long-term repositories for radioactive waste in shales and claystones.

Submarine landsliding in carbonate ooze along low-angle slopes (Inner Sea, Maldives)

Multichannel seismic and hydroacoustic data from the Maldives carbonate platform in the northern Indian Ocean document 4–8 km wide and are 70–170 m thick submarine landslides at water depths greater than 500 m, potentially associated with extensive fluid escape and the occurrence of gas hydrates. The mobilized sediments are part of giant drift bodies that consist of fine-grained carbonate ooze. Their downslope extent is located adjacent to large gateways that connect the semi-enclosed inner basin of the carbonate platform with the open Indian Ocean. Exposed at the sea-floor are headwall and translational domain of the landslides, showing different kinematic indicators typical for mass transport complexes. The submarine landslides evolved at slightly inclined slopes (<1°) along a listric basal shear surface (BSS) by downslope progradation and retrogressive failure. Styles of motion include sediment creeping, sliding and slumping. Where the BSS is flat, sediments translate as a large body. Where the BSS is irregular, folded, thrusted and rotated blocks/ridges occur, associated with secondary listric shear faults. Characteristically for the headwall domain are arcuate depressions which are related to rollover structures in the displaced slide block. The distal toe domain of the submarine landslides is located at the steep (>20°) part of the platform slope which faces the Indian Ocean. Data show that landslide activity is ongoing.The likely trigger of slope failure is a combination of gravitational forces and the reduction of sediment shear strength moderated by fluid ascent and the destabilization of gas hydrates. Gas hydrates are suggested to play an important rule, because the top of the gas hydrate stability zone coincides with a significant steepening of the platform slope from 1-3° to 10–20°. It is therefore proposed that the dissociation of the gas hydrates, for example during sealevel lowstands might have been an additional trigger of slope failure. During sealevel highstands, gas-hydrate coating of sediment grains may facilitate low-velocity plastic deformation of the strata.

Geomechanical properties of gas hydrate-bearing sediments in Shenhu Area of the South China Sea

It is essential to understand the in-situ geomechanical properties of natural gas hydrate-bearing reservoirs for hydrate drilling, the design of reservoir stimulation and production schemes, geologic disaster prevention, and environmental impact assessment. In this way, it serves as the basis for the development and utilization of gas hydrate resources. In 2016, the Guangzhou Marine Geological Survey conducted the fourth natural gas hydrate drilling (Expedition GMGS4) in the north of the South China Sea, during which the effective stress of gas hydrate-bearing reservoirs in the study area was ascertained and gas hydrate-bearing pressure cores were collected. The effective stress was determined to be 700–800 kPa by in-situ piezocone penetration tests. The saturation, permeability, and shear strength of gas hydrates in the pressure core samples were measured under the in-situ stress state using the shipboard multi-sensor core logger (MSCL), pressure core analysis and transfer system (PCATS), and PCATS Triaxial system. Based on this, the distribution characteristics and the relationship of the permeability and gas hydrate saturation of the gas hydrate-bearing reservoirs were explored. Furthermore, the effects of effective stress and gas hydrate saturation on shear strength of gas hydrate-bearing clayey-silt sediments were mainly analyzed, as well as the stress–strain characteristics of gas hydrate-bearing sediments. The results are as follows: (i) the gas hydrate saturation and reservoir permeability in the study area are 0.7%–55% and 10−3–10−1mD, respectively, indicating low-permeability reservoirs; (ii) the gas hydrate saturation and reservoir permeability have obvious anisotropy in horizontal and vertical directions; (iii) the undrained shear strength of gas hydrate-bearing sediments is 0.24–0.52 MPa, and the shear strength of fine-grained sediments is more sensitive to the changes of in-situ stress. In the case that deviatoric stress continues increasing, micro-fractures may occur inside gas hydrate-bearing sediments, and this can lead to drastic changes in the strength of gas hydrate-bearing reservoirs. Therefore, it is necessary to take into account the effects of micro-fractures in gas hydrate-bearing reservoirs on engineering safety during long-term hydrate production.

Liquid consistency of organic soils under the bridge construction

The paper presents the case study of geotechnical investigations reporting connected with expert’s opinion on undrained shear strength of organic sediments in the river valley at the north of Poland according to reinforcement methodology of c.a. 0, 7 km highway embankment. Diversification of primary results caused additional research which revealed and confirmed liquid consistency of organic soils – several meters of organic and calcareous sediments – gyttja under the thin cover of peat. The main goal of the paper is a discussion on standardization of consistency of organic soils itself and additionally to point out very rare possibility of liquid consistency, finally not pointed in International Standards ISO 14688. The application of Atterberg limits in organic sediments is moot but even organic soil with water content higher than liquid limit can’t be classified and interpreted as very soft with description as soil which exudes between the fingers when squeezed in the hand. Such identification is practically impossible when it’s hard to squeeze because of gravitationally leaking through the fingers.

Chemical weathering and progressing alteration as possible controlling factors for creeping landslides.

Landslides can behave as dynamic processes, which emerge from the complex interplay of tectonics, erosion, weathering and gravitational influences, triggered by various hydrological, mineralogical, biological and geotechnical factors. Integral studies to assess the mechanisms underlying landslide initiation and progression are mainly focussed on specific cases with high geohazard potential. The landslide near Stadtschlaining (Austria) represents a key study site to elucidate the impacts of pelitic sediment composition, weathering regime, alteration patterns and hydrochemistry on recurrent damage progression in the local infrastructure. Based on field work, soil-mechanical logging (Atterberg limits, undrained strength, friction angles), water chemistry (ICP-OES, IC, hydrochemical modeling), solid-phase characterization (XRD, XRF, SEM) and sorption experiments we establish a conceptual model for initiating and progressing of landslides: Infiltration of low mineralized meteoric water (EC: <200μS/cm) in permeable limonitic gravels triggers chemical weathering of greenschist-derived detritus and promotes its transformation into kaolinite and smectite. The clayey strata (>50wt% of clay minerals) create zones of mechanical and chemical weakness in the underground (~4-6m below ground level), which are characterized by particle disintegration/delamination, slip bedding and deformations, and development of porous layers depicting water flow paths. Subsequent Na+ exchange for bivalent ions in the smectite interlayer delivered by percolating, highly mineralized water (EC: 1600-5100μS/cm) is caused by de-icing salt and fertilizer applications during winter and late summer, and yield in i) decohesion and physical breakdown of the particle aggregates and ii) swelling of the clay matrix in early spring and autumn. These processes reduce the shear strength of the pelitic sediments, resulting in failure and initiation of landslides (deformation: ~500mm within a month) and subsequent steady creeping motion (deformation: ~100mm in 6months). Customized engineered solutions to prevent landslides in this area are presented, which can be conveyed to analogous landslide-affected areas worldwide.