Sediment Strength Research Articles

Normal-incidence bottom loss measurements over a sand-mud gradient and comparison to direct seabed measurements

Seabed variability introduced by physical or biogenic processes contributes to multi-scale horizontal and vertical variability in seabed acoustic and geotechnical properties. An experiment was conducted near the mouth of Mobile Bay to investigate such spatial variability over a mud-sand gradient. A normal-incidence bottom loss survey was conducted on a 750 × 1500 m2 grid with 50-m track-line spacing, in which a broadband acoustic source transmitted frequency-modulated chirps (1–100 kHz) every second, resulting in dense sampling of the seabed reflection coefficient over a wide frequency band. Diver cores were collected in the survey area for direct analysis of near-surface seabed properties. Cores were acoustically logged to obtain vertical profiles of sound speed and attenuation (10–1000 kHz) and then analyzed for physical properties (porosity, grain size, carbon content), infauna community composition, and sediment strength properties. Additionally, in situ portable free fall penetrometer measurements were conducted within the survey area to characterize geotechnical properties within the upper 50 cm of the sediment. Spatial distributions of properties estimated from bottom loss measurements (e.g. porosity and sound speed) will be compared with vertical profiles from the cores and from the penetrometer to examine potential connections between seabed acoustic, geotechnical, and biological parameters. [Sponsored by ONR.]

Statistical damage constitutive model based on self‐consistent Eshelby method for natural gas hydrate sediments

AbstractNatural gas hydrate (NGH) is widely distributed in marine sediments and continental permafrost, which is a promising energy resource. The sustainable and safe exploration and production of NGH requires fully understanding of its mechanical behaviors, which is still a challenge to our community. In this paper, a statistical damage constitutive model named SC‐SDCM is developed for NGH sediments. The NGH sediments are regarded as two phases: the matrix phase of solid mineral grains, pore fluid, and gas, and the inclusion phase of hydrate crystal. The effective elastic parameters of such two‐phase composite are estimated by self‐consistent method (SC) according to the equivalent inclusion principle of micromechanics. The mesoscopic element strength of the NGH sediments is described by the Weibull statistical distribution and the damage theory of composite materials. And then combined with the Drucker‐Prager failure criterion of microelement, the damage constitutive model of NGH sediments is established. The new SC‐SDCM is tested to be reliable and robust by triaxial experimental observations on artificial cores and naturally occurring samples under different confining pressures and hydrate saturations. The SC‐SDCM could properly describe the stiffness, peak strength, and strain softening properties of the NGH sediments. Moreover, the predictions of SC‐SDCM are closest to the experiment observations compared to several published constitutive models, especially for the near‐ and after‐damage stages with relatively high hydrate saturation.

Strength and Deformation Behaviors of Methane Hydrate-Bearing Marine Sediments in the South China Sea during Depressurization

Strength and Deformation Behaviors of Methane Hydrate-Bearing Marine Sediments in the South China Sea during Depressurization

Effect of organic matter content on Atterberg limits and undrained shear strength of river sediment

The Atterberg limits and the undrained shear strength are crucial soil properties in the preloading design of reclamation foundation projects using dredged river sediment. These properties can be significantly affected by the presence of organic matter in the sediment. This study investigates the physical properties and undrained shear strength of river sediment, based on data from a series of fall cone tests performed on river sediment with different organic matter contents, and undrained shear strength data for reconstituted clay collected from the literature. It was found that the liquid limit, plastic limit, and plasticity index increase with an increase in organic matter content. The effect of organic matter on the undrained shear strength of remolded soil at a given water content can be attributed to the effect of organic matter on the liquid and plastic limits. The undrained shear strength is a function of the liquid and plastic limits; it can be normalized by the liquidity index, irrespective of the organic matter content. Highlights Investigating role of organic matter in the Atterberg limits Investigating role of organic matter in shear strength Understanding key factors of controlling shear strength Establishing organic matter–liquidity index–shear strength interrelations

Combined portable free fall penetrometer and chirp sonar measurements of three texas river sections post hurricane harvey

The US Gulf of Mexico coastal region has repeatedly been subjected to major flood events. Local geotechnical site characteristics and geomorphology can change due to sediment transport processes during such events. However, field measurements during extreme conditions are challenging. This paper discusses initial attempts at a combined geotechnical and geophysical site investigation of the uppermost layers of riverbeds following severe flooding events at three different rivers in Texas: the Guadalupe, Brazos, and Colorado Rivers in terms of sediment strength derived from a portable free fall penetrometer, backscatter intensity recorded by a chirp sonar, and soil sample characterization. Results show low strength sediments (<40 kPa) along the investigated sections of the Guadalupe and Brazos riverbanks. Although sediments in the center of the Brazos River were characterized with higher strength (>50 kPa) and larger grain sizes (d50 ∼ 0.3 mm), sediment strength of the Guadalupe and Colorado Rivers displayed more variations around bridge piers. The spatial variations likely resulted from sediment remobilization processes and local scour under severe hydrodynamic conditions. Both, geotechnical and geophysical results, reflected the observed variations in the riverbed sediments; nonetheless, a quantitative correlation among the rivers was impeded by challenges primarily related to limitations of spatial accuracy and the significant riverbed heterogeneity, as well as shallow water limitations of the chirp sonar.

Investigation of the effect of cementing ratio on the mechanical properties and strain location of hydrate-bearing sediments by using DEM

This study investigates the strain location and particle-scale contact properties of methane hydrate-bearing sediments with different cementing ratios λ through adopting discrete elements method (DEM). In this paper, a new numerical samples preparation technique that can reflect the real microstructure of methane hydrate-bearing sediments based on the nucleation and growth mechanism of methane hydrate is proposed, and a new hydrate bonding model that can consider the coupling effects of temperature and pore pressure is established. Then a series of biaxial shear tests in which flexible particle membrane boundaries are adopted to capture inhomogeneous deformation on the methane hydrate-bearing sediments with different cementing ratios are carried out to provide new insight on the formation and development of shear bands and the evolution of particulate-scale information inside hydrate-bearing sediments. The simulation results show that the increase of hydrate cementing ratios λ results in enhanced strength and volumetric dilatancy of hydrate-bearing sediments. The different hydrate cementing ratio creates several important differences about strain location modes of bond breakage, contact force chains, particles displacement and rotation, local void ratio, local coordination number, and local volumetric strain with the samples of lower cementing ratio exhibit two shear bands while the samples of higher cementing ratio exhibit single shear band. The main mechanical behaviors of hydrate-bearing sediments are controlled by three microscopic contacts (sand-sand contacts, sand-pore-filling hydrate contacts, and sand-cementation hydrate).

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.

Use of Nanosilica for Increasing Dune Erosion Resistance during a Sea Storm

Dune recovery interventions that integrate natural, sustainable, and soft solutions have become increasingly popular in coastal communities. In the present study, the reliability of an innovative non-toxic colloidal silica-based solution for coastal sand dunes has been verified for the first time by means of laboratory experiments. An extensive experimental campaign aimed at studying the effectiveness of the use of nanosilica has been conducted in the 2D wave flume of the EUMER laboratory at the University of Salento (Italy). The study was first based on a horizontal seabed and then a cross-shore beach-dune profile was drawn similar to those generally observed in nature. Detailed measurements of wave characteristics and observed bed and cross-shore beach-dune profiles were analyzed for a wide range of wave conditions. In both cases, two sets of experiments were carried out. After the first set of experiments performed resembling the native conditions of the models composed with natural sand, the effects of the injection of the mineral colloidal silica-based grout were investigated. The observations show that mineral colloidal silica increases the mechanical strength of non-cohesive sediments reducing the volume of dune erosion, thus improving the resistance and longevity of the beach-dune system.

Repetitive Late Pleistocene soft‐sediment deformation by seismicity‐induced liquefaction in north‐western Lithuania

AbstractLiquefaction can cause deformation of unconsolidated sediment, but specific processes involved and the trigger mechanisms often remain obscured. This study describes multiple deformed sediment layers in a succession of lacustrine sand, silt and clay deposited during the Marine Isotope Stage 5d in north‐western Lithuania. The deformation structures (load casts, pseudonodules, ball‐and‐pillow structures, broken‐up laminae and injections) are embedded in ten separate layers of fine‐grained, laterally continuous sediments. Detailed mesoscale sedimentological analyses suggest that each deformation event consisted of numerous successive stages of sediment advection facilitated by liquefaction. Low‐permeability fine‐grained laminae contributed to localized pore‐water pressure build‐up and lowering of sediment strength. Erosional top surfaces that truncate layers with soft‐sediment deformation structures suggest that at least seven deformation events were separated by successive periods of initial erosion and then uninterrupted deposition in the lake. The most likely trigger of the deformation was recurrent palaeoseismic activity possibly linked to a late glacial isostatic adjustment following the Scandinavian Ice Sheet melting after the Saalian glaciation. This study emphasizes the potential role of seismic processes in shaping the sedimentary record of the intraplate region of north‐eastern Europe and contributes to constraining the depth of liquefaction, regardless of the actual trigger mechanism.

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.

The depressurization of natural gas hydrate in the multi-physics coupling simulation based on a new developed constitutive model

Natural gas hydrate has become a hotspot for researchers because of its considerable economic benefits and potential strategic significance. In the hydrate exploitation process, the hydrate-bearing sediments get damaged, and the coupling effect of multi-physical fields is involved. In this study, a damage constitutive model of hydrate-bearing sediments is established by considering decomposition damage, loading damage, and the influence of residual strength. Based on this damage constitutive model, a Thermo-Hydro-Chemo-Mechanical (THCM) multi-field coupling model considering the sediment damage is constructed on the COMSOL simulation platform. Compared with the experimental data, the THCM coupling model is demonstrated and employed to study the depressurization process of horizontal wells in permafrost regions. During the hydrate exploitation model, changes in displacement, saturation, average pore pressure, and temperature at different mining positions are recorded and analyzed. The results indicate that the damaging of sediments has a significant effect on soil deformation and temperature. After the sediment damaging effect is considered, the minimum temperature of the extraction points is reduced by 22.5% at most. The maximum deformation of the extraction points is about 11.8% greater than that without considering the sediment damage effect. Moreover, the model reveals the self-preservation effect of gas hydrate. The temperature of the surrounding soil at the exploitation site decreases due to hydrate decomposition, which would inversely promote hydrate formation. These numerical models in this study can be applied to estimate the strength of sediments and predict the reservoir settlement in the process of hydrate depressurization.

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.

Effects of bed longitudinal inflexion and sediment porosity on basal entrainment mechanism: insights from laboratory debris flows

Channel morphology and bed sediment erodibility are two crucial factors that significantly affect debris flow entrainment processes. Current debris flow entrainment models mostly hypothesize the erodible beds are infinite with uniform slopes. In this study, a series of small-scale flume experiments were conducted to investigate the effects of bed longitudinal inflexion and sediment porosity on basal entrainment characteristics. Experimental observations revealed that sediment entrainment is negligible at early stages and accelerates rapidly as several erosion points appear. Continual evolution of flow-bed interfaces changes interactions between debris flows and bed sediments, rendering the interfacial shear action involved into a mixed shear and frontal collisional action. Lower bed sediment porosity will change the spatial arrangement and orientation of particle mixture, strengthen the interlocking and anti-slide forces of adjacent sediment particles, and promote the formation of particle clusters, all of which will increase bed sediment resistance to erosion. By examining the post-experimental bed morphology, the slope-cutting amounts and topographic reliefs are determined to positively correlate with longitudinal transition angles. These high topographic reliefs may indicate the propensity of triangular slab erosion, rather than strip-shaped slab erosion, in non-uniform channels with relatively steep erodible beds. Empirical formulas are obtained that denote the relationships among bed sediment strength, channel curvature radius, and sediment porosity through a multi-parameter regression analysis. This study may aid in clarifying the complex coupling effects of spatial variations in debris flow dynamics as well as sediment erodibility and bed morphology in non-uniform channels with abundant seismic loose material.

Investigating the Interaction Effects between Reservoir Deformation and Hydrate Dissociation in Hydrate-Bearing Sediment by Depressurization Method

Natural gas hydrate (NGH) has been widely focused on having great potential for alternative energy. Numerous studies on gas production from hydrate-bearing sediments have been conducted in both laboratory and field. Since the strength of hydrate-bearing sediments depends on the saturation of NGH, the decomposition of NGH may cause the failure of sediments, then leading to reservoir deformation and other geological hazards. Plenty of research has shown that the reservoir deformation caused by hydrate decomposition is considerable. In order to investigate this, the influence of sediment deformation on the production of NGH, a fully coupled thermo-hydro-chemo-mechanical (THMC) model is established in this study. The interaction effects between reservoir deformation and hydrate dissociation are discussed by comparing the simulation results of the mechanical coupling and uncoupled models on the laboratory scale. Results show that obvious differences in behaviors between gas and water production are observed among these two models. Compared to the mechanical uncoupled model, the mechanical coupling model shows a significant compaction process when given a load equal to the initial pore pressure, which leads to a remarkable decrease of effective porosity and reservoir permeability, then delays the pore pressure drop rate and reduces the maximum gas production rate. It takes a longer time for gas production in the mechanical coupling model. Since the reservoir temperature is impacted by the comprehensive effects of the heat transfer from the boundary and the heat consumption of hydrate decomposition, the reduced maximum gas production rate and extended gas production process for the mechanical coupling model lead to the minimum reservoir temperature in the mechanical coupling model larger than that of the mechanical uncoupled model. The reduction of the effective porosity for the mechanical coupling model causes a larger cumulative water production. The results of this paper indicate that the reservoir deformation in the gas production process should be taken into account by laboratory and numerical methods to accurately predict the behaviors of gas production on the field scale.

Swell-shrink behaviour of cement with fly ash-stabilised lakebed sediment

Dredged river or lake sediments are ideal for repurposing in engineering applications. Unfortunately, the low strength of these materials makes it necessary to amend them with chemical stabilisers before utilisation. This research attempted to improve the shrinkage and swelling potentials of cement-stabilised dredged sediments by addition of fly ash (FA). Material mixtures were created from combinations of sedimentary soil from Phayao Lake in Northern Thailand, Ordinary Portland cement (OPC) type I, and FA. Mechanical tests determined that the unconfined compressive strength of cement-stabilised sediments with 7% and 10% OPC increased with the addition of 25% FA. Digital imaging technology and free swell tests determined that the 3D volumetric shrinkage and swelling of OPC-stabilised sediment reduced by 40% and 2.8%, respectively, when 20% FA was added. With this additional OPC and FA, the treated sediment satisfied both strength and durability criteria for reuse as road material as set by the Thailand Department of Highways.

Study on shear wave attenuation characteristics of shallow sediments in gas hydrate pilot production area of South China Sea

Shear wave attenuation of sediment is an important acoustic feature of seafloor sediment, which reflects the ability of sediment to absorb and dissipate shear wave energy, which is closely related to sediment structure. In order to explore the change rule and influencing factors of shear wave attenuation in sediments, the absorption coefficient was selected as the characterization of shear wave attenuation characteristics, and unconsolidated undrained triaxial shear tests were carried out with two typical sediment silty sand and fine sand in Shenhu sea area of South China Sea as samples. Through comparative analysis, the experimental results show that the shear wave absorption coefficient of sediment is in the process of shear failure at first, it fluctuated slightly, then gradually stabilized, and finally continued to rise in the fluctuation. The shear wave absorption coefficient increases with the increase of porosity and decreases with the increase of confining pressure. In addition, the relationship between the shear wave absorption coefficient and the undrained shear strength of sediment presents a quadratic function. The smaller the shear wave absorption coefficient is, the higher the undrained shear strength of sediment is.

Seasonality in cold coast bluff erosion processes

High water levels in the Laurentian Great Lakes are causing widespread erosion of unlithified coastal bluffs and damaging infrastructure. Measurements of bluff recession along Wisconsin's Lake Michigan coast are mostly limited to estimates generated from comparisons of historical orthophotos with little information about the short-term scales (months to years) that affect many management and hazard-based decisions. Here we investigate the physical factors driving coastal bluff response to toe erosion at seasonal timescales using a multi-method case study of a mixed lithology bluff in Southeastern Wisconsin, an area that undergoes annual freezing. Five drone photogrammetry surveys were conducted from the winter of 2018 through the winter of 2020, and time lapse photography was collected from March 2019 through February 2020. The bluff face volumetric erosion rate between December 2018 and June 2019 was 3800 m3 a−1, which is 300% larger than the erosion rate between June 2019 and November 2019. Statistical analysis of the time lapse photography indicates that high magnitude erosion events are significantly correlated with environmental factors, including freeze-thaw events. These observations suggest that bluff erosion occurs episodically and is dominated by freeze-thaw related mass wasting events during the winter and spring. To investigate the physical processes driving this pattern, we model transient pore pressure response to a frozen bluff face as an input to a 3-D moment-balance slope stability model. Results show that increases in pore pressures up to one meter are possible at the face of the bluff over 5 days, which corresponds to 4–7% reduction in slope stability, indicating that observed increases in upslope bluff erosion during the winter and spring are a result of increased pore pressures, in combination with sediment strength reduction and high vadose zone saturation. As elevated lake levels continue to steepen bluff toes, upslope impacts will likely be temporally clustered during times of freeze-thaw. To accurately predict the seasonality of bluff recession on cold coasts, landscape change models must incorporate transient conditions caused by freeze-thaw events as they appear to be a dominant process.

Lithologic, geomorphic, and permafrost controls on recent landsliding in the Alaska Range

AbstractBecause landslide regimes are likely to change in response to climate change in upcoming decades, the need for mechanistic understanding of landslide initiation and up-to-date landslide inventory data is greater than ever. We conducted surficial geologic mapping and compiled a comprehensive landslide inventory of the Denali National Park road corridor to identify geologic and geomorphic controls on landslide initiation in the Alaska Range. The supplemental geologic map refines and improves the resolution of mapping in the study area and adds emphasis on surficial units, distinguishing multiple glacial deposits, hillslope deposits, landslides, and alluvial units that were previously grouped. Results indicate that slope angle, lithology, and thawing ice-rich permafrost exert first-order controls on landslide occurrence. The majority (84%) of inventoried landslides are &amp;lt;0.01 km2 in area and occur most frequently on slopes with a bimodal distribution of slope angles with peaks at 18° and 28°. Of the 85 mapped landslides, a disproportionate number occurred in unconsolidated sediments and in felsic volcanic rocks. Weathering of feldspar within volcanic rocks and subsequent interactions with groundwater produced clay minerals that promote landslide initiation by impeding subsurface conductivity and reducing shear strength. Landslides also preferentially initiated within permafrost, where modeled mean decadal ground temperature is −0.2 ± 0.04 °C on average, and active layer thickness is ∼1 m. Landslides that initiated within permafrost occurred on slope angles ∼7° lower than landslides on seasonally thawed hillslopes. The bimodal distribution of slope angles indicates that there are two primary drivers of landslide failure within discontinuous permafrost zones: (1) atmospheric events (snowmelt or rainfall) that saturate the subsurface, as is commonly observed in temperate settings, and (2) shallow-angle landslides (&amp;lt;20° slopes) in permafrost demonstrate that permafrost and ice thaw are also important triggering mechanisms in the study region. Melting permafrost reduces substrate shear strength by lowering cohesion and friction along ice boundaries. Increased permafrost degradation associated with climate change brings heightened focus to low-angle slopes regionally as well as in high-latitude areas worldwide. Areas normally considered of low landslide potential will be more susceptible to shallow-angle landslides in the future. Our landslide inventory and analyses also suggest that landslides throughout the Alaska Range and similar climatic zones are most likely to occur where low-cohesion unconsolidated material is available or where alteration of volcanic rocks produces sufficient clay content to reduce rock and/or sediment strength. Permafrost thaw is likely to exacerbate slope instability in these materials and expand areas impacted by landslides.

Study on the Optimal Production Pressure of Different Types of Natural Gas Hydrate Based on Formation Stability

Natural gas hydrate is an ice-like crystal formed by methane and water, which is a new type of strategic energy with huge reserves. The exploitation of deep-sea hydrate will cause a large amount of decomposition of hydrate, which will decrease the sediment strength. In this paper, production models of different types of hydrate reservoirs under different BHP (bottom hole pressure) were established. Then, sensitivity analysis was conducted to determine parameters which have the most significant influence on formation subsidence. After that, hydrate formation failure was discussed using two different criteria: formation subsidence and stress-strain curve. Then, critical production pressure was determined. Through comparison, it was found that the criterion of formation subsidence is more suitable. Finally, based on this criterion, the optimal production pressure of different types of hydrate reservoir was determined. The work of this paper will provide a certain reference value for the efficient and safe production of hydrate in the future.

Coastal sand dune restoration with an eco-friendly technique

Dune restoration projects that integrate vegetation efforts with natural, sustainable, and soft solutions have become increasingly popular in coastal communities. Eco-restored dunes have high aesthetic value and are likely to be more resilient to near-surface wind effects and/or minor storms events. However, environmental-friendly restoration strategies still lack scientific insight from systematic research and the role of eco-restoration in dune erosion resistance and resilience has not yet been rigorously investigated. The question is: how can we optimize eco-friendly solutions in dune ecosystem restoration to maximize the resistance and resilience of coastal dunes against erosion from wind, wave, and storm surge attack? The present paper proposes an innovative colloidal silica-based consolidation technique for coastal sand dunes. A special focus is placed on the description of the experience gained from two pilot field studies started in Salento coastal area, in the south of Italy. The preliminary observations clearly show that mineral colloidal silica increases the mechanical strength of non-cohesive sediments allowing the adopted solution to reduce the volume of dune erosion and the dune scarp retreat rate, thus improving the resistance and resilience of the dune system. Besides furthering our knowledge of coastal erosion, the results of these and future studies will be of value to coastal managers and policy makers responsible for dune restoration projects.