Sediment Shear Strength Research Articles

Flocculation-bio-treatment of heavy metals-vacuum preloading of the river sediments

Heavy metals pollution in river sediments is irreversible, and it is not easy to be found to be concealed. The pollution of heavy metals for river sediments is currently a serious environmental problem. In the paper, the Bacillus subtilis was selected to remove heavy metals by bio-mineralization method in river sediments. Optimal content of Bacillus subtilis powder and organophosphate monoester was 20g and 0.2mol in 1L of bacterial solution, respectively. The optimum reaction time and temperature were 36h and 30°C, respectively. The optimal reaction conditions were applied to zinc ions in river sediments. After heavy metals treatment, there was little effect on the water content before and after flocculation and vacuum preloading. Treatment of heavy metals had no effect on the cross-plate shear strength of river sediments after vacuum preloading. After treatment of heavy metals, the effect of purifying water quality was the group B(Polyacrylamide+Polysilicate aluminium ferric) bigger than the group A (Polyacrylamide). The removal efficiency of zinc ions (Zn2+) in the group B was 89.59% and 74.99% before and after the vacuum preloading, respectively, which was better than that in the group A.

Undrained triaxial tests on water-saturated methane hydrate–bearing clayey-silty sediments of the South China Sea

Approximately 90% of gas hydrates are buried in fine-grained sediments, especially in the South China Sea. The potential instability of fine-grained sediments induced by hydrate dissociation requires investigation of the shear strength and pore pressure response of sediments during hydrate recovery. To date, most studies have focused on the undrained mechanical behavior of gas hydrate–bearing sand or gas hydrate–free clay — few studies have examined gas hydrate–bearing fine-grained sediments. Because of the low-permeability and water-saturated characteristics of the sediments in the South China Sea, a series of undrained triaxial shear tests were performed on water-saturated methane hydrate-bearing clayey–silty sediments in this area. The experimental results show that the failure strength of methane hydrate–bearing sediments (MHBSs) increases with the increase in hydrate saturation and initial effective mean stress. The excess pore-water pressure of MHBSs remains positive during shear. Cohesion in the Mohr–Coulomb model increases with the increase in hydrate saturation, while the internal friction angle in the Mohr–Coulomb model has little dependence on the hydrate saturation.

Abyssal macrofaunal community structure in the polymetallic nodule exploration area at the easternmost region of the Clarion-Clipperton Fracture Zone, Pacific Ocean

Benthic communities in the deep sea are often characterized by high diversity, contrary to the once held view of the deep sea as an environment desolate of life. Heightened commercial interest in polymetallic nodule mining in the abyssal Clarion-Clipperton Fracture Zone (CCFZ) have seen a number of exploratory expeditions to the area. However, macrobenthic community structure is still poorly characterized across this extensive habitat. This study aims to fill in the knowledge gap at the easternmost region of the CCFZ where sea surface productivity is known to be relatively higher compared to other areas. A total of 5199 metazoan individuals (macrofauna sensu lato, > 250 μm) were collected from the top 10 cm sediment layer in 12 box core samples obtained from 12 stations assigned randomly within a 30 by 30 km survey stratum in the Ocean Mineral Singapore (OMS) contract area. Mean macrofaunal abundance (635 ± 54 ind.m−2, n = 12; macrofauna sensu stricto, > 300 μm) in the OMS survey area was higher than all but one locality west of the OMS area in the CCFZ. Total metazoan diversity (Hʹ) was positively and significantly correlated with volume and surface area of nodules, whilst macrofaunal abundance was significantly but negatively correlated with sediment shear strength. Not unexpectedly, the Nematoda was the most abundant metazoan group (29.3%). Other major infaunal groups comprised the Copepoda (21.2%), Polychaeta (17%), Tanaidacea (8.9%), Ostracoda (5.9%), Isopoda (4.4%) and Bivalvia (2.8%). We identified four of these groups (i.e., Polychaeta, Copepoda, Isopoda and Tanaidacea) to family level. Polychaetes exhibited the highest family richness (42 families) while the copepods (15 families), tanaids (10 families) and isopods (8 families) recorded high to moderate richness. Multivariate analyses suggested that whilst all major infaunal groups occurred in the 12 stations, the family composition of these groups (polychaetes, isopods and tanaids) differed between the stations, showing that community structure was not the same across all the stations. Small differences in sediment shear strength, nodule volume and surface cover might contribute to dissimilarities in the community structure of isopods and tanaids. The 900 km2 OMS survey area located at the easternmost region of CCFZ is characterized by a high abundance of abyssal macrofauna that is spatially variable taxonomically even when examined at family level.

Geotechnical assessment of near-surface sediments and their hazardous impact: Case study of Jizan city, southwestern Saudi Arabia

The geotechnical parameters of near-surface sediments in Jizan city have been assessed through shear wave velocity (Vs) from twenty-two profiles of multi-channel analysis of surface waves (MASW) through Jizan city. The Vs varies between 106 and 560 m/s indicating wide range of soil stiffness properties. While the allowable bearing capacity, qa varies from less than 1 kg/cm2 at sites till depth 21 m indicating very soft/loose materials; these are considered by building codes as special cases and increased into 2.5 kg/cm2 suggesting stiff or medium dense materials Moreover, qa reaches 6.5 kg/cm2 indicating dense materials, while reaches 8.5 kg/cm2 suggesting the presence of very dense materials. Accordingly, the allowable bearing capacity at a depth of 13 m is less than the recommended values for foundation materials by building codes. Jizan city differentiated into two zones; Jizan salt dome to the west and sabkha deposits to the east. Several weakness zones in the Jizan salt dome zone have been detected with significant variations in the topography of salt rock. While sabkha deposits have low shear strength which requires engineering treatment before construction. Furthermore, shear modulus varies both vertically and laterally through Jizan city where values of shear strength of the near-surface sediments overlying salt rocks are greater than those of sabkha zone. Therefore, deep foundation and ground improvement are highly recommended.

Hydrate growth in quartzitic sands and implication of pore fractal characteristics to hydraulic, mechanical, and electrical properties of hydrate-bearing sediments

Gas hydrate pore habits significantly impact the pore-scale structure within hydrate-bearing sediments, and thus, play a central role in the physical property evolution. Characterization and quantification of the effective pore space within hydrate-bearing sediments at different hydrate saturations have not been well offered. This study performs random simulations of hydrate nucleation and growth in quartzitic sands to understand effects of hydrate saturation and hydrate pore habits on fractal characteristics of the effective pore space. Normalized pore-size fractal dimension and normalized maximal pore diameter are characterized and found to decrease with increasing hydrate saturation. In order to predict hydrate saturation dependent pore fractal characteristics, theoretical and empirical models are proposed and further extended to give implications to hydraulic, mechanical, and electrical properties of hydrate-bearing sediments during hydrate dissociation. Implications include that hydrate dissociation facilitates the absolute permeability and the electrical conductivity, and enhances first and then reduces the saturation exponent of Archie's law; hydrate dissociation also lowers the capillary pressure, and this promotes relative permeability to gas but inhibits relative permeability to water even the water saturation remains as a constant; shear strength of unsaturated hydrate-bearing sediments drops down due to the decreasing capillary pressure as hydrate dissociation. These implications all meet with conclusions in previous literatures.

Geotechnical properties and stability of the submarine canyon in the northern South China Sea

The upper part of the continental slope in the northern South China Sea is prone to submarine landslide disasters, especially in submarine canyons. This work studies borehole sediments, discusses geotechnical properties of sediments, and evaluates sediment stability in the study area. The results show that sediment shear strength increases with increasing depth, with good linear correlation. Variations in shear strength of sediments with burial depth have a significantly greater rate of change in the canyon head and middle part than those in the canyon bottom. For sediments at the same burial depth, shear strength gradually increased and then decreased from the head to the bottom of the canyon, and has no obvious correlation with the slope angle of the sampling site. Under static conditions, the critical equilibrium slope angle of the sediments in the middle part of the canyon is 10° to 12°, and the critical slope angle in the head and the bottom of the canyon is 7°. The results indicate that potential landslide hazard areas are mainly distributed in distinct spots or narrow strips on the canyon walls where there are high slope angles.

A constitutive model for bentonite–water mixture and the effect of wall slip boundary conditions on its mechanical response

This paper presents the development of a constitutive model for describing the nonlinear viscoelastic behavior of deep-seabed sediments. To quantify the nonlinear response, shear rheological tests are carried out on deep-sea sediments substitute at different shear rates. These substitute samples are prepared by mixing bentonite with water based on the in-situ vane shear strength of deep-sea sediments. A compressible viscoelastic fluid model is formulated based on the thermodynamic framework developed by Rajagopal and Srinivasa (2000) to describe the experimental response of bentonite–water mixture. Experimental responses indicate that the effects of slip are significant in the shear rheometry of bentonite–water mixture. Hence a slip model is proposed, which relates the shear stress to slip velocity at the wall and the imposed shear rate values. The slip boundary conditions coupled with the viscoelastic model is validated using experimental data and is observed to be in good agreement. Further, the influence of shear rate on interfacial slip has been numerically analyzed and slip effects are found to be significant in defining rheological behavior with increasing shear rates.

Modeling the mechanical response of gas hydrate reservoirs in triaxial stress space

Exploitation of gas from deep-sea methane hydrate-bearing layers might lead to some geological disasters, including marine landslides and excessive settlement of marine ground. The first offshore gas production tests for methane hydrate-bearing sediments were carried out in eastern Nankai Trough. However, knowledge on mechanical behavior of gas hydrate reservoirs with similar gradation and minerology component to the marine sediment is still insufficient. Consequently, proper modeling of geomechanical properties of methane hydrate-bearing sediments is crucial for reservoir simulation and deep ocean ground stability analysis for long-term gas production in the future. This study conducted a series of triaxial shear tests to examine the shear response of methane hydrate-bearing sediments with a similar grading curve and minerology components to the hydrate-rich sediments in Nankai Trough. The test results demonstrated that the presence of hydrate mass between sand grains altered the stress-strain pattern from strain-hardening to postpeak strain-softening. A simple constitutive model based on several empirical relationships of granular materials is proposed to describe the stress-strain relationship of methane hydrate-bearing sediments under triaxial stress condition. This model can reproduce the enhancement of shear strength, initial stiffness, and dilation behavior of methane hydrate-bearing sediments containing different amounts of fines content with a rise in the methane hydrate saturation at a wide range of effective confining pressures. The numerical results indicate that the parameter A associated with initial stiffness of stress-strain curve and the parameter α related with dilation properties are jointly governed by the confining pressure, fines content, and hydrate saturation.

Assessing Risk of E. coli Resuspension from Intertidal Estuarine Sediments: Implications for Water Quality.

Estuarine sediments are a reservoir for faecal bacteria, such as E. coli, where they reside at greater concentrations and for longer periods than in the overlying water. Faecal bacteria in sediments do not usually pose significant risk to human health until resuspended into the water column, where transmission routes to humans are facilitated. The erosion resistance and corresponding E. coli loading of intertidal estuarine sediments was monitored in two Scottish estuaries to identify sediments that posed a risk of resuspending large amounts of E. coli. In addition, models were constructed in an attempt to identify sediment characteristics leading to higher erosion resistance. Sediments that exhibited low erosion resistance and a high E. coli loading occurred in the upper- and mid-reaches of the estuaries where sediments had higher organic content and smaller particle sizes, and arose predominantly during winter and autumn, with some incidences during summer. Models using sediment characteristics explained 57.2% and 35.7% of sediment shear strength and surface stability variance respectively, with organic matter content and season being important factors for both. However large proportions of the variance remained unexplained. Sediments that posed a risk of resuspending high amounts of faecal bacteria could be characterised by season and sediment type, and this should be considered in the future modelling of bathing water quality.

Prediction of nonlinear viscoelastic behaviour of simulative soil for deep-sea sediment using a thermodynamically compatible model

ABSTRACTThis work focuses on the development of a suitable theoretical model for defining the nonlinear viscoelastic behaviour of deep-seabed sediments. To that end, shear rheological tests are performed on simulative soil samples at different shear rates. These samples are prepared by mixing bentonite with water based on the in situ vane shear strength of deep-sea sediments. A nonlinear viscoelastic fluid model based on a thermodynamic standpoint is derived by considering two scalar functions. The two scalar functions, namely, the stored energy and the rate of dissipation are found using the nonlinear response of the samples collected from experiments. This novel approach identifies the model parameters having physical significance and has considerable advantages over previous models which are predominantly empirical. This is carried out as an inverse problem in which the parameters are estimated by minimizing the deviation between the predicted and measured torque response of the sample when subjected to shear rheometry with the shear rate as the input. The efficacy of the model has been investigated and reasonable agreement with the peak as well as steady-state values of torque is observed.

Modeling the Mechanical Behavior of Gas Hydrate Bearing Sediments Based on Unified Hardening Framework

Gas Hydrate Bearing Sediments (GHBS) are natural soil deposits that contain methane hydrate inside their pores and they have been considered as a potential energy resource. Deformation behavior and change in shear strength of sediments are not clearly understood on the occasion of methane hydrate production and its modeling poses great challenges. In this study, a new GHBS model based on unified hardening framework has been proposed with less and easily determined model parameters. This analytical model incorporates modified Cam-clay yield surface, plus the effect of hydrate saturation parameters to capture the mechanical behavior of GHBS. Comparisons of tri-axial compression tests with model simulations have been conducted to show the proposed model is able to predict the mechanical behavior of GHBS at different hydrate saturations and confinement conditions.

Shear strength performance of marine sediments stabilized using cement, lime and fly ash

The combined effects of the chemical stabilization of sediments using lime, cement and fly ash on the sediment shear strength behavior are studied for geotechnical purposes. An elemental analysis is carried out to examine the chemical aspects of stabilized sediments resulting from a series of chemical reactions. Direct shear strength measurements are performed to investigate shear strength improvements and stress-strain behavior of mix samples. A statistical analysis using the principal component analysis and the ANOVA (ANalysis Of Variance) method is also conducted to examine the correlation between cohesion and elemental ratio resulting from the chemical reactions. The analysis of the principal components suggests that Ca and Si and Al and K have greater impact on cohesion than the other elements. However, the ANOVA method reveals that the elemental ratios of Si, Al and Ca produce less impact on the sediment overall gain in cohesion at 28 days of curing. These results demonstrate that the gain in cohesion mostly depends on some other physical parameters, which need to be yet examined. The prediction equation of the cohesion indicates each elemental component gain weight on the overall cohesion gain resulting from the chemical reactions. In short, the considered equation still needs to be refined, notably in terms of performing further laboratory testing.

Prediction Of Tensile And Shear Strength Of Friction Surfaced Tool Steel Deposit By Using Artificial Neural Networks

Friction surface treatment is well-established solid technology and is used for deposition, abrasion and corrosion protection coatings on rigid materials. This novel process has wide range of industrial applications, particularly in the field of reclamation and repair of damaged and worn engineering components. In this paper, we present the prediction of tensile and shear strength of friction surface treated tool steel using ANN for simulated results of friction surface treatment. This experiment was carried out to obtain tool steel coatings of low carbon steel parts by changing contribution process parameters essentially friction pressure, rotational speed and welding speed. The simulation is performed by a 33-factor design that takes into account the maximum and least limits of the experimental work performed with the 23-factor design. Neural network structures, such as the Feed Forward Neural Network (FFNN), were used to predict tensile and shear strength of tool steel sediments caused by friction.

Shear Strength of Aeolian Sand Sediments

Abstract. The anti-erodibility of aeolian sand sediments represents their resistance to airflow shear and is closely associated with soil shear strength. However, there have been few studies on the anti-erodibility of sediments from the perspective of shear strength. In this study, we performed shear strength experiments for four categories of sediments at different values of normal stress. All samples were oven-dried prior to the shear tests. The main conclusions were as follows. The shear strength of sediments increased linearly with normal stress and grain diameter. Organic matter may weaken the shear strength. The impact of bulk density and CaCO3 content on the shear strength did not follow consistent rules, but RCaCO3/RBulk (where RCaCO3 is the ratio of CaCO3 content between two samples, and RBulk is the difference in bulk density) determines the relative contribution of each factor to shear strength. The cohesion, internal friction angle, and shear strength increased with increasing grain diameter. They also increased with increasing sorting coefficient when the sorting coefficient was greater than 0.6, while the opposite trend occurred with sorting coefficients less than 0.6. There was a positive linear correlation between cohesion and threshold friction velocity (R2 = 0.91). This study improves our understanding of shear strength, providing basic data with which to construct a dynamic model of aeolian sand transport by wind action. Keywords: Bulk density, Cohesion, Grain diameter, Internal friction angle, Shear strength, Sorting coefficient, Threshold friction velocity.

Pressure-core-based reservoir characterization for geomechanics: Insights from gas hydrate drilling during 2012–2013 at the eastern Nankai Trough

Pressure coring and analysis technology has progressed remarkably over the last decade. This technology allows for methane-hydrate-bearing sands, silts, and/or clays to be recovered from the deep-sea gas hydrate reservoir and then maintained in the laboratory within the hydrate stability phase boundary for evaluation and characterization. In this study, a series of pressure-core-based undrained/drained confined compression tests, unconfined (uniaxial) compression tests, isotropic loading and unloading tests, and permeability tests are conducted to investigate and characterize the intact strength, compressibility, and permeability of the gas hydrate reservoir. The consolidation tests and fluid flow tests showed that clayey silt sediments have permeability values of approximately tens of microdarcies, and sandy sediments with and without hydrates have permeability values of tens of millidarcies. The sediment from the bottom of the reservoir, which is sandy in nature with fines, also has permeability values of tens of microdarcies. From previous reports, the undrained shear strength of clayey silt sediments with low gas hydrate saturation is relatively low. Results from the drained shear tests and uniaxial compression tests show the first Mohr−Coulomb failure criteria for natural gas hydrate-bearing sediments, in which both the effective cohesion c' and the friction angle ϕ’ increase with increasing hydrate saturation. The results indicate in situ existence of pore filling and patchy gas hydrate. Finally, a new failure envelope for hydrate-bearing sediment is proposed that includes both hydrate saturation and confining pressure; it shows a good relation with the experimental results.

Influence of diatom microfossils on sediment shear strength and slope stability

AbstractDiatom microfossils have been detected in many natural marine sediment deposits around the globe and are held responsible for the disobedience to well‐established geotechnical relationships between index‐properties and shear strength. We revisit the static shear strength and present the first cyclic undrained shear strength experiments on diatom microfossil—clayey‐silt mixtures to study the role of diatoms on submarine slope stability. It is attested that the angle of internal friction (Φ) increases with diatom content, however, we provide evidence for a significant overestimation of Φ in previous studies. Based on direct shear tests at varying normal stresses ≤ 600 kPa we find Φ = 32° in contrast to 43° in pure diatom. Similarly, to static shear strength, cyclic shear strength increases with diatom content, however, in contrast to static shear strength the most drastic increase does not occur from 0% to 25% diatoms but from 75% to 100%. Interestingly, diatomaceous sediments tend to fail by liquefaction although well‐established relations between index properties and liquefaction susceptibility predict the opposite. Liquefaction failure is observed solely in samples containing ≥ 50% diatoms whereas samples with lower diatom content fail by cyclic softening. We conclude diatom microfossils in marine sediments significantly contribute to an increased slope stability under static and cyclic loading conditions since diatoms lead to higher resistance independently of the loading mode. The strength increase is interpreted as a result of particle interlocking and surface roughness, which is very efficient given the highly variable habitus of diatom species.

Factors influencing the initial establishment of salt marsh vegetation on engineered sea wall terraces in south east England

Sea walls provide vital flood protection for lowland coastal property. We investigated the integrity of a cost-effective method of repairing sea defences, which has potential to create habitat for coastal and salt marsh flora. Experimental stone-gabion and clay-filled terraces were installed as a soft engineered approach to repair damaged sea walls in estuarine embayments in south east England. Changes in the surface heights of sediment and vascular plant colonisation were monitored over a 22 month period. Seven of the 12 terraces were colonised, by 12 species of plant, reaching a maximum of 85% cover. The main drivers of plant colonisation were sediment stability, elevation, exposure and sediment shear strength. Terraces with least change in the surface height of sediments were favourable for plant colonisation. Ordination (Canonical Correspondence Analysis) showed 72% variation in plant distribution explained by elevation (37%), exposure (30%), terrace length and sediment shear strength (5%). Elevation was the most influential variable; recruitment increased as terrace height approached the height of existing marsh (r2 = 0.43). This cost-effective approach has the potential to provide protection to sea walls and create additional habitat for wildlife. Key considerations for the improvement of terrace design and construction are discussed.

Influence of fibers on the shear strength and dewatering performance of geotextile tubes

Geotextile tubes have been widely used for the dewatering of high water content fine sediments. Fine sediments include lake sediments, organic soils and coal combustion products, which generally have low shear strength. Several researchers have emphasized the effect of the retained sediment (filter cake) properties on dewatering performance and stability of the geotextile tubes. Maintaining retained sediment properties, in particular permeability and shear strength, allows for faster dewatering time, better retention of sediments and improved stability of the geotextile tubes during multiple filling stages and during the tube stacking process. Several studies have shown that the use of synthetic fibers can increase the shear strength of sand, silt and clays. However, the use of fibers to improve the strength of retained sediments in geotextile tubes has not been explored. In this study, three synthetic fibers varying in length and diameter were used to evaluate the effect of fibers on the dewatering performance and shear strength of the fine sediments. A cationic polyacrylamide flocculant was used as an optimum conditioner for the selected sediment. The fibers were mixed with the flocculant conditioned slurry at 0.25, 0.5, 1 and 2% fiber concentration by mass of the dry solids. The effect of fibers on the undrained shear strength of the dewatered sediments was evaluated using a lab vane test, fall cone test and undrained unconsolidated triaxial test. Jar tests and pressure filtration tests were also conducted to evaluate the effect of fiber on the required flocculant dosage and on the dewatering performance. Based on the results of this study, 0.5% of F1 fiber type was found to be the optimum fiber concentration, yielding an approximately 100% increase in undrained shear strength. It was also found that the diameter and length of the fibers play a significant role in altering the undrained shear strength of the tested sediments. The relatively short (6 mm) and thick (38 μm) fiber did not yield an increase in shear strength for all the tested fiber concentrations, whereas thin fibers (9 μm) of similar length increased the shear strength by 100%. Furthermore, it was found that the use of fibers at any concentration decreases the required optimum doses of flocculant by about 15%, and increases the dewatering rate of the effluents by about 50%.

Dynamic propagation criteria for catastrophic failure in planar landslides

SummaryQuantitative assessment of the risk of submarine landslides is an essential part of the design process for offshore oil and gas developments in deep water, beyond the continental shelf. Landslides may be triggered by a reduction in shear strength of subsea sediments over a given zone, caused for example by seismic activity. Simple criteria are then needed to identify critical conditions whereby the zone of weakness could grow catastrophically to cause a landslide. A number of such criteria have been developed over the last decade, based either on ideas drawn from fracture mechanics, or considering the equilibrium of the initial weakened zone and adjacent process zones of gradually softening material. Accounting for the history of the weak zone initiation is critical for derivation of reliable propagation criteria, in particular considering dynamic effects arising from accumulating kinetic energy of the failing material, which will allow the failure to propagate from a smaller initial zone of weakened sediments. Criteria are developed here for planar conditions, taking full account of such dynamic effects, which are shown to be capable of reducing the critical length of the softened zone by 20% or more compared with criteria based on static conditions. A numerical approach is used to solve the governing dynamic equations for the sliding material, the results from which justify assumptions that allow analytical criteria to be developed for the case where the initial softening occurs instantaneously. The effect of more gradual softening is also explored. Copyright © 2016 John Wiley & Sons, Ltd.

On the viscous resistance of marine sediments for estimating their strength and flow characteristics

To examine debris flow mobility with respect to the runout distance and velocity, strength parameters (e.g., the Bingham and Herschel-Bulkley yield stresses, undrained shear strength, flow behavior index) are needed. In modeling debris flow, the Bingham and Herschel-Bulkley yield stresses are often used, but they lead to large differences in the runout distance and velocity of the failed masses. It is very difficult to determine the true yield stress. In this study, we propose using the viscous resistance, obtained from the normalized flow curve resulting when the shear stress and shear rate are divided by reference shear stress and shear rate values (i.e., \(\log \tau /\tau _0 - \log \gamma /\gamma _0 \)). The viscous resistance and strength parameters are compared from the viewpoint of geotechnology and rheology. The shear strength and flow characteristics of marine sediments can be characterized in terms of the viscous resistance and the viscous resistance is linearly related to other strength parameters (e.g., viscosity, yield stress and undrained shear strength). Thus, the viscous resistance provides a simple and easy approach for approximating strength and flow parameters in debris flow dynamics.