Temperature-dependent undrained behaviour of sand in an ultra-deep marine environment

In the present study, the loading frequency dependencies of cyclic shear strength and elastic shear modulus of reconstituted clay were examined by performing undrained cyclic triaxial tests and undrained cyclic triaxial tests to determine deformation properties. The result of undrained cyclic triaxial test of reconstituted and saturated clay shows that a faster frequency leads to higher stress amplitude ratio, but when the frequency becomes fast up to a certain point, the stress amplitude ratio will reach its maximum limit and the frequency dependence becomes insignificant. And also, the result of undrained cyclic triaxial deformation test shows a fact that a faster loading frequency leads to higher equivalent shear modules and smaller hysteresis damping ratio, and confirms the frequency dependence of cohesive soil. Meanwhile, the result of the creep test shows that continuing creep is created in the undrained cyclic triaxial test with slow loading frequency rate, and since loading rate becomes slower at the vicinity of the maximum and the minimum deviator stress due to sine wave loading, the vicinity of the maximum and the minimum deviator stress shall be more influenced by creep.

DEM analysis of monotonic and cyclic behaviors of sand based on critical state soil mechanics framework

Undrained Cyclic Shear Behaviour of Reconstituted Scoria Deposit

Cyclic triaxial and simple shear tests are both commonly used to study the behavior of granular assemblies subjected to dynamic loading. These two stress paths have also been simulated with the discrete element method (DEM) using varying boundary conditions. In this work, stiff boundaries and periodic boundaries were implemented in DEM simulations of constant volume cyclic triaxial and simple shear tests. The model response was analyzed at the specimen scale by comparing stress paths, decrease in mean effective stress, and number of cycles to failure across simulations. Particle scale analyses, including quantification of the contact force network, entropy of the local void ratio distribution, and spatial variability in the distribution of void ratio, contact number, and normal contact force are used to provide insight into model response. Simulation results imply that periodic boundaries are more appropriate for consolidating specimens to a homogenous state and minimizing boundary effects during cyclic loading. Significant uneven “squeezing” where particles are pinched between the horizontal and lateral plates in the corners of the specimen with stiff boundaries was observed in the cyclic simple shear test. After liquefaction initiation, increased entropy indicated a more disordered particle assembly in all cases. In addition, specimens in cyclic simple shear tests and cyclic triaxial tests behave significantly differently at the microscale. With the same level of deviatoric shear strain amplitude, the simple shear specimens liquefied in fewer cycles than the triaxial specimens. Cyclic triaxial tests are not able to produce pure shear waves, while cyclic simple shear tests do, micromechanically indicating the better suitability of cyclic simple shear tests to simulate shear wave induced liquefaction. The findings from this work will have implications for the future modeling of cyclic element tests using DEM.

This paper presents the yielding characteristics of overconsolidated Bootlegger Cove Formation (BCF) clays extracted from the Port of Anchorage construction site. Additionally, it extends the critical shear strain concept to define the point at which irrecoverable deformations begin (Y2 yielding) to the case of fully reversed loadings during undrained cyclic triaxial (CyTX) tests. On-specimen linear variable differential transformers (LVDTs) and an internal load cell with high accuracy were employed to define stress–strain responses and yield points of the specimens. This paper also presents Y2 yield points obtained using standard procedures available in the technical literature based on monotonic triaxial testing to show that the size and location of the Y2 yield surface can be determined from undrained CyTX tests.

Mixing of waste rock and tailings as a homogeneous mixture (referred to as “paste rock”) has been suggested as a favourable approach to overcome deficiencies associated with traditional methods of mine waste disposal. In consideration of the current limited understanding of the fundamental mechanical response of paste rock, a laboratory research program was undertaken to investigate the monotonic and cyclic shear response of paste rock. This paper presents the findings from undrained cyclic triaxial shear testing conducted on saturated paste rock specimens reconstituted such that the tailings would “just fill” the void spaces between the coarse particles of the skeleton. During undrained cyclic loading, paste rock typically exhibited a cumulative decrease in effective stress along with a progressive degradation of shear stiffness. The material generally displayed a higher tendency for strain development under cyclic loading when compared with rock-only and tailings-only specimens subjected to similar consolidation and cyclic loading conditions. However, no strain-softening or loss of shear strength was observed, suggesting that the material is unlikely to experience flow failure under undrained cyclic loading. The cyclic shear resistance was relatively insensitive to the initial effective confining pressure ([Formula: see text]) for the tested stress levels of [Formula: see text] < 400 kPa.

Based on a non-equilibrium thermodynamic approach, this paper has established a constitutive model for saturated soils different from the classical elasto-plastic model and other thermodynamics-based models. It does not require concepts such as yield surface and flow rule. Instead, through a quantitative description of the energy in the reversible and irreversible processes in materials, the constitutive relations have been obtained theoretically. Using the elastic potential energy function, the stress is expressed as a function of elastic strain. By introducing the granular entropy and locked energy concepts, the non-elastic strain evolution is determined according to the non-equilibrium thermodynamics. The model can describe many basic mechanical properties of saturated soils under monotonic and cyclic shear loadings, such as the stress state boundary, the critical state and especially the cyclic mechanical behavior. Based on this model, the paper simulates the cyclic undrained triaxial shear tests for saturated clays and analyzes the impact of factors that include cyclic stress ratio, OCR value and cyclic stress frequency. The simulation results are compared with experimental results in order to validate the model.

Two series of undrained cyclic triaxial strain-controlled tests were performed on two different Imperial Valley, California, silty sands which liquefied during an earthquake in 1981. Both intact and reconstituted specimens were tested, and the testing procedures are described. The experimental data confirm that cyclic shear strain is the fundamental parameter governing pore pressure buildup, because strain-controlled tests essentially eliminate the influence of specimen fabric and sample disturbance. Also, the results indicate that the cyclic triaxial test can be used to model cyclic simple shear (similar to seismic field conditions), if the cyclic simple shear strain, γcy, is related to the cyclic triaxial axial strain, εcy, by either of two similar analytical expressions: γcy = 1.5 εcy or γcy = √3 εcy. Consequently, a unique pore pressure model is developed and recommended to simulate the seismic pore pressure buildup at the site. This model is applicable to reconstituted and intact specimens of the two sands, despite their different void ratios and nonplastic silt contents, and is valid for both cyclic triaxial and cyclic simple shear strain-controlled conditions.

Particle breakage evolution during cyclic triaxial shearing of a carbonate sand

The dynamic behavior of saturated sands has been sufficiently studied from different points of view, but in laboratory tests only the stress conditions, prior to dynamic stress, are reproduced at the level of the surface; that is, for conditions of zero static shear stresses (τ o). For this reason, the main objective of this research is to reveal the influence that static shear stresses (τ o) have on the response of the dynamically saturated sands. The experimental phase included: identification and classification tests and six monotonic simple shear tests and 17 cyclic simple shear tests with different combinations of monotonic (τ o) and cyclic shear stresses (τ c) and in undrained conditions. The tested samples were extracted from the ‘La Bocana Norte’ sector of the Port of Barcelona–Spain. The samples were taken at depths between 31 m and 55 m, with a corresponding effective vertical stress (σ’ov) between 290 kPa and 421 kPa, respectively. Based on the results of the tests, a detailed analysis was carried out on the influence of the combination of monotonic (τ o) and cyclic (τ c) shear stresses on the variation of the dynamic shear modulus (G). Specifically: in the stress–strain behavior, the development of both permanent (γ p) and cyclic (γ c) shear strain and the reduction of the effective stresses (σ′v) as a consequence of the generation of pore water pressure (u). The results of the laboratory tests generally show that: (a) the combination of τ o and τ c controls the stress–strain behavior, the degradation of the cyclic shear modulus, the development of permanent and cyclic shear strain, the generation of pore water pressures and the liquefaction risk, (b) in the monotonic case, an empirical function was established to evaluate the internal friction angle (ϕ') as a function of the shear strain (γ m) (mobilized friction angle), (c) in the combined case of monotonic and cyclical stress, it was possible to establish an empirical function among the G/σ′ov ratio, the cyclic strain (γ c) and the number of cycles (N) and among the σ′v/σ′ov ratio, the cyclic strain (γ c) and the number of cycles (N), (d) based on empirical functions, a simple methodology can be proposed to assess liquefaction risk and (e) it is not always true that the liquefaction phenomenon occurs when the effective vertical stress (σ’v) is reduced to zero. Depending on the cyclic stress ratio (τ c/σ’ov) liquefaction can occur in the first cycle with a reduction in vertical effective stress of just 70%.

In order to examine the effects of fabric anisotropy on the relationships between the cyclic triaxial and torsional shear strengths, a series of cyclic undrained triaxial and torsional shear tests was performed on three kinds of sands made by three different sample preparation methods. The test results showed that the cyclic undrained strengths varied with sample preparation methods. In particular, the effects of sample preparation methods were clearly observed in the triaxial test. The relationships between the cyclic triaxial and torsional shear strengths also varied with sample preparation methods. In addition, it was found that the effects of fabric anisotropy due to the difference in the sample preparation methods appear in the axial deformation and the developed pore water pressure characteristics during the cyclic triaxial test. In order to evaluate quantitatively the effects of fabric anisotropy on the relationships between the cyclic triaxial and torsional strengths, new index parameters were introduced, which express the degree of the anisotropy of specimens. Consequently, it was indicated that there exists a unique rule between these parameters and the ratio of torsional strengths to triaxial strengths.

A series of undrained cyclic triaxial liquefaction tests were conducted to observe directly and indirectly the local deformations of sand specimen through a transparent membrane. Black-colored silica sand, mixed in white-colored silica sand, was used for the direct evaluation. Black-colored dots, used for observing the local deformations indirectly, were pasted on membrane with a constant interval of 5 mm. Digital photographs were taken in front of the triaxial cell to evaluate the deformations of sand particles patterns and dots by Particle image Velocimetry method. The results indicated that image analysis is an efficient method for direct and indirect measurements of local deformation. Comparison of direct and indirect evaluations revealed that relative movement between sand particles pattern and an adjacent dot on the membrane had a leap when excess pore water pressure ratio reached unity.

It is common to employ a traditional double cell system, of which an open-ended inner cell is installed in an ordinary triaxial apparatus, to measure the volume change of unsaturated specimens. In such a system, the total apparent volumetric strain of the specimen (εv) is deduced from the water level change in the inner cell, monitored by a differential pressure transducer (DPT) considering, meanwhile, the top cap intrusion into the inner cell recorded by a vertical displacement transducer (VDT). Severe apparent volumetric strain, caused by the compliance of the double cell system (εv,SC), was observed during the undrained cyclic loading tests in a previous study. Test results on a steel-spring dummy specimen revealed that εv,SC was induced not only by such as the meniscus effect, but unexpectedly also by the asynchronous responses between the DPT and the VDT (i.e., the response of the DPT was delayed compared with that of the VDT). By doing some treatment εv,SC could be reduced to some extent, whereas the magnitude of εv,SC was still too high to be acceptable when the tested specimen approached the liquefied state. To radically solve these technical difficulties, a modified double cell system, named the linkage double cell system, was developed in this study. In this modified system, a linkage rod moving simultaneously with the loading shaft was introduced, through which the DPT could directly measure εv without considering the top cap or loading shaft intrusion. Test results for the steel-spring dummy specimen as well as for saturated and unsaturated soil specimens demonstrated that the linkage double cell system has major advantages in measuring accurately the volume change of the specimen during undrained cyclic triaxial loading tests compared with the traditional double cell system.

In the 2016 Kaikoura earthquake, liquefaction of gravelly soils from reclaimed fills occurred in CentrePort, Wellington, New Zealand. This study presents constant volume monotonic and cyclic simple shear tests on well-graded gravel with sand collected from CentrePort. A large-scale cyclic simple shear device is utilized to evaluate the monotonic, cyclic, and postcyclic responses of the sandy gravel soils. Specimens prepared at various relative densities were subjected to a vertical effective stress of 100 kPa and then monotonically and cyclically sheared. After the cyclic loading, the postcyclic response was evaluated, including volumetric compression or monotonic shear with or without dissipation of excess pore water pressure. Shear wave velocity was measured before and after the cyclic loading. The results show that the well-graded sandy gravel has a high potential for liquefaction, with higher relative density specimens having higher liquefaction resistance. Postcyclic volumetric strain is primarily correlated with density and maximum shear strain during cyclic loading. Postcyclic reconsolidation causes densification of the liquefied specimens, resulting in higher monotonic shear resistance, while postcyclic monotonic shear without dissipation of excess pore water pressure reveals that substantial shear strain is required to develop the shear resistance. Shear wave velocity was significantly reduced after liquefaction, but recovered to slightly higher than its precyclic shear values after reconsolidation. Compared to other gravelly and sandy soils, the well-graded sandy gravel showed a similar or slightly higher liquefaction resistance than gap-graded and uniform gravels. Moreover, the well-graded sandy gravel had a relatively lower ultimate postcyclic volumetric strain due to a small variation between its maximum and minimum void ratios. The results advance our understanding of the liquefaction resistance and subsequent postcyclic responses of the well-graded sandy gravel soils.

The behavior of sand largely depends on the initial state, water content, drainage conditions, and type of action on the sand. The dominant mechanism that governs sand behavior is dilatancy, the volume change during shearing. Owing to dilatancy, dense sand samples attain greater strength during shearing compared to loose samples. Investigation of the sand behavior shows that the final strength during shearing, for the sand samples starting from the same initial state of stress, tends to be independent of the initial void ratio. This final state is called the critical state, or steady state, and can be well described by the critical state line. The main goal of this research is to investigate the behavior of clean sand from Jerovec under triaxial shearing and to derive its critical state line. The behavior of sand is investigated for different initial states (initial densities and state of stress) and for different drainage conditions (consolidated isotropically drained (CID) test and consolidated isotropically undrained (CIU) test). The test results are presented along with all the laboratory tests performed on Jerovec sand (grain-size distribution, specific gravity, minimum and maximum void ratio). The critical state line is compared with those of other sands available in the literature.