Strain-Softening Characteristics of Hydrate-Bearing Sediments and Modified Duncan–Chang Model

Marine hydrate exploitation may trigger the seabed geological disaster, such as seafloor collapse and landslide. It is critically important to understand the mechanical properties of hydrate-bearing sediment. Strain-softening observation is a typical behavior of hydrate-bearing sediment (HBS) and exhibits more significant at higher hydrate saturation. This paper performed a series of triaxial compression tests on methane hydrate-bearing sand to analyze the influence rule and mechanism of hydrate saturation on the strain-softening characteristic, stiffness, and strength and introduced the strain-softening index to quantificationally characterize the strain-softening behaviors of HBS with different hydrate saturations. Based on the analyses on the mechanical behavior of HBS, the Duncan–Chang model is extended to address the stress-strain curves of HBS. Two empirical formulas with hydrate saturation embedded are used to characterize the enhanced initial modulus and strength for HBS, respectively. To address the strain-softening behavior of HBS, the modified Duncan–Chang model introduced a damage factor into the strength of HBS. To validate the modified Duncan–Chang model, four different triaxial compression tests are simulated. The good consistence between simulated result and experimental data demonstrates that the modified Duncan–Chang model is capable of reflecting the influence of hydrate saturation not only on the stiffness and strength but also on the strain-softening characteristics of HBS.

An Experimental Study Into the Effects of Cumulative Energy Damage on the Mechanical Properties of Coal

The accumulated damage of the surrounding rocks induced by the existing coal mining activities (such as excavation and extraction) is the initial mechanical background of the rock and coal for the further mining operations. An energy-based Cumulative Initial Damage (CID) variable was proposed to account for such existing damage. With the MTS815 electro-hydraulic servo-controlled system, coal samples with different CID value were prepared by cyclic pre-loading and unloading process, and a novel experimental framework was presented to investigate the effect of CID on the further mechanical properties of the CID coal samples. The deformation characteristics, peak strength, peak strain, dilatancy characteristics, brittle-ductile transformation behavior, and microscopic structure of the CID coal samples were investigated in detail. The triaxial compression tests showed that with the increased of CID value, the compaction part of the coal samples was shorter and the strain softening stage became longer. When the CID value was increased from 0 to 0.521, the deviation stress and peak strain of the coal decreased by 31.4 and 37.7%, respectively and the main characteristic of the fracture morphology changed from cleavage steps to dimpling.

Triaxial compression behavior of 3D printed and natural sands

Different particle properties, such as shape, size, surface roughness, and constituent material stiffness, affect the mechanical behavior of coarse-grained soils. Systematic investigation of the individual effects of these properties requires careful control over other properties, which is a pervasive challenge in investigations with natural soils. The rapid advance of 3D printing technology provides the ability to produce analog particles with independent control over particle size and shape. This study examines the triaxial compression behavior of specimens of 3D printed sand particles and compares it to that of natural sand specimens. Drained and undrained isotropically-consolidated triaxial compression tests were performed on specimens composed of angular and rounded 3D printed and natural sands. The test results indicate that the 3D printed sands exhibit stress-dilatancy behavior that follows well-established flow rules, the angular 3D printed sand mobilizes greater critical state friction angle than that of rounded 3D printed sand, and analogous drained and undrained stress paths can be followed by 3D printed and natural sands with similar initial void ratios if the cell pressure is scaled. The results suggest that some of the fundamental behaviors of soils can be captured with 3D printed soils, and that the interpretation of their mechanical response can be captured with the critical state soil mechanics framework. However, important differences in response arise from the 3D printing process and the smaller stiffness of the printed polymeric material. Graphic abstract Artificial sand analogs were 3D printed from X-ray CT scans of sub-rounded and sub-angular natural sands. Triaxial compression tests were performed to characterize the strength and dilatancy behavior as well as critical staste parameters of the 3D printed sands and to compare it to that exhibited by the natural sands.

Influence of plastic fines content on the liquefaction susceptibility of sands: monotonic loading

The paper presents an experimental study on the effect of plastic fines content on the undrained behavior and liquefaction susceptibility of sand–fines mixtures under monotonic loading. The results of undrained monotonic triaxial compression tests conducted on mixtures of Hostun sand with varying amount (0–20%) and type (kaolin and calcigel bentonite) of plastic fines are presented. The specimens were prepared with different initial densities using the moist tamping method and consolidated at two different isotropic effective stresses. The results demonstrate that for both types of plastic fines, an increase in the fines content leads to a more contractive response and lower values of mobilized deviatoric stress. Despite similar relative density and fines content, the sand–kaolin mixtures showed a more contractive behavior than the sand–calcigel specimens. The steady-state lines (SSLs) in e–p´ space generally move downwards with increasing clay content. While the slopes of the SSLs for the clean Hostun sand and the mixtures with 10 and 20% kaolin are quite similar, the SSL lines for the specimens containing 10% or 20% calcigel run steeper or flatter, respectively. The inclination of the SSL in the q–p′ plane was found independent of clay type and content. The sand–kaolin mixtures were observed to be more susceptible to instability and flow liquefaction than the sand–calcigel mixtures.

Evolution of Strength Parameters for Sandstone Specimens during Triaxial Compression Tests

Despite the lack of test data of the coefficient of pressure sensitivity α and the shearing cohesion k, the Drucker–Prager criterion is commonly applied for numerical analyses of geotechnical engineering. To bridge the gap between the wide application and insufficient knowledge of strength parameters of the Drucker–Prager criterion, this study presents experimentally calibrated strength parameters of this criterion for the first time. This paper proposes a new method to measure α and k in the Drucker–Prager criterion. The square root of the second invariant of the deviatoric stress tensor J 2 is linearly fitted with the first invariant of the stress tensor I 1 in the stress space. The parameters φ and c in the Mohr–Coulomb criterion and α and k in the Drucker–Prager criterion are calibrated to the same set of triaxial compression tests of sandstones. With these testing results, five pairs of conversion formulae (which are most commonly used in the literature) are examined and the most appropriate pair of conversion formulae is identified. With parameters indicating cohesive strength (c and k) and parameters indicating frictional strength ( φ and α ), the evolutions of different strength components are compared with those in the cohesion-weakening friction-strengthening model. With an increase in plastic deformation, the cohesive strength parameters c and k firstly increase to a peak value and then decrease. The frictional strength parameters φ and α gradually increase at a decreasing rate after the initial yield point.

Effect of Compaction Banding on the Hydraulic Properties of Porous Rock: Part I—Experimental Investigation

The mechanical behavior and the influence of compaction banding on the hydraulic properties in soft porous rocks were studied. The tested rock was Calcarenite Tuffeau de Maastricht. In the frame of experimental investigations, triaxial and oedometric tests were conducted under dry and drained conditions. The results demonstrated that the rock is forming discrete compaction bands under high confining stresses and steep angle shear bands under low confining stresses. Permeability measurements during the oedometric and triaxial compression tests under drained conditions demonstrated that the axial permeability decreases with increasing axial strain. The maximum permeability decrease was three orders of magnitude for 40% of axial strain.

Relationship between monotonic and cyclic behavior of saturated soft clay in undrained triaxial compression tests

Cyclic loading-induced deformation of soil is a common problem in the engineering practice. In the current practice, however, monotonic triaxial tests are more commonly used in the practice, due to the availability of apparatus and ease of operation. Thus, it will be very useful and practical if the monotonic triaxial tests can be used to evaluate the behavior of soil under cyclic loading. This study aims to find an explicit relationship between monotonic and cyclic behavior of saturated soft clay. Six monotonic and nine cyclic triaxial compression tests were conducted on undisturbed saturated soft clay under an undrained condition. The test results showed that the monotonic and cyclic tests shared the same stress-strain surface in a three-dimensional space p^'-q-ε_a. It is also found possible to evaluate the effective stress states of cyclic tests at two specific numbers of cycles, using corresponding monotonic tests. Based on these two findings, a simple procedure was then proposed to predict the peak axial strain for the saturated soft clay under different cyclic loadings based on the monotonic tests and only one cyclic test, which was further verified against more test data from the previous literature.

Interpretation of undrained shear strength observed in confined triaxial compression tests on compacted clay

This paper presents an interpretation technique to quantify the effects of compaction state and matric suction on the undrained shear strength of compacted clay under confined undrained triaxial compression. This novel technique is based on the mathematical frameworks of SHANSEP (Stress History and Normalized Soil Engineering Property) method for saturated soil and BBM (Barcelona Basic model) for unsaturated soil. Test data of compacted Calgary till were analyzed and interpreted using the proposed technique. The interpretation technique is very useful in delineating the relative impacts of the factors on the behavioral trends in measured undrained shear strength. It was found that in addition to the initial compacted void ratio and suction, soil structure and failure mode exert significant influence on the undrained shear strength of compacted clay. This technique is attractive to engineering practitioners because the confined undrained compression tests (with no pore air and water pressure measurement) are much simpler and less time consuming compared to rigorous laboratory tests on unsaturated soil.