Results Of Triaxial Compression Tests Research Articles

A new state-dependent constitutive model for cyclic thermo-mechanical behaviour of unsaturated vegetated soil

Climate change has resulted in increasing attention being paid to the effects of temperature and humidity on vegetated soil. However, existing constitutive models rarely investigate the thermo-mechanical behaviour, such as accumulation of plastic strain under non-isothermal conditions, of unsaturated vegetated soil. To address this issue, this study develops a comprehensive constitutive model by adopting the state-dependent dilatancy in conjugation with loading, memory, and bounding surfaces. Moreover, root-induced hardening due to pore occupancy and internal bonding is modelled with dependence on root volume ratio. The performance of this newly developed constitutive model is then evaluated using previous laboratory element tests. Comparisons between the computed and measured results reveal that the model is capable of capturing variations in soil state at a range of temperatures, suctions, and root volume ratios. The shearing behaviour of vegetated soil, especially its volumetric response, is well predicted by the model, as confirmed by comparing with triaxial compression test results. Moreover, the model predicts that the accumulated volumetric strain of unsaturated vegetated soil (0.2%) is significantly less than saturated bare soil (1.1%) after 4 thermal cycles. These computed results imply vegetation can mitigate the effects of climate change on soil through both transpiration-induced suction and mechanical reinforcement.

CPT-based design of pile foundations in sand and clay: Perspectives

This paper provides some perspectives and guidance for the design of pile foundations in sand and clay using cone penetration test (CPT) results. A key variable in the estimation of the limit unit shaft resistance qsL of piles in sand is the critical-state interface friction angle δc, which is a function of the critical-state friction angle ϕc of the sand. In the absence of direct shear or triaxial compression test results, which is often the case for routine infrastructure projects, engineers typically assume a conservative value for ϕc in pile design. In addition, effective stress-based methods for estimation of qsL of driven piles in clay rely on the residual interface friction angle δr, among other variables; however, in these methods, δr does not vary with the normal effective stress on the pile operative at the time of shearing. In this paper, we present relationships and approaches to address these issues. Finally, a relationship between the cone resistance qc and the corrected standard penetration test (SPT) blow count N60 was also developed so that engineers may obtain an estimate of qc for use in a CPT-based design method when only SPT blow counts are available for a site.

Anisotropy in the Liquefaction Resistance of Fibre Reinforced Sand.

Adding discrete fibres to sand has been seen as a feasible technique to improve sand's strength as well as liquefaction resistance. Considering the anisotropic distribution of fibre orientations, the anisotropy in the liquefaction resistance of the reinforced sand is also introduced using fibres. Here, the triaxial compression and extension test results of unreinforced and fibre-reinforced sand in different density states are provided, from which the anisotropy in the liquefaction resistance of fibre-reinforced sand is demonstrated. Fibre reinforcement improves the liquefaction resistance of sand by introducing both the densifying effect and the confining effect. The inclusion of fibres increases both the slope and the intercept of the strength envelope in comparison with the unreinforced sand under triaxial compression, while the strength envelope is not affected by fibres under triaxial extension. Stress contribution of fibres makes the ESP of the composite under undrained loading reverse its direction to develop even though the phase transformation is absent. The stress ratio initiating the ESP reversal is irrespective of the fibre content but dependent on the density state under triaxial compression. Under triaxial extension, the stress ratio initiating the ESP reversal remains the same in the samples with varied density states and fibre contents. The mechanism correlating to the strength envelope and ESP reversal of the fibre-reinforced sand was demonstrated following a rule of mixture based constitutive modelling framework. By introducing an alternatively defined pore pressure ratio that incorporates the stress contribution of fibres, the liquefaction state of the fibre reinforced sand is reasonably assessed. Liquefaction remains absent in the sand once the fibres are mixed. The anisotropy in the liquefaction resistance of fibre-reinforced sand arises, as the predominant role played by the fibres to suppress the liquefaction is different when varied loading paths are involved, which is sourced from the anisotropic distribution of fibre orientations.

An elastoplastic damage model for concrete considering the influence of mesostructure on transverse deformation

The deformation of mesostructures such as micropore collapse or microcrack expansion within concrete can hugely influence its macro transverse deformation. This deformation characteristic is not effectively characterized in existing elastoplastic damage constitutive models based on micromechanics. This discrepancy can result in substantial disparities between the calculated results and the actual situation when carrying out the stress and deformation analysis of concrete structures. Through an in-depth examination of how micropore collapse and microcrack shear expansion affect transverse measurable strain, a new calculating method for concrete transverse deformation is established according to its actual evolution characteristics. The reliability of the proposed method is verified based on the triaxial compression test results. Research results indicate that the transverse deformation of concrete increases slightly before the peak deviatoric stress, and significantly in the post-peak stage. If deformation is restricted, changes in transverse deformation can profoundly affect the stress state of concrete. Micropore collapse reduces the macroscopic transverse deformation of concrete, while microcrack expansion has the opposite effect. The strain caused by micropore collapse and microcrack expansion can be deemed as derivative strain of plastic strain, and an influence coefficient can be used to characterize the impact of these two factors on macroscopic transverse deformation. It is proposed that the total transverse strain consists of elastic strain and the product of plastic strain and plastic strain influence coefficients. The development characteristics of concrete transverse deformation can be effectively captured when describing the plastic strain influence coefficient using an arctangent function.

Breakage critical state of gravels with different gradings. Part II: Constitutive modelling

A unified critical state surface for gravels considering the effect of initial grading and particle breakage was proposed in Part I based on the test results of large-scale triaxial compression tests. In this study, we proposed a state-dependent elastoplastic constitutive model to predict the shear behaviors of gravels in the framework of bounding surface plasticity and critical state theory. A simple state parameter with a reference to the unified critical state surface was applied to the dilatancy equation and the bounding stress ratio in the proposed model. The proposed constitutive model can well capture the grading state index, the relative breakage index, the deviatoric stress and the void ratio at the end of tests, and the stress–strain relationships during the shearing process. The model can still be improved by considering the influence of the confining pressure on the critical state strength.

An experimental and numerical investigation on mechanical behavior of sandstone specimens saturated in brine

The mechanical behavior of reservoir rock saturated in brine is of great interest to scientists and engineers for geological CO2 sequestration in deep saline aquifers. However, the mechanical behavior of brine-saturated sandstone under triaxial compression has not been fully investigated, particularly with numerical simulations. In this study, an experimental study and numerical simulation were carried out on brine-saturated sandstone specimens to explore the mechanical properties in sandstones subjected to various brine salinities. Laboratory uniaxial compression tests were conducted on the brine-saturated sandstone specimens, and enhancements in strength and elastic modulus were observed with increasing brine salinity. Scanning electron microscopy results showed NaCl crystals deposited in the pore spaces, which provides evidence of strength and modulus variation in sandstone after brine saturation. Microparameters in a PFC3D numerical model were calibrated for each brine salinity condition, and the numerically simulated results were compared with the experimental results. The numerical results agreed well with the triaxial compression test results, indicating the appropriateness and reasonability of the calibrated models.

Experimental and DEM Simulation Study on the Mechanical Characteristic and Strain Energy Evolution of Longmaxi Shale under a Confining Pressure Unloading Path

Drilling vertical and horizontal wellbores in the shale reservoir may trigger the in-situ stress release around the wellbore walls and change the original stress equilibrium state, leading the wellbores to instability. This stress change in the wellbore corresponds to the stress paths of confining pressure unloading and axial stress loading under laboratory conditions. In this paper, according to the conventional triaxial compression test results, laboratory experiments and DEM simulations by PFC2D were conducted to deeply study the strength, failure, strain energy evolution, and micro-crack damage mechanism of shale specimens under confining pressure unloading conditions. The shale specimens at different bedding inclinations were tested under different initial axial stress levels and confining pressure unloading rates, with fixed initial unloading confining pressure. This research revealed that confining pressure unloading induces greater plastic deformation, more micro-crack damage and strain energy dissipation, and a more complex failure pattern. The strain energy dissipation and dilatation under confining pressure unloading conditions are mainly induced by the generation and accumulation of tensile cracks. Moreover, the unloading rate has a significant effect on the mechanical properties, and the high unloading rate enhances the failure strength and induces more strain energy dissipation and micro tensile cracks. For the wellbore drilling in shale formations, when the buried depth and vertical stress are fixed, the lower the lateral stress is, the easier it is to form tensile failure around the wellbore wall in the drilling process, and the more induced fractures will be generated in the formation around the wellbore.

Triaxial compression experiment and damage constitutive model of microbially modified strongly weathered phyllite

Microbial improvement of strongly weathered phyllite roadbed can effectively solve the problems of insufficient filling and waste soil in roadbed construction while protecting the environment and considering economic benefits. This paper uses microbially induced carbonate precipitation (MICP) technology to improve the strongly weathered phyllite in the Nanchang area and conducts the porosity and triaxial compression tests on the samples before and after the improvement. According to the strain strength theory of rock and the Druker-Prager yield criterion, the damage constitutive model of the improved sample is established considering the effects of the initial porosity and cementation degradation coefficient on the sample’s mechanical properties. The results show that the strengths of the samples improved by MICP technology are increased by more than 35%, and the porosities are reduced by 15%, which meets the requirements of roadbed filling. The stress–strain curves of the modified samples under triaxial compression can be divided into three stages: elastic stage, elastic–plastic stage and ductile flow stage. The triaxial compression test results of the improved samples show that the damage constitutive model established can describe the stress–strain relationship of the samples well, which verifies the rationality of the model. This model provides a theoretical basis for studying damage softening and mechanical properties of microbially modified strongly weathered phyllite samples.

A constitutive model of expansive concrete and steel fiber-reinforced expansive concrete under tri-axial compression

Expansive concrete (EC) or steel fiber-reinforced expansive concrete (SF-EC) have been adopted for enhancing the performance of concrete-filled steel tube (CFST) columns, while the investigations on confinement mechanism and theoretical model for EC and SF-EC are limited. This study aims to develop a constitutive model for EC and SF-EC under tri-axial compression. In the first part, the results of tri-axial compression tests on 52 concrete cubes, were summarized and collected to form a database. In the second part, the failure criterions of concrete cubes, and performances of four existing stress–strain models were assessed and discussed using the database. The results of tri-axial compression with equal lateral confining stresses generally complied well with the compressive meridians of Ottosen failure criterion, whereas the Lode angles were around 56°–58° for the true tri-axial compression results. However, the existing models substantially underestimated the axial strains at peak stresses by at least 40%, and thus the predicted stress–strain curves deviated considerably from the test results with much steeper initial ascending slopes. To this end, a modified constitutive model was proposed with Montoya et al.’s model as the base form and the key parameters updated as per the result database. The proposed model had good performances in predicting the peak axial stresses and the corresponding strains (average error at 8% and 19%, respectively), as well as the full axial stress–strain curves.

Оценка удароопасности скальных горных пород Ждановского месторождения (Кольский полуостров)

he paper presents the results of uniaxial and triaxial compression tests of hard rock samples from the Zhdanovskoe deposit. Two criteria were used to assess the tendency of rocks to fail in dynamic mode (rockburst hazard). In the first case, the A.N. Stavrogin's criterion was used, which requires testing of specimens using “rigid” presses. In the second case, the criterion developed at the Mining Institute of the Kola Scientific Center of the Russian Academy of Sciences was applied to estimate the rockburst hazard of rocks based on the test results of samples using the standard presses. It is shown that the studied rocks of the Zhdanovskoe deposit are prone to failure in dynamic mode according to the two criteria considered. It was found that with the transition from uniaxial to triaxial compression conditions at the lateral pressures up to 40 MPa, the tendency of rocks to dynamic failure is maintained.

Preparation and Performance of Microencapsulated Sealing Material for Coal Mine Gas Drainage.

The performance of coal mine gas extraction borehole sealing material plays an essential role in the efficiency of gas extraction. Microcapsule technology was proposed to delay the expansion time of sealing materials to address the ineffective expansion of cement-based sealing materials in hydration. Based on conventional cement-based sealing materials, delayed-expansion microcapsules were prepared by phase separation with ethyl cellulose (EC) as the capsule wall material and montmorillonite (MMT) as the core material. A single-factor experiment showed that the delayed-expansion microcapsule had the best comprehensive effect when the EC content was 3%, the stirring rate was 400 rpm, the MMT content was 3.5%, and the core-wall ratio (CWR) was 10:3. Second, the slow release effect of the cement samples under the action of microcapsules is remarkable through physical tests. In addition, the triaxial compression test results show better mechanical properties of the delayed-expansion sealing materials. Then, nuclear magnetic resonance scanning was applied to the coal samples injected with different sealing materials. It was found that the proportions of macropores and mesopores in the total pore volume in the coal sample at 50 and 100 mm from the borehole wall decreased by 12.28 and 11.82%, respectively, indicating that the delayed-expansion sealing material has a better sealing effect.

The Stress-Dilatancy Behaviour of Artificially Bonded Soils.

In this study, the results of triaxial compression tests of some naturally and artificially bonded soils presented in the literature were analysed. It was shown that the three characteristic stages of plastic flow during shear can be identified. In all stages, the stress–dilatancy behaviour could be approximated by the general linear stress–dilatancy equation of the Frictional State Concept. For many shear tests, the failure states and newly defined dilatant failure states are not identical. The points representing dilatant failure states lie on a straight line, for which the position and slope in the η-D plane depend on the soil type and the amount of cement admixture. This line defines the critical frictional state angle, and its slope for bonded soils is greater than for unbonded soils.

A constitutive model of freeze–thaw damage to transversely isotropic rock masses and its preliminary application

A constitutive model was developed to study damage in transversely isotropic rock masses caused by freeze–thaw cycles. The model was based on the Weibull distribution function, the macroscopic bedding plane damage variable and a new freeze–thaw damage variable. The model was verified by the results of uniaxial and triaxial compression tests of Carboniferous slates at various bedding angles that corresponded to different numbers of freeze–thaw cycles. The predictions of the model and the experimental results followed similar trends. Thus, the model objectively reflected the complete evolution of the stress and strain of a transversely isotropic rock mass due to freeze–thaw cycling. Then, the constitutive model was embedded into UDEC numerical analysis software to analyze the bedding–freeze–thaw stress coupling response of a whole section of the Guanjiao Tunnel during excavation. The results of the calculation accurately reflected the failure mechanism of the rock surrounding the tunnel induced by excavation and correlated with field monitoring data.

A general plastic model for rockfill material developed by using Caputo fractional derivative

The gradient direction and fractional derivative direction of one function can be coaxial or non-coaxial, and this depends on the order value of fractional derivative. A fractional plasticity model was developed by using Caputo fractional derivative and a dilatancy equation pondering particle breakage for rockfill material in this study. In this fractional plasticity model, the gradient direction and fractional derivative direction of the plastic potential function derived from a dilatancy equation were deduced and further used as the plastic flow direction and loading direction respectively, and the order controls the non-associated feature between the plastic flow direction and loading direction. For validation, drained triaxial test results under different stress paths of five rockfill materials are simulated by the proposed fractional plasticity model. It can be found from comparisons between the drained triaxial compression test results of rockfill materials and corresponding model predictions that the proposed model can well capture the stress-strain characteristics of rockfill material, including strain hardening and volume change behaviors etc.

Stability Analysis of Karst Tunnels Based on a Strain Hardening–Softening Model and Seepage Characteristics

There are more and more tunnel projects in the karst-developed areas in Southwest China. Affected by karst caves and water, karst tunnels often experience geological disasters such as local collapses and water inrush. A simplified rock stress hardening-softening model was established based on the triaxial compression test results of two kinds of carbonatite to accurately analyze the deformation and water inrush characteristics of the surrounding rocks after karst tunnel excavation. The total stress–strain curve of rocks was simplified into four linear stages: the linear elastic stage, strain hardening stage, strain-softening stage, and residual stage. The volumetric strain–axial strain curve was simplified into four corresponding linear stages: the elastic expansion stage, slow expansion stage, rapid expansion stage, and stable expansion stage. The stress hardening–softening model was used to deduce the relationship between the rocks’ mechanical parameters such as cohesion, internal friction angle, dilatancy angle, and plastic strain, as well as the relationship between seepage characteristic parameters such as permeability coefficient, porosity, and volumetric strain. A karst tunnel in Chongqing, China was taken as the engineering background. The stress hardening–softening constitutive model and seepage characteristic parameters were applied to the FLAC3D numerical simulation by the programming language FISH to analyze the stability and water inrush characteristics of karst tunnels in overlying confining caves. The results showed that rock masses between the cave and tunnel were prone to overall sliding instability. Confined water in the karst cave intruded into the tunnel through the shear-slip rupture zone on both sides instead of the shortest path. Two water inrush points existed on the tunnel surface. The variation law of the permeability coefficients of the surrounding rocks could more truly reflect whether there was a seepage channel between the tunnel and karst cave, as well as the permeable area and water inrush speed of the seepage channel. The work provides a new idea for the stability control of karst tunnels.

Effect of Confining Pressure on Mechanical and Energy Conversion Properties of Gas-Containing Coal under Loads

Confining pressure is closely related to the deformation and failure characteristics of gas-containing coal and is one of the key factors for gas outburst-rockburst coupled dynamic disaster. Firstly, this paper briefly summarized the triaxial compression test results of gas-containing coal under different confining pressures, including deformation and stiffness and failure and strength. Then, based on the second developed MatDEM gas-solid coupling program, the macro- and microdeformation and failure process of coal samples under different confining pressures and constant gas pressure were simulated, and the distribution of particle stress, pore stress, and crack, as well as energy conversion characteristics, was obtained. Finally, the influence mechanism of confining pressure on deformation and failure of gas-containing coal was discussed, and the occurrence conditions of coal and gas outburst, rockburst, and their coupled dynamic disasters were distinguished. The results show that the peak strength and residual strength after failure of gas-containing coal both increase approximately linearly with the increase of confining pressure, while the elastic modulus does not change much or increases continuously, which is closely related to the number of microdefects in the coal sample. The strengthening effect of confining pressure on rock stiffness and strength mainly comes from external support and compaction. The total number of cracks decreases with the increase of confining pressure, while the number of shear cracks increases, and the inclination angle between the formed macroscopic main crack and the horizontal plane gradually decreases. The maximum cumulative elastic energy and residual elastic energy both increase with the increase of confining pressure, which leads to the increase of the impact tendency index of coal samples.

Nonlinear unified strength criterion for frozen soil based on homogenization theory

To establish a strength criterion that can consider the composite properties and internal non-homogeneity of frozen soil, a load sharing model is first defined based on homogenization theory and the binary-medium structure model, and then the strength functions of frozen soil on the p-q plane and the π plane are established, respectively. Finally, a three-dimensional nonlinear unified strength criterion for frozen soil is proposed. The applicability of the proposed strength criterion is verified based on the multi-group triaxial compression test results and the directional shear test results at different stress Lode angles.

A Simple Elastoplastic Damage Constitutive Model of Porous Rock Materials

Traditional macroscopic phenomenon constitutive model is not easy to describe in the non-linear mechanical properties of porous rock materials, since the effect of porosity does not incorporate into the strength criterion. This paper presents a simple elastoplastic damage constitutive model of porous rock material based on micromechanical theory. To consider the heterogeneities of the studied porous rock, a simplified representative volume element is introduced, and it is assumed that it is made up of randomly distributed spherical pores embedded in a solid matrix obeying Drucker-Prager yield function. Thus, a homogenized plastic criterion considering the effect of porosity is introduced to describe the macroscopic plastic mechanical properties of porous rock materials. In this model, the non-associated flow rule and isotropic strain hardening law are used, and then the degradation of elastic and plastic properties is employed by adopting a damage criterion. This criterion is related to the evolution of equivalent plastic strain. In order to verify the accuracy of the model, the corresponding numerical program was used to realize the micro-macro constitutive model, and the results were compared with the triaxial compression test results of sandstone under different confining pressures. It is observed that the numerical simulation results are in great agreement with the experimental data, indicating that the proposed model is able to predict the main mechanical behavior of porous sandstone.

Stable Crack Propagation Model of Rock Based on Crack Strain

The establishment of a rock constitutive model considering microcrack propagation characteristics is an important channel to reflect the progressive damage and failure of rocks. The prepeak crack strain evolution curve of rock is divided into three stages based on the triaxial compression test results of granite and the definition of crack strain. According to the nonlinear variation characteristics of crack strain in the stage of rock crack stable propagation, rock deformation is expressed as the sum of matrix strain and crack strain. Then, the exponential constitutive relationship of rock crack stable propagation is deduced. The axial crack strains of the rock sample and its longitudinal section are equal. Thus, the longitudinal symmetry plane of the rock sample is abstracted as a model containing sliding crack structure in an elastic body, and the evolution equation of crack geometric parameters in the process of stable crack propagation is obtained. Compared with the experimental data, results show that the rock crack stable propagation model based on crack strain can adequately describe the evolution law of crack strain and wing crack length. In addition, the wing crack propagates easily when the elastic body with small width contains an initial crack with a large length and an axial dip angle of 45° under compressive load. This study provides a new idea for the analysis of the stable propagation characteristics and laws of rock cracks under compressive load.

Study on the mechanical properties of coral sands with different particle gradations

The properties of coral sands are significantly affected by the gradation changes caused by the particle breakage. In this paper, a series of triaxial consolidated drained (CD) shear tests were carried out to investigate the influence of gradation on the particle breakage and mechanical properties of coral sands. The results showed that the softening and dilatancy during shearing are more significant for coral sands with smaller average particle sizes under low confining pressure. The peak friction angle decreases with the confining pressure and can be fitted by a power function for different gradations. The particle size distribution of the “hyperbolic” or “straight” curve has a trend to transform into a “sigmoidal” type after shearing; and the variation is affected by the confining pressure and the initial gradation. The critical state lines of the coral sands with different gradations are parallel straight lines on the plane. In addition, the relative breakage of coral sand particles increases with the confining pressure and the mean particle size. Based on the triaxial compression test results, the expressions relating relative breakages to the confining pressure and the gradation parameters were proposed.