Multistage Triaxial Tests Research Articles

Investigation of Fluid-Injection-Induced Coal Stiffness Alteration using a Single-Core Multistage Triaxial Test

Nonlinear stress-strain behavior and failure envelopes are critical rock mechanical data for gas reservoir evaluation and development, especially when the basic rock properties are measured under actual reservoir conditions that include pore pressure. For coalbed methane (CBM) reservoirs, the mechanical properties of coal may be different under actual reservoir conditions because the in situ coal is fluid saturated. Multistage triaxial testing of coal can generate a full failure envelope using a single core instead of a set of destructive core tests—as is the case with single stage testing. This multistage technique is necessary because of the scarcity of actual reservoir specimens. Consequently, in this study, a series of multistage triaxial compression tests on a fluid-saturated coal specimen was conducted to determine the mechanical properties of reservoir coal formations as a function of confining stress and fluid compositions and pressures. An oven-dried coal core 50 mm in diameter and 100 mm in length was prepared. Different fluids were employed in the test, including the non-sorbing gas He and sorbing gases, such as N2, CH4 and CO2, and also H2O. Coal sorption behavior was investigated during the triaxial compression test, and the peak stresses under various pore pressures and confining pressure conditions were measured. In addition, the competition between the fluid-density-induced hardening and adsorption-induced softening of the coal core is discussed. The coal strength under gas-saturated conditions was sensitive to the confining pressure. The coal under a higher confining pressure exhibited a stiffer response and the peak load increased with higher confining pressure. Strength weakening due to gas adsorption was observed for N2, CH4, CO2, and water injection. Both CH4 and CO2 had a strong influence on coal stiffness alteration mainly because the higher adsorption capacity of coal for them. Stress-strain relationships behaved in a non-linear when a sorbing-gas was involved. The softening effect of CO2 injection was complicated by a CO2 phase change at higher pressure conditions. This study is applicable to a wide range of engineering applications, including reservoir stress response, permeability evolution, hydraulic fractures, coal mining, and borehole stability, and fault slipping.

Mechanical Characteristics of the Hydrate-Bearing Sediments in the South China Sea Using a Multistage Triaxial Loading Test

Methane hydrate is being widely considered as an energy source by countries worldwide due to its to environmentally friendly and large reserves. However, during the exploitation process, a series of geological engineering disasters can be induced. Therefore, it is necessary to comprehensively study the mechanical characteristics of methane hydrate before commencing with commercial development. Many researchers have used conventional single-stage triaxial loading tests to study the mechanical characteristics of hydrate-bearing sediments, and these have the disadvantages of being time-consuming and labor-intensive. A multistage triaxial loading test can shear a single specimen under different effective confining pressures to obtain a series of mechanical parameters (strength, cohesion, internal frictional angle, etc.). In this study, a high-pressure and low-temperature triaxial apparatus was used to perform multistage and single-stage triaxial loading tests under effective confining pressures of 0.25, 0.5, and 1 MPa, and the specimens were remolded from the South China Sea had porosities of approximately 40% and a hydrate saturation of approximately 30%. The results show that under the relatively low effective confining pressure (0.25 and 0.5 MPa) the stress–strain and volumetric strain–axial strain curves between multistage and single-stage triaxial loading tests are highly coincident, and stress accumulation and compaction seem to have little effect on the strength and volumetric strain in the multistage triaxial loading test. However, under a relatively high effective confining pressure (1 MPa), the stress–strain and volumetric strain–axial strain curves of the two methods present a certain difference. In addition, the elastic modulus obtained by the multistage triaxial loading tests was larger than that from the single-stage triaxial loading tests due to stress accumulation and compaction.

Cyclic behavior of semi-rigid recovered plastic blends in railway track substructure

Plastic deformation of granular layers in railway tracks under cyclic loading is a key factor controlling the substructure’s stability. In particular, the granular subballast capping layer placed between the ballast and subgrade can undergo permanent deformations due to the moving wheel load of trains. This study was conducted to evaluate the cyclic behavior of recovered plastic (RP) blended at different percentages with construction and demolition (C&D) aggregates as railway subballast capping layer. Recycled concrete aggregate (RCA) and crushed brick (CB) were used as the parent C&D aggregates. Multistage triaxial compression tests were initially undertaken to determine the shear strength and the effect of RP inclusion on strength characteristics. The results of multistage triaxial compression tests were also used to find a link between static shear strength and cyclic deformation of the granular recycled materials. The plastic deformability of the RP + C&D blends was also assessed through multistage cyclic triaxial tests at three confining pressures and incremental deviator stress at each confinement. The shakedown theory was employed to characterize the changes in permanent axial and radial deformations, resilient axial strain, and permanent axial strain rate. Inclusion of RP in the RP + C&D blends was found to increase the sample’s deformability particularly in the secondary phase of deformation, reduced the stiffness, and most importantly increased the rate of permanent strain accumulation.

Varying least principal stress along lithofacies in gas shale reservoirs: effects of frictional strength and viscoelastic stress relaxation

Compositional variation of rock properties in unconventional shale reservoirs impacts not only hydraulic fracturing stimulation but also the variability of induced shear slip on pre-existing fracture networks. Lithological layering is also responsible for varying stress states in various unconventional reservoirs subjected to normal, strike-slip, reverse or hybrid faulting regime (a combination of any two of them). Three possible mechanisms are considered to evaluate the layer-based stress profile in the sub-surface, that is viscoelastic stress relaxation, elevated pore pressure and variation in rock frictional strength. We conducted multistage triaxial deformation tests with varying strain rates (5×10−7 to 10−5s−1) on shale sample originating from three lithological units of the Goldwyer shale formation, that is G-I, G-II and G-III in the Broome platform of onshore Canning basin. Additionally, pore pressure was estimated from wireline logs, and tectonic stress accumulation was modelled using time-dependent deformation (creep) data. The above analysis suggests that varying frictional strength alone could not explain the variation of the least principal stress; however, the inclusion of stress relaxation also contributes, whereas elevated pore pressure has no effect. Using this approach, we could also relate the changes in minimum principal stress (Shmin) from one lithofacies to the other and anticipate how this will impact the optimum design of landing zone during horizontal drilling and completion in unconventional field development.

Evolution of shear bands and cracks in multi-stage triaxial tests with water-saturated sandstone: A study of micro-tectonics with a fractal perspective

A series of multi-stage triaxial compression tests revealed quite chaotic reactions of water-saturated Permotriassic sandstones on changing pore water pressures. Not before focussing the various test stages with respect to occurring critical phenomena, which strike the fabric structures due to their fractal nature during any triaxial testing, eventually an approach became tractable: Pore systems with cracks and shear bands can be widened in a quasi-static way, but only up to a stability limit. Thereafter the fabric collapses with a seismogenic contraction of the pore system causing a rise of the pore water pressure. Changes of structural patterns occur as formation either of fissures up to collapse with acoustic emission, or of a rather fractal pattern of crossing shear bands generated without audible noise but with fine grit due to comminution. Dilation and contraction of the pore system are visible from overall volume changes. Both modes of pattern formation can be understood as critical phenomena of the mineral solid. Scale-independent features of these changes of fabric patterns can be transferred to the meso- and macro-scales in the field and in the lithosphere, respectively. However, questions of scale-dependent features related to cohesion should be further investigated.

Investigation of static and dynamic bulk moduli in a carbonate field

Bulk modulus which relates seismic attributes to rock and fluid properties is considered as an important parameter for seismic reservoir characterisation workflows, and can provide useful information for such studies. However, its measurement and modelling approaches are yet to be sufficiently addressed and discussed especially for carbonate rocks. In this research, in order to provide more information about bulk modulus, nine samples from a carbonate oilfield were subjected to static and dynamic investigations under dry and saturated conditions. Then, the dynamic modulus was calculated using well log data and ultrasonic measurements. In order to obtain static data, multi-stage triaxial compression tests were performed on the vertical plugs extracted from conventional cores taken at different selected depths. The results showed that static and dynamic bulk moduli follow similar trends, although the value of dynamic modulus is usually higher. Furthermore, several empirical relationships based on simple linear regression and forward and backward stepwise multiple linear regressions were developed to relate measured static and dynamic bulk moduli for the given field, then proposed equations were evaluated by ANOVA analysis. Besides, dry and saturated bulk moduli were modelled using Xu and Payne rock physics model. Here, saturated bulk modulus was modelled using either dynamic or static dry bulk moduli within the Gassmann’s theory. It is observed that Gassmann’s equation gives a more accurate result using dynamic data rather than static ones. The poor Gassmann’s equation prediction under static condition could be attributed to microcracks in the samples and the uncertainty associated with this theory for complex pore geometry. The outcomes of this study can, furthermore, provide the necessary information and relationships for rock physics (e.g. shear wave estimating and fluid substitution modelling) and geomechanical (e.g. CO2 injection and compaction forecast) studies for exploration and development programs.

Stiffness and strength characteristics of demolition waste, glass and plastics in railway capping layers

The increased generation of demolition waste has led to the successful implementation of its utilization in civil engineering projects. The combination of recycled aggregates and supplementary materials can potentially improve the quality of geomaterials when constructing alternative railway capping layers. In this research, two types of demolition waste, namely, Recycled Concrete Aggregates (RCA) and Crushed Brick (CB), were studied in comparison to two Conventional Capping Materials (CCMs), which are currently used for railway track construction. Recycled Glass (RG) and Mixed Recovered Plastic (MRP) were also blended with RCA to assess their performance. All the materials and mixtures were evaluated in terms of both stiffness and strength. A new Repeated Load Triaxial (RLT) testing protocol was introduced based on the stress induced in the capping layers to determine the stiffness of the materials. A comparison was made among the current resilient modulus prediction models to find a model that would better fit the results for the demolition waste and mixtures. Multistage triaxial tests were also conducted to determine the strength, friction, stiffness and energy absorption capacity of the materials. It was found from this research study that RCA, CB and mixtures of RCA with RG and MRP have equivalent or higher levels of stiffness and strength than CCMs and are suitable alternatives for sustainable railway capping layer construction.

Suction-controlled multistage triaxial testing on clayey silty soil

Multistage triaxial testing offers a potentially reliable and time-saving alternative method of determining the shear strength of soils under suction-controlled conditions, while addressing the issues related to soil variability and lack of replicates. However, evidence of its suitability for testing geomaterials in unsaturated conditions is rather limited. In this work, a series of suction-controlled multistage triaxial tests were performed on compacted specimens of clayey silty soil, wherein each test was performed on the same soil specimen at a constant suction under drained condition, but at varying net confining pressures. The point of termination of the shearing stage was defined through a selection process involving either the stiffness or the volume change in monotonically sheared specimen, based on whichever prevents premature failure of the specimen. This was the first attempt where all the stages of a multistage triaxial test were compared against that from single-stage triaxial tests for validation of the approach. Moreover, the influence of errors introduced by multistage triaxial testing on the shear strength predictions of unsaturated soil was considered by constructing a three-dimensional failure envelope on p-s-τ space using a well-known relation and comparing them against those obtained from single-stage triaxial tests. It was also noted that for determining the shear strength parameters of soil, the multistage testing, reduced the testing times to half of that required by single-stage triaxial testing, which would translate into significant reduction of cost of testing, thereby making unsaturated soil testing much more appealing to the practitioners.

Use of instability curves for the assessment of post-liquefaction stability and deformation of sloping grounds

The paper presents a simplified approach to determine the post-cyclic deformation of liquefied sloping grounds. The approach uses instability curves derived from undrained multi-stage (cyclic+monotonic) triaxial tests. It is shown that the salient aspects of the post-liquefaction deformation can be expressed as a function of the state parameter ψ, defined as the void ratio difference between the current state of the soil and its critical state at the same mean stress level, and amplitude of accumulated cyclic strain. As the proposed approach predicts deformations, rather than residual strength or factor of safety, the method can be used for the definition of the performance criteria following a performance-based design approach. The application of the proposed method is illustrated through a real case study.

Experimental investigation on strength and failure behavior of carbonate rocks under multistage triaxial compression

Conventional or single stage triaxial compression experiments for obtaining failure envelope of rocks require several specimens to be tested under different confining pressures. As a replacement, multistage triaxial tests using one specimen have been employed in experimental rock mechanics for situations where preparing several specimens is difficult or expensive. The results obtained by the multistage triaxial test may deviate from those obtained by the single stage. However it is believed that using multistage triaxial test can decrease the cost and time and the obtained results are still reliable due to less variability in the rock properties. Several methods have been proposed for conducting multistage triaxial test including ISRM suggested method and a modified version of the ISRM method proposed by Crawford and Wylie. The results of this study demonstrate that both multistage methods give reasonable results. However, the modified method provides closer results to the single stage, because the specimen is subjected to less damage. As the number of the loading stages rises, both ISRM and the modified version provide lower strength parameters comparing to the single stage.

Numerical prediction of the creep behaviour of an embankment built on soft soils subjected to preloading

This work analyses the ability of the Singh-Mitchell and Taylor creep laws associated with a Modified Cam Clay model to predict the creep behaviour of a soft soil subjected to preloading. Firstly, the creep laws and constitutive model are validated by the results of CIU triaxial tests, creep oedometer tests and multi-stage triaxial tests, carried out in order to reproduce the effect of the preloading. After that, the creep laws/model are used to predict the long-term behaviour of an embankment built on soft soils subjected to preloading. The numerical predictions incorporating the creep phenomenon show a constant rate of increase in the settlement and the horizontal displacements, with a constant relationship between the settlement and the horizontal displacements.

Wetting effects and strength degradation of swelling shale evaluated from multistage triaxial test

The multistage triaxial compression test was mainly utilized in this research to investigate the degradation in the strength of swelling shale under different wetting conditions. Shale samples were tested in four different conditions: (1) under their natural moisture content denoted as intact, and after soaking in, (2) water, (3) polymer solution, and (4) bentonite solution, separately. The experimental results showed that a considerable decrease in the shale’s strength was noted after soaking in water and the lubricant fluids (LFs). The polymer solution caused the least decrease in the strength compared to other fluids. Strength envelopes of the tested shale were constructed for intact and soaked states. These envelopes showed fair agreement between the single stage and the multistage triaxial compression test results, indicating suitability of the multistage triaxial compression test for the tested swelling shale.

Laboratory scale characterization of brittleness and permeability enhancement due to rock failure.

In this study, the multi-stage triaxial compression testing method is adopted to obtain geomechanical parameters without heterogeneity effect. Using the geomechanical characterisation we calculate various brittleness indices previously suggested in the literature. Before the triaxial test and after the ultimate rock failure, we also measure the permeability enhancement ratio caused by rock failure with characteristic time obtained from pulse decay permeameter. Analysing the relationship between the various brittleness indices and the permeability enhancement, we assess the ability of the former to predict the latter. The results show that no unique brittleness index can completely explain the permeability enhancement, but those indices perform better which contain information on post-peak behaviour in direct or indirect manner. We conclude that at least three aspects of the failure behaviour are important and hence at least two indices are required to characterise the potential of permeability enhancement. [Received: July 1, 2017; Accepted: August 17, 2017]

Post-liquefaction behavior of natural pumiceous sands

Pumice particles are volcanic soils which are highly crushable, compressible and lightweight, due to their vesicular nature. After a series of volcanic eruptions in the North Island of New Zealand and due to flooding and erosion, some pumice particles have become mixed with hard-grained materials and have been distributed over some parts of the Waikato Basin; these mixtures are referred to herein as natural pumiceous (NP) sands. The post-liquefaction behavior of NP sands is important not only for the purpose of investigating ground deformation induced by liquefaction but also for assessing the impact of these deformations on buried structures. For this purpose, reconstituted NP sands were obtained from three sites along the Waikato River in order to perform multi-stage triaxial testing. Three parameters – initial Young's modulus (E1), Young's modulus at recovery (E2) and recovery strain (Ɛr) – were used to express the stress–strain behavior of NP sands that had undergone liquefaction. For comparison purpose, similar tests were performed on hard-grained Toyoura sand. The liquefied NP sands recovered their strength at a considerably lower strain level than Toyoura sand due to the formation of stable soil skeleton during cyclic loading and the high angularity of the pumice particles. In addition, the E1 values of liquefied NP sands were higher than those of Toyoura sand while the values of E2 were lower; such differences in the stress-strain behavior of liquefied NP sands can be attributed to the unique characteristics of pumice components, i.e. their irregular particle shape and crushability features.

Deformation and Damage Failure Behavior of Mudstone Specimens Under Single-Stage and Multi-stage Triaxial Compression

In tunnel engineering, due to the effect of excavation disturbance, the surrounding rock mass can produce an excavation damage zone with different damage extents. Therefore, knowledge of rock deformation and damage behavior is especially significant for the design of deep tunnel support. However to date, a few experiments and numerical simulations have been conducted to investigate the deformation and mechanical failure behavior of damaged rocks. Therefore, in this research, multi-stage triaxial compression test was used to investigate the mechanical behavior of mudstone specimens with different damage extents by experiment and two-dimensional particle flow code. First, a group of micro-parameters was calibrated by single-stage triaxial compression experiments of mudstone, and the numerical results agree very well with the experimental results. Then, multi-stage triaxial compression experiment and discrete element modeling of mudstone specimens were carried out. The more axial strain the specimens sustained, the less strength they had (because the degree of damage increased). A damage variable was defined by the ratio of the area of micro-cracks to the total area of the specimen. As the post-stress reducing ratio increases, the damage variable increases rapidly until the post-stress reducing ratio reaches 0.4; then, it remains constant. The force field were analyzed to reveal the damage evolution mechanism in the mudstone specimens under multi-stage triaxial compression.

Elastic, viscoelastic, and strength properties of Marcellus Shale specimens

Shale gas rocks are characterized as clastic sedimentary rocks, with features such as obvious bedding planes, presence of micro-cracks, and high clay and organic content. These rocks are anisotropic and inhomogeneous, exhibiting a nonlinear response under loading. In order to optimize the required energy for hydraulic fracturing operation and production in shale gas reservoirs, and for constitutive and numerical modeling, it is important to characterize these shale gas rocks. In this study, the hysteresis, elastic-plastic, viscoelastic, and strength properties of Marcellus Shale specimens, retrieved from a deep well located in West Virginia, were evaluated through performing a series of creep, cyclic, and triaxial multi-stage failure tests on these specimens. Both elastic moduli and plastic deformations show significant levels of pressure dependency. Moreover, higher creep compliance and lower Young's modulus values were observed for clay-rich specimens. Both Power-Law and Burgers' models were found to capture the creep response of these specimens reasonably well. The dynamic moduli estimated from the ultrasonic velocity measurements at different stress levels were found to be higher than static moduli estimations. In addition, the changes in the internal micro-structure of the specimens resulted from variations in the stress condition, were found to affect the ultrasonic velocity measurements. The strength properties of the calcite/quartz-rich specimen, using multi-stage triaxial failure test, were estimated through both Mohr-Coulomb and Hoek-Brown failure criteria.

Multistage Triaxial Tests on Laboratory‐Formed Methane Hydrate‐Bearing Sediments

AbstractA sound understanding of the geomechanical behavior of hydrate‐bearing sediments (HBSs) is essential not only for assessing reservoir and wellbore stability during methane gas production but also for projecting the impact of global warming on the stability of geological settings that contain hydrates. This study experimentally investigated the geomechanical responses of laboratory‐formed methane‐HBS to triaxial shearing under drained conditions. The hydrate pore habit formed in the sediments is noncontact‐cementing, which is different from contact‐cementing or grain‐coating hydrate pore habits formed when the excessive gas method is used. The multistage triaxial test method, which shears a single specimen under different confinements, was employed to examine its applicability in measuring geomechanical properties of the HBS. Conventional single‐stage triaxial tests were also performed to serve as baselines. The results show that both the strength and stiffness of HBS increase with increased hydrate saturation and confining stress. Shear dilatancy increases with increased hydrate saturation and decreased confining stress. The peak strength and peak/postpeak shear dilation measured using the multistage tests are comparable to those using the single‐stage tests. However, the stiffness measured in later stress stages in the multistage tests was enhanced by the stress‐strain history of earlier stages. Therefore, the multistage test offers an efficient way of measuring the strength and volumetric response of the HBS with a much smaller number of specimens than the single‐stage test. This benefits the geomechanical characterization of the HBS, as obtaining the pressure cores is extremely costly and preparing lab samples is usually sophisticated and time consuming.

Anisotropic Behaviour of Opalinus Clay Through Consolidated and Drained Triaxial Testing in Saturated Conditions

This paper investigates the anisotropic hydro-mechanical behaviour of Opalinus Clay shale, the host material currently being considered for the construction of a nuclear waste repository in Switzerland. Consolidated and drained triaxial tests on Opalinus Clay from the Mont Terri URL have been conducted in order to derive information on its strength and stiffness properties. Opalinus Clay specimens were tested both parallel to bedding (P-specimens) and perpendicular to bedding (S7 specimens). The considered effective confining stress range (from 2 to 12 MPa) has been selected in order to reproduce possible in-situ stress conditions for the repository. In this work, particular attention has been paid to the experimental procedure in order to ensure consolidated conditions and avoid generation of unwanted excess pore water pressure during drained shearing. The Skempton B parameter has been determined for all the tested specimens in order to ensure saturation. Both single-stage and multistage triaxial testing procedures were adopted in the experimental campaign. The results of the triaxial tests highlight an anisotropic elastic response of Opalinus Clay: S specimens present a more compliant behavior than P-specimens. The values of the Young modulus are found to increase with the increase in mean effective stress. The analysis of the peak and ultimate shear strength results reveals that the material behaves in a similar manner regardless of the considered direction of loading (P and S directions) with respect to the bedding orientation. Peak and ultimate failure envelopes for Opalinus Clay were derived for the investigated stress range.

A Simplified Direct Shear Testing Procedure to Evaluate Unsaturated Shear Strength

Abstract The work in this paper presents a simplified and expedient procedure to evaluate unsaturated shear strength using conventional direct shear testing with tensiometer-measured suctions. Two types of residual soils are utilized in this testing program. These are classified as elastic silt (MH) and silt (ML) and were obtained from a test site in Greensboro, North Carolina. Unsaturated triaxial tests were performed to provide reference values for comparison with the measured values from the proposed direct shear testing approach. Both single stage and multistage triaxial tests were performed, and an insignificant difference in the peak stress from both approaches was found. The total cohesion intercept was used to represent the strength contribution of the matric suction in order to normalize the effect of the confining pressure. The applicability of the simplified expedient procedure in assessing shear strength of residual soils used herein was demonstrated based on the total cohesion intercepts obtained from both testing approaches. The results from the proposed approach were very similar to those obtained from the unsaturated triaxial compression tests.

Multistage triaxial testing of various rock types: A case study of East Attica Prefecture, Greece

This paper presents the statistical processing of data obtained from multistage triaxial tests of selected rock types from East Attica Prefecture of Greece. Two strength models for the peak and residual strength were fitted to the experimental data, i.e., the Mohr - Coulomb criterion and the Hoek - Brown criterion. The derived strength parameters for these distinct states of stress are compared with findings of other researchers and their dramatic change during the residual state of the rock samples is discussed.