Confining Stress Levels Research Articles

X-ray microstructural insight into the mechanical behaviour of ligth-weight cemented soils

Light-weight cemented soils (LWCS) – that is, materials prepared by mixing natural soil, water and cement with air foam, are characterised by a heterogeneous microstructure, consisting of large foam-induced voids distributed in a cemented porous matrix. The reduced volume weight coupled with a good mechanical strength makes LWCS suitable for many geotechnical applications. However, the role of the microstructure on their macroscopic mechanical behaviour is not completely clear. A novel experimental investigation on the mechanical response of LWCS under triaxial loading paths using in situ x-ray microtomography is presented here, focusing on the evolution of foam-induced porosity. The tomographies are acquired during the shear phase of multiple triaxial compression tests, performed at different confining stress levels. Image analysis is employed for obtaining porosity maps and incremental strain fields of the samples. At low confining stress, the sample deformation essentially consists in the progressive opening of sub-vertical dilatant fractures connecting the foam-induced voids. At higher confining stresses, localised compactions develop in the cemented matrix along the directions coherent with the deviatoric load path. The results presented above indicate that the deformation and failure mechanisms of LWCS strongly depend on the mean stress level.

Shear modulus of sand-rubber mixtures: element testing and constitutive modeling

In this study, a series of resonant column tests was conducted to measure the shear modulus of sand-rubber mixtures at small strain amplitudes (i.e. between 10−4% and 10−2%), considering different rubber percentages and confining stress levels. The results were then combined with data obtained by dynamic hollow cylinder tests to investigate shear modulus degradation of the mixtures over a wider shear strain range. Based on the test results, a new expression was proposed to improve the prediction of maximum shear modulus of sand-rubber mixtures using the modified equivalent void ratio concept. A new constitutive model was also developed for estimation of strain-dependent shear modulus of the mixtures based on the modified hyperbolic framework. The shear modulus of the mixtures was found to be a function of rubber percentage, confining stress, the modified equivalent void ratio and the relative shear stiffness of rubber and sand. The experimental data and the developed models showed that the shear modulus decreased with rubber percentage and increased with confining stress. Moreover, the reference shear strain of the modified hyperbolic model increased with both rubber percentage and confining stress while its curvature coefficient increased more considerably with rubber percentage compared to the confining stress.

Rock slope stability analysis under Hoek–Brown failure criterion with different flow rules

The stability analysis of homogeneous rock slope following the Hoek–Brown failure criterion under the hypothesis of different flow rules is performed based on limit equilibrium and finite element methods. The applied failure criterion is the generalized Hoek–Brown that can be introduced as a shear/normal function in analysis applying different flow rules. The results are compared with those obtained by the application of equivalent shear strength parameters of the Mohr–Coulomb criterion, considering that this is still the most widely used criterion in rock slope stability analysis and is still the base for the shear strength reduction method applied in finite element modelling. Different proposals for estimating the equivalent strength parameters based on confining stress level are evaluated. The limitation of stress-dependent linear Mohr–Coulomb parameters is emphasized by analysing the vertical cut problem, for which, depending on the chosen stress level, different critical heights are obtained for the same material. Sensitivity analysis of geotechnical parameters used as input for failure criterion is performed to determine their influence on slope stability. Probabilistic analysis is conducted to determine the probability of failure when different flow rules are applied. If slope stability analysis is performed with an assumption of associative flow rule, the probability of failure is within the acceptable limits for the considered case study, while employing non-associative flow rule, the probability of failure is rather high. The chart is presented that could be readily used to estimate the combination of σci, GSI, and mi values that produce failure for the analysed case study.

Examining the Role of Liquefiable Layer Thickness and Depth on the Seismic Lateral Response of Piles through Numerical Analyses

In nature, soil layers possess large variability during their geological process. This variability may also lead to differences in the location and thickness of nonliquefiable and liquefiable soil layers. In practice, the impact of liquefaction on the pile can be ignored at depths greater than 20 m due to high confining stress levels and a lack of liquefaction triggering data. On the other hand, this approach may underestimate design loads in many cases, especially in deep-seated and embedded engineering structures. This paper presents the bending response of piles installed through liquefiable layers located at depths beyond 20 m, and parametric analysis was conducted for a wide range of liquefiable layer thicknesses and depths by using OpenSeesPL (version 3.0.2) software. The numerical results were evaluated considering the inelastic concrete pile behavior under different earthquake records and different peak ground accelerations. The findings show that liquefaction can lead to a failure of piles even at depths greater than 20 m, and thus, a design consideration of piles may require a more comprehensive view considering the liquefiable layer depth and thickness effect.

Nonlinear criterion for strength mobilization in brittle failure of rock and its extension to the tunnel scale

As underground excavations are getting deeper and field stresses increase, the behavior of intact rock blocks plays an increasingly important role in understanding and estimating the overall rock mass strength. To model the brittle behavior of intact rock blocks, the stress–strain curve is usually idealized considering a linear strength mobilization approach (cohesion-weakening-friction-strengthening, CWFS), however, it is well recognized that rock presents a nonlinear behavior in terms of the confining stress. This study extends the strength mobilization in brittle failure of rock using nonlinear criteria. To determine the model parameters, a standard statistical method that uses the complete laboratory stress–strain curves of the intact rock is employed. Several hypotheses of linear and nonlinear models are statistically compared for different types of rock and confining stress levels. Results demonstrate that the best approach to model the brittle failure of rock is to consider a nonlinear strength envelope, such as the Hoek-Brown criterion assuming a residual uniaxial compressive strength different from zero and a m i parameter that increases, both with simultaneous mobilization. This model helps to recreate high-confining conditions and a more realistic transition between peak and post-peak strength. The obtained parameters are discussed and compared with literature values to verify the validity and to develop guidelines for the estimation of parameters, providing an objective mobilization criterion. Finally, the nonlinear model was applied to a finite element code and extended to a tunnel scale in the brittle rock under high-stress conditions. A reasonable fit between the simulations and the in-situ overbreak measurements was found.

Shear Properties of Cemented Paste Backfill under Low Confining Stress

Cemented paste backfill (CPB) plays an important role in the mining industry due to safety, cost efficiency, and environmental benefits. Studies on CPB have improved the design and application of paste backfill in underground mines. Direct shear is one of the most fundamental parameters for assessing backfill strength. This study harnesses direct shear tests to explore the low confining stress behavior of CPB. We perform all the tests in a standard apparatus on the combination of three binder contents of 4.2%, 6.9%, and 9.7% CPB with four curing times of 3, 7, 14, and 28 days, respectively. The applied confining stress levels vary in a range according to the in situ regime. Results are presented by strength envelope, stress-strain property, and shear strength with curing time and binder content. The data suggest that the shear strength follows the Mohr–Coulomb envelope in which the shear strength and behavior are time and binder content dependent. In addition, the results show that shear strength is strongly related to the binder content than the curing time, namely, the higher the degree of binder hydration, the higher the cementation binding force between CPBs.

A NEW APPROACH FOR INVESTIGATING THE ENGINEERING BEHAVIOR AND MECHANICAL PROPERTIES OF BONDED SANDY SOILS

Cemented soils are widely encountered in the world, and with the growth of population, their exploitation becomes a necessity, especially, by building on diverse engineering infrastructures. The stability of those infrastructures is directly related to the nature and the properties of bonding in presence. This paper aims to investigate a new approach in studying the engineering behavior and mechanical properties of bonded sandy soils of different bonding levels, by using artificial soils similar to natural ones. The investigation was performed on four types of artificially bonded materials and one unbonded soil. The experiment was conducted in the laboratory under consolidated undrained isotropic triaxial compression tests subjected to three different levels of confining pressure. A new parameter, bonding index ( ), was defined to implement the new approach. It is found that the variation of bonding and confining pressure alters significantly the response of soils. The cohesion intercept, the friction angle, the shear stress, and the brittleness index increase together with , especially at low confining stress. However, decreases when the confining pressure increases, restraining the role of bonding strength. Soils with higher are identified to be more brittle. The determination of the confining stress level is found to be related to the bonding degree, which is expressed in terms of . The bonding index is assessed as an efficient parameter whereby the bonded soil response can be investigated. Thus, the new approach offers an alternative for studying the engineering behavior of bonded soils.

Consolidated Undrained Monotonic Shearing Response of Hydrophobic Kızılırmak Sand

Geotechnical properties of hydrophilic (wettable) sands have been widely discussed in the literature. However, sands may gain hydrophobic (non-wettable) properties after being exposed to a hydrophobic agent in the nature. The number of available studies regarding the response of hydrophobic sands is very limited, and mostly focus on their environmental and hydrological aspects. To close this gap, a controlled laboratory testing program, consisting of 18 monotonic strain-controlled consolidated undrained triaxial shear tests, was designed. Tests were performed on fully saturated hydrophilic and hydrophobic re-constituted Kızılırmak sand samples of different relative densities with pore water measurements. Hydrophobic samples were prepared by using 1 and 2 % WD-40 lubricant by mass. The effect of hydrophobic agent was assessed by comparing the stress – excess pore water pressure - strain responses of hydrophobic sand samples with those of conventional (hydrophilic) sand samples. Test results revealed that addition of hydrophobic agent increases the dilatancy of sands at low confining stresses (~100kPa) by decreasing the excess pore water pressure generation. At higher confining stresses (~400kPa) this effect is less pronounced. Moreover, the addition of hydrophobic agent up to 2% by mass does not systematically and significantly change the effective angle of shearing resistance of sand samples, independent of their initial relative density and confining stress levels.

Effect of inclined interface angle on shear strength and deformation response of cemented paste backfill-rock under triaxial compression

Cemented backfilling has been widely used in underground mines worldwide. The stability of cemented backfill must be sufficient to maintain the subsequent stope. Investigation into the shear strength and deformation of cemented paste backfill-rock (CPB-rock) under triaxial compression is prominent important for mine operators and managers, while the knowledge of the effect of the interface between cemented paste backfill (CPB) and rock on the shear strength and deformation behavior is limited. The objective of this paper is to experimentally investigate the effect of interface angle on the triaxial deformation behavior and shear strength of the CPB-rock. Two interface angles (45° and 60°) and three cement contents (2.5, 5 and 7.5 wt%) were designed. All tests were carried out by a standard triaxial compression apparatus, and the confining stress levels were varied in a range corresponding to the in-situ horizontal pressures (σc ≤ 200 kPa). The results show that the interface angle plays a dominant role in the failure patterns of CPB-rock. Increasing the cement content improves the brittleness of CPB matrix, and increases the shear strength difference between CPB-rock and CPB. The failure pattern of CPB-rock changes from composite failure to slipping failure. At last, the bilinear peak strength envelope for CPB-rock specimens is proposed. The results are expected to quantify the effect of CPB-rock interface on stability of cemented backfill.

Experimental investigation on shear performance of fiber-reinforced high-strength concrete dry joints

In order to consider the seven parameters including joint types, confining stress levels, fiber lengths, fiber shapes, fiber types, volume ratios of fibers, and reinforcement of key specimens, seventeen pairs of shear key specimens were cast and tested. The effects of the above seven parameters on the shear behavior and shear capacity of fiber-reinforced high-strength concrete (FHSC) dry joints were obtained. Analysis of the test results shows that the type of joint has a great influence on the shear behavior of specimens. Compared with HSC specimens, the cracking stress, ultimate stress, and ductility of FHSC specimens were improved. The average cracking stress is increased by 46.4%, and the average ultimate stress is increased by 25.3%, and the average ductility is increased by 30.9%. Within a certain range, the ultimate stresses of FHSC specimens increase with the increases of confining stresses, fiber lengths, and fiber volume ratios. The end-hook steel fibers have a greater effect on the ultimate stresses than the two other types. Then, the calculating formula for the shear capacity of dry joints was summarized. Based on the existing test data, the calculated results of different formulas were compared and analyzed. It was found that the calculated results of the formula suggested by AASHTO agreed best with the test results for single-keyed specimens, while the formulas suggested by Buyukozturk predicted the shear capacity well for multi-keyed specimens. Also, considering the effect of fibers on the concrete strength, a calculating formula for FHSC was proposed based on Mohr's stress circle principle, which was in a good agreement with the test data.

Acoustic emission behavior of granular soils with various ground conditions in drained triaxial compression tests

Since the mechanical failure of granular soils is the macro-manifestation of complex internal particle interactions, studying this relevant issue from micro-scale perspective is of fundamental importance. Acoustic Emission (AE) has become a promising approach for this purpose; however, its synthetic and validating study on granular soils with various ground conditions is far from substantive. In an attempt to verify and correlate the relationship between AE behaviors and mechanical behaviors of granular soils with various conditions, results of drained triaxial compression tests along with AE measurements recorded during testing are discussed. These results revealed intrinsically different propagation properties of acoustic waves between saturated and unsaturated/dry sands. The water in saturated sands facilitates acoustic waves to propagate with less attenuation, whereas discontinuities (i.e. particle-particle contacts and particle interfaces with water) in dry/unsaturated sands lead acoustic waves to propagate with larger attenuation. The AE manifestation showed its high sensitivity to different saturation degrees, package densities and confining stress levels, which allows the evaluation of ground conditions to be made by AE. The congruous AE evolution in saturated sands was considered closely related to particle interactions during drained triaxial compression; whereas the changeable AE evolutions in accordance with different failure patterns in dry sands were considered relating to soil structure behaviors.

A Model for Pore Pressure Response of a Claystone due to Liberated Residual Stress Dilation

The hydromechanical response of Opalinus Clay is being studied as a potential host formation for the long-term geological storage of nuclear waste. Two separate mine-by field experiments were conducted at Mont Terri rock laboratory, Switzerland: one in 1997–1998 (HD-B) and one in 2008 (MB Niche) to observe the claystone’s response to tunnel excavation. The data from both mine-by tests revealed an undrained pore pressure response dominated by significant pore pressure drop (i.e., a dilational tendency), even in instrumentation locations where stress-induced damage (‘failure’) was not anticipated. This response is not reproducible by conventional hydromechanical models unless very low yield envelopes are used to predict the onset of dilation. An alternative mechanism for the undrained dilational response of Opalinus Clay due primarily to unloading in the low confining stress regime is presented in this paper. The mechanism is derived from observed behaviour of the Opalinus Clay in the field and laboratory, and an understanding of the composition and geological history of the material. The model is based on mechanisms associated with liberation of latent strain energy, and associated residual stress (‘locked-in stress’ or ‘stress memory’) through microcrack formation. Mobilization of this internal stress on the undrained response of Opalinus Clay results in distinct regimes of hydromechanical response, dependent on confining stress levels, that is accounted for in the model. The model was implemented into the continuum finite difference code FLAC3D and produced good agreement with mine-by test observations.

A DEM investigation of water-bridged granular materials at the critical state

The critical state is an important concept for saturated and partially saturated granular materials as the strength and volume become constant and unique under continuous shear. By incorporating the water bridge effect, the mechanical behaviours of wet granular matters can be studied by the discrete element method (DEM). A series of DEM simulations are performed following the conventional triaxial loading path for dry and wet granular materials, and different suction values are applied at various confining stress levels. Unique critical state behaviours have been observed in both macroscopic and microscopic scales. It shows that the confining stress level plays an important role in the critical state behaviour of wet granular materials. The critical stress ratio for a wet material is not a constant value at different stress levels, and it is found that both the critical stress ratio and void ratio in wet granular matters are also much higher with a low confining stress. A framework is proposed by considering both the contact stress and the capillary stress effects to model the critical state lines. At large strain, the coordination number, the mean inter-particle force and fabric anisotropies evolve to constant critical state values for both dry and wet materials. The macro-parameters formulating the critical state stress ratio are found to be associated with the critical state anisotropies in solid skeleton and water phase fabrics, respectively.

Rock cone penetration test under lateral confining pressure

To assess the effect the field stress on the in situ cone penetration test of rock, a series of laboratory cone penetration tests was conducted to study the fundamental relationship between the cone penetration test results and the state of stress of rock to be tested. Bidirectional confining pressures were applied on rock samples to reproduce the field stress state of the rock at the borehole wall. The test results show that the specimen under confining pressure shows a different failure mode and mechanical property. The fracture pattern of the specimen was affected by the lateral confining pressure. The load displacement curve under different confining pressures shows that the peak load and ultimate displacement tend to increase with increasing confining stress level. The minor confining pressure plays a dominant role in the process. The failure mechanism was analyzed, and attempts were made to relate the improvement effect of the confining pressure in this test to the increase in compression strength obtained in the conventional triaxial test.

Influence of intensity & eccentricity of posttensioning force and concrete strength on shear behavior of epoxied joints in segmental box girder bridges

Precast concrete segmental bridges have the advantages of rapid construction speed, excellent quality control and low cost. The structural behavior of segmental bridges largely depends on the behavior of the joints between segments. In this study, a series of specimens with epoxied joints, were tested to assess shear behavior of box girder segmental bridges, under direct shear loading. The study parameters were: confining stress level, prestressing eccentricity, and the concrete compressive strength. Shear behavior, shear capacity, and crack patterns of the joints were investigated as well. Mechanisms of sequential shear failure and a detailed analysis of failure events are also presented. It was found that the shear capacity of the joints increased approximately 40% for each increased 50% of the confining stress. Eccentricity of prestressing force wasn’t affecting the elastic behavior of the joints, but its role was appeared obviously at the plastic stage. By increasing concrete compressive strength up to 70 MPa, the joint shear capacity can be increased significantly. The experimental results of the current study and results of other researchers were compared with design provisions. It was seen that the relationships tended to underestimate the shear strength of keyed epoxied specimens by an average value of 11.3%.

Experimental investigation of hard rock fragmentation using a conical pick on true triaxial test apparatus

This study aims to determine the influences of confining stresses and cutting parameters on hard rock fragmentation using a conical pick. Using a true triaxial testing apparatus, single/double pick forces and static/coupled static-dynamic pick forces were applied to granite rock specimens to break them confined by biaxial, uniaxial and no lateral stress conditions. The corresponding cuttabilities were estimated and compared by the peak pick force, insertion depth and disturbance duration at rock failure and the associated failure patterns. The results showed that excavation-induced unloading and cracking, which can change the biaxial confining stress conditions into the uniaxial and decrease uniaxial confining stress level, respectively, can significantly improve the hard rock cuttability. The experimental, theoretical and regressive results indicated that the hard rock cuttability initially decreases and then increases as the level of uniaxial confining stress increased. A moderate uniaxial confining stress instead improves the rock cuttability, but a high stress level may induce rock burst triggered by conical pick penetration. Therefore, only the hard rocks under stress-free and low uniaxial confining stress conditions can be easily fragmented with high safety and efficiency, as a complete splitting failure occurs. In addition, the hard rock cuttability can be also improved by the dynamic pick disturbance and the artificial defects such as excavation-induced fractures, pre-slits and boreholes in rock mass.

Experimental Investigation of the Influence of Confining Stress on Hard Rock Fragmentation Using a Conical Pick

High geostress is a prominent condition in deep excavations and affects the cuttability of deep hard rock. This study aims to determine the influence of confining stress on hard rock fragmentation as applied by a conical pick. Using a true triaxial test apparatus, static and coupled static and dynamic loadings from pick forces were applied to end faces of cubic rock specimens to break them under biaxial, uniaxial and stress-free confining stress conditions. The cuttability indices (peak pick force, insertion depth and disturbance duration), failure patterns and fragment sizes were measured and compared to estimate the effects of confining stress. The results show that the rock cuttabilities decreased in order from rock breakages under stress-free conditions to uniaxial confining stress and then to biaxial confining stress. Under biaxial confining stress, only flake-shaped fragments were stripped from the rock surfaces under the requirements of large pick forces or disturbance durations. As the level of uniaxial confining stress increased, the peak pick force and the insertion depth initially increased and then decreased, and the failure patterns varied from splitting to partial splitting and then to rock bursts with decreasing average fragment sizes. Rock bursts will occur under elastic compression via ultra-high uniaxial confining stresses. There are two critical uniaxial confining stress levels, namely stress values at which peak pick forces begin to decrease and improve rock cuttability, and those at which rock bursts initially occur and cutting safety decreases. In particular, hard rock is easiest to split safely and efficiently under stress-free conditions. Moreover, coupled static preloading and dynamic disturbance can increase the efficiency of rock fragmentation with increasing preloading levels and disturbance amplitudes. The concluding remarks confirm hard rock cuttability using conical pick, which can improve the applicability of mechanical excavation in deep hard rock masses.

Potentially exploitable supercritical geothermal resources in the ductile crust

The hypothesis that the brittle–ductile transition (BDT) drastically reduces permeability implies that potentially exploitable geothermal resources (permeability >10−16 m2) consisting of supercritical water could occur only in rocks with unusually high transition temperatures such as basalt. However, tensile fracturing is possible even in ductile rocks, and some permeability–depth relations proposed for the continental crust show no drastic permeability reduction at the BDT. Here we present experimental results suggesting that the BDT is not the first-order control on rock permeability, and that potentially exploitable resources may occur in rocks with much lower BDT temperatures, such as the granitic rocks that comprise the bulk of the continental crust. We find that permeability behaviour for fractured granite samples at 350–500 °C under effective confining stress is characterized by a transition from a weakly stress-dependent and reversible behaviour to a strongly stress-dependent and irreversible behaviour at a specific, temperature-dependent effective confining stress level. This transition is induced by onset of plastic normal deformation of the fracture surface (elastic–plastic transition) and, importantly, causes no ‘jump’ in the permeability. Empirical equations for this permeability behaviour suggest that potentially exploitable resources exceeding 450 °C may form at depths of 2–6 km even in the nominally ductile crust. The brittle–ductile transition is thought to control crustal permeability. Laboratory experiments and model simulations show that permeability is also stress dependent and ductile granitic rocks may have enough permeability to host geothermal resources.

Strain Rate Effect on Shear Behavior of Open-Graded Aggregates

Despite their wide application as construction materials in various earthworks built by state and local transportation agencies, the role of physical and mechanical factors in the strength and deformation behavior of crushed, manufactured open-graded aggregates (OGAs) is not well studied. In this investigation, the strain rate dependency of strength–deformation behaviors of two commonly employed crushed aggregates with small (12.7 mm) and large (38.1 mm) sizes is investigated. A 150-mm diameter triaxial testing device was used to conduct a drained compression test at five strain rates, ranging from 0.000083%/s to 0.0083%/s. To evaluate the significance of confining stress and density on the effect of strain rates, the shear tests were conducted at 34 kPa and 207 kPa effective confining stress levels, with the samples compacted at loose (30%) and dense (95%) relative densities. The peak friction angle, maximum dilation angle, secant modulus, and axial strain at which the aggregates started to dilate were determined to evaluate the strain rate effect on the shear behavior of OGAs. The results demonstrate that within the imposed quasistatic strain rate ranges, only the dilation angle showed an increasing trend with the increase in strain rate, whereas other extracted strength parameters were less sensitive to strain rate for both OGAs tested. Hence, the selection of strain rates according to ASTM specifications is appropriate for conducting strength parameter tests, used by practitioners for the design of geotechnical structures, on OGAs under quasistatic conditions.

Impact of confining stress on permeability of tight gas sands: an experimental study

Stress influence on permeability has been extensively studied by various authors, as the stress can significantly affect reservoir’s productivity. This paper displays the features of permeability stress sensitivity in tight gas sandstone in Kirthar fold belt lower Indus Basin, Sindh, Pakistan. The experiments performed under a range of pore pressure and confining stress, and the results were analyzed by integrating with microstructural observations. The results obtained were used, to explore the combined effects of changing pore pressure on slippage and absolute permeability. The results revealed that the stress sensitivity increases as the permeability decreases; this is because of existence of microfractures and the presence of larger pore throat radius. In addition, the effective pore size was calculated from the gas slip parameter, and at low confining stress levels, this value was in the same order of magnitude as the microfracture width. Moreover, the pore size calculated from gas slip parameters was reduced at higher stress levels, which indicated grain boundary fractures closures.