Unconfined State Research Articles

Influence of confining pressure on rubberised cement bound aggregate behaviour during static triaxial test

The aim of this research was to better understand the influence of adding waste rubber in cement bound aggregate (CBA) by testing both the unconfined and triaxial stress state. CBA with waste rubber was tested in this manner aiming to analyse influence of the confining pressure on the material behaviour. The results indicate that adding 20% rubber to a high cement content mix can assimilate the high stiffness caused by the high cement paste content, resulting in a mixture with increased cement paste content to facilitate in situ compaction. Additionally, a low confining pressure of 40 kPa has the most impact on the elastic properties/stiffness and it would be more appropriate to measure the CBA elasticity modulus with moderate lateral (confining) pressure. Finally, the practical implication of this research is that, in pavement mechanistic-empirical design, a correction of the measured uniaxial static modulus of elasticity by 10–15% should be adopted.

Geometric distribution characteristics and mechanical reinforcement effect of herbaceous plant roots at different growth periods

The growth form of herbaceous plant roots and their mechanical reinforcement effect on soil play a positive role in preventing soil erosion. Herbaceous plant roots grow with increasing growth periods, thus resulting in different root geometry and mechanical reinforcement in different growth periods. To explore the difference in root system characteristics of herbaceous plants and their effect on soil reinforcement in different growth periods, two herbaceous plants, Chenopodium album, and Setaria viridis, were taken as the research objects. First, the diameters of the two herbaceous plant roots in each growth period were measured by pixel conversion, and then the root area ratio (RAR) was calculated, and the tensile force of the roots in each growth period was measured with a tensile force meter. The measurement results were substituted into the Wu-Waldron model (WWM), and the WWM was corrected by the direct shear test. The modified WWM was used to calculate the mechanical reinforcement effect of roots at different growth periods. Second, the average root area ratio (ARAR) and the average root additional cohesion (Acr) in different growth periods of the two herbaceous plants were calculated. Finally, to characterize the reinforcement effect of herbaceous plant roots in different growth periods, FLAC3D was used to simulate the compressive strength of the root-soil composite in the unconfined compressive state. The results showed that the RAR of Chenopodium album and Setaria viridis increased with the increase of the growth period, and the standard deviation of RAR after 78d was higher than that in the previous period. The root tensile force of Chenopodium album on 14d and Setaria viridis on 21d was the smallest, and the tensile force of roots increased significantly after continued growth for 7d, and the effect of further growth was not obvious. The root additional cohesion of Chenopodium album and Setaria viridis fluctuated with increasing growth periods. The ARAR and Acr of Chenopodium album and Setaria viridis increased with the increase of the growth period and showed a significant positive correlation (p < 0.01). With the increase in the growth period, the compressive strength of the root-soil composite increased linearly. When the herbaceous plant grew to 131d, the compressive strength of the root-soil composite of Chenopodium album and Setaria viridis increased by 24% and 21% compared with that of the rootless soil. This study can provide a reference for the study of using the mechanical reinforcement of herbaceous plant roots to inhibit soil erosion.

Assembly of an iron-based complex into a metal-organic framework: a space confinement strategy for isolation of mono-iron complexes to protect from dimerization.

Non-heme mononuclear iron complexes, especially when supported by tripodal tetradentate ligands, show promising C-H bond activation efficiency in catalytic reactions. Nevertheless, they intrinsically decay readily to their dinuclear form, and the dimerization process is inevitable in homogenous solution, which dramatically hinders their further application. Hence, we demonstrate that the mononuclear iron complex [(TPA)FeII-2L]2+ (L = labile ligands, mainly solvent molecules) was successfully encapsulated in a highly robust metal-organic framework UiO-66 via a two-step "ship-in-a-bottle" strategy. The nearly perfect size matching of the octahedral cages of the host UiO-66 provides ideal space confinement for the guest complex to protect from dimerization and dramatically increases the mono-nuclear complex stability compared to its un-confined state. The successful encapsulation of [(TPA)FeII-2L]2+ in UiO-66 was verified thoroughly by spectroscopy, microscopy, N2 adsorption, and electrochemistry characterization techniques. This work shows that encapsulating an unstable molecular complex in MOFs via a two-step "ship-in-a-bottle" strategy highlights opportunities for extending the heterogenization of homogeneous complexes.

Characterization of basaltic rock aquifer parameters using hydraulic parameters, Theis’s method and aquifer test software in the hard rock area of Buchakewadi watershed Maharashtra, India

The twelve wells were selected to carry out the various test, duration of pumping (min), maximum draw drown (m), duration of recovery (min), residual drawdown, and aquifer type in the basaltic rock aquifer parameters of Buchakewadi watershed. The source and flow of groundwater are essential concerns in hydrological systems that concern both spatially and temporally components of groundwater discharge and water supply problems. The content and temperature of groundwater flowing through an aquifer might change depending on the aquifer environment. As a result, hydrodynamic analyses can provide valuable information about a region's subsurface geology. The present research attempts of aquifer variables such as transmissivity (T) and storativity (S) estimation are significant for groundwater resource development and evaluation. There are numerous approaches for calculating precise aquifer characteristics (i.e., hydrograph analysis, pumping test, etc.). A most frequent in situ analysis is a well-pumping test, which accurately measures the decline and rise of groundwater levels. During an aquifer pumping test, to characterize aquifer properties in an undiscovered location to forecast the rate of depletion of the groundwater table/potentiometric surface. The shallow, weathering subsurface water accessible above the Deccan traps in an unconfined state is insufficient to satisfy the ever-increasing pressure on water supplies. Maharashtra is similarly dominated by hard rocks, whose rainfall susceptibility is limited by weathering and primary porosity, as is their volume to store and convey water. Based on the hydraulic parameters and Theis method, results are optimized. Aquifer mapping and pumping test results can be more important for solving problems such as water scarcity, nonpolluting water, health issues, and source of fresh water on the earth surface. However, the characterization of aquifer parameters should be significant role in the scientific planning and engineering practices.

Mechanical behavior of Malan loess under unconfined compression conditions

Loess landslides result in heavy casualties and economic losses. Among them, toppling is one of the predominant failure modes in Malan loess. Loess toppling failure normally arises from deep vertical cracks, which tend to develop in the shoulder areas of loess slopes and extend to a depth of several meters. Under natural conditions, loess’ static lateral pressure coefficient is low and the loess mass within a 2 m horizontal depth from the slope surface approximates an unconfined compression stress state. In this work, we sought to further examine this unconfined compression state using acoustic emission monitoring and in situ computed tomography scans to investigate the mechanical behavior of Malan loess. Results indicated that Malan loess failed under compression-induced tension in unconfined compression condition. It exhibited the brittle failure with an abrupt release of elastic energy. The failure plane was at large angles and occurred nearly along the loading axis. This result coincided well with the vertiloess structure, which is characterized by vertically aligned strong units and weak segments. The above failure mode supports the formation of vertical cracks in the loess strata and consequently the toppling and crack sliding slope failures, as well as the formation of loess walls and columns.

Vibration-induced segregation characteristics of binary pebble beds in confined containers

This paper presents the study of the segregation characteristics of bi-disperse mixed pebble beds under different confined conditions and vibration conditions by the Discrete Element Method. The Lacey index and the λ index are used to evaluate the degree of particle mixing or segregation. Results show that confined containers can suppress the segregation of the pebble beds when the top free space height is below a threshold. Further simulation reveals that this threshold of the free space height is related to the maximum bed height increment in an unconfined state and impacted by the vibration amplitude and frequency. However, the particle diameter ratio as well as the container shape and size seem to have a limited effect on the threshold. These findings are helpful to clarify the suppression effect of container confinement on the pebble bed segregation and its main resulting factors, and hence provide solutions to controlling vibration-induced segregation.

Modelling of Irrigation Return Flow in Unconfined Aquifer of Dharoi Command in Mehsana District of Gujarat, India

Surface water flow and subsurface flow have traditionally been investigated separately and simulators have been developed over the years, to model each of these systems. Growing interest in conjunctive water management, and need for simulations of surface/subsurface flow and their interactions has lead to the linking of models of the respective domains. This study from Dharoi command area, using MODFLOW, showed pertaining to the second layer of the calibrated model. The first layer is a thin soil cover layer of about 3 m uniform thickness. Top layer is kept under unconfined water table condition, where as the next layer is given the scope of transforming between water table to confined state and vice versa depending upon the fluctuation of the computed water level with respect to the bottom boundary of the top layer. No recharge and discharge options other than a single boundary condition of average May water level in the extreme north-east grid has been allowed. This calibration is based upon matching the observed and computed average gradient of the water table (hydraulic gradient) and nothing to do with matching the point to point values of the observation wells as that could lead to misappropriation in respect of hydrologic condition. Over all gradient of the water table in the area of interest is computed as 0.23 m per km in comparison to an observed hydraulic gradient of 0.3 m per km. The rivers such as Sabarmati downstream of Dharoi dam and up to Bijapur, Rupen and Pushpawati are activated in the transient run. Conductance value is considered as 0.0001 m/sec for all the rivers. proximity of the second layer under unconfined state. Comparisons of water levels in selected locations. Overall match between the trends and the point values indicate that the calibrated model transient run can be considered as base case and various options can be studied with this model. Visual MODFLOW is very effective to know the present scenario of ground water flow, water level of aquifer, wells, also point values indicate that the calibrated model transient run can be considered as base case and various options can be studied with this model. Visual MODFLOW is very effective to know the present scenario of ground water flow, water level of aquifer, wells, also distribution of spatial recharge, specific yield distribution. It gives very close result to the observed value.

Modified Time-Dependent Model for Flexural Capacity Assessment of Corroded RC Elements

The corrosion of embedded steel bars in concrete is one of the severe deterioration processes that can escalate the risk and reduce the durability and lifetime of concrete structures. Based on the literature, simultaneous corrosion of longitudinal and transverse bar is considered to estimate section capacity. In this research, a flexible multi-stage approach is proposed to differentiate between corrosion levels on longitudinal and transverse reinforcement based on their corrosion initiating time. The result indicated that the multi-stage approach provides higher capacity and curvature for the sections. A significant reduction in reinforced concrete (RC) sections’ ductility after corrosion of confining bars and before starting corrosion of longitudinal rebars was observed, which illustrates the importance of stirrups corrosion effects on the sections’ ductility. In the higher values of corrosion intensity, confined concrete’s ultimate strain tends to reach the unconfined state. Using the energy-based method of calculating ultimate concrete compressive strain instead of approximate methods, a higher value of curvature ductility of corroded RC cross-sections is obtained.

Evaluation of the safety parameters for a permitted explosive type emulsion

When preparing a permitted explosive recipe an energy or an explosive heat is considered, which should ensure the detonability of the system and at the same time a power that satisfies the purpose for which it will be used, under the conditions of firedamp hazardous mines. The safety parameters for the explosive charges used in the firedamp hazardous mines are decisive, in order to ensure the safety and health requirements at work together with the efficient performance of the blasting operation. The permitted explosive type emulsion is recommended to be used in underground mines, open pit mines as a special methane explosive and can be used where a coal dust and/or methane explosion hazard exists can be loaded into dry and wet blasting holes and it can be used for mechanical loading. The permitted explosive type emulsion is a Detonator-sensitive explosives that can be reliably initiated in an unconfined state by a No. 8 strength detonator it have safety handling characteristics because of the relatively low sensitivity to friction, shock and impact. Technological changes due to the change of suppliers of certified explosives for civil use for underground use in the firedamp hazardous mines, involve reassessing the safety and efficiency of the loads made with these products, which have not been tested and evaluated for the conditions from the Jiu Valley mines.

A Review of mathematical models for prediction of Stress-strain and moment –curvature behaviour in concrete

In this paper, a mathematical model for predicting the stress –strain and moment curvature relations in concrete is developed. A good number of empirical equations were proposed to represent stress-strain behaviour of conventional concrete. Most of the equations can be used for the ascending portion of the curve only. In 1997 Mansur et al. have adopted Carriera and Chu (1985) model, which was based on the model proposed by Popovics (1973). As such, model proposed by Mansur et al includes both ascending and descending portions of the stress-strain curve for the confined concrete with introduction of two constants for the descending portion of the curve. Several researchers proposed various empirical equations for stress-strain behaviour as briefly reported in the previous chapter. An attempt has been made in this study to develop mathematical models for concrete in unconfined state. These analytical equations can be applied to any concrete with slight modifications. These models are developed to validate the experimental values obtained.

Wall effects in flow past a rigid stationary sphere enclosed in a rectangular channel

The effect of finite boundaries on the drag coefficient applied in a sphere enclosed in a rectangular channel has been investigated numerically. The three-dimensional Navier-Stokes equations along with continuity equation in Cartesian coordinates have been solved numerically. The investigation covers the Reynolds number range from 0.1 up to 300 and channel half width-to-sphere radius ratio R/r from low value to unconfined case. It is found that when the sphere is close to channel walls the flow is unsteady and when the sphere lies away from the walls the flow becomes steady for Re ≤ 200. For Re = 300 the flow is unsteady for confined and unconfined cases. For Re = 300, both the drag coefficient and Strouhal number decrease as the R/r ratio increases and reach a constant value at the unconfined state.

Assessment of the Effects of Lime and Cement on Geotechnical Properties of Laterites

Two different samples of soil tested as A-7-5 and A-7-6 were stabilized respectively with lime and cement. The samples were stabilized with the additives in steps of 2% and subjected to laboratory tests such as consistency limits, compaction, California bearing ratio (CBR) and unconfined compressive strength (UCS) until it reached 10% additives by weight of dry soil sample. The Proctor energy type of compaction and soaked method of CBR was adopted for the research while the compressive strength of the soils were measured in an unconfined state at days 1, 7 and 14 respectively. The plasticity index (PI) of the soil samples increase at 2% additives which later reduces with further increase as the additives contents. The soil samples attained their maximum dry densities at 6% cement and lime respectively, with cement being more effective compared to lime and the water content needed to achieve maximum dry density of the soil were higher in cement than in lime. The CBR values of the soil appreciate with increase in the additives content as indicated in the rise from 1.12% at natural to 7.26% at 6% cement content. The UCS of the lime stabilized soil improved better than that of cement stabilized soil with respect to the age of curing. Keywords- Stabilization, Lime, Cement, Soil, Laterite.

Stress-strain relationship of cement mortar under triaxial compression

This paper presents a study on the stress-strain relationship of cement mortar under triaxial stress state. The influence of confining pressure on peak-stress, peak-strain, volume change, modulus of elasticity, and Poisson’s ratio were studied using experimental results and data from limited studies in the literature. Cement mortar samples having compressive strengths of 38 MPa and 45 MPa were tested under four levels of confining pressures (i.e., 0, 5, 10, and 15 MPa) and a stress-strain model for confined cement mortar was subsequently developed. It has been shown that the model has good agreement with both the experimental results. The results not only show that confinement significantly improves the strength and ductility of the cement mortar samples, but also reveal a linear relationship between the observed lateral and axial strain. In this regard, the mathematical relationship for confined cement mortar was developed as a function of the stress and strain parameters of the unconfined state and a new linear relationship between modulus of elasticity, Poisson’s ratio and lateral stress is proposed.

An Empirical UCS Model for Anisotropic Blocky Rock Masses

The Hoek–Brown (HB) failure criterion is one of the most widely used failure criteria in rock engineering. Based on the Geological Strength Index (GSI) system, a number of empirical models have been proposed in parallel with this criterion to estimate the strength and deformation properties of rock masses such as uniaxial compressive strength (UCS) and deformation modulus. However, the GSI system does not incorporate the effects of joint orientation β on the quality of a rock mass. This means that these empirical models cannot capture anisotropic rock mass strength caused by joint orientations. In this research, UDEC rock mass models, which are calibrated by laboratory data, are used to investigate the effects of joint orientation on rock mass strength in an unconfined state. The values of UCS obtained from the numerical simulation are then compared with those calculated from traditional empirical UCS models based on the GSI system. The comparison study shows that the value of UCS is significantly overestimated by the traditional empirical model when 10° < β < 45°, which will have serious safety implications for engineering designs. To rectify the problem, based on the analysis of numerical simulation results, an anisotropic weighting factor fβ is proposed to be used to refine the empirical UCS model. The modified UCS model is demonstrated to be capable of giving conservative but more accurate prediction of the rock mass strength for various joint orientations, which will result in more optimal and safer engineering designs.

Confinement-Induced Alteration of Morphologies of Oil-Water Emulsions.

Reversible alteration between different emulsion morphologies like core-shell and Janus is conventionally triggered by altering the interfacial energy between different phases. In contrast, here, we show that the morphology of dispersed droplets can be changed also when the emulsion is sufficiently confined between two parallel plates. In particular, we use three immiscible phases: silicone oil, paraffin oil, and aqueous solution of surface-active agents like agarose, sodium dodecylsulfate, dioctyl sodium sulfosuccinate, and cetyl trimethylammonium bromide to generate oil-in-water emulsions consisting of complex morphologies of the dispersed droplets. In the unconfined state, the core-shell drops appear with paraffin oil at the core and silicone oil at the shell. However, the morphology of oil droplets changes to Janus when the emulsion is confined between two parallel plates. We have shown that the meniscus of the continuous phase that forms between the parallel plates alters the pressure field in the emulsion and the total energy of the system, which trigger such morphological transition.

Quantitative Analysis of Micro-structural Changes in a Bituminous Coal After Exposure to Supercritical CO2 and Water

High-volatile bituminous coal samples were reacted in deionized water with supercritical CO2 (ScCO2–water) under simulated in situ pressure and temperature conditions (8 MPa and 40 °C) in unconfined stress state for 14 days, in order to characterize potential CO2–water–coal reactions. Micro-structural changes were identified pre- and post-experiment using X-ray powder diffraction (XRD) analysis for powdered coal (mineralogical changes), optical microscopy and scanning electron microscopy (SEM) for polished thin sections (surface feature changes) and micro-CT scanning for a small core (porosity and permeability changes). XRD analysis revealed that carbonic acid leaches out mineral matters in coal, including carbonates (calcite) and silicate minerals (albite, illite and kaolinite). Optical microscopy, SEM and CT images confirmed that the interaction of coal with ScCO2–water causes an abundance of micro-cracks to open or propagate in unconfined coal samples. Most micro-cracks preferably propagated along maceral–mineral and maceral–maceral interfaces, which demonstrates that the micro-cracking was caused by differential swelling of different coal lithotypes. Wormhole formation was observed in coal caused by mineral dissolution and hydrocarbon mobilization, which significantly increases coal porosity compared with swelling-induced cracking. 3-D pore network models extracted from CT images show that ScCO2–water treatment enlarges the pore and throat size, increases the numbers of pores and throats and improves pore network connectivity. Overall, CO2–water–coal interactions under unconfined conditions enhance coal porosity, connectivity and permeability, which can be attributed to the combined effect of micro-cracking, mineral dissolution and hydrocarbon mobilization.

CO2 sorption induced damage in coals in unconfined and confined stress states: A micrometer to core scale investigation

Gas sorption in coal, in particular CO2, is known to cause swelling and internal structural change or damage. For instance, CO2 adsorption on coal causes swelling which in turn can close the fractures or cleats whereas its desorption can open pre-existing fractures as well as create new fractures, thus enhancing gas flow paths in coal seams. Gas release and the associated induced damage are relevant for several applications such as coal burst in coal mining operations and for geological sequestration of CO2 into coal seams to reduce both the carbon footprint and increase the efficiency of energy extraction. However, the sorption-induced internal structural damage in coal, in particular the link between the micro-scale damage phenomena to macro-scale processes in confined and unconfined stress states, is not yet fully understood.We carried out a series of experiments to study the potential sorption-induced damage in both confined and unconfined stress states at micro (μm) and macro (cm) scale. 3D images, obtained by X-ray microcomputed tomography (micro-CT) technique, showed that in the unconfined stress state, adsorption of CO2 closes some pre-existing fractures while new fractures form in the specimen of coal. This is well supported by high pressure CO2 adsorption analyses that were independently conducted. After CO2 release, the overall fracture intensity, defined as the area of fractures per volume of rock mass, was considerably increased by opening both pre-existing fractures together with new fractures. In the confined condition, the gas adsorption was significantly lower confirming the closure of the initial fractures by swelling without creation of induced fractures. However, the gas release under shearing stress was significantly higher showing that the shearing stress causes further damage assisted by gas release and its induced micro-structural damage.

Analysis-oriented stress–strain model of CRFP-confined circular concrete columns with applied preload

The compressive behavior of FRP-confined concrete is a current issue in the field of structural retrofitting. The available models well predict the stress–strain behavior under monotonic and cyclic loads. However, in the practical applications, columns that need an increasing of bearing capacity are often strengthened under serviceability load conditions, with a stress and strain state that could change the response of the reinforced systems with respect to the case of the unloaded state. In this paper, the compressive behavior of circular FRP-confined concrete columns with preload is analyzed with the introduction of a modified analysis-oriented model. Differently from the classical formulations in which stress–strain model is aimed to the evaluation of the confinement capacity of non-preloaded elements in monotonic regime, the proposed model is also suitable for the determination of the combined response of unconfined and confined concrete subjected to an established stress/strain state in the unconfined state. The proposed model is also compared with the experimental results available in the literature under different assigned preloading levels.

Exploring the effect of confinement on water clusters in carbon nanotubes.

Using armchair-type single-walled carbon nanotubes (SWCNTs) of different sizes as model compounds for lignite, the effect of water molecule confinement on the water-holding capacity of lignite pores was investigated. Results indicated that the water-holding capacity of pores with diameters of <10nm was eight times larger than that of pores with diameters of 100nm. The configuration of the cluster of water molecules in each SWCNT and the binding energy between each SWCNT and the water molecules within it were calculated by means of density functional theory using a hybrid functional: M06-2X/6-311+G**, 6-31G*. The results prove that the configurations of the water molecules in the SWCNTs are very different to their configuration in the unconfined state. In vacuum, the cluster of three water molecules adopted a trimer configuration, while they presented a linear configuration in the 6.78Å SWCNT. Similarly, in vacuum, the cluster of five water molecules formed a five-membered ring, while they favored a linear configuration in the 6.78Å SWCNT, a zigzag configuration in the 8.14Å SWCNT, and a trimer + 1 + 1 configuration (i.e., a trimer plus two isolated water molecules) in the 9.49Å, 10.85Å, and 13.75Å SWCNTs. There was found to be a degree of competition between the coupling energy of the water molecules with the SWCNT and the hydrogen bonding among the water molecules. When the diameter of the SWCNT was >1nm, the hydrogen bonding among the water molecules dominated, while the coupling energy of the water molecules with the SWCNT amounted to only 30-40% of the total interaction energy of the water molecules. Graphical Abstract Computed equilibrium structures of five water molecules confined in SWCNTs with diameters of 6.78Å, 8.14Å, 9.49Å, 10.85Å, and 13.75Å, and in vacuum.

Characterizing coal cleats from optical measurements for CBM evaluation

In this study we apply fracture characterization techniques, common to many other reservoir rocks, to coals. Additionally, the data gathered from this technique is used to determine fracture permeability and porosity based on a cubic relationship between permeability and porosity, as a function of aperture and spacing. However, these data are in an unconfined stress state. To relate this information to the subsurface, which is in a confined state, the results are then compared and calibrated to permeability data determined from laboratory triaxially confined samples, using the same sample material. A 1D scanline technique is used to gather data on cleat aperture, spacing, height, and frequency. Sixteen different coals from several European basins were examined, creating a large dataset (>8000 individual data measurements). This data was gathered from blocks of coals and from polished sections. The polished sections were digitally scanned under the microscope to create an image of the whole section. As a result, we examined a range of scales from micrometers to centimeters. Maximum, minimum and median values are determined for each attribute. To clearly define the limits of resolution of the data, a practical pore resolution is applied to a power law function of the data. Cleat attributes were further examined to determine a relationship with cleat type, e.g., face or butt, and lithotype. Power law distributions of butt cleat attributes are similar to the distributions found in lithotype. It was concluded that butt cleats are more strongly related to lithotype, and thus bed thickness and mechanical strength of the bed, than face cleats. Cleat aperture and spacing depend on cleat type. A face to butt cleat ratio of 5:1 given an aperture of about 12μm was found. This is similar to a published anisotropy in permeability, where face cleats are five times more permeable than butt cleats. Cleat porosity from aperture and spacing is several times greater than cleat porosity calibrated to laboratory permeability (under stress). The calibrated porosity ranged from 0.01% to 1.8% and agrees with published values of in-situ cleat porosity. Thus aperture width due to an increase in stress is predicted to decrease four times. This data can be used to calibrate reservoir simulators for CBM production.