Loss Of Post-peak Strength Research Articles

Investigation of constitutive properties of high plasticity clay soils mixed with crushed rubber tire waste

Addressing the enormous waste resulting from discarded worn rubber tires is an environmental challenge. Recycling and using crushed rubber tire waste (CRTW) in construction materials can help in addressing this challenge. This study investigates the effect of addition of CRTW on the engineering properties of high plasticity clay soils (HPCS). There is a paucity of research in the application of CRTW in HPCS. This research tries to fill this research gap. Specifically, this study seeks to investigate the effect of mixing CRTW on the constitutive properties of HPCS. After identifying a locally available HPCS, mixtures of the clay and several percentages (0%, 6%, 12%, 18%, and 24%) by weight of CRTW were prepared. A range of CRTW shapes and sizes were investigated. Three different particle shapes of CRTW (granular rubber, rubber chips, and rubber fiber), and two particle sizes (fine and coarse) were studied. The parameters studied included unconfined compressive strength (UCS), strain at failure, post-peak strength loss (PPSL), modulus of elasticity, failure modes/mechanisms, repeatability of tests results, and examination of CRTW particles and mixtures via binocular and SEM microscope. Our findings unveiled that the highest level of repeatability was observed in granular CRTW, with a maximum variability of only 5%. Moreover, the mixtures containing granular CRTW exhibited, on average, 10% and 15% higher strength and modulus of elasticity, respectively, in comparison to mixtures incorporating other shapes of CRTW. In general, the HPCS-CRTW mixtures displayed higher shear strains, averaging 25% greater than pure HPCS. Furthermore, the addition of CRTW to HPCS resulted in a reduction of its PPSL and a transition in behavior from brittle to slightly ductile. Examination of failed specimens revealed the existence of two primary failure modes: shear plane failure and shear plane failure accompanied by multiple vertical cracks within the mixtures. These results suggest that the utilization of granular CRTW in HPCS can improve certain properties of HPCS. However, it is advisable to limit the rubber content in this mixture to 6% to mitigate significant adverse effects on its strength.

Impact of directly accounting for post-peak strength loss on the dynamic response of a soil column during earthquake loading

Impact of directly accounting for post-peak strength loss on the dynamic response of a soil column during earthquake loading

Back-analyses of soft soil failure with "strain-softening" behavior by the "equivalent sensitivity" concept

Stability analyses of slopes in soft soils are usually affected by strain-softening, resulting in unrealistic (unconservative) safety factors. The loss of post peak strength cannot be accounted for by classic limit equilibrium analyses. In practice, however, the overall loss of soil strength is generally approximated by Bjerrum correction factor μ ≤ 1, which is believed to account for the different failure velocities during the field tests (usually vane tests) and the actual failure in the field, in addition to anisotropy (Schnaid & Odebrecht, 2012). The objective of this work is to demonstrate that strain-softening reduces the overall safety factor to a value nearly equivalent to the application of Bjerrum’s correction factor. To accomplish this, a simple constitutive model (Mohr-Coulomb with residual stress) is used for total stress finite element analyses by means of the concept of “equivalent sensitivity” proposed by Pereira Pinto (2017). The results showed that equivalent sensitivity can be a great instrument to simulate the strain-softening behavior of soft soils.

GMX/GDC strength loss mechanisms

This paper provides insight into the causes of post-peak strength loss for textured geomembrane (GMX) and nonwoven geotextile (NGT) interfaces. The NGT can be part of a geosynthetic drainage composite (GDC) or a stand-alone NGT. The study used ring shear tests where one of the two interface materials was replaced after reaching a residual strength condition and the test was restarted to measure the change in interface strength. The interface strength loss from peak to large displacement (LD) strength primarily comes from three mechanisms: (1) geomembrane wear, (2) breakage and combing of fibers in the NGT, and (3) reduction of the hook and loop effect between GMX asperities and fibers of the NGT. The source of interface strength loss from LD strength to the residual value mainly comes from breakage and continuous combing of NGT fibers parallel to the direction of shear in ring shear tests. Scanning electron microscopy photographs of the GMX and NGT before and after shearing confirm wear and smoothing of GMX asperities and the combing of NGT fibers in the direction of shear.

Strength and deformation behaviors of cemented tailings backfill under triaxial compression

It is of great significance for safety reason to obtain the triaxial compressive properties of cemented tailings backfill(CTB). The influence of cement content, curing age and confining pressure on strength and deformation properties of CTB was examined and discussed. Results indicate that the triaxial compressive and deformation behavior of CTB is strongly affected by the cement content, curing age and confining pressure. The increase in cement content, curing age and confining pressure leads to a change in stress-strain behavior and an increase in the axial strain at failure and post-peak strength loss. The cohesion of CTB rises as the curing age and cement content increase. However, the enhancement in internal friction angle is trivial and negligible. It should be noted that the failure pattern of CTB samples in triaxial compression is mainly along a shear plane, the confining pressure restrains the lateral expansion and the bulging failure pattern is dominantly detected in CTB samples as curing age length and cement content increase. The results will help to better understand the triaxial mechanical and deformation behavior of CTB.

Behaviour of fibre-reinforced cemented sand under flexural tensile stress

In soil improvement, the insertion of cement to the soil matrix provides an increase in strength and rigidity of the material, whereas fibre addition provides an increase in ductility, reducing post-peak loss in strength. This research aims to experimentally analyse the mechanical behaviour of unreinforced and fibre-reinforced cemented sands under flexural tensile monotonic loading conditions. In order to do so, an experimental program was planned using silty sand (Arenito Botucatu), early strength Portland cement – CPIII, and polypropylene fibres 24 mm long and 0.023 mm thick. Three different dry unit weights (γd) (18, 19 and 19.7 kN/m3), three cement contents (3, 5 and 7%), and two fibre contents (0 and 0.5%) were chosen. Analysing the results, an increase in cement content generates an increase in flexural strength (qf) for all concentrations and the increase in γd generated smaller qf gains, especially for lower cement contents. The addition of fibres generated a qf increase for lower γd but at higher γd there was a reduction in the influence of fibre addition on strength gain. The reduction can be attributed to an entanglement of the fibres in higher compaction strengths, disrupting the formation of cementitious bonds. However, based on a qualitative assessment, there was a noticeable reduction in post-peak strength loss due to fibre insertion. The parameter porosity/volumetric cement content (η/Civ0.28) was considered suitable to predict the qf mechanical behaviour of the studied composite.

Strength enhancement of geotextile-reinforced carbonate sand

The mechanical behavior of carbonate sand reinforced with horizontal layers of geotextile is invetigated using a series of drained compression triaxial tests on unreinforced and reinforced samples. The main factors affecting the mechanical behavior such as the number of geotextile layers, their arrangement in specimens, confining pressure, particle size distribution, geotextile type and relative density of samples were examined and discussed in this research. To make a precise comparison between the behavior of reinforced siliceous and carbonate sand, triaxial tests were performed on both types of sands. Results indicate that geotextile inclusion increases the peak strength and strain at failure, and significantly reduces the post-peak strength loss of carbonate specimens. The amount of strength enhancement rises as the number of geotextile layers increases while two other parameters including confining pressure and particle size affect adversely. The strength enhancement of reinforced carbonate sand is greater than the corresponding siliceous sample at high axial strains. Reinforced and unreinforced carbonate specimens exhibit more contractive behavior than their corresponding siliceous samples and tend to dilate at higher axial strains. By increasing the relative density of the samples, the peak strength of reinforced specimens rises due to enhanced interlocking between geotextile layers and sand particles. This process continues as long as the geotextile is not ruptured. The utilization of geotextiles with high mass per unit areas was found to be uneconomical due to slight differences between the strength augmentation of geotextiles with high and low mass per unit areas. It should be noted that geotextile layers limit the lateral expansion of specimens which leads to changing the failure pattern from a shear plane to bulging between the adjacent layers of geotextile.

Strength Behaviour of Clayey Soil Mixed with Pond Ash, Cement and Randomly Distributed Fibres

An experimental program was undertaken to study the effect of inclusion of pond ash and randomly distributed polypropylene fibres on the strength behaviour of cement-stabilized clayey soil. In the present investigation, 19 combinations of fibre-reinforced cement-stabilized clayey soil-pond ash mixes at three different percentages of pond ash (i.e. 10, 20 and 30%) and two different percentages of polypropylene fibres (i.e. 0.5 and 1%) of length 6 and 12 mm with 5% cement content were prepared. The combinations of clayey soil-cement-pond ash-fibres were subjected to a series of standard proctor test, unconfined compressive strength and split tensile strength tests. Based on favourable test results obtained, it can be concluded that partial replacement of cement-stabilized clayey soil by pond ash with the inclusion of polypropylene fibres causes a significant increase in strength, decrease in the stiffness, and rate of loss of post-peak strength. The fibre inclusion changed the brittle behaviour of the composite to ductile. The inclusion of 1% fibres of 6 mm length or 0.5% fibre of 12 mm length has the same impact on the unconfined compressive strength and split tensile strength of the mixtures. The study demonstrates that pond ash can be used successfully as partial replacement of the clayey soil by using reinforcement and cement.

Effect of waste rubber fibres on the geotechnical properties of clay stabilized with cement

The voluminous generation of discarded waste rubber tires of about 1.5 million per year has created an aggravating situation across the globe because it exerts the deleterious effect on the human health and ecological systems. This paper focuses on the influence of inclusion of waste rubber tire fibres on some of the geotechnical properties of uncemented/cemented clay. For this investigation, three percentages of cement (0%, 3%, and 6%) and five percentages of rubber fibre (0%, 2.5%, 5%, 7.5%, and 10%) were considered. The tests namely, compaction, unconfined compressive strength, split tensile strength, California bearing ratio, swelling pressure, wet/dry cycles durability along with the scanning electron microscopy were conducted on the clay-cement-rubber fibre mixtures to ascertain the suitability of rubber fibres with cement stabilized clay. The test results revealed that the incorporation of rubber fibre reduces the unconfined compressive strength and split tensile strength of cement-stabilized clay but prosperously improves the rate of loss of post-peak strength and change the brittle failure behavior of cemented clay to ductile. The results of California bearing ratio test showed that the resistance against the penetration of the cement stabilized clay decreases as the content of rubber fibre increases. The swelling pressure of the clay was found to decrease with the increase in cement and rubber fibre content. The encouraging results of wet/dry cycles durability test proves the propriety of other tests on clay-cement-rubber fibre mixtures. Based on the test results, the maximum percentage of rubber fibre content mixed with cement-stabilized clay was found to be 7.5%. The study has shown that clay-cement-rubber fibre mixtures containing 6% cement and upto to 7.5% rubber fibre can be use as fill material, backfill material for retaining wall, sub-base of low volume traffic roads, and side slope of the canal.

Unconfined compressive strength and freeze-thaw resistance of sand modified with sludge ash and polypropylene fiber

In recent years, the amount of sludge ash (SA) has considerably increased due to rapid urbanization and population growth. In addition, its storage in landfills induces environmental pollution and health problems. Therefore, its disposal in an environmentally friendly way has become more important. The main goal of this study is to investigate the reusability of sludge ash as an additive with polypropylene fiber (PF) to stabilize marginal sand based on the compressive strength performances from UCS tests. For this purpose, a series of UCS tests was conducted. Throughout the experimental study, the used inclusion rates were 10, 15, 20 and 30% for sludge ash and 0, 0.5 and 1% for polypropylene fiber by total dry weight of the sand+sludge ash mixture and the prepared samples were cured for 7 and 14 days prior to the testing. Freezing and thawing resistance of the mixture including 10% sludge ash and 0, 0.5 and 1% polypropylene fiber was also examined. On the basis of UCS testing results, it is said that sludge ash inclusion remarkably enhances UCS performance of sand. Moreover, the addition of polypropylene fiber to the admixtures including sand and sludge ash significantly improves their stress-strain characteristics and post-peak strength loss as well as UCS. As a result of this paper, it is suggested that sludge ash be successfully reused with polypropylene fiber for stabilizing sand in soil stabilization applications. It is also believed that the findings of this study will contribute to some environmental concerns such as the disposal problem of sludge ash, recycling, sustainability, environmental pollution, etc. as well as the cost of an engineering project.

Behaviour of cement stabilized treated coir fibre-reinforced clay-pond ash mixtures

In a comprehensive laboratory study, different tests namely, standard Proctor tests, unconfined compressive strength (UCS) and split tensile strength (STS) tests were performed for evaluating the compaction and strength behaviour of the cement-stabilized, and sodium hydroxide treated coir fibre reinforced clay-pond ash mixtures. The treated coir fibres and pond ash of 0.5–1.5% and 10–30%, respectively, were added to the stabilized cement (2% and 4%) clayey soil. A series of compaction tests were carried out for studying the compaction characteristics. The cylindrical specimens of dimension 38.1 and 76.2mm were prepared at desired densities and cured for 7, 14, and 28 days, after which they were, subjected to a series of unconfined compressive strength and split tensile strength tests. The investigation showed that the dry unit weight of the mixtures decreases and water content increases with the addition of pond ash and fibres. The inclusion of fibres and pond ash in the cementitious clay caused an increase in the unconfined compressive strength, split tensile strength and axial strain at failure. Compared with the clayey soil, the incorporation of pond ash (10%) and treated coir fibres (1%) in the cement-stabilized clay with 4% cement content improved the unconfined compressive strength and split tensile strength, i.e. 3.72 and 3.83 times growth in unconfined compressive strength and split strength respectively. The incorporation of treated coir fibre reduced the stiffness, rate of loss of post-peak strength and changed the behaviour of the composite from brittle to ductile.

Effects of Discrete Short Polypropylene Fibers on Behavior of Artificially Cemented Kaolinite

To meet construction demands, reinforcement and stabilization methods have been widely used to improve properties and mechanical behavior of clays. Although cement stabilization increases soil strength, at the same time reduces ductility which is of paramount importance in roads, landfill covers, etc. In the current study, kaolinite was stabilized with 1, 3, and 5 % cement and mixed with 0.05, 0.15, 0.25, and 0.35 % polypropylene fibers to increase ductility. Samples were cured at 35 °C for 1, 7, and 28 days and subjected to unconfined compression tests. Results showed that the inclusion of discrete fibers to uncemented and cemented kaolinite reduced stiffness and the loss of post-peak strength and changed brittle behavior of cemented samples to a more ductile behavior. Cement and fiber contents as well as curing period were found to be the most influential factors, and fiber–soil interaction was influenced by binding materials.

Behavior of cement-stabilized fiber-reinforced pond ash, rice husk ash–soil mixtures

In order to study the effect of addition of rice husk ash, pond ash, cement and fiber on the compaction and strength behavior of clay, a series of tests were performed. Modified proctor tests were conducted to evaluate the compaction behavior, while unconfined compression tests (UCS) and split tensile strength (STS) tests were conducted to evaluate the strength properties of clay. For testing purpose specimens were prepared with different amount of admixtures. Pond ash (PA) and rice husk ash were added to clayey soil at ranges of 30–45% and 5–20%, respectively. Polypropylene fibers of length 6 mm and 12 mm are used in this study with different contents as 0, 0.5, 1.0 and 1.5% by dry weight of mix whereas cement contents used are 0, 2 and 4%. To study the effect of curing on strength property, the specimens were cured for 7, 14, and 28 days and tested. Test results have shown that the maximum dry density decreases and optimum moisture content increases with the addition of admixtures, but fiber inclusion have marginal effect. The results shows that the inclusion of fiber reinforcement within un-cemented and cemented soil caused an increase in the UCS, STS and axial strain at failure, decreased the stiffness and the loss of post-peak strength, and changed the cemented soil brittle behavior to a more ductile one. Strength of clay improves by the addition of admixtures and fibers. The study has shown that combination of clay, RHA, PA, cement and fiber can be used as light weight fill material in different structures like embankment, retaining wall etc.

Compressive behaviour of fibre-reinforced cemented paste backfill

Reinforcement of cemented paste backfill (CPB) with polypropylene fibres was investigated as a way of improving the stability of backfilled underground mine stopes. A series of unconfined compressive strength (UCS) tests were carried out on both non-reinforced and fibre-reinforced cemented tailings. Sandy silt tailings from a nickel mine in Western Australia were used in the study. Ordinary Portland cement at concentrations of 3–5% by weight of tailings and 0–0.5% Adfil-Ignis polypropylene fibres by weight of total solids were used for specimen preparation. The stress–strain curves from the UCS tests showed the inclusion of fibres increased the UCS and significantly reduced the post peak strength loss. Accordingly, the fibre-reinforced specimens were found to be much more ductile than unreinforced specimens, which is highly desirable in many backfill applications. Sliced images acquired from X-ray computed tomography (CT-scan) demonstrated that the observed ductile behaviour of reinforced specimens could be explained by the restraint to crack growth provided by the mobilised fibre tensile strength. At large strains, fibre-reinforced specimens had virtually zero dislodged fragments and retained their integrity as shown in both experimental photos and CT sliced images. This was different from unreinforced specimens, which developed large, wide cracks that resulted in fracturing of the tested specimens. The potential for improving the self-supporting capacity of the fill mass using fibre reinforcing but less cement is discussed and potential advantages, such as reduced ore dilution when excavating adjacent stopes are discussed.

Experimental Investigation of Mechanical Properties of Sands Reinforced with Discrete Randomly Distributed Fiber

The addition of cementitious admixtures and/or inclusion of fibers are frequently used in practice to stabilize soils and to improve their mechanical properties. In this study, ring shear tests were conducted to investigate mechanical properties such as shear strength, angle of friction and cohesion values of randomly distributed discrete fiber-reinforced sand mixtures. The length and aspect ratio of the fibers used in the current study were 12 mm and 120, respectively. Specimens were prepared at four different fiber ratios (0.1, 0.3, 0.6, and 0.9 % by weight of sand). A series of ring shear tests were carried out on sand alone and fiber-reinforced sand mixtures at different normal stresses. The test results indicated that the addition of fiber had a significant effect on the shear strength of the sand. Shear stress of the unreinforced sand increases 1.29–2.32, 1.16–1.39, and 1.07–1.5 times at a normal stress of 50, 150, and 250 kPa, respectively with fiber inclusion. Fiber content had positive effects on improving the shear strength parameters (angle of internal friction and cohesion) of the mixtures. The cohesion and angle of internal friction of fiber-reinforced sand prepared at different ratios of fiber increased by 5.3–27.4 kPa and 2.0°–7.3° respectively. The inclusion of fibers improves the ductility of the soil by preventing the loss of post-peak strength.

Effectiveness of Two Sequential Mixing of Lime -Portland Cement in Stabilizing Expansive Soils

The existence of expansive soils in the study area that undergo swelling and shrinkage when subjected to moisture variations has caused major deterioration and distresses on lightweight construction such as shallowly founded structures ad roads founded on them. Unfortunately, the treatment of these highly plastic clay soils has attracted minimal attention despite the damages they are able to induce. Stabilization of highly plastic clay soils with lime or cement alone is generally not satisfactory. While significant increase in workability and adequate strength can be achieved by lime treatment, the process is characterised by brittleness. On the other hand, soil treatment with cement alone can somehow increase strength but with the danger of leaving pockets of uncemented material in the modified mass. Therefore, substantial increase in both unconfined compressive strength (UCS) and tensile-split strength can be achieved through a two sequential mixing operation whereby soil brittleness is reduced by providing smaller loss of post-peak strength. The operation is carried out by firstly treating the soil with lime, leaving it to mellow for some hours to ensure adequate workability and then secondly treating it with cement to obtain lime-cement treated soil with adequate strength and durability. The analysis of test results of the two sequential lime-cement treated soils showed markedly improvement in the unconfined compressive strength at a large strain when highly plastic soils were first treated with lime followed with cement treatment. This indicates that the use of two-sequential lime-cement soil stabilization is the cost effective techniques that effectively improves the engineering properties of expansive problematic soils.

Unconfined Compression Strength of Reinforced Clays with Carpet Waste Fibers

This paper presents results of a comprehensive investigation on the utilization of carpet waste fibers in reinforcement of clay soils. Effects of adding proportionate quantities of two different types of shredded carpet waste fibers to clay soils (i.e., 1, 3, and 5% by dry weight of the soil) were investigated and evaluated. The investigation was conducted on specimens prepared at their maximum dry unit weight and optimum moisture content, as well on specimens prepared at variable conditions of dry unit weight and moisture content. A comparison was also made on specimens prepared at the same fiber content by changing dry unit weight while moisture content was kept unchanged or by changing both dry unit weight and moisture content. The investigation revealed that inclusion of carpet waste fibers into clay soils prepared at the same dry unit weight can significantly enhance the unconfined compression strength (UCS), reduce postpeak strength loss, and change the failure behavior from brittle to ductile. The results also showed that the relative benefit of fibers to increase the UCS of the clay soils is highly dependent on initial dry unit weight and moisture content of the soil. Failure patterns were gradually transformed from the apparent classical failure for unreinforced soil specimens to barrel-shaped failures for reinforced specimens at 5% fiber content.

Influence of Plastic Waste Fibers on the Strength of Lime-Rice Husk Ash Stabilized Clay Soil

A study has been undertaken to investigate the strength of stabilized clay-soil reinforced with randomly distributed discrete plastic waste fibers by carrying out unconfined compressive strength and tensile-split strength test. In this study, the clay soil was stabilized with lime and rice husk ash mixtures. The effect of the fiber length and content on the compressive and split tensile strength was investigated. The laboratory investigation results show that inclusion of the plastic waste fiber increased significantly both the unconfined compressive strength and tensile-split strength of the stabilized clay soil. The fiber length plays a significant contribution in increasing the soil strength. To contribute for any significant improvement on compression as well as tensile strength, the fiber length should be in range of 20 mm to 40 mm. Fiber reinforcements also reduced soil brittleness by providing smaller loss of post-peak strength.

Strength and mechanical behavior of short polypropylene fiber reinforced and cement stabilized clayey soil

An experimental program was undertaken to investigate the effects of discrete short polypropylene fiber (PP-fiber) on the strength and mechanical behavior of uncemented and cemented clayey soil. In the present investigation, 12 groups of soil samples were prepared at three different percentages of PP-fiber content (i.e. 0.05%, 0.15% and 0.25% by weight of soil) and two different percentages of cement content (i.e. 5% and 8% by weight of soil), and unconfined compression and direct shear tests were carried out after 7-, 14- and 28-day curing periods. The test results indicated that the inclusion of fiber reinforcement within uncemented and cemented soil caused an increase in the unconfined compressive strength (UCS), shear strength and axial strain at failure, decreased the stiffness and the loss of post-peak strength, and changed the cemented soil's brittle behavior to a more ductile one. The interactions at the interface between fiber surface and soil matrix were analyzed by using scanning electron microscopy (SEM). It is found that the bond strength and friction at the interface seem to be the dominant mechanism controlling the reinforcement benefit. The behavior at the interface in fiber-reinforced uncemented soil was different from that in fiber-reinforced cemented soil. The micromechanical properties of fiber/matrix interface were influenced by several factors, e.g. binding materials in soil, normal stress around the fiber body, effective contact area of the interface and fiber surface roughness, etc.

A study on bearing capacity of randomly distributed fiber-reinforced sand fills overlying soft clay

Laboratory California Bearing Ratio (CBR) tests were performed to investigate the load–penetration behavior of sand fills reinforced with randomly distributed discrete fibers overlying soft clay. The effect of fiber reinforcement content on bearing capacity, stiffness and ductility of the fiber-reinforced sand fill–soft clay system was determined. The test results indicated that adding fiber inclusions in sand fill resulted in an appreciable increase in the peak piston load. The reinforcement benefit increased with an increase in fiber content. However, the initial stiffness of load–penetration curves was not significantly affected by fiber reinforcement. The penetration value at which the piston load was the highest tended to increase with increasing fiber reinforcement content. In addition, the test results showed that increasing fiber reinforcement content could increase the brittleness of the fiber-reinforced sand fill–soft clay system providing higher loss of post-peak strength.