Fiber-reinforced Soil Research Articles

The assessment of innovative geotechnical slope repair techniques

Work to evaluate the effectiveness of innovative geotechnical slope repair techniques used on UK national highways is summarised. The techniques assessed were the planting of live willow poles, electrokinetic geosynthetics (EKG) and fibre-reinforced soil (FRS). These were used instead of conventional approaches to reduce the overall impacts of various challenges, including environmental constraints (habitat and visual) and access and utility constraints, and to reduce the scale and/or cost of traffic management and traffic delays. Trials have been undertaken over the last 20 years or so, but monitoring was generally limited to just a few years post-construction; longer term evaluation was not generally undertaken. The evaluated success – or otherwise – of the techniques led directly to recommendations for future use, ranging from the development of live willow pole design guidance and specification information, guidance on further EKG trials and cessation of the use of FRS, primarily on environmental grounds but also acknowledging the construction difficulties encountered. Life cycle analysis revealed that each technique provides significant carbon dioxide savings compared with rock fill replacement. More generic lessons learnt from the trials and the practical applications reported were used to produce guidance for future trials of innovative geotechnical repair techniques, including those for slopes.

Effect of Nano Additive on Mechanical Properties of Natural Fiber Reinforced Soil

ABSTRACT Organic soil is a poor bearing medium and is susceptible to uncharacteristic volume change and settlement behavior. Geotechnical design of substructures in organic soil is often of high economic value to account for its weak geotechnical properties. Fiber reinforcement improves the strength of the soil. However, fibers act as a mechanical reinforcement in soil mass rather than enhancing the intrinsic nature of the soil. But chemical additives alter its behavior, thereby improving its strength. This study investigates the strength and hydraulic conductivity (HC) of low plastic organic silt reinforced with banana fiber and nano-silica. Banana fiber was added in dosages of 0.25%, 0.50%, 0.75% and 1%. The optimum fiber dosage was 0.75%. Nano-silica was added to the 0.75% banana fiber reinforced soil at dosages of 0.2%, 0.4%, 0.6% and 0.8% to improve the strength of the reinforced soil. The effect of aging on HC and unconfined compressive strength (UCS) was studied for a period of 90 d. Results showed that nano-silica and banana fiber beneficially complemented each other in the enhancement of geotechnical properties. Nano-silica increased the strength of fiber-reinforced soil, and banana fiber averted the brittle failure nature imposed by the nano-silica. Also, it improved drainage characteristics through aggregation action.

Performance Evaluation of Lime, Cement and Alkali-Activated Binder in Fiber-Reinforced Expansive Subgrade Soil: A Comparative Study

Abstract Expansive subgrade soils show early distresses causing distortion and premature failure to pavement structures. In the present study, an attempt is made to enhance the subgrade strength characteristics of the expansive soil by treatment with an alkali-activated binder (AAB) and reinforcement with four types of fibers (polypropylene [PF], glass, hemp, and coir fiber [CF]). The research also compares the effectiveness of fiber reinforcement in geopolymerized alkaline soil with the traditional lime and cement binders. AAB is produced by the reaction in an alkaline activator of sodium silicate and sodium hydroxide with aluminosilicate precursors (Class-F fly ash and slag) by maintaining a 0.4 water-to-solid ratio. The effect of varying dosages of fibers and slag/fly ash ratio in the alkaline, lime, and cemented soil mixture on unconfined compressive strength (UCS), split tensile strength (STS), and flexural and penetration resistance is evaluated through a series of geomechanical and microstructural characterization tests. California bearing ratio (CBR) and flexural strength (Sf) are chosen as a subgrade strength performance indicator for fiber-reinforced soil. It is observed that PF-AAB soil shows higher interfacial bonding with strong interlocking density and tensile cracking resistance. The combined inclusion of fiber reinforcement (> 0.5 %) with 0.66 slags/fly ash ratio (S40 F60) in the alkaline mixture increased the subgrade strength performance indicator (flexural and elasticity) by around 35 %–40 % over the lime-cement soil-fiber mixture. The corresponding CBR, UCS, and STS values were also found to have increased by 55 %–65 %. Furthermore, the uncertainties associated with subgrade strength failure are predicted in the form of the CBR reliability index using Monte Carlo simulation through a nonlinear regression model developed from experimental results. The study shows that the optimum dosages of fiber reinforcement and slag/fly ash are essential factors for regulating the volumetric instability and tensile cracks of expansive soil.

Fibre-Reinforced Soil Mixed Lime/Cement Additives: A Review

Soil modification is a technique for improving poor soil properties to make them suitable for engineering projects. Regarding the previous studies, various types of stabilisations were used to improve mechanical properties in soil. Several methodologies and experimental tests were used to study the positive and negative effects of utilising fibre on lime/cement-modified soil. This paper reviews the strength behaviour and microstructural properties of Fibre-Reinforced Lime Stabilised (FRLS) soil and Fibre-Reinforced Cement Stabilised (FRCS) Soil. First, the impact of FRLS/FRCS soil on strength behaviour under freeze-thaw conditions, the California Bearing Ratio (CBR) value, and compression/tensile strength are all examined. Then synthetic and natural fibres are compared at the microstructure level. FRCS/FRLS soil has been studied for its influence on geotechnical characteristics such as peak strength, residual strength, ductility, bearing capacity, stiffness, and settlement values. In addition, the micro-level evidence demonstrates that lime/cement affects the interlocking between soil particles and fibre. Although lime/cement improves soil strength by making it solid and compact, it makes stabilised soil brittle. Fibre as reinforcement in lime/cement stabilised soil transforms the brittleness of the soil into ductility. Hence building various infrastructures on poor soils is possible if fibre with lime/cement is used as an improvement method. Here, these three most used soil additive materials are investigated in terms of strength, microstructural, mineralisation, and some open issues are suggested for further research.

Experimental Study on the Mechanical Behaviors of Loess Reinforced with Randomly Distributed Basalt Fiber

Loess has the structural characteristics of porous, weakly cemented and under compacted, leading to its collapsible, disintegrative and dissolute features. To study the mechanical behaviors of basalt fiber-reinforced loess, consolidated undrained triaxial tests were carried out to investigate the effects of fiber length (FL), fiber content (FC) and cell pressure (σ3) on the shear strength. Based on the test results, a constitutive model considering the effects of the σ3, FL and FC was established using regression analysis, and the estimation method for the model parameters was proposed. The results show that the stress–strain curve of the unreinforced loess exhibited a strain-softening type, while the reinforced loess displayed a strain-hardening type. The peak strength of the reinforced loess was significantly higher than that of the unreinforced soil, and increased with increasing of FL, FC and σ3. Compared with the peak strength when FL was 8 mm, the peak strength increased slightly when the FL was 12 and 16 mm, respectively. The anchoring effect and bridging effect between soil particles and fibers improved the cohesion and friction of reinforced soil, resulting in the increment in the shear strength. The experimental results are in good agreement with the model predictions, indicating that the established model and the parameter estimation method are suitable for describing the relationship between the stress and strain of basalt-fiber-reinforced loess. The research results can provide guidance of the design and construction of fiber-reinforced soil in loess areas.

Suffusion restraint in gap-graded soil reinforced with fibers

To study how gap-graded soil reinforced with fibers of different types restrains suffusion in embankment dams, the hydraulic response of fiber-reinforced soil (FRS) containing polypropylene fibers (PPF), coir fibers (CF) or basalt-tape fibers (BTF) was investigated in one-dimensional seepage tests. The critical hydraulic gradient and fine-particle loss of the FRS were analysed quantitatively by changing the fiber type, fiber content, and reinforcement-layer thickness. The test results show that the three types of FRS all had restraining effect on the development of suffusion. The netting reinforcement effect of PPF was the best, followed by the water absorption and expansion of CF to reduce pore volume, and finally the effect of BTF was to block migration channels under the same reinforcement conditions. With increasing fiber content and the reinforcement-layer thickness, the critical onset hydraulic gradient (icr,onset), and critical failure hydraulic gradient (icr,failure) increased, improving the internal erosion resistance of gap-graded soils against suffusion. The fine-particles loss decreased and the restraint rate of fine-particle loss (RF) increased meant that the permeability stability of soil was enhanced. Linear regression analysis showed that the reinforcement-layer thickness has greater influence on restraining effect parameters than fiber content.

Desiccation characteristics and direct tension attributes of thin clayey soil containing discrete natural fibers

The use of thin clayey soil as a liner plays an important role in many geotechnical and geo-environmental engineering applications, such as open channel and reservoir sealant, contaminant barrier etc. Their functional performance and sustainability depend primarily on the desiccation characteristics of these liners and barriers. A number of studies have been undertaken to quantify the degree of improvement achieved by using natural and synthetic fiber reinforcement. However, there is a lack of studies to understand the desiccation behavior of reinforced clay. This study aimed to explore the desiccation and cracking behavior of clayey soil reinforced with two natural fibers (coir and jute fiber) in addition to the degree of improvement in tensile strength. A series of direct tension and desiccation cracking tests have been conducted in the laboratory on clay-coir and clay-jute fiber mixes. The results demonstrate that when coir and jute fibers are used, the tensile strength of fiber-reinforced soil rises by up to 475 percent and 215 percent, respectively, when compared with the tensile strength of unreinforced soil at the same moisture content. Desiccation test results also show that blending of fibers reduces the breadth and depth of cracks significantly. The characteristics of unreinforced and fiber-reinforced clayey soil under desiccation and direct tension are briefly discussed in this paper. Findings of the present study will be important for professionals dealing with clay liners and trying to reduce cracking problems associated with drying soil.

Shear Strength Behavior of Fiber-Reinforced Soil: Experimental Investigation and Prediction Model

This paper presents a prediction model for the shear strength of fiber-reinforced soil. A series of triaxial compression tests were first conducted to investigate the influences of fiber content and fiber length on the shear strength behavior of fiber-reinforced soil under different confining pressures. Experimental results on the stress–strain relationship, shear strength improvement ratio, shear strength parameters, and reinforcement mechanism are presented and discussed. Results indicate that the shear strength of fiber-reinforced soil increases significantly with increasing fiber content, but there exists an optimal value for strength improvement. The fiber content and fiber length have little influence on the friction angle of fiber-reinforced soil, while the apparent cohesion increases significantly with increasing fiber content and fiber length. Based on the experimental data, a prediction model was proposed for the shear strength of fiber-reinforced soil using the concepts of equivalent apparent cohesion and equivalent normal stress. The model can capture the decrease in shear strength improvement effect due to excessive fibers and can predict the shear strength of fiber-reinforced soil with good accuracy using fiber parameters and shear strength parameters of unreinforced soil.

Estimation of seepage velocity and piping resistance of fiber-reinforced soil by using artificial neural network-based approach

Estimation of seepage velocity and piping resistance of fiber-reinforced soil by using artificial neural network-based approach

Laboratory investigation of hydraulic conductivity of lignocellulosic fiber-reinforced soil

Laboratory investigation of hydraulic conductivity of lignocellulosic fiber-reinforced soil

Soil-water characteristic curve of polypropylene fibrereinforced sandy soil

Fibre reinforcement is considered a good alternative for improving the geotechnical properties of soil. However, studies that investigate its behaviour, accounting for the unsaturated condition, and the hydraulic behaviour of soil mixtures with fibre, are limited. Therefore, the current study evaluates the impact of the inclusion of polypropylene fibres on the hydraulic behaviour of soil through geotechnical characterisation, scanning electron microscopy (SEM), macroporosity and microporosity tests, and filter paper tests. The soil-water characteristic curve (SWCC) of different mixtures of fibre-reinforced soil was adjusted by the models enshrined in the literature, using polypropylene fibres of length 6 mm, diameter 18 μm, and fibre contents 0.25% (SF025), 0.75% (SF075), 1.0% (SF100), and 1.25% (SF125) relative to the dry weight of the soil. The results indicated a transition from unimodal to bimodal shape in the SWCC for the polypropylene fibre-reinforcement, suggesting that their inclusion altered the soil structure. The same bimodal behaviour of SWCC was observed in all reinforced samples that produced similar values of air-entry suction and residual volumetric water content, but with increased water retention for the same level of suction for higher fibre content. The results of the tension table test indicated an increase in the volume of macropores with an increase in fibre content and a decrease in micropore volumes. These results agree with the compaction tests, which showed a decrease in the dry maximum density with an increased fibre content, whereas the optimum water content increased.

Disturbed State Concept–Based Model for the Uniaxial Strain-Softening Behavior of Fiber-Reinforced Soil

A uniaxial strain-softening constitutive model for fiber-reinforced soils is developed based on the disturbed state concept (DSC). The response in the relatively intact state is assumed to satisfy the Duncan–Chang model obtained from the prepeak stress–strain curve, while the fully adjusted state response satisfies the linear model obtained by an extension of the residual strength. The apparent stress–strain curve is a weighted average response derived from the two aforementioned response curves by a disturbance function that acts as the weight. The peak of the stress–strain curve and the postpeak stable point are assumed as the starting and ending points of the disturbance, respectively, which assign a reasonable physical sense to the parameters in the disturbance function. Comparisons of stress–strain curves and peak strength reveal that for a specified fiber, fiber content exhibits a greater influence on the reinforcement effect than fiber length. Five required parameters that vary with fiber content are used in the DSC model. Five sets of uniaxial compression test data of different fiber-reinforced soils are evaluated, and a high consistency between the stress–strain curves predicted by the DSC model and the test curves is noted. Both the consistency index δ and the energy absorption capacity reveal a satisfactory description of the fiber reinforcement effect.

Experimental studies on compressive and tensile strength of cement-stabilized soil reinforced with rice husks and polypropylene fibers

This paper innovatively proposes the use of natural rice husk for reinforcing stabilized soil, and the experimental results exhibit the application potential. The reinforcement effects of rice husks and polypropylene fibers on mechanical properties in three types of cement-stabilized soils (clayey sand (SC), sandy clay of low liquid limit (CLS), and low liquid limit clay (CL)) were comparatively studied by conducting compressive strength tests and splitting tensile strength tests. The influence parameters evaluated were the fiber content, the initial moisture content, the curing time, and the fiber type. The experimental results reveal that the addition of rice husks and polypropylene fibers in cement-stabilized soil significantly improve the unconfined compressive strength (UCS) and split tensile strength (STS). The UCS and STS of fiber-reinforced soil exhibit the same trends of increasing first and then decreasing with the increase of fiber content, and the most effective strengthening range of fiber content is 0.3%–0.5%. Rice husk shows the better reinforcement effect on UCS in cement-stabilized SC and CLS, and shows better reinforcement effect on STS in three types of cement-stabilized soils, while polypropylene fiber only shows the better reinforcement effect on UCS in cement-stabilized CL.

Compaction Characteristics of Fiber-Reinforced Soils

Compaction Characteristics of Fiber-Reinforced Soils

A study on the shear strength and dry-wet cracking behaviour of waste fibre-reinforced expansive soil

Expansive soils have typical expansion and contraction properties. Under the action of dry-wet cycles, the mechanical properties and cracking behaviour of expansive soil change significantly due to repeated expansion and contraction, which results in engineering problems in expansive soil areas. The resource utilization of solid waste in civil engineering is in line with environmental protection policies and is an important solution to engineering problems. In this study, two types of fibres that can be recycled from solid waste are selected: glass fibre (GF) and polypropylene fibre (PF). Through indoor shear strength and dry-wet cracking tests, the influence of fibre content on the shear strength parameters of expansive soil is quantitatively analysed, the enhancement effect of fibres on the crack resistance of expansive soil is evaluated, and the crack characteristics of fibre-reinforced expansive soil as a function of different drying times, numbers of dry-wet cycles and soil layer thicknesses are explored. The results show that both fibre types significantly enhance the shear properties of expansive soils, and types of fibre-reinforced soil exhibit a peak in shear strength parameters at a certain fibre content. Fibre reinforcement inhabits expansive soil cracking to some extent, and both fibre types exhibit the best crack inhibition effect at a fibre content of 0.5%. The fundamental factor controlling the performance of fibre-improved expansive soil is the adhesion of the expansive soil particles to the fibres, and the PF surfaces are much rougher than the GF surfaces, which can further facilitate the interactions between the fibres and soil grains. In general, the improvement effect of PF is better than that of GF.

Experimental Investigation on the Deformation and Noncoaxial Characteristics of Fiber-Reinforced Aeolian Soil under Traffic Load

The cumulative plastic deformation of a foundation associated with traffic loading causes subgrade instability, and eventually, failure of retaining structures. In the present study, hollow torsional shear tests were conducted on fiber-reinforced aeolian soil involving varying fiber contents, cyclic deviator stresses, cyclic shear stresses, and consolidation confining pressures using the Small-Strain Hollow Cylinder Apparatus, developed by GDS Instruments Ltd. (GDS SS-HCA). This enabled an investigation of the deformation characteristics and noncoaxial angle changes of fiber-reinforced aeolian soil under a heart-shaped stress path. The results indicate that fiber-reinforced soil deformation under cyclic loading involves axial (ɛz), radial (ɛr), circumferential (ɛθ), and shear (γzθ) strains, and deformation increases with increasing the cyclic number. The strain increases rapidly during the first 200 cycles and then gradually. The axial deformation decreases with increasing fiber content when the fiber content is less than 2‰ and increases when the content is more than 2‰. The plastic strain and total deformation decrease with increasing confining pressure but increase as the cyclic dynamic stress and cyclic shear stress amplitudes increase. Functional equations of the cumulative axial plastic strain and vibration numbers are proposed. The relationships between the model parameters, including the fiber content and cyclic torsional shear stress, are analyzed. The principal stress axis rotation causes variations between the major principal strain increment and the principal stress directions, representing the noncoaxial angle (β). The noncoaxial angle fluctuates during each loading cycle, exhibiting a V shape, for principal stress axis angles (ασ) from −90° to −45° and −45° to 30°, and decreasing at angles varying from 30° to 90°, with a maximum noncoaxial angle of approximately 30° per cycle.

Effect of alkali-treated sisal fibres on expansive clay

In this study, the synergic effects of alkaline activation (AA) and fibre inclusions on the mechanical behaviour of fly-ash-stabilised black cotton soil were investigated. The natural fibre used in this research was treated with sodium hydroxide solution as an expedient solution to attain durability. The fibres are added in two forms: discrete fibre and geotextile. Assessment of engineering behaviour of the AA-treated soil reinforced with surface-coated sisal fibres was carried out by conducting various laboratory tests such as free swell test, unconfined compressive strength test and cyclic triaxial test. Microstructural characterisation was carried out by scanning electron microscopy analysis and Fourier transform infrared spectroscopy analysis. The test results conclusively showed that the alkaline-activated fly-ash-treated sisal fibre-reinforced soil exhibits better mechanical behaviour in terms of peak stress and post-peak behaviour and appreciable resistance to swelling. Overall, the study shows that amendment of fibre-reinforced AA-fly-ash could effectively enhance the strength, stiffness and toughness of the soil and could subsidise the anticipated detrimental effects that could cause by dynamic loading. This study affirms the potential of alkali-treated sisal fibre to be an alternative to traditional stabilisers used in construction involving expansive soil.

Combined Effect of Biopolymer and Fiber Inclusions on Unconfined Compressive Strength of Soft Soil.

The utilizing of traditional chemical stabilizers could improve soil engineering properties but also results in brittle behavior and causes environmental problems. This study investigates the feasibility of the combined utilization of an ecofriendly biopolymer and fiber inclusions as an alternative to traditional cement for reinforcing soft soil. A series of unconfined compression tests were conducted to examine the combined effect of the biopolymer and fibers on the stress–strain characteristics, strength improvement, failure pattern, and reinforcement mechanism of soft soil. The results show that the biopolymer associated with fibers has an unconfined compressive strength similar to that of fiber-reinforced soil. However, it then increases with different curing times and conditions, which can be up to 1.5 MPa–2.5 MPa. The combined effect of fibers and the biopolymer is not simply equivalent to the sum of the effects of each individual material. The fiber shows its role instantly after being mixed into soil, whereas the effect of biopolymer gradually appears with sample curing time. The biopolymer plays a dominant role in increasing the peak unconfined compressive strength and brittleness of soil, while the amount of fiber is crucial for reducing soil brittleness and increasing ductility. It is shown that the biopolymer not only contributes to the particle bonding force but also facilitates the reinforcement efficiency of fibers in the soil. The fibers in return assist in reducing the soil brittleness arising from biopolymer cementation and provide residual resistance after post-peak failure.

Influence of fibre type on unconfined compressive strength of fibre-reinforced cemented soil under freeze-thaw cycling

Influence of fibre type on unconfined compressive strength of fibre-reinforced cemented soil under freeze-thaw cycling

Contributions to the Study of Fracture Mechanics of Synthetic Fiber-Reinforced Soils Based on Flexural Tests and Imaging Techniques

Contributions to the Study of Fracture Mechanics of Synthetic Fiber-Reinforced Soils Based on Flexural Tests and Imaging Techniques