Showing papers in "International Journal of Geosynthetics and Ground Engineering in 2015"

Influence of Particle Size on the Friction and Interfacial Shear Strength of Sands of Similar Morphology

Abstract: Size and morphological characteristics of particles play vital role on the shear and interfacial shear strength of sands. Often, effects of these parameters are merged and cannot be easily separated. Effect of size of the particles on the shear and interfacial shear strength of sands is presented in this paper through direct shear and interface direct shear tests complemented with image analyses and surface roughness studies. To eliminate the effect of morphological characteristics, three sands of different particle sizes with similar morphological characteristics like angularity, roundness, sphericity and roughness were selected for the study. These morphological characteristics for all three sands were determined from the analysis of scanning electron microscope images and were found to be similar for all three sands. It was observed from the symmetric direct shear tests that the particle size has no effect on the peak friction angle when the tests were carried out at same void ratio. However, ultimate friction angles were affected by the particle size. Shear band thickness was estimated from image segmentation analysis of the profiles of colored sand columns during shear and the same was correlated to the particle size. Interface direct shear tests were carried out on sand–geomembrane interfaces to study the effect of particle size on the interfacial shear strength. Microscopic images of geomembranes were captured after the interface shear tests to understand the change in surface roughness of the geomembrane due to particle indentations. Surface roughness studies on geomembrane samples after the tests confirmed that the plowing and groove formation on geomembranes during interface shear tests depend on the particle size as well as the relative roughness of the sand particles with respect to the membrane. Sand of medium particle size showed highest interfacial strength because of more number of effective contacts per unit area of the interface.

Recycled Tire Chips Mixed with Sand as Lightweight Backfill Material in Retaining Wall Applications: An Experimental Investigation

Abstract: This paper presents studies on use of recycled tire shreds in sand–tire chips (STC) mixture for retaining wall applications. Small-scale physical model tests were performed on rigid retaining wall with different STC mixtures. Rigid retaining wall model of 600 mm height was constructed in a Perspex container. The wall was made with hollow rectangular steel sections. STC mixtures with different tire chips proportions, such as 10, 20, 30, 40, and 50 % along with pure sand were considered as backfill materials. Static surcharge load, up to 10 kPa, was applied using concrete blocks. Model wall behaviour in terms of displacements and earth pressures has been discussed for sand alone (control case) and STC mixtures as backfill materials. The experimental results indicate that the horizontal displacements and lateral earth pressures are reduced to about 50–60 % of that of control case by using STC mixtures which functioned as light weight backfill materials.

A Critical Appraisal of the Role of Clay Mineralogy in Lime Stabilization

Abstract: The stabilization of problematic fine-grained soils using lime as an admixture is a widely accepted practice, owing to its simplicity and cost-effectiveness. The optimal quantity of lime required for soil stabilization primarily depends upon the reactive nature of soil as well as the degree of improvement desired. The term ‘optimum lime content’ (OLC) defines the amount of lime required for satisfying the immediate/short-term soil–lime interaction, and still providing sufficient amount of free calcium and high residual pH necessary to initiate long-term pozzolanic reaction. Previous researchers proposed various empirical correlations and experimental methodologies for determining OLC, in terms of clay-size fraction and plasticity characteristics of virgin soil. However, the limiting lime content obtained using various conventional methods does not account for the most influencing inherent clay mineralogy of the soil; and hence, the results of these methodologies are observed to be quite disagreeing with each other. In view of these discrepancies, the present study attempts to validate the existing conventional methodologies for OLC determination at an elementary level, by comprehending the fundamental chemistry following soil–lime interactions. Based on the theoretical and experimental observations, it is quite evident that the accuracy of conventional tests is limited by combined influence of chemical and mineralogical properties of soils. Hence, it is proposed to develop a precise methodology to ascertain the required optimal lime dosage based on scientific criteria, by incorporating the influence of soil properties such as clay mineralogy, specific surface area, soil pH, cation exchange capacity, soil acidity, base saturation capacity, and buffer capacity.

Underground Mine Backfilling in Australia Using Paste Fills and Hydraulic Fills

Abstract: Underground mine backfilling is a form of ground improvement that has to be carried out in the mine sites. The backfilling provides ground support and regional stability, thus facilitating ore removal from nearby regions. The large underground voids created by the ore removal are backfilled with the waste tailings in the form of paste fills, hydraulic fills, and others. The tailings are placed in the form of slurry that undergoes self-weight consolidation. A small dosage of binder is added to paste fill and cemented hydraulic fill to enhance strength. Considering the high cement cost, mines are using fly ash and slag to partially replace cement with blended cements. This paper gives a practical overview of underground mine backfilling in Australia using paste fills and hydraulic fills. The mining methods and different types of backfills are briefly discussed, with major focus on paste fills and hydraulic fills.

Experimental and Numerical Studies on Response of the Stone Column in Layered Soil

Abstract: This paper describes the soil layering effects on response of the stone column and stone column improved ground through a series of small scale laboratory tests and numerical analyses. Two types of layering systems, i.e. soft clay overlying stiff clay and vice versa are considered for the present study. The entire laboratory tests were carried out on 88 mm diameter stone columns installed in a two layered soil systems. Unit cell concept is used to idealize the behaviour of a single column within an infinite group of stone columns. Entire unit cell and only the stone column area were loaded to evaluate the stress versus settlement response of the entire improved ground and that of the stone column. Effects of the top soft and stiff clay layer thickness on the axial stress of the whole improved ground and stone column only are evaluated through laboratory tests. A detailed parametric study using finite element based software Plaxis was also carried out. Elastic-perfectly plastic Mohr–Coulomb failure criterion with drained conditions was used for the soil and stone columns in the numerical analyses. Result shows that the limiting axial stress of the stone column is found to be influenced by the top clay layer thickness up to two times the diameter of the stone column beyond which it remains constant for both the layering systems. The limiting axial stress of the whole improved ground is found to be influenced by the presence of the top layer up to a depth of four times the diameter of the stone column for both the layering systems. The stiffness improvement factor of the improved ground increases with increase in the thickness of the top soft clay layer and attains maximum value for the full depth of soft clay whereas it remains constant for different depth of the top stiff clay. The vertical extent of the bulging increases with increase in the thickness of the top soft clay up to two times the diameter of the stone column for both the layering systems.

Experimental and Analytical Studies on Soft Clay Beds Reinforced with Bamboo Cells and Geocells

Abstract: This manuscript deals with the experimental and analytical studies carried out to explore the possibility of using naturally available bamboo to increase the bearing capacity of the soft soil. In order to extract the additional confinement effect on the soil, 3 dimensional-cells are formed from the locally available bamboo known as bamboo cells. The performances of the bamboo cells are compared with the commercial geocells. Further, a planar reinforcement in the form of bamboo grid was provided at the base of bamboo cells and the performance was compared with the clay bed reinforced with the combination of geocell and geogrid. The results of the laboratory plate load tests suggested that the ultimate bearing capacity of the clay bed reinforced with combination of bamboo cell and bamboo grid was about 1.3 times higher than the geocell and geogrid reinforced clay beds. In addition, a substantial reduction in the settlement was also observed. An analytical model was also proposed to estimate the bearing capacity of the clay bed reinforced with bamboo cells and bamboo grids. The model comprised of three mechanisms, namely the lateral resistance effect, vertical stress dispersion effect and membrane effect. The results predicted from the analytical model were found to be in good agreement with the experimental results. In a larger perspective, this study proposes a cost effective ground improvement technique in soft soils as an alternative to geocells and geogrids.

Effects of Degradation on Geotechnical Properties of Municipal Solid Waste from Orchard Hills Landfill, USA

Abstract: In bioreactor landfills, geotechnical properties of municipal solid waste (MSW) are believed to be affected by increased moisture content and accelerated biodegra- dation due to leachate recirculation; however, studies to quantify the changes in the MSW properties are scarce. This study quantifies the change in geotechnical properties of field MSW as a function of level of degradation. Fresh MSW samples were collected from the working phase of Orchard Hills landfill (Davis Junction, Illinois, USA) and were subjected to leachate recirculation and enhanced anaerobic degradation in specially designed laboratory bioreactors. Samples were exhumed from the bioreactors at different stages of degradation as determined by the amount and composition of biogas generated, and subse- quently tested for moisture content, organic content, unit weight, hydraulic conductivity, compressibility, and shear strength. Moisture content of MSW increased significantly, while organic content decreased with degradation. Bulk unit weight increased with degradation which led to decrease in saturated hydraulic conductivity. Primary compression ratio showed slight increasing trend with degradation, while the secondary compression ratio was not affected significantly with the degradation. The friction angle decreased from 30� to 12� , but cohesion increased from 29 to 65 kPa with degradation based on direct shear test results. The testing of saturated MSW in triaxial con- solidated undrained conditions resulted in lower shear strength with no distinct correlation of friction angle and cohesion with degradation. Additional large-scale, long- duration testing is recommended using the field MSW samples with the consistent composition to establish the correlations between the engineering properties and degree of degradation. Overall this study showed that the engi- neering properties of field MSW are affected by degrada- tion and these changes should be properly accounted in the analysis and design of bioreactor landfills involving lea- chate recirculation.

Evaluating the Influence of Additives on Swelling Characteristics of Expansive Soils

Abstract: Stabilization is one of the most preferred techniques of dealing with expansive soils. Several types of additives have been evolved and are successfully being used for this purpose. This paper evaluates the performance of a variety of additives categorized into (a) cementitious: lime and fly ash (b) non-cementitious: stone dust, and (c) chemical additives: CaCl2 and Na2SiO3, when employed to stabilize three types of expansive soils used in the study. Attempts were also made to investigate the influence of valence of cations (viz., monovalent, divalent and trivalent) and mean particle diameter (d50) of additive(s) on percentage reduction of swelling characteristics. Results reveal that each additive exhibits distinct response on the swelling behavior of expansive soils. It has been observed that chemical additives exhibit superior performance over cementitious and non-cementitious additives in reducing the swelling characteristics. Further, it has also been found that valence has profound influence on the swelling characteristics of expansive soils. As such, the degree of reduction of swelling was found significantly high when employed chemical additive consists of trivalent cations than its counterpart additive consists of mono- or divalent cations. Further, efforts were also devoted to correlate mean particle diameter of additives with swelling characteristics, and it was clearly evident from trends that an appreciable decrease in swelling characteristics occurs with decrease in mean particle diameter.

Comparative Study on Performance of Tire Crumbles with Fly Ash and Kaolin Clay

Abstract: In this study, the impact of tire crumbles as an admixture on compaction and strength behavior of two different materials, kaolin clay and fly ash is considered. In order to understand the compaction and strength behavior, proctor compaction tests and California bearing ratio (CBR) tests respectively were performed on the mixture of clay and fly ash with tire crumbles. Test results have shown that maximum dry unit weight decreases when tire crumbles are added in fly ash or clay. Increment in the optimum moisture content was also observed, when tire crumbles were added in clay. But no significant changes have been observed in case of fly ash. Test results have shown that the fly ash-tire crumble mix can sustain greater load than the clay-tire crumble mix. Improvement in CBR value is 5 times for clay and 3 times for fly ash with the addition of tire crumbles.

Behavior of Geogrid Reinforced Foundation Systems Supported on Clay Subgrades of Different Strengths

Abstract: This paper presents an experimental study investigating the behaviour of geogrid reinforced sand-clay foundation systems with clay subgrades of different strengths. Model tests were carried out on a circular footing of 150 mm diameter (D) resting on 1 × 1 × 1 m foundation bed having clay subgrades of different undrained shear strengths (c u), ranging from 7 to 60 kPa. Different series of laboratory model tests were performed on homogeneous and layered foundation systems. The layered systems were comprised of dense sand of varying layer thicknesses (H = 0.63–2.19D) overlying the clay subgrades. Pressure-settlement responses obtained indicated that the foundation performances were largely influenced by footing settlement (s/D %), layer thickness (H), and subgrade strengths (c u). The results indicated that the planar geogrid reinforcement, placed at the sand-clay interface, can substantially improve the performance of the foundation beds depending on layer thickness and subgrade strength. A maximum of about 5.6-fold improvement in bearing capacity was observed in the study, for very soft clay subgrade of 7 kPa.

On the Utilization of Fly Ash and Cement Mixtures as a Landfill Liner Material

Abstract: Cement was added to class F type of fly ash in the proportion of 0, 2, 5 and 10 % to study for its suitability as a landfill liner material. Mixtures were compacted to their optimum moisture content (OMC) and maximum dry density (MDD). The results from the consolidation tests showed a relatively lower value of compression index for all the mixtures indicating the settlement due to application of overburden pressure would be small. Hydraulic conductivity of the samples were found to be decreased with the increasing the load. However, none of the mixtures exhibited a hydraulic conductivity value less than 10−9 m/s, a limiting criterion set by various environmental agencies for the material to be used as a landfill liner. However, mixtures of 90 % fly ash + 10 % cement compacted at 5 % wet of OMC-MDD exhibited a hydraulic conductivity value less than 10−9 m/s. On drying, all the mixtures shrunk marginally. The unconsolidated undrained test results indicated that the shear strength parameters increase with increase in the cement content in the mixtures.

Stress Transfer Mechanism in 2D and 3D Unit Cell Models for Stone Column Improved Ground

Abstract: Stone column has been used widely to improve the foundation for many structures. Many designs of stone column are based on the unit cell concept. However, the intrinsic mechanism of stress transfer between the column and the surrounding soil has not been investigated thoroughly. This paper presents the important features of stress sharing mechanism in unit cell concept under an embankment loading. The arching effect, the deformation mode, the stress concentration ratio and the plastic straining in the unit cell are the main focus of this paper. Finite element software PLAXIS was used to examine these features. Unit cell was simulated as a two-dimensional (2D) axisymmetrical model and a representative three dimensional model in the numerical analysis. Drained loading condition was analyzed in this study in which the embankment is assumed to be built slowly with no excess pore pressure buildup. The change of the stress concentration ratio as the embankment height increases was also studied. From this study, it was found that the bulging happened near the column head accompanied by multiple shear bands progressing along the column. Generally, stone column in the unit cell shared about 4–5 times more the loads than the surrounding soils throughout the column depth. In most cases, 2D and 3D models give results that are similar to each other especially on the settlement performance and the failure mechanism.

Pond Ash Based Controlled Low Strength Flowable Fills for Geotechnical Engineering Applications

Abstract: Granular materials are conventionally used as backfills behind retaining walls and many other filling applications. Normally, the backfills are compacted in layers at a relative compaction of not less than 95 % of the Standard Proctor unit weight at desired water content. Depending on the type of backfill material, suitable compaction equipments are selected for compacting the material. Usually, coarse grained materials like gravel, sand, Pond ash, crushed rock pieces and cobble are used as backfill materials. As a replacement to the conventionally used backfill materials, controlled low strength materials (CLSM or flowable fills) are also used, especially in areas where compaction equipments cannot be mobilized. This paper reviews the effective utilization of different types of ashes in flowable fill production and the main properties, advantages and applications in geotechnical engineering practice. Experimental results of a flowable fill made of a local Pond ash are also presented in this paper.

Performance of Chemically Treated Natural Fibres and Lime in Soft Soil for the Utilisation as Pile-Supported Earth Platform

Abstract: This work presents the effect of lime and treated coir fibre on the mechanical behaviour of soft clay soil as a pile-supported earth platform The experimental programme comprised three types of test (flexural strength, indirect tensile strength and triaxial compression strength) Experimental results were used in a numerical analysis in order to observe the performance of the treated soil as a load-transfer base layer depending on the height of the earth platform and the material properties of the treated soil Two-dimensional physical model experiments were performed to validate the numerical model of the pile-supported load transfer platform The numerical analyses showed the importance of the mechanical properties of the treated soils for the efficacy and effectiveness of the reduction of the settlement of the earth platform, as well as to enhance the bending performance of the earth platform The efficacy of limed soil reinforced with chemically treated coir fibres is up to 30 % under various loadings of structures when the effective height of the earth platform is 03 m The differential settlement at the elevation of the pile head is significantly reduced by up to 100 % Present study concluded that this treatment technique can not only increase the mechanical performance of the coir fibres and lime-reinforced soil, but can also improve the interfacial mechanical interactions between the coir fibre surface and the soil particles, resulting in higher performance of the composites used as a pile-supported earth platform

Prediction of Deviator Stress of Sand Reinforced with Waste Plastic Strips Using Neural Network

Abstract: The paper presents a study conducted on sand-waste plastic strip mixture for carrying out consolidated drained triaxial compression tests and to use the experimental data in training, testing, and prediction phases of neural network-based soil models. The input variables in the developed neural network models were strip content, tensile strength of strip, thickness of the strip, elongation at failure of the strip, aspect ratio, dry unit weight of the composite specimen, confining pressure and strain at failure of the composite specimen and the output was the deviator stress. These variables were considered to construct 8-6-1 topology of neural network in the prediction of the deviator stress. Further, using the mean squared error, root mean squared error, mean absolute error, mean absolute percentage error, correlation coefficient (r) and coefficient of determination (R 2) for the training and testing data, the predictability of neural networks was analysed using various activation functions. The neural network model obtained had an acceptable accuracy. Sensitivity analysis revealed that the contribution of the input variables such as strip thickness, tensile strength of the strip and dry unit weight does not have much impact on the output deviator stress. After the sensitivity analysis, neural network structure was revised. The revised model having 5-4-1 topology gives a better prediction of the output deviator stress than the previous model with 8-6-1 topology. Further, the revised neural network model having 5-4-1 topology is superior to the one obtained using multiple regression analysis in predicting the output deviator stress. Finally a model equation is presented based on trained weights in the revised neural network.

Strength Characterization of Fiber Reinforced Cement–Fly Ash Mixes

Abstract: The experimental investigation for the characterization of strength of fiber reinforced cement–fly ash mixes is presented in this paper. The objective of the investigation was to quantify the optimum quantity of cement contents and randomly distributed polypropylene fibers on the performance of cement stabilized and fiber reinforced fly ash mixes. Four samples of the mixes were considered in this investigation. For each sample, the fly ash was replaced by 5, 10, 15 and 20 % cement content, respectively. Similarly, the contents of the fibers were also varied by 0, 0.5, 1, and 1.5 %, respectively. The aspect of soaking was also considered. The samples were cured for 7, 14 and 28 days respectively. For the purpose of evaluation, unconfined compressive strength test and Brazilian (i.e., indirect) tensile strength test were conducted. The results obtained indicate significant improvement in strength with the inclusion of fibers and cement. The investigation underscores the effective utilization of pozzolanic waste such as fly ash as a civil engineering material.

Bearing Capacity and Settlement Characteristics of Sand Subgrades with Vertical Reinforcement Supporting a Square Footing

Abstract: The restraining effects in the sand subgrade under the footing can be provided by inserting the vertical reinforcing bars beyond the footing base without disturbing the subgrade below the footing. This technique of soil reinforcement can be quite beneficial for the footings where improvement is necessary. This paper presents the results of model square footing tests with vertical reinforcements inserted into the sand subgrade beyond the footing base only. The variables selected for the investigation were spacing of vertical reinforcement, extent of reinforcement from the edge of footing, length of reinforcement, diameter of reinforcement, width of footing and surface characteristics of the reinforcement. The values of theoretical bearing capacity of reinforced sand subgrade were determined using the method of slices and compared with experimental results. It was found that results were comparable particularly when the length of reinforcement and extent of reinforcement were two times the footing width.

Influence of Synthetic and Natural Fibers on Dewatering Rate and Shear Strength of Slurries in Geotextile Tube Applications

Abstract: This study investigates the effect of fibers on the dewatering time, filter cake properties, and shear strength of filter cakes with fine-grained silty clay. Synthetic nylon fibers with lengths of 6, 12, and 18 mm and one natural jute fiber with an average length of 8 mm were added to slurries with 33 % fines in concentrations of 0.5 % by soil mass. Optimum dose tests for particle flocculation showed that slurries reached a turbidity of 20 NTU with up to 19 % less flocculatant material compared to slurries with no fibers. Pressure filtration tests showed that the dewatering time decreased significantly with both synthetic and jute fibers, but were not dependent on fiber length. However, fall cone tests and unconsolidated undrained tests showed that the increase in shear strength was dependent on fiber length. Increases in shear strength were over 100 % with 12 mm nylon fibers, while filter cakes with 6 and 18 mm nylon fibers increased the shear strength by 43 %. Jute fibers did not show as high of a strength gain as the nylon fibers, but did increase dewatering times by an average of 14–22 % compared to filter cakes with nylon fibers.

Liquefaction Potential of Roorkee Region Using Field and Laboratory Tests

Abstract: Before taking up the ground improvement of a site, assessment of liquefaction potential of a region is very important. This also helps in seismic microzonation of the area. It can be rationally performed if the data of site (using either field tests or laboratory tests) are available. The aim of the present study is to evaluate liquefaction potential of Roorkee (India) region. For this purpose the liquefaction resistance of the soil, within the radius of 30 km of Roorkee, was evaluated using two different approaches. First is the field approach based on standard penetration test (SPT) N-Value and second is the laboratory approach employing mean grain size distribution (D 50). Investigation was carried out at five different locations by conducting SPT tests and collecting soil samples at regular interval. The cyclic shear stress due to earthquake loading was examined using simplified method as well as using ground response analysis. The factor of safety against liquefaction was evaluated at different depths for all the sites using both field and laboratory data. It was found that the factor of safety against the liquefaction using field approach is marginally greater than that using the laboratory approach for all the sites. Also the factor of safety using ground response analysis is significantly smaller than that using simplified method. Thus it was concluded that use of simplified method may not be adequate.

Strain Behavior of Backfill Soil in Rigid Faced Reinforced Soil Walls Subjected to Seismic Excitation

Abstract: The reinforced soil walls offer an excellent solution to the problems related to the earth retaining structures, especially under seismic condition. This paper presents the strain behavior of backfill soil subjected to seismic excitation. Full height concrete faced reinforced soil retaining wall which is termed as rigid faced reinforced soil retaining wall is considered. Numerical models of shaking table tests on full height rigid faced walls are simulated using FLAC3D and validated with the laboratory test results. The octahedral strains developed on backfill soil during the dynamic excitations are determined from the numerical simulations and analyzed. Two types of strained zones are observed: high strain zone near the wall facing and low strained zone extending into the retained backfill indicating localized displacements near the facing. Parametric studies are also conducted to observe the behavior of soil strains for different reinforcement configurations and backfill soils. These parameters have marginal effect on strain behavior. The results indicate that the facing modulus affects the response of rigid faced walls.

Effect of Submergence on Settlement and Bearing Capacity of Surface Strip Footing on Geotextile-Reinforced Sand Bed

Abstract: This paper presents the effects of submergence on the settlement and the bearing capacity of a surface strip footing resting on the reinforced sand bed at a relative density of 90 % by conducting the laboratory model tests. The reinforcement layers used were woven geotextile layers, without and with wraparound ends. The number of reinforcement layers was varied from 1 to 4. The test tank had an arrangement for the water table rise, from the bottom of the sand bed constructed in the tank. The model strip footing was placed at the surface of the sand bed, and measurement of the settlement occurring with the rise of the water table, was taken to observe the effect of water table rise. When the water table reached the top surface of the sand bed, that is, the sand bed was fully submerged under water; the measurement of settlement of the footing was continued, by applying the load incrementally through the hydraulic jack. The results show that the rise of the water table causes a significant settlement of the footing for both unreinforced and reinforced cases. However, the beneficial effect of reinforcement layers was observed in terms of increased load-bearing capacity. An increase in the number of reinforcement layers, from 1 to 4 as well as providing the wraparound ends to the reinforcement, brought a significant increase in load-bearing capacity. When compared to the dry situation, there is a significant decrease in the load-bearing capacity as well as in the modulus of subgrade reaction of the unreinforced and reinforced sand beds with the rise of water table and full submergence.

Effect of Geosynthetic on the Performance of Road Embankment over Sabkha Soils in Algeria: Case Study

Abstract: This paper focuses on the construction technique of road embankment which crosses a section of about 11 km on sabkha soils of Chott El Hodna in Algeria. Due to the poor bearing capacity of sabkha surface and the rising water table over the surface, serious difficulties were encountered during the investigation of the subsurface soil and the construction of the first embankment layers. In this construction site, the use of basal geosynthetic layer to maintain the performance of embankment was found to have relevant reinforcing effect by increasing the soil bearing capacity to allow safe construction of the first layers and improving the compaction quality. Based on the in situ observations and the geotechnical investigation, numerical simulations were performed using the software PLAXIS. The results have shown the contribution conditions of geosynthetics to improve the bearing capacity of sabkha soils.

Effect of Soil Grain Size Distribution on the Mechanical Damage of Nonwoven Geotextiles Under Repeated Loading

Abstract: Installation processes (which induce mechanical damage) may cause undesirable changes on the properties of geosynthetics, affecting their performance. This work evaluates the effect of mechanical damage on the short-term tensile behaviour of two nonwoven geotextiles (with different masses per unit area). The geotextiles were damaged in laboratory using a standardised procedure and an artificial aggregate (corundum) and eight other soils. The damage induced was characterized using wide-width tensile tests. Results showed reductions of the tensile strength of both geotextiles, which depended on the grain size distribution and uniformity of the soils and on the mass per unit area of the geotextiles. The reduction in tensile strength provoked by corundum was higher than the decreases caused by most of the other soils. The mechanical damage tests also led to a reduction of elongation at maximum load and an increase of stiffness.

Optimization of Design Parameters and Cost of Geosynthetic-Reinforced Earth Walls Using Harmony Search Algorithm

Abstract: This paper proposes a new approach to optimize the design of geosynthetic-reinforced retaining walls. Minimizing the cost of construction was considered as the optimization criterion. A metaheuristic technique, named Harmony Search Algorithm (HSA), is applied in optimizing the design of geosynthetic-reinforced earth walls. The involved optimization procedures are discussed in a step-wise approach and their applicability is demonstrated on geosynthetic-reinforced walls of height 5, 7 and 9 m. The effects of static and dynamic loads are considered. Results are compared between this study and studies that used Sequential Unconstrained Minimization Technique (SUMT). It is found that the construction cost, for a geosynthetic-reinforced walls optimized by HSA, showed as high as 9.2 % reduction from that of SUMT.

In-Situ Stabilization of Soil Slopes Using Nailed or Anchored Geosynthetics

Abstract: The incidence of soil slope instability is, and has been, a natural hazard of major proportions and is ongoing to this day. While stabilization can be afforded by excavation and reconstruction, oftentimes structures and utilities are involved requiring in situ stabilization methods. This paper is focused accordingly. The method uses soil nails or soil anchors connected to a geosynthetic surface covering which is tensioned thereby stabilizing the encapsulated soil and providing tensile reinforcement. The paper is subdivided as follows: (i) introduction and theoretical background, (ii) required theoretical modifications, (iii) basics of soil nailing and anchors along with surface geosynthetics effects, (iv) implementation of the technique, (v) current activities by manufacturers, suppliers and ground modification companies, and (vi) summary and conclusions.

Modelling of Non-linear Shear Displacement Behaviour of Soil–Geotextile Interface

Abstract: Behaviour of reinforced soil structures depends on the material properties of soil and geosynthetics. Apart from individual properties of soil and reinforcement, the interaction between reinforcement and soil also plays important role in deciding the behaviour of reinforced soil structures. The modelling of the interface characteristics is a very important aspect in developing a correct numerical model for predicting the performance of reinforced soil structures. For simulating the interface behaviour a linearly elastic model with Mohr–Coulomb criterion is commonly used. Experimental observations made from direct shear tests usually show that the force–displacement relationship is non-linear till a peak is attained, beyond which softening behaviour is observed. In this paper a constitutive model appropriate for geosynthetic interfaces has been implemented in FLAC3D and used to simulate the shear stress displacement behaviour of different sand–geotextile interfaces. In the present study, shear stress–displacement behaviour of interfaces of different types of sands and geotextiles (monofilament woven, multifilament woven and nonwoven) have been considered for simulation. The simulated curves using the FLAC3D showed very good agreement with the experimental data over the complete stress-displacement ranges.

Seismic Stability Analysis of Reinforced Soil Wall Using Horizontal Slice Method

Abstract: The seismic stability of a reinforced soil wall has been analyzed using the horizontal slice method considering the pseudo-static seismic forces. The effect of various parameters including wall inclination angle, angle of internal friction of soil, horizontal seismic loading, cohesion of the backfill and surcharge loading has been examined. It is found that the stability of a reinforced soil wall is largely affected by the horizontal seismic forces. The tensile resistance which has to be mobilized by the reinforcement to maintain the stability of wall, increases with an increase in surcharge, horizontal seismic forces, whereas the same decreases with an increase in the cohesion of soil. The normalized geosynthetic tensile reinforcement force is used for comparing the effect of different parameters.

Settlement Prediction of Stone Column Group

Abstract: Stone columns can be designed to bear on the hard stratum or as a floating system where the toe is embedded in the soft layer. Most of the existing design methods for stone columns adopt unit cell idealization which is not applicable to spread footing. From the view of settlement analyses, current available methods to evaluate the settlement performance of a footing on a limited number of stone columns are more or less rough approximations derived from the elastic theory or using simple empirical approach. None of these methods incorporate the idea of optimum length and the yielding effect in the plastic zone. This paper introduces a new mechanical method to account for these. This method works for homogenous and non-homogenous ground condition. The settlement predictions made by the proposed method are compared with the finite elements results and field measurement. Good agreements are obtained not only for load settlement response but also the displacement profile.

A note on void ratio of fibre-reinforced soils

Abstract: This technical note extends the concept of void ratio, presented traditionally in soil mechanics, for fibre-reinforced soils. Phase relationships related to the void ratio of fibre-reinforced soils are presented along with their definitions. A simple analytical model verified with experimental data for estimating the void ratio of fibre-reinforced soils is developed which can be used to express the compressibility of fibre-reinforced soils in geotechnical engineering applications. The results indicate that the void ratio of fibre-reinforced soils is dependent on the volume ratio of fibre-soil solid.

Measured Performance and Stability Analysis of Large-Scale Reinforced Model Embankments at Different Moisture Contents

Abstract: This paper describes construction and instrumentation of two 1.65-m high reinforced embankment models that were tested at two different gravitational water contents (GWC) on the dry and wet sides of the optimum moisture content (OMC); i.e. OMC−2 % and OMC+2 %. The embankment models were constructed using a clayey sand that was reinforced with four layers of instrumented geotextile with a uniform vertical spacing of 300 mm. Each model embankment was subjected to a line surcharge load (strip footing) near its crest until failure. The magnitudes of GWC, earth pressure, geotextile strains, footing settlement and embankment deformations were measured using EC-5 sensors, earth pressure cells, wire potentiometers, linear variable deferential transformers and reference plates, respectively during the construction and loading phases of each test. The GWC values in each model embankment were measured using EC-5 sensors and the oven drying method to ensure that they were close to the target values. Slope stability analysis was carried out to study the equilibrium of embankment models and to validate the predicted bearing capacity and factor of safety results against the experimental data. Results of this study show that the embankment model constructed on the dry side of OMC (i.e. OMC−2 %) had a 10 % greater serviceability failure load than the model built at OMC+2 %. Meanwhile, stability analysis results indicate that the bearing capacity of an unreinforced model embankment at OMC−2 % would be 72 % lower than that of the corresponding reinforced model. It is also found that compaction-induced energy could increase the earth pressure within the embankment by up to 70 % of the vertical stress in the soil mass due to gravity.