Showing papers in "International Journal of Geosynthetics and Ground Engineering in 2016"

Correlation Between Shear Wave Velocity (Vs) and SPT Resistance (N) for Roorkee Region

Abstract: For ground improvement, assessment of damage during an earthquake is very important issue which in turn depends on the ground motion. The characteristics of an earthquake motion at a site depend on the shear wave velocity (V s ). The shear wave velocity profile at a site may not be readily available however, the numbers of blows (N) from standard penetration test (SPT) are readily available. This paper presents a development of reliable correlation between V s measured by multi channel analysis of surface wave tests and N measured using SPT at various sites in Roorkee region. These tests have been carried out at ten different sites in Roorkee region (within a radius of 30 km). The SPT samples are tested in the laboratory for index properties. Roorkee is situated in high seismic zone, therefore the study is important for this region. Based on the statistical assessments, an empirical correlation between V s and N was developed. This is done separately for all types of soils, sands only and clays only. The developed relations fall within the range of other relations developed worldwide for other sites. A comparison with available relations is also presented. The proposed relations will be helpful in seismic microzonation of the region as ground motion is one of the important parameters.

Analysis of a Nailed Soil Slope Using Limit Equilibrium and Finite Element Methods

Abstract: This paper aims at the two most common methods used for slope stability analysis An attempt has been made to bring out the differences in results of reinforced slope stability analysis obtained from SLOPE/W (limit equilibrium based) and PLAXIS 2D (finite element based) The analysis is carried out on two slope angles of 45° and 60°, which are reinforced with nails at three different inclinations of 0°, 15° and 30° respectively Both the slope angles and all nail inclinations are taken from the horizontal A comparative study on stability parameters such as factor of safety, critical slip surfaces and nail forces has been carried out The limit equilibrium method is found to yield higher values of factor of safety in comparison to finite element method The failure surfaces from both methods are found to vary significantly Large nail forces are observed by limit equilibrium method for 45° slope with all nail inclinations, whereas for 60° slope angle, finite element method shows an increase in the nail forces The effect of other parameters like bond length in limit equilibrium, soil–nail interaction and bending stiffness in finite element are also studied

Model Study of Alkali-Activated Waste Binder for Soil Stabilization

Abstract: This study assesses the laboratory investigation to evaluate the feasibility of using alkaline activation technique for engineering improvement of soils. The originality of this paper stems from the novel two-stage approach. The first stage investigates the effectiveness of locally available precursor in the alkaline activation process by focusing on soil strength improvement. As such, in presence of high alkali solutes (Na-based and Ka-based alkaline activators), palm oil fuel ash (POFA) was used as a precursor due to its amorphous nature and high silica-to-alumina ratio. In the second stage of this study, geotechnical model procedure of interaction between a strip footing model and stabilized soil by column technique and the most effective percentage of POFA was performed. According to the test results, applying alkaline activators to soil induced low strengths of up to 159 kPa after 7 days curing. When the POFA content used in alkaline activation increased from 0 to 15%, the UCS values increased up to 226% after similar curing duration. This assertion reflects the fact that the addition of POFA enriched the reactive Si and Al in the matrix, which allowed stronger Si–O–Si and Al–O–Si bonds to form. Curing condition, type and quantity of the alkaline activators were also shown to have significant strengthening effects on the treated soil. In this respect, the use of moderate 10 M NaOH and 10 M KOH were found to be viable as the best concentration for strength improvement of investigated soil when economy and practicality were considered. In terms of using alkali-activators, the use of the NaOH for soil treatment is beneficial in terms of lower cost, since the price of KOH solution is higher than that of the NaOH solution. Results of the second phase showed that a considerable settlement reduction up to 192% of treated columns by means of alkaline activation could be achieved.

Experimental Investigation of Interface Behaviour of Different Types of Granular Soil/Geosynthetics

Abstract: In this paper, influence of different types of soils and geosynthetics on soil/geosynthetics interface behaviouris investigated by direct shear and pullout tests. Three different types of cohesionless soils and three different types of geosynthetics materials are adopted for experimental investigation. A series of large direct shear tests and pullout tests were conducted to investigate the interface behaviour of soil/geosynthetics. The test equipment, soils, and geosynthetics properties are described. The influence of soil particle size (D 50) and geosynthetic structure are discussed by analysing tests results. Results are presented and discussed in terms of peak shear resistance, peak pullout resistance, interface friction angle, efficiency factors and interaction coefficient for different soils and geosynthetics. It could be seen that the interface friction angle from both direct shear and pullout tests linearly increases with increase in (D 50) of soil. The pullout interaction coefficients (C i ) are found to be in the range of 0.62–1.72 for different tests conditions.

Prediction of Maximum Dry Density and Unconfined Compressive Strength of Cement Stabilised Soil Using Artificial Intelligence Techniques

Abstract: The soft soil that has not enough in situ bearing capacity needs proper stabilization before any construction can be done on this soil. Cement stabilization has been found to be an effective method to improve the soil properties by many researchers. The strength development in a cement stabilized mix depends on a number of factors such as the soil properties, the water–cement ratio and the percentage of cement in the mix. In the present study an attempt is made to develop prediction model to determine the maximum dry density (MDD) and the unconfined compressive strength (UCS) of cement stabilized soil with the use of two recently developed artificial intelligence (AI) techniques; functional networks (FN) and multivariate adaptive regression splines (MARS). Database previously available in the literature was used to develop the prediction models. Based on different statistical performance criteria, it was found that the FN and MARS techniques, are better at prediction of MDD and UCS as compared to previously used AI techniques, artificial neural network and support vector machine. The prediction model presented here is more comprehensive and can be used by professional engineers.

Experimental and Numerical Analysis of Geosynthetic-Reinforced Floating Granular Piles in Soft Clays

Abstract: The installation of reinforced granular piles is a commonly adopted technique to improve load carrying capacity and reduce settlements in very soft clayey soils. This paper presents results of a series of laboratory model tests and numerical analysis carried on geosynthetic reinforced granular pile under short term loading. Unit cell concept has been adopted. Laboratory model tests were conducted on unreinforced, vertical encased, reinforced with horizontal strips and combined vertical-horizontal reinforced granular piles. The loading was applied either over the entire cylindrical tank area or only over the area of granular piles. The effects of various parameters such as reinforcement, encasement stiffness, shear strength of clay, length and diameter of granular piles have been studied. Experimental results in the form of vertical load intensity-settlement relationship have been compared with that obtained from PLAXIS 3D. The results of laboratory model tests indicated significant influence of reinforcement on the ultimate load intensity of granular piles and ultimate bearing capacity of treated ground. Lateral bulging in reinforced granular piles has also been controlled by incorporating geosynthetic materials.

Shear Strength Behavior of Different Geosynthetic Reinforced Soil Structure from Direct Shear Test

Abstract: This paper presents the results of direct shear test on soil samples reinforced with geosynthetics, conducted with the aim of characterize the shear strength of reinforced soil composite. Two types of granular soil (well graded sand and silty sand) and four types of geosynthetic (woven and nonwoven geotextile—uniaxial and biaxial geogrid) were selected. Laboratory testing program were performed in two shear boxes, circular box with 63 mm in diameter and square box with 100 mm in length; the samples were made with loose and dense sand; the reinforcement layer was placed perpendicular to the failure surface; tests are conducted with three vertical confining pressures: 15.7, 31.4 and 62.8 kPa. The effect of different factors that influence the results of the shear tests is analyzed, such as: the particle size of soils, density of soils, shear box size and type of geosynthetics. The test results reveal that the maximum value of shear strength improvement was achieved for dense silty sand samples reinforced with biaxial geogrid. In general, the improvement was more favorable for samples reinforced with geogrid compared to samples reinforced with geotextile.

Strength Characterization of Cement Stabilized and Fiber Reinforced Clay–Pond Ash Mixes

Abstract: This paper presents the experimental results obtained from tests conducted on clayey soil specimen stabilized with pond ash (PA) and cement and reinforced with randomly distributed fibers. The amount of PA and cement were varied from 0–50 and 0–6 % respectively by dry weight of the soil. In order to understand the influence of admixture on the strength properties of clay, compaction tests, unconfined compression tests (UCS), split tensile strength (STS) tests and California bearing ratio tests (CBR) were conducted. In addition scanning electron microscopy and X-ray diffraction tests were carried out on certain samples in order to study the surface morphological characteristics and hydraulic compounds, which were formed. The specimens were tested for their strength behaviour at different curing periods. The obtained results have shown that addition of mixtures leads to a decrease in the maximum dry density and increase in optimum moisture content. The results also indicate that the proposed method is very effective to improve the strength of the clayey soil in terms of UCS, STS and CBR tests.

A Laboratory Study on the Geotechnical Characteristics of Sand–Bentonite Mixtures and the Role of Particle Size of Sand

Abstract: Mixtures of sand and bentonite are generally used as a liner material at the waste disposal site. Sand is added to bentonite to achieve a higher compaction density and lower desiccation shrinkage. A review of the literature showed that most of the past research works has focused to study the influence of bentonite on the various geotechnical properties of sand–bentonite mixtures. Although sand occupies a larger proportion in a sand–bentonite mixture, the effect of proportion of sand and its particle size on the characteristics of the sand–bentonite mixture is still unclear as very little information is available. This study is an attempt to understand the effect of the particles size of the sand on the behaviour of various sand–bentonite mixtures mixed in different proportions. Various mixtures of fine sand–bentonite and medium sand (MS)–bentonite were prepared by varying the sand content from 50 to 90 % by dry weight of the mix. Mixtures were tested for Atterberg limits, compaction characteristics, swelling and hydraulic behaviour. Results indicated that variation in liquid limit is not linear even though the clay content in the mixes varied linearly; mixtures with fine sand (FS) displayed relatively higher liquid limits compared to MS counterparts. FS mixtures exhibit relatively high optimum moisture content and low maximum dry density. Consolidation test indicated that irrespective of the sand particle size, mixes with bentonite content <20 % showed a general lack of appreciable swelling. This implied that bentonite content available was not sufficient for filling the voids created by sand skeleton. For the same bentonite content, mixtures with FS displayed relatively higher swelling pressure and lower hydraulic conductivity. For a given void ratio, FS mixtures exhibited lowest hydraulic conductivity for all bentonite contents.

Load-Settlement Response of Square Footing on Geogrid Reinforced Layered Granular Beds

Abstract: Experimental studies were carried out to obtain the load-settlement response of a model square footing resting on unreinforced and reinforced granular beds. The response was obtained for two cases: (a) geogrid-reinforced sand layer, and (b) geogrid-reinforced layered system consisting of aggregate layer overlying a sand layer. The parameters considered in the experimental study include the thickness of the aggregate layer, the depth of geogrid reinforcement placed in sand layer and in aggregate layer, width of the reinforcement, and relative density of bed. Plate vibrator was used to compact uniform sand beds to relative densities equal to 50 % and 70 % inside large-size test chamber of dimensions equal to 1 m × 1 m × 1 m (in length, in width, and in depth). Load was applied on square footing using a 100 kN capacity actuator in displacement-controlled mode, and the improvement in the load carrying capacity of the footing resting on reinforced sand layer and layered system was quantified in terms of load improvement factors. In addition, the optimum embedment depth and width of reinforcements were proposed for various cases considered in the study. The optimum depth of reinforcement for the case of aggregate layer overlying sand layer decreased to 0.30 times the width of the footing from 0.45 times the width of the footing for sand only case.

Seismic Response of Soil Slopes in Shaking Table Tests: Effect of Type and Quantity of Reinforcement

Abstract: To study the effect of reinforcement type and quantity on the response of model slopes in this study, a series of shaking table tests were carried out on model slopes reinforced with different quantities of geotextile and geogrid. Model slopes were constructed to an angle of 45° using poorly graded sand. Acceleration of base shaking and shaking frequency were varied in different tests. The response of model soil slopes is compared in terms of the acceleration amplifications and horizontal displacements of the slope measured at different elevations. Results from these model tests revealed that the acceleration amplifications were slightly lesser in case of geogrid reinforced slopes because of higher interfacial friction of cohesionless soil with the geogrid. Acceleration amplifications were not affected by varying the quantity of reinforcement. However, horizontal displacements reduced drastically with the inclusion of reinforcement. Though the difference was not substantial, geotextile reinforced slopes were more effective in reducing the deformations compared to geogrid reinforced slopes. With the increase in the quantity of reinforcement, deformations decreased linearly, until reinforcement saturation occurred, beyond which the rate of decrease of deformations was less.

Assessing the Influence of Some Soil–Reinforcement Interaction Parameters on the Performance of a Low Fill on Compressible Subgrade. Part I: Fill Performance and Relevance of Interaction Parameters

Abstract: Soil–geosynthetic interaction is a complex subject, particularly in geogrid reinforced structures. Geosynthetic reinforcement can be used to improve the performance of granular layers on compressible subgrades in situations such as paved and unpaved roads and in reinforced foundations. This paper presents results of large scale cyclic loading tests on reinforced and unreinforced granular layers on a compressible subgrade. A woven geotextile and different geogrids were used as reinforcement. The geogrids were chosen aiming at achieving a wide range of values of some physical and mechanical properties to allow the investigation of the influence of some relevant properties related to soil–reinforcement interaction on the geogrid performance. The results obtained showed that geogrid reinforcement tensile stiffness and some of its physical properties such as aperture–fill particle diameter ratio, thickness and fraction of grid area available for bearing are important properties for grid performance as reinforcement in granular layers. A dimensionless parameter taking into account several geogrid properties has been introduced and shows good correlation with test results. On the other hand, no correlation was noted between geogrid aperture stability modulus and granular layer performance for the conditions of the tests carried out. The results show that a geogrid reinforcement should not be specified based only on its tensile stiffness and strength, since other properties play important roles in the gravel layer performance. This is particularly relevant for reinforced unpaved roads and railway tracks.

Effect of Liquid Acrylic Polymer on Geotechnical Properties of Fine-Grained Soils

Abstract: The present paper investigates the effect of liquid polymer on the geotechnical properties of fine-grained soil. Commercially available liquid polymer (acrylic polymer) was used to stabilize natural Carbondale soil (Soil A) and commercially available soil (Soil B). The polymer was mixed at various percentages (i.e., 2, 3, 4, and 5 %) of the dry weight of both soils. Tap water was added corresponding to its OMC (optimum moisture content) for a particular soil-polymer mixture and compacted to achieve its maximum dry unit weight. The compacted samples were allowed to cure for 7, 14, and 28 days under confined and open air environment. Unconfined compressive strength (UCS) test was performed to evaluate the strength of polymer stabilized soil. The results show that with the addition of polymer; UCS value for Soil B samples prepared at OMC increases from 30 to 75 % in open air environment and the UCS value increases from 12 to 14 % in confined air environment. Soil A samples prepared at OMC (i.e., 23.50 %) show cracks while curing in open air environment and there is no significant change (i.e., 1.2–13.8 %) of strength in confined air environment. For the Soil A samples prepared with reduced moisture contents (less than OMC i.e., 12.50 %) and cured in open air environment shows increase in UCS strength from 7 to 10 %. Also, California bearing ratio (CBR) test was performed for both soils and there was marginal increase (i.e., 14 %) in CBR value for Soil A but a significant increase (i.e., 340 %) in CBR value for Soil B.

Improved Performance of Rail Track Substructure Using Synthetic Inclusions: Experimental and Numerical Investigations

Abstract: Ballasted rail tracks offer the most important means of transporting bulk freight and passengers in terms of the sheer tonnage of traffic. Ballast is a prominent component of conventional rail infrastructure because it controls the stability and performance of track. Repetitive train loads degrade ballast grains due to breakage and the progressive accumulation of external fines or mud-pumping from the softer subgrade. They decrease the shear strength and drainage capacity of track embankments, while adversely affecting its safety and efficiency as speed restrictions are imposed and track maintenance becomes more frequent. Although synthetic inclusions such as geogrids and rubber mats placed between the ballast and subballast definitely improve track performance, further study is needed before incorporating them into existing design routines catering for future high speed trains and heavier haul trains. This paper presents the very latest knowledge of rail track geomechanics, including several important concepts and topics related to laboratory testing and discrete element modelling approaches to study the load and deformation of ballast improved by rubber mats and synthetic geogrids. This paper focuses on studies carried out at the University of Wollongong on track infrastructure, and includes examples whereby innovation progresses from theory to practice. Discrete element modelling is also used to carry out a micromechanical analysis of the ballast and geogrid interface to provide further insight into ballast subjected to shearing.

Seismic Response of Geocell Retaining Walls Through Shaking Table Tests

Abstract: Performance of geocell retaining walls under seismic shaking conditions is studied in this paper. Shaking table tests on geocell retaining walls subjected to ground shaking conditions of different accelerations and frequencies are discussed. Retaining wall models of 600 mm height were constructed using a sand backfill with layers of geocells stacked one above the other to form the facing. Geocells in these tests were constructed using planar geonets by stitching them into a honeycomb network. Material used for making geocells, number of geocells in each layer, slope of the facing and infill material of geocells were varied in different tests. These geocell retaining walls were subjected to 100 cycles of sinusoidal base shaking at accelerations ranging between 0.2 and 0.3 g and frequency range of 1–7 Hz. Response of retaining walls was monitored in terms of acceleration amplifications and wall deformations at different elevations. It was observed that all geocell retaining walls were extremely strong to seismic shaking. Acceleration amplifications and displacements increased with increase in base acceleration or shaking frequency. It was observed that the detrimental effect of increasing ground motion parameters on the wall response was more significant than the beneficial effect of improvement in geocell parameters.

Environmental Impact and Physicochemical Assessment of Pond Ash for its Potential Application as a Fill Material

Abstract: Thermal power plants (TPPs) produce a large quantity of coal ash, whose disposal is a big environmental issue. A major portion of coal ash is dumped as pond ash in ash pond near TPPs. Properties of pond ash vary from TPP to TPP and also within the same ash pond at inflow and outflow point. To assess the feasibility of pond ash as a fill material, pond ash samples were collected from inflow and outflow points of three TPPs in Haryana. This paper presents a detailed characterization study on the physico-chemical, mineralogical and morphological properties of pond ash samples. Leaching study was also carried out to assess the potential contamination of ground water from coal ash fills. Results reveal that all inflow and outflow pond ashes have low specific gravity (2.03–2.27) as compared to soil (2.6–2.7), i.e. natural fill material, low amount of unburned carbon content (1.79–3.49 %) and the values of maximum dry density and optimum moisture content are within the permissible limits as per design standards of embankment construction. The results of leaching study indicate that concentration of lead (Pb) and chromium (Cr) are quite high from standard land disposal limit, prescribed by Ministry of Environment and Forest in all pond ash samples. So it can be effectively used as construction fill materials for low-lying areas and as embankment materials etc., with caution to protect ground water from contamination due to the high concentration of lead (Pb) and chromium (Cr).

Numerical Modeling of Granular Anchor Pile System in Loose Sandy Soil Subjected to Uplift Loading

Abstract: During the last few decades various researchers have proposed appropriate experimental and numerical methods to estimate the uplift capacity of granular anchor piles (GAPs) in expansive soils. Surprisingly, very few studies have been performed to determine the uplift capacity of GAPs in loose sands. This paper presents the results of the numerical study to estimate the ultimate uplift capacity of group piles. Numerical analysis is performed using finite element software PLAXIS-3D. The foundation system is assumed to consist of a different number of regularly spaced GAPs installed in loose sandy soils. The analysis examines the influence of factors such as number of piles n and length L to width D ratio, and properties of the granular pile material and compares the efficiency of group of GAP systems of different configurations.

Unconfined Compressive Strength of Compacted Disturbed Cement-Stabilized Soft Clay

Abstract: Pneumatic flow mixing method is a new land reclamation method, developed in Japan to meet the persistent lack of space. In this method dredged soft soil is mixed with a small amount of stabilizing material (such as cement) during transporting the soft soil in a pipe using compressed air to be used for land reclamation. In some cases, the soil/cement mixture is stored in temporary place for days and then transported and compacted at the required place. Basically, the cement chemical reaction starts immediately after the mixing with the soft soil and the mixture starts to gain its strength, therefore disturbing the mixture after days from the mixing influences the mixture strength. However, the soil/cement mixture is still able to gain extra strength after disturbance, transportation, and compaction. This study aims to evaluate the effect of dynamic compaction on the shear strength of disturbed cemented soft soil mixture experimentally. The mixture was fully disturbed after one week from mixing with cement. Three cement/soil ratios were used in this study under different dynamic compaction energies. Unconfined compression test was conducted at various curing times for both disturbed and non-disturbed specimens.

Pullout Tests Using Modified Direct Shear Test Setup for Measuring Soil–Geosynthetic Interaction Parameters

Abstract: Soil–geosynthetic interaction parameters and their determination play a vital role in the design of reinforced soil structures. Direct shear test and/or pullout test are commonly used to determine the interaction parameters. Often, it is economically viable to obtain these parameters through existing test setups that are conventionally used in geotechnical engineering. However, the existing test setups need certain modifications, to facilitate the requirement for the specialized tests. This paper introduces modifications to the large size (300 mm × 300 mm) direct shear test setup for evaluating the soil–geosynthetic interaction parameters under pullout. The shear box in the existing test setup is replaced by a rectangular box having internal dimensions of 400 mm × 400 mm wide and 230 mm height, with a slot in the front face. Additional amendments for achieving smooth stress transfer, over entire displacement range, are explained. Typical pullout test results using the modified direct shear test set up are presented. Pullout friction coefficient values are observed to be within the range 0.55–1.69. In general, it is observed that the pullout behavior is sensitive to the normal stress and the type of geosynthetics in terms of its surface roughness.

Interface Shear Strengths Between Geosynthetics and Clayey Soils

Abstract: Interface shear behavior between geosynthetics and clayey soil was investigated by large scale direct shear tests. The interfaces investigated are geomembrane (GM) and clayey soil; GM and geotextile (GT); and GT encased geosynthetics clay liner (GCL) and clayey soil. For GM/clayey soil interfaces, a softer GM resulted in a higher apparent adhesion, and higher water content of the soil yielded lower interface strength. A GM/bentonite interface had a small friction angle of 3 –4° only. For all cases tested, the interface shear strength (τ f) was lower than the shear strength of the corresponding soil (τ fs), and the lowest τ f /τ fs ratio was about 0.55. For GM/GT interfaces, the stiffer a GM, the lower the interface shear strength. Also a GT with a woven slit film layer, which is smoother than a randomly aligned nonwoven fiber surface, had a lower interface shear strength. The moisture content of a cover silty soil layer also had a considerable effect on the interface shear strength. Higher water content of the cover soil promoted soil particles entering the openings of the GT and increased the strength. For GCL/clayey soil interfaces, increase the water content of the bentonite in the GCL, reduced the interface friction angle, but increased apparent adhesion. The ratios of τ f /τ fs was about 0.8–1.0, and it reduced with the increase of the water content of the bentonite and overburden pressure possibly due to migration of water from the GCL to the interface.

FEM Analysis of Back-to-Back Geosynthetic-Reinforced Soil Retaining Walls

Abstract: Reinforced soil retaining walls with two opposite sides (back-to-back) are commonly used for embankments approaching bridges. This type of walls has complex geometry. The distance between two opposing walls is a key parameter used for determining the analysis methods in the federal highways administration guidelines. The objective of this research is to study under static conditions, the effect of the distance between the two back-to-back geosynthetic-reinforced soil retaining walls of embankment approaching bridges on the internal and external stability. Finite element method incorporated in the Plaxis software and analytical methods were used for this research. Parametric studies were carried out by varying the distance between side walls, to investigate their effects on the critical failure surface, the lateral earth pressure behind the reinforced zone, the wall displacement and on the required tensile strength of reinforcement.

Frictional and Interface Frictional Characteristics of Multi-layer Cover System Materials and Its Impact on Overall Stability

Abstract: The municipal and hazardous landfills nearing its design capacity need to be isolated from the atmosphere using multi-layered cover system (MLCS). These MLCS constitutes different layers of soil and geosynthetics with widely varying properties. Each of these layers fulfil specific requirements by acting as a surface protection, drainage, separation, filtration and hydraulic barrier layers of MLCS. Failure of these MLCS leads to waste–atmosphere interaction and results in extremely hazardous situation to biosphere. The stability of MLCS significantly depends on the shear strength characteristics of materials used viz., the internal frictional characteristics of individual soil materials and interface frictional characteristics of soil–soil or soil–geotextile combination. In view of this, frictional characteristics of four type of soils, four soil–geotextile interfaces and one soil–soil interface was determined using direct shear and modified direct shear testing methods. The modification of geomembrane used in barrier layer was also attempted, for improving its interface shear characteristics. The usefulness of the above parameters and the influence of its variability on the slope stability of MLCS of a near surface low level radio-active waste disposal facility (NSDF) is demonstrated in this study.

Internal Failure of Deep Mixing Columns Reinforced by a Shallow Stabilized Soil Beneath an Embankment

Abstract: The isolated deep mixing columns, particularly the columns near the slope of the embankment, were reported to experience a minor horizontal resistance while supporting an embankment. In order to reinforce these columns and increase the stability of the improved area, which includes the group of columns, a shallow layer produced by shallow mixing method was proposed as a countermeasure. While many studies reported that the isolated columns may fail either by the external failure or the internal failure, the failure of this combined structure including the deep mixing columns and the shallow layer has not well studied yet. In the previous study, overturning and sliding failures were observed as the leading patterns in terms of the external stability, when the combined structure to supports embankment slope. In addition to the study of the external stability, the internal failure pattern of the columns reinforced by the shallow layer was concentrated in this study. It is worth to note that the low-strength columns are used here. Four centrifuge model tests were performed by varying the column strengths in models with and without the shallow layer of reinforcement. The results showed a significant influence of the shallow layer on the failure pattern of the deep mixing columns. The study further discusses the effect of the shallow layer and the column strength on the stability of the supported embankment.

Experimental Investigations on Biological Resistance of Surface Modified Coir Geotextiles

Abstract: Results of detailed experimental studies carried out to investigate the resistance of coir geotextiles modified using cashew nut shell liquid (CNSL) to attack of biological organisms are reported. A mixed spore suspension composed of four fungal stains, Chaetomium indicum, Curvularia lunata, Aspergillus fumigatus and Penicillium rubrum was used to study microbial susceptibility of coir geotextiles in humid warm atmosphere. Soil burial studies were carried out under controlled conditions for a period of 240 days to learn the behaviour of geotextiles in contact with specially prepared soil containing a variety of microorganisms. A field study was carried out to investigate the behaviour of coir geotextiles in subterranean termite mounds. The improved properties of modified coir geotextiles were substantiated on the basis of weight loss, moisture absorption, tensile strength and surface morphology. The results show that biological resistance of coir geotextiles was greatly improved by modification with CNSL. Tensile strength of unmodified samples reduced to 19 % whereas modified geotextiles retained 76 % of the initial tensile strength at the end of 240 days of soil burial. SEM images affirm that modification of coir with CNSL obtained in the present work could close the pores on fibre surface and delay biological degradation considerably.

Assessing the Influence of Soil-Reinforcement Interaction Parameters on the Performance of a Low Fill on Compressible Subgrade. Part II: Influence of Surface Maintenance

Abstract: Geosynthetics can be effectively used as reinforcement of low fills on weak subgrades, as is the case of unpaved roads. They can reduce fill deformation and increase its life. This paper investigates the performance of unreinforced and reinforced low fills on a loose sand subgrade, with particular emphasis on the behaviour of the fills after surface maintenance. Different types of geosynthetics (12 geogrids and a woven geotextile) were tested in a large equipment where the fills were subjected to cyclic loading. The results obtained showed that the presence of the reinforcement improved the performance of the fill during the first and second (after surface maintenance) loading stages. The gain in performance depended on the reinforcement type and characteristics. Optimum ranges for the ratio between geogrid aperture dimension and fill particle diameter were identified for which less fill particle breakage and greater load spreading angles were obtained. The results show that the specification of a geogrid reinforcement for applications such as in unpaved roads or railway tracks based on its tensile stiffness is necessary but not sufficient to obtain maximum efficiency from the reinforcement.

Geosynthetics Under Cyclic Pullout and Post-cyclic Monotonic Loading

Abstract: Several real life reinforced soil structures such as traffic supporting embankments and pavements are subjected to cyclic loads. The behaviour of soil-reinforcement interface under cyclic load is different from that under monotonic load. In the investigation reported herein, cyclic pullout tests were conducted on geogrid embedded in sand. A load controlled pullout test apparatus complying with the dimensions suggested by ASTM D-6706-01 was designed and fabricated in-house. Modular units for applying both cyclic and monotonic load were also designed and fabricated. Cyclic load was applied through a pneumatic double acting air cylinder. In addition, the setup consisted of a signal generator and a filter lubricator regulator volume booster. The effect of normal stress and cyclic load on the pullout behaviour was studied. It was found that cyclic loads of a lower magnitude than the monotonic capacity could cause failure. In the case of cyclic loads of small magnitude, the displacements showed tendency to stabilize. In the case of higher magnitudes of cyclic loads, the displacements progressively increased to failure. An initial stiffness was noticed in the system due to the initial densification achieved near the interface due to the dynamic nature of the cyclic pullout forces. Normal load as well as cyclic load played important roles in the number of cycles to cause failure. Post-cyclic monotonic tests showed the effect of degradation of the soil-geosynthetic interface after subjecting the same to certain number of load cycles.

Comparison of 1-D and 2-D Tests in Geotextile Dewatering Applications

Abstract: In this study, two different one-dimensional tests (only vertical flow) (pressure filtration test and suction filtration test) were compared to determine variability in the results due to different forms of pressure application. In addition, an innovative two-dimensional filtration apparatus (with both vertical and radial flow) was developed in order to determine the effect of radial flow on the results. This apparatus will more accurately imitate the real-life dewatering application of geotextile tubes. Unlike on-site “hanging bag” and “pressurized geotextile dewatering” tests, this laboratory apparatus was designed in a form that will facilitate the studying of dewatering rate vertically and radially separately. It will also be used by mathematicians, as its geometric form will be more conducive to the analysis of dewatering. This will aid in the creation of a simple and fast mathematical model to determine geotextile dewatering rate, which will reduce the need and cost of experimental testing during prior to actual dewatering in the site.

Control of the Local Scouring Around the Cylindrical Bridge Pier Using Armed Soil by Geotextile

Abstract: Flood currents are considered threatening factors by creating local scour along bridge piers. One method for decreasing local scour is to strengthen the bed against imposed tensions. Among methods which can be appropriate in decreasing and controlling local scour of bridge piers directly is to put riprap beside bridge piers and to employ geotextile around them. Geotextiles form a large group of geosynthetic products produced from polypropylene and polyester fibers, and are used in separation, strengthening and reinforcing, filtration, and drainage. In the present study, the effect of geotextile layer in decreasing local scour of cylindrical single-pier was investigated and the best coverage pattern with the most effect was obtained so that layers with circular and oval shapes were put around the pier relative to its diameter, and the performance of each was compared with the unprotected pier. Test results showed by using geotextile with an appropriate cover, the scour location is transferred to downstream and the scour depth is decreased. These advantages can reduce the risk of pier failure when the duration of flood is short. The results indicate that the scour reduction increases as the layer area increased. In this case the oval layer has a better efficiency compared to the circular one.

Long-Term Intruding Effects of Acid Rain on Engineering Properties of Primary and Secondary Kaolinite Clays

Abstract: In this study, the effects of simulated acid rain (SAR) on the engineering properties of primary and secondary kaolinite clays (PK and SK) were investigated. Tests of consistency limit, compressive strength, compressibility, hydraulic conductivity, and compaction characteristics were conducted to investigate the changes in the physico-chemical and engineering properties of PK and SK clays after being exposed to SAR at different pH levels of 2, 3, 4, 5, and 5.6 and at different fluxes of SAR equivalent to precipitation for 1, 5, 10, and 20 years. In addition, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX), zeta potential and atomic adsorption spectroscopy (AAS) studies were conducted to identify the underlying mechanisms involved. The results revealed that for both PK and SK, the low pH value of SAR and higher flux of SAR resulted in a reduction in soil strength, maximum dry density, and increased compressibility, hydraulic conductivity, liquid limit, and optimum moisture contents. The AAS tests confirmed that reduction of more traced elements in PK and SK specimens when they were infiltrated by the SAR. Also, the formation of relatively a weak structure was observed through the SEM tests. The sensitivity of the PK specimen was higher than that of the SK specimen for SAR intrusion.

Sensor-Enabled Geogrids for Stabilization and Performance Monitoring of Earth Structures: State of Development

Abstract: The sensor-enabled geogrid (SEGG) technology has been introduced and developed by the authors in the past few years as a new category of geogrid products that possess built-in strain-sensing capability in addition to their conventional reinforcement/stabilization function in geotechnical and transportation applications. The SEGG strain-sensing function arises from their tensoresistivity, which is the sensitivity of their electrical conductivity to tensile strain. This paper reports the state of development of the SEGG technology and reports latest findings on both the in-isolation and in-soil tensoresistivity performance of SEGG specimens. Results indicate that the technology holds promise to serve as an alternative to conventional instrumentation for the performance monitoring of geotechnical structures.