Showing papers in "Transportation geotechnics in 2016"

A review of sustainable approaches in transport infrastructure geotechnics

Abstract: Transportation geotechnics associated with constructing and maintaining properly functioning transportation infrastructure is a very resource intensive activity. Large amounts of materials and natural resources are required, consuming proportionately large amounts of energy and fuel. Thus, the implementation of the principles of sustainability is important to reduce energy consumption, carbon footprint, greenhouse gas emissions, and to increase material reuse/recycling, for example. This paper focusses on some issues and activities relevant to sustainable earthwork construction aimed at minimising the use of energy and the production of CO2 while improving the in-situ ground to enable its use as a foundation without the consumption of large amounts of primary aggregate as additional foundation layers. The use of recycled materials is discussed, including steel slag and tyre bales, alongside a conceptual framework for evaluating the utility of applications for recycled materials in transportation infrastructure.

Three-dimensional numerical modelling of ballasted railway track foundations for high-speed trains with special reference to critical speed

Abstract: Due to recent congestion of highways in many countries around the world, railways have become the most popular means of public transportation, which has increased the demand for heavier and faster trains. High speeds and heavy loads of trains are usually accompanied with large vibrations in the train–track–ground system, especially when train speed reaches its critical value, leading to possible train derailment and track damages. This unwanted scenario makes it important for railway geotechnical engineers to investigate the behaviour of ballasted railway track foundations for high-speed trains, with special reference to critical speed. In the current paper, a sophisticated three-dimensional (3D) finite element (FE) modelling was developed to simulate the dynamic response of ballasted railway tracks subjected to train moving loads, and the critical speed was investigated for various train–track–ground system conditions. The results were presented in terms of the evolution of the coefficient of dynamic amplification of sleeper deflection versus train speed, which have been synthesized into simple sensitivity charts that can be used to determine the critical speed corresponding to the conditions of a particular train–track–ground system.

Railway critical velocity – Analytical prediction and analysis

Abstract: When high speed trains travel close to the wave propagation velocity of the supporting track-ground system, large amplitude track deflections are generated. This has safety implications, and also results in a significant increase in track maintenance due to subgrade deterioration. Thus, this paper presents a method to rapidly predict the speed at which these ‘critical velocity’ effects occur. The method is based upon a dispersion analysis of both the track (either ballast or non-ballasted/slab track) and the underlying ground, which are treated as uncoupled systems. Unlike previous approaches, the new calculation approach is fully automated thus not requiring any post-processing to extract the soil dispersion curve. It also works for soil layers of arbitrary depth, uses minimal computing power and can calculate critical speeds associated with higher soil modes. The dispersion based method can be deployed on new/existing lines via a drop-weight test, or using existing geotechnical data. Its accuracy is tested by comparing the results against an alternative semi-analytical, quasi-static railtrack model, and found to be 97% accurate. The code is useful for railway track infrastructure design and its short run times mean it can be used as a scoping tool for newly proposed high speed railroad lines. To obtain new insights into the key variables effecting critical velocity, a sensitivity analysis is undertaken using 1000 random soil profiles. It is found that on average, for the same track height, slab tracks are less likely to encounter critical velocity issues than ballasted tracks because their critical speed is typically 11% higher. It is also shown that track height plays an important role with increases in slab track thickness and reductions in ballasted track thickness both causing increases in critical velocity. Furthermore, it is found that soil saturation affects critical speed considerably (by up to 12–17% depending on track type) because changes to Poisson’s ratio alter the dispersion characteristics of layered soils in the mid-frequency range, where critical velocity effects occur. Finally, it is shown that railpad stiffness has a low influence, and that increasing the rail bending stiffness on ballasted tracks increases critical speed.

Advances in ground modification with chemical additives: From theory to practice

Abstract: This keynote paper describes recent advances in chemical stabilization with a particular focus on applications related to transportation infrastructure. Chemical stabilization advances ranging from stabilization design guidelines with incorporation of fundamental soil chemistry principles to novel chemical additives for future stabilization are covered. This paper focuses on chemical treatments of shallow to moderate subgrade depths for supporting transportation pavement infrastructure. A background summary of stabilization methods along with modifications to the current design practices is presented. This section is followed by a comprehensive experimental program on chemical treatments of expansive subsoils. Both clay mineralogy influence and durability issues are fully addressed. Stabilization leachate studies and their implication in relation to real field performance are also addressed. The final outcome of this research is a more comprehensive stabilization methodology that addresses soil types and performance based testing methodologies. The last two sections cover a brief summary of new treatments of immense interest to addressing sustainable elements in ground improvement practices.

Effect of geogrid on railroad ballast particle movement

Abstract: Individual ballast particle movement significantly affects ballast performance. In order to improve ballast performance, geogrids are widely used to create interlocking of ballast particles to reduce particle movement. To better quantify the effect of geogrid on particle movement, two types of ballast box tests were conducted in this study: one without geogrid as a control and the other one with a layer of multiaxial geogrid placed at 25 cm below the top of the ballast. The ballast box represents a half section of a railroad track structure consisting of a ballast layer, two crossties, and a rail (I-beam). A wireless device – “SmartRock” was embedded underneath the rail seat and the edge of a tie in the ballast layer to monitor particle translation and rotation under cyclic loading. The results indicate that (1) horizontal translation and rotation are important modes of movement for ballast particles under cyclic loading; (2) particle translational movement and rotation were higher beneath the edge of the tie than those beneath the rail seat; (3) the particle movement, such as translation and rotation, were significantly reduced with an inclusion of the geogrid under 500 load cycles. This investigation also demonstrates that the SmartRock is capable of recording and visualizing real-time particle movement including translation and rotation and, hence, can be used as a fundamental research and monitoring tool in railroad.

Performance improvement of rail track substructure using artificial inclusions – Experimental and numerical studies

Abstract: Large and frequent loads from heavy freight and passenger trains often lead to the progressive track deterioration. The excessive deformation and degradation of ballast and unacceptable differential settlement of track and/or pumping of underlying soft subgrade soils necessitates frequent and costly track maintenance. However, artificial inclusions such as geogrids and shockmats can mitigate ballast degradation and improve track performance. A quantitative assessment of the influence of breakage, fouling, and the effects of artificial inclusions on the shear behaviour of ballast can be performed either experimentally or numerically. Numerical modelling can simulate these aspects subject to various types of loading and boundary conditions for a range of material properties so in this study, the stress–strain and degradation response of ballast was analysed through discrete element (DEM) and finite element (FEM) methods. In DEM, irregularly shaped ballast aggregates were simulated by clumping together spheres in appropriate sizes and positions. In FEM, a composite multi-layer track system was simulated and an elasto-plastic model with a non-associative flow rule was used to capture ballast degradation. These DEM and FEM simulations showed a good agreement with large-scale laboratory tests. This paper outlines the advantages of the proposed DEM and FEM models in terms of capturing the correct stress–strain and degradation response of ballast with particular emphasis on particle breakage and fouling, as well as applications of geosynthetic grids and shockmats.

Role of suction stress on service state behavior of geosynthetic-reinforced soil structures

Abstract: Several design guidelines adopt limit state methods (e.g., earth pressure or limit equilibrium slope stability methods) to estimate the reinforcement loads for geosynthetic-reinforced soil structures (GRSSs). Field measurements usually reveal lower tensile loads in the reinforcements than that predicted by conventional design methods. Consequently, limit state methods have been criticized for being conservative or inaccurate. However, these lower-than-expected loads are primarily due to redundancy in design, attributed to several factors such as toe resistance, soil volumetric dilation, underestimation of soil shear strength, and the effect of suction stress. While these factors commonly contribute to the performance of the GRSSs, they are not accounted for in design procedures. This disregard is accredited to complexities and uncertainties associated with reliable quantification of these factors during the life span of the structure. By properly quantifying the role of suction stress, this study aims to quantitatively explain a part of the discrepancy which exists between the tensile loads in the reinforcement, which are estimated by working-condition design methods versus those by limit state design methods. A suction stress-based formulation is presented for calculating the active earth pressures coefficient ( K a ) of unsaturated backfill under unsaturated steady flow conditions. The formulation is derived by implementing an analytical solution of one-dimensional steady flow into a limit equilibrium-based effective stress analysis. The proposed formulation is used in conjunction with the earth pressure method to illustrate the effect of suction stress on predicted reinforcement loads. Two backfills, referred to as marginal and high quality backfill, along with three representative annual rain events are examined. The results are compared with two classical earth pressure methods as well as two empirical methods. The results show that the suction stress contribution can lead to significantly lower reinforcement loads than that predicted by classic earth pressure methods for backfill types. Empirical methods lead to lower loads in some cases and higher predicted loads in others. The proposed formulation provides a mechanics-based approach to explain a part of the discrepancy which exists between the measured tensile loads in the reinforcement in the field versus those by limit state design methods.

Modelling geogrid-reinforced railway ballast using the discrete element method

Abstract: Rail ballast is an unbounded granular material that spreads laterally when subjected to train loading. Railroads can be reinforced by geogrids to reduce lateral movement and to optimize track performance. This paper presents a study of the behaviour of geogrid-reinforced ballast subjected to monotonic and cyclic loading using a large-scale direct shear box and a novel Track Process Simulation Apparatus (TPSA). The shear stress–strain response of fresh and fouled ballast reinforced by geogrid was investigated using large-scale direct shear tests subjected to normal stresses from 15 kPa to 75 kPa, where the levels of fouling varied from 20% to 95% Void Contamination Index (VCI). Cyclic tests for fresh and fouled ballast were conducted using the TPSA to realistically simulate real track conditions. The experimental results showed that a geogrid provides extra internal confinement and interlocks the aggregates in its apertures, hence reduces ballast deformation. The discrete element method (DEM) was used to model geogrid-reinforced fresh and fouled ballast subjected to monotonic and cyclic loading. Irregularly-shaped particles and geogird were simulated by clumping spherical balls together, while the coal fines were simulated by adding 1.5 mm diameter spheres into the pore spaces of ballast. The predicted stress-displacement responses obtained from the DEM were in good agreement with those measured in the laboratory, where the peak shear stress of fouled ballast decreased and the dilation of fouled ballast increased with an increasing level of fouling. The contact force distributions and the orientations of normal and shear force were analyzed to provide more insight into the behaviour of ballast subjected to shearing.

A Laboratory Investigation to Assess the Functioning of Railway Ballast with and without Geogrids

Abstract: Understanding the complex load transfer mechanism and the subsequent accumulation of deformation in ballast and subballast layers under repeated wheel loading is essential to design resilient rail tracks. Large-scale cyclic tests have been conducted on railroad ballast instrumented with optical based fibre Bragg grating (FBG) sensors, LVDTs, pressure plates and the settlement pegs to explore the role of geogrid and its interaction with ballast in improving the track performance. Latite basalt and geogrids with different aperture sizes were used for the investigations. The laboratory experimental results indicate that the geogrid inclusions enhance track performance by arresting the lateral spreading of ballast and thereby significantly reducing the extent of its vertical settlement. In contrast, the reinforcement of ballast with geogrid has only a marginal effect on reducing the settlement in the subballast layer. The results also show that geogrid minimises the amount of particle breakage, the effectiveness of which is governed by its placement position, with lowest breakage occurring when the geogrid is placed at a location 130 mm above the subballast. In addition, geogrids also reduce the extent of vertical stress in the subgrade soil. The laboratory test results establish beyond doubt the effectiveness of FBG sensing system in capturing the ballast movement under cyclic loading.

Behavior of geocell-reinforced granular base under repeated loading

Abstract: Major factors that influence the design of paved and unpaved roads are the strength and stiffness of the pavement layers. Among other factors, the strength of pavements depends on the thickness and quality of the aggregates used in the pavement base layer. The strength of pavement increases with increase in the thickness of the base which has a direct implication on construction cost whereas decreasing the thickness of the base makes it weak which results in low load bearing capacity especially for unpaved roads. Due to the lack of availability of aggregates, there is a need to identify alternative methods to minimize the thickness of the base without compromising on the strength of base. The use of geocell, a type of geosynthetics is a potential and reliable solution and studies are conducted in this direction. In the current study the performance of geocell reinforced base layer subjected to repeated loading is investigated. The results of the study are examined in terms of the resilient deformation and permanent deformation for different thicknesses of base layers under different repeated loading. The results showed that geocell reinforces the unbound aggregate thus reducing the deformation along with the reduction in the required thickness of the aggregate layer of unpaved roads. The amount of reduction of permanent deformation is found to be more for reinforced layer with higher thickness. The results also showed that the reinforced layers achieved maximum rut depth reduction compared to unreinforced section. The permanent deformation model modified for the geocell reinforced condition is calibrated using numerical analysis and the model can be used to predict the deformations for higher number of load cycles.

Non-destructive testing of cementitiously stabilized materials using ultrasonic pulse velocity test

Abstract: In this paper, non-destructive testing of cementitiously stabilized materials (CSMs) was studied using ultrasonic pulse velocity instrumentation. Flexural strength and flexural modulus tests were conducted on CSMs and their constrained modulus were recorded. The effect of compaction, curing time, and binder content was evaluated. The results indicate that the P-wave velocity decreases with decrease in density, whereas P-wave velocity increases with increase in curing time and binder content. A strong relationship (R2 = 0.89) was observed between flexural strength and constrained modulus, while the relationship between flexural modulus and constrained modulus at 30% stress level was also high (R2 = 0.70). Non-destructive testing is thus proposed as a convenient and expedient method for determining the flexural properties of CSMs in comparison to destructive methods such as third-point bending beam tests.

1st Proctor Lecture of ISSMGE:: Railroad performance with special reference to ballast and substructure characteristics

Abstract: Ballasted rail tracks are widely used throughout the world because they are economical, readily drained, and have sufficient load bearing capacity. Despite these advantages, geotechnical concerns such as ballast degradation, fouling (e.g. coal and subgrade soil), poor drainage of soft subgrade, pumping of clayey subgrade, differential track settlement and track misalignment due to excessive lateral movements exacerbate the cost of track maintenance. Globally, billions of dollars are spent annually on the construction and maintenance of rail tracks. Existing industry design standards are often unable to address these problems because they ignore true cyclic loading patterns, track vibrations, and the onset of plasticity and degradation of track materials. The mechanisms of ballast breakage and deformation, understanding the interface behaviour using geosynthetics, the need for effective track confinement using geocells, time-dependent drainage and filtration properties of track materials require further research to improve existing design guidelines. In view of this, large scale laboratory tests have been carried out using state-of-the-art facilities designed and built at the University of Wollongong and in other proactive rail institutes worldwide in Europe, America, Japan and China. Based on these tests, various factors governing the stress–strain behaviour of ballast, the strength and degradation of ballast, the ability of various geosynthetics and synthetic energy absorbing mats to minimise ballast breakage and track settlement, the effectiveness of subballast as a granular filter and its stabilisation with geocell have been analysed. In Australia, field studies on instrumented tracks at Bulli (near Wollongong), Singleton and Sandgate (near Newcastle), have been carried out to assess the performance of railroad embankments stabilised with geosynthetic grids, rubber mats, and prefabricated vertical drains. This inaugural Ralph Proctor Lecture focuses on the current state of research encompassing deformation and degradation assessment of railroads and the benefits of geo-inclusions, highlighting examples of innovations from theory to practice, predominantly based on the own experience of the Author.

Application of geosynthetics on low-volume roads

Abstract: There are numerous cost-effective applications for geosynthetics on low-volume roads, yet geosynthetic materials use on these roads is typically underutilized. The USDA Forest Service has been using geosynthetics on its low-volume roads for the past 40 years in applications of separation, reinforcement, drainage, filtration, and others. The objective of this paper is to document many of these uses on low-volume forest roads, both traditional and unique, and discuss the many cost-effective advantages of geosynthetic use. Uses in low-volume road applications are similar to those used in many highway projects, and have many of the same benefits and cost savings, yet utilization is variable and inconsistent. In many developing countries, engineers and technicians designing rural roads have barely heard of geosynthetic materials, much less taken advantage of their benefits. Low volume roads make up roughly two thirds of all the roads worldwide, or roughly 30 million kilometers of roads, yet they do not receive the attention and appropriate technologies deserving of such a major amount of infrastructure. Significant cost savings and improvements in design and ultimately roadway performance can be realized with the increased use of geosynthetics in underdrains, for subgrade reinforcement, in geosynthetically reinforced retaining structures, and in improved erosion control. Materials used include geotextiles, geogrids, geocells, geofoam, netting, and other geosynthetics.

Receptance of railway tracks at low frequency: Numerical and experimental approaches

Abstract: This paper presents numerical simulations and experimental studies on the frequency domain behavior of railway track below 100 Hz, focusing on the link between the substructure properties of the track and its global dynamic response. A numerical method in the frequency domain is first proposed and used to understand the frequency response of a railway track with a French High Speed Line (HSL) design. Then, low-frequency receptance measurements, performed in a specific HSL test site with different designs, are presented. These experimental results are used to characterize a change in the track substructure. Further analysis of the full track responses associated with peaks visible in the receptance test is conducted using numerical simulations. In the considered test case, these simulations demonstrate the existence of the superstructure and ballast resonance on relatively soft mats.

“Critical particle size” and ballast gradation studied by Discrete Element Modeling

Abstract: In this paper, a Discrete Element Modeling (DEM) approach is used to evaluate the impacts of gradation on both ballast void space and load carrying performances. All existing ballast gradations were first represented by a set of “characteristic gradation curves”. The effect of “characteristic gradations” on aggregate assembly volumetric properties is then studied by using DEM. It is found that ballast particles with size around half of the nominal maximum size are not favorable as they separate major particle contacts and introduce extra voids i.e. decrease the overall density of the ballast. To test the effect of this particle size on ballast load carrying capability full-scale ballast layers with common gradations listed in the American Railway Engineering and Maintenance-of-Way Association (AREMA) specifications for main line railroads are generated and tested in DEM. Also, the lab test for the modeling was conducted to verify the modeling results. Repeated train loading is applied to investigate the structural performances by means of comparing settlements occurred after certain volume of traffic. Introducing ballast particles with size finer than half of the nominal maximum size was again not favorable because it yields the maximum ballast settlement. This finding is believed to provide insight into optimizing ballast layer aggregate gradations for better railroad track performances.

The influence of tree root water uptake on the long term hydrology of a clay fill railway embankment

Abstract: This paper uses a numerical model to investigate the influence of tree root water uptake and tree removal on pore water pressures and the vertical movement of a clay fill railway embankment. Simulated results of soil wetting and drying are compared with field measurements from an instrumented railway embankment before and after tree removal. A parametric study compares the influence of vegetation on the seasonal movement of the embankment slope. The simulations and field measurements show that while trees cause significant seasonal variations in pore water pressure and water content near the soil surface, they can maintain persistent soil suctions at depth within the tree rooting zone. Demonstration of this result using a numerical model requires a root water uptake function that separates spatially the processes of water infiltration, evaporation and transpiration. When all of the trees are removed, the persistent soil suctions established by the trees are lost as water infiltrates from the soil surface. Leaving the trees in place over the bottom third of the slope can maintain persistent suctions at the slope toe, while potentially also reducing seasonal ground movements at the crest that may adversely affect railway track geometry.

Machine vision based characterization of particle shape and asphalt coating in Reclaimed Asphalt Pavement

Abstract: Reclaimed Asphalt Pavement (RAP) particles are created from the impact removal and/or reprocessing of existing asphalt layers. RAP particles contain a combination of asphalt and aggregates with varying degrees of coating and morphology. Particle size and shape properties, amount of asphalt coating the RAP particles, and the binder content of the RAP are among the important engineering properties that control the performance of this material. This paper introduces an innovative machine vision-based inspection system to quantify the percentage of asphalt coating in different RAP aggregate sources. The Enhanced-University of Illinois Aggregate Image Analyzer (E-UIAIA) is used to acquire the color RGB images of RAP particles from six different sources with sizes between 1/4 in. (6.35 mm) and 1/2 in. (12.5 mm). The influence of asphalt coating percentage on the RAP particle size and shape properties are quantified in this paper. Then, using the advanced color image thresholding scheme incorporated in the E-UIAIA, the corresponding segmented binary images of RAP particles are generated. A newly defined image mean property is used as an automatic variable threshold limit to segment the bright areas in the associated grayscale version of RAP images to detect the uncoated areas on each particle. A relationship was found between the results of the proposed image processing technique in terms of asphalt coating percentages and the asphalt content of the RAP. Furthermore, the asphalt surface coating percentages could be successfully correlated to the fracture energies of concrete specimens containing these RAP particles blended with other virgin aggregates.

Research and construction of geosynthetic-reinforced soil integral bridges

Abstract: Geosynthetic-reinforced soil (GRS) integral bridge was developed to overcome several inherent serious problems with conventional type bridges comprising a simple-supported girder (or multiple girders) supported via bearings typically by RC abutments retaining unreinforced backfill (and a pier or piers for multiple girders). The problems include: (a) relatively high construction and maintenance costs with relatively long construction time resulting from the use of bearings and massive abutment structures usually supported by piles; (b) bumps immediately behind the abutments; and (c) a relatively low stability of the girders supported by roller bearings and the approach embankment against seismic and tsunami loads. For a GRS integral bridge, a pair of GRS walls (and an intermediate pier or piers if necessary for a long span) are first constructed. After the deformation of the supporting ground and the backfill of the GRS walls has taken place sufficiently, steel-reinforced full-height-rigid (FHR) facings are constructed by casting-in-place concrete on the wall face wrapped-around with the geogrid reinforcement. Finally a continuous girder is constructed with both ends integrated to the top of the FHR facings. The girder is also connected to the top of an intermediate pier, or piers, if constructed. The background and history of the development of GRS integral bridge is described. The first four case histories, one completed in 2012 for a new high-speed train line and the other three completed in 2014 to restore a railway damaged by a great tsunami of the 2011 Great East Japan Earthquake, are reported.

Effect of water content on the resilient behavior of non standard unbound granular materials

Abstract: The results of an experimental study on non-standard road granular materials are presented, including small strain precision triaxial tests under cyclic loading and wetting tests. The materials have attrition coefficients, plasticity index, and fines content above the recommended limits. The influence of each parameter and the influence of the water content were studied. This work emphasizes the role of suction in the interpretation of the results, and the possibility to use an effective stress approach to express the changes in normalized resilient modulus as a function of a single parameter representing both total stresses and suction. Among the three effective stress definitions tested, those of Terzaghi and Taibi were shown to yield satisfactory results. This approach noticeably simplifies the characterization of UGM specimens at different water contents in the perspective of a more rational design of pavement layers.

Characterization of granular lateritic soils as pavement material

Abstract: Due to the scarcity of crushed stone aggregates, the feasibility of using locally available granular materials and marginal aggregates as pavement material is being explored in most part of the world. Granular lateritic soil (locally known as Moorum) has been used as a sub-base material for road construction in India. The characteristics of lateritic soils vary considerably based on their mineralogical composition, microstructure of soil particles, climate, parent rock and degree of laterisation. Therefore a detailed characterization of granular lateritic soils available locally in India is very much essential for its use as pavement material. In recent years, there is a sway towards mechanistic design of pavements, which requires the mechanical properties of all the materials being used in the pavement layers. Material property in terms of elastic modulus or resilient modulus and Poisson’s ratio are two major input parameters for mechanistic design. Granular materials show stress hardening characteristics and behave non-linearly under repeated loads. Considering the requirement of characterization of granular lateritic soils in India, a comprehensive laboratory study was taken up. This paper presents the results of various tests conducted on these soils collected from the eastern part of the country and the correlations established between different parameters for easier determination of complex soil properties.

Dynamic simulation of a flexible pavement layers considering shakedown effects and soil-asphalt interaction

Abstract: This research evaluates the effects of shakedown and soil asphalt interaction on the dynamic simulation of flexible layered pavement structure. In a newly developed assessment methodology the shakedown concept is implemented for the granular layer under asphalt concrete. Under this assessment granular material behaviour changes from plastic to elastic as a function of the number of loading cycles. Interactional forces are considered through contact elements placed between the base and the asphalt layer. A nonlinear stress dependent model in the elastic domain and the Mohr–Coulomb constitutive model in the plastic domain are utilised to simulate the behaviour of granular layers and the results of the simulation are validated with previously published literature. One static simulation and three different dynamic simulations under cyclic Haversine loading of a single tyre dual axle are then compared with each other. The first dynamic simulation assumes Mohr–Coulomb plasticity, the second simulation considers the shakedown effects and the third simulation calculates soil asphalt interactional forces.

A study on the deformation characteristics of ballasted track at structural transition zone by multi-actuator moving loading test apparatus

Abstract: At a structural transition zone between concrete structure and embankment, settlements of ballasted tracks generally become larger than those in a general section. To investigate the deformation characteristics of ballasted track at the transition zone, scale model tests using multi-actuator moving loading test apparatus were carried out. This test apparatus is equipped with sixteen electric–hydraulic actuators to simulate a moving train load traveling on the rails. In the test case without any buffering structure at the transition zone, large local settlement appeared even under the condition of constant moving load without any dynamic wheel load change. Conversely, by installing an approach block (wedge-shaped backfill with well graded and well compacted crushed stone) at the transition zone, local large settlement did not appear. Similarly by installing an approach slab (reinforced concrete slab) or a resilient mat (resilient polyurethane mat installed on the concrete structure), settlement at the transition zone became less than that in the case without any buffering structure.

Resilient modulus of fine-grained soil and a simple testing and calculation method for determining an average resilient modulus value for pavement design

Abstract: The resilient modulus has been recognised as an important property that governs the performance of pavement materials under the dynamic wheel loads. The resilient modulus can be obtained from a repeated load triaxial test with a series of a combination of deviator and confining stresses. It is traditionally reported as a function of the deviator and confining stresses, which can be a practical challenge in selecting the appropriate resilient modulus value for a pavement design. In this study, eight different Victorian fine-grained soils with different moisture contents were used for the determination of the resilient modulus and the evaluation of a proposed simplified method. The resilient modulus from the simplified method is reported as a single value, which is taken as the average value from the simplified testing method. It has been found that the resilient modulus obtained from the proposed simplified and the standard testing method are almost identical.

Effect of clay invasion on shear behavior and dilatancy of unbound aggregate subbase

Abstract: Unbound aggregate subbase is occasionally fouled by the invading or pumping of fine subgrade soils, and evaluation of clay invasion on shear and dilative behaviors become practically valuable. This paper presents an experimental investigation of the effect of clay invasion on shear and dilative behavior of a tested granite aggregate, used as subbase fill. To deepen the understanding of dilation, the large-scale triaxial tests (TX) tests were conducted on aggregate–clay mixtures in saturated conditions. The invasion of clay into unbound aggregate subbase leads to decreasing peak strength and critical strength, with a 20–30% strength loss. Both the clay inclusion and the increasing confining pressure decrease the volumetric strain in the form of dilation suppression. For the mixtures, the linear relationship was observed between the constant parameter of dilatancy angle and shear stress ratio. A hyperbolic model was proposed for the dilation rate and shear stress ratio, and the parameters, a, b, k, were identified. Results show a good agreement between the experimental diagrams and the modeling trends. The findings would benefit the application of unbound aggregate materials in pavements.

Effect of plasticity index and dust ratio on moisture-density and strength characteristics of aggregates

Abstract: Unbound aggregate base and subbase layers distribute wheel load induced stresses to protect subgrade. Standards such as AASHTO and individual states specifications have set several quality control limits for a variety of properties of aggregates used in the pavement base and subbase courses. Most standards and DOT specifications suggest that the ratio of percent passing No. 200 sieve to percent passing No. 40 sieve should not be greater than two thirds, and plasticity index of the material passing No. 40 sieve should be less than 6%. The ratio of percent passing No. 200 to percent passing No. 40 sieves is called the dust ratio (DR). There are many states that have either waived the standard limits or adopted their own quality control method. In this study the effect of dust ratio, percent passing No. 200 sieve and plasticity index (PI) affecting moisture-density and strength characteristics of crushed limestone aggregates are investigated. The prepared samples represented one of the most commonly utilized dense-graded standard aggregate gradation in Illinois, i.e. CA6. Standard Proctor compaction and soaked California Bearing Ratio (CBR) tests were conducted on specimens to determine the moisture-density and strength characteristics of the material with moisture content, respectively. In addition, two staged triaxial tests using three confining pressures were performed to study the effect of confining pressure and fine quantity on strength of aggregates. According to the results, the effect of percent passing No. 200 sieve on strength variation is significant in unbound aggregates. The sensitivity of aggregate strength to moisture content variation is also discussed.

Investigation of kinematic behavior and earth pressure development of geogrid reinforced soil walls

Abstract: Geogrid reinforced soil is an advanced composite material that is widely used for the construction of retaining walls as frequently needed in transportation structures. Most standards and proposed design methods have included a beneficial effect of geogrid reinforcement on the earth pressure that acts on the facing of a retaining wall. However, field measurements at applications often show much lower deformations and stresses as estimated from the design. This indicates a certain improvement potential for the design of geogrid reinforced soil structures and thereby for the general acceptance of geogrid reinforced soil. Therefore, at RWTH Aachen University model tests with geogrid reinforced soil retaining walls have been carried out. The design of the developed test apparatus allows the determination of various parameters, such as the determination of the earth pressure distribution at the facing, geogrid connection loads at the facing and specimen deformations throughout the whole cross section of the test wall. This article deals with the results of parametric studies focusing on the reduction of the active earth pressure in geogrid reinforced retaining structures due to geogrids with varying reinforcement ratio, i.e. number of reinforcement layers per specimen height, and reinforcement stiffness. A reduction of the earth pressure due to a surface load was apparent already underneath the topmost reinforcement layer. This effect was observed for both, structures with and without a facing connection of the reinforcement. With increasing reinforcement ratio, the sliding soil wedges decreased in size and only an unconfined soil area, beneath a developing soil arch in between two reinforcement layers, caused a horizontal earth pressure on the facing. All observations were merged to formulate a mechanical model idea that is presented at the end of this article.

Investigation of approach slab settlement for highway infrastructure

Abstract: Approach slabs serve as a transitional system between an approach road and a bridge. Settlement of bridge approach slabs and their supporting backfill has been reported by more than ten Departments of Transportation throughout the United States. According to current Wyoming Department of Transportation inspection reports, bridge approach slab settlements occurred not only on existing bridges but also on newly built bridges that were recently opened to traffic. These settlements typically create voids ranging from 150 mm to 305 mm (6–12 in.) between the base of the approach slab and the geotextile reinforced backfill. This research presents factors causing bridge approach slab settlements and provides appropriate design and construction recommendations. A comprehensive literature review pertaining to approach slabs was performed to examine outcomes of research conducted by 12 states. The current specifications and standards on bridge approach slabs of the corresponding states’ Departments of Transportation were also evaluated. A nationwide survey was conducted to fill in the missing knowledge identified in the literature review. The results of the survey are categorized into three groups based on the percentage of bridges experiencing approach slab settlements in that state. Results of the survey show that 46% of the total 28 states are not satisfied with their current approach slab designs. The survey results revealed that the most common cause of approach slab settlement is poor construction practices. The most important finding of the survey is that performing in-situ tests to control backfill compaction reduces the amount of approach slab settlement. Using the lessons learned from the literature and the results of the survey, recommendations were developed for transportation agencies nationwide.

Evaluation of the Effect of Toe Restraint on GRS Walls

Abstract: The effect of toe restraint on the behavior of geosynthetic-reinforced soil (GRS) walls was evaluated using the results of three large-scale walls constructed at the COPPE/UFRJ Geotechnical Laboratory. The walls were similar, with the exception of the toe restriction. The walls were well-instrumented in order to monitor the values of the reinforcement load, toe load, horizontal facing displacement, horizontal stress at the back of the block facing and vertical displacement on the top of the walls. Results clearly showed the influence of the toe restriction on the walls’ behavior. A higher toe restraint led to a greater toe load and lower maximum reinforcement load, as well as lower horizontal facing displacement and vertical displacement. Furthermore, the measured values of the reinforcement load were compared to the values predicted by design methods found in the literature, in order to verify the prediction accuracies of those methods. Results are presented and some limitations of those methods are discussed.

Design and construction of the first GRS integrated bridge with FHR facings in Europe

Abstract: The bridge across the Pavlovski potok stream in the village of Žerovinci in northeast Slovenia is a part of the investment into the modernisation of the Pragersko–Hodos railway line, one of the biggest investments in the infrastructure in Slovenia at the moment. Its design was accompanied by very short deadlines and a deep layer of soft foundation soil. For this reason the reinforced concrete abutments of a nearby railway bridge were founded on 24 m deep piled foundations. Short deadlines and a limited budget forced the authors of this paper to find an alternative solution. Deep piled foundations were replaced by shallow foundations made of compacted fill material reinforced with geosynthetics. The bridge was completed by the end of 2014. From the results of previous laboratory tests that were obtained within the scope of the EU co-funded research project “Research voucher”, the basic characteristics of the building materials for the geosynthetic reinforced soil (GRS) bridge abutments, as well as the deformation properties of the typical reinforced soil were obtained. These data were used for the design of the abutments. The staged construction procedure of full height rigid (FHR) facings for the GRS retaining walls (RW) was used for the construction of this GRS integrated bridge. Partial pre-stressing of geogrids and consequently the increased stiffness of the reinforced soil was achieved by following the staged construction procedure. The bridge system consisted of a cast-in-situ RC slab, which was placed on top of the GRS immediately behind the FHR facings, i.e. the bridge was constructed as a simply-supported slab supported by a pair of GRS abutments, without the use of bearings. Thus the described bridge across the Pavlovski potok combines two approaches for GRS integrated bridge design, one of which has been used in Japan (Tatsuoka et al., 2009), with full structural integration of the deck onto the pair of FHR facings, other being proposed by the FHWA (Adams et al., 2010), without full integration of the deck onto the GRS RWs. A system for the monitoring of structure performance was established in order to ensure optimization of this kind of structure. Data for this GRS integrated bridge, which were obtained during the construction works were compared to similar data obtained from the construction of a nearby railway bridge with reinforced concrete abutments. The comparison provides a good basis for future decisions when choosing the type of bridge abutments.

Estimation of Resilient Modulus of Unbound Granular Materials Using Clegg Impact Value and Field Stress Levels

Abstract: Resilient modulus (Mr) is an important property that controls the performance of pavement materials under dynamic wheel loads. Mr can be determined in the laboratory from repeated load triaxial tests and is defined as the ratio of the deviator stress to the recoverable strain after a number of load applications. Inherently, it is a challenge to perform repeated load triaxial tests as routine tests due to their rather complicated, time-consuming and expensive procedure. Hence, researchers have attempted to develop empirical estimation models based on the mechanical properties of pavement materials, such as the California bearing ratio and the unconfined compressive strength or physical properties of materials. This study examines the correlation between the Clegg impact value (CIV) and the Mr of unbound granular materials (UGM) of pavements. The Clegg impact hammer test provides a parameter based on the response of the material to dynamic, rather than static or gradual, loading. Fourteen different unbound granular materials with particle sizes ranging from 7 mm to 19 mm used in the study. The results indicate that there exist a good to strong correlation between the resilient modulus Mr and Clegg impact hammer value, CIV, with the highest R2 value of 0.82 and the average R2 value of 0.76, from 48 stress levels used in Mr tests.