Subgrade Stiffness Research Articles

Using accelerated pavement testing to examine traffic opening criteria for concrete pavements

The risk of cracking in a concrete pavement that is opened to traffic at early ages is related to the maximum tensile stress σI, that develops in the pavement and its relationship to the measured, age dependent, flexural strength of a beam, fr. The stress that develops in the pavement is due to several factors including traffic loading and restrained volume change caused by thermal or hygral variations. The stress that develops is also dependent on the time-dependent mechanical properties, pavement thickness, and subgrade stiffness. There is a strong incentive to open many pavements to traffic as early as possible to allow construction traffic or traffic from the traveling public to use the pavement. However, if the pavement is opened to traffic too early, cracking may occur that may compromise the service life of the pavement. The purpose of this paper is two-fold: (1) to examine the current opening strength requirements for concrete pavements (typically a flexural strength from beams, fr) and (2) to propose a criterion based on the time-dependent changes of σI/fr which accounts for pavement thickness and subgrade stiffness without adding unnecessary risk for premature cracking. An accelerated pavement testing (APT) facility was used to test concrete pavements that are opened to traffic at an early age to provide data that can be compared with an analytical model to determine the effective σI/fr based on the relevant features of the concrete pavement, the subgrade, and the traffic load. It is anticipated that this type of opening criteria can help the decision makers in two ways: (1) it can open pavement sections earlier thereby reducing construction time and (2) it may help to minimize the use of materials with overly accelerated strength gain that are suspected to be more susceptible to develop damage at early ages than materials that gain strength more slowly.

Numerical Simulation of Moving Load on Concrete Pavements

Abstract The knowledge of the development with time of the strain and stress states in pavement structures is needed in the solution of various engineering tasks as the design fatigue lifetime reliability maintenance and structure development. The space computing model of the truck TATRA 815 is introduced. The pavement computing model is created in the sense of Kirchhof theory of the thin slab on elastic foundation. The goal of the calculation is to obtain the vertical deflection in the middle of the slab and the time courses of vertical tire forces. The equations of motion are derived in the form of differential equations. The assumption about the shape of the slab deflection area is adopted. The equations of the motion are solved numerically in the environment of program system MATLAB. The dependences following the influence of various parameters (speed of vehicle motion, stiffness of subgrade, slab thickness, road profile) on the pavement vertical deflections and the vertical tire forces are introduced. The results obtained from the plate computing model are compared with the results obtained by the FEM analysis. The outputs of the numerical solution in the time domain can be transformed into a frequency domain and subsequently used to solve various engineering tasks.

Soil Resilient Modulus Regressed from Physical Properties and Influence of Seasonal Variation on Asphalt Pavement Performance

Subgrade soil, as the critical underlying support for other pavement layers and traffic loads, should be stiff enough to maintain the integrity of pavement structures and the smoothness of pavement surface. The resilient modulus, as an indicator of subgrade stiffness, is an essential input in the American Association of State Highway and Transportation Officials (AASHTO) Mechanistic-Empirical Pavement Design Guide (MEPDG). At input level 1 of MEPDG, the MEPDG generalized model is required to describe resilient modulus of subgrade soil, and the coefficients of this model are used for pavement design. The change of the resilient modulus model has raised the interest of many state highway agencies and made it necessary to convert old resilient modulus test data into new ones required for the MEPDG model. In this study, the coefficients of the generalized and the universal models for soil resilient modulus were obtained through regression of the results of 13 soils in Tennessee. The coefficients of the two models were also compared. There is a potential risk that the coefficients from the universal model may be mistakenly used in the MEPDG instead of those coefficients from the generalized model. The consequence of this improper use was demonstrated in the comparison between the miscalculated and the real resilient moduli. The coefficients of the generalized model were correlated to soil physical properties, which provided an alternate time-saving and economical method to obtain soil resilient modulus as level 2 inputs. The coefficients were obtained at different post-compaction water contents, to allow the estimation of pavement response under seasonal moisture variation of subgrade. Rutting and roughness of two typical pavement sections were analyzed to investigate the influence of the seasonal variation of soil resilient modulus on pavement performance. The results showed that moisture variation had a significant effect on subgrade resilient modulus and, subsequently, on pavement performance. It is recommended that seasonal change in soil resilient modulus be considered in the analysis on pavement performance.

An investigation of the dynamic behaviour of track transition zones using discrete element modelling

Previous investigations have shown that an abrupt stiffness change in track support is often associated with accelerated rates of deterioration of track geometry, high maintenance demand and poor ride quality. However, at present, there is no detailed understanding of the mechanisms of the deterioration of track geometry at transition zones. This paper aims to use the discrete element method to investigate transition zones from a micromechanical perspective. A simple track transition model with dimensions 2.1 m × 0.3 m × 0.45 m was simulated by using PFC3D. In order to identify and evaluate appropriate mitigation methods, two kinds of transition patterns, including a single step change and a multi step-by-step change for subgrade stiffness distribution were tested. The influence of train direction, speed and axle load on the transition was also investigated. In addition, geogrid was used in the ballast layer to examine the effects of geogrid reinforcement. This paper provides insight into the factors that can cause or accelerate track degradation at the transition zones, in order to identify and evaluate appropriate mitigation design.

Evaluation of Bearing Capacity of Low-Volume Roads in Minnesota

Deflection testing and analysis are routinely used to evaluate the spring load capacity of pavements and to design structural overlays. The current falling-weight deflectometer (FWD) deflection analysis process used by the Minnesota Department of Transportation was found to be unreliable. Traditionally, the process used to interpret FWD deflection measurements converted the measured maximum FWD deflection to an equivalent Benkelman beam deflection and compared it with the allowable deflection for a pavement with given asphalt surface thickness and anticipated traffic. Because the maximum FWD deflection is greatly affected by the subgrade stiffness, the computer program TONN may underestimate the allowable axle load for subgrades depending on the subgrade material. As a result, the researchers determined that the allowable axle load rating procedure should be revised. Those revisions and the development of a new rating procedure for low-volume roads are detailed.

Experimental and numerical investigation of slabs on ground subjected to concentrated loads

AbstractAn experimental program is presented where a slab on ground is subjected to concentrated loading at the centre, the edges and at the corners. Analytical solutions for the ultimate load capacity fit well with the results obtained in the tests. The non-linear behaviour of the slab is captured by performing nonlinear finite element analyses. The soil is modelled as a no-tension bedding and a smeared crack approach is employed for the concrete. Through a parametric study, the finite element model has been used to assess the influence of subgrade stiffness and shrinkage. The results indicate that drying shrinkage can cause severe cracking in slabs on grade.

Estimating the Elastic Settlement of Piled Foundations on Rock

AbstractThis research introduces a simple method for preliminary evaluation of the elastic settlement of piled foundations resting on rock. Despite a significant number of publications in this area, the present effort is distinguished by adopting a unique representation of rock nonhomogeneity. It also considers pile–subgrade stiffness ratios that have not been covered in the literature for piled foundations on nonhomogeneous media. The settlement behavior was investigated using an extensive three-dimensional finite-element (FE) analysis of different piled foundation models. Elastic settlement of single piles was also studied for comparison with the pile-group behavior. Charts were developed to estimate the elastic settlement of piled foundations and to help in choosing the optimum pile spacing and length that limit foundation settlement to tolerable levels. Settlement values yielded from the FE analysis were compared with those calculated from equations commonly used in estimating elastic settlement of fo...

Impact of Traffic Overload on Road Pavement Performance

Traffic on a road pavement is characterized by a large number of different vehicle types, and these can be considered in pavement design by using truck factors to transform the damage they apply to the pavement to the damage that would be applied by a standard axle. The truck factors to convert trucks into standard axles or the load equivalent factors to convert axles into standard axles are defined by considering the average loads for each axle. This process includes the vehicles that travel with axle loads above the maximum legal limit. There are also a substantial number of overloaded vehicles in terms of total vehicle weight. These axles/vehicles cause significant damage to the pavements, increasing the pavement construction and rehabilitation cost. Thus, this paper investigates the impact of overloaded vehicles on road pavements by studying the truck factors for different vehicle cases applied to a set of pavements composed of five different asphalt layer thicknesses and five different subgrade stiffness moduli. The study revealed that the presence of overloaded vehicles can increase pavement costs by more than 100% compared to the cost of the same vehicles with legal loads.

Three-dimensional modelling of high speed ballasted railway tracks

In the present paper, a three-dimensional dynamic railway finite-element model, is utilised to study the geotechnical performance of high speed railway tracks. This model takes into account many of the railway track components such as multi-layered ballast and subgrade, nonlinearity in resilient behaviour of the sub-soil, and track–train interaction. Different features of the developed model are briefly presented. The effects of train speed, ballast and subgrade stiffness, and nonlinearity of the soil behaviour are then investigated. The results of the numerical simulations regarding the track response and required maintenance levels, in terms of the stress and vibration level experienced by the ballast and subgrade are also discussed and compared to site measurements. Track design methods are also reviewed, and a design method based on the dynamic finite-element analysis is proposed and discussed, which aims at minimising railway track maintenance levels by protecting the subgrade from dynamic ground amplification for speeds approaching the Rayleigh wave velocity, and also aims at minimising ballast maintenance by setting a maximum ballast particle velocity.

A study on the correlation between soil properties and subgrade stiffness using the long-term pavement performance data

This paper presents the statistical analysis results for correlating the stiffness characteristics of subgrade with subgrade soil properties using the long-term pavement performance database in the USA. The stiffness of subgrade soils was characterised by the falling weight deflectometer deflection basin parameters and subgrade modulus calculated using the backcalculation technique. For the statistical approach, both linear fixed- and mixed-effects models were applied to estimate the correlation between subgrade stiffness and soil properties. It is found from this study that the base curvature index and subgrade modulus are highly correlated with subgrade soil properties and variation of water content. The results also indicate that the mixed-effect model can accurately estimate the stiffness of subgrade soils using the soil properties and water content. Region-specific factors considered as the sources of random effects in the mixed-effects model can help to improve the degree of correlation.

Railway track performance study using a new testing facility

This paper presents results from full-scale laboratory research on railway tracks using the geopavement and railway accelerated fatigue testing (Graft) facility at Heriot-Watt University. A typical ballast depth of 300 mm has been considered, overlying subgrade soil (a typical situation on site). Sub-ballast has not been used, to ensure that only ballast and subgrade behaviour are investigated. The influence of load level and subgrade stiffness for a typical UK track is presented, and from the results a new railway track settlement equation is proposed that fits the measured data in Graft well. The proposed equation can predict track settlement in Graft after a number of cycles for any applied load, and for the clay and subgrade Young's modulus considered in this paper. The threshold stress concept used in fine-grained subgrade materials, granular materials and within railway track design processes has been considered, and a new track parameter (t) within the track settlement equation has been proposed to account for this. The equation has been compared with other previously published equations with different parameters that are based on both laboratory and field measurements, and similarities have been found. This paper demonstrates how the Graft facility can accurately simulate the railway track response.

Analysis of the subgrade stiffness effect on the behaviour of ground‐supported concrete slabs

AbstractThis paper confirms that the structural behaviour of ground‐supported slabs is a non‐linear function of the structural properties of slabs as well as the supporting soil. The findings reported emphasize that the suggested equations used in design codes pay insufficient attention to the effect of the supporting ground stiffness within the context of the mechanical behaviour of slabs as far as ductility is concerned. The results presented demonstrate that ground stiffness has a significant effect on the ductility of ground‐supported slabs. It also pays particular attention to the possibility of determining the ductility limit of slabs.

Influence of Design Parameters of Highway on the Deformation

Based on the geometry irregularity of pavement, an expression of the traffic load was put forward. The influence law of design parameters of each part of highway and pavement geometry irregularity on vertical deformation of highway was discussed by the three-dimension finite element numerical analysis. The research shows that the vertical deformation range of highway under the action of traffic load is generally semi-circular; the influence of surface stiffness on vertical deformation is different in different horizontal positions of surface; the influence of subgrade stiffness and foundation stiffness on vertical deformation is remarkable, but that of course stiffness is small; the rational matching of all parts of highway can effectively reduce the vertical deformation; with the increase of geometric irregularity index, the vertical deformation increases significantly.

Effects of overlay designs on reliability of flexible pavements

Management of flexible pavements requires timely application of preventive maintenance and rehabilitation actions such as overlays. These actions are an integral part of a design strategy that minimises the use of recourses over the pavement's life-cycle. The objective of this paper is to develop a reliability model for flexible pavements that accounts for the effects of overlay designs. In this paper, the pavement responses are determined by the linear theory of elasticity for the system of pavement layers, while the pavement reliability is represented as a series system of two failure criteria: fatigue cracking and rutting. To illustrate the developed model, a numerical study is presented. Sensitivity and importance measures are computed for the parameters and the random variables included in the limit state functions. The sensitivity measures suggest that the initial asphalt layer thickness and the sub-grade stiffness are the critical predictors of the pavement performance after application of a structural overlay.

Stress Analysis of Cement Concrete Pavement with Special Heavy Mine Vehicle

With the rapid development of opencast mines, there is an increasing demand of extra-heavy duty vehicle for mines. The physical road structure model of vehicle SF31904 acting on the cement concrete pavement is established in this paper. The tensile stress in different situations about pavement thickness, subgrade stiffness, base rigidity and thickness was analysed using the ABAQUS finite element software. The recommended values about the cement concrete slab thickness are proposed and the factors affecting the stress are analyzed through this simulation analysis. The theoretical basis for design of the mine road was suggested.

Study on Impact Compaction of Aeolian Sand Subgrade and its Effect Evaluation

In order to solve the problem of aeolian sand subgrade compaction, we studied the technology of impact compaction, applied it to the engineering practice and analyzed its effect with Rayleigh wave. The technology of impact compaction can combine the compaction of potential energy and kinetic energy and make it easier for the materials to reach their elastic stage. With the combined function of "knead-roll-impact", the impact compaction road roller can compact the soil body and offer 6～10 times impact force and 3～4 times the depth of influence more than the vibratory roller. The impact compaction methods of aeolian sand subgrade were put forward. The comparative field compaction tests between impact and vibratory compaction are carried through, which are detected by Rayleigh wave. The results show that the impact compaction can make the density of the aeolian sand subgrade 2～5% higher than the vibratory compaction, and reach the influence depth of 7 metres. To sum up, the impact compaction can clearly increases the strength and stiffness of aeolian sand subgrade with a dynamic elastic modulus of 202.63MPa.

Postconstruction Evaluation of Subgrades Chemically Treated with Lime Kiln Dust

An extensive field investigation was carried out to determine the properties of subgrade soils treated with lime in pavements that had been in service for at least five years. Six sites were selected for the field tests. At each site, standard penetration test (SPT), dynamic cone penetration test (DCPT), and falling weight deflectometer (FWD) tests were performed to evaluate the in situ stiffness and/or strength properties of the lime-treated subgrade. In addition, laboratory tests on soil samples taken from the SPT spoon were done to obtain index properties. The long-term performance of the subgrade was evaluated by comparing the soil indexes and stiffness and/or strength properties of the lime-treated subgrade soil with those of the natural soil. In addition, pH, X-ray diffraction (XRD), and thermogravimetric analysis (TGA) tests were conducted for both lime-treated and natural soils. The field and laboratory investigation showed that (1) the lime remains in the soil even after 11 years of service of the road; (2) the addition of lime decreases the plasticity of the soil and increases its California bearing ration (CBR); and (3) the construction quality determined from the field tests was highly variable.

Backcalculation of subgrade stiffness under rubblised PCC slabs using multilevel FWD loads

Rubblisation is an effective rehabilitation method for deteriorated Portland cement concrete (PCC) pavements due to its low initial costs, minimal traffic disruption and ability to minimise reflective cracking in asphalt overlays. However, the loss of strength in the PCC slab due to rubblisation creates the demand for a subgrade that is strong enough to handle traffic after rubblisation. This paper presents a research effort to develop an analysis method that allows the realistic estimation of subgrade stiffness after rubblisation from deflection measurements on intact PCC slabs before rubblisation. As a forward model, stress- and strain-dependent nonlinear subgrade models are incorporated into a finite element analysis (FEA). Based on the synthetic database generated from the FEA, different methodologies are developed to predict the coefficients in the nonlinear subgrade model from deflections under multilevel falling weight deflectometer loads.

Behavior of soil retaining walls on deformable foundations

Cost-effectiveness evaluations for soil retaining structures require better knowledge of the integrity and serviceability of these structures subjected to foundation settlements. As a preliminary study on the performance-based design of soil retaining structures that takes into account the influence of foundation settlements, the settlement-tolerating characteristics of cantilever soil retaining walls (CWs) placed on deformable foundations were explored using a 0.6 m-high model cantilever wall backfilled with rhombically stacked 1.96 mm uniform diameter steel rods used as two-dimensional backfills of the wall. Screw jacks were used to simulate a non-yielding foundation and three types of spring with various spring constants were used to simulate deformable foundations with maximum foundation settlements of approximately 2%, 5%, and 9% of the total wall height. It was found that the measured values of the lateral pressure coefficient ( K f) increased significantly with a decrease of subgrade stiffness, k v (or an increase of maximum foundation settlement, S max). The measured values of K f for the CWs placed on deformable ground may be greater than those for the at-rest ( K o) condition when S max is greater than 3% of the total height of the wall ( H t). For the tested cantilever wall with moderately high safety factors against sliding and overturning based on a commonly adopted assumption of an unyielding foundation, significant decreases in safety factors associated with the increase in S max were observed. Equations for additional margins of design lateral pressure coefficient and/or safety factors against sliding and overturning for the wall, expressed as a function of maximum foundation settlement, are proposed to substantiate the safety evaluation of soil retaining walls situated on a deformable foundation.

Carbon Dioxide Emissions due to Highway Subgrade Improvements

Improving poor subgrades is essential to facilitate highway construction and ensure long-term stability. Improvements include enhanced load distribution by using crushed rock “capping” layers or increasing the sub-base thickness if the bearing capacity of the subgrade remains unchanged, and lime stabilization of soft clay subgrades improves the subgrade stiffness and strength to reduce or remove the need for additional crushed rock. However, the relative sustainability of subgrade improvement options as determined by indicators such as carbon dioxide (CO2) emissions is not immediately apparent. If capping is used, the relative depth requirements, together with variations due to different aggregates, make the lowest emissions option unclear. Lime production incurs far greater CO2 emissions than aggregates production, but it is used in smaller quantities and the quantity truly required, as opposed to that commonly specified, requires consideration. To address these decision-making dilemmas, a methodology is presented that allows the relative sustainability, according to CO2 emissions, to be evaluated. This methodology is illustrated through analysis of available improvement options for a typical low California bearing ratio clay subgrade by using capping, subbase thickening and lime stabilization, or a combination of these solutions. It is concluded that analysis of the relative sustainability of subgrade improvements can be informed by emissions calculations and that the lowest emissions option is heavily dependent on associated highway haulage vehicle movements. However, wider appreciation of financial, social, and environmental factors and a fundamental understanding of the role of lime stabilization are necessary if road foundations are to be constructed sustainably.