Subgrade Soil Layer Research Articles

Assessing strength properties of stabilized soils using dynamic cone penetrometer test

Abstract The subgrade soil layer is the most essential part of the pavement system. Many pavement failures can be associated with subgrade weak strength and stiffness qualities. Therefore, it is necessary to strengthen this layer before building the other pavement layers. One of the essential methods utilized to enhance the engineering characteristics of this layer is soil stabilization. Stabilization methods are many and varied, but chemical and mechanical stabilization are the most common. This research aims to evaluate the strength of stabilized soils by using a dynamic cone penetration (DCP) test. To achieve this aim, subgrade soil was provided from a roadway project in Kerbala city and stabilized with 2.5 and 5% of Portland cement by weight of the dry soil. Then, the cemented subgrade soil was reinforced with 6 and 12 mm of polypropylene discrete fiber. The fiber was added to the soil with the following contents: 0.5, 1, and 2%. The characteristics of stabilized soils were evaluated by determining three parameters: dynamic cone penetration index (DCPI), in-situ California bearing ratio (CBR), and bearing capacity obtained from the DCP test. The results showed that when Portland cement was increased from 0 to 2.5% and then to 5% by weight of the dry soil, the DCPI value gradually decreased while increasing both CBR and bearing capacity. However, adding 6 and 12 mm fiber contents (0.5, 1, and 2) to the cement–sand mixture containing 2.5 and 5% cement led to increased DCPI and a gradual decrease in CBR and bearing capacity. Also, the results showed that the DCPI for a 12 mm fiber is lower than that for 6 mm fibers, while the CBR and bearing capacity for a 12 mm fiber are greater than those of 6 mm fibers for all ratios. Accordingly, the results of the DCP tests showed that the most significant support for the soil is obtained when the soil is stabilized using 5% cement with 12 mm of fiber by weight of dry soil.

Strengthening Soft Clayey Subgrade Soil Layer Using Cement Kiln Dust and Palm Kernel Ash Materials

The product materials which are generated from industrial manufacturing are used to enhance the engineering and chemical properties of soil. There are three phases of soil improvement are improvement without the addition of any other material, adding certain materials, and providing reinforcement. This study used materials product from industrial and waste materials as additives to improve the physical properties and strength of clayey soil subgrade under foundation and flexible pavement structure. This study has been investigated the effect of isolated and combined Cement Kiln Dust (CKD) and Palm Kernel Ash (PKA) materials on strength of clayey soil as tested by California Bearing Ratio (CBR) test. Various percentages of CKD and PKA materials were added to clayey soil samples to determine the optimum percent of CKD and PKA. The results showed that CKD and PKA have improved soil strength at the optimum compound percent of CKD content was 8% and PKA was 2% by weight of dry soil.

Simulation and Experiments on Signals of Ground-Penetrating Radar in Soil Cement

Subgrade bedding is a key element of railway structure in stability and durability, and it is almost made of soil cement. The dielectric constant, compaction and curing ages of soil cement with different moisture contents were measured. The relation among curing ages, moisture content, compaction of soil cement and electromagnetic features was analyzed. The model of layered soil cement was established to simulate ground-penetrating radar (GPR) electromagnetic waves in the soil cement. The variation rules of dielectric constant and amplitude of radar signals were obtained. The electromagnetic features, i.e. velocity, frequency and amplitude, were analyzed. Ground-penetrating radar was used to detect the railway subgrade soil layer. Based on the center line of the subgrade, there were nine detection lines for every 1[Formula: see text]m to detect the density and smoothness of the subgrade soil layer. Based on the radar wave events and reflected waves, the subgrade condition can be judged.

The cold accumulative effect of expressway embankment with a combined cooling measure in permafrost zones

The thermal performance of an experimentally built expressway embankment with a combined cooling measure at Beiluhe Basin in the Qinghai-Tibet Plateau was investigated based on long-term field monitoring. Six-year records of soil temperatures showed that the embankment cooled by a measure combining hollow concrete bricks and ventilation ducts had a significant cooling effect on the subgrade soil. Over the investigated time period, the permafrost table beneath the embankment moved upward to the natural ground surface, and the subgrade soil layers down to a depth of −15 m underwent a considerable cooling. Then, the long-term thermal performance of this kind of cooled embankment with different surface widths (13 and 26 m) is comparatively studied using numerical simulations. The simulated results show that both embankments perform well in cooling the subgrade soil. However, the annual heat loss of the subgrade soil beneath the wider cooled embankment is greater than that of the narrow ones. Via the wider cooled embankment, more cold accumulates in the subgrade soil after the embankment's construction, which we term as the “cold accumulative” effect. This cold accumulative effect can ensure the underlying permafrost thermal stability better in the context of climate warming and strength the long-term embankment stability.

Experimental Simulation and Quantification of Migration of Subgrade Soil into Subbase under Rigid Pavement Using Model Mobile Load Simulator

AbstractRigid pavement structure typically consists of a surface layer (concrete), underlying granular layers (subbase and/or base), and a compacted subgrade soil layer. Because the subgrade is sat...

Strength Enhancement of Silty Sand Soil Subgrade of Highway Pavement Using Lime and Fines from Demolished Concrete Wastes

The highway pavement quality and lifetime depend on its different layers such as subgrade, sub-base and base courses. It is of great importance for subgrade soil layer to have the excellent properties as it is the one to lower or increase the project cost. This paper discusses the utilization of lime and fines from concrete waste to enhance the strength of silty sand soil. California Bearing Ratio (CBR) and shear strength were evaluated. The content such as 0%, 2%, 4%, 6%, 8% and 10% were used. The CBR and shear strength of soil increased with the increase of lime or concrete content. The optimum lime and concrete content which maximized CBR were 6% and 8% respectively. The shear strength improvement was also noticed. No optimum content from both stabilizers which gave maximum shear strength.

Numerical Simulation for Mechanical Behavior of Asphalt Pavement with Graded Aggregate Base

The performance of asphalt pavement is determined by the combination of its material properties, road structure, and loading configurations. A DEM numerical simulation study was conducted to determine stress distribution and deformation behavior of asphalt pavement with graded aggregate base under standard traffic loading. Stress contour and displacement contour were presented via a self‐made program. Compressive stress concentrated area located in both sides of wheel, while tensile stress concentrated area appeared in lower part of the asphalt layer. The traffic loading transferred downward by graded aggregate base and to both sides at the same time, and has a trend to expand gradually with increasing depth within graded aggregate base. Therefore, stress was well distributed in the subgrade soil layer with a great action scope, and the value decreased obviously because of the stress dispersion of graded aggregate base. Vertical displacement was the main displacement of the asphalt layer, and on the both sides of traffic loading, displacement was downward and inclined slightly to the central of loading. Vertical and horizontal deformations included in both graded aggregate base layers, and displacement extended to both sides gradually with increasing depth corresponding to stress‐distribution trends. Vertical displacement was dominated in the subgrade soil layer which was relatively small.

Measuring the migration of subgrade fine particles into subbase using scaled accelerated flexible pavement testing – a laboratory study

Pavement structure typically consists of an asphalt concrete or concrete surface layer, underlying granular layers (subbase and/or base) and compacted subgrade soil layer. During wet seasons when the subgrade becomes saturated, cyclic traffic loading may result in pumping of fine particles from the subgrade up into the granular layer, which may lead to failure of the pavement. The objective of this research is to investigate occurrence of migration of fines and to determine the magnitude and rate of such migration. A one-third scale accelerated pavement testing device is utilised to simulate the cyclic traffic loading on two geometrically scaled flexible pavements representing typical collector roadways and interstate highways. The pavement sections are constructed on partially saturated aggregate subbase overlying non-plastic saturated silt as subgrade. The lab testing reveals that, under cyclic loading conditions, a considerable amount of fines migrated into the subbase. The subgrade fines migration in mass percentage increases with the simulated traffic loading cycles. The resulting gradation from the migration of subgrade fines into the subbase also varies with the depth in the subbase: more fines are deposited in the lower section (closer to the subgrade) than in the upper section of the subbase.

Field-Monitoring System for Suction and Temperature Profiles under Pavements

The purpose of this paper is to describe the development of and typical results from a new field-monitoring system to evaluate changes in matric suction and temperature profiles in subgrade soil layers beneath constructed pavement systems over an extended period of time. This monitoring system involves the placement of sensors capable of inferring the volumetric water content and temperature of soils into a borehole in the shoulder of an existing pavement. High-permeability silica flour is used to backfill the borehole around the sensors so that changes in matric suction with depth in the subgrade can be inferred through the soil-water retention curve of the silica flour. The monitoring results from a pavement site in Arkansas with low-permeability clay subgrade are compared with predictions from the Enhanced Integrated Climatic Model (EICM). The measured and predicted temperature distributions match well, but the EICM did not capture the fluctuations in matric suction inferred from the monitoring system. This can be attributed not only to the fact that the monitoring system captured the two-dimensional water flow near the pavement shoulder but also to the possibility that the EICM may not have adequately represented the water-flow process through the asphalt and base-course layers. Profiles of matric suction and temperature obtained using the monitoring system provide real-time, site-specific feedback on interactions between the atmosphere and pavement systems. Comparison of results from multiple pavement sites with clay subgrades across Arkansas demonstrates the variability in matric suction and temperature changes in different climatic settings.

Nonlinear Pavement Foundation Modeling for Three-Dimensional Finite-Element Analysis of Flexible Pavements

Pavement foundation geomaterials, i.e., fine-grained subgrade soils and unbound aggregates used in untreated base/subbase layers, exhibit nonlinear behavior under repeated wheel loads. This nonlinear behavior is commonly characterized by stress-dependent resilient modulus material models that need to be incorporated into finite element (FE) based mechanistic pavement analysis methods to predict more accurately the pavement resilient responses, such as stress, strain, and deformation. Many general-purpose FE programs have been used to predict such pavement responses under various traffic loading conditions while not considering properly material characterizations of the unbound aggregate base/subbase and subgrade soil layers. This paper describes the recent pavement FE modeling research efforts at the University of Illinois focused on using both the specific-purpose axisymmetric and general-purpose three-dimensional (3D) FE programs for flexible pavement analyses. To properly characterize the resilient beh...

Pollution Intrusion on Soil-Pavement System

The performance of both flexible and rigid pavement is a function of the constituent materials and their susceptibility to physical and chemical stress and degradation. To that end, specific phenomena that accompany and characterize the interaction of the soil-pavement system with the environment are discussed. Mechanisms of pollution intrusion through the pavement wearing surface, base, subbase, and subgrade soil layer are evaluated. Liquid movement phenomena and processes in the soil mass such as infiltration, percolation, and hydraulic conduction related to the soil-pavement system are examined. Laboratory data that reflect the effects of pollution intrusion on the behavior, stability, and performance of soil-pavement systems are presented and remedial actions on these effects are discussed. Pollution intrusion is manifested by changes in system variables, including solution pH, dielectric constant, ionic composition, temperature, and moisture content. These changes are shown to influence pavement design parameters, such as soil physical properties, compressibility, and stress.

Characteristics of Geogrid-Reinforced Aggregate Under Cyclic Load

The characteristics of geogrid-reinforced aggregates over soft subgrade soil were investigated through a laboratory-testing program. Nine cyclic plate load tests were conducted with varying base layer thickness and reinforcement type. Results indicated that aggregate base course (ABC) degraded under cyclic loading as manifested by an increase in stresses at the interface between the ABC and the subgrade with increasing number of cycles. The improvement in stress distribution due to geosynthetic inclusion at the interface of ABC and subgrade soil is indicated by a decrease in the measured maximum stress (under the center of the loaded area) and the measurement of a more uniform stress distribution on the subgrade soil layer. The higher modulus geogrid provided a better load-spreading effect compared with a lower modulus geogrid used in the testing. The improvement in plastic surface deformation was related to two aspects: ( a) decrease in vertical deformation of the subgrade and ( b) decrease in lateral spread of the ABC. A model that incorporates the mechanics of reinforcement contributions to deformation and stresses and the effect of ABC degradation as a function of number of load cycles needs to be developed for the transportation community to fully realize the benefits of reinforcement in such an application.

Elastic Pavement Analysis Using Infinite Elements

A numerical analysis of a multilayered rigid pavement system is presented. To efficiently model the behavior of the subgrade soil layers in the horizontal direction, infinite elements are used to represent the decaying feature of the field variables (displacements and stresses) in the far field. Ordinary finite elements are used in a relatively small region around the applied loads to accurately capture the stress field. Compared with a conventional finite-element analysis of pavements, wherein a finite size of the computational domain must be chosen and artificial boundary conditions must be assumed at the truncated boundary, the current approach can significantly reduce the total number of elements, saving considerable computation time and thereby making feasible more extensive analysis.