Subgrade Strength Research Articles

The effect of coal bottom ash as a replacement on subgrade strength

Coal Bottom Ash (CBA) which is one of coal combustion product (CCPs) obtained from thermal power plant will negatively affect the ecological system which human health ultimately depends on. The increase in demands for landfill requirements and the limitations natural aggregates is quite disturbing unless the new alternative to cut the usage of natural aggregates is found. Therefore, this paper present the laboratory investigation work to improve the strength of subgrade by the replacement of CBA with different percentage varies (0%, 15%, 30% & 40%) by weight. California Bearing Ratio (CBR) test is carried out to obtain the optimum percentage replacement CBA and then some physical which are particle size distribution, Atterberg limit test, specific gravity test and compaction test also carried out to observe the mechanical properties of optimum sample. The optimum percentage of CBA replacement obtained is 40.65% at 30% CBA replacement compared to 17.99% of subgrade soil alone. The moisture dry-density of unmodified soil sample is higher (1.99Mg/m3, 15.34%) than optimum sample (1.95Mg/m3, 10.2%). Moreover, 30% CBA replacement resulted in intermediate plasticity than the subgrade soil which is highly plastic. In conclusion, strength of subgrade soil is highest at 20-30% of CBA replacement.

Utilization of Support Vector Models and Gene Expression Programming for Soil Strength Modeling

The subgrade strength of roads and highways is based on the California bearing ratio (CBR) value. In this investigation, attempts have been made to overcome the limited boundary condition approach by using advanced methods, support vector machine (SVM) and gene expression programming (GEP) for prediction of CBR value. A large and wide range of datasets of different types of soils have been utilized in the analysis. The grain size distribution, Atterberg’s limits and compaction characteristics of soils have been used as the input variables. Best models with different variables were developed by using GEP and the same were used for SVM analysis. The advantage of SVM over others is that it works on the principle of statistical risk minimization. A comparative study of SVM and GEP models indicates that the SVM has better predictability than GEP. Further, it was found that the five-input variable (including gravel content, sand content, plasticity index, maximum dry density and optimum moisture content) model is the best one to predict the CBR value. The detailed statistical analysis including Pearson coefficient correlation (R) and Error analysis have also been carried out. Based upon the statistical analysis, overfitting ratio of SVM was found to be 0.630 against the value of 1.02 in GEP analysis. Further, sensitivity analysis was carried out and it was found that the CBR value is highly dependent on gravel and sand contents. On the other hand, plastic limit plays an insignificant role in determining the CBR value of soils.

Determining field bearing resistance of subgrade soils using physical characteristics

In highway design, subgrade strength is mostly affected in pavement layers’ thickness. California Bearing Ratio (CBR) is one of the simple testing methods that is commonly used to determine the strength of unbounded paving materials; i.e., subgrade, subbase, and base course materials of highways and airfield pavement. In-situ CBR test is intended to determine the static properties of unbound pavement layers represented by bearing capacity of subgrade soils without requiring the digging of test pits. The in-situ CBR test is laborious, time-consuming, and relatively expensive, therefore an alternative methodology is proposed for correlating CBR value with physical properties of subgrade soils such as field density and moisture content. Three testing methods were used: in-situ CBR test, sand replacement method for determining the field density and moisture content of granular soils and clayey soils, respectively. Selected types of soils were nominated from the local area having soil’s type of A-l-b, A-3, and A-6-7. Statistical analyses were performed to evaluate bearing resistance of subgrade soils depending on water content and dry density obtained from field density tests. The results of two statistical models indicated that both field density and water content correlate well with the bearing ratio for various subgrade soils.

Enhancement of strength characteristics of Clayey Sand using fly ash and geonet

India being a developing country, tones of industrial waste byproducts are generated every year and the quantity of waste keeps on escalating with time. In recent times, improving the subgrade strength by using soil reinforcement such as geosynthetics and by utilizing industrial wastes which if left unutilized pollutes the environment, is gaining lot of importance. In the present work, an experimental investigation has been carried out to improve the soil strength by adding fly ash (FA) and by reinforcing the soil using geonet. Locally available soil used in the experimental work was classified as SC soil (Clayey Sand) based on its index properties. Soil was mixed with variable percentages of FA, 10%, 14%, 18%, 22% and 26% to decide the ideal percentage of FA for which maximum gain in soil strength was witnessed in Unconfined Compression test and soaked CBR test. Geonet was placed at varying depths of one fourth the depth (H/4), half the depth (H/2) and three fourth the depth (3H/4) from top of soil specimen to determine the ideal depth of placement of geonet for which the soil exhibited maximum gain in strength in soaked CBR test. Based on the experimental results, soil with 18% FA showed maximum increase in UCC strength of 156.19 kPa and soaked CBR value increased by approximately 2.55 times that of control specimen. Soil reinforced with geonet placed at three fourth depth from top showed maximum strength gain in soaked CBR test.

A model for predicting the deterioration of the pavement condition index

ABSTRACT Road pavements continuously deteriorate mainly due to the combined influence of the traffic load and the environmental conditions. The pavement ability to satisfy the road user demands over its design life, expresses its performance level. The prediction of pavement performance level or performance deterioration curves is a basic component of any Pavement Management System (PMS). This paper presents the development of a model for predicting the deterioration of the Pavement Condition Index (PCI), calibrating its parameters based on the pavement structural factors such as structural number, asphalt layer thickness, subgrade strength, and environmental conditions. The approach that was adopted for the PCI deterioration model development was based on the combination of the empirical-mechanistic approach and the regressive empirical approach. The model's predicted results were compared with the measured results of road sections located in varied climate zones, which are embedded in the PMS of Netivei Israel (NETI), National Company for Transport Infrastructures. The comparison shows a very good correlation, and most of the predicted results are deviant within the measurement and interpretation error range.

Land Stability at The Final Closing of Waste Final Disposal Supit Urang in Malang City

Amount of waste that enters waste final disposal (TPA) Supit Urang is 833.625 m3/day, with a gradual pile pattern up to ± 12 meters. From 2018 until 2020, new cell was being established. On 13th July 2018, landslide hit the active cell area so that it was necessary to close the landfill zone at the TPA. Based on this background, it is necessary to calculate the landfill stability in landfill management until the new cell is done. The stability calculation also anticipates the subgrade strength and remaining waste cell capacity. Research method was carried out by observing the amount of incoming waste, characteristics of waste at the TPA, topography measurement and the results of observations of subgrade strength. In determining the calculation of land stability analysis, Geo 5 program was used. For technical data, topographic measurement was conducted directly at the TPA and compared with existing topography obtained from secondary data. Based on the topography result, it will be determined the contour slice in the field and a total slice of landfill plan. The result of this study shows that the landfill stability depends on subgrade type and landfill structuring at the TPA.

A Comparison between the Empirical and Mechanistic-Empirical Pavement Design Methods

In this paper a comparative study has been carried out between the Mechanistic-Empirical Pavement Design Methods and the Empirical Pavement Design Methods. For the purpose of this paper, nine pavement design sections have been prepared using the Mechanistic-Empirical Methods represented by the Asphalt Institute Design Method (SW-1 software of the Asphalt Institute) and the Empirical Design represented by AASHTO 1993 and TRL Road Note No.31, using the same traffic loading and subgrade strength. Three traffic loading scenarios have been considered for this study.
 (Key words: Mechanistic-Empirical, Empirical, axle loads, Subgrade Strength, pavement design)

Time-Domain Analysis of Deformation Characteristics of Asphalt Concrete Pavement Considering Vehicle–Pavement Coupled Effect

Analysis of the vehicle-pavement couple system was mainly based on the linear elastic model in frequency domain considering pavement roughness. The nonlinear dynamic numerical model for the vehicle-pavement coupled system was established using finite element mothed. Using the model, the deformation characteristics of asphalt concrete pavement were studied under the conditions of different pavement roughness, different vehicle speeds, different vehicle loads and different subgrade strength. The pavement roughness was worse and the vibration deformation of the pavement was larger, the max deformation of the C level pavement roughness was 1.31 times than that of the A level pavement roughness. With the faster of the vehicle speed, the deformation of pavement was smaller, but the trend was not obviously. The deformation of pavement increased obviously for overload, and the ratio of the overload reached by 100%, the ratio of deformation increased by 50%~109%. The deformation of pavement increased obviously for the subgrade strength reduction, the subgrade strength was reduced by 28%, and the deformation of the pavement increased by about 15%.

Finite Element Modeling of Ballasted Rail Track Capturing Effects of Geosynthetic Inclusions

This paper presents a two dimensional finite element (FE) approach to investigating beneficial aspects of geogrids in the railway track. The influences of different factors including the subgrade strength, the geogrid stiffness, the placement depth of geogrid, the effective width of geogrid, the strength of ballast-geogrid interface and the combination of double geogrid layers were investigated under the monotonic loading. The results indicated the role of geogrid reinforcement is more pronounced over the weak compressible subgrade. A stiffer geogrid reduces ballast settlement and produces a more uniform stress distribution along a track. The placement location of a geogrid is suggested at the ballast-sub-ballast interface to achieve better reinforcement results. Although the width of a geogrid layer should be sufficient to cover an entire loaded area, excessive width does not guarantee additional benefits. Higher interface strength between a ballast and a geogrid is beneficial for effective reinforcement. Increasing the number of geogrid layers is an effective way to reinforce the ballast over weak subgrades. The results of the limited cyclic FE simulations revealed the consistency of the reinforcement effect of the geogrids under monotonic and cyclic loads.

Assessing subgrade strength using an instrumented dynamic cone penetrometer

Subgrade strength is an important aspect to be considered when designing unsurfaced roads and working platforms for pavement structures. Furthermore, dynamic cone penetration tests are used to characterize the subgrade strength. However, the results of dynamic cone penetration tests, including the penetration depth per blow, can be affected by transferred energy and the verticality of the cone penetrometer. The objective of this paper is to characterize the subgrade strength using an instrumented dynamic cone penetrometer (DCP). The instrumented DCP includes a load cell and an accelerometer, located on the cone tip, and measures the dynamic responses of force and velocity over elapsed time. Weathered soil is compacted with three different dry unit weights in a square-shaped container for lab-scale dynamic cone penetration tests. Using instrumented and standard DCPs, several dynamic cone penetration tests are performed, and the dynamic cone penetration index (DCPI) is recorded along the depth profiles. Based on the dynamic responses at the cone tip of the instrumented DCP and the principles of energy conservation, the dynamic cone resistance is suggested as a new strength index. The experimental results show that the dynamic cone resistance increases with the depth of the instrumented DCP, providing a reliable subgrade strength profile in the soil. In addition, the correlation between the DCPI obtained from the instrumented DCP and the dynamic cone resistance suggests that the dynamic cone resistance is more sensitive than the DCPI for stiff soil. Based on the correlation values of the dynamic cone resistance obtained from the instrumented DCP, the internal friction angle and California bearing ratio values can be estimated with a high degree of reliability. This study demonstrates that use of the instrumented DCP may be a promising in situ testing method for the reliable characterization of subgrade strength.

The significance of concrete slab flexural strength inference variation based on its compression strength characteristics in apron pavement analysis and design

Rigid pavement of apron in the airport is a complex structure which is designed based on the sense of balance of sub-grade strength, pavement aggregates, applied load characteristics, and climate. Various sub-grade condition and concrete slab flexural strength values results on the pavement design thickness that have a direct impact on the cost construction. In this study, the rigid pavement design of an apron with various sub-grade condition and concrete flexural strength values are presented. As a reference, the Federal Aviation and Administration (FAA) method is used. Dynamic Cone Penetration (DCP) test value is used to estimate California Bearing Ratio (CBR) values to determine the sub-grade reaction modulus (k). The flexural strength of concrete slab analyzed by several empirical models with constant values range of 0.72-0.9. The pavement structure analysis conducted by FAARFIELD. The CBR values from DCP’s test vary between 6-10% which equal to k values between 31.4-46.8 MN/m3. Concrete slab flexural strength of 4.6 MPa results on the concrete slab thickness of 550-510 mm. Based on k value of 46.8 MN/m3 and K400 concrete strength, the calculated flexural strength varies between 4.15-5.17 MPa and the concrete slab thickness is 570-540 cm. The inference variation of flexural strength based on the same value of concrete compression strength characteristics will produce different concrete slab thickness. The concrete slab thickness tends to increase with the smaller values of inference of flexural strength.

EVALUATION OF SUBGRADE CAPACITY OF JIMMA SOILS USING DCP TEST: A CORRELATION OF CBR AND DCPI

Subgrade strength of soils is usually evaluated using California Bearing Ratio (CBR) values. As the cost and time required to conduct CBR test are high, dynamic cone penetrometer (DCP) would be recommended and CBR value can be estimated later from DCP result using a correlation formula. In this paper, laboratory CBR of Jimma fine-grained soils has been correlated with field DCP values referring to the physical properties such as natural moisture content and field density; as these factors significantly influence the behaviour of subgrade soils. Different techniques were used to demonstrate relations that best suit to find values of CBR from DCP test. Equations were developed between CBR and dynamic cone penetrometer index (DCPI) for the total of 36 sample points and adjusted coefficient of determination becomes 0.84. A validation was also done to test the applicability of the developed correlation formula for the local soils with the given physical conditions. The correlation gave a promising relationship between CBR and DCP and can be applicable for preliminary design purpose with the due consideration of the locality circumstances.

EVALUATION OF SUBGRADE CAPACITY OF JIMMA SOILS USING DCP TEST: A CORRELATION OF CBR AND DCPI

Subgrade strength of soils is usually evaluated using California Bearing Ratio (CBR) values. As the cost and time required to conduct CBR test are high, dynamic cone penetrometer (DCP) would be recommended and CBR value can be estimated later from DCP result using a correlation formula. In this paper, laboratory CBR of Jimma fine-grained soils has been correlated with field DCP values referring to the physical properties such as natural moisture content and field density; as these factors significantly influence the behaviour of subgrade soils. Different techniques were used to demonstrate relations that best suit to find values of CBR from DCP test. Equations were developed between CBR and dynamic cone penetrometer index (DCPI) for the total of 36 sample points and adjusted coefficient of determination becomes 0.84. A validation was also done to test the applicability of the developed correlation formula for the local soils with the given physical conditions. The correlation gave a promising relationship between CBR and DCP and can be applicable for preliminary design purpose with the due consideration of the locality circumstances.

Chemically Altered Natural Fiber Impregnated Soil for Improving Subgrade Strength of Pavements

Abstract This study explores the possibility of using sustainable materials in the form of natural fibers for reinforcing and improving the subgrade strength of pavements. Natural fibers with suitable biochemical properties were used for subgrade reinforcement in the past. Recently, the use of a waste weed, water hyacinth (WH), has garnered popularity as it can reinforce soil with the added advantage of waste utilization. It is well known that natural fibers have limited life when used in soils due to their degradation with time. For improving the longevity of the fiber and enhancing the mechanical performance of the soil-fiber composite, an effort was made in this study to chemically coat the natural fiber surface with nanoparticles of ferric hydroxide. The chemical coating can alter the short-term, as well as long-term, mechanical and chemical characteristics of WH fiber-impregnated soil, which is not well understood. The primary objective of the current study focuses on the short-term behavior of ferric hydroxide-coated WH fiber-impregnated soil that can be used as pavement subgrade. The effect of the chemical coating on WH fibers was initially analyzed by field emission scanning electron microscopy and energy dispersive X-ray tests. The impregnation of nanoparticle on the fiber surface increases surface roughness, coats the porous lumen of the fiber, and increases the tensile strength of the material. A set of tensile strength and moisture absorption tests was done for both untreated and treated WH fiber. The fiber tensile strength of treated fiber (TF) increased by 1.25 times as compared to untreated fiber (UF). The moisture absorption of TF decreased significantly from 580 % for UF to 255 %, indicating that the modified fiber became more hydrophobic. Unconfined compressive strength and direct shear tests were performed to evaluate the improvement in mechanical characteristics of chemically altered randomly distributed fiber-reinforced soil. The increase in cohesion, friction angle, and compressive strength at various compaction states has been discussed for soil reinforced with treated fibers (TF + S), untreated fibers (UF + S), and unreinforced soil (BS). For demonstrating the subgrade performance, soaked and unsoaked California bearing ratio tests were conducted on fiber-reinforced soil. For all the tests conducted on soil combinations (BS, (UF + S), and (TF + S)), the TF-reinforced soil composite performed the best. The results demonstrate the efficacy of using chemically altered natural fiber in increasing the subgrade strength of pavements.

Strength and mechanical behaviour of coir reinforced lime stabilized soil

Soil stabilization is an essential engineering process to enhance the geotechnical properties of soils that are not suitable for construction purposes. This study focuses on using coconut coir, a natural fibre to enhance the soil properties. Lime, an activator is added to the reinforced soil to augment its shear strength and durability. An experimental investigation was conducted to demonstrate the effect of coconut coir fibers and lime on the consistency limits, compaction characteristics, unconfined compressive strength, stress-strain behaviour, subgrade strength and durability of the treated soil. The results of the study illustrate that lime stabilization and coir reinforcement improves the unconfined compressive strength, post peak failure strength, controls crack propagation and boosts the tensile strength of the soil. Coir reinforcement provides addition contact surface, improving the soil-fibre interaction and increasing the interlocking between fibre and soil and thereby improve strength. Optimum performance of soil is observed at 1.25% coir fibre inclusion. Coir being a natural product is prone to degradation and to increase the durability of the coir reinforced soil, lime is used. Lime stabilization favourably amends the geotechnical properties of the coir fibre reinforced soil.

Experimental Modelling of a Reinforcement Theoretical Model on Peaty Soils

Peaty soils are highly organic soils with high moisture contents, low shear strengths and high compressibility. It was shown through years of experience that construction on such soils will yield unexpected failures. Several researchers have undertaken studies to investigate the reinforcement effects of various geosynthetics. Peat however was less investigated especially with regards to geogrid reinforcement. Since peat covers an appreciable area in Malaysia and several other countries, this study is an attempt to evaluate the applicability of using Giroud and Han’s [1] model for the design of unpaved roads on Malaysian peaty soils. It was found that as the tire inflation pressure increased and as the subgrade shear strength decreased, more aggregate thickness was required to offset settlements. It was shown that, although this model was applicable to these soils, a separate theoretical model addressing the particular qualities of peaty soils, should be attempted. Geotechnical engineers working in this field are advised to analyze and document tire inflation pressures and subgrade strengths as a prerequisite to constructing reinforced unpaved roads. Also it is advisable to investigate the effect cyclic loading has on reinforced peaty soils.

A rational approach to select the number of field tests required to determine subgrade properties

This paper presents a rational approach to answer the questions of how many field tests need to be performed at a test site to determine its subgrade strength properties, and what inferences can be drawn from limited number of field tests. MATLAB simulations were performed to generate various data sets that corresponded to site variability, with the coefficient of variation (CoV) ranging from 10 to 80%. Studies were performed based on coefficient of variation (CoV) and regression analysis. This study provides two important charts for evaluating subgrade properties at test sites: (1) to determine the number of field tests to be performed, after which there would not be any significant change in the inferences drawn from the test results; (2) inferences pertaining to subgrade soil property of a field test site, based on a limited number of tests. In order to demonstrate the applicability of the charts developed through this research, light weight deflectometer (LWD) spot tests that were performed at a test site, were considered. The results were used to draw inferences about the modulus of the subgrade material and the possible variability of the test site, in addition to the maximum number of tests that might be performed.

ICAO Overloading Practice versus Airport Pavement Design Life Using FAARFIELD 1.3 and COMFAA 2.0, 3.0

The objective of the study is to evaluate the impact of New Large Aircraft (NLA) on airport flexible pavement versus overloading in ICAO Practice. The study is conducted using Hurghada, Egypt Airport air fleet composition at four annual departure levels, four standard subgrade strength categories, and adding NLA at five different percentages. On the basis of the results of this research, it is concluded that, The statement in ICAO Practice “the annual number of overload movement should not exceed approximately 5 per cent of total annual aircraft movement” must be re-evaluated due to many factors as new large aircraft type ,annual departures and soil characteristics which High reduction in pavement life that has an average of 8.2% of 20 years design life due to introduction of A380-800 by 5% of annual departures, occurred at 3% CBR ., and The statement in ICAO Practice “For flexible pavement occasional movement by aircraft with Aircraft Classification number (ACN) not exceeding 10 per cent above the reported Pavement Classification number (PCN) should not adversely affect the pavement” must be re-evaluated because there is no restriction for adding new large aircraft with 5% Annual departure at any annual departures, soil characteristics comparing between COMFAA 2,3 software which High reduction in pavement life that has an average of 8.2% of 20 years design life due to introduction of A380-800 by 5% of annual departures, occurred at 3% CBR.

Comparison of AASHTO 1993 and MEPDG considering the Egyptian climatic conditions

The current flexible pavement design system in Egypt was adopted primarily from the AASHTO 1993 Design Guide. It is an empirical design method based on the results of the late 1950s AASHO Road test with many limitations. Thus, the Mechanistic-Empirical Pavement Design Guide (MEPDG) which is now called Pavement ME Design was developed to overcome those limitations. Unlike the AASHTO 1993 method, MEPDG considers the variation in moisture and temperature on the mechanical properties of the pavement layers. The main objective of this paper is to compare the AASHTO 1993 and MEPDG methods based on a recently developed Egyptian climatic data. The AASHTO 1993 method was used to design the thickness of a typical flexible pavement structure, using two different levels of traffic (low and high) and two different types of subgrade strength (weak and strong). Then, the AASHTO 1993 designed structures were simulated with MEPDG to predict performance over 20 years of service life. The comparative analysis of AASHTO 1993 design guide and MEPDG revealed that although all pavement sections in this study were designed with the AASHTO 1993 method for the same serviceability loss, they exhibited different performance as predicted by MEPDG. The variation of the MEPDG-predicted performance of the AASHTO 1993 designed pavement structures increased with the increase in traffic level and decrease in the subgrade strength. This variation was different for different climatic conditions. For the Egyptian conditions, the predominant distress was rutting. Finally, the climatic conditions showed a significant effect on distress occurrence and time to failure, especially the AC rutting.

Effect of Varying Subgrade Strength on Cost of a Rigid Pavement

It is generally believed that increase in subgrade strength leads to decrease in the thickness of a rigid pavement which helps in reducing the cost of pavement. The guidelines of Indian Roads Congress (IRC) for the design of rigid pavements also recommend the use of select soil of minimum CBR 8% in the subgrade for design of cement concrete pavement for roads having traffic greater than 450 CVPD. An attempt has been made in this paper to study the effect of variation in subgrade strength on thickness and cost of a rigid pavement. It is found that the thickness and cost of pavement may not decrease with an increase in the strength of subgrade soil. The provision of using select soil of minimum CBR 8% in the subgrade of the road as recommended by IRC, therefore, needs to be applied carefully.