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- Authors : Anteneh Geremew
- Paper ID : IJERTV7IS120017
- Volume & Issue : Volume 07, Issue 12 (December – 2018)
- Published (First Online): 05-01-2019
- ISSN (Online) : 2278-0181
- Publisher Name : IJERT
- License: This work is licensed under a Creative Commons Attribution 4.0 International License
Effect of Construction of Pavement on Expansive Subgrade Soil. A Case Study of Jimma-Agaro Road Segment
Effect of Construction of Pavement on Expansive Subgrade Soil. A Case Study of Jimma-Agaro Road Segment
Jimma University Institute of Technolgy Faculty of Civil and environmental Engineering Jimma,Ethiopia
Abstract:-The properties of a soil along the alignment of a road was quite differ from place to place; this affects the performance of the pavement. The research study has been undertaken at Jimma to Agaro road section with the main objective to investigate the effect of construction of pavement on expansive subgrade soil. In order to attain the objective, systematic methodology was adopted which includes field investigation and laboratory test while purposive sampling techniques was used to extract soil samples from road section of embankments of non- damaged section and also damaged section based on severity level road section. There are several types of asphalt failure observed on road section. The analyses of soil samples were carried out based on the Geotechnical properties such as wash gradation, hydrometer analysis, Atterberg limit, Modified proctor test and California Bearing Ratio (CBR) as well as Axle load analysis. It was found out that the liquid limit of subgrade soil varies from 53.9% -72.9% and the plasticity index from 24.4% -38.20% while there was a significant increase in moisture content at field and observed beyond the OMC in the laboratory result due to the distress of pavement affected by water infiltration through cracks, that tend to reduce the dry density. The recorded soaked CBR values of subgrade soil materials range between 4.34% -6.0%, which was below the 15% minimum value specified by ERA manual. However, the existing CBR values of the subgrade soils indicate a poor material used for pavement structures. According to ASSHTO, the soil is classified as A-7-5 and A-7-6 category which means the existing soils are fair to poor as a sub-grade material while USCS classification shows that the soil falls into CH, MH and ML group. In addition, the failures that are frequently observed on the road surface were significantly influenced by subgrade soil. The overall results showed that, heavy vehicles or traffic loads are one of the major causes of road failure along the study road section. Hence, the effect of other factors such as poor drainage courses, level of ground water table, variety of geologic materials along the road route and poor construction materials should be thoroughly addressed before the start of the rehabilitation of the road section in the future. Finally a possible remedial measure is recommended for every observed failure or distress on the pavement condition of the study area in order to sustain the design life of the pavement.
Keyword-Expansive subgrade soil, Plasticity index, swelling properties, CBR value, ERA standard, pavement performance, traffic load.
BACKGROUND OF STUDY
Ethiopia one of fasts growing country in East Africa has been scoring two digit economic growth rates for the past years. There are different types of abundant natural resource distribution in the country; the accessibility of
untouched surface and subsurface material it makes the country the second famous in Africa further it makes the nation and nationality to be beneficial with this resources. One of the popular sectors for development was the construction; it increases participation of population during operation that makes beneficial for the society and changes there living standards. Know a day the economic back ground of the country was gradually shifted from agricultural-economy to industrial , which needs shifting agricultural labors to that of the industrial workers, that makes them beneficially from their country as a nation. Ethiopian government gives more attention on constructs different industrial parks across the country that gives more opportunity those graduate and increase the utilization natural resources of the country. Therefore the construction of industries plays a great role for the development of the country and also the nation .
Road transport was one of typical modes of transportation especially in Ethiopia. In order to transport the raw material from the source or area of cultivated to industry parks, to distribute the product to different customers and for passenger it requires comfortable a road segments. The distribution of expansive soil in Ethiopia covers about 40% of the total surface area of the country . Therefore most of the road constructed in expansive soil shows different types of distress and this leads to fail before there design period due to the swelling the shrinkage properties of expansiveness of the soil . Thus, the selection process of route corridor influence the pavement structure and the construction costs, a thorough investigation should be done on the characteristics of subgrade soil .According to the report of addis ababa city road authority in 2004 the annual allocation for road construction and maintenance about 300 million Ethiopian birr ; out of this 30 million Ethiopian birr was allocated for routine maintenance which was obviously huge budget and it needs special attention on the subgrade soil, construction material and design of the projects.
The properties of soil may vary from place to place due to the variation in soil formation, drainage condition and climate. When the soils within the possible corridor for the road vary in strength significantly from place to place, it is clearly desirable to locate the pavement on the stronger soils, if this does not have other constraints. Thus, since selection process of route corridor influence the pavement structure and the construction costs, thorough investigation should be done on the characteristics of subgrade. Failures of roads are being observed before their design period and are greatly
affecting the economic growth of the country. Such failures could be overcome by undertaking through investigation on the subgrade material and the materials overlaying the subgrade and incorporating it in the design .Due to the Economic growth of a country the movement of traffic volume and loads on roads are going on increasing from year to year with alarming rate all over the world. Such heavy traffic growth demands need better performance roads for efficient transport of agricultural, commercial and industrial products without delay from one location to others. The repetitive traffic loading that the road experiences during its service life combined with environmental factors causes deformation, fatigue cracking, instability and other forms of deterioration which ultimately degrade/reduces the serviceability and durability of pavement structures .The major function of subgrade soils is to provide support to pavement structures. Under heavy traffic loads, subgrade soils may deform and contribute to distress in the overlying pavement structure. In asphalt pavement, this distress normally takes the form of cracking and rutting. It has been well documented that the subgrade soils plays a critical role in the initiation and propagation of permanent deformation of pavement structures and directly influences the pavement performance .
Statement of the Problem
Pavement is an engineering structure placed on natural soils and designed to withstand the traffic loading and the action of the climate with minimal deterioration and in the most economical way. Asphalt pavement roads are designed and constructed to serve the upcoming traffic that reveal during the service life of the road. Different factors taken in to account in the design and construction of pavements include the characteristics of the traffic, climatic conditions, material as well as structural properties and other elements which have significant impact on the overall performance of the road .
There was approximately 45km Jimma to Agaro asphalt pavement where sample was taken. From this point of view the load bearing capacity of flexible pavement is depend on load distribution characteristics of pavement structural layers which consist of the pavement structures resting on multiple foundation layers including sub grade. The pavement constructed on poor, soft and highly compressible sub grade soils causes pavement deteriorations which leads to premature failures interims of cracking, rutting distortional settlement that result frequent and costly rehabilitation .
Road failure of along the study area could be in the forms of cracks, potholes, surface deformation, surface defects which make the road network unsafe and not suitable to the road users. The performance of a pavement depends on the quality of its subgrade and existing condition of road bed 
The strength of subgrade soil is a major factor for the performance of the pavement. So the movement of the sub- grade is one of the causes of road pavement failure. Road failure could be in the forms of cracks, potholes, deformation, disintegration, surface defects etc. which makes the road network unsafe and not suitable to the road users. The performance of a pavement depends on the quality of its embankments and existing condition of road bed (ERA
The effects of sub grade materials have different negative consequence currently in different road construction. It has been noticed that construction of road on poor sub grade soils face numerous problems and the causes of the failure of road project and also increases vehicle operation cost. The aim of this thesis is to study effect of sub grade materials on pavement that have already deteriorated. Proper understanding of the causes of failure of roads may lead to proper remedial measures, designing methods and construction methods of roads appropriate for the study area. This in turn will be helpful for the people using the road in particular and the whole of the country at large by reducing the maintenance and vehicle running costs. Attempt made to analyze the causes of failure of roads constructed on poor sub grade soils had been investigated by representative test pit samples from the selected sites for the study area. The study focused on Jimma to Agaro road section.
Currently Condition of a road some part of road is failed by different types of failures such as reflective crack, block crack , pothole, rutting and alligator crack , so its not suitable for driving even also affect the vehicle operating cost for the road users. Misunderstanding the nature of soils and their properties can lead to construction errors that are costly in effort and material. It is for this reason that this research study to undertake investigation of subgrade soil on asphalt pavement performance along Jimma to Agaro road section.
The main objective of the research project was to investigate to investigate the effect of construction of pavement on expansive subgrade soil along Jimma-Agaro Road Segment.
To identify the location and level of severity of asphalt layer along road section
Determining the geotechnical properties of soil based on severity level.
To compare the existing properties of soil with the standard specification.
To determine the strength of subgrade soil and its relation with the thickness of asphalt pavement layers.
Significance of the study
The main significance of the study as follows:
Minimize the distress of the upper layer of the pavement.
Reduce the traffic accident, otherwise leads to loss of life and property.
Enhance easy movements of vehicles.
Reduce socio-economic problems of the surrounding community.
Use the result of the research study for rehabilitation of asphalt pavement.
Proper understanding the types of distresses and possible causes of damage on asphalt pavement may lead to correct application of remedial measures.
Provide detail information on how the geotechnical properties of soil affect pavement performance.
The study area started at Jimma town with an elevation of 1,722m above mean sea level (a.s.l.) and ends Agaro town at an elevation of 1675 m ASL. Then route of the Road, passing through different woredas or kebeles of various elevations. Therefore, the route of the road descends and ascends thereafter from the starting to end points. Generally the altitude ranges on the order of 1500-2300 m above mean sea level. The topography of the road terrain can be classified as flat and rolling terrain .
The procedure utilized throughout the conduct of this research study is as follows: Reviewed related literatures literature and standard specifications such as ERA, AASHTO and ASTM. Necessary data collection, organization, comparison and analysis were obtained, and then subsequently compared the results to pre-existing literature and standard specifications. A conclusion and recommendation are drawn based on the results, as well as appropriate remedial measure to be taken on each distress
type of pavement failures.
Data collection technique
The purposive collection technique was used by selecting particular parameters to have certain characteristics as applied in this research project. It is projected to be normally targets at particular Geotechnical parameters.
The research study was conducted by using both experimental and analytical methods. Qualitative and quantitative study was employed in this study area. Qualitative study gives impression of the findings where a quantitative study was used to describe the numerical aspects of the research finding.
Statement of the problem
Formulation of research objectives
Continuous review of Literature
Sample collection along road section
Particle size distribution
Result and discussion
Air dried the soil sample
Sieve dry soil sample using
0.425mm for Atterberg limit and hydrometer
2.00mm for specific gravity
Sieve size 9.5for compaction
Sieve size 9.5mm for CBR
Conclusion and recommendation
Figure 2.1 Research Design Chart.
Dependent variable: Effect of construction of pavement on expansive subgrade soil.
Independent variable: Pavement distresses, Atterberg limit tests, Compaction test, CBR tests, Grain size tests, Hydrometer test, Specific gravity, thickness of asphalt pavement, traffic load.
Data collection process
Quantitative and qualitative data were utilized based on the necessary input parameters for the analysis by comparing with standard specification and ERA manuals. Data collection process included but not limited to: -Desk study (reviewing letter correspondences, reports, design documents such as template of the road, working drawings etc.), Field visual inspection, Field investigation, Sampling represntative samples and finally preparation of samples for Laboratory tests. The study populations by grouped were classified as Normal, low damage, intermediate damage High damage and extremely damages. Finally the results from laboratory test were compared with Standard Specification.
Preliminary visual survey was under taken on the existing road in Jimma to Agaro town. Field observations and representative samples were taken to laboratory tests. Results from laboratory tests were compared with ERA Standard Specifications.
Field operation sequence
During the field observation, it was necessary to begin by conducting visual inspection and site inventory of the whole stretch of the Jimma to Agaro town road section. The initial site visit was taken on the whole portion of the road and at the same time the damaged sections were identified for further detailed site observation. There were different damaged level on their locations were inspected and the level of the damages were recorded to obtain the extent of defects on damaged.
After finishing the initial visual inspection and categorizing the conditions of the road into five, the damages were categorized accordingly as extremely damage, high damage, intermediate damage, low damage and slightly damage and
Normal condition. The next step was then to select the representative locations for sampling based on their failure conditions. The researcher selected five (5) test pits that represents the fives conditions. For each condition five test pits were taken for laboratory testing as. Since the detailed Field investigation was carried out at dry season and at the beginning of the rainy season. Test pits were taken to assess the strength sub grade level and the suitability of materials for existing road. Samples were extracted at five sections at Jimma to Agaro road section.
Existing pavement condition survey
Condition surveys are essentially required to assess a pavements physical distress and form the basis of a diagnosis regarding the maintenance or rehabilitation needs . The main objective of the pavement condition survey for this study was to evaluate the state of the existing pavement and that of the sub grade by inspecting the physical conditions of the existing pavement. Before the commencement of the detailed pavement evaluation, the entire road length was visually assessed and an attempt was made to identify the types of distresses occurred on the road section. The structural adequacy of pavement is highly depends on sub grade strength. The visual condition survey of existing pavement was evaluated prior to commencing investigation of sub grade. This was used as input data for intervals of sampling and testing of sub grade materials. The detailed study of existing pavement includes 
Visual assessment of existing pavement: This was done at start of detailed condition investigation of existing pavement. The objective of this survey is to identify sections with serious damage such as rutting, corrugation, pothole and deformation. The following were addressed during visual assessment of existing pavement such as Rutting, Corrugation, Pothole, Loss of camber, Worn out of gravels, Erosion gully (transversal or longitudinal), Cracking, Deformation and Surface disintegration
Test pitting and sampling of pavement
Laboratory testing of samples
RESULT AND DISCUSSION
Field Test results
Pavement Condition Survey results
In order that on the extent of damage observed from the visual inspection would become reliable, proper identification was made to select five representative sections. These sections were categorized as section where extreme damage (more severe) was observed, section 2 high damage, section 3 where intermediate damage (severe) was observed , section 4 where observed relatively low damage (slightly damage) and section 5 where observed relatively normal road section.
Table 3.1 Level of damage
Types of failure
Level of damages
Pothole , Alligator cracking
Longitudinal shoulder cracking ,swell
Note: Starting point of station at jimma City municipality.
Field investigation of the existing pavement thickness.
Existing thickness of the materials of the road layers Based on the field observation and investigation, the width of the existing road surface is measured using a meter tape during test pitting and sampling. The road has an average of 6m carriageway, while the pavement edges were difficult to establish because the camber of the road had changed due to repetitive raveling and erosion. Hence the width of the road is established mostly by judgment and measurement. The thickness of the road materials is measured in each test pit using a meter tape.
Table 3.2 Average thickness of layers
Type of Vehicle
AADTo in one directional flow
Traffic Growth Rate(i)
Truck and trailer
Table 3.3 Average Annual daily Traffic in JimmaAgaro road section in 2005 (G.C)
Source: Ethiopian Road Asset Management System; Addis Ababa
Level of damages
Average Thickness of road layers(cm)
Type of vehicles
one directional flow
Truck and trailer
Table 3.4 Result of Traffic Analysis.
3.1.3 Visual Sub-grade Soil Extension
In order to group homogeneous sections, during the field work, te visual sub-grade soil extension survey along the road alignment has been carried out to assess the nature, type and extent of existing sub-grade soil that makes the road bed. Sub-grade soils with similar soil type were grouped together and their extent was determined. Thus, the type of sub-grade soil encountered along the route corridor is found to be mainly dependent on topography and geology of the study area. The study area generally, lies on rolling and flat terrain.
From the visual inspection of the sub-grade soil during the site visit, in most of the cases it was found that it is red clay, dark brown to yellowish silty clay type and in few stretches of the existing road alignment it was reddish brown silt clay/clayey silt soil mixed with weathered laterite gravel. The soil extension survey was carried out in such a way that different soil types along the existing road can be recorded and classified according to color, texture and composition. It should be noted that in many cases, clear distinction between the individual soil types could not be made due to their similarity in origin and soil properties, and gradual transitions from one soil type to the other which was common in relation to the topography of the area.
The observation results show that crack, pumping, bleeding, pothole and rutting types of distresses were recognized along the selected sections. The causes of the observed distresses could be moisture fluctuation, poor sub grade materials. Similar observation has been reported by ERA (2000) in which moisture fluctuation is the main cause of distress in tropical climate. These may indicate that the analyzed road sections are in bad conditions.
Assume: n=is a years between traffic survey (AADT0 in 2005 and Opening of traffic 2006) =1years
Investigation period for research(x) =13 years up to now.
From the CBR test, value ranges 4.34%-6% according to ERA road design manual, the thickness of the base course and sub-base course for traffic class T6 with ESAs of 6.0-
10.0 million should be 20cm and 25cm respectively and Average thickness of the existing road layer of the base course was 11.56 cm and that of the sub-base course is 16.04 cm; this shows that the base and the sub-base course will not be able to carry the traffic loading at its service time.
Natural Moisture Content of the soil under study. The natural moisture content obtained from a disturbed sample range in between 15.5% to 20.4%.The plasticity index of the soil on which the road under study was constructed varies from 24.4% to 38.2%. The plasticity limits of the sub grade soil are 24.4% to 34.7% ranges.
Level of damages
Types of damages
Pothol e, Alligat or crackin g
Subsiden ce, Raveling
Longitudi nal shoulder cracking, swell
Natural moisture contents (%)
Table 3.5 Natural moisture content of soils of study area.
This table indicates subgrade materials of selected sections have high content of moisture which causes pavement failures due to moisture fluctuations.
Atterbergs limit test
Atterbergs limit of subgrade
Table 3.6 Atterbergs limit of sub grade materials.
Table shown above indicate all sections liquid limit, plastic limit and plastic indexes of subgrade layers are higher which not suitable materials. High plastic index indicate high content of clay which is not good for existing pavement.
These figures shows atterberg limit of subgrade materials for damaged road sections which have more than 50% liquid limit and more than 20% plastic indexes . These materials are not suitable materials existing pavement. Standard specifications stated that LL<50% and PI<30%.
All damaged road section and normal road section liquid
limit are greater than 50% which indicates subgrade materials are unsuitable materials. And also their plastic indexes are greater 30% for all damaged road section and greater than 24% for normal road section. These indicate subgrade materials are unsuitable. These high moisture content of subgrade materials caused pavement deterioration on existing pavement condition on study area and Atterberg limits (liquid limit, plastic limit) were determined according to AASHTO T 89 and 90 standard test method. The results of the Atterberg limits were shown in appendix B. Measured liquid limit was found in the range of 53% to 74% and plasticity index in the range of 26% to 39%. Based on the Atterberg test results soil classification is made based on AASHTO soil classification system classified as A-7-5 and A-7-6. This soil groups have high liquid limit and are highly plastic as well as causes considerable volume change upon moisture fluctuation.
Sieve analysis test
0.05 0.5 5
High Intermediate Low
Figure3.1 sieve analysis test
The particle size analysis and plasticity characteristics of the soils are required to classify soil. According to AASHTO soil classification Soils with 35% minimum percent pass sieve no. 200 are classified as silt-clay materials. The minimum percent pass sieve no. 200 for the sub grade soil under study is 48.75% which is normal section of road. The rest of sections are above 60% pass sieve no. 200 soil is categorized as poor clay sub grade soil.
Soil particle sizes smaller than 0.075 mm (passing 200 mesh sieves) are determined hydrometer method analysis. It is based on the process of sedimentation of soil particles in water by gravity. The hydrometer analysis as follows based on a combined passing of wet sieve analysis and hydrometer. Table 3.7 combined % of passing of subgrade soil.
Level of damage
combined % of passing of subgrade soil
Hydrometr analysis and wet sieve analysis combined passing results which are listed on table 4.4 indicated that all section have high content of clay which results swelling and shrinkage of sub grade layers. These characteristics of sub grade layers lead to premature cracking, rutting and long term settlement of pavement structures. For extremely damaged
road section, intermediate damaged road section and low damaged road section percentage of clay content in combined passing wet sieve analysis and hydrometer analysis is above 50%. For high damaged road section silt content is above 50% and clay content above 30%. For normal road section clay content is below 50% and silt content is almost below zero. Generally sub grade soils of selected sections are unsuitable materials.
3.3.5 Compaction Test
The most common measure of compaction of soil is its density. Soils density and optimum moisture content should be determined according to AASHTO T180. Optimal engineering properties such as shear strength for a soil type occur near its maximum dry density (MDD) and optimum moisture content (OMC). At this state, soils void ratio, potential to shrink and swell is minimized.
The sub-grade soil samples were subjected to the determination of maximum dry density (MDD) and optimum moisture content (OMC) in the laboratory. The laboratory test result reveals that the range of maximum dry density of the Sub grade lies in the range of 1.29g/cc to 1.7g/cc and optimum moisture content (OMC) lies in between 24% to 36.6%.
Table 3.8 Laboratory compaction test result of sub grade materials.
Level of damage
Based on the ERA Pavement Design Manual, it is recommended that the top 25 cm of all sub grades should be compacted to a relative density of at least 95% of the maximum dry density achieved by heavy compaction. Based on the test pits conducted, it was found out that sub grade material at extremely damaged, high damaged, intermediate damaged, low damaged and normal road sections were fine grained soils. At Extreme damaged section, the average MDD for the sub grade material was 1.7 gm/cc and average OMC is 24.5% .All of the rest damaged sections MDD are below 1.5 gm/cc which minimum standard specification is recommended. Their average corresponding OMC are above 30% which shows weak sub grade layers.
California Bearing Ratio (CBR) Test
The CBR is the most widely used methods for designing pavement structures. The method is primarily intended, but not limited to evaluate the strength of cohesive materials. The test procedure is based on, American society for testing and materials, AASHTO T193. The CBR value for a soil depends upon its density, molding moisture content and moisture content after soaking. Three points CBR was conducted for subgrade soil in the laboratory and design CBR value considered at 95% of MDD as summarized below.
CBR for sub grade materials were conducted at 3 point
system for the existing road and the value considered for design is a CBR value of 95% of MDD. Accordingly, the determination of CBR at 95% maximum dry density is given as follows.
Level of damage
CBR swell (%)
CBR at 95% MDD
Table 3.9 Average values of CBR and CBR swell (%) test results for the five sections
CBR can be performed either through one point or with three points for these study area. For one point system, the CBR value at100%MDD was considered. But for three points, usually the CBR value at 95% of MDD (Arora, 1997).
The average CBR obtained for Sub grade material when compacted at its optimum moisture content and maximum dry density is greater than 15. From Ethiopian Road authority Manual, the sub grade strength class for CBR range of 3-4 is S2 and for 5-7 is S3. The CBR values obtained from the dry density versus CBR curve at extreme damaged section is 4.6, whereas, at high damaged section is 4.8, at intermediate damaged section is 4.4, at low damaged section is 4.34 and at normal road section is 6.
Sub grade strength for pavement structural design is evaluated in terms of California Bearing Ratio (CBR). Strength indicators other than CBR may be used provided they are adequately correlated of CBR values. Design CBR of all damaged section of road the sub grade under study is sub grade strength class according to ERA pavement design manual is S2 except for normal road section which is S3.The test was conducted for sub grade soil CBR at optimum moisture content and laboratory test result shows that the CBR value lie in between 3% – 6%. This shows that sub grade soil is poor soil to support structures constructed on it so that pavement deterioration was caused .Therefore, special treatment like removal and replacement of soil should be made.
Specific Gravity test result.
The specific gravity of solid matter in a soil particle may be defined as the ratio of the unit weight of solid matter to the unit weight of water. The specific gravity of the solid particles without the void spaces is called the true or absolute or real specific gravity and is usually designated by a letter Gs. The specific gravity sub grade normally used in road construction ranges from about 2.5 to 3.0 with average value of about 2.68.
Table 3.10 Specific gravity test result of sub grade material.
Level of damages
Clay Lateritic soil
Silty clay Black clay soil
Silt and red clay
Silty clay, red laterite
Silt and red clay soils
The result of specific gravity of damaged section of existing road sub grade materials are ranges between 2.67 to 2.9 as shown on table 4.9. According ASTM D 854 of laboratory specific gravity ranges 2.67 to 2.9 clay and silt clay which is unsuitable materials.
Table 3.11 Summary of all laboratory test result of all selected damaged sections and normal section
Level of damages
Grain size analysis
standard proctor test
at 95% MDD
swell at MDD
Figure 3.2 Laboratory test activity
Overall characterizations of sub-grade materials.
The characterization or classification of the sub-grade soils of the study area, for its suitability to be used for pavement, was evaluated based on the properties, such as; (i) CBR swell values, (ii) Plasticity index, (iii) Liquid limits, (iv) California Bearing Ratio (CBR) and (v) Maximum dry Density (MDD). Thus, based on the results following inferences were made;
The CBR swell values are considered as the first parameter for classifying the sub-grade material, because the moisture fluctuation in the bottom of the sub-grade will have a significant influence for swell and shrinkage of these soils that causes pavement deterioration. Therefore, taking extremely damaged section of existing road is 1.2% CBR swell value which indicates unsuitable materials.
The fourth criteria for classifying the sub-grade soils in to suitable/unsuitable are the value of CBR at 95% of the MDD. The minimum value of CBR for sub-grade soils in this study is defined as less than5%. Taking this in to account samples are considered to be unsuitable.
The last criterion used for the classification of the sub- grade soils is the Maximum Dry Density. As mentioned above in section, the maximum dry density is obtained by AASHTO T 180 (D) with standard proctor methods of 25 blows. The minimum values of MDD for sub- grade material are supposed to be 1.5gm/cc  hence, taking this in to account the samples is below the minimum requirement.
Plasticity indices are also taken as the second main factor for classifying the sub-grade soils in to suitable/unsuitable ones. Taking the maximum PI value above 30 %, of the samples are unsuitable.
The third parameter is the Liquid limit value, according to the standard specification; sub-grade material with LL value more than 50% are unsuitable.
Comparison of Field density and laboratory compaction test of subgrade materials.
Water content (%)
Dry density (g/cm3 )
Water content (%)
Dry density ( g/cm3 )
Table 3.12 Comparison of field density and laboratory compaction test of subgrade material.
The moisture contents in failed sections (6.36% to 12.3%) and non-distress pavement (3.56%) were found out to have an increase of moisture content in the field than the optimum moisture content obtained in the laboratory test. This is due to poor drainage during the rainy season and due to infiltration through the cracks and pothole on the road surface. At the same time, decrease in dry density is observed. The compaction at section at failures section (81.58% to 85.51%) and non-distress (89.13%) The decrease in dry density could be due to the increase of moisture content above the optimum or due to poor compaction effort during construction.
CONCLUSION AND RECOMMENDATION
Poor compaction, poor subgrade materials of construction are the major causes of asphalt damages along the studied sections. Based on the study results, the extent of damages along the studied asphalt road sections can be categorized as extreme damage (at station 35+530), High damages (at station 19+620), intermediate damage (at station 12+350), Low damage (at station 1+250) and Normal road (at station 0+900).
(A) Areas of extremely damaged road sections, high damaged road section, Intermediate damaged road section and low damaged road sections the moisture
contents obtained for sub grade materials are less than in the moisture contents of relatively non affected sections. However, this condition is good for materials the observed CBR values for sub grade layer materials are less for Low damaged, high damaged road section, intermediate damage road sections and extremely damaged road sections. These indicate materials used for sub grade layer for existing road are unsuitable as compared to standard specifications.
(B)According to AASHTO classification system, the sub grade soil of the study area is classified as A-7-5 and A-7-6.Furthermore, the CBR, plasticity index, Hydrometer, Sieve analysis, compaction and specific gravity test results of the soil shows that the sub grade soil has a very low bearing capacity and unsuitable for sub grade layers. The laboratory test results proved that the subgrade materials used for the existing constructed road not as per the specification, and fulfill all the requirements
(C) The test results observed during laboratory test conducted showed that the dry density values of the sub grade layers are less than the values stipulated in the standard specification. The CBR values found related dry density for sub grade are less for high damaged road sections, extremely damaged road sections, Intermediate damaged road section and Low damaged road section.where as, in areas of Normal road sections, even if the dry density is lower than what is required, the CBR values are less than the standard specifications.
Increase the thickness of subgrade layer and quality of subgrade materials in order to minimize moisture fluctuations at bottom of subgrade layers.
For unsuitable sub-grade soils, whose CBR values at 95% of MDD is below 5% and maximum plasticity
index of 30%, it is recommended to excavate down up to 600 mm depth and replace it with soil having a minimum CBR of 5% and PI between maximum 20% which is equivalent to an S3 type material indicated as per ERA, 2002. The work shall be executed in three layers of equal thickness and must be compacted to the required minimum density. To achieve high-quality subgrade, proper understanding of soil properties and quality control testing should be done since pavement performance depend on sub grade strength.
Use Geosynthetic product that have successfully used for sub grade reinforcement and improving the performance of pavement constructed over soft and poor foundations.
Reconstruction or replacement of an existing pavement structure by the placement of the
equivalent of a new pavement structure. Reconstruction usually involves complete removal and replacement of the existing pavement structure and may include new or recycled materials. And also reconstruction contractors team and staff must
monitor roadway condition during threatening weather condition for sub grade lacked strength to carry heavy load due to excess moisture of study area.
Remove and replace of unsuitable subgrade materials by: i) suitable materials which are economically feasible, drainable and granular
materials ii) Existing subgrade materials should be recycled with special treatment and granular materials
Great glory goes to the almighty God who is always standing at the right of my side in each and every step of my life. My gratitude also goes to the laboratory technicians of Ethiopian Road authority Jimma district for their supporting all the times during the laboratory activities
Finally, my appreciation and thanks goes to all my family, friends and relatives who have helped me in any form of support which are greatly needed for the advancement and completion of this work.
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