Effect of Subgrade Properties on Pavement ...

‫اﻟﻤـﺆﺗـﻤﺮ اﻟﺨﻠﻴـﺠﻲ ﻟﻠﻨـﻘـﻞ‬

‫‪Gulf Conference on Transportation‬‬ ‫‪23-24 January 2008 , Dubai, UAE‬‬

‫‪ 24-23‬ﻳﻨﺎﻳﺮ ‪ ، 2008‬دﺑﻲ ‪ ،‬اﻹﻣﺎرات اﻟﻌﺮﺑﻴﺔ اﻟﻤﺘﺤﺪة‬

‫ﺩﺭﺍﺴﺔ ﺘﺄﺜﻴﺭ ﺨﺼﺎﺌﺹ ﺘﺭﺒﺔ ﺍﻟﺘﺄﺴﻴﺱ ﻋﻠﻰ ﺤﺎﻟﺔ ﺴﻁﺢ ﺍﻟﺭﺼﻑ ﺍﻟﻤﺭﻥ‬

‫ﻤﻨﺎل ﻋﺒﺩﺍﷲ ﺃﺤﻤﺩ‬

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‫ﻤﺤﻤﻭﺩ ﺍﻟﺴﻌﻴﺩ ﺴﻠﻴﻤﺎﻥ‬

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‫ﻁﺎﺭﻕ ﻨﺠﻴﺏ ﺴﺎﻟﻡ‬

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‫ﻣﻠﺨﺺ‬ ‫ﺇﻥ ﺨﺼﺎﺌﺹ ﺘﺭﺒﺔ ﺍﻟﺘﺄﺴﻴﺱ ﻭ ﻤﺤﺘﻭﺍﻫﺎ ﺍﻟﻤﺎﺌﻲ ﻻ ﺘﺩﺭﻙ ﺒﻁﺭﻴﻘﺔ ﻤﺒﺎﺸﺭﺓ ﻓﻲ ﺘﻘﻴﻴﻡ ﺤﺎﻟﺔ ﺭﺼﻑ ﺍﻟﻁﺭﻕ ﻭﺃﺩﺍﺌﻬـﺎ‪،‬‬

‫ﻭﻤﻊ ﺫﻟﻙ ﻓﺈﻥ ﻜﺜﻴﺭ ﻤﻥ ﻫﻴﺌﺎﺕ ﺍﻟﻁﺭﻕ ﻭﺠﺩﺕ ﺃﻥ ﺯﻴﺎﺩﺓ ﺍﻟﻤﺤﺘﻭﻯ ﺍﻟﻤـﺎﺌﻲ ﻟﺘﺭﺒـﺔ ﺍﻟﺘﺄﺴـﻴﺱ )ﺨﺎﺼـﺔ ﺍﻟﺘﺭﺒـﺔ‬ ‫ﺍﻟﻤﺘﻤﺎﺴﻜﺔ( ﺇﻟﻰ ﺠﺎﻨﺏ ﺯﻴﺎﺩﺓ ﺍﻷﺤﺠﺎﻡ ﻭﺍﻷﺤﻤﺎل ﺍﻟﻤﺭﻭﺭﻴﺔ ﻴﺅﺩﻱ ﺇﻟﻲ ﺍﻟﺘﺩﻫﻭﺭ ﺍﻟﺴﺭﻴﻊ ﻟﻠﻁﺭﻕ ﺫﺍﺕ ﺍﻟﺭﺼﻑ ﺍﻟﻤﺭﻥ‪.‬‬ ‫ﻭﻴﻬﺩﻑ ﻫﺫﺍ ﺍﻟﺒﺤﺙ ﺇﻟﻲ ﺩﺭﺍﺴﺔ ﺘﺄﺜﻴﺭ ﺍﻟﻤﺤﺘﻭﻯ ﺍﻟﻤﺎﺌﻲ ﻟﺘﺭﺒﺔ ﺍﻟﺘﺄﺴﻴﺱ ﻭﺍﻟﺨﺼﺎﺌﺹ ﺍﻷﺨﺭﻯ ﻟﻬﺎ ﻋﻠﻰ ﺃﺩﺍﺀ ﺍﻟﻁﺭﻴﻕ‬

‫ﻭﺫﻟﻙ ﻋﻥ ﻁﺭﻴﻕ ﺇﺠﺭﺍﺀ ﺒﺭﻨﺎﻤﺞ ﺤﻘﻠﻲ ﻭﻤﻌﻤﻠﻲ ﻟﻠﻭﺼﻭل ﻷﻫﺩﺍﻑ ﺍﻟﺒﺤﺙ‪ .‬ﻓﻔﻲ ﺍﻟﺠﺯﺀ ﺍﻟﺤﻘﻠﻰ ﻤﻥ ﺍﻟﺩﺭﺍﺴـﺔ ﺘـﻡ‬ ‫ﻤﺴﺢ ﻋﻴﻭﺏ ﺍﻟﺭﺼﻑ ﻓﻲ ﺍﻟﻘﻁﺎﻋﺎﺕ ﻤﻭﻀﻊ ﺍﻟﺩﺭﺍﺴﺔ ﻭﺤﺴﺎﺏ ﻤﻌﺎﻤل ﺤﺎﻟﺔ ﺍﻟﺭﺼﻑ )‪ (PCI‬ﻟﻬﺎ‪ ،‬ﺜـﻡ ﺘـﻡ ﺃﺨـﺫ‬ ‫ﻋﻴﻨﺎﺕ ﻤﻥ ﺘﺭﺒﺔ ﺍﻟﺘﺄﺴﻴﺱ ﻓﻲ ﺍﻟﻘﻁﺎﻋﺎﺕ ﺍﻟﻤﺨﺘﺎﺭﺓ ﻭﺫﻟﻙ ﻟﺩﺭﺍﺴﺔ ﺘﺄﺜﻴﺭ ﺴﻠﻭﻙ ﺘﺭﺒﺔ ﺍﻟﺘﺄﺴﻴﺱ ﻋﻠـﻰ ﺠـﻭﺩﺓ ﻭﺃﺩﺍﺀ‬

‫ﺍﻟﻁﺭﻴﻕ ﺃﺜﻨﺎﺀ ﺍﻟﺨﺩﻤﺔ‪.‬‬

‫ﻭﻗﺩ ﺃﻅﻬﺭﺕ ﺍﻟﻨﺘﺎﺌﺞ ﺃﻥ ﺯﻴﺎﺩﺓ ﺍﻟﻤﺤﺘﻭﻯ ﺍﻟﻤﺎﺌﻲ ﻟﺘﺭﺒﺔ ﺍﻟﺘﺄﺴﻴﺱ ﻭﺨﺎﺼﺔ ﺘﻠﻙ ﺍﻟﺘﻰ ﺘﻨﺘﺞ ﻋﻥ ﻤﺼﺩﺭ ﺨﺎﺭﺠﻰ‪ ،‬ﻟﻬـﺎ‬ ‫ﺘﺄﺜﻴﺭ ﺭﺌﻴﺴﻰ ﻋﻠﻰ ﺤﺎﻟﺔ ﺍﻟﺭﺼﻑ ﻟﻠﻁﺭﻴﻕ‪ ،‬ﻜﻤﺎ ﺃﻥ ﺍﻟﻬﺒﻭﻁ ﺍﻟﻤﺘﻔﺎﻭﺕ ﺍﻟﺤﺎﺩﺙ ﻓﻰ ﺍﻟﺘﺭﺒﺔ ﺒﺴﺒﺏ ﺯﻴـﺎﺩﺓ ﻤﺤﺘﻭﺍﻫـﺎ‬

‫ﺍﻟﻤﺎﺌﻰ ﻟﻪ ﺘﺄﺜﻴﺭ ﻜﺒﻴﺭ ﻓﻲ ﻅﻬﻭﺭ ﺍﻟﺸﺭﻭﺥ ﻭ ﺍﻨﻬﻴﺎﺭ ﺍﻟﺭﺼﻑ‪.‬‬

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‫ﺃﺴﺘﺎﺫ ﻤﺴﺎﻋﺩ ﺒﻘﺴﻡ ﻫﻨﺩﺴﺔ ﺍﻟﺘﺸﻴﻴﺩ ﻭﺍﻟﻤﺭﺍﻓﻕ – ﻜﻠﻴﺔ ﺍﻟﻬﻨﺩﺴﺔ – ﺠﺎﻤﻌﺔ ﺍﻟﺯﻗﺎﺯﻴﻕ – ﺍﻟﺯﻗﺎﺯﻴﻕ – ﻤﺼﺭ‪.‬‬

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‫ﺃﺴﺘﺎﺫ ﻤﺸﺎﺭﻙ ﺒﻘﺴﻡ ﺍﻟﻬﻨﺩﺴﺔ ﺍﻹﻨﺸﺎﺌﻴﺔ – ﻜﻠﻴﺔ ﺍﻟﻬﻨﺩﺴﺔ – ﺠﺎﻤﻌﺔ ﺍﻟﺯﻗﺎﺯﻴﻕ – ﺍﻟﺯﻗﺎﺯﻴﻕ – ﻤﺼﺭ‬

‫‪Gulf Road Engineering Society‬‬


Gulf Conference on Transportation 23-24 January 2008 , Dubai, UAE

‫ اﻹﻣﺎرات اﻟﻌﺮﺑﻴﺔ اﻟﻤﺘﺤﺪة‬، ‫ دﺑﻲ‬، 2008 ‫ ﻳﻨﺎﻳﺮ‬24-23

Effect of Subgrade Properties on Pavement Performance Manal A. Ahmed 1

Mahmoud E. Solyman 1

Tarek N. Salem 2

Abstract Properties of the subgrade soil and its water content are not directly considered in assessing the performance or serviceability of pavements. However, most highway agencies recognize that the increased water content in subgrade soils (especially the cohesive ones) combined with increased traffic volumes and loads often leads to premature pavement distresses. This paper is concerned with the effect of subgrade soil water content along with other soil properties on the overall pavement performance. Field and laboratory study programs are preformed to achieve the study objectives. Pavement performance in chosen distressed sections is assessed, and then soil samples from different sites are extracted and tested in the laboratory to study the effect of subgrade soil behavior on the pavement performance. Results indicated that increasing the water content in the subgrade soils, especially that induced by an external source, has a major role in the pavement distress during the serviceability state. Results also indicated that differential settlement in the subgrade soil is the major source of such pavement distress. KEY WARDS: Subgrade soil, pavement performance, water content, soil modulus, pavement distress.

1

Assistant Prof., Dept. of Construction Eng. & Utilities, Faculty of Eng., Zagazig University, Zagazig, Egypt.. 2

Associate Prof., Dept. of Structural Eng., Faculty of Engineering, Zagazig University, Zagazig, Egypt.

Gulf Road Engineering Society

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Effect of Subgrade Properties on Pavement Performance

Manal A. Ahmed

Mahmoud E. Solyman

Tarek N. Salem

1. Introduction: Environmental changes have a direct impact on the pavement performance. It can be considered as a major factor in causing pavement deterioration. Dore and Savard (1998) concluded that; 1) Transverse cracking, longitudinal cracking, and winter roughness are related to environmental factors. 2) Fatigue cracking and rutting are associated with heavy loads circulating on the pavement. While the modulus of the asphalt concrete layer is more sensitive to temperature variations, the modulus of the unbound materials (soil) is sensitive to the variation in moisture content. Although, most highway agencies incorporate the subgrade soil properties in the pavement design process, the properties of the subgrade soil and its water content are not directly considered in assessing the performance or serviceability of pavements. Li (2004) stated that subgrade soil should be adequately; 1. Provide a stable platform on which to construct the track. 2. Limit progressive settlement from repeated traffic loading. 3. Limit consolidation settlement. 4. Prevent massive slope failure. 5. Restrict swelling or shrinking from water content change. The resilient (soil) modulus is the main property representing the bearing capacity of the subgrade. One of the major differences between the AASHTO (1986) guide and previous interim guides was the adoption of the soil resilient modulus as the preferred parameter for characterizing the quality of subgrade support. Laboratory test results reported in the literature showed that the resilient modulus of all classes of unsaturated Gulf Road Engineering Society

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‫اﻟﻤـﺆﺗـﻤﺮ اﻟﺨﻠﻴـﺠﻲ ﻟﻠﻨـﻘـﻞ‬ ‫ اﻹﻣﺎرات اﻟﻌﺮﺑﻴﺔ اﻟﻤﺘﺤﺪة‬، ‫ دﺑﻲ‬، 2008 ‫ ﻳﻨﺎﻳﺮ‬24-23

granular materials decreases to some extent with increasing the moisture content. Edris and Lytton (1977) suggested that resilient modulus differences due to variations in water content are significant only when the water contents are greater than the optimum compaction water content plus or minus two percent. Elliott and Thornton (1988) concluded that both the low and high plasticity soils exhibit resilient modulus decrease with increasing the water content. Chaohan (2004) evaluated the effect of high groundwater level on pavements subgrade performance through testing eight subgrade soils using open test-pits to simulate the different field moisture conditions. The author concluded that the moisture content in the subgrade soil is a major factor affecting the soil resilient modulus, then its dry weight, and coefficient of uniformity, if applicable. Furthermore, in seasonal frost areas variation of the modulus of unbound materials and moisture content are incorporated in the design process of flexible pavements, where pavements are likely to heave during winter and then lose part of their bearing capacity during spring thaw. Jano and Berg (1990) concluded that this problem is the prominent seasonal phenomena leading to pavement deterioration. White and Coree (1990) reported that 60% of the failures during the AASHTO road test occurred during spring are due to loss of subgrade strengh due to thawing. Salem et al. (2003) and Bayomy et al. (2003) developed regression models to relate the changes in subgrade modulus to the changes in moisture content for various types of soils. These models are then used to predict the seasonal changes in subgrade modulus.

Most highway agencies recognize that high groundwater table exerts detrimental effects on roadway base and the whole pavement. Their main concern is the way to prevent water from intruding into the pavement system. Water can intrude into the pavement through several ways such as cracks, infiltration, through shoulder and ditches, and high groundwater. The Florida Department of Transportation (FDOT) has developed high groundwater clearance guidelines which are intended to satisfy two concerns: 1- To prevent potential damage to the roadway base. 2- To achieve the required compaction and stability during construction operations.


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Drainage is the single most important factor governing the performance of track substructure. A properly functioning drainage system provides the following, as presented in Li (2004): 1- Intersects the water seeping up from the subgrade. 2- Diverts the surface water flowing toward the track. 3- Removes water falling onto the track. 4- Carry off stone dust, sand, and other debris that otherwise could foul the track. NCHRP (1998) stated that drainage of water from pavements is the key element in the design of pavement systems and indiscriminate exclusion of this element will assuredly lead to the premature failure of the pavement systems. The authors concluded that subsurface drainage design should be part of the pavement structural procedure. Li (2004) stated that deflection measures the overall mechanical response of pavement system to load. Any factor affecting the mechanical properties of pavement layers has a direct impact on the measured deflection. For example, water content and high temperature can cause a dramatic reduction of the moduli of subgrade soil and asphalt layers, respectively, resulting in high measured deflection. Due to the limited pavement dimensions and variation of pavement distress, different testing locations in the same section can cause a significant difference in deflection measurement. For flexible pavement, deflections measured near cracks are normally much higher than the measurements in non-distressed areas. Similarly, deflection measurements near longitudinal joints, transverse joints, or corners are higher than those measured at midslab for concrete (rigid) pavements. Thermal and moisture gradient in the vertical direction of concrete slabs cause curling and warping and have a significant influence on deflection measurements. Measurements taken at night or in the early morning are considerably different from those obtained in the afternoon. The main goal of this paper is to quantify and assess the effects of subgrade soil water content along with other soil properties on the overall pavement performance.

2. Experimental Testing Program: A comprehensive testing program is planned and performed to obtain as much data as possible to assess the effect of subgrade water content on the pavement distress. Field and laboratory study programs are preformed to achieve the study objectives. Gulf Road Engineering Society

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2.1. Field Study Program: A quick visual survey of a large number of agriculture highway links suffering from premature distresses for the selection of the study test sites was performed based on specific criteria. The criteria is selected such that: all links have good quality control during construction, all highways are new with service life less than 5 years, all test sites are located within the same area presumably with the same subgrade soil type and stratification, the distresses are mainly attributed to the subgrade soil. 2.1.1. Pavement Condition Index: A complete survey of the selected sites is performed using Pavement Condition Index (PCI) method developed by Shahin and Kohn (1984), which assesses the present pavement surface condition based on specific criteria. In this procedure deduct values are assigned to certain observed distress types, according to their density and severity, and then subtracted from a perfect score to give the Pavement Condition Index (PCI) value and the pavement rating. The procedure basically consists of six steps which are summarized below as follows: (1) The inspection unit inspects the target highways using a distress identification guide, and the approximate amount of each distress type/severity combination is recorded as a percentage by dividing the distress type/severity combination quantities by the total area of the segment and multiplying by 100. (2) The deduction values for each distress type/severity combination are determined from special deduct curves. The PCI procedure uses a set of "deduct curves" to calculate the numerical impact of each distress type/severity combination on the overall PCI. (3) The number of distress type/severity combinations with deduct values larger than 5 are counted. The obtained q-value is used later in the calculations to correct the curves because research found that if occurrences of small deduct values are included, the final value would be too small, or over estimated. (4) The total deduct value is computed by summing all the deduct values for the distress type/severity combinations. (5) When multiple distress type/severity combinations are present, the deduct units must be corrected as more distress type/severity combinations occur in the same inspection unit, they have less and less impact. To account for this nonlinearity, the total deduction and q-values are used with correction curves to determine the corrected deduct value. (6) The corrected deduct value is subtracted from 100 to determine the inspection unit PCI in percentage. Gulf Road Engineering Society

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2.1.2. Soil Sampling: After visual inspection of the highways, the distressed zones are located and open pits are made to allow for extracting undisturbed specimens for the subgrade soil. Subgrade specimens are obtained from different test sites using special cylindrical thin thickness samplers to minimize the sample disturbance, as per ASTM D1587-00. At the specified depths, the samplers are pushed steadily into the soil, then trimmed and sealed to obtain the field unit weight as per ASTM D2937-04, and also to maintain their consistency and moisture contents and transported to the laboratory. 2.2. Laboratory Study Program: Undisturbed field subgrade soil specimens which are obtained from the different test sites are tested in the laboratory. The laboratory test program includes; the natural water content, Atterberg limits as per ASTM D4318-00, unit weight, and the unconfined compressive strength tests as per ASTM D2166-00E01, standard Proctor test as per ASTM D0698-00AE01 on the collected soil specimens.

3. Results and Analysis: The results obtained from field and laboratory programs are plotted in the forthcoming diagrams to assess the effect of subgrade water content on the pavement behavior. A key element in explaining the effect of water content on the behavior of the tested soils is their classification according to their plasticity index. Highly plastic soils tend to be stiffer clayey soils, while soils of intermediate or low plasticity tend to be weaker with higher silt content. In addition, the presence of silt will induce little friction and higher permeability. The high permeability values allow for the groundwater to ingress, affect, and dissipate more freely from the soil. Thus, the repeated traffic loads have more severe effect and cause permanent deformations in the underlying soils. Soil deformations under flexible pavements are a direct cause of surface rutting and deformations in the highway pavement itself. Table (1) shows summary of the test results showing the field water content (W.C.), liquid limit (L.L.), plasticity index (P.I.), maximum dry unit weight (γdry max.), optimum moisture content (O.M.C.), classification according to the Casagrande Plasticity Chart (Pl. Class.), subgrade soil unconfined compressive strength (qu), and the initial soil tangent modulus (Es), the Pavement Condition Index (PCI), and the long crack density Gulf Road Engineering Society

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(L.C.D.) of the tested specimens. The table also shows the plasticity classification of the tested specimens. The soil plasticity classification indicates that the in place soils are mainly silts (M) of high plasticity (H), with only one specimen of clay (C) with intermediate plasticity (I) and another one of silt with intermediate plasticity also. The presence of higher ratio of silt in the clayey soil specimens interprets the greater effect of water content on the soil behavior. In addition, according to the unconfined compressive strength values, the soil specimens are mainly medium stiff to stiff with natural water contents higher than the optimum moisture contents in almost all the tested specimens. Table (1): Summary of the Field and Lab Testing Results. No.

W.C.

L.L.

P.I.

γdry max.

OMC

Pl.

(%)

(%)

(%)

(kN/m3)

(%)

Class.

qu

Es,

(kPa) (MPa)

PCI LCD

1

16.04 49.0 35.0

15.71

24.0

CI

69.9

33.33

75

7

2

21.04 92.0 52.0

15.71

19.0

MH

132.0

30.77

69

10

3

24.06 58.0 26.0

15.84

11.0

MH

104.8

31.87

69

10

4

23.64 45.0 15.0

15.49

12.0

MI

50.6

34.84

53

25

5

31.25 53.0 20.0

12.97

15.0

MH

57.0

16.69

28

20

6

27.72 53.0 15.0

14.86

16.0

MH

65.6

11.07

20

7

7

18.75 61.0 28.0

16.03

14.0

MH

89.9

31.51

28

16

3.1. Effect of Moisture Content on the Soil Initial Tangent Modulus: Figure (1) shows the relation between the tested specimen's natural water content and the soil initial tangent modulus. The trend of the test results indicated that increasing the water content over 24% resulted in a noticeable decrease in the soil modulus. A well known concept in soil mechanics is that increasing the water content in fine grained specimens results in decreasing its consistency till reaching a liquid like state when the water content is approximately equal the liquid limit of that soil. Moreover, the presence of higher ratio of silt in the clay soil specimens tends to amplify such behavior.


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Soil Initial Tangent Modulus (MPa)

40.0 35.0 30.0 25.0 20.0 15.0 10.0 5.0 0.0 14.0

16.0

18.0

20.0 22.0 24.0 26.0 Natural Water Content (%)

28.0

30.0

32.0

Figure (1): Effect of Water Content on the Soil Initial Tangent Modulus. 3.2. Effect of Moisture Content on the Dry Unit Weight of the Subgrade: Figure (2) shows the effect of increasing the water content of the subgrade soil on the dry unit weight of the tested specimens. The results indicated a moderate increase in the dry unit weight for water contents from 16% up to 20%, followed by a moderate recess of the dry unit weight values till a water content value of about 22%. Increasing the water content more than 22% resulted in a pronounced decrease in the dry unit weight of the subgrade soil specimens.

Dry Unit Weight (kN/m3)

17.0 16.0 15.0 14.0 13.0 12.0 14

16

18

20 22 24 26 Natural Water Content (%)

28

30

32

Figure (2): Water Content - Dry Unit Weight Relation for the Subgrade Soil. Gulf Road Engineering Society

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It should be noted that the results shown in Figure (2) is closer to the normal laboratory compaction test results. 3.3. Effect of Moisture Content on the Unconfined Compressive Strength of the Subgrade Soil: The effect of moisture content on the unconfined compressive strength of the subgrade soil is shown in Figure (3). The trend line shows a slight increase in the unconfined strength of the soil specimens up to water content of 22%. However, the reduction in the unconfined compressive strength of the soil samples became noticeable when increasing the water content over 24%. Samples with lower water content tend to be closer to the partially saturated soils with ease in cracking and failure under moderate loads. On the other hand, higher water contents tend to weaken the soil even further to the point of becoming in the liquid state, when the water content is equal to the liquid

Soil Unconfined Compressive Strength (kPa)

limit. 140 120 100 80 60 40 14.0

16.0

18.0


28.0

30.0

32.0

Figure (3): Effect of Water Content on the Soil Unconfined Compressive Strength. 3.4. Effect of Moisture Content on the Pavement Condition Index (PCI): Figure (4) shows the effect of moisture content on the pavement condition index. Increasing the moisture content of the subgrade soil resulted in noticeable deterioration in the PCI value as shown in Figure (4). Lower moisture contents showed higher PCI values and higher water contents showed lower PCI values. Thus, the subgrade soil water content or to a wider perspective, the subgrade soil overall status including moisture content, mechanical and physical properties should be directly incorporated in the Pavement Condition Index. Gulf Road Engineering Society

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Pavement Condition Index, PCI (%)

80 70 60 50 40 30 20 10 14.0

16.0

18.0

20.0 22.0 24.0 26.0 28.0 30.0 32.0 Natural Water Content (%) Figure (4): Effect of Water Content on the Pavement Condition Index (PCI).

3.5. Field Water Content and Proctor O.M.C.: Figure (5) shows a graph presenting the field water content against the laboratory proctor optimum moisture content of all the studied cases, the field water contents were moderately to considerably higher the optimum moisture contents (O.M.C.). The O.M.C. was higher than the field water content in one case only. The presence of these roads within agriculture areas imposed the subgrade soils to increased water contents from the irrigation water and consequently lower compressive strength and the resulting pavement distresses. 35

Water Content (%)

31

Field W.C. (%) Proctor O.M.C. (%)

30

24

24

25

19

20

24

21 19

16

15

28

15 11

10

16

12

14

5 0 0

1

2

3 4 5 Specimen Number

6

7

8

Figure (5): Graph Showing the Field Water Content Vs. the Laboratory O.M.C. Gulf Road Engineering Society

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3.6. Effect of Subgrade Soil Water Content on the Long Crack Density: Figure (6) shows the effect of the subgrade soil water content on the long crack density. It is clear that the long crack density increases with increasing the subgrade water content as shown in Figure (6). This may be attributed to the decrease in the subgrade soil modulus due to increasing its water content over OMC as shown in Figure (5). This decrease in the subgrade modulus leads to deferential settlement of pavement causing both longitudinal and transverse cracks. 28

Long Crack Density

24 20 16 12 8 4 14.0

16.0

18.0


28.0

30.0

32.0

Figure (6): Effect of Water Content on the Long. Crack Density.

4. Conclusions: Based on the results of the current research the following conclusions could be drawn: 1- Increasing the subgrade soil water content has an adverse effect on the behavior of pavements supported on such subgrade. 2- In addition to the AASHTO soil classification system, the subgrade soils should be classified according to the plasticity index, because such classification may interpret or predict the overall performance of the pavement under working conditions. 3- A well selected and efficient drainage system could prevent, to a noticeable degree, deterioration of the pavement under working conditions. 4- The subgrade soil behavior have a direct influence in the Pavement Condition Index (PCI) of the pavement and thus, the subgrade soil overall status including moisture Gulf Road Engineering Society

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content, mechanical and physical properties should be directly incorporated in the Pavement Condition Index. 5- The pavement layers thickness should be increased in cases of highways surrounded by agriculture lands to accommodate the higher distresses caused by variation in water contents during the highway service life.

5. References: AASHTO Guide for Design of Pavement Structures American Association of State Highway and Transportation Officials, Washington, D.C., 1986. ASTM, American Standards for Testing and Materials. Chahan, Z., (2004), "The Effect of High Ground Water Level on Pavement Subgrade Performance”, Ph. D. Dissertation, Dept. of Civil and Environmental Engineering, college of engineering, University of Florida, . Dore, G., and Savard, Y., (1998), "Analysis of Seasonal Pavement Deterioration", Transportation Research Record, Transportation Research Board, Washington, D.C. Edris, E.V., and Lytton, R.L., (1977), "Climatic Materials Characterization of FineGrained Soils", Transportation Research Record 642, Transportation Research Board, Washington, D.C., pp39-44. Elliott, R.P., and Thornton, S.I., (1988), "Simplification of Subgrade Resilient Modulus Testing", Transportation Research Record 1192, TRB, Washington, D.C., pp.1-7. Highway Research Program [NCHRP] Synthesis of Highway Practice 96, (1998), "Pavement Subsurface Drainage Systems", Road Management & Engineering Journal. Janoo, V.C., and Berg, R.L., (1990), "Thaw Weakening of Pavement Structures in Seasonal Frost Areas," Transportation Research Record 1286, TRB, Washington, D.C., pp. 217-233. Li, Y., (2004), "Handbook of Transportation Engineering: Chapter (15): Pavement Testing and Evaluation", in Kutz, M. Ed., McGraw Hill Book, Inc., New York.


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Salem, H.M., Bayomy, F.M., Al-Taher, M.G., (2003), "Prediction of Seasonal Variation of Subgrade Resilient Modulus Using LTPP Data", Transportation Research Board, TRB 82nd Annual Meeting, pp. 02-3642, Washington, D.C.2003. Shahin, M. Y., and Kohn, S. D., (1984), "Pavement Maintenance Management for Roads and Parking Lots", Construction Engineering Research Lab., United States army, Corps of Engineering, Technical Report, M-294. White, T.D., and Coree, B.J., (1990), "Threshold Pavement Thickness to Survive Spring Thaw", Proceedings of the 3rd International Conference on Bearing Capacity of Roads and Airfields, Trondheim, Norway, pp. 41-51. Zhang, C., (2004), "The Effect of High Groundwater Level on Pavement Subgrade Performance", Ph.D. Thesis, College of Engineering, Florida state University, U.S.A.


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