Influence of subgrade condition on rutting in flexible pavements

International Journal of Civil Engineering and Technology (IJCIET), ISSN 0976 – 6308

(Print), ISSN 0976 – 6316(Online) Volume 4, Issue 3, May - June (2013), © IAEME

30

INFLUENCE OF SUBGRADE CONDITION ON RUTTING IN

FLEXIBLE PAVEMENTS- AN EXPERIMENTAL INVESTIGATION

Dr. K.V.Krishna Reddy

Professor & Principal, Chilkur Balaji Institute of Technology, Hyderabad-75, AP, India

ABSTRACT

In the present study, an attempt is made to investigate the influence of subgrade

condition on the rutting phenomena in flexible pavements. Flexible pavement section is

formed in a steel box section with a standard pavement section over a clayey subgrade. The

subgrade condition is varied by varying the CBR of the same by additives. Conventional and

strengthened surface courses were considered to check the influence of strengthened surface

courses in lowering the rut depth. The results indicated that the rutting phenomenon is

initiated in the subgrade itself and surface strengthening alone has low influence on limiting

rut formation.

Key Words: Rutting, Subgrade stabilization, Clay subgrades, CBR

1. INTRODUCTION

In recent years, Highways have experienced an increase in the severity and extent of

permanent deformation (rutting) in bituminous pavements. Rutting reflects not only the

structural condition of the pavement but also the functional condition and hence life cycle

costs. Need was recognized as to find which layer of the pavement needs attention to limit the

permanent deformation. Literature indicated that rutting could be ascribed to shear

deformation within the asphalt layer (primary rutting) or subgrade deformation (secondary

rutting). This paper investigates the influence of subgrade strength and surface modification

on rutting.

INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND

TECHNOLOGY (IJCIET)

ISSN 0976 – 6308 (Print)

ISSN 0976 – 6316(Online)

Volume 4, Issue 3, May - June (2013), pp. 30-37 © IAEME: www.iaeme.com/ijciet.asp

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IJCIET

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31

2. RESEARCH METHODOLOGY

2.1 Subgrade strength variation Laboratory experimentation is done to determine the optimum additives content for

stabilizing the clayey subgrade with pond ash-lime, sand and gravel to obtain subgrade

strength variation.

2.2 Surface course strengthening 80/100-penetration grade bitumen is considered for experimentation and aggregates

confirming midpoint gradation of grade II specifications as per MORTH specification have

been used. Hydrated lime was used to improve the strength of the bituminous concrete to be

used in the surface course.

2.3 Laboratory pavement setup Laboratory based pavement sections with conventional materials and that with

different subgrades and modified surface course is prepared in a prefabricated box type

arrangement made of mild steel of size 40cm X 30 cm X 30 cm. Eight Laboratory based

multi layer sample pavement sections were formed, four of them namely pavement section

with clay subgrade and conventional surface course, pavement section with sand stabilized

subgrade and conventional surface, pavement section with gravel stabilized subgrade and

conventional surface and that with pond ash+lime stabilized subgrade and conventional

surface were formed. Another four pavement sections were formed a with the above

subgrades and lime modified surface course. All of them were soaked for 96 hours by passing

water continuously through the water inlet and draining the same through the water outlet

drainage pipes.

3 DATA ANALYSIS

3.1 Material Properties Clay soil of highly expansive nature has been used as subgrade material. The liquid

limit and plasticity index were 79.3 and 47.84 respectively with a soaked CBR of 2.65.

Pondash+ lime , gravel and sand were used as additives to vary the strength of the subgrade

layer. The optimum additives content is evaluated by a series of laboratory tests. The

stabilized soil properties are depicted in Table 1 along with the basic soil properties used for

modification.

Aggregates corresponding to grade II specifications for base course materials

(MORTH) have been used in formation of water bound macadam (WBM). Gravel screenings

were used to fill the voids and the properties are as depicted in Table 2. Bitumen of 80/100-

penetration grade with grade II aggregates for bituminous concrete mix (MORTH) was used

in the formation of the surface course. Hydrated lime was used as modifier to strengthen the

bituminous concrete surface layer. The properties of the bituminous concrete along with lime

modified material are as in Table 3.



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Table 1 Properties of clay and gravel used in laboratory sample pavement preparation

S.

N

o.

Property Clay Gravel

Clay +

25%PA +

5%lime

Clay+ 30%

gravel

Clay +

25% Sand

1 Grain Size

Distribution

Gravel (%)

Sand (%)

Silt size (%)

Clay size (%)

-

1.2

31.4

67.4

6

80

9.6

4.4

-

-

-

2 Atterberg Limits Liquid Limit (%)

Plastic Limit (%)

Plasticity Index

Shrinkage Limit (%)

79.3

31.46

47.84

12.20

35

18.1

16.9

14.0

56.50

44.30

12.20

39.80

79.3

31.46

47.84

12.20

79.3

31.46

47.84

12.20

3 Compaction

properties Optimum moisture

content (%)

Maximum Dry

Density (g/cc)

17.10

1.683

11.7

1.95

17.50

1.660

15.00

1.767

15.62

1.758

4 Soaked CBR (%) 2.65 11.4 16.6 5.18 4.22

5 Free swell index (%) - - 30

6 Swell potential (%) 1.20

7 UCC kN/m2 310 (7D)

Table 2 Properties of aggregate used for WBM of laboratory sample pavement

Property Value Property Value

Crushing value 18% Specific gravity 2.79(CA)/2.76(FA)

Impact value 14% Water absorption 0.8%

Abrasion value 22%

Table 3 Properties of bituminous concrete used for surface course of laboratory multi

layered sample pavement section

S.

No Mix / Property Conventional

Lime

modified

1 Optimum Bitumen content

/lime content

4.3% 2.80%

2 MSV (Kg) 1300 2650

3 Air voids (%) 3.875 4.05

4 Flow value (mm) 2.375 3.5

5 Bulk density (g/cc) 2.520 2.446



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3.2 Laboratory based multi layer sample pavement section

The thickness of the pavement layers have been designed to ensure that the stresses

reach the subgrade level. It was proposed to form the multi layer sample pavement section

with the 40mm thick bituminous concrete, 100mm thick WBM layer and 75 mm thick

subbase. The subbase was formed of 25mm thick well-graded sand overlain by 50mm of

gravel layer to facilitate free drainage of water during soaking. Figure 1 represent the

laboratory based pavement section. Figure 2 depict the laboratory conventional pavement and

the testing of the same

4 RESULTS

Hamburg wheel tracking device (Germany) was used to evaluate the rut depth. These

laboratory based pavement sections were subjected to wheel tracking on the wheel-tracking

device under a contact pressure of 5.6 kg/cm2 for a set of 1,10,000 revolutions.

GRAVEL SUBBASE

Fig 1(b) Cross section of modified multi layer pavement section

Drainage

185mm

25 mm

100 mm

50 mm

40 mm

W B MACADAM

MODIFIED BITU CONC

WELL-GRADED SAND

STABILIZED SUBGRADE

W B MACADAM

WELL-GRADED SAND

GRAVEL SUBBASE

BITUMINOUS CONCRETE

CLAY SUBGRADE Drainage

Fig 1(a) Cross section of conventional multi layer pavement section

Fig 2 (b) Photograph showing

wheel tracking of the laboratory

based pavement

Fig 2 (a) Photograph showing

laboratory based multilayer pavement



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The wheel tracking test results in terms of rut depth for all pavement sections are noted down

for every 1000 revolutions. The rut depth at the end of 110000 revolutions is as presented in

Table 4. The results are plotted in terms of rut depth to number of wheel load repetitions as

depicted in Fig.3 for the conventional surfaced pavement sections and Fig.4 represent the rut

depth to number of wheel load repetitions for the lime modified surface pavement sections.

Table 4 Rut depth for pavement sections at the end of one lakh revolutions

S.No Pavement section Rut depth

(mm)

1 Clay subgrade with conventional surface 1.91

2 PA+Lime stabilized subgrade with conventional surface 1.15

3 Gravel stabilized subgrade with conventional surface 1.40

4 Sand stabilized subgrade with conventional surface 1.51

5 Clay subgrade with lime modified surface 1.62

6 PA+Lime stabilized subgrade with Lime modified

surface 0.86

7 Gravel stabilized subgrade with lime modified

bituminous surface 1.09

8 Sand stabilized subgrade with lime modified bituminous

surface 1.21

Fig3. Rutdepth vs no. of wheel load repetions for pavement sections with conventional

surfaces

0.00 40000.00 80000.00 120000.00

No. of wheel load repetitions

0.00

400.00

800.00

1200.00

1600.00

2000.00

Rut depth

in 0

.01m

m

1 clay subgrade + conventional surface Y = 0.0176931 * X + 57.9239 0.998302

Sno. Pavement Model selected Fit R Sq value

2 Sand stabilized subgrade + conventional surface Y = 0.0135704 * X + 38.7609 0.998868

3 Gravel stabilized subgrade + conventional surface Y = 0.0130083 * X + -17.2826 0.998938

4 PA + lime stabilized subgrade + conventional surface Y = 0.0109846 * X + -40.2391 0.998302

1

2

3

4



35

0.00 40000.00 80000.00 120000.00

No. of wheel load repetitions

0.00

400.00

800.00

1200.00

1600.00

2000.00

Rut depth

in 0

.01 m

m

1

2

3

4

Sno. Pavement Model selected Fit R Sq value

1 clay subgrade + lime modif surface Y = 0.0082419 * X + -30.4783 0.998233

2 Sand stabilized subgrade + lime modif surface Y = 0.0108273 * X + 33.1957 0.998841

3 Gravel stabilized subgrade + lime modif surface Y = 0.0101453 * X + -13.3804 0.998944

4 PA + lime stabilized subgrade + lime modif surface Y = 0.0082419 * X + -30.4783 0.998233

Fig4. Rutdepth vs no. of wheel load repetions for pavement sections with lime modified

surfaces

5 ACKNOWLEDGEMENT

At the outset the author would thank the Head, CED, Vasavi college of Engineering,

SE R&B Department and Head CED & TE Division and other professors at NIT Warangal

for their valuable guidance and encouragement during experimentation.

6 CONCLUSION

1. Rut depth resulted in the conventional pavement section under test conditions is more

by 66%,36% and 26.5% as compared to that resulted in conventional surface on

stabilized subgrade with pond ash-lime ,gravel and sand respectively clearly

highlighting the involvement of subgrade in rutting phenomena.


by 122%, 75% and 47% as compared to that resulted in lime modified surface surface

on stabilized subgrade with pond ash-lime, gravel and sand stabilized subgrade

respectively. This highlights that though surface modification result in taking more

wheel load repetitions, subgrade modification results in achieving better rut control.


by 18% as compared to the conventional subgrade with lime modified surface

showing that the improvement by modifying surface is less than that achieved by

improving the subgrade alone (66%).

4. Effective pavement performance is to understand the cause of rutting. Sound

judgment should be used to determine which part of the existing pavement structure is

weak and total reconstruction of the pavement or full depth reclamation should be

considered rather than just improving the surface if the subgrades are poor.



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