New Layer Coefficients for Geogrid-Reinforced Pavement Bases


Stabilizing the base layers of flexible pavements is gaining tremendous attention due to the lack of suitable construction materials. A geogrid reinforcement could offer a reduction in granular layer thickness or enhance the service life of the pavement. However, there is no standard direct methodology available for the design of a flexible pavement with a geogrid-reinforced base layer. The current design approaches adopt the base layer coefficient ratio (LCR) derived from the layer coefficient equation proposed by the American Association of State Highway and Transportation Officials (AASHTO 1993), which was initially developed for an unreinforced base layer. Moreover, the accuracy of the existing model for determining the base layer coefficient needs a reassessment since it varies for different subgrade conditions. Hence, an attempt was made to propose a new model which emphasizes on unreinforced and geogrid-reinforced base layer coefficients for weak-to-moderate subgrade conditions. Prior to the analysis, large-scale model pavement experiments were conducted to realize modulus improvement factor (MIF) and range of values of LCR of different geogrids, which are crucial parameters used in the design. In addition, design examples, validation and the MIF and LCR values of geogrid-reinforced base layer were provided for obtaining the base layer coefficients. It was noticed that the MIF and LCR value for the geogrid-reinforced base layers range between 1.6–3.33 and 1.23–1.59, respectively. The newly proposed equation for the base layer coefficients accounted for about a 33% reduction in the base layer thickness compared to the unreinforced case. Hence, a safe and economical pavement section may be obtained from the proposed model.

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a :

Radius of circular loading plate

a 1, a 2 :

Layer coefficients of asphalt and base layers

a 2r :

Base layer coefficient of reinforced section

a 2u :

Base layer coefficient of unreinforced section

d 1, d 2 :

Asphalt and base layer thicknesses

d 2r :

Reinforced base layer thickness

D :

Diameter of circular plate

E 1 :

Elastic modulus of layer 1 (base and subbase together)

E 2 :

Elastic modulus of layer 2 (subgrade)

E bcr = E 1r :

Elastic modulus of reinforced base course

E bcu = E 1u :

Elastic modulus of unreinforced base course

E eq :

Equivalent elastic modulus

S e :

Elastic settlement of plate

ε t :

Horizontal tensile strain below the asphalt layer (fatigue strain)

ε v :

Vertical compressive strain on top of the subgrade (rutting strain)

I :

Influence factor

I f :

Improvement factor

μ :

Poisson’s ratio

H :

Total height of base and subbase layers

H 1 :

Base layer thickness

H 2 :

Subbase layer thickness

H 3 :

Subgrade layer thickness

m 2 :

Drainage coefficient of base layer

M ra :

Resilient modulus of asphalt layer

M rr :

Improved resilient modulus

M rs :

Subgrade resilient modulus

M ru :

Resilient modulus of unreinforced base layer

q :

Bearing pressure

S N :

Required structural number

S Na :

Actual structural number

S o :

Overall standard deviation

Z R :

Standard normal deviate


Allowable loss of serviceability


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This research work has been carried out under active funding from the national highway authority of India (NHAI). The authors are thankful for NHAI. Authors also extend their gratitude for TechFab India Ltd. and Strata (I) Geosystems Ltd. for providing material for the study. The authors would like to thank Prof. M. R Madhav and Dr. B. Umashankar for providing valuable inputs during the experimental program.

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Saride, S., Baadiga, R. New Layer Coefficients for Geogrid-Reinforced Pavement Bases. Indian Geotech J (2021).

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  • Subgrade
  • Geogrid
  • Modulus improvement factor
  • Layer coefficient ratio
  • Base layer coefficients
  • Pavement design