Methods Of Pavement Design

Methods Of Pavement Design

a) Empirical methods – Methods are based on physical properties and strength parameters of soil subgrade. Ex is bG.I/CBR/Stabilometer/Mc-Leod Methods
b) Semi-Empirical – Methods are based on stress-strain function. Ex Triaxial Method
c) Theoretical Method- Based on mathematical computation. Ex Burmister Method which is based on Elastic theory

G.I. Method

  • Computation is explained in Highway material
  • To design the pavement thickness first G.I value of soil is found. Then anticipated traffic is estimated and designated as light, medium, or heavy
  • G.I value is related to thickness by design charts
  • Limitation- Method does not consider the quality of material used for pavement as thickness suggested same for poor or good quality material

The group-index method was devised by the U.S. highway Engineers (Highway analysis Board) in 1945. The group-index of a soil, an arbitrary index allotted to completely different soil varieties based on the percent fines, liquid limit and physical property index, is defined by the subsequent equation –

GI = 0.2 a + 0.005 ac + 0.01 bd
The higher the group index, the weaker is that the subgrade soil. the planning curves are given in Fig

CBR Method

The California bearing ratio (CBR) value of the subgrade soil was the basis for the method of design of flexible pavements, developed originally by the California State Highway Department, and adopted by The Road Research Laboratory, London, for developing their own design procedure and design charts.

  • Computation is explained in Highway material
  • This method is based on the strength parameter of subgrade soil and subsequent pavement material
  • From the charts prepared by the California state highway department and IRC, the Total thickness of pavement needed to cover the subgrade of the known CBR value is obtained knowing the value of Design wheel load for CSHD and Traffic Volume for the IRC approach
  • By the U.S. Corps of Engineers thickness of pavement for different layers can also be computed using:

 t=\sqrt{P}[\frac{1.75}{CBR}-\frac{1}{p\pi}]^{\frac{1}{2}} \: or\:t=[\frac{1.75P}{CBR}-\frac{A}{p\pi}]^{\frac{1}{2}}

Where
t is pavement thickness in cm,
P wheel load in kg,
CBR is in percent,
p is tyre pressure in kg/cm2,
A is the area of contact in cm2

California Resistance value method

  • Total thickness is computed using:
  •  t=\frac{K(TI)(90-R)}{C^\frac{1}{3}}
    • Where K is numerical constant=0.166, R Stabilometer value and C is Cohesinometer value
    •  TI\: (Traffic\: Index)=1.35(EWL)^0.11,\: EWL \: is \: Equivalent \: wheel \: load \: computed \: as
  • The individual thickness of each layer can be calculated from an equivalency relationship using
  •  \frac{t_1}{t_2}=(\frac{C_2}{C_1})^\frac{1}{5}

TRIAXIAL Method

  • Pavement thickness Ts consisting of a material with modulus E is given as
    •  T_s=\sqrt{(\frac{3PXY}{2\pi\Delta})^2}-a^2
  • Where P is wheel load in kg, a is the radius of contact area(cm), X is Traffic Coefficient, Y is Rainfall coefficient, is design deflection (0.25 cm), E is the modulus of elasticity in Kg/cm2
  • If pavement and subgrade are considered as a two-layer system a stiffness factor has to be introduced
  • Pavement thickness is modified using  S.F=(\frac{E_S}{E_P})^\frac{1}{3}
  • Hence modified Thickness would be
    •  T_s=\sqrt{(\frac{3PXY}{2\pi\Delta})^2}-a^2*(\frac{E_S}{E_P})^\frac{1}{3}
  • Relation between pavement layer of thickness t1 and t2 of elastic modulus of E1 and E2 is given by
    •  \frac{t_1}{t_2}=(\frac{E_2}{E_1})^\frac{1}{3}

Mc Leod Method

Emperical design equation  T=K\:log_{10}\frac{P}{S}

Where
K is base course constant,
T is required thickness,
P is gross wheel load (Kg),

Burmester’s (Layered equation) Method-

  • Boussinesq’s equation is a special case of Burmister’s layered system, which take the homogenous modulus of elasticity for all the layers,  E_s=E_{sb}=E_b
  • Burmester’s takes  E_p=10E_s
  • Where Ep is pavement elastic modulus
  • Es is a subgrade elastic modulus, because of it vertical stress on subgrade reduces from 70 to 30 %
  • Assumptions
    The surface layer is infinite in the horizontal direction and finite in vertical direction whereas 2 bottom layers considered infinite in both the direction
  • Deflection factor is introduced as F which depends upon Es/Ep and h/a ratio, h iss the thickness of reinforcing layer and a is the radius of contact area
  • Where deflection is denoted as
    •  \Delta=1.5\frac{pa}{E_S}.F_2 for Flexible pavement
    •  \Delta=1.18\frac{pa}{E_S}.F_2  for Rigid pavement

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