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**Introduction**

Flexible pavements are those, which on the whole have low or negligible flexural strength and are rather flexible in their structural action under the loads.

A typical flexible pavement consists of four components:

- soil subgrade
- sub-base course
- base course
- surface course.

**(i) Stress Under Road Surface as per Boussineq’s Equation,**

where,

σ_{z} = vertical stress at depth z.

q = surface pressure.

z = depth at which σz is computed.

a = radius loaded area.

**(ii) As per IRC**

Maximum legal axle load = 8170 kg

Equivalent single wheel load = 4085 kg.**(iii)** **Contact pressure****(iv)** **Rigidity factor (R.F)**

**(v) Equivalent Single Wheel Load (ESWL)**

**Methods of Flexible Pavement Design**

**(i) Group Index Method**

G.I = 0.2a + 0.005ac + 0.01bd

**(ii) C.B.R Method****(a) **

**(b) The thickness of Pavement, (T)**

where, P = Wheel load in kg.

CBR = California bearing ratio in percent

p = Tyre pressure in kg/cm^{2}

A = Area of contact in cm^{2}.

A = πa^{2}

a = Radius of contact area.

**(c) Number of a heavy vehicle per day for design (A),**

A = P[1 + r]^{(n + 10)}

where, A = No. of vehicles at the end of design period.

P = Number of heavy vehicles per day at least count.

r = Annual rate of increase of heavy vehicles

n = Number of years between the last count & the year of completion of construction.

**(d) CBR Method of pavement design by cumulative standard axle load,**

where,

N_{s} = Cumulative number of standard axle load

A’ = Number of the commercial vehicle per day when construction is completed considering the number of lanes.

n = Design life of the pavement, taken as 10 to 15 years.

F = Vehicle damage factor.

D = Lane distribution factor

**(iii) California Resistance Value Method**

where, T = Total thickness of pavement, (cm)

k = Numerical constant = 0.166

T.I = Traffic Index

T.I = 1.35(EWL)^{0.11}

R = Stabilometer resistance value

C = Choesiometer value.

where, T_{1} & T_{2} are the thickness values of any two pavement layers & C_{1} & C_{2} are their corresponding Cohesiometer values.

**(iv) Triaxial Method**

(a) Thickness of pavement required for single layer, (T_{S})

where, TS = Thickness in cm

P = Wind load in kg

X = Traffic coefficient

Y = Rainfall coefficient

E_{S} = Modulus of elasticity of subgrade soil (kg/cm^{2})

a = Radius of contact area (cm)

Δ = Design deflection (0.25 cm)

(b) Thickness of Pavement Consist of Two layer system,

where, E_{P} = Modulus of elasticity of pavement material

T_{1}/T_{2} = (E_{S})/(E_{P})^{1/3}

**(v) MC Load Method**

T = k.log_{10}(P/S)

where, T = Required thickness of gravel base (cm)

P = Gross wheel load, (kg)

k = Base course constant.

**(vi) Burmister Method (Layered System)**

Displacement equations given by Burmister are,

where, μ_{s} and μ_{p} are Poisons ratio for soil subgrade & pavement.

For single layer, F_{2} = 1

P = Yielded pressure

E_{S} = Subgrade modulus

a = Radius of loaded area

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