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: 1. soil subgrade 2. sub-base course 3. base course 4. surface course.
(i) Stress Under Road Surface as per Boussineq’s Equation,
σz = 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.
(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
(b) The thickness of Pavement, (T)
where, P = Wheel load in kg.
CBR = California bearing ratio in percent
p = Tyre pressure in kg/cm2
A = Area of contact in cm2.
a = Radius of contact area.
(c) Number of a heavy vehicle per day for design (A),
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,
Ns = 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, T1 & T2 are the thickness values of any two pavement layers & C1 & C2 are their corresponding Cohesiometer values.
(iv) Triaxial Method
(a) Thickness of pavement required for single layer, (TS)
where, TS = Thickness in cm
P = Wind load in kg
X = Traffic coefficient
Y = Rainfall coefficient
ES = Modulus of elasticity of subgrade soil (kg/cm2)
a = Radius of contact area (cm)
Δ = Design deflection (0.25 cm)
(b) Thickness of Pavement Consist of Two layer system,
where, EP = Modulus of elasticity of pavement material
(v) MC Load Method
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, are Poisons ratio for soil subgrade & pavement.
For single layer, F2 = 1
P = Yielded pressure
ES = Subgrade modulus
a = Radius of loaded area
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