  # Flexible Pavement

By BYJU'S Exam Prep

Updated on: September 25th, 2023 Flexible pavement is one of the types of pavement, Pavement is the surface designed to transfer the vehicle’s load. Pavement can be categorised into the rigid pavement and flexible pavement. Flexible pavement is designed with bituminous materials and rigid pavement is designed with concrete materials.

Flexible pavement is composed of different materials, which makes it flexible in nature. The settlement at different points will be different for the flexible pavement. This article contains fundamental notes on the “Flexible Pavement” topic of the “Highway Engineering” subject.

## What is Flexible Pavement?

Flexible pavement is the type of pavement designed to transfer the vertical load of vehicles to underlain soil. A pavement is a hard surface which can resist or transfer the load of traffic to the below layers which are sufficient to bear. Pavement can be classified into rigid pavement and flexible pavement. Flexible pavements consist of many layers of bituminous materials and WBMs.

A flexible pavement transfers the vertical load to the below layers through the grain-to-grain transfer mechanism. In this mechanism above the layer transfer the load to the below layer which is in their contact. This load transfer mechanism can be understood by knowing the internal structure of the flexible pavement.

## Flexible Pavement Structure

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, which are listed below.

• Sub-base course
• Base course
• Surface course ## Different Terminologies in the Design of Flexible Pavement

Terminologies are the different parameters that are necessary to know before designing flexible pavement. These parameters consist of the stress-induced, thickness of different layers, etc. These parameters are explained below:

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

• σz = vertical stress at depth z.
• q = surface pressure.
• z = depth at which σz is computed.

(ii) IRC Considerations

Maximum single axle load = 8170 kg

Equivalent single wheel load = 4085 kg.  (v) Equivalent Single Wheel Load (ESWL)

ESWL at d/2 depth = P

ESWL at 2s depth = 2P ## Methods of Design of Flexible Pavement

Flexible pavement can be designed with different methods based on different parameters. These include the Group index method, CBR method, IRC method, etc. These methods are explained below.

(i) Group Index Method

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

(ii) C.B.R Method

The CBR method of pavement design consists of determining the CBR value for load conditions and then determining the thickness. It can be calculated with the following steps.

 Penetration Standard load 2.5 mm 1370 kg 5.0 mm 2055 kg

(b) 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=πa2

a = Radius of the contact area.

(c) Number of heavy vehicles per day for design (A)

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

where, A = No. of vehicles at the end of the 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) The cumulative standard axle load where,

• Ns = Cumulative number of standard axle load
• A’ = Number of commercial vehicles 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

T = k(TI)(90 – R)/C(1/5)

where,

• T = Total thickness of the 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.

or, T = 0.22(EWL)0.11(90-R)/C0.20

T1/T2 = (C2/C1)0.2

where T1 and T2 are the thickness values of any two pavement layers and C1 and 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 Consisting Two-layer system where,

• EP = Modulus of elasticity of pavement material

T1/T2 = (Es/EP)(1/3)

T=k log10(P/S)

where,

• T = Required thickness of gravel base (cm)
• P = Gross wheel load, (kg)
• k = Base course constant.

(vi) Burmeister Method (Layered System)

Displacement equations given by Burmeister are

(a) Δ = 1.5 Pa/Es → for flexible plate

(b) Δ = 1.18 Pa/Es → for rigid plate

μsp=0.5

Where,

• μs and μp are the Poisons ratio for soil subgrade and pavement respectively.
• P = Yielded pressure

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