Transportation Engineering

Pavement Design

Flexible (CBR, layered elastic) versus rigid (Westergaard slab on subgrade) pavements — the two design philosophies and the stresses they control.

PART 1

Topic Breakdown & Traps

The Engineering Principle

Flexible pavements transmit wheel loads to the subgrade through successive layers in compression; their thickness is governed by the CBR of the subgrade and traffic repetitions. Rigid pavements are concrete slabs whose flexural stiffness spreads the load, resting on a subgrade characterised by the modulus of subgrade reaction

k

; Westergaard's analysis gives the critical edge/corner/interior stresses through the radius of relative stiffness

l

The Core Formula Matrix

Tyre contact area:

A = \frac{P}{p}

(wheel load ÷ tyre pressure)

Radius of relative stiffness:

l = [\frac{E h ^{3}}{12 ( 1 - μ ^{2} ) k}]^{1/4}

Flexible design: CBR method ⇒ total thickness above subgrade for a design traffic.

Rigid design: Westergaard interior/edge/corner stresses depend on

l

, slab thickness

h

and

k

The ‘IIT Traps’

⚠
** $k$ for rigid, CBR for flexible.** The modulus of subgrade reaction governs slabs; the CBR governs layered flexible pavements.
⚠
Radius of relative stiffness rises with slab thickness ( $l \propto h^{3/4}$ ) and falls with stiffer subgrade $k$ .
⚠
Contact area, not contact pressure, is load ÷ tyre pressure — keep the ratio the right way up.

📚 Standard references

Highway Engineering — S.K. Khanna & C.E.G. Justo · Pavement Design

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

The CBR method of design directly yields the:

Q2MEDIUM2 Marks · NAT

A wheel load of

40 kN

acts at a tyre pressure of

0.7 MPa

. The tyre contact area is _____ mm².

Q3HARD2 Marks · MCQ

In Westergaard's rigid-pavement analysis, the radius of relative stiffness

l

increases when: