Test: McLeod and Burmister Method - Civil Engineering (CE) MCQ

McLeod is an empirical method used for the design of flexible pavements. The results from the plate load bearing test are used for the McLeod method. Bearing stress is not a test, it’s a strength parameter. The shear test is not used to compute the bearing capacity of the soil.

Burmister Method:
(i) This method is based on young's modulus of elasticity of different layers of pavement. As flexible pavement composed of layers and elastic modulus of top most layer is maximum.
Assumptions:

• Materials in each layer are isotropic, homogeneous, and elastic.
• Pavement forms a stiffer layer having higher value of E than that of subgrade.
• Layers are in constant contact.
• The surface layer is infinite in horizontal direction but finite in vertical direction means infinite area with finite depth.

This method gives the following design deflection values for pavements with different layers –
For flexible pavement:

For rigid pavement:

Where, p = contact pressure at road surface due to wheel load (kg/m²)
E_s = modulus of elasticity of subgrade (kg/cm²), a = radius of contact area(cm) and
F₂ = deflection factor which depends upon E_s/E_P  and h/a,  for single layer system F₂ = 1.

For computing the thickness of gravel base using McLeod method is given by  The equation uses the base course constant, the wheel load and the subgrade support.

Burmister’s Method:
Assumptions

• Materials in each layers are isotropic, homogeneous and elastic.
• Pavement forms a stiffer layer having higher value of E than that of subgrade.
• Layers are in constant contact.
• Surface layer is infinite in horizontal direction but finite in vertical direction.

This method gives the following design deflection values for pavements with different layers

Where, p = contact pressure at road surface due to wheel load (kg/m²)
E_s = modulus of elasticity of subgrade (kg/cm²), a = radius of contact area(cm) and
F₂ = deflection factor which depends upon E_s/E_P  and h/a,  for single layer system F₂ = 1.
Formula used:-
Design deflection for single-layered flexible pavement is given by,

Radius of contact area is given by,

Calculation:
Wheel Load, P = 40kN
Modulus of elasticity for subgrade and pavement, E_S = E_P = 20MPa
Tyre pressure, p = 0.50 MPa

Design deflection for flexible pavement

The surface layer is considered to be infinite in the horizontal direction and finite in the vertical direction. The surface layer is the topmost layer and hence it has a particular thickness and it extends infinitely along the horizontal direction. It is elastic and is homogeneous.

In order to compute the design subgrade support, there are design charts available. The ratio of design subgrade support to the standard subgrade support is obtained from the chart and the ratio is found out. On multiplying the ratio by the contact pressure of the design load, the design subgrade support can be found out.

The equation for the thickness of pavement using McLeod method is 
From the question, P = 4100 kg; K = 85 and S = 1810 kg 

The equation for the thickness of pavement using McLeod method is 
From the question, P = 4100 kg; K = 80 and S = 2050 kg 

The radius of the contact area can be found out using the relation between wheel load (P), tyre pressure (p) and radius (a) as πa² = P/p. So, the radius of the contact area can be obtained as 
In order to find subgrade support, perimeter area ratio and the plate load test details are required. The perimeter area ratio can be obtained as 
The design chart defined by Norman W. McLeod through the Canadian Department of Transport is used. A sample is shown below with the details for 0.5 cm deflection. The ratio for subgrade is obtained as 1.1 from the same.

Therefore, the design subgrade support is = Ratio obtained from chart X contact pressure of design load = 1.1 X 2.5 = 2.75 kg/cm².

For a single layer, the pavement thickness or the thickness of the reinforcing layer, h is taken as 0. In the design chart, the value of the ratio of deflection factor is read for h/a = 0 and modulus of elasticity 1. It is obtained as 1.