Page 101 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

• Adjustment of the ELTS for different climatic regions (wet, dry or moderate), according to Table 39.
• Adjustment of the stiffness determined to take account of depth of subgrade cover using the chart or equation
shown in Figure 39.

Table 38. Stiffness Determination for the Subgrade

Design Equivalent Material Stiffness Value (MPa)
Class for Subgrade
G6 or better 180
G7 140
G8 100
G9 90
G10 70
Note:
Subgrade stiffness value should be adjusted for climate (Table 39)
and cover depth (Figure 39).

Table 39. Climate Adjustment Factors
Climate and Weinert N Values Adjustment Factor
(after TRH4, 1996)
Wet (Weinert N < 2) 0.6
Moderate (Weinert N = 2 to 5) 0.9
Dry (Weinert N > 5) 1.0

15
Adjustment to Subgrade Stiffness (MPa)

10

5

0

-5
If Cover Thickness (in mm) is:
> 800, then Adjustment = +10 MPa
-10 < 500, then Adjustment = -10 MPa
else:
Adjustment = -10 + [(Cover-500)/300] * 20 MPa
-15
400 450 500 550 600 650 700 750 800 850 900
Thickness of Subgrade Cover (mm)

Figure 39. Adjustment of Subgrade Stiffness Based on Cover Thickness

(iii) The Modular Ratio Limit and Maximum Stiffness
The modular ratio is defined as the ratio of a layer’s stiffness Modular Ratio
relative to the stiffness of the layer below it. Thus, if the stiffness The modular ratio is the ratio of a layer’s
of a base layer is 300 MPa, and the stiffness of the support below stiffness relative to the stiffness of the layer
it is 200 MPa, then the modular ratio of the base layer is 1.5. below it. Thus, if the stiffness of a base
layer is 300 MPa, and the stiffness of the
The modular ratio accounts for the stress-sensitive stiffness of support below it is 200 MPa, then the
granular materials, and, albeit to a lesser extent, BSM materials. modular ratio of the base layer is 1.5.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 90