Page 92 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

7.1.3 Waterbound Macadam

Waterbound macadam layers are a special type of crushed stone layers, however, they were not included in the
original SAMDM. However, in 2000 CSIR published “Guidelines for the Selection, Design and Construction of
Waterbound Macadam Base Layers” in which recommendation for mechanistic-empirical analysis of waterbound
macadam layers are given. As with unbound granular layers, waterbound macadam layers are assumed to
accumulate permanent deformation, from shear deformation. The resilient properties for waterbound macadam
bases are given in Table 32. The stiffness in a waterbound macadam layer depends on the strength of the support;
the stronger the underlying layer, the stiffer the layer. For rehabilitation investigations, it is important to realise that
wide ranges can exist for the same material, depending on the in situ state.

Table 32. Elastic Moduli for Waterbound Macadam Layers
Material Layer thickness Dry Condition Moist Condition
Code (mm) (30 % saturation) (60 % saturation)
2
1
2
1
Good support Weak support Good support Weak support
100 1200 800 1100 640
WM1
> 100 1080 620 1010 550
100 1000 600 900 440
WM2
> 100 880 420 790 330
Note
1. Well supported by an intact cemented-treated subbase that creates confinement of the waterbound macadam layer.
2. Relatively weak support provided by a granular or equivalent granular subbase layer

Waterbound macadam layers are also analysed by determining the shear stress state in the middle of the layer, as
illustrated in Figure 32, and comparing this to the shear strength, in terms of the cohesion and friction angle using
the Mohr-Coulomb model. However, the shear strength state parameter is the stress ratio, which is essentially the
inverse of the safety factor. The transfer function is given in Equations (22) and (23) in Table 33, along with the
shear strength parameters (cohesion and friction angle).

Waterbound Macadam for Road Widening
Waterbound macadam layers are useful when widening a
road or working in restricted areas, such as wedges.
Lighter equipment can be utilised to achieve the required
“strength” of the layer without damaging the adjacent
granular pavement. In addition, waterbound macadam
layers allow lateral drainage of moisture, and thus
prevent the development of the so called “bath-tub”
situation, which invariably results in premature failures.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 81