Page 23 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

3.4 Pavement Balance

3.4.1 Flexible Pavements
One of the design principles for flexible pavements is that
the material quality gradually, and smoothly, increases from Pavement Balance
the in situ subgrade up to the structural layers and One of the design principles for flexible pavements
surfacing. Such a pavement structure is referred to as a is that the material quality gradually, and
well-balanced pavement. Pavement balance may be smoothly, increases from the in situ subgrade up
quantified from the analysis of Dynamic Cone Penetrometer to the structural layers and surfacing. Such a
(DCP) data for which pavement balance parameters have pavement structure is referred to as a well-
been defined, or by calculating the modular ratio of the balanced pavement.
stiffnesses of successive layers. Refer to Section 7.3 for
DCP analyses and Section 7.2.2 for modular ratios.

If the strength of a pavement is concentrated in one or two strong layers, those layers initially carry most of the
loading, but deteriorate to achieve balance with the rest of the pavement structure. This process of the relative
strength of the pavement layers being forced into balance is referred to as “traffic moulding”. If the bulk of the
strength of a pavement is concentrated in only the
uppermost layers, the pavement is referred to as a shallow
pavement. If the pavement strength is distributed
throughout the depth of the pavement, the pavement is Avoid Shallow Flexible Pavements
referred to as a deep pavement. Deep, well-balanced Shallow pavements with a cemented or a hot mix
pavement designs are normally less sensitive to high loads asphalt base layer on a subbase or subgrade of
than where all the strength is concentrated towards the top much lower stiffness should be avoided. Such a
of the pavement, with poor support from below. pavement not only deteriorates rapidly under
normal traffic, but is also more sensitive to
Excellent performance has been obtained from deep, well- overloading than a well-balanced, deep pavement
balanced granular pavement structures in South Africa structure.
using only thin asphalt surfacing layers, provided water is
prevented from entering the base through the surfacing. Shallow pavements with a cemented or a hot mix asphalt
base layer on a subbase or subgrade of much lower stiffness should be avoided. Such a pavement not only
deteriorates rapidly under normal traffic, but is also more sensitive to overloading than a well-balanced, deep
pavement structure.

The exception to well-balance pavements is inverted pavements. This is where a granular base is placed on a
stronger, lightly cemented subbase. In this situation the pavement is unbalanced. With the action of traffic, the
lightly cemented layer deteriorates to an equivalent
granular state, and the pavement becomes balanced.
These pavements are commonly, and very
Inverted Pavements successfully, used in South Africa. See Section 7.1.3
An inverted pavement is when the base layer is a high for further discussion.
quality granular layer, and the subbase a cement
stabilized layer. A thin asphalt layer or seal provides Structural design methods do not generally yield a
the surfacing. The term “inverted” is used because the balanced pavement design automatically. Care must
strength of the pavement does not decrease with be taken to ensure that a design is balanced.
pavement depth, because of the stiff cemented layer.
This means that the pavement is not in balance.

The idea behind an inverted pavement is that the Deep and Shallow Pavements
cemented layer provides an anvil upon which the
granular base can be well compacted. This achieves a • In shallow pavements, the strength of the
high quality, dense base. pavement is concentrated in the uppermost
layers.
Over time, the cemented layer weakens to an • In deep pavements, the strength is distributed
equivalent granular state. The pavement is then in throughout the pavement.
balance.
Deep, well-balanced flexible pavement designs are
Inverted or “upside-down” pavement structures are normally less sensitive to high loads than designs
commonly used in South Africa, and are included in the where all the strength is concentrated towards the
TRH4 catalogues. top of the pavement, with poor support from
below.

Section 3: Design Considerations
Page 12