Page 35 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

3.8.2 Compaction

Insufficient compaction may result in field densities below the minimum
required. In such cases, the strength of the material is not fully utilised, and
densification or failure may occur under traffic. Quality control on site must Poor Drainage and
ensure that the design specifications are met through proper construction and Insufficient Compaction
compaction practices. TRH14 and the Standard Specifications provide
recommendations on the minimum density requirements for pavement layers. Poor drainage and insufficient
Any special density requirements should be discussed with the relevant road compaction are probably
authority. Chapters 12 and 13 discuss many aspects of compaction. responsible for more pavement
failures than poor structural and
Compaction problems may result from material grading deficiencies or poor material design.
construction practices, such as not compacting at the optimum moisture
content, poor mixing of the material and compaction fluid, or insufficient or inappropriate compaction energy.
Blending of material from different sources, to improve the grading and compaction potential of the material, may be
better than trying to achieve density with excessive rolling.

When compacting a layer, the support layer needs sufficient support to act as an anvil, otherwise the compaction
energy is transmitted and lost through the pavement structure. The use of impact rollers can improve the strength
and support from the subgrade substantially. Impact rollers are discussed in Chapter 12: 2.10.1.

3.8.3 Variable Cross Sections and Paved Shoulders

On multi-lane roads or roads with climbing lanes, the traffic loading may be significantly different between the lanes.
In these cases, the pavement design may be adjusted accordingly. For example, the slow lane of heavily trafficked
roads is often constructed in concrete and the remainder of the pavement in granular, cemented, BSM or asphalt
layers. Care should be exercised not to trap water in the pavement when using layers of different thickness or
material type across the width of the road, including the shoulder. The complexity of implementing such a design
should also be considered, with reference to construction sequences, accommodation of traffic, and the construction
period.

Category A and B roads normally have paved shoulders. There is a zone of seasonal moisture content variation
towards the edge of the pavement (Emery, 1984 and 1985). The recommended minimum paved shoulder widths in
Table 6 should be used to prevent the zone of influence of the outer wheel-path overlapping with the seasonal
moisture content variation zone. A paved shoulder is more important in wet regions than moderate or dry regions.
Low volume roads seldom justify a surfaced shoulder. The decision to pave road shoulders also depends on the
traffic expected on the road, locations where vehicles pull of the road, and erosion protection requirements.

Table 6. Recommended Minimum Paved
Shoulder Widths

Road Category Paved Shoulder Width
(mm)
A 1 200
B 1 000
C 800

If a paved shoulder is provided, the structural capacity of the paved shoulder should be sufficient to carry traffic that
may use the shoulder. See Section 4.4.1 on design traffic estimation for lane distribution factors, including paved
shoulders.

Different Pavement Structures Across
the Width of a Road

When the traffic levels are different across different
lanes, the use of varying pavement structures is
sometimes justified.

However, care should be exercised not to trap water
in the pavement when using layers of different
thickness or material type across the width of the
road, including the shoulder.

Section 3: Design Considerations
Page 24