Page 100 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

− For pavements with thin surfacings, the base layer is the most critical component, and failure in this layer
effectively constitutes pavement failure.
• Specific pavement layers
− The load spreading potential of an individual layer is a product of its thickness and its effective long term
stiffness under loading.
− The Effective Long Term Stiffness (ELTS) of a layer depends on the material type and class, and on its
placement in the pavement system.
− Fine-grained subgrade materials act in a stress-softening manner. For these materials, the ELTS is
determined mainly by the material quality and by the climatic region. Owing to the stress softening
behaviour, subgrade materials generally soften with decreased cover thickness.
− Coarse-grained, unbound layers act in a stress-stiffening manner. For these materials, the ELTS is
determined mainly by the material quality and the relative stiffness of the supporting layer. The ELTS of
these materials increases with increasing support stiffness, by means of the modular ratio limit, up to a
maximum stiffness, determined mainly by the material quality.
− BSMs are assumed to act in a similar way to coarse granular materials, but with a higher cohesive strength.
However, owing to the higher cohesive strength in bituminous stabilized materials, these layers are less
sensitive to the support stiffness than unbound granular materials and can therefore sustain higher modular
ratio limits. If the cement content of a BSM mix exceeds 1 percent, then the material is assumed to behave
as a cemented material.

These rules-of-thumb introduce several concepts such as the ELTS, modular ratio limit, maximum stiffness and
stress-stiffening behaviour, which are briefly described in the following sections.

(i) The Effective Long Term Stiffness (ELTS)
The ELTS is a model parameter which serves as a relative indicator of the average long term in situ stiffness of a
pavement layer. As such, the ELTS averages out the effects of decreasing stiffness owing to traffic related
deterioration, as well as seasonal variations in stiffness. Thus, the ELTS does not represent the stiffness of a
material at any specific time. This concept is illustrated in Figure 38 for cemented materials, but is applicable to all
materials included in the Pavement Number.

Effective Stiffness Pre-Cracked Phase (E > 3000 MPa) Effective Stiffness Note: For hot mix asphalt and stabilized layers, the modular ratio limit ensures

that faster breakdown of stiffness due to weaker support is incorporated

Effective Fatigue Phase (E = 800 to 2000 MPa) ELTS of cement stabilized material on stiff support

Equivalent Granular Phase (E = 200 to 400 MPa)
ELTS of cement stabilized material on soft support
ELTS

Axles Accommodated Axles Accommodated

Figure 38. Effective Long Term Stiffness (ELTS): Example for Lightly Cemented Material

The ELTS is also not a stiffness value determined by means of a laboratory or field test. It is a model parameter,
calibrated for use in the PN design method. It may therefore differ from stiffness values typically associated with
material classes.

(ii) Modelling of Subgrade Materials
Characterization of the support is critical to the pavement design of all pavements. For new construction, the TRH4
procedure for delineation of the in situ subgrade and for importing selected subgrade material, if necessary when the
structural strength of the in situ subgrade is insufficient, applies to the PN method. For rehabilitation projects, the
guidelines in TRH12 for evaluating and designing for changing support conditions should be followed.

The first step in the calculation of the PN-value is the determination of the subgrade material class using the Material
Classification System for Design described in Chapter 9: 15 and Appendix C of TG2. Once the subgrade class has
been determined, the ELTS for the subgrade is calculated. This involves the following steps:
• Assignment of an ELTS based on the materials class. The values are given in Table 38.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 89