Page 108 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

Advantages of the DCP Method Disadvantages of the DCP Method
• Suited to new and rehabilitation design • Need DCP Data
• Relatively simple and easy to apply • Empirical: derived from CBR cover design
• Tested for South African conditions and materials • Mostly applicable to unbound and lightly
cemented pavements
• Variable results (need many repeats)
• Dependent on in situ moisture conditions
(seasonal)
• Influenced by large aggregates in the pavement
structure

7.4 The AASHTO Structural Number (SN) Method
The AASHTO Guide for Design of Pavement Structures (AASHTO, 1993) gives a full description of the structural
number (SN) method. The method can be used for new and rehabilitation pavement design. The method is based
on the results of the AASHO road test done in Ottawa, Illinois during the late 1950s to early 1960s. This method
must be applied with caution for a number of reasons:
• The method is an empirical method, based on performance data collected almost 50 years ago.
• The subgrade and pavement materials, as well as the pavement structures, used in the AASHO road test are
foreign to South Africa.
• The method is in imperial units and conversion to metric units must be done correctly.

The structural number method is, however, important because it is the only structural design method used in South
Africa based on a reduction in the functional level of service of the pavement. The structural capacity estimation is
based on a reduction in the Present Serviceability Index (PSI), a measure of riding quality. The pavement
deterioration models of the Highway Development and Management system, HDM-4 (ISOHDM, 2004) also uses the
structural number approach and these models will be incorporated in the revision of the South African flexible
pavement design method. The method also provides a good check of designs completed using more complex
methods.

The basic formula to estimate the structural capacity of a pavement is given by Equation (30). Each layer in the
pavement contributes to the structural number according to a layer coefficient depending on material type, the
thickness of the layer and a drainage coefficient for the layer, calculated using Equation (31). Note these equations
use imperial units.

Refer to Table 2 for the initial and terminal serviceability indices for the road categories used in South Africa.
Variability is accounted for by using the standard normal deviate and overall standard deviation, for which
recommended values related to the design reliability are given in the references for the method.

The first step in the process of estimating the structural capacity of a pavement with the SN method is to determine
the effective roadbed resilient modulus, which is an average subgrade resilient modulus adjusted for seasonal
changes. The AASHTO design guide makes provision for dividing a year into half-month periods to accommodate
seasonal moisture content variation in the subgrade. This division is believed to be too fine for subgrade moisture
content variation in South Africa and a monthly or quarterly division is recommended, if such data are available.

Each layer is assigned a layer coefficient, representing
the strength of the material. The value of the layer
coefficients increases with increasing material quality.
Typical ranges for the layer coefficients of the main AASHO Road Test
South African material groups are provided in Table 43, The AASHO Road Test is described in Chapter 1: 2,
and are based on the AASHTO 1993 design guide and “History of Pavement Design”. The test was an
local research. enormous effort to systematically quantify the
complex interaction between road deterioration,
traffic and composition of the pavement structure on
a closed loop test track with trucks.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 97