Page 110 - SAPEM-Chapter-10-2nd-edition-2014

South African Pavement Engineering Manual
Chapter 10: Pavement Design

7.5 Deflection Bowl Parameter Analysis

Although not listed as an independent rehabilitation design method in its own right in TRH12, the interpretation of
Falling Weight Deflectometer (FWD) deflection results has evolved to the point where it is quite a widely used
method for evaluating pavements and for crude estimates of the remaining life. The method is described in more
detail in TRH12, Maree (1990) and Horak (2008). Much of the discussion in this section is taken from Horak (2008).

When a flexible pavement deflects under the load of a heavy vehicle, the influence of the load extends to about 1 to
2 metres in three dimensions. The deflection area is typically a circular deflected indentation known as a deflection
bowl. See Section 3.5.1 for a brief discussion on deflection bowls and FWD measurements. Deflection bowls are
often used to backcalculate stiffness moduli for the pavement layers. To do this however, requires good knowledge
of the materials in the layers and the layer thicknesses. The deflection bowl can be used to identify weak areas in the
depth of a pavement structure and over the length of a uniform section, without detailed knowledge of the pavement
structure and without backcalculation.

Deflection bowl parameters have been developed for analysing the deflection bowl. These are the base layer index
(BLI), middle layer index (MLI) and lower layer index (LLI). The formulae for calculating these parameters are
shown in Table 44.

Table 44. Deflection Bowl Parameters
Parameter Formula
1
Base layer index (BLI) BLI = D 0 – D 300 (32)
2
Middle layer index (MLI) MLI = D 300 – D 600 (33)
3
Lower layer index (LLI) LLI = D 600 – D 900 (34)
= 2
Radius of curvature (RoC) RoC � (L) � (35)
2 D 0 (1−D 200 /D 0 )
4
where D 0 = Maximum (peak) deflection , measured under the load
D 300 = Deflection at 300 mm sensor
D 600 = Deflection at 600 mm sensor
= Deflection at 900 mm sensor
D 900
L = 200 mm for the FWD
Notes
1. Previously referred to as surface curvature index (SCI)
2. Previously referred to as base curvature index (BCI)
3. Previously referred to as base damage index (BDI)
4. Also known as Y-max.

A deflection bowl measured under a load can be divided into three zones, as reflected in Figure 44 (from Horak,
2008):
• Zone 1 is the closest to the load, and generally lies within 300 mm from the load. In this zone, the curvature of
the bowl is positive. This zone is mainly surface and base layers and correlates well with the base layer index
(BLI).
• Zone 2 is typically between 300 mm to 600 mm, although the exact positions depend on the pavement
structure. In this zone, the curvature switches from a positive to reverse curvature. This zone is mostly the
subbase layers, and correlates well with the middle layer index (MLI).
• Zone 3 lies furthest away from the load, and is generally from 600 mm to up to 2 000 mm from the load. The
curvature is reverse and the deflection eventually reduces to zero. The extent of the deflection bowl depends on
the pavement structure. Zone 3 is mostly selected and subgrade layers, and correlates well with the lower layer
index (LLI).

Condition classification criteria have been developed for a number of FWD deflection bowl parameters. Limiting
criteria, relating the cumulative number of E80s to a number of deflection bowl parameters are available. The
criteria given in TRH12 are shown in Table 45. Criteria for different behaviour states are given, including a crude
estimate of remaining life for pavements with deflection bowl parameters within those ranges. These correlations to
remaining life must be used with great care, as they can lead to an over simplification and inaccuracies.

Horak has also suggested criteria for assessing pavements in terms of sound, warning and severe, shown in
Table 46. By using these assessment criteria, deficiencies in the structural layers are identified. By assessing a
length of road, the possible cause of structural deficiencies can be deduced. An example is given in the stripmap in
Figure 29.

Section 7: Structural Capacity Estimation: Flexible Pavements
Page 99