15 March 2001
DISTRESS 56, SWELL
18.1. Description. Swell is characterized by an upward bulge in the pavement's surface. A
swell may occur sharply over a small area or as a longer, gradual wave. Either type of swell can
be accompanied by surface cracking. A swell is usually caused by frost action in the subgrade or
by swelling soil, but a small swell can also occur on the surface of an asphalt overlay (over PCC)
as a result of a blow-up in the PCC slab. Deduct curves for swell are shown in Figure 18.1.
18.2. Severity Levels.
18.2.1. L. Swell is barely visible and has a minor effect on the pavement's ride quality as
determined at the normal aircraft speed for the pavement section under consideration.
(Low-severity swells may not always be observable, but their existence can be confirmed by
driving a vehicle over the section at the normal aircraft speed. An upward acceleration will occur
if the swell is present) (Figure 18.2.).
18.2.2. M. Swell can be observed without difficulty and has a significant effect on the
pavement's ride quality as determined at the normal aircraft speed for the pavement section
under consideration (Figure 18.3.).
18.2.3. H. Swell can be readily observed and severely affects the pavement's ride quality at the
normal aircraft speed for the pavement section under consideration (Figure 18.4. and
18.3. How to Measure. The surface area of the swell is measured in square feet (square
meters). The severity rating should consider the type of pavement section (i.e., runway, taxiway,
or apron). For example, a swell of sufficient magnitude to cause considerable roughness on a