DHARM434 GEOTECHNICAL ENGINEERING
front is the common pattern. The compaction pressure are relatively low because of a large
contact area. This type appears to be more suitable for compacting granular base courses and
paving mixtures for highway and airfield work rather than for compacting earth fill. The rela-
tively smooth surface obtained acts as a sort of a ‘seal’ at the end of a day’s work and drains off
rain water very well. The roller is self-propelled by a diesel engine and has a weight distribu-
tion that can be altered by the addition of ballast to the rolls. The common weight is 80 kN to
100 kN (8 to 10 t), although the range may be as much as 10 kN to 200 kN (1 to 20 t). The
pressure may be of the order of 300 N (30 kg) per lineal cm of the width of rear rolls. The
number of passes varies with the desired compaction; usually eight passes may be adequate to
achieve the equivalent of standard Proctor compaction.
(b) Pneumatic-tyred rollers: This type compacts primarily by kneading action. The usual
form is a box or container—mounted on two axles to which pneumatic-tyred wheels are fitted;
the front axle will have one wheel less than the rear and the wheels are mounted in a stag-
gered fashion so that the entire width between the extreme wheels is covered. The weight
supplied by earth ballast or other material placed in the container may range from 120 kN (12 t)
to 450 kN (45 t), although an exceptionally heavy capacity of 2000 kN (200 t) may be occasion-
ally used. Some equipment is provided with a “Wobble-wheel” effect, a design in which a slightly
weaving path is tracked by the travelling wheels; this facilitates the exertion of a steady pres-
sure on uneven ground, which is very useful in the initial stages of a fill.
The weight of the roller as well as the contact pressure is an important parameter for
the performance; the latter may be varied from 0.20 to 1 N/mm^2 (2 to 10 kg/cm^2 ) through the
adjustment of air pressure in the tyres. Although this type has originated as a towed unit, self-
propelled units are also available. The number of passes required is similar to that with smooth
wheeled-rollers.
This type is suitable for compacting most types of soil and has particular advantages
with wet cohesive materials.
(c) Sheepsfoot rollers: This type of roller consists of a hollow steel drum provided with
projecting studs or feet; the compaction is achieved by a combination of tamping and knead-
ing. The drum can be filled with water or sand to provide and control the dead weight. As
rolling is done, most of the roller weight is imposed through the projecting feet. (Fig. 12.7):
Longitudinal section End view
Fig. 12.7 Sheepsfoot Roller
It is generally used as a towed assembly with the drums mounted either singly or in
pairs; self-propelled units are also available.