Arrester Bed Concept and Aggregate Characteristics

Low interparticle friction is needed so that the truck wheels sink deeper into the bed and as a result more energy is transferred. As the truck sinks into the bed, if the aggregate is of sufficiently low shear resistance, the axles will also drag on the upper surface further slowing the truck. The lower the interparticle friction, the more rapid the slowing of the truck.

It was recognized that in winter conditions, the contamination by ice control sand and water that might accumulate and freeze at the bottom of the bed would stop it functioning through the full depth. Theoretically, only a depth of 450 mm was required, although the bed was designed with a depth of about 600 mm with side slopes of 2:1 and a taper at the upper or entry end.

Aggregate having smooth rounded particles of substantially the same size with little or no sand or smaller or larger size particles is ideal. Another critical requirement is that the aggregate be resistant to both degradation by abrasion and freezing and thawing.

There are a number of characteristics that determine interparticle friction: sphericity, roundness, and surface texture. Gravel consisting of a high proportion of spherical particles will have a higher void space and lower internal friction thus giving a more efficient gravel arrester bed than one made with a low proportion of spherical particles.

In terms of roundness, there should be few or no fractured or crushed surfaces or sharp edges. Being rounded is not the same as being spherical. A cube has equal principle axis lengths and thus has a sphericity of 1, but such a particle is not going to have the desirable properties of roundness for use in a truck arrester bed.

Surface texture is determined by the grain size of the rocks and the degree of polish given by interparticle grinding. Grading is also important; poorly graded material preferably of a single sieve fraction is needed to reduce the stability of the material.