Surfaces with a high degree of firmness and stability are critical for long distance trails so users may expend a minimum amount of energy over a given distance.  A high degree of stability would be desirable for areas with multi-directional traffic.

The following test methodology is based on a preliminary test procedure for the measurement of surface firmness and stability which is one of the ways that firmness and stability can be measured.  For more information, consult the “Accessible Exterior Surfaces Technical Report” available through the Access Board. 

The recommended test equipment for determining firmness and stability on outdoor surfaces is the rotational penetrometer, a device consisting of three main components:  penetrator, frame, and reference base.  The penetrator consists of an 8 by 1  inch (20 cm by 3 cm) pneumatic caster and a means to press the caster into the surface with a known force.  The frame is an attachment to the reference base that provides a means for allowing the penetrator to move freely, perpendicular to the reference base.  The reference base is a flat, rigid, surface used to position and anchor the testing equipment relative to the test surface.  It has an area through which the penetrator can pass and rotate freely without hindering the movement of the surface material being tested or interfering with the test results.  The reference base may also provide a platform for the device operator during testing.  The rotational penetrometer is instrumented with a method to measure the amount of vertical displacement of the penetrator into the test surface.

To test surface firmness and stability, the rotational penetrometer is placed on the surface to be tested.  A person stands on the reference base of the rotational penetrometer to stabilize its position during testing.  The penetrator is lowered onto the test surface and an initial vertical displacement measurement is taken.  A load of 44 + 1 pound (20 + 0.5 kg) is applied to the penetrator and then a second measurement of the amount of vertical displacement is completed.  Then, with the load still applied, the penetrator caster is rotated through four 90 degree rotations about an axis perpendicular to the surface, alternating the direction of rotation (clockwise, counter-clockwise) after each 90 degree rotation. The final amount of vertical displacement is then measured.  This test procedure is repeated on the same surface in a different test area until a total of five trials have been completed.