Which factors affect shock absorption in play sand, rounded washed gravel, wood chips and bark mulch?

Shock absorption in play sand, rounded washed gravel, wood chips and bark mulch comes from the loose fill and the voids between the individual grains and chips, not from the construction of a tile. Grading, particle shape, fines, layer depth, sub-base, moisture and temperature therefore all determine how well the fill performs in service.

Grading and cleanliness are laid down in the standard. The table of ground surfacing types in BS EN 1176-1 gives 0.2 to 2 mm for washed play sand, 2 to 8 mm for gravel that is both rounded and washed, 5 to 30 mm for wood chips, and 20 to 80 mm for bark mulch made from shredded conifer bark. Wood chips must contain no engineered wood products, bark or leaves, and silty or clayey fines are not permitted. Fines from abrasion, soil or winter grit fill the voids and make the particles interlock, which significantly reduces shock absorption.

Layer depth is the second factor. The installed depth is 30 cm for a maximum free height of fall of up to 2 m and 40 cm for up to 3 m. A free height of fall of more than 3 m is not permitted for playground equipment. Each of these depths includes an allowance of 10 cm on top of the minimum layer depth of 20 or 30 cm respectively, because loose fill is displaced and carried away during play. Depressions still form in the impact areas, precisely where the depth is needed. A mark on the edging makes the specified fill level easy to check.

The sub-base and the weather have an effect of their own. Without a water-permeable sub-base, the fill becomes waterlogged from below. Without a separation layer, it mixes with the soil and roots grow into it. Wet sand compacts and absorbs impact less effectively. In prolonged rain, bark mulch turns slimy, decomposes fastest and loses volume. Fill that has frozen through no longer absorbs impact at all, and the item of play equipment then has to be taken out of use. Grit and de-icing salt must not be spread on impact attenuating playground surfacing. The different types of loose fill must not be mixed with one another, as this reduces shock absorption.

Maintenance follows from these effects. The depth is checked in the impact areas and topped up to the mark, and the fill is loosened and redistributed. Leaves, glass, animal faeces and decomposed bark mulch are removed, and contaminated sand is cleaned or replaced. Evidence of compliance for loose-fill surfacing does not rest on a test to BS EN 1177. It rests on the type of fill, the grading and the installed depth, and it holds only for as long as those values are maintained. Under BS EN 1176-7 the checks consist of routine visual inspections, operational inspections every one to three months and an annual main inspection carried out by competent persons.