Trafficked Surfaces That Stay Green or Loose

In a permeable pavement the wearing course is made of rigid units that hold their own shape and pass load to each other across their joints, with nothing confining them except the perimeter. In a confined surface the wearing course is loose or living material with no shape of its own, so confinement is distributed through the entire area rather than concentrated at its edge.

That is the whole difference, and the entries state a viewing test for it: from above, one shows a pattern of units and joints, the other shows fill with a grid rim occasionally visible within it. Unbound paving sits alongside both as the answer where interlock between units, rather than confinement of a fill, is what holds the surface together.

Confinement is set to work by the vertical load arriving on the fill. Fill loaded from above tries to spread sideways, the cell wall stops it, and the reaction develops friction between the fill and the wall so that the filled cell becomes stiffer than the same material lying loose.

That stiffness exists only while the cell is full. Fill lost from a cell leaves a hollow, and the wheel that finds the hollow loads the cell wall directly instead of loading it through the material it was meant to restrain. Fill is consumable in this assembly, and topping up is a routine operation rather than a repair.

The entry is direct about this: a confinement grid is not a structural layer and cannot compensate for a foundation that would rut without it. The grid is thin, it follows the shape beneath it, and it has little stiffness in its own plane, so ground that would rut without a grid will rut with one, more neatly and in the same places.

That is why the junction with the foundation is the one that decides whether the surface behaves. The grid cannot bridge, cannot stiffen the layer beneath and cannot correct its shape, so the foundation has to be finished as though it were the surface, because in every respect that matters it is.

The same grid serves two surfaces that have little else in common. Filled with angular stone it reads as loose gravel and behaves as a hard, free-draining surface. Filled with a growing medium and grassed, it becomes a reinforced lawn in which the cells spread wheel loading and limit the compaction reaching the root zone beneath.

The construction beneath is similar; what is being asked of it is not. The published misunderstanding is that a grassed grid surface is still grass, so it remains subject to the light, drainage and rest that any lawn under traffic needs, and the grass has to survive being driven over. Whether it will is a use-pattern question rather than a product one.

These constructions admit water rather than shedding it, which means their behaviour depends on the foundation continuing to accept what the surface admits. A construction that admits water at the surface and then meets a foundation unable to take it holds that water in the cells, and saturated fill under traffic is exactly the condition in which fines washed down from the surface begin filling the voids.

Where the fill is living, that same water is what the grass needs, so the balance between draining and retaining is what separates a grass surface surviving traffic from bare compacted soil in the wheel tracks. A confined surface holding water is a surface whose voids have filled with fine material.

Every cell is restrained by its neighbours, the outer row has neighbours on one side only, and everything inside is pressing outward against it. A restraint that is discontinuous, or that finishes lower than the fill, lets the outer cells move, and once the outer row has lost its fill the row behind it becomes the outer row.

Boundaries are also where material is exchanged in both directions. Fill escapes into planting and soil is drawn back into the cells, and soil arriving in a stone-filled cell is what turns a free-draining surface into a slowly closing one. Where the field meets hard paving, a moving surface meets a fixed one and the joint has to restrain both.

The published design question asks what traffic will cross the surface and how much of it involves turning, which is what concentrates wear on a confined field. An access route that is driven straight along and a parking area where vehicles manoeuvre are different problems wearing the same appearance.

The other question worth answering in advance is who tops up the fill, and whether the appearance of a surface that is partly depleted has been discussed with whoever will live with it. Loose surfaces with a grid are often assumed to be maintenance free, when in reality the fill is consumed and has to be replaced.