Cellular Confinement Surfacing System

Loose gravel does not stay where it is put. Under a turning wheel or a dragging foot it migrates, and an even surface becomes ruts along the wheel lines, a ridge between them and a drift of stone at the edges. A cellular grid changes that by dividing the area into small closed compartments, so material that would otherwise travel across the whole surface can move only within the cell holding it.

The nearest sibling is the permeable paving system, and the difference lies in what the wearing course is made of. Permeable paving uses rigid units that hold their own shape and pass load to each other across their joints, with nothing confining them except the perimeter. Here 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. From above, one shows a pattern of units and joints; the other shows fill with a grid rim occasionally visible within it.

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 are there to spread wheel loading and limit the compaction reaching the root zone beneath, and the grass has to survive being driven over. The construction beneath is similar; what is being asked of it is not.

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.

The grid depends entirely on what it sits on. It is thin, it follows the shape beneath it, and an uneven or soft foundation shows through as an uneven surface however carefully the cells were filled. It also has little stiffness in its own plane, which is why it is no substitute for a foundation: ground that would rut without a grid will rut with one, more neatly and in the same places.

Water passing through the fill has to keep moving. A construction that admits water at the surface and then meets a foundation unable to accept 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, the same water is what the grass needs, so the balance between draining and retaining is what separates a grass surface that survives being driven over from bare compacted soil in the wheel tracks.

The perimeter is where a confined field holds or unravels. 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.