Roadside Management Toolbox

Rock Blanket

What is This Treatment?

Rock blanket is 4 - 6 inch rock (typically river cobble) joined together with mortar to form a continuous and uniform hard surface.

When to use This Design Concept?

Best use of rock blanket (mortared cobble) is in high visibility areas such as interchanges, ramp loops, median strips and road edge treatments on lower speed roadways. Cobble can also be used as an accent for other hard surface applications, such as concrete or stamped asphalt. Cobble has a variety of natural colors that blend well with the natural and built environments and can be used in urban and non-urban areas. A large amount of hand placement makes rock blanket more expensive than other hard surface treatments.

Benefits

Cobble is a common treatment in highway landscaping, commercial areas and local streetscapes allowing for consistent corridor aesthetics
Cobbles are readily available in most areas of the state and have natural color tones that are consistent and easy to match if repairs are required.
When cobbles 4 inche and larger are used they have a good load bearing capacity for maintenance operations and errant vehicles.

Limitations

Repairing cracked mortar or loose cobbles is more difficult than concrete slab removal and replacement.
Cobble texture must be reviewed and approved by District Traffic Engineering if placement is within recovery zone limits.
Installation of cobble and mortar is often by hand and may be labor intensive.
Construction inspection is required to ensure that the majority of the visible surface is cobble rather than mortar.
Do not place Rock Blanket adjacent to guardrail posts without first talking to HQ Office of Traffic Safety Hardware.

Specifications and Details

Cost

$85 - $120/yd² at minimum thickness with no subbase or reinforcement
BEES Item(s) - 200114