Rip rap calculator for Illinois

A ton of riprap will cover around 17 square feet. The number is accurate. Because armor rock is placed in layers of 12 inches or more, each ton covers only a small share of the area covered by landscape stone. This calculator estimates by layer depth, using 3,100 lb per cubic yard for rock commonly ranging from 4 to 12 inches and up.

How to measure an armored bank

1. Take the measurement down the sloped bank face, not across the level ground, then multiply it by the shoreline or channel length.
2. Use a layer 1.5 to 2 times the biggest rock dimension. For erosion-control work, 12 inches is the minimum depth.
3. Divide cubic feet by 27 for cubic yards, then multiply by 1.55 for tons.

Tons of coverage

Installation details for stable stone

• Put fabric or bedding below the rock: use non-woven geotextile, or install 3-4 inches of #4 stone. Rock laid directly over soil may hold for a season, then settle when the underlying bank erodes away.
• Install a toe key at the bottom. Set the first row below the waterline or expected scour depth, since erosion can undermine RipRap from its base.
• Prepare the bank at a 2:1 slope or flatter before placing stone. Faces that are steeper need engineering rather than extra rock.
• Use equipment or hand placement for the exposed top course, arranging stones so they lock together rather than sit where they landed.
• At culvert exits, extend the apron 4-6 pipe diameters downstream. That is the area where water scours the ground.

Example: 50 feet of pond bank

For 50 feet of shoreline with a 6-foot slope face, the area comes to 300 square feet. Using an 18-inch depth, calculate 300 x 1.5 = 450 cubic feet = 16.7 cubic yards. That is about 26 tons of riprap, or 27 including a small allowance. Use geotextile beneath it, or plan on about 4 tons of #4 stone for a rock bedding layer. Check the delivery path before you place the order. The final 20 feet near a pond can be difficult for a loaded truck, especially in slow-draining prairie clay, and moving stone a second time doubles the placement work.

Following the first major storm

When water levels fall, inspect the protected edge. At the Illinois River or Mississippi River, check for bare earth in the rock, exposed fabric, and stones pushed away near the bottom of the slope. Reset loose pieces, fill shallow areas with saved stone, and examine the outfall apron for erosion. Ten minutes, a couple of times a year, usually covers the routine check. Riprap keeps working with limited upkeep.

Stone size classes

Select stone size for the water conditions. Four to eight inch RipRap is used on pond banks, downspout outfalls and gentle streams. Use eight to twelve for lake shorelines with boat wake and quicker channels. Engineered projects such as highways and spillways have their own stone classes and layer depths, so follow the drawing instead of this guide. If two sizes seem possible, choose the larger one and use a deeper layer. More weight allows for more tolerance.

Riprap quantity, sizing and installation

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