Weighs 25 to 150 lbs per rock; standard for drainage aprons.
Depth must equal or exceed maximum stone caliper to prevent scour.
Supplier-reported loose bulk density (2,800 lbs/yd³). Note: ASTM C29 standard applies to aggregates up to 5 inches.
Mandatory geotextile prevents soil migration through rock voids.
Engineering & Permitting Notice
Culvert outfalls, stream-bank armoring, and shoreline revetment work frequently alter natural hydrology and are governed by local conservation districts, state Departments of Transportation (DOT), and the U.S. Army Corps of Engineers. Always verify local riprap gradation classifications and obtain required environmental permits before commencing heavy equipment placement.
Hydraulic Principles of Riprap Slope Protection
Riprap (Rock Armor) is composed of heavy, angular quarry-blasted stones placed along shorelines, drainage channels, culvert outfalls, and steep highway slopes to protect soil against hydraulic scour and wave erosion.
Unlike smooth decorative rock, riprap relies on individual stone mass (typically 25 to 150 pounds per piece for Class 1, and 150 to 500 pounds for Class 2) and angular particle interlock to resist hydraulic shear stresses generated by fast-flowing stormwater.
Volume (Cubic Yards) = [Length (ft) × Width (ft) × Depth (in)] / 324Scale Tonnage = Volume (yd³) × 1.40 Tons/yd³Critical Rule: Thickness must be at least 1.0× the maximum stone size (12 inches for Class 1).Installation Protocol for Riprap Swales and Aprons
- Excavation: Over-excavate the channel by 14 inches to accommodate 2 inches of bedding aggregate and 12 inches of armor stone without restricting channel capacity.
- High-Strength Geotextile: Lay a heavy-duty 6 to 8 oz non-woven needle-punched polypropylene drainage fabric. Pin with 12-inch steel spikes and washers.
- Bedding Layer (Optional but Recommended): Spread a 2-inch cushion of #57 crushed stone over the fabric. This prevents heavy riprap stones from puncturing the geotextile when dropped into place.
- Stone Placement: Place riprap starting from the bottom of the slope and work upward to create an interlocking mass. Never dump stones from heights exceeding 3 feet onto bare fabric.
Frequently Asked Questions About Riprap
How many tons of riprap do I need for a 15x20 foot culvert outfall apron?
A 15x20 foot apron equals 300 square feet. Per civil drainage standards, Class 1 riprap (6-12 inch) must be installed at a minimum depth equal to the largest stone caliper (12 inches). At 12 inches depth, you need 11.11 cubic yards. At 1.40 tons per cubic yard plus a 10 percent buffer, order approximately 17.1 tons of riprap.
What is the minimum thickness for riprap installation?
Civil engineering standards specify that the minimum thickness of a riprap layer must be at least 1.5 times the average stone diameter (D50) or 1.0 times the maximum stone diameter. For Class 1 riprap (6 to 12 inches), the absolute minimum installed layer thickness is 12 inches.
Does riprap require geotextile fabric underneath?
Yes. A heavy non-woven 6 to 8 oz needle-punched geotextile fabric is mandatory under riprap. Without fabric, high-velocity storm runoff will erode the fine soil particles between the stones (piping), causing the heavy riprap to sink into the mud and destabilize the bank.
What is the difference between Class 1 and Class 2 riprap?
Class 1 riprap consists of stones ranging from 6 to 12 inches (weighing 25 to 150 lbs each) used for residential culverts and ditch linings. Class 2 riprap consists of 12 to 18 inch stones (weighing 150 to 500 lbs each) designed for high-energy riverbanks, highway bridge abutments, and lake shorelines.