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Sitework & Earthworkaka: rock armoringaka: erosion stoneaka: slope protection

Riprap

In Plain English

Large rocks or boulders placed to protect a slope or waterway bank from erosion.

Definition

A layer of large, loosely placed angular rock or stone used to protect soil surfaces from erosion caused by flowing water, wave action, or surface runoff. Riprap is commonly used at culvert outlets, stream banks, channel slopes, and at the base of retaining walls. Stone size is selected based on the velocity of water flow to prevent displacement.

Why It Matters in Bidding

Riprap is typically bid by the ton or cubic yard, and the gradation (stone size class) called out in the plans drives both material cost and the haul/placement labor an estimator must carry. Underestimating quantities or pricing the wrong stone class against the spec is a common takeoff miss because riprap often requires an underlying filter fabric or bedding stone that must be quantified separately.

Example

An estimator pricing a highway culvert outlet measures the apron dimensions, applies the specified stone gradation and an 18-inch placement thickness, and adds 25 tons of riprap plus geotextile filter fabric to the earthwork line item.

Related Terms

Erosion ControlGravelGeotextileSwale

Frequently Asked Questions

It depends on the spec. Most DOT and public projects pay by the ton based on weighed truck tickets, while some private jobs pay by the cubic yard in place or by square yard of covered surface. Always read the measurement-and-payment section so your unit price matches how the owner will verify quantities.
Usually yes. Most specs require geotextile filter fabric or a granular bedding layer beneath the stone to prevent soil migration and undercutting. These are separate pay items or incidental costs, so review the detail carefully; omitting fabric can leave a gap between your bid and the actual scope.
Larger gradation classes cost more per ton because they require quarrying, sorting, and heavier placement equipment. Hand-placed or grouted riprap carries far higher labor than dumped stone. Match the exact class and placement method in the plans, since substituting a smaller or cheaper stone makes a bid non-responsive.

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