Guidelines for Construction of Revetment Test Plots




Revetment or rip-rap is rock or other material used to armor bridge abutments, pilings, and rivers or shorelines against scour and water erosion. The Iowa DOT uses five Classes of Revetment based on the size of the aggregate. See the table below for nominal top size. The Engineer may approve revetment containing material larger than the nominal top size. For this product, individual beds are approved at each source based on quality and bed thickness.


Revetment Class

Nominal top size

Class A

400 pounds

Class B

650 pounds

Class C

450 pounds

Class D and Class E

250 pounds


Commonly, Class E revetment is used to protect structures, such as bridges, from the erosive destruction of river water. The design expectation is for Class E revetment to last the life of the structure or about 60 years without failure. Class D revetment is the same weight gradation as Class E, but is intended to be temporary erosion control and only last several years before failure. Class A, B, and C revetment are larger weight gradations, and may be required on larger rivers or rivers with very high-water velocities. For revetment specifications and approval requirements see Section 4130 of the Iowa DOT Standard Specifications and the body of this IM.


The guidelines below are intended to assist Aggregate Producers and District personnel in the site selection and construction of revetment test plots. The District Materials Engineer may require revetment test plots before an Aggregate Producer can secure an approval to produce a specific class of revetment from an aggregate source or specific beds. Sources are considered suitable for all classes of revetment when no more than 25% of the stones crack or split in service after an exposure time of 24 months in actual stream bank rip-rap service or in a natural weathering test plot which simulates an actual stream bank.




1. Choose a site that will provide a water-saturated condition for a portion of the revetment stones.

In choosing a site for a revetment test plot it is important that a portion of the revetment stones are in a water-saturated or frequently-saturated environment. It is also important that the test plots are not placed in locations where approved revetment is required, for example, protecting a structure such as a bridge. Good locations include the edge of ponds (e.g., settling ponds) or along streams or riverbanks. Figures 1 and 2 show good examples of test plots. Figure 3 shows a poor and unacceptable location for test plot placement. The revetment in the test plot in Figure 1 is being used as erosion control (near a box culvert) along a stream that is at the edge of the quarry property. Figure 2 shows a test plot protecting a drive over a stream that cuts through land leased for quarry development. With the agreement of the District Materials Engineer, a non-critical construction site may be used, such as a County revetment project.  In choosing the plot location, it is important for the Aggregate Producer to work with the District Materials Engineer and Geology Section to establish mutual agreement regarding the suitability of the location.


2. Revetment stones must represent all the beds in the requested approval, must be processed to remove minus 3-inch material, and should match the Class E revetment gradation.

It is important that stones used in the revetment test plot are representative of the beds in the requested approval. Approvals are usually not considered if the ledge contains beds with excessive shale or clay or beds containing breccia. Beds within the ledge must also meet dimensional requirements (50% minimum of the beds thicker than 5 inches) to assure the revetment stone will remain in place during flooding events. Class E gradation is probably the most common revetment used in Iowa. Class E is processed to remove the minus 3-inch material and has a nominal top size of 250 pounds. Figures 4 through 6 will give an indication of stone size and weight. The scale bar in the photographs has 12-inch white and red stripes. Removal of the minus 3-inch material before construction of the test plot also removes doubt about whether the small stones occurred due to revetment failure.

3. Make a test plot that uses a statistically valid number of revetment stones placed in a single or “close-to-single” layer of stones and place signage (or document) placement dates and bed numbers.

Figures 1 and 2 show good examples of test plot placement. Figure 3 is an example of a poor and unacceptable test plot. Test plots should use a “statistically significant” number of stones. This may be judged as: several truck loads, or greater than 50 tons, or enough material to cover an area greater than 750 square feet. Stones should be carefully placed, typically using an excavator. If a Producer is constructing multiple test plots (e.g., to test different bed combinations) signs may be placed at the test plots to indicate placement dates and bed numbers. If there is a chance the plots may be disturbed (e.g., unintended end loader mistakes) fencing around the test plot may be beneficial. 

4. After construction of the test plot, contact the District Materials Engineer to document the test plot placement and aggregate condition.

The District Materials Engineer and Geology Section will visit the test site, make additional construction suggestions (if necessary), and document the condition of the revetment stones, usually by photography.

5. Contact the District Materials Engineer after 24 months for aggregate evaluation.

After two years, the Producer should contact the District Materials Engineer for evaluation of the test plot. Typically, evaluation of the test plot will involve Geology Section and will include review of the photographs taken of the test plot two years before. For approval, the stones exposed to a water-saturated environment should have less than 25% failure. Any stone with a fracture is considered failed. Stones will also be examined to determine if secondary bedding fractures have developed.


Figure 1.  Example of a revetment test plot along a stream on quarry property.



Figure 2. Revetment test plot protecting a drive on quarry property.



Figure 3. Example of a test plot that is not using a statistically significant amount of revetment stones and is not located in a water-saturated environment.



Figure 4. This revetment stone is 250 lbs, the nominal top size for Class E revetment.  The range pole is in 12-inch increments.


Figure 5. A 500 lb revetment stone.



Figure 5. Comparison of 100, 250, 300, and 1400 lb revetment stones.