813 - Slope Protection

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Slope Protection



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Precast Slope Paving

To obtain satisfactory results with precast slope blocks, a uniform and properly compacted sand grade is necessary. The front slopes are most easily compacted when this fill is placed before erection of the structural steel or prestressed beams. If the compaction operations are left until after the beams are in place, special efforts and equipment are often necessary to obtain proper density. Backfill placed in the header trenches will require compaction and density tests before placing blocks.

Toe headers should be analyzed to ensure that the area will be properly drained. When toe headers at grade separation are not placed on the front of the columns, there is a possibility of trapping surface water between columns and keeping the entire shoulder saturated for long periods. A positive system to maintain correct grades should be used on header forms. This may be accomplished by use of a cant strip or other suitable means, but final grades should not be left to the ability of the finisher to hand float.

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Concrete Slope Paving and Headers

Forms must extend the full depth of the concrete for slope paving. Slope paving headers may be cast to against existing soil below the slope paving subgrade if the soil can be trimmed neat. The form material must be strong enough and staked well enough to resist deflection during placement of concrete. The concrete must be thoroughly compacted along the forms and struck off to the required finished grade and cross section. The surface is finished with a wood float and all edges are finished with a 1/4 inch (6 mm) radius. Curing compound is applied at a rate of 1 gallon per 25 square yards (1 liter per 5 m2).

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Plain riprap is most often called for where wash or scouring action may be expected and the availability of stones to drop in and plug voids is desirable. Geotextile blankets may be used under the riprap to minimize the scour action. Grouted riprap, slope blocks or paved slopes are best suited to areas not subject to wash action, but perhaps subject to vandalism such as stone rolling, etc.

Heavy riprap is commonly used below spillways and outlet headwalls as energy dissipating structures where high velocity turbulent flow is reduced to non-eroding velocities to prevent damage to downstream channels. Stone should be sound, durable rock or sound pieces of broken concrete without exposed steel reinforcement.

Material considered should be pounded with a hammer to check its soundness. If it does not ring under a hammer blow and breaks up easily, it is probably unsatisfactory. Minimum dimensions are taken from Section 916 of the Standard Specifications for Construction.


Natural stone or broken concrete - least dimension must be 8 inches (1200 mm). Plain precast must be 6 inches (150 mm) thick and 15 square feet (1.5 m2) surface area.


Natural stone or broken concrete - length or width must be 16 inches (400 mm). Ratio of 3 to 1 for maximum to minimum dimension. Heavy precast must be 16 inches (400 mm) thick and 20 square feet (1.75 m2) surface area.


Dimensions same as for plain riprap. Spaces filled with Type R-3 mortar.

An adequate record of the thickness of the material used will be maintained in the project records as supporting data and these measurements submitted with the balancing recommendation.

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