13.4 Riprap and Gabion Inspection

Structural Slope Stabilization and Velocity Dissipation Controls

When hydraulic shear stresses in steep channels, culvert outfalls, shorelines, or cut-and-fill slopes exceed the permissible threshold of vegetative linings and rolled erosion control products ($> 8\text{ to }10\text{ ft/s}$), engineers specify heavy structural armoring and energy-dissipation systems. Under SAOP 11: Management Practices, a CESSWI must know the design principles, material specifications, installation criteria, and failure mechanisms for five structural runoff control BMPs in the CESSWI General Principles Review Manual: Rip-Rap (12.5.8), Gabions (12.5.4), Retaining Walls (12.5.7), Outlet Velocity Protection (12.5.5), and Wet Ponds (12.5.10).

1. Rip-Rap (12.5.8)

Rip-Rap (12.5.8) is a permanent, flexible ground-armor blanket of well-graded, hard, dense, durable, angular quarry stone placed on steep channel banks, high-velocity swales, bridge piers and abutments, shorelines, embankment toes, and spillway outlets to protect the underlying soil from tractive shear stress, wave action, and turbulent eddies.

Riprap Material, Filter, and Placement Inspection

Part B exam questions and field photographs repeatedly test three non-negotiable engineering requirements for rip-rap installations:

1. Angular and Well-Graded Stone Centered on $D_{50}$ (Never Rounded River Cobbles!):

   • Angularity: Rip-rap stone must be crushed quarry rock with sharp, fractured angular faces that mechanically interlock with adjacent stones to resist rolling and sliding. Smooth, rounded river rock or field cobbles must NEVER be used for rip-rap (except in specialized architectural dry-stream aesthetic features outside high-velocity flow), because rounded stones have minimal friction angle and roll downstream like marbles during high-velocity storm flows.
   • Well-Graded Mix ($D_{50}$): Rip-rap sizing is specified by its median stone diameter $D_{50}$—the sieve size for which $50\%$ of the rock mass by weight is smaller and $50\%$ is larger (typically ranging from $D_{50} = 6\text{ in.}$ Class I rip-rap up to $D_{50} = 18\text{ to }24\text{ in.}$ heavy rip-rap). The rock mixture must be well-graded across a range of stone sizes (from $D_{15}$ chinking stones up to $D_{100}$ maximum stones). If a contractor installs uniformly graded (single-sized) boulders, huge open voids remain between the rocks, allowing turbulent water to penetrate and scour the subgrade. In a well-graded mix, smaller stones chink and lock the voids between larger stones.

2. Mandatory Underlayment Filter (Non-Woven Geotextile or Granular Filter Blanket):

   • Rip-rap must NEVER be placed directly onto bare fine soil, silt, or clay!
   • When water flows rapidly over rough rip-rap, pressure differentials and turbulent eddies reach down through the rock interstices. Without an underlying non-woven geotextile filter fabric (or an engineered graded granular gravel filter bedding layer $4\text{ to }8\text{ inches}$ thick), water sucks fine soil particles out from beneath the rock blanket. This subsurface erosion process—known as "piping" or "winnowing" failure—creates cavities under the armor layer, causing the heavy rip-rap stones to sink into the mud and collapse.

3. Toe Key Trench and Proper Placement Thickness:

   • Toe Key Trench: Excavate a toe key trench at the base of the bank slope (extending below the anticipated channel bed scour depth) and thicken the rock at the toe so the rip-rap blanket cannot slide downslope under gravity or be undermined by channel bed degradation.
   • Minimum Blanket Thickness: Place the rip-rap layer to a uniform thickness of at least $1.5 \times D_{50}$ (or equal to the maximum stone diameter $D_{100}$, typically $12\text{ to }24+\text{ inches}$ thick) on side slopes no steeper than $2\text{H}:1\text{V}$.
   • Placement Method: Place stone carefully using an excavator bucket, orange-peel grapple, or clamshell near ground level. Never end-dump rip-rap from high up out of a dump truck bed down a long slope—high-drop dumping punctures and tears the underlying geotextile fabric and causes size segregation (heavy boulders roll all the way to the bottom while small stones stay at the top).

2. Gabions (12.5.4)

Gabions (12.5.4) are modular, compartmentalized rectangular wire mesh containers tightly packed with clean, durable stone to form flexible, permeable, monolithic structural blocks for steep channel linings, bridge abutments, check dams, weirs, and gravity earth-retaining walls.

Box Gabions vs. Gabion Mattresses (Reno Mattresses)

• Box Gabions: Rectangular wire baskets typically measuring $3\text{ ft}$ high $\times 3\text{ ft}$ wide $\times 6\text{ to }12\text{ ft}$ long, stacked in stepped courses like massive building blocks to form gravity retaining walls, drop structures, and steep bank buttresses.
• Gabion Mattresses (Reno Mattresses): Thin, wide wire mesh units ($6\text{ to }18\text{ inches}$ thick by $6\text{ ft}$ wide by $9\text{ to }12\text{ ft}$ long) laid flat across high-velocity channel beds, chute spillways, and streambanks where loose rip-rap would be swept away.

Wire Mesh and Stone Fill Specifications

• Double-Twisted Hexagonal Wire Mesh: Standard gabions are fabricated from heavy double-twisted hexagonal galvanized steel wire mesh (additionally coated with extruded PVC or Galfan [zinc-aluminum alloy] when installed in corrosive, acidic, brackish, or coastal saltwater environments). Double-twisted hexagonal mesh is engineered so that even if a single wire strand is cut or corroded, the hexagonal weave will not unravel.
• Stone Size Relative to Mesh Aperture: Gabions must be filled with hard, dense, durable, angular-to-subangular $4\text{- to }8\text{-inch}$ stone (typically $3\text{ to }6\text{ inches}$ for thin Reno mattresses). Crucial Rule: Every stone's smallest dimension must be larger than the wire mesh opening (typically $1.5\times\text{ to }2\times$ the mesh aperture of $3.25 \times 4.5\text{ inches}$) so rocks cannot fall out through the wire mesh and internal void space is kept below $30\%$.

Installation Details and Inspection Checkpoints for Gabions

1. Geotextile Filter Fabric Backing/Underlayment: Install non-woven geotextile filter fabric behind all vertical soil-contact faces and beneath the base of the gabion structure so backfill soil cannot wash through the porous stone baskets.
2. Perimeter Edge Lacing: Unfold empty baskets on a level foundation and wire adjacent baskets together along all four perimeter edges (top, bottom, and vertical corners) using continuous alternating single- and double-loop lacing wire every $4\text{ to }6\text{ inches}$ (or approved high-tensile pneumatic C-rings) so the entire installation acts as a single monolithic structure.
3. Internal Diaphragms Every $3\text{ Feet}$ & Cross-Tie Stiffeners (Preventing "Pillowing"):
   • Each gabion basket is divided into $3\text{-foot}$-long internal cells by vertical wire diaphragms to prevent stone from shifting inside the basket.
   • Box gabions must be filled in three $12\text{-inch}$ vertical lifts. After placing the first $12\text{-inch}$ lift and again after the second $12\text{-inch}$ lift, workers must install horizontal internal cross-connecting tie wires (stiffeners / windlass ties) linking the front face to the back face.
   • #1 Visual Defect on Part B Photos: If workers dump $3\text{ feet}$ of rock into a gabion all at once without installing internal cross-tie stiffeners, the lateral weight of the rock pushes the front wire mesh outward into a rounded belly ("bulging" or "pillowing"), which overstresses the wire and causes structural failure. Overfill baskets by $1\text{ to }2\text{ inches}$ before stretching and lacing the lid shut so the lid remains under taut tension.
4. Stepped Batter Angle: Multi-course gabion walls must be stepped slightly backward into the retained slope (typically a $6^\circ$ batter, or $1\text{H}:10\text{V}$) to lower the center of gravity and resist overturning.