Wall drainage, backfill, and geogrid

Drainage Aggregate, Perforated Pipe, and Geotextile Separation

Water behind a wall adds hydrostatic loading and can weaken soil or erode support. Water's unit weight is about 62.4 pounds per cubic foot; pressure increases by 62.4 pounds per square foot for each foot of water depth. At three feet depth, pressure is 187.2 psf at that point, not 62.4 pounds per cubic foot of pressure. Drainage is part of the design, not a substitute for checking groundwater and outlet feasibility.

Drainage aggregate and unit fill

Provide the design's drainage zone and clean compatible aggregate. Its thickness, height, gradation, and connection to outlets follow the wall system and site design. Fill hollow units as the manufacturer requires; not all units have the same cores or fill detail.

Keep soil fines from contaminating the drainage zone. A separator must be selected for filtration and flow with the actual soils, not only fabric weight or the woven/nonwoven name. Verify placement as the wall rises, since later correction may require dismantling the structure.

Drain pipe and outlet

Use the specified diameter, material, elevation, slope, and outlet arrangement. Two-row perforated pipe commonly places holes near the lower quadrants so collection begins near the drain bed; other products have different perforation patterns. Follow the product and design rather than require all holes to face one direction.

Provide an approved outlet that remains open and protected from erosion and pests as specified. A pipe with no effective lower outlet cannot drain the wall as intended. Surface runoff above the wall must also be managed so it does not continually saturate the backfill. Do not discharge onto a neighbor or road without authorization.

Filters and interfaces

Use the approved separator where native fines could enter drainage aggregate. The design may specify a fabric envelope, a graded filter, or another detail appropriate to the soil and water. Pipe socks are neither universally required nor universally banned.

Avoid folds, tears, and unapproved overlaps that obstruct the path or allow soil bypass. Keep filter placement coordinated with reinforcement connections and wall units. If a filter clogs, the system can retain water even though the pipe itself is clear, so diagnose the complete interface.

Retained Backfill Placement and Compaction Zones

Improper backfilling and compaction can displace units and reduce wall performance. Backfill material must be placed and compacted progressively as each block course is erected.

Lift Thickness and Soil Types

• Backfill must be placed in uniform horizontal lifts not exceeding 6 to 8 inches of loose thickness.
• Acceptable backfill soils include granular sands, gravels, or low-plasticity silty gravels. Heavy organic soils, expansive high-plasticity clays, and construction rubble must never be used in the reinforced zone.
• The project may specify 95% Standard Proctor maximum dry density (ASTM D698), as in common CMHA specifications; follow its actual density, moisture tolerance, and testing requirements.

Compaction near the facing

Use light equipment near the wall face according to the system's installation instructions and engineered exclusion distance. Heavy equipment can displace units or overload an incomplete wall. Keep fill lifts level, avoid pushing a large mound against the facing, and monitor alignment during compaction.

The permitted machine mass and distance depend on the wall system and design; a universal three-foot boundary and 250-pound limit are not Oregon law. Follow the specified density and moisture requirements while maintaining unit alignment. Stop and correct movement before additional courses conceal the defect.

Geosynthetic Reinforcement (Geogrid): Mechanics, Sizing, and Tensioning

When a retaining wall exceeds its allowable gravity height or supports external surcharges, geogrid provides the tensile strength that soil naturally lacks.

Reinforcement direction and connections

Geogrid products have specified tensile properties and directions. Uniaxial products emphasize one direction; biaxial products distribute strength differently. Read the actual manufacturer data and wall design. Roll direction does not automatically prove the required strength direction.

Lay the specified strength axis into the retained mass, maintain the required wall connection, and follow approved overlap, tensioning, and coverage details. Do not splice or truncate the load path without design approval. A grid oriented wrongly can lose the reinforcement function even though it visually covers the soil.

Reinforcement length and orientation

Geogrid length, vertical spacing, strength, and connection capacity come from the wall design. Do not size reinforcement by a universal $0.6H$ shortcut. Surcharges, slopes, water, soil strength, wall height, and the facing connection can change the requirement. A reinforcement zone must fit within property and utility constraints; grid cannot simply be cut short at a boundary.

Place the grid in its specified principal strength direction, normally extending into the retained soil, and remove slack according to the installation instructions. A roll's long direction is not automatically its strong direction. Keep overlapping and connection details consistent with the design. If a pipe or foundation interrupts the reinforcement, obtain a revised detail rather than cut an opening without review.

Geogrid Tensioning and Backfilling Protocol

1. Ensure the soil lift beneath the geogrid elevation is compacted to 95% Standard Proctor density and screeded completely flat, flush with the top of the receiving block course.
2. Lay the geogrid flat over the modular blocks, hooking it over the shear pins, rear lip, or lugs according to manufacturer instructions. Geogrid should extend forward to within 1 inch of the front face.
3. Pull the geogrid taut away from the wall face into the excavation zone to eliminate all slack, folds, or wrinkles.
4. Stake the back edge of the geogrid firmly into the compacted backfill using metal U-pins or mounds of aggregate to maintain continuous tension.
5. Spread backfill aggregate starting at the back of the wall blocks and pushing backward toward the cut hillside. Never push soil toward the wall face, as this creates ripples and slack in the reinforcement.
6. Provide the specified compacted cover before equipment crosses the grid. Six inches is a common minimum in installation guidance; the wall system, grid, equipment, and approved design control. Do not turn or brake equipment in a way that displaces or damages reinforcement.