7.3 Wall System Selection, Fences, Screens & Gates
Key Takeaways
- Wall system selection is driven by retained height, available space behind the wall for geogrid or a footing, anticipated settlement, and groundwater, not by appearance alone.
- Fence and gate posts must be embedded below the local frost depth and set in concrete sized for lateral load, because frost jacking and post rotation are the dominant fence failure modes.
- Gate leaves require a swing radius and a level landing on both sides; an accessible gate needs the same maneuvering clearance as an accessible door.
- Screen walls are opaque barriers that block sight and some sound, while fences may be transparent; specifying a fence where a screen is required is a common program failure.
- Chain link, ornamental steel, wood, and masonry differ principally in maintenance interval and lifespan, so enclosure selection should be evaluated on life-cycle cost rather than installed cost.
Last updated: September 2026
1. Retaining Wall Systems: Comprehensive Comparison
| Wall System | Practical Height Limits | Structural Resistance Action | Drainage Characteristics | Differential Settlement Tolerance | Relative Cost Index |
|---|---|---|---|---|---|
| Stone Gravity Wall | Up to 4–5 ft | Mass dead weight only | Moderate; requires gravel backfill and weep holes | Low; rigid stone joints crack if subgrade shifts | High (labor-intensive masonry) |
| Cantilevered RC Wall | 5 to 25+ ft | Reinforced concrete stem in bending; soil weight on heel | Low permeability; strictly requires aggregate backfill, weep holes, and drain pipe | Extremely low; requires rigid, uniform foundation bearing | High (forming, rebar, engineering) |
| Segmental Wall (SRW) | 3–4 ft unreinforced; up to 30+ ft with geogrid | Coherent gravity mass of geogrid-reinforced soil matrix | High; dry-stacked joints allow minor weep; requires gravel backfill zone | Moderate to high; dry-stacked blocks flex with minor ground settlement | Moderate (economical modular installation) |
| Gabion Wall | 3 to 20+ ft | Wire-basket aggregate mass dead weight | Extreme; 100% permeable free-draining stone baskets | Exceptionally high; flexible wire mesh deforms without structural collapse | Low to Moderate (uses local stone fill) |
| Crib Wall | 5 to 20 ft | Internal cellular gravity mass of compacted gravel | High; open-faced cells promote natural drainage | Moderate; modular framework accommodates minor settling | Moderate |
2. Fences, Screen Walls, and Gates
Structural Anchoring & Wind Load Mechanics
Vertical site enclosures must resist severe lateral wind forces (ASCE 7) and physical impacts:
- Post Footing Depth: Post footings must extend below the local frost line (typically 30 to 48 inches in northern climates) to prevent frost-heaving displacement. In frost-free zones, minimum footing depth should equal one-third to one-half of the total fence height above ground ($D \ge H/3$).
- Footing Diameter: Concrete piers should have a minimum diameter equal to three times the post diameter (e.g., a 4-inch square post requires a 12-inch diameter cylindrical footing).
- Screen Wall Masonry: Concrete masonry unit (CMU) screen walls require vertical reinforcing bars tied directly into continuous reinforced concrete strip footings. Wind-induced overturning is prevented by introducing structural piers, pilasters, or serpentine curved wall layouts.
Gate Hardware & Universal Accessibility
- Gate Latches: Gate latches in accessible paths must operate with one hand without tight grasping, pinching, or twisting of the wrist (lever or push-bar action), positioned between 34 and 48 inches above finished grade.
- Clear Width: Compliant accessible gates must provide a minimum clear opening width of 32 inches (clear opening measured between face of gate leaf opened at 90 degrees and the opposite stop), with smooth lower bottom kickplates (10 inches high minimum) on the push side.
- Vehicular Cantilever Slide Gates: Preferable over swinging vehicular gates where entry driveways slope upward into the site, preventing uphill bottom-drag conflicts.
3. Real-World Case Scenario: The Waterfront Promenade Plaza
Scenario: A landscape architect is designing a 1-acre public riverfront plaza in a cold-climate Midwestern city. The site features a 12-foot grade change from the adjacent municipal street down to the riverwalk boardwalk. The client requires: (1) a multi-tiered pedestrian plaza with unit pavers, (2) cast-in-place concrete amphitheater steps, (3) a 10-foot tall retaining structure to terrace the hillside, and (4) continuous accessibility.
Design Development Solution:
- Paving Assembly: The main pedestrian plaza is detailed with high-density precast concrete pavers (ASTM C936) laid in a 45-degree herringbone pattern. To resist heavy winter maintenance vehicles, the designer details a bituminous-set assembly (pavers bedded in 3/4" neoprene-modified asphalt adhesive over a 5-inch reinforced concrete slab, primed with asphalt primer, edged with continuous cast-in-place concrete flush curbs).
- Concrete Flatwork: Amphitheater steps and concrete sidewalks specify a 4,000 psi mix with 6.0% air entrainment and a maximum w/c ratio of 0.42 to resist freeze-thaw scaling and deicing salts. Control joints are spaced at 10 feet on center (panel aspect ratio 1:1) and saw-cut to a depth of 1.25 inches within 8 hours of pouring.
- Retaining Wall Engineering: Rather than a single 10-foot tall concrete wall, the landscape architect details an SRW system terraced into two 5-foot tiers separated by a 6-foot horizontal planted bench. Uniaxial geogrid is specified with an embedment length of 4 feet ($0.8H$) for each tier. Subsurface drainage includes 12 inches of clean ASTM No. 57 crushed stone wrapped in non-woven geotextile and a 4-inch perforated collector pipe pitched at 1.5% with perforations facing downward.
4. Exam Traps & Pitfalls
- The Perforated Pipe Orientation Trap: A pervasive LARE trap presents a cross-section detail showing the perforated drainage pipe behind a retaining wall with the holes oriented at the top (12 o'clock). This is completely incorrect. The holes must face DOWN (4 o'clock and 8 o'clock) so that rising groundwater enters from below without dragging surface silts into the pipe.
- The Control Joint Spacing & Depth Trap: Remember the two critical mathematical rules for cast-in-place concrete control joints: (1) Depth must be at least one-fourth of the slab thickness ($D/4$); (2) Spacing in feet must not exceed 2 to 3 times the slab thickness in inches (a 4-inch slab has joints spaced 8 to 12 feet apart, never 15 or 20 feet).
- The Unreinforced SRW Height Trap: Segmental Retaining Walls cannot be built as unreinforced gravity walls up to arbitrary heights. In professional practice and on the LARE, any SRW exceeding 3 to 4 feet in height requires engineered geogrid reinforcement.
- The Sand-Set Unit Paver Bedding Trap: Questions often ask whether fine sand or stone dust can be used as the 1-inch bedding layer for permeable pavers (PICP). Sand-set beds in PICP are a critical design failure because fine sand clogs the open reservoir. PICP requires ASTM No. 8 or No. 89 washed open-graded aggregate, never concrete sand.