Retaining Wall Guide

Block selection, base preparation, drainage requirements, and height limitations for segmental block walls

What is a Segmental Block Retaining Wall?

A segmental retaining wall (SRW) is a gravity-type wall system built from interlocking concrete blocks that are dry-stacked (no mortar) to retain soil and create level areas on sloped terrain. These walls rely on their mass and built-in setback (batter) to resist soil pressure.

SRW blocks feature lips, pins, or interlocking mechanisms that create stability while allowing flexibility for settling and movement. The blocks are hollow or solid and typically weigh 40-80 pounds each, making them manageable for DIY projects up to certain heights.

Key Advantage: Segmental retaining walls are modular, requiring no mortar or specialized masonry skills. They can handle minor ground movement without cracking, unlike poured concrete or mortared stone walls.

Block Selection Guide

Block Type	Typical Size	Weight	Best Use
Standard SRW Block	12"H × 18"W × 12"D	75-85 lbs	Walls up to 4 feet without reinforcement
Hollow Core Block	8"H × 18"W × 12"D	50-60 lbs	Lighter handling, can fill cores for added weight
Solid Block	6"H × 16"W × 12"D	40-50 lbs	Garden walls, planters, low terracing
Cap Blocks	3"H × 18"W × 12"D	30-40 lbs	Finishing top course of wall

Block Design Features

Setback lip: Creates automatic batter (typically 3/4" per course) for stability
Pin holes: Accept fiberglass pins to secure courses together
Hollow cores: Allow for vertical reinforcement with rebar and concrete fill
Split face: Natural stone appearance on front face
Textured finish: Various colors and textures to match landscape

Mixing Brands: Do not mix block brands within a single wall. Different manufacturers have different setback angles and interlocking mechanisms that are incompatible.

Base Material Depth and Compaction

Base Depth Requirements by Wall Height

Wall Height	Trench Depth	Base Material Depth	Buried Courses
Up to 2 feet	6-8 inches	6 inches compacted	1 course (optional)
2-4 feet	8-10 inches	6-8 inches compacted	1 course minimum
4+ feet	12+ inches	8-12 inches compacted	1-2 courses

Critical Foundation Rule: The base course (and sometimes second course) should be buried below grade. This provides stability and prevents the wall from being undermined by erosion.

Base Material Specifications

Material type: 3/4" crushed angular stone (also called Class 5, road base, or CA6)
Not suitable: Round river rock, pea gravel, sand - these do not compact or interlock properly
Why crushed stone: Angular edges lock together when compacted, creating stable base
Drainage benefit: Crushed stone allows water to drain through base

Compaction Requirements

Compaction tool: Use plate compactor (rental) for walls over 2 feet. Hand tamper acceptable for small walls.
Lift thickness: Compact in 2-3 inch lifts (layers), not all at once
Minimum passes: 2-3 passes with plate compactor per lift
Target compaction: 95% modified proctor density (firm and unyielding)
Test method: Base should not move or sink when walked on

Base Material Calculator

Formula:

Cubic yards = (Length × Width × Depth in feet) ÷ 27

Example: 30-foot wall, 2-foot wide trench, 8-inch (0.67 ft) depth:

30 × 2 × 0.67 = 40.2 cubic feet
40.2 ÷ 27 = 1.5 cubic yards
Order 2 cubic yards to account for compaction

Leveling Pad Preparation

Step-by-Step Leveling Process

Excavate trench: Dig to required depth along entire wall length, maintaining consistent depth
Remove loose soil: Compact native soil at bottom of trench with plate compactor
Add base material: Spread first 2-3 inch lift of crushed stone
Rough level: Use rake to distribute stone evenly along trench
Compact first lift: Make 2-3 passes with plate compactor
Add subsequent lifts: Repeat until reaching desired base depth
Fine leveling: Use straight 2×4 or screed board to level final surface
Final compaction: Compact final lift, checking for level every few feet

Critical Leveling: The base course must be perfectly level side-to-side and along its length. An unlevel base course will compound with each additional course, creating a visibly crooked wall.

Leveling Tools and Techniques

String line: Stretch taut string line along planned wall face at base block height
Line level: Hang line level on string to verify level along entire run
4-6 ft level: Check compacted base every 3-4 feet for level
Transit or laser level: For long walls, use surveying equipment for precise leveling
Adjustment: Add or remove base material to achieve level. Re-compact after adjustments.

Dealing with Slopes

Stepped base: On slopes, create level steps in base rather than following slope
Step height: Each step should be one block height (8" typical)
Step length: Minimum 4 feet per step, longer is better for appearance
Overlap: Each step overlaps previous by at least half a block length

Block Setback Per Course (Batter)

Batter is the backward lean of a retaining wall, where each course is set back slightly from the course below. This setback tilts the wall into the retained soil, increasing stability and helping the wall resist overturning.

Block System	Setback Per Course	Total Batter (4 ft wall)	Method
Lip-style blocks	3/4" - 1" automatic	4-5 inches	Blocks have built-in setback lip on bottom rear
Pin-style blocks	3/4" via pins	4 inches	Fiberglass pins create automatic setback
Flat blocks	User-controlled	Varies	Manual setback required, typically 3/4" per course

Why Setback Matters: A 4-foot tall wall with 3/4" setback per course leans back about 4 inches total. This lean redirects soil pressure downward into the wall rather than pushing it forward, dramatically increasing stability.

Installing Blocks with Setback

First course: Place on leveling pad with front face aligned to string line
Subsequent courses: Place block so rear lip or pins engage with course below
Check alignment: Verify each block aligns with those adjacent before moving on
Tap into place: Use rubber mallet to seat blocks firmly on course below
Verify setback: Measure from a vertical reference point to ensure consistent batter

Common Setback Issues

❌ Inconsistent setback: Creates wavy, unprofessional appearance
❌ Blocks tilting forward: Indicates unlevel base or improper seating
❌ Setback too extreme: Wall leans back excessively, wastes space
✓ Even, consistent lean: All courses parallel, uniform appearance

Drainage Gravel and Pipe Placement

Critical Success Factor: Proper drainage is the most important factor for retaining wall longevity. Water buildup behind the wall creates hydrostatic pressure that can cause wall failure. Never skip drainage!

Drainage Gravel Backfill

Material: 3/4" clean crushed stone (no fines or sand)
Width: Minimum 12 inches behind wall blocks
Height: From base to within 6 inches of top of wall
Purpose: Creates drainage zone that prevents water from saturating retained soil
Installation: Place as each course is installed, compact lightly

Perforated Drain Pipe Installation

Pipe type: 4" perforated drain pipe (with sock to prevent clogging)
Location: At base of wall, on top of compacted base, behind first course
Slope: Minimum 1/8" per foot slope toward outlet
Holes orientation: Holes facing down (contrary to common belief)
Outlet: Pipe must daylight at end of wall or connect to drainage system
Gravel cover: Surround pipe with 6 inches of drainage gravel

Complete Drainage System Cross-Section

From front to back of wall:

Retaining wall blocks
12+ inches of 3/4" crushed drainage stone
Geotextile fabric (optional but recommended)
Native soil backfill, compacted in 6-8 inch lifts

The geotextile fabric prevents fine soil particles from migrating into drainage gravel and clogging it over time.

Wall Height	Min Drainage Width	Drain Pipe Required?	Geotextile Fabric?
Up to 2 feet	12 inches	Recommended	Optional
2-4 feet	12-18 inches	Yes	Recommended
4+ feet	18-24 inches	Yes	Yes

Geogrid Reinforcement Requirements by Height

Geogrid is a synthetic mesh material that extends from the wall back into the retained soil, creating a reinforced soil mass. It dramatically increases the effective weight and stability of taller retaining walls.

Wall Height	Geogrid Required?	Placement	Grid Length
Up to 3 feet	No	N/A	N/A
3-4 feet	Recommended	Every 2-3 courses	3-4 feet (0.6-0.7 × height)
4-6 feet	Yes	Every 2 courses	4-6 feet (0.6-0.7 × height)
6+ feet	Yes, engineered	Per engineering specs	Per engineer (typically 0.7 × height)

Engineering Required: Walls over 4 feet tall typically require engineering calculations to determine proper geogrid type, spacing, and length. Many jurisdictions require permits and engineering for walls over 4 feet.

Geogrid Installation Steps

Place grid: Unroll geogrid on top of designated course, extending back into excavation
Position: Front edge should be 2-3 inches behind front face of block
Overlap: Adjacent grid sections should overlap by 6-12 inches
Place next course: Install blocks on top of geogrid, trapping it
Backfill: Cover grid with 6 inches of soil and compact
Continue: Add more soil lifts, compacting each before next grid layer

Geogrid Types

Biaxial geogrid: Strength in two directions, used for most retaining walls
Uniaxial geogrid: Strength in one direction, used in specific engineered applications
Strength rating: Measured in lbs/ft. Common ratings: 1,500-4,000 lbs/ft
Specifications: Use manufacturer-recommended grid for your block system

Cap Installation

Cap blocks are solid, flat-topped units that finish the top of the wall. They provide a clean appearance, protect the cores of blocks below, and can be adhered for additional security.

Cap Block Installation Steps

Clean top course: Remove debris and dust from top of final course
Apply adhesive: Run two beads of concrete adhesive along top of blocks
Place cap block: Center cap block over wall blocks, press firmly into adhesive
Check alignment: Use string line to keep caps aligned along length of wall
Overhang: Caps typically overhang wall face by 1-1.5 inches on front
Corner caps: Use provided corner cap units or miter-cut standard caps
Cure time: Allow adhesive to cure 24-48 hours before backfilling behind wall

Cap Block Adhesive

Product: Concrete block adhesive or construction adhesive (tubes or cartridges)
Purpose: Prevents caps from shifting or being removed
Coverage: One tube covers approximately 8-10 cap blocks
Application: 1/4" bead along center and front edge of top course
Alternative: For extra security, caps can be pinned with rebar through cores

Cap Type	Typical Size	Best Use	Cost
Standard Cap	3"H × 12"W × 18"L	Most retaining walls	$3-5 per block
Beveled Cap	3"H × 14"W × 18"L	Wider overhang, decorative	$4-6 per block
Column Cap	4"H × 18"W × 18"L	Pillar caps, wall columns	$8-12 per block

Cap Color Matching: Caps may not perfectly match wall block color due to different manufacturing processes. Review color samples before purchasing if color matching is important.

When Engineering is Required

Situations Requiring Professional Engineering

Wall height over 4 feet: Most jurisdictions require engineering for walls exceeding 4 feet
Slopes above wall: Surcharge loads from slopes require engineering analysis
Poor soil conditions: Clay, soft soil, or high water table conditions
Tiered walls: Multiple walls in close proximity create complex load conditions
Heavy loads: Driveways, structures, or equipment behind wall
Limited space: Insufficient room for proper geogrid length
Building codes: Local codes may require engineering regardless of height

Code Compliance: Always check local building codes before starting. Many areas require permits for retaining walls over 2-4 feet tall. Unpermitted walls may need to be removed and can affect property sales.

What Engineers Analyze

Soil bearing capacity and stability
Wall sliding and overturning resistance
Proper geogrid type, spacing, and length
Drainage system adequacy
Seismic considerations (earthquake zones)
Foundation depth and width requirements
Global stability of entire slope

DIY-Friendly Wall Guidelines (No Engineering)

Maximum height: 3-4 feet (check local codes)
Level or gently sloping terrain above wall
Good drainage, well-draining soil
No surcharge loads (structures, vehicles, slopes)
Stable soil conditions
Follow manufacturer installation guidelines exactly

Tiered Walls: Instead of one tall wall, consider two shorter tiered walls with a terrace between them. Keep walls under 4 feet and terrace width at least 2× the height of lower wall. This may avoid engineering requirements while achieving desired height change.

Tools and Materials Needed

Tools

Plate compactor (rental)
4-6 ft level
String line and stakes
Line level
Tape measure
Wheelbarrow
Shovel and rake
Masonry saw or angle grinder (for cuts)
Rubber mallet
Circular saw with masonry blade
Chalk line
Safety glasses and gloves

Materials

Retaining wall blocks
Cap blocks
3/4" crushed stone for base
3/4" clean drainage stone
4" perforated drain pipe with sock
Geotextile fabric
Geogrid (if required)
Concrete adhesive for caps
Native soil or fill for backfill
Landscape fabric (optional)

Safety and Code Considerations

Call Before You Dig: Contact 811 at least 2-3 business days before excavating to have underground utilities marked. Buried power, gas, water, and communication lines can cause serious injury or death if struck.

Heavy Lifting: Retaining wall blocks weigh 40-80 lbs each. Use proper lifting technique (bend knees, straight back). Consider using mechanical assistance or helpers for walls requiring 100+ blocks.

Slope Stability: Excavating at the base of a slope can cause the slope to fail. If the slope shows signs of movement (cracks, bulging), stop work and consult an engineer immediately.

Permits and Engineering: Walls over 4 feet typically require engineering and permits. Walls supporting structures or near property lines have additional requirements. Check local codes before starting.

Note: Retaining wall regulations vary significantly by jurisdiction. Height limits, setback requirements, engineering thresholds, and permit requirements differ between municipalities. Always verify local requirements before beginning construction.

Quick Reference Summary

Standard 3-Foot Wall (No Geogrid)

Base: 6-8 inches compacted crushed stone
Buried course: 1 course below grade
Drainage: 12" gravel backfill + drain pipe
Setback: 3/4" per course (automatic with lip blocks)
Cap: Glued with concrete adhesive

Key Success Factors

Level base is critical - check every 3 feet
Compact base in 2-3 inch lifts
Always install drainage gravel and pipe
Use geogrid for walls over 3 feet
Bury first course for stability
Get engineering for walls over 4 feet

Disclaimer: This guide provides general information about retaining wall construction. Soil conditions, site-specific factors, and local building codes vary significantly. For walls over 4 feet, walls supporting structures or slopes, or walls in poor soil conditions, always consult a licensed engineer. Improperly built retaining walls can fail catastrophically. When in doubt, hire a professional.