7.2 Retaining Walls, Earth Pressure & Wall Drainage
Key Takeaways
- Active earth pressure acts on the wall from the retained soil, passive pressure resists at the toe, and at-rest pressure governs when the wall is restrained from any rotation, such as a basement wall.
- Hydrostatic pressure behind an undrained wall can exceed the soil pressure it was designed for, which is why drainage aggregate, a geotextile separator, and a perforated collector pipe with a positive outlet are non-negotiable.
- Retaining walls must be checked separately for overturning, sliding, bearing capacity, and global (deep-seated) slope stability; passing three checks and failing the fourth still produces a wall failure.
- Surcharge loads from adjacent vehicles, structures, or stockpiled soil substantially increase lateral pressure and must be declared to the wall designer rather than assumed away.
- Segmental retaining walls above the manufacturer's unreinforced height limit require geogrid reinforcement extending into the retained soil mass, making the reinforced soil block, not the facing units, the structural element.
Last updated: September 2026
1. Retaining Wall Systems & Earth Pressure Theory
Retaining walls hold back soil and water where site grades change abruptly. Choosing the correct wall system depends on retained height, available right-of-way, foundation soil bearing capacity, and aesthetic context.
Retaining Wall Typologies
- Gravity Walls: Rely entirely on their own massive dead weight to counteract lateral soil thrust. Constructed of dry-stacked stone, cyclopean concrete, or heavy stone masonry. Economically and structurally practical only for low wall heights (typically under 4 to 5 feet); taller gravity walls require an impractical base footprint.
- Cantilevered Reinforced Concrete Walls: Composed of a vertical reinforced concrete stem tied rigidly to a horizontal base footing containing a "toe" (extending outward in front of the wall) and a "heel" (extending backward beneath the backfill soil). The structural heel uses the massive weight of the backfill soil resting on top of it to prevent overturning. Economical for heights from 5 to 25+ feet.
- Segmental Retaining Walls (SRWs): Dry-stacked, mortarless modular concrete blocks that interlock vertically using integrated concrete shear lips, fiberglass pins, or keys. Wall faces are battered (set back) at 1:8 to 1:12 slope (roughly 3 to 7 degrees from vertical). Walls over 3 to 4 feet require geogrid reinforcement extending into the backfill soil mass.
- Gabion Walls: Rectangular wire mesh baskets (double-twisted galvanized steel or PVC-coated wire) filled with durable, weather-resistant stone (4- to 8-inch size). Units are wired together in courses. Highly flexible, free-draining (100% permeable), and capable of tolerating substantial differential settlement without structural failure. Ideal for riverbanks, swales, and remote terrain.
- Crib Walls: Interlocking frameworks of precast concrete, steel, or timber "headers" and "stretchers" forming open cells backfilled with free-draining crushed gravel.
CANTILEVERED REINFORCED CONCRETE SEGMENTAL RETAINING WALL (SRW)
| | [Block] <--- Finished Grade
Retained | | [Block]=======(Geogrid)====
Backfill | | Stem [Block] Reinforced
Soil | | [Block]=======(Geogrid)====
| | [Block] Soil Zone
Toe | | Heel [Block]=======(Geogrid)====
[=======| |=============] Footing [Leveling Pad (Crushed Stone)]
Geogrid Reinforcement Mechanics in SRWs
When an SRW exceeds 3 to 4 feet in height, unreinforced gravity block stacking becomes unstable. High-tensile polymeric geogrids (polyester yarns coated with PVC, or punched-and-drawn polypropylene sheets) are installed horizontally between block courses:
- Length of Geogrid: Standard engineering practice mandates a minimum geogrid embedment length equal to 60% to 70% of the total wall height ($0.6H$ to $0.7H$), measured from the wall face back into the slope.
- Vertical Spacing: Placed every 2 to 3 block courses (typically 16 to 24 inches maximum on center).
- Soil Zone: The soil within the geogrid embedment envelope is compacted in 6- to 8-inch lifts to 95% Standard Proctor density. This creates a coherent, unified composite soil mass that resists lateral sliding and overturning.
Lateral Earth Pressure Theory
Lateral earth pressure is the horizontal force exerted by retained soil against the back of a wall. It is governed by the soil's unit weight ($\gamma$), internal angle of friction ($\phi$), and the physical deflection of the wall structure:
- Active Earth Pressure ($K_a$): Occurs when the retaining wall moves or tilts slightly away from the retained backfill soil (as little as 0.001H in granular soil). The soil expands horizontally, mobilizing internal shear strength and reducing horizontal lateral pressure to its theoretical minimum.
- At-Rest Earth Pressure ($K_o$): Occurs when the wall is completely rigid and unyielding, with zero deflection (such as building basement walls, pedestrian tunnel vaults, or pool tanks restrained rigidly at top and bottom). Lateral pressures are significantly higher than active pressure.
- Passive Earth Pressure ($K_p$): Occurs when the wall structure is actively pushed into or against the soil mass (e.g., against the footing shear key beneath a wall or the front toe footing). Soil is compressed until it shears along failure planes, generating the maximum lateral soil resistance.
Exam Tip: Retaining walls that are free to rotate or deflect at the top (cantilevered walls, SRWs, gravity walls) are designed using Active Earth Pressure ($K_a$). Walls that are pinned or braced at the top and cannot rotate (basement walls, sunken garden vaults) must be designed using At-Rest Earth Pressure ($K_o$).
2. Subsurface Retaining Wall Drainage Systems
Hydrostatic pressure—the pressure exerted by standing water trapped behind a wall—is the single primary cause of catastrophic retaining wall failures. Water weighs 62.4 pounds per cubic foot ($pcf$), dramatically magnifying lateral forces against the wall. Retaining structures are not engineered to hold water unless detailed as dams.
Retaining Wall Drainage Assembly
=======================================================
/ / Impervious Clay Cap / Turf Swale (Slope away)
| |
| | +----+ Free-Draining Aggregate Backfill
| | | | (Clean ASTM No. 57 stone, 12" min width)
Stem | | | :: |
| | | :: | <-- Geotextile Filter Fabric Envelope
| | | :: | (Prevents silt clogging)
| | | :: |
Weep Hole | | | :: |
[===O===] | | |====| Perforated Collector Pipe
| | |(oo)| (4" PVC/HDPE, PERFORATIONS DOWN, 1% slope)
==========+=+==+====+==================================
Footing / Foundation
Essential Drainage Components
- Free-Draining Granular Backfill: A continuous zone (minimum 12 inches wide, extending from the base footing to within 12 inches of the top) of clean, washed crushed stone (such as ASTM No. 57 aggregate with zero fines).
- Geotextile Filter Fabric: A non-woven needle-punched geotextile wrapping the gravel backfill envelope. It allows water to pass freely while preventing native soil silt and fine sands from migrating into and clogging the aggregate voids.
- Perforated Drain Pipe: A minimum 4-inch diameter rigid perforated PVC (ASTM D3034) or smooth-interior corrugated HDPE pipe placed along the base of the drainage gravel behind the wall. The pipe must be pitched at a minimum 1.0% slope to daylight or into a stormwater catch basin. Crucially, the pipe must be oriented with its perforations facing DOWN (this forces the groundwater table to rise up into the pipe from below and enter cleanly without washing surrounding gravel into the openings).
- Weep Holes: Through-wall drainage openings (2 to 4 inches in diameter) spaced every 6 to 10 feet horizontally along the base of the wall (placed 4 to 6 inches above finished front grade). Weep holes must be fitted with galvanized mesh or insect screens to prevent animal nesting and aggregate loss.
- Impervious Surface Cap: The top 6 to 12 inches of backfill above the gravel zone must be capped with low-permeability native clay or compacted topsoil pitched away from the wall to divert surface runoff into a positive drainage swale.
3. Retaining Wall Structural Failure Modes
On the LARE, landscape architects must diagnose and prevent four primary retaining wall structural failure modes:
1. SLIDING FAILURE 2. OVERTURNING FAILURE
Lateral thrust > Friction Earth pressure pivots wall around toe
---------> --------->
---------> Wall pushes ---------> Wall rotates
---------> forward __ forward
[====================] \_______/ \__
3. BEARING CAPACITY FAILURE 4. GLOBAL SLOPE FAILURE
Excessive toe soil pressure Deep rotational shear circle beneath
Toe sinks into soft subsoil entire slope carrying wall with it
\ . - ~ ~ ~ - .
\_ [Wall tilts] / [ Wall ] \
[ ] ( ___ )
[ _ ] \____/ \______/
| Failure Mode | Physical Mechanism | Preventative Detailing & Safety Factors |
|---|---|---|
| Sliding | Total horizontal active earth pressure ($F_a$) exceeds the frictional shear resistance developed between the base footing and foundation soil. | - Minimum Factor of Safety: $FS \ge 1.5$.<br>- Install a cast-in-place concrete shear key projecting downward beneath the footing into undisturbed subsoil.<br>- Widen the footing or embed the footing deeper below finished grade to engage passive resistance ($K_p$). |
| Overturning | Lateral soil thrust generates an overturning moment around the wall's front toe that exceeds the resisting moment generated by the dead weight of the wall stem and the soil resting on the heel. | - Minimum Factor of Safety: $FS \ge 1.5$ to $2.0$.<br>- Lengthen the structural heel footing backward into the slope to capture more soil overburden.<br>- Widen the overall footing width ($B$), typically detailed as $0.5H$ to $0.7H$. |
| Bearing Capacity Failure | The combined eccentric vertical load and overturning moment concentrates extreme compressive stress at the front toe of the footing, exceeding the allowable soil bearing capacity ($q_a$) and causing soil shear settlement. | - Minimum Factor of Safety: $FS \ge 2.0$ to $3.0$.<br>- Widen the footing toe to distribute loads across a larger soil contact area.<br>- Excavate unsuitable subsoil and replace with engineered crushed aggregate backfill; utilize drilled micro-piles or grade beams in soft soils. |
| Global Slope Failure | A deep rotational slip surface (shear circle) forms through the hillside soil beneath and behind the entire wall system, shearing the entire hillside mass and displacing the wall intact. | - Minimum Factor of Safety: $FS \ge 1.3$ to $1.5$.<br>- Requires global slope stability analysis (Bishop or Spencer method).<br>- Lengthen geogrid reinforcement layers beyond the failure circle; terrace tall grades into multiple tiered retaining walls separated by intermediate horizontal benches (bench width equal to or greater than wall height, $W \ge H$). |
Test Your Knowledge
During a site construction observation for an 8-foot tall segmental retaining wall (SRW) supporting a civic plaza terrace, the contractor has begun backfilling behind the modular concrete units. Which of the following observed construction practices represents a critical structural defect that must be rejected?
A
B
C
D
Test Your Knowledge
A civil engineer and landscape architect are evaluating the structural stability of a proposed cantilevered reinforced concrete retaining wall. The calculations reveal that the horizontal active earth pressure exceeds the frictional resistance between the footing base and the underlying subgrade soil, resulting in a Factor of Safety below 1.5. Which structural modification is the most direct and effective method to resolve this specific failure condition?
A
B
C
D