1 Scope
NOTE This standard covers the materials, design basis, drainage, execution, and inspection of permanent site retaining walls that separate grades, stabilize slopes, and terrace the site. (1.1)
NOTE A retaining wall is a structure that holds back soil and the loads the soil carries, converting a slope that would not stand on its own into a stable vertical or near-vertical face. (1.2)
NOTE Retaining walls fail in ways that are sudden, expensive, and dangerous — by overturning, sliding, bearing failure, or deep-seated global slope failure — and the great majority of those failures trace back to two omissions: inadequate drainage that allows hydrostatic pressure to build behind the wall, and the absence of an engineered design where the wall height or the loads on it demanded one. (1.3)
NOTE This standard treats drainage and the engineered-design threshold as the two controlling requirements, because they are the two failure modes that recur on site walls. (1.4)
NOTE This standard addresses segmental retaining walls (SRW) in both gravity and geogrid-reinforced (mechanically stabilized earth) configurations, cast-in-place reinforced concrete cantilever walls, gravity walls of mass concrete, gabion baskets, or stacked boulders, and modular block walls, together with their foundations, drainage systems, backfill, and caps. (1.5)
1.6All retaining wall work shall conform to the recommendations of the project geotechnical investigation report and to the wall design prepared for the project.
1.7Where this standard, the geotechnical report, the wall design drawings, and the adopted building code conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
1.8The Contractor shall coordinate work under this standard with Earthwork for site excavation, subgrade preparation, structural fill placement, and compaction behind and beneath the wall. 1.10The Contractor shall coordinate the wall drainage system with Foundation Drainage where the wall drainage discharges into or shares the building foundation drainage system. 1.11This standard does not govern building foundation or basement walls, temporary excavation support walls, or below-grade waterproofing of habitable spaces.
2 Referenced Standards
2.1Materials, design, testing, and execution shall comply with the latest adopted edition of the following standards.
2.2Where project documents, adopted codes, and referenced standards conflict, the more stringent requirement shall govern unless the Engineer of Record directs otherwise in writing.
| Standard |
Title |
| NCMA SRW Design Manual |
Design Manual for Segmental Retaining Walls (National Concrete Masonry Association / The Concrete Masonry & Hardscapes Association) |
| ASTM C1372 |
Standard Specification for Dry-Cast Segmental Retaining Wall Units |
| FHWA-NHI-10-024 (GEC 011 Vol. I) |
Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes |
| FHWA-NHI-10-025 (GEC 011 Vol. II) |
Design and Construction of Mechanically Stabilized Earth Walls and Reinforced Soil Slopes — Volume II |
| AASHTO LRFD |
AASHTO LRFD Bridge Design Specifications (retaining wall and MSE wall design provisions) |
| ACI CODE-318 |
Building Code Requirements for Structural Concrete (cast-in-place cantilever and gravity walls) |
| IBC Chapter 18 |
Soils and Foundations (International Building Code, currently adopted edition) |
| IBC Section 1807 |
Foundation Walls, Retaining Walls, and Embedded Posts and Poles |
| ASTM D6637 |
Standard Test Method for Determining Tensile Properties of Geogrids by the Single- or Multi-Rib Tensile Method |
| ASTM D6638 |
Standard Test Method for Determining Connection Strength Between Geosynthetic Reinforcement and Segmental Concrete Units (Modular Concrete Blocks) |
| ASTM D6916 |
Standard Test Method for Determining the Shear Strength Between Segmental Concrete Units (Modular Concrete Blocks) |
| ASTM D448 |
Standard Classification for Sizes of Aggregate for Road and Bridge Construction (drainage aggregate gradations) |
| ASTM D698 |
Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Standard Effort |
| ASTM D1557 |
Standard Test Methods for Laboratory Compaction Characteristics of Soil Using Modified Effort |
| ASTM D2487 |
Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) |
| ASTM D6938 |
Standard Test Methods for In-Place Density and Water Content of Soil and Soil-Aggregate by Nuclear Methods |
| ASTM A615 |
Standard Specification for Deformed and Plain Carbon-Steel Bars for Concrete Reinforcement |
3 Submittals
3.1 Action Submittals
3.1.1The Contractor shall submit the following for the Engineer of Record's review prior to procurement and construction:
- Engineered wall design — calculations and drawings, signed and sealed by a professional engineer licensed in the jurisdiction of the project — for every wall that exceeds the height triggering engineered design under this standard or the adopted code, or that carries a surcharge, supports a slope above, or retains water; the design shall address external (sliding, overturning, bearing), internal (for reinforced walls), and global (deep-seated) stability
- Geotechnical investigation report or the geotechnical parameters used in the wall design, including the assumed soil friction angle, unit weight, cohesion, allowable bearing pressure, and global stability findings for the wall locations
- Product data for the segmental retaining wall units, including ASTM C1372 compliance, unit dimensions, compressive strength, and absorption
- Product data for the geogrid reinforcement where used, including ASTM D6637 long-term design strength, ASTM D6638 connection strength with the specified SRW unit, and the reduction factors used in design
- Product data for the drainage aggregate (ASTM D448 gradation), the perforated drainage pipe, the geotextile filter fabric, and the reinforced backfill source with gradation and plasticity testing
- Shop drawings showing wall layout, plan and profile, exposed and embedded heights, batter, leveling pad detail, geogrid layout (length, vertical spacing, and coverage), drainage system, cap detail, and the relationship of the wall to adjacent structures and utilities
☑ Engineered wall design (PE-stamped calculations and drawings) where required
☐ Geotechnical report / design soil parameters and global stability findings
☐ SRW unit product data (ASTM C1372)
☐ Geogrid product data (ASTM D6637 strength, ASTM D6638 connection strength)
☐ Drainage aggregate, pipe, filter fabric, and reinforced backfill product data
☐ Shop drawings (layout, profile, batter, geogrid layout, drainage, caps)
3.1.2Wall construction shall not proceed until the submittals relevant to that wall are reviewed and returned.
3.1.3Submittal review does not relieve the Contractor of responsibility for compliance with the contract documents and the engineered wall design.
3.2 Closeout Submittals
3.2.1Prior to substantial completion the Contractor shall provide the following:
- Field compaction testing reports for the reinforced and retained backfill, including test locations, lift number, tested dry density, moisture content, and percent compaction against the applicable Proctor
- As-built survey confirming wall alignment, top-of-wall and bottom-of-wall elevations, batter, and embedment within the tolerances of the design
- Certification from the geotechnical engineer or the special inspector that the wall foundation, drainage, geogrid placement, and backfill compaction were observed and conform to the engineered design
- Manufacturer warranty for the SRW units and geogrid, and the Contractor warranty for the installed wall
☑ Field compaction testing reports for reinforced and retained backfill
☑ As-built survey (alignment, elevations, batter, embedment)
☑ Geotechnical engineer / special inspector wall conformance certification
☐ Manufacturer and Contractor warranties
4 Quality Assurance
4.1 Engineered Design Threshold
NOTE The single most consequential decision for a site wall is whether it requires an engineered design, because a wall built by feel that needed a PE design is the most common path to a wall failure. (4.1.1)
NOTE IBC Section 1807 and most jurisdictions require a permit and an engineered design for retaining walls retaining more than 4 ft of unbalanced backfill measured from the bottom of the footing to the top of the wall, and for any wall of any height that carries a surcharge, supports a slope above, or retains water — these conditions add load or remove the simplifying assumptions that gravity-wall charts rely on. (4.1.2)
● Required — wall exposed height greater than 4 ft (unbalanced backfill)
○ Required — wall carries surcharge, supports a slope above, or retains water (any height)
○ Required — geogrid-reinforced / MSE wall of any height
○ Not required — wall 4 ft or less, no surcharge, no slope above, level backfill (verify with AHJ)
4.1.4A retaining wall of any height that carries a surcharge load, supports a sloped backfill above the wall, or retains water shall be designed by a professional engineer regardless of its exposed height.
4.1.5A geogrid-reinforced or mechanically stabilized earth wall of any height shall be designed by a professional engineer.
4.1.6The engineered design shall demonstrate adequate external, internal, and global stability for the wall and the loads acting on it.
NOTE The Contractor shall not substitute a manufacturer's standard gravity-wall chart for an engineered design where an engineered design is required. (4.1.7)
4.2 Global Stability
NOTE Global stability is the resistance of the entire soil mass — including the wall, the reinforced zone, and the soil beneath and behind it — against a deep-seated failure surface that passes below or behind the wall. (4.2.1)
NOTE A wall can be internally and externally sound and still fail by global instability if it sits on a weak foundation soil, on a slope, or above a lower wall, and this failure mode can only be evaluated by the geotechnical engineer with the site soil profile. (4.2.2)
4.2.3The geotechnical engineer shall evaluate global (deep-seated) stability for every engineered wall, for tiered walls, for walls on or above a slope, and for walls founded on soft or compressible soils.
4.2.4Where global stability does not meet the required factor of safety, the design shall be revised by lengthening reinforcement, improving the foundation soil, flattening the slope, or relocating the wall, as directed by the geotechnical engineer.
4.2.5Tiered walls, where one wall is set above another, shall be analyzed for the surcharge that the upper wall imposes on the lower wall and for global stability through both tiers.
4.3 Special Inspection
4.3.1Where the adopted building code, the structural drawings, or the wall design require special inspection of retaining walls, the Owner shall retain a special inspector approved by the Authority Having Jurisdiction.
● Required — engineered wall; foundation, geogrid, drainage, and backfill compaction inspected
○ Required — as directed by the Engineer of Record or AHJ
○ Not required — wall below threshold requiring special inspection (verify with AHJ)
4.3.2The special inspector shall observe the foundation bearing surface, the leveling pad or footing, the placement and coverage of geogrid reinforcement, the installation of the drainage system, and the placement and compaction of reinforced and retained backfill.
4.3.3The special inspector shall document observations and test results and report non-conformances to the Owner and to the Engineer of Record.
4.4 Pre-Construction Conference
4.4.1Prior to beginning wall construction the Contractor shall participate in a pre-construction conference attended by the Contractor's superintendent, the wall installer, the Owner's geotechnical engineer or special inspector, and the Engineer of Record.
4.4.2The conference shall review the engineered wall design, the foundation and global stability requirements, the geogrid layout and compaction requirements, the drainage system and its outlets, the backfill materials and testing program, and the requirements for handling differing site conditions at the wall foundation.
5 Geotechnical and Site Conditions
5.1 Foundation Soil and Bearing
NOTE The retaining wall is only as stable as the soil it bears on, and a wall designed for one bearing capacity that is built on weaker soil will settle, rotate, or fail in bearing regardless of how well the wall itself is built. (5.1.1)
5.1.3The bearing surface shall be confirmed to match the allowable bearing pressure assumed in the wall design.
5.1.4Soft, loose, wet, or organic material encountered at the foundation elevation shall be undercut and replaced with compacted structural fill, or the wall shall be redesigned, as directed by the geotechnical engineer.
5.1.5The Contractor shall provide the geotechnical engineer at least 24 hours notice before each wall foundation will be ready for observation.
5.2 Frost Depth and Embedment
NOTE A wall founded above the frost line in a frost-susceptible soil heaves and settles seasonally, opening joints and racking the wall, so the foundation must extend below the local frost depth. (5.2.1)
648
61218243648
Default: 18 inches
Per drawings — wall profile
5.2.2The wall foundation, leveling pad, or footing shall be embedded below the local frost depth in frost-susceptible soils, and not less than the embedment shown on the wall design.
5.2.3The minimum embedment of the bottom of the wall below the finished grade at the toe shall be the greater of the design value or a minimum of one-tenth the exposed wall height, unless the engineered design establishes a different value.
NOTE Embedment shall be increased where the wall toes out onto a slope, because a wall fronting a descending slope has less passive resistance and a shallower path to a global failure surface. (5.2.4)
5.3 Surcharge from Adjacent Structures
NOTE A surcharge is any load applied to the soil behind the wall in addition to the soil weight — a building footing, a roadway, a parking area, a stockpile, or construction equipment — and an unaccounted surcharge is a frequent cause of overstress and failure. (5.3.1)
☑ None — level, unloaded backfill
☐ Sloped backfill above the wall
☐ Traffic / parking surcharge (vehicular)
☐ Adjacent building foundation within the zone of influence
☐ Construction equipment / stockpile surcharge (temporary)
5.3.2The wall design shall account for every surcharge that acts within the zone of influence behind the wall, including sloped backfill, traffic and parking loads, adjacent building foundations, and construction loading.
5.3.3The Contractor shall not operate heavy equipment or place stockpiles within the zone of influence behind a wall during or after construction unless the wall is designed for that surcharge.
5.3.4Where an adjacent building foundation falls within the zone of influence behind the wall, the wall design shall include the foundation surcharge, and the wall shall not undermine the lateral or vertical support of the existing foundation.
6 Wall Type and Configuration
6.1 Wall Type Selection
NOTE The wall type is selected from the exposed height, the available space behind the wall for reinforcement, the foundation conditions, the surcharge, and the appearance, and these constraints usually narrow the choice more than aesthetics do. (6.1.1)
NOTE A gravity SRW wall resists overturning by its own mass and is economical to roughly 3 to 4 ft of exposed height; above that, geogrid reinforcement extends the same units into a mechanically stabilized earth mass that can reach much greater heights; a cast-in-place cantilever wall is used where reinforcement cannot extend behind the wall or where a thin footprint is required; and gravity walls of mass concrete, gabions, or boulders suit specific site and appearance conditions. (6.1.2)
Segmental (SRW) gravity — modular block, no soil reinforcement
Segmental (SRW) geogrid-reinforced (MSE) — modular block with geogrid
Cast-in-place reinforced concrete cantilever
Gravity — mass concrete
Gravity — gabion basket
Gravity — stacked boulder / rock
6.1.3The wall type shall be as shown on the contract drawings and confirmed by the engineered wall design.
6.1.4Where the Contractor proposes an alternative wall type, it shall be supported by an engineered design meeting the same performance requirements and approved by the Engineer of Record.
6.2 Exposed Height Range
NOTE The exposed height — the vertical face from the finished grade at the toe to the top of the wall — together with the embedment, sets the total design height and governs the wall type and the reinforcement. (6.2.1)
230
246812162030
Default: 6 ft
Per drawings — wall profile
6.2.2The exposed wall height shall be as shown on the wall profile.
6.2.3Where the exposed height varies along the wall, the design and the geogrid layout shall be developed for the controlling (tallest) section and stepped down as the height decreases.
6.3 Batter and Setback
NOTE Batter is the backward lean of the wall face into the retained soil, and it improves stability by directing the resultant of the wall weight back toward the heel; segmental walls achieve batter through the setback built into each course of units. (6.3.1)
Near-vertical (1 to 3 degrees) — minimal setback units
Moderate (4 to 8 degrees) — standard SRW setback
Steep (9 to 15 degrees) — high-setback / large-batter units
Vertical — cast-in-place cantilever (battered face per design)
6.3.2The wall face shall be constructed to the batter shown on the design, established by the unit setback for segmental walls or by the formed face for cast-in-place walls.
6.3.3The batter shall be maintained uniformly course to course, and shall be checked as the wall rises so that drift does not accumulate.
7 Materials
7.1 Segmental Retaining Wall Units
NOTE SRW units shall conform to ASTM C1372. (7.1.1)
● 3000 psi (ASTM C1372 minimum)
○ 4000 psi (severe weathering / freeze-thaw exposure)
● Per ASTM C1372 for the local weathering region (standard)
○ Reduced absorption for severe freeze-thaw exposure
7.1.2SRW units shall have a minimum net-area compressive strength of 3000 psi, increased where severe weathering exposure requires.
7.1.3SRW units shall meet the maximum water absorption of ASTM C1372 for the weathering region of the project.
7.1.4Units shall be sound, free of cracks and chips that impair structural integrity or appearance, and consistent in dimension within the ASTM C1372 tolerances so courses seat uniformly.
7.1.5Units shall provide the design connection capacity to the geogrid where the wall is reinforced, verified by ASTM D6638 testing of the specific unit-and-geogrid combination.
7.2 Concrete and Reinforcing (Cast-in-Place and Gravity Concrete Walls)
3000 psi
3500 psi
4000 psi
4500 psi
Per drawings
● ASTM A615 Grade 60
○ ASTM A615 Grade 75 (where shown on design)
7.2.2Cast-in-place concrete shall achieve the specified compressive strength shown on the wall design and shall meet the durability requirements for soil and weather exposure under Cast In Place Concrete. 7.2.3Reinforcing steel shall conform to ASTM A615 of the grade shown on the design, and shall be placed with the cover and at the locations shown.
NOTE The wall stem reinforcement shall be placed on the tension (retained-soil) face, and shall not be displaced toward the exposed face during placement, because cover error on the tension face directly reduces the wall's flexural capacity. (7.2.4)
7.3 Geogrid Soil Reinforcement
NOTE Geogrid is the tensile reinforcement that converts a stack of facing units into a mechanically stabilized earth mass; it carries the lateral earth pressure into the soil through friction and pullout resistance, and its long-term design strength, after reduction for creep, installation damage, and durability, must meet the design demand. (7.3.1)
● Required — geogrid-reinforced (MSE) wall
○ Not required — gravity wall (no soil reinforcement)
0.61.1
Default: 0.7 fraction of total wall height (H)
Per drawings — wall design
Per drawings — wall design
7.3.2Geogrid reinforcement shall be the type and strength shown on the engineered wall design, with long-term design strength established per ASTM D6637 and the design reduction factors.
7.3.3Geogrid length shall be not less than the value shown on the design, and not less than 0.6 times the total wall height; walls with a surcharge or a slope above commonly require lengths of 0.8 to 1.0 times the height or greater.
7.3.4Geogrid vertical spacing shall not exceed the value shown on the design.
7.3.5The geogrid-to-unit connection strength shall be verified for the specific unit-and-geogrid combination by ASTM D6638 testing, and shall meet the connection demand at every reinforcement level.
NOTE Geogrid shall be installed with the strong (machine) direction perpendicular to the wall face, because the design strength is the strength in that direction; geogrid laid with the strong direction parallel to the wall provides little reinforcement. (7.3.6)
7.4 Leveling Pad
NOTE The leveling pad is the prepared base course that the first course of units is set on; it spreads the wall load, establishes a level and true starting course, and a wall that starts out of level only gets worse as it rises. (7.4.1)
● Compacted crushed stone (ASTM D448 aggregate), 6 in. minimum thickness
○ Unreinforced concrete leveling pad (where shown on design)
7.4.2The leveling pad shall be a compacted crushed-stone pad not less than 6 inches thick, or an unreinforced concrete pad where shown on the design.
7.4.3The leveling pad shall extend beyond the front and back of the base units by the dimension shown on the design, and not less than 6 inches in each direction.
7.4.4The leveling pad shall be compacted and screeded level so the first course of units seats fully and uniformly without rocking.
7.5 Backfill
7.5.1 Reinforced Fill
NOTE The reinforced fill is the soil within the geogrid-reinforced zone behind an MSE wall; its friction angle and compaction are design assumptions, so it must be a free-draining granular soil placed and compacted exactly as the design requires. (7.5.1.1)
☐ GW — Well-graded gravel
☐ GP — Poorly graded gravel
☑ SW — Well-graded sand
☐ SP — Poorly graded sand
☐ SM — Silty sand (with geotechnical engineer approval, fines limited)
7.5.1.2Reinforced fill within the geogrid zone shall be a free-draining granular soil meeting the gradation, friction angle, and plasticity limits of the engineered design.
7.5.1.3Reinforced fill shall have a fines content (material passing the No. 200 sieve) not exceeding the design limit, commonly 15 percent or less, because excess fines reduce drainage and friction and raise the risk of moisture-driven failure.
7.5.1.4Reinforced fill shall be free of organics, debris, and particles larger than the design maximum, which can damage the geogrid during placement.
7.5.2 Retained Fill
NOTE The retained fill is the soil behind the reinforced zone or behind a gravity wall; it shall meet the parameters assumed in the design and shall not be a high-plasticity expansive clay placed against the wall. (7.5.2.1)
7.5.2.2Retained fill shall meet the soil parameters assumed in the wall design for friction angle, unit weight, and drainage.
7.5.2.3High-plasticity clays (USCS CH) and other expansive soils shall not be placed as retained fill directly behind the wall unless the design specifically accounts for their swelling pressure and reduced strength.
7.6 Cap Units and Coping
NOTE The cap finishes the top of the wall, sheds water away from the wall face, and conceals the top course; an unsecured cap that is kicked off leaves the top course exposed and the wall looking unfinished. (7.6.1)
● Adhered with manufacturer-approved flexible concrete adhesive (standard)
○ Cast-in-place concrete coping (cast-in-place and mass concrete walls)
○ Mechanically pinned cap units (where provided by the system)
7.6.2Cap units shall be secured to the top course with a manufacturer-approved flexible concrete adhesive, or by the system's mechanical means, so they are not displaced by traffic, maintenance, or freeze-thaw.
7.6.3The top of the wall shall be detailed to shed surface water away from the wall face and away from the drainage aggregate behind it.
8 Drainage System
8.1 Drainage Behind the Wall
NOTE Inadequate drainage is the leading cause of retaining wall failure: water that accumulates in the retained soil generates hydrostatic pressure that the wall was very likely not designed to resist, and a saturated backfill also loses shear strength, so the drainage system is not an accessory but a primary structural element. (8.1.1)
NOTE A free-draining aggregate zone immediately behind the wall, a perforated collector pipe at its base, a filter separation from the retained soil, and a positive outlet together keep the backfill drained and the pressure on the wall at the active (drained) value the design assumes. (8.1.2)
● Drainage aggregate zone + perforated collector pipe + filter fabric + outlets (standard)
○ Drainage aggregate zone + perforated pipe to daylight, no fabric (clean free-draining backfill only)
○ Engineered subsurface drainage per geotechnical / wall design
8.1.3Every retaining wall shall be provided with a drainage system that prevents the buildup of hydrostatic pressure behind the wall, unless the wall is specifically designed to resist full hydrostatic load.
8.1.4The drainage system shall coordinate with Foundation Drainage where the wall drainage discharges into or shares the foundation drainage system. 8.2 Drainage Aggregate
NOTE A clean, open-graded crushed stone immediately behind the wall face provides a free-draining zone that intercepts water before it reaches the retained soil and conveys it to the collector pipe. (8.2.1)
● No. 57 stone (3/4 in. nominal, open-graded, clean)
○ No. 67 stone (3/4 in. to No. 4, open-graded, clean)
○ No. 8 stone (3/8 in. nominal, open-graded, clean)
8.2.2The drainage aggregate shall be a clean, open-graded crushed stone conforming to an ASTM D448 size such as No. 57 stone, with negligible fines so it drains freely.
8.2.3The drainage aggregate zone shall be placed immediately behind the wall face and within the SRW unit cores where the units are hollow, in a continuous zone not less than 12 inches wide measured from the back of the units.
8.2.4The drainage aggregate shall be placed and lightly compacted in lifts as the wall rises, kept free of fines and soil contamination, and not allowed to mix with the retained or reinforced fill.
8.3 Perforated Drainage Pipe
NOTE The perforated collector pipe at the base of the drainage aggregate gathers the intercepted water and carries it to an outlet; without a positive outlet the aggregate zone fills with water and the system fails. (8.3.1)
● 4 in.
○ 6 in. (long runs or high flow)
Daylight to grade at low points along the wall
Connect to site storm drainage system
Connect to foundation drainage system per [[sync/foundation-drainage]]
8.3.2A perforated drainage pipe not less than 4 inches in diameter shall be placed at the base of the drainage aggregate zone, behind and at or below the bottom of the lowest course of units.
8.3.3The drainage pipe shall be laid with positive slope, not less than 1 percent, to outlets that daylight to grade, connect to the site storm drainage, or connect to the foundation drainage system.
8.3.4Pipe outlets shall be located at the low points of the wall, spaced so no run is dead-ended, and shall be marked and protected so they are not buried or blocked.
8.4 Filter Separation
NOTE A geotextile filter fabric between the drainage aggregate and the retained soil lets water pass into the aggregate while holding back the soil fines that would otherwise migrate in and clog the drainage path over time. (8.4.1)
● Required — non-woven geotextile filter wrapping the aggregate/soil interface (standard)
○ Not required — reinforced fill is itself a clean, filter-compatible granular soil
8.4.2A geotextile filter fabric shall separate the open-graded drainage aggregate from the adjacent finer soil, unless the adjacent fill is itself a clean, filter-compatible granular material.
8.4.3The filter fabric shall be lapped at all seams in the direction of water flow and carried up the full height of the drainage aggregate zone so soil cannot enter the aggregate over the top.
9 Structural Stability and Loading
9.1 Factors of Safety
NOTE A retaining wall is checked against three external failure modes — sliding along its base, overturning about its toe, and bearing failure of the foundation soil — each with a required minimum factor of safety, and the wall is not adequate unless all are satisfied. (9.1.1)
NOTE IBC Section 1807 establishes the minimum static factors of safety of 1.5 against both sliding and overturning; bearing pressure shall not exceed the allowable bearing capacity. (9.1.2)
1.52.5
Default: 1.5 factor of safety
1.52.5
Default: 1.5 factor of safety
1.32
Default: 1.5 factor of safety
9.1.3The wall shall be designed for a minimum static factor of safety against overturning of 1.5.
9.1.4The wall shall be designed for a minimum static factor of safety against sliding of 1.5.
9.1.5The maximum bearing pressure beneath the wall shall not exceed the allowable bearing capacity established by the geotechnical engineer.
9.1.6The wall and the surrounding soil mass shall be designed for a minimum global (slope) stability factor of safety as established by the geotechnical engineer, commonly 1.5 for static conditions.
9.1.7Where seismic loads govern, the seismic factors of safety shall be those required by the adopted code and the geotechnical report, which are lower than the static values because the seismic load is a short-duration event.
9.2 Lateral Earth Pressure and Loading
NOTE The lateral earth pressure the wall resists depends on the soil parameters, the backfill slope, the surcharge, and whether the wall can yield (active condition) or is restrained (at-rest condition); using the wrong pressure coefficient, or omitting a surcharge, under-designs the wall. (9.2.1)
9.2.2The wall shall be designed for the lateral earth pressure determined from the soil parameters in the geotechnical report, the backfill geometry, and all surcharge loads acting within the zone of influence.
9.2.3Drained backfill that is positively drained may be designed for the active or at-rest earth pressure as appropriate to the wall's restraint; a wall without effective drainage shall be designed for the additional full hydrostatic pressure.
9.2.4Walls restrained at the top against rotation, such as walls tied to a structure, shall be designed for at-rest earth pressure rather than active pressure.
10 Installation
10.1 Leveling Pad and Base Course
NOTE The base course controls the alignment, batter, and level of the entire wall, so disproportionate care at the bottom pays off through the full height. (10.1.1)
10.1.2The Contractor shall excavate to the base elevation, observe and accept the foundation soil with the geotechnical engineer, and construct the leveling pad to a level, true, compacted surface.
10.1.3The first course of units shall be set on the leveling pad, leveled front-to-back and side-to-side, checked for alignment and batter, and seated fully so no unit rocks.
10.1.4Errors in the base course shall be corrected before proceeding, not carried up the wall.
10.2 Unit Placement
10.2.1Units shall be placed in running bond, each course set back to produce the design batter, and seated and aligned course by course.
10.2.2Each course shall be swept clean of debris and aggregate before the next course is placed so units seat fully and the courses do not wedge open.
10.2.3Hollow units shall be filled with drainage aggregate within the cores as each course is placed.
10.3 Geogrid Placement
NOTE Geogrid placed at the wrong elevation, too short, with gaps in coverage, or with the weak direction perpendicular to the wall, does not reinforce the soil even though it appears installed; the layout on the design is exact, not approximate. (10.3.1)
● Continuous (100 percent) coverage at each design elevation (standard)
○ Per design where partial coverage is specifically engineered
10.3.2Geogrid shall be placed at the elevations, lengths, and coverage shown on the engineered design, with the strong (machine) direction perpendicular to the wall face.
10.3.3Geogrid shall be laid flat, pulled taut to remove slack and folds, staked or tensioned before the reinforced fill is placed over it, and connected to the units by the system's connection detail.
10.3.4Equipment shall not run directly on the geogrid; fill shall be placed over the geogrid and tracked from the placed fill, not from the bare grid.
10.3.5Geogrid shall provide continuous coverage across the wall length at each design elevation, with no unreinforced gaps except where the design specifically engineers partial coverage.
10.4 Backfill Placement and Compaction
NOTE The reinforced and retained fill carry the design soil parameters only if compacted to the design density at the design moisture, and over-compaction immediately behind the face can also push the wall out of alignment, so compaction near the face is done with light equipment. (10.4.1)
9098
90929598
Default: 95 percent of Standard Proctor (ASTM D698) maximum dry density
6 in.
8 in.
Per wall design
● 3 ft behind the wall face (hand-operated equipment only)
○ Per wall design
10.4.2Reinforced and retained fill shall be placed in uniform lifts not exceeding the design lift thickness and compacted to the percent compaction shown on the design, commonly not less than 95 percent of Standard Proctor (ASTM D698) or as referenced to Modified Proctor (ASTM D1557) where the design so states.
10.4.3Within 3 feet of the wall face, compaction shall be performed with hand-operated equipment so the wall is not pushed out of alignment or batter.
10.4.4Heavy compaction equipment shall not operate within the zone behind the face reserved for light equipment, nor closer to the wall than the design permits.
10.4.5No fill shall be placed on frozen subgrade and no compaction shall be performed on frozen material.
10.5 Drainage Installation Sequence
NOTE The drainage system shall be built up with the wall — the collector pipe set at the base, the drainage aggregate placed and kept clean as each course rises, and the filter fabric placed so soil cannot enter the aggregate — because the drainage cannot be added after the wall is backfilled. (10.5.1)
10.5.2The Contractor shall place the perforated collector pipe at the base before backfilling begins, and shall place the drainage aggregate and filter fabric continuously as the wall rises.
10.5.3The Contractor shall keep the drainage aggregate free of soil and fines during placement and shall not allow the retained fill to contaminate the aggregate zone.
10.5.4The Contractor shall verify that every drainage outlet is open, sloped, and discharging before the wall is accepted.
11 Testing and Inspection
11.1 Foundation and Leveling Pad
11.1.1The geotechnical engineer or special inspector shall observe and accept the foundation bearing soil and the leveling pad before the first course of units is set.
11.1.2The leveling pad shall be verified level and true, and the base course shall be verified for alignment, level, and batter before the wall proceeds.
11.2 Geogrid and Reinforced Fill
11.2.1The special inspector shall verify the elevation, length, orientation, and coverage of each geogrid layer against the engineered design before it is covered with fill.
11.2.2Field compaction testing of the reinforced and retained fill shall be performed by an Owner-retained testing agency using nuclear density testing per ASTM D6938, with sand-cone verification, at the frequency directed by the geotechnical engineer.
1 test per lift per 50 lf of wall
1 test per lift per 100 lf of wall
Per geotechnical engineer
11.2.3Failing compaction tests shall require the affected lift to be reworked, re-compacted, and re-tested before placement continues.
11.3 Alignment and Tolerance
11.3.1The Contractor shall check wall alignment, batter, and level as the wall rises so that drift is detected and corrected early rather than accumulated.
● Plus or minus 1.25 in. over 10 ft (NCMA SRW guidance)
○ Per wall design
11.3.2Wall face alignment and batter shall be within the tolerance of the design, commonly plus or minus 1.25 inches over a 10-foot length for segmental walls.
11.3.3The as-built wall shall be surveyed to confirm alignment, top and bottom elevations, batter, and embedment conform to the design.
11.4 Drainage Verification
11.4.1The Contractor shall confirm, before acceptance, that the collector pipe is continuous and sloped, that every outlet daylights or connects as designed and is open, and that the drainage aggregate zone is continuous and uncontaminated.
12 Delivery, Storage, and Handling
NOTE SRW units, geogrid, drainage aggregate, and reinforcing must reach the wall undamaged and uncontaminated, because a cracked unit, a UV-degraded geogrid roll, or a fines-contaminated aggregate compromises the assumptions the design relied on. (12.1)
12.2SRW units shall be delivered and stored on stable, well-drained ground, protected from damage, and handled so units are not chipped or cracked.
12.3Geogrid shall be stored protected from prolonged ultraviolet exposure and physical damage, kept in its labeled rolls until installed, and not used if degraded, cut, or of unverified type.
12.4Drainage aggregate shall be stockpiled and handled so it is not contaminated with soil or fines that would reduce its permeability.
12.5Reinforcing steel shall be stored off the ground and protected from contaminants that impair bond, under Concrete Reinforcement. 13 Warranty
13.1The Contractor shall warrant the retaining wall against defects in materials and workmanship for the project warranty period beginning at substantial completion.
1 year from substantial completion
2 years from substantial completion
○ Required — survey monuments read at intervals per geotechnical engineer
● Not required on this project
13.2The Contractor shall warrant the retaining wall, including its alignment, drainage performance, and stability, for the project warranty period beginning at substantial completion.
13.3Warranty obligations shall include correction of wall movement, settlement, bulging, joint opening, drainage failure, and other defects attributable to non-conforming materials or workmanship.
13.4The warranty shall not relieve the Contractor of liability for concealed non-conforming work, nor limit the Owner's remedies for latent defects, including movement or failure attributable to inadequate drainage or backfill compaction discovered after the warranty period.
13.5Where directed by the geotechnical engineer, survey monuments shall be installed on the wall and read at specified intervals to provide objective data on wall movement during the warranty period.