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Height of the reinforced mass from the top of the levelling pad to the top of the wall.
Measure the design height, which includes any embedded courses below finished grade. Toe embedment provides passive resistance at the base but does nothing for a sliding plane part way up the wall, which is exactly why the upper levels can govern.
Uniform vertical spacing at which grid layers are placed up the wall.
Spacing normally lands on a whole number of block courses or compaction lifts. Where a design uses variable spacing — closer at the base, wider at the top — run this at the tightest spacing to locate the critical level, then confirm that level by hand against the real layout.
Horizontal length of each grid layer, which is also the width of the reinforced block being checked.
The reinforced mass behaves as a coherent block of this width, so the length that resists sliding is the same length that carries the weight. Truncated or stepped layouts break that assumption and need the sliding plane checked against the actual length at each level.
Average density of the compacted reinforced fill together with the facing units it stands behind.
Facing units are denser than the fill behind them, so a wall with a wide block and a narrow reinforced zone sits above the fill's own figure. Where the two differ materially the honest approach is to weight them by their share of the width rather than to use the fill alone.
Uniformly distributed load carried on the retained soil behind the reinforced zone.
A surcharge behaves very differently from the soil's own weight in this check: it adds to the driving thrust at every level without adding to the mass that resists sliding at any of them, so it hurts the upper levels proportionally more than the base.
Angle of internal friction of the retained soil behind the reinforced zone.
This is the retained soil's property, not the reinforced fill's, and the two are frequently different materials on the same job. A geotechnical report is the proper source; a low value quietly raises the thrust at every level at once.
Coefficient of interaction for sliding along a grid layer, from the manufacturer's direct-shear testing.
A grid layer creates a plane of reduced friction through the fill, and how reduced depends on the aperture geometry and on the fill gradation it was tested against. Test data developed with a well-graded sand does not transfer to an open-graded drainage stone.
Coefficient for sliding of the whole wall across its levelling pad.
This is a block-on-aggregate or block-on-concrete interface and is usually a different number from the grid one. Whether the wall slides at the pad or at a grid layer is decided by which of the two coefficients falls furthest short of the height it has to resist.
Governing sliding factor of safety
1.97 (factor of safety)
The levelling pad governs: the whole wall slides before any single grid layer does. Levels are numbered from the base upward, and the factor rises as you go up because less mass sits above each successive plane.
- Factor of safety against sliding at the levelling pad
- 1.97 (factor of safety)
- Factor of safety at reinforcement level 1
- 2.4 (factor of safety)
- Factor of safety at reinforcement level 2
- 2.71 (factor of safety)
- Factor of safety at reinforcement level 3
- 3.13 (factor of safety)
- Factor of safety at reinforcement level 4
- 3.69 (factor of safety)
- Factor of safety at reinforcement level 5
- 4.5 (factor of safety)
- Reinforcement levels in the wall
- 7 levels
- Driving thrust at the governing plane
- 5,363.05 lbf/ft
- Active earth pressure coefficient
- 0.31 (Ka)
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Reinforced SRW Internal Sliding Check: 1.97 (factor of safety) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- NCMA Design Manual for Segmental Retaining Walls sets out the internal stability checks for a geogrid-reinforced SRW, of which sliding of the mass above a reinforcement level is one; connection strength and pullout are separate checks it also requires and this page does not perform
- Rankine active earth pressure, Ka = tan squared (45 degrees minus phi/2), with thrust on the back of the reinforced mass taken as 0.5 x Ka x gamma x h squared plus Ka x q x h for a uniform surcharge
- Sliding resistance at a reinforcement level is the weight of the mass above it times a coefficient of interaction from the geogrid manufacturer's direct-shear data — that is a tested interface property, not a soil friction angle, and it is entered here rather than derived
Inputs used
- Total Wall Height
- 15 ft
- Vertical Spacing Between Reinforcement Levels
- 24 in
- Reinforcement Length Into the Fill
- 10.5 ft
- Unit Weight of the Reinforced Mass
- 121.73 pcf
- Uniform Surcharge Behind the Wall
- 250.63 psf
- Retained Soil Friction Angle (degrees)
- 32
- Interface Friction Coefficient at the Grid
- 0.6
- Friction Coefficient at the Levelling Pad
- 0.55
Intermediate steps
- Factor of safety against sliding at the levelling pad
- 1.97 (factor of safety)
- Factor of safety at reinforcement level 1
- 2.4 (factor of safety)
- Factor of safety at reinforcement level 2
- 2.71 (factor of safety)
- Factor of safety at reinforcement level 3
- 3.13 (factor of safety)
- Factor of safety at reinforcement level 4
- 3.69 (factor of safety)
- Factor of safety at reinforcement level 5
- 4.5 (factor of safety)
- Reinforcement levels in the wall
- 7 levels
- Driving thrust at the governing plane
- 5,363.05 lbf/ft
- Active earth pressure coefficient
- 0.31 (Ka)
Confidence note: The levelling pad governs: the whole wall slides before any single grid layer does. Levels are numbered from the base upward, and the factor rises as you go up because less mass sits above each successive plane.
What this calculation does not cover
- Sliding only. Connection capacity, grid tensile rupture, pullout beyond the failure surface, bearing and global stability are separate checks this page does not perform.
- Assumes a level backslope, a uniform surcharge and a single reinforcement length at every level. A broken backslope or a truncated layout changes both the thrust and the resisting mass.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-30 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations3
- NCMA Design Manual for Segmental Retaining Walls sets out the internal stability checks for a geogrid-reinforced SRW, of which sliding of the mass above a reinforcement level is one; connection strength and pullout are separate checks it also requires and this page does not perform
- Rankine active earth pressure, Ka = tan squared (45 degrees minus phi/2), with thrust on the back of the reinforced mass taken as 0.5 x Ka x gamma x h squared plus Ka x q x h for a uniform surcharge
- Sliding resistance at a reinforcement level is the weight of the mass above it times a coefficient of interaction from the geogrid manufacturer's direct-shear data — that is a tested interface property, not a soil friction angle, and it is entered here rather than derived
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