SRW Geogrid Layer Spacing and Layout

The spacing limit is set by the BLOCK, not the soil, and three independent standards say the same thing: no more than twice the block depth, and never past 32 in (NCMA 7.2.2 also suggests 24 in, AASHTO 11.10.2.3.1 says 2 x Wu or 2.7 ft, FHWA NHI-10-024 4.4.7.d says 2 x depth or 32 in). Grid lands on a course JOINT, so the limit divides into whole courses and rounds DOWN -- a 9 in deep by 8 in tall unit is limited to 18 in but works at 16, and a 10 in deep by 6 in tall unit is limited to 20 but works at 18. Reports the levels, where the first and top ones sit, the unreinforced facing left above the top layer (the height AASHTO asks you to check for bulging), grid length at 0.6H with a 4 ft floor, square feet per linear foot, and the compaction lifts -- 8 in loose maximum, the rule most often broken on a fast job. Layout and quantity only: it does not size the grid, and a wall over 4 ft needs an engineered design.

spacing limit = min(2 x block depth, 32 in, 2.7 ft, suggested max); working spacing = floor(limit / block height) x block height; levels = floor((floor(H/hb) - 1) / courses_per_layer) + 1, first level one course above the base; unreinforced crest = H - top level height; grid length = max(0.6 H, 4 ft); grid area = levels x length; compaction lifts = ceil(spacing / 8 in).

NCMA Design Manual for Segmental Retaining Walls, 3rd edition, Section 7.2.2; AASHTO LRFD Bridge Design Specifications 2016, Section 11.10.2.3.1; FHWA NHI-10-024, Section 4.4.7.d. All three are quoted side by side in Keystone Retaining Wall Systems TIS-15 (2021), whose two published spacing examples this tile reproduces exactly.

The spacing rule is stated identically in three independent design documents and is reproduced in manufacturers' free technical bulletins. No proprietary geogrid strength table, reduction factor, or connection curve is reproduced here - none is used.

Estimate. AHJ and licensed professional govern.

Field names used by the API: wall_height_ft, block_depth_in, block_height_in, ncma_suggested_max_in, grid_length_basis, spacing_limit_in, actual_spacing_in, courses_per_layer, layer_count, top_layer_height_in, unreinforced_crest_in, grid_length_ft, compaction_lifts

• Twice the unit depth the controlling spacing rule in all three standardsNCMA 7.2.2 / AASHTO 11.10.2.3.1 / FHWA NHI-10-024 4.4.7.d
• Absolute ceiling 32 in (NCMA, FHWA); AASHTO 2.7 ft = 32.4 in; the most restrictive is appliedthe three cited sections
• NCMA suggested maximum 24 in, editable; reduces construction stability issuesNCMA 7.2.2
• Whole-course rounding grid lands on a joint, so the limit rounds DOWN to a course multipleKeystone TIS-15 worked examples
• Minimum grid length 0.6 times wall height, with a 4 ft practical floorNCMA global-stability minimum
• Compaction lift 8 in loose maximum regardless of grid spacingNCMA 7.2.2
• Not a strength design no LTDS, pullout, connection, or global-stability checkstated scope limit