Retaining Wall Takeoff: Block Count, Base Width, Geogrid, Drainage, and the Code Checks

Block count with the batter and the tiers, the base width and the base stone, the geogrid layers and length, the pipe, the gravel, the fabric, the weep holes, the cost per foot, and the height line where a wall becomes an engineered drawing.

A gravity retaining wall is a mass, a base, and a drainage package. The manufacturer tables give the maximum height at a base width, and the base width is 0.4 to 0.6 of the wall height with a 1 ft key in front and 12 in of compacted stone under the wall. Batter is 1 in of setback per 12 in of height, which shortens the face area as the wall goes up, and the tiers are the block courses with the half-block offset at the corners and the returns.

—Block count with batter, tiers, and waste
—Base width, key, and base stone
—Geogrid layers, length, and square feet
—Pipe, gravel, fabric, and weep holes
—Material, installed, and cost per foot
—Height line, surcharge, fence, and permit
How the math works
  • Face area and blocks: face sq ft = length times height, and blocks per sq ft are 1.125 on an 8 in block, 0.9 on a 10 in, 0.75 on a 12 in, 0.56 on a 16 in. Batter at 1 in per 12 in of height reduces the face area by up to 12 percent on a tall wall, and 8 percent is ordered for the cuts, corners, returns, and breakage.
  • Base width: 0.4 times the wall height plus a 1 ft key in front on a wall with no surcharge, 0.45 to 0.6 times the height with a driveway, a patio, a pool, and a structure surcharge, with 4 in of compacted 3/4 in clear stone under the wall and the key at 95 percent compaction.
  • Geogrid: layers starting at 2 ft up at every 8 in of wall height, layer length of 0.8 times the wall height with an 8 ft minimum on a wall with no surcharge, and 0.8 to 1.2 times the height with a surcharge, in 6 in of cover on a level compacted base with the facing side pinned to the block.
  • Drainage: 4 in perforated pipe with a fabric sock pitched 1/8 in per ft to daylight, 12 in wide column of 3/4 in clear washed stone behind the full height, non-woven fabric behind the block and up the slope, weep holes at 4 ft on center at the first course, at the corners, and at the ends of returns.
  • Cost: material at 4.50 to 9 dollars per linear foot per foot of wall height on an 8 in block wall with the base, the grid, and the drainage, installed at 26 to 32 dollars per sq ft of face on a 4 ft wall, 32 to 40 on 5 to 6 ft, and 40 to 55 above 6 ft, with excavation at 45 to 75 dollars per cubic yard and an engineered drawing at 1,500 to 4,500 dollars.
Retaining wall takeoff tool on published segmental gravity wall practice with manufacturer base-width and height tables, standard geogrid spacing and length bands, and standard drainage detail. Manufacturer height tables by block face and base width, the geogrid schedule on the manufacturer design chart, a wall over 4 ft in height, a wall over 4 ft with a permit, a wall over 6 ft, a wall with a surcharge within the failure wedge, a wall on a slope, a wall with a slide history, a wall in a seismic design category, a wall in a flood hazard area, a wall on a property line, a wall over a utility, a wall over a septic system and a drain tile, a wall with a required engineered drawing and a stamped calculation, a wall with a required geotechnical report, a required soils compaction test, a required inspection, a required fence at a 30 in drop with a 4 in sphere clearance and a 36 in height, a barrier at a pool with a 48 in height and a self-closing gate, a required guard at a walkway, a required drainage outlet with a required connection to a storm system, a required permit at a slope, a required notice to a neighbour, a required tree and a required easement clearance, a wall in a historic district, and a wall on a shared driveway are all set by the authority having jurisdiction, the manufacturer design manual, a licensed design professional, and the geotechnical engineer of record. Overturning, sliding, bearing capacity, global stability, a seismic pseudo-static check, a compound failure surface, a wedge analysis, a surcharge on the engineered chart, a soil nail wall, an anchored wall, a cantilever retaining wall, a soil grid wall, a gabion, a crib wall, a bin wall, a reinforced soil slope, and a mechanically stabilized earth wall are engineered design and a stamped drawing. Block, grid, aggregate, excavation, and installed prices vary by region, by block brand, and by the distance to the quarry, and a wall quote with no drainage line in it is a wall quote with a failure line in it.

How many blocks do I need for a 4 ft by 60 ft retaining wall?

About 265 face blocks on an 8 in block system: 240 sq ft of face at 1.125 blocks per sq ft, reduced about 8 percent for the 1 in per 12 in batter, plus 8 percent for the cuts, corners, returns, and breakage, plus 60 cap blocks on the 60 ft run. A wall with a corner and two returns takes 12 percent more blocks than the face area suggests.

How wide does a retaining wall base need to be?

0.4 times the wall height plus a 1 ft key in front on a wall with nothing above it, and 0.45 to 0.6 times the height with a driveway, a patio, a pool, or a structure in the surcharge zone. A 4 ft wall is a 2.6 to 3.4 ft base with 4 in of compacted 3/4 in clear stone under it, in a level trench with the base compacted to 95 percent.

When does a retaining wall need geogrid?

At the manufacturer gravity height limit, which is 3 to 4 ft on most 8 in face block systems with a granular backfill and a 12 in base, and at any surcharge within 3 to 5 ft of the top of the wall: a driveway, a patio, a pool, a structure, a road, and a fill. Layers start at 2 ft up at every 8 in of height, and each layer is 0.8 times the wall height with an 8 ft minimum, longer with a surcharge.

Do I need drainage behind a retaining wall?

Yes, and it is the line that decides wall life: 4 in perforated pipe with a fabric sock pitched 1/8 in per ft to daylight, a 12 in wide column of 3/4 in clear washed stone behind the full wall height, non-woven filter fabric behind the block and up the slope, and weep holes at 4 ft on center at the first course. A saturated clay backfill behind a 4 ft wall is a hydrostatic load, and hydrostatic load is what bulges a wall in its first wet winter.

How tall can a retaining wall be without an engineer?

Most jurisdictions require a permit at 4 ft of vertical rise and many require an engineered wall at 4 ft, and a wall over 6 ft is engineered everywhere with a stamped drawing, a grid schedule, a base detail, a drainage detail, a compaction spec, and soils inspection. The manufacturer gravity chart on the specific block with the specific base width and a granular backfill is the un-engineered height line, at 3 to 4 ft on most face systems.

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