The job
Modelled, not recorded. Every figure on this page is re-run through the calculator it names whenever the site is built; the estimate is worked, but no job was carried out, so the last section gives the mechanism of each likely error rather than a measured overrun.
A segmental retaining wall is bought by the pallet and built mostly out of things nobody sees again. Of the quantities on this page — blocks, pad stone, geogrid, drainage stone, filter fabric, perforated pipe, solid outlet pipe, the compaction behind the face and the cap — only the blocks and the cap show once the lawn has grown back, and most of the rest hang on one decision made before a single unit is counted: how much of the wall stands below the ground as well as above it. The calculators run in the order that decision travels through the job, so each takes its inputs from a step before it.
The job: one straight 40 ft (12.19 m) run of 18 × 6 × 12 in (457 × 152 × 305 mm) segmental units, standing 3 ft (0.91 m) out of a lower lawn that is level along the wall and falls away beyond one end, with one course buried beneath it, cut into the toe of a bank whose top is lawn and carries nothing else. The wall sits on a 6 in (152 mm) compacted crushed-stone pad and 12 in (305 mm) of clean drainage stone stands behind the block. A perforated pipe along the heel falls towards the end where the lawn drops away — the outlet end — and hands its water to a solid outlet that comes out at the surface 25 ft (7.62 m) beyond it.
THE BURIED COURSE IS WHY THE WORKING HEIGHT IS 3.5 FT, NOT 3. The block count, the geogrid, the drainage stone and the lift count all run on the full height of block from the pad up, 3.5 ft (1.07 m). The drainage and geogrid calculators say so in their own field help; the block calculator asks for the exposed height and counts a buried course only when it is entered, so here it is entered. Feeding the exposed 3 ft (0.91 m) into any of them loses at least a seventh of what it counts, and loses it at the bottom of the wall, where the soil pushes hardest and the water collects.
THE DESIGN IS NOT ON THIS PAGE. Whether 3.5 ft (1.07 m) of this unit stands on its own weight or needs grid depends on the unit's depth, the soil and what sits above, and the block manufacturer's chart for that unit decides it; the model assumes grid, the heavier case and the one that sets how far back the dig goes. The site's preliminary stability check cannot stand in for that chart. It asks for the wall's self-weight, which needs the weight of the unit, and this model has none; and it checks a gravity wall, where a gridded wall stands or falls as a reinforced block of soil. The site's retaining-wall footing calculator sizes a concrete footing, and a segmental wall sits on a stone pad. Many building departments draw an engineering and permit line at about 4 ft (1.2 m), but they differ on where it is measured from: measured from the bottom of the pad, this wall is 4 ft (1.2 m) before the cap, and over that line once the cap is on.
WHERE THE FIGURES COME FROM, AND WHERE THEY STOP. The wall is a model built from the measurements below, and each quantity is what the calculator linked at its step returns on the inputs printed with it. Four things the job needs have no calculator on the site and appear without a number where they fall: the cut into the bank, imported reinforced fill if the dug soil fails the specification, the adhesive under the cap, and the pipe fittings. The last section says which of these figures a real wall is likely to beat or miss, and why: a reasoned direction, since no wall was built to measure against.
What was measured, and how
Length of the face line, end to end
40 ft (12.19 m), straight, full height, square ends
Pulled along a string set at the front face of the base course, not along the crest of the bank or the back of the wall, both of which wander. The model runs the full 40 ft (12.19 m) at full height with square ends; a real bank usually needs its ends turned back into the slope or stepped down with it, and either one changes the block count.
Exposed height, from the finished grade in front
3 ft (0.91 m) exposed as six 6 in (152 mm) courses, plus one buried course: 3.5 ft (1.07 m) of block
Measured from the finished level of the lower lawn, not the ground as it stands before digging, up to the top of the last full course, with the cap above that. One course is then added for embedment. Carrying the exposed figure into the later steps is how the stone, the grid and the blocks all come up a course short together.
The unit, measured off a block rather than a brochure
18 in face × 6 in high × 12 in deep (457 × 152 × 305 mm)
Face width, height and depth taken with a tape from a unit on the pallet. The face sets the count, the height sets the courses and the grid spacing, and the depth sets the pad width. A rounded nominal size, or a catalogue page for a sister unit in the same range, moves all three at once.
A daylight point below the heel
25 ft (7.62 m) beyond the outlet end of the wall
Levels taken with a laser from the heel drain's planned invert at the outlet end — pad-top level less the fall the heel run needs — out across the lower lawn where it drops away beyond that end, until the ground has fallen far enough for the outlet, laid to a fall of its own, to come out at the surface. An outlet picked by eye can sit higher than the heel it drains, and then the pipe holds water against the wall instead of taking it away.
What stands on the ground behind the wall, and what the soil is
Level lawn, no surcharge; native soil taken as granular, low in clay and free of roots and debris, reused as the reinforced fill behind the stone zone
The bank is walked back well beyond the wall's height, looking for anything that loads it: more slope rising above, a path, a parking space, a shed base. A lawn that will carry a hot tub or a car later is a surcharge now; it changes the design, and with it the grid length and layer count this page works out. The soil dug on the way in is checked too, because the model sends it back as the fill the grid is laid in, and that only works if it is granular and low in clay, as the block manufacturer's specification for the reinforced zone requires.
The takeoff, in order
Each step needs something from the one before it, which is why the order is part of the answer.
Count the blocks, buried course included
- Needs
- The 40 ft (12.19 m) face line and the 18 × 6 in face from the measurements, with the height entered as 3.5 ft (1.07 m) — the exposed 3 ft plus the buried course — because this calculator's height field leaves the buried course out unless it is put in.
- Produces
- 206 blocks over 7 courses. The wall is 140 sq ft (13.0 m²) of face, or 186.7 units at 0.75 sq ft (0.07 m²) each, and the default cut-and-breakage allowance takes it to 206. The seven courses, and the 3.5 ft (1.07 m) they stand to, are the height every later step works on. The cap course is not in the count: it is a separate product sold by length, measured in the last step, and the adhesive that holds it down is counted by no calculator here.
Result206 blocks over 7 courses (18 × 6 in / 457 × 152 mm face)
Retaining Wall Block CalculatorSize the levelling pad under the buried course
- Needs
- The buried course from the block count, which puts the pad one course below the lower lawn, and the 12 in (305 mm) unit depth it was counted on, carried 6 in (152 mm) past the unit at front and back for a 24 in (610 mm) pad — over the same 40 ft, at a 6 in compacted thickness.
- Produces
- 2.75 US tons (2.49 metric tons) of dense-graded crushed stone over 80 sq ft (7.43 m²), at the calculator's compacted density of about 137 lb/ft³ (2,200 kg/m³) — the 1.85 US tons per cubic yard a highway department pays on for compacted dense-graded base. The figure is the in-place mass and carries no allowance. The pad's top is the level the rest of the job is measured up from: the grid height, the drainage stone and the lifts all start there, and the heel drain starts on it at the far end before falling below it towards the outlet end.
Result2.75 US tons (2.49 metric tons) of base stone, 80 sq ft (7.43 m²) at 6 in (152 mm)
Gravel Base Layer Tonnage CalculatorCount the geogrid layers, and with them the depth of the dig
- Needs
- The 3.5 ft (1.07 m) of block from the count, measured up from the pad top set in the step before; two 6 in courses as the spacing, 12 in (305 mm), because grid can only lie on a bed joint; and a 3 ft (0.91 m) grid length — the calculator's starting rule of 0.7 × height, 2.45 ft (0.75 m), taken up to the next whole foot.
- Produces
- 4 layers, and 12 ft of grid for every foot of wall (3.66 m per m), which is 480 sq ft (44.6 m²) along the 40 ft (12.19 m) before any roll end is wasted. The calculator spreads the four evenly at 10.5 in (267 mm), which a wall of 6 in courses cannot build. Kept to bed joints no more than two courses apart, they go on top of the first, third, fifth and sixth courses: a fourth on the seventh would lie in the cap's adhesive bed, and a manufacturer's chart that stops the grid at the fifth course would ask for three. The 3 ft is also the least distance back from the face the dig has to reach, which is why the grid is settled before any stone behind the wall is counted.
Result4 layers, 12 ft of grid per foot of wall (3.66 m per m)
Geogrid Reinforcement Layer Count CalculatorQuantify the drainage stone behind the block
- Needs
- The full 3.5 ft (1.07 m) of block from the first step — buried course included, since the bottom of the zone is the part that has to drain — the pad from step 2 that the front half of the zone stands on, and a 12 in (305 mm) width that sits inside the 3 ft grid zone from step 3.
- Produces
- 5.70 yd³ (4.36 m³) of clean angular stone, from 5.19 yd³ (3.96 m³) of zone before the calculator's spillage and settlement allowance. The cores of a hollow unit take the same stone and are not in this figure; neither is any width the cut leaves beyond 12 in (305 mm), nor the stone in the channel that lets the heel drain fall below the pad top in step 7. The zone's back face and top are the area the fabric in the next step has to cover.
Result5.70 yd³ (4.36 m³) of drainage stone, 12 in (305 mm) wide over the full 3.5 ft
Retaining Wall Drainage Gravel CalculatorQuantify the filter fabric between the stone and the soil
- Needs
- The drainage zone from the step above: its back face, 40 × 3.5 ft (12.19 × 1.07 m), where it meets the retained soil, plus its top, 40 × 1 ft (12.19 × 0.30 m), where the fabric folds over before the soil cap goes on — 180 sq ft (16.7 m²) in all.
- Produces
- One roll. The calculator was written for weed barrier under beds; here it reads as the separator that keeps the retained soil from washing into the stone, and it counts area and 300 sq ft (27.9 m²) retail rolls, not the grade of fabric, which the block manufacturer's detail names. The laps are set at 20% rather than the default 10% because the strips run the short height of the zone and are joined as the wall rises, giving 216 sq ft (20.1 m²) — and the count stays at one roll even at the field's 40% ceiling.
Result1 roll (216 sq ft / 20.1 m² to cover, laps included)
Landscape Fabric CalculatorCount the perforated pipe along the heel
- Needs
- The foot of the stone zone from step 4, where the pipe runs just behind the back edge of the step-2 pad in the rear half of the zone, and the full 40 ft (12.19 m) run, entered in the calculator's perimeter field — which here is one straight line with no corners.
- Produces
- 4 lengths of 10 ft (3.05 m) rigid perforated 4 in (102 mm) pipe, laid holes down and bedded in the stone. The calculator was written for footing drains round a house; on a straight run its perimeter is simply the length of pipe. Four is the count for lengths butted end to end. Rigid perforated pipe commonly comes bell-ended, each of the three joints swallows a socket's depth of the next length, and the calculator's own FAQ says to add that take-up to the run before counting — any take-up at all carries 40 ft past four lengths, so four is a known shortfall and the order is a fifth length. The bend at the outlet end and the cap at the far end are fittings it does not include.
Result4 lengths of 10 ft (3.05 m) perforated pipe, butted
Perimeter Weeping Tile Pipe Segment CalculatorSet the heel drain's fall towards the outlet end
- Needs
- The 40 ft (12.19 m) heel run and the 4 in (102 mm) pipe from the step above — the size that puts it in the calculator's band for pipe larger than 3 in — with the fall running towards the outlet end, where the lower lawn drops away.
- Produces
- 5 in (127 mm) of drop from end to end. The calculator gives the plumbing-code minimum for building drains; a land drain behind a garden wall is not under that code, and the figure is borrowed here as a floor, not a rule. The pipe starts on pad-top level at the far end, where it is capped, and finishes 5 in below it at the outlet end, 11 in (279 mm) under the lower lawn: that invert is where the outlet in the next two steps begins. The channel the pipe lies in deepens to match, and the stone that fills it is extra to the step-4 figure.
Result5 in (127 mm) of drop over the 40 ft (12.19 m) heel run
Drain Pipe Slope CalculatorCount the solid outlet pipe to daylight
- Needs
- The outlet end of the heel run from step 6, at the depth step 7 left it, and the 25 ft (7.62 m) to the daylight point found in the measurements, in the same 10 ft (3.05 m) lengths — but solid-wall 4 in (102 mm) rigid drain pipe, 4.215 in (107 mm) outside diameter, because a perforated outlet would hand the water back to the ground at the toe of the wall.
- Produces
- 3 lengths of 10 ft (3.05 m) solid pipe, with 5 ft (1.5 m) in hand after the cut, enough to cover its own bell joints. It is the same division as the heel run, and the diameter plays no part in it; the outside diameter matters to the trench in the next step. The outlet needs a fall of its own, at no less than the heel's rate, which is why the daylight point sits where the lawn has dropped further than the heel is deep. The rodent guard on the open end is a fitting and is not counted here.
Result3 lengths of 10 ft (3.05 m) solid pipe
Perimeter Weeping Tile Pipe Segment CalculatorSize the outlet trench and what goes back into it
- Needs
- The 25 ft (7.62 m) outlet run and the pipe's 4.215 in (107 mm) outside diameter from the step above — the field takes the outside diameter, not the 4 in nominal size — in a spade-width trench 8 in (203 mm) wide, far narrower than the field's help suggests because nobody climbs into a trench this shallow. Its depth follows the pipe: 11 in (279 mm) below the lower lawn where it leaves the wall, the invert step 7 set, shallowing to about the pipe's own 4 in (102 mm) at the daylight point, 7.5 in (191 mm) on average. For a straight taper at one width the average depth gives the true volume.
- Produces
- 0.34 yd³ (0.26 m³) of loose fill by the calculator's reckoning, out of 0.39 yd³ (0.29 m³) dug. That headline assumes imported granular fill, bulked for compaction. Here the trench's own spoil goes back instead: the void to refill is 0.30 yd³ (0.23 m³) compacted, the dug volume less the pipe, so the spoil covers it with some to spare even after dug earth shrinks as it is packed back, and nothing is brought in. This is the only earthwork on this page run through an excavation calculator: the cut into the bank behind the wall is a wedge whose size depends on the slope's profile, and no calculator here computes its volume.
Result0.34 yd³ loose (0.26 m³) to backfill round the outlet pipe
Trench Excavation & Backfill Volume CalculatorCount the compaction lifts behind the wall
- Needs
- The 3.5 ft (1.07 m) of fill from the pad top in step 2 to the top of the block in step 1, placed one course at a time so every lift finishes on a bed joint where the step-3 grid can lie flat, with the step-4 stone and the step-5 fabric rising alongside it. The course height, 6 in (152 mm), is entered as the lift: the calculator reads it as a loose thickness, and here it is the compacted height each lift has to finish at.
- Produces
- 7 lifts, one to each course. The calculator's own note warns that entering a compacted lift where it expects a loose one under-counts; here the course fixes the count instead, so each lift is placed a little proud of its joint and compacted down to it, which keeps it near the 6 to 8 in (150 to 200 mm) the field's help gives for a hand-operated plate. The stone zone and the reinforced fill behind it come up together behind the course just laid, with the plate kept off the units, so the lift count is also the number of stops in the programme: no course goes on until the one below it is filled and compacted. A thicker lift would leave its lower part soft, and once the next course is on, nothing reaches it again.
Result7 lifts, one to each 6 in (152 mm) course
Backfill Compaction Lift Count CalculatorMeasure the cap along the top course
- Needs
- The top of the seventh course from step 1, which the last of the step-10 lifts finishes level with, over the same 40 ft (12.19 m) face line, at the calculator's default allowance for the cuts at the two square ends.
- Produces
- 42 ft (12.80 m) of cap, the form the block calculator's own FAQ says cap units are sold in: by linear length. The calculator was written for parapet coping, and only its arithmetic — the run plus a cut allowance — carries over; its notes on metal coping, movement joints and inward fall belong to a roof, not a garden wall. The cap goes on last, bonded to the top course with an adhesive no calculator on the site counts, once the soil cap over the fabric fold behind it is in.
Result42 ft (12.80 m) of cap along the top course
Parapet Coping Cap Linear Footage Calculator
The figures
| Step | Calculator | Figure |
|---|---|---|
| Count the blocks, buried course included | Retaining Wall Block Calculator | 206 blocks over 7 courses (18 × 6 in / 457 × 152 mm face) |
| Size the levelling pad under the buried course | Gravel Base Layer Tonnage Calculator | 2.75 US tons (2.49 metric tons) of base stone, 80 sq ft (7.43 m²) at 6 in (152 mm) |
| Count the geogrid layers, and with them the depth of the dig | Geogrid Reinforcement Layer Count Calculator | 4 layers, 12 ft of grid per foot of wall (3.66 m per m) |
| Quantify the drainage stone behind the block | Retaining Wall Drainage Gravel Calculator | 5.70 yd³ (4.36 m³) of drainage stone, 12 in (305 mm) wide over the full 3.5 ft |
| Quantify the filter fabric between the stone and the soil | Landscape Fabric Calculator | 1 roll (216 sq ft / 20.1 m² to cover, laps included) |
| Count the perforated pipe along the heel | Perimeter Weeping Tile Pipe Segment Calculator | 4 lengths of 10 ft (3.05 m) perforated pipe, butted |
| Set the heel drain's fall towards the outlet end | Drain Pipe Slope Calculator | 5 in (127 mm) of drop over the 40 ft (12.19 m) heel run |
| Count the solid outlet pipe to daylight | Perimeter Weeping Tile Pipe Segment Calculator | 3 lengths of 10 ft (3.05 m) solid pipe |
| Size the outlet trench and what goes back into it | Trench Excavation & Backfill Volume Calculator | 0.34 yd³ loose (0.26 m³) to backfill round the outlet pipe |
| Count the compaction lifts behind the wall | Backfill Compaction Lift Count Calculator | 7 lifts, one to each 6 in (152 mm) course |
| Measure the cap along the top course | Parapet Coping Cap Linear Footage Calculator | 42 ft (12.80 m) of cap along the top course |
The waste factors, and why these ends of the ranges
| Material | Applied | Why |
|---|---|---|
| Blocks | The calculator's default cut-and-breakage allowance and no more, because the modelled wall is straight with square ends | The field's help says a straight wall between two square ends cuts few blocks, and the default 10% is left alone on that footing; curves, steps in the base and outside corners are where it asks for fifteen or more, because splitting a unit on site usually spoils one of the two pieces. The model has none of those. Turning the ends back into the bank would add corners and split units, and the allowance would have to rise with them. |
| Levelling pad stone | The computed tonnage, rounded up to the supplier's delivery increment, with the trench floor checked before ordering | The calculator adds nothing for spillage or over-dig: it is the in-place mass of a perfect 6 in (152 mm) layer. The pad fills whatever the trench floor actually is, and a base trench dug along 40 ft (12.19 m) is rarely dead level, so the check that earns its keep is a laser across the floor before the stone is ordered, not a percentage added afterwards. |
| Drainage stone | The calculator's spillage and settlement allowance, with unit cores and the heel channel added from their own figures rather than guessed | The 10% default covers stone that settles as it is tamped and what spills in front of the wall. It does not cover a cut taken wider than the 12 in (305 mm) zone, which the field's help says to measure rather than raise the allowance for; the channel that deepens under the heel drain towards the outlet end; or the cores of a hollow unit, which hold a volume that belongs to the particular block. |
| Geogrid | Whole pieces cut to the full design length, never made up from roll ends | The calculator's 12 ft of grid per foot of wall is the exact area of four 3 ft (0.91 m) layers and knows nothing about roll widths or lengths. On the uniaxial grid block manufacturers usually specify, the strength runs along the roll, so each piece is unrolled from the face back into the fill and cut at 3 ft. A roll end shorter than that is waste, not grid, and the last piece in each layer is usually a part-width offcut. |
| Filter fabric | Laps raised above the calculator's default, then whole rolls | Strips run the short height of the zone and are joined as the wall rises, so laps take a bigger share than on a wide bed; the 20% used here reflects that. The roll count does not move either way — 180 sq ft (16.7 m²) plus laps fits one 300 sq ft (27.9 m²) roll even at 40%. Where the block manufacturer names a particular non-woven, the roll it comes in replaces the calculator's retail roll. |
| Drain pipe | Whole 10 ft (3.05 m) lengths, with the heel run's exact count raised by one length for its bell joints | Forty feet (12.19 m) of heel is exactly four lengths butted end to end, and bell-ended pipe loses a socket's depth at every joint, so four cannot reach; the fifth length covers that take-up, and its spare is a part length, not cover for a whole one cracked in handling. The outlet has 5 ft (1.5 m) in hand for its own joints. The fittings — the bend at the outlet end, the end cap at the far end, any couplers and the rodent guard at daylight — come from the layout, not from any calculator. |
| Cap units | The coping calculator's default cut allowance on a straight run, then whole units | Its 5% default is written for straight parapet runs, and a straight garden wall with square ends is the same case: a cut unit at each end and little else. A curved or stepped top, or ends returned into the bank, cuts more units and needs more. The length converts to units at the cap's own laid length, taken off a unit like the block face was. |
Where this estimate is most likely to be wrong
The cut into the bank behind the wall
usually underNothing on this page computes it, and it is the largest earthwork on the job. The pad trench under the units goes down to pad-bottom level; behind it, the cut reaches past the end of the grid with room to run a plate, at pad-top level, and lower along the heel drain's channel towards the outlet end. A bank of loose or wet soil will not stand vertical at that height, so the cut face gets laid back further still. Planned from the wall's footprint, the hole comes out smaller on paper than on site.
Narrow it by: Staking the back line of the dig at the grid length plus working room, and profiling the bank at several points along the wall before hiring a machine.
Reinforced fill behind the stone zone
usually underThe model sends the soil dug from the bank back as the fill the grid is laid in, and counts nothing for it. That holds only if the soil is granular and low in clay, as the block manufacturer's specification for the reinforced zone requires. Clay pushes harder, drains slowly and swells, and puts a load on the wall the design never allowed for. Where the dug soil fails that test, the whole grid zone is filled with imported granular material no step here quantifies, and the spoil it replaces has to go off site.
Narrow it by: Having the dug soil checked against the manufacturer's reinforced-fill specification before digging starts, and sizing imported fill for the grid zone as the fallback.
Drainage stone
usually underThe calculator fills a neat 12 in slot on a flat floor. The channel under the heel drain deepens towards the outlet end and takes stone below the pad top that the slot never reaches; a cut face that slumps or is dug wide takes stone into every extra inch across the full height; and hollow units take more into their cores. Each of those adds stone, and the pipe's own volume takes back very little of it.
Narrow it by: Measuring the actual width of the cut once the first courses are down, and taking the core fill per unit from the block manufacturer's sheet.
Blocks
either wayThe count is geometry and is close for a straight wall at full height. What moves it is the ends: turned back into the bank, each return adds a corner and split units; stepped down with the slope, whole courses come off. A lower lawn that turns out not to be level along the wall moves the buried course with it, and the pad has to step in whole courses to follow.
Narrow it by: Setting out both ends on the ground against the bank's real profile before ordering, and counting the courses at each end rather than using one height.
Geogrid
either wayTwo things pull in opposite directions. The 3 ft length comes from a rule of thumb applied to this height; design practice sets a floor under that ratio on short walls, clay soil or anything above the wall lengthens it, and roll ends shorter than the design length are waste the per-foot figure never sees. All of that pushes the grid up. The layer count can go the other way: four comes from dividing the height by two courses, and a chart that ends the grid at the fifth course asks for three. The block manufacturer's chart for the actual unit and soil is the figure that stands.
Narrow it by: Getting the block manufacturer's chart for the unit and the soil before the grid is ordered, and choosing a roll width that divides the wall length sensibly.
Levelling pad stone
usually underThe calculator weighs a perfect 6 in layer. The trench floor is rarely that tidy: over-dig, a soft patch dug out and replaced, and a pad carried a little further at the ends all take stone, and a well-graded base can compact denser than the density assumed. Soft ground under the pad is also the first sign the design needs another look.
Narrow it by: Proving the trench floor with the plate before the stone is ordered, and asking the yard for the compacted density of the product it actually supplies.
Programme rather than material
usually underSeven courses mean seven rounds of laying, placing stone, filling behind, compacting and checking level, and the grid goes down on four of them. None of it can be batched: a course set on uncompacted fill is the very fault the lifts exist to prevent. The buried course, set level on the pad in both directions, is the slowest of the seven, and the heel drain has to be in and falling before the stone reaches it.
Narrow it by: Planning the wall course by course with the compaction written into each, and treating the pad, the heel drain and the buried course as a stage of their own.
Tools this job needs
Frequently asked questions
- How many blocks does a 40 ft retaining wall 3 ft high need?
- On this modelled wall, 206 blocks with an 18 × 6 in (457 × 152 mm) face. The 3 ft (0.91 m) that shows is six courses and one more is buried, so the count runs on 3.5 ft (1.07 m) of wall: 140 sq ft (13.0 m²) of face, or 186.7 blocks, plus the calculator's default cut-and-breakage allowance. The cap is separate, 42 ft (12.80 m) of it with its cut allowance, and ends that return into the bank add more blocks.
- How much gravel goes behind and under a 40 ft retaining wall?
- Two different stones. Behind the block, a 12 in (305 mm) zone of clean angular drainage stone over the full 3.5 ft (1.07 m) takes 5.70 yd³ (4.36 m³) with the calculator's settlement allowance, plus whatever fills the cores of a hollow unit. Under it, a 24 in (610 mm) wide pad of dense-graded base, 6 in (152 mm) thick when compacted, takes 2.12 US tons (1.93 metric tons). Neither covers an over-dug trench or the channel that lets the heel drain fall.
- Does a 3 ft retaining wall need geogrid?
- Sometimes. A short wall of deep units on good soil, carrying nothing, can stand on its own weight; the manufacturer's chart for the unit and soil decides. This model assumes grid: at 12 in (305 mm) spacing and 3 ft (0.91 m) long, 3.5 ft (1.07 m) of wall takes 4 layers, or 480 sq ft (44.6 m²) along 40 ft (12.19 m). From the pad's bottom it is 4 ft (1.2 m) before the cap and over once capped: the line where many building departments want an engineered design.