Cantilever Retaining Wall BOQ — Worked Example for a 20 m × 3 m High RCC Wall

Cantilever retaining walls are the dominant retaining-wall type in Indian civil engineering — used wherever a soil cut needs vertical retention from 1.5 m to 6 m height. NH / SH slope cuts, hill-station road embankments, multi-storey-basement perimeter walls, plot levelling on sloping ground, sports-ground level changes. The IS 14458 + IS 456 standard configuration: RCC vertical stem + RCC base slab (heel + stem + toe) + drainage backfill + weep holes. This article walks through a complete BOQ for the most common configuration — 20 m long × 3 m retained height.

Project Scenario

You're estimating a retaining wall for a plot-development project in Lonavala (Western Ghats) — a 20 m long stretch of road embankment needs retention up to 3 m above the lower-level finished ground. Soil is medium-dense decomposed laterite (SBC ~ 200 kN/m²). Design: cantilever RCC wall with 300 mm stem (bottom), 2.4 m base slab (350 mm thick), foundation 0.5 m below NGL, weep holes every 1.5 m.

Design specification at a glance

• Length: 20 m
• Stem height (retained): 3.0 m
• Stem thickness: 300 mm at base (tapered to 200 mm at top per IS 14458)
• Base slab length: 2.4 m (heel + stem + toe per IS 14458 sizing)
• Base slab thickness: 350 mm
• Foundation depth: 0.5 m below NGL
• Weep holes: 75 mm PVC pipes @ 1.5 m c/c
• Backfill: excavated earth + filter cloth behind stem

The Complete BOQ — 10 Items in CPWD DSR 2023

Total RCC volume = 32.1 m³ split between base slab (16.8) and stem (15.3). At 150 kg/m³ that's ~4.8 tonnes of steel — much higher per-cubic-metre than non-flexural RCC because cantilever walls are designed for full flexural action (the stem cantilevers off the base; the base cantilevers off the stem). The live BOQ Builder evaluates these formulas in real time.

How Each Quantity Was Computed

Geometry per IS 14458

Stem height H = 3.0 m
Base slab length ≈ 0.6H to 0.8H = 1.8 to 2.4 m → use 2.4 m
Heel + toe split: heel = 0.5 × base = 1.2 m, toe = 0.25 × base = 0.6 m, stem occupies middle 0.25 = 0.6 m
Stem thickness: bottom 300 mm, top 200 mm (tapered) → average 255 mm
Base thickness ≈ H/10 to H/12 = 250-300 mm → use 350 mm (conservative)

1. Excavation — Item 2.8.1

Trench width = base_l + 0.30 = 2.7 m
Trench depth = depth_below_NGL + base_thk + PCC = 0.5 + 0.35 + 0.10 = 0.95 m
Excavation = 20 × 2.7 × 0.95 = 51.30 m³

2. PCC bed — Item 4.1.8

PCC width = base_l + 0.15 = 2.55 m
PCC volume = 20 × 2.55 × 0.10 = 5.10 m³

3-4. RCC base + stem

Base slab volume = 20 × 2.4 × 0.35 = 16.80 m³
Stem volume = 20 × stem_avg_thk × stem_height = 20 × 0.255 × 3.0 = 15.30 m³

5-6. Centering & shuttering

• Base slab (5.9.1): under-side only = 20 × 2.4 = 48 m²
• Stem (5.9.2): both faces = 2 × 20 × 3.0 = 120 m²

7. Reinforcement — Item 5.22.6

Total RCC = 16.80 + 15.30 = 32.1 m³
Steel = 32.1 × 150 = 4,815 kg (~4.8 tonnes)

150 kg/m³ is typical for 3 m high cantilever walls. For walls > 4 m, push to 180-200 kg/m³. The split: ~60 % goes into the stem (vertical bars + curtailment at base), ~40 % into base slab (heel-side tension steel + toe-side compression steel + temperature steel).

8. Weep holes

Number of weep holes = floor(length / 1.5) = floor(20 / 1.5) = 13 nos

75 mm PVC pipes embedded through the stem at the base, with filter fabric on the backfill side to prevent soil ingress while allowing water through. Critical for stability — without weep holes, the active earth pressure on the wall increases 60-80 % from hydrostatic buildup behind the stem.

9. Backfill — Item 2.25

Backfill volume = length × (base_l − stem_thk) × stem_height
= 20 × (2.4 − 0.3) × 3.0 = 20 × 2.1 × 3.0 = 126 m³

Backfill is excavated earth from the trench plus topsoil from the level-raise area. For premium projects, granular backfill (gravel-sand mix, GSB material) reduces lateral earth pressure further but adds ~15 % cost. Set the BOQ Builder backfill code to DSR 16.79 (GSB) for granular spec.

When Cantilever Beats Gravity / Gabion / RE Wall

For the bread-and-butter Indian civil retaining wall (1.5-6 m + soil retention + paved area / road on top), cantilever is the right answer.

What This BOQ Excludes

• Friction key / shear key at base slab heel (only for sliding-stability design check on poor soils)
• Strip-drain pipes at base (collect water from weep holes + discharge laterally)
• Coping on top of stem — small concrete fillet, ~₹500/m
• External painting / weather-protective finish on exposed face
• Railings on top (if pedestrian access — MS angle iron, ~₹800/m)
• Approach / road overlay on top of backfill
• Drainage chamber at base of weep hole discharge

Common Estimation Mistakes

• Using 100 kg/m³ steel "RCC standard" — cantilever walls need 150 kg/m³ minimum for flexural design. Under-steeling cracks the stem at base within first monsoon.
• Skipping weep holes — without drainage, hydrostatic pressure behind the stem can DOUBLE the lateral load. Walls without weep holes routinely fail in the second monsoon.
• Wrong heel/toe proportions — IS 14458 sizing puts heel = 0.5 × base for surcharge loaded walls. Some estimators put toe = 0.5 × base (mirror of footing) which is wrong for cantilever design.
• Under-sized base slab — a 3 m wall with base < 1.8 m fails sliding or overturning checks. The Builder defaults to 2.4 m (which is on the conservative side).
• Treating base + stem as one RCC item — different DSR rates (5.1.2 base < 5.2.2 stem in cost per m³). Lumping misses 5-8 % cost.
• Forgetting the centering on both stem faces — stem needs shuttering on BOTH sides (unlike a wall on a building, which has plaster on one side). Frequent under-estimate.
• Stem thickness uniform top-to-bottom — IS 14458 specifies a tapered stem (300 mm at base, 200 mm at top) for efficiency. Constant thickness adds 15-20 % concrete unnecessarily.

What Changes for a Different Wall

• Shorter wall (1.5 m) — base slab drops to 1.2 m, stem 200-250 mm. Steel down to ~120 kg/m³.
• Taller wall (5 m) — base slab 4 m, base thickness 500 mm, stem 400 mm tapered to 250 mm. Steel up to 180 kg/m³.
• Highway slope retention (sloping backfill) — heel grows to 0.6-0.7 × base for the higher surcharge. Add lateral surcharge load in design.
• Expansive / black-cotton soil — bump foundation depth to 0.75-1.0 m below NGL + use lime/cement-stabilised backfill. Add shear key at heel for sliding resistance.
• Seismic zone IV / V — design includes earthquake-induced lateral pressure (per IS 1893 Mononobe-Okabe method). Steel ratio bumps to 180-200 kg/m³.
• Counterfort wall (6-9 m height) — adds vertical buttress ribs (counterforts) on backfill side at 3-4 m c/c. Stem becomes thinner; counterforts carry the cantilever load. Different template.
• Granular backfill (gravel-sand) — switch backfill DSR code to 16.79 (GSB Grade-II). Adds 15 % cost but reduces stem flexural design moments.

Get Your Retaining Wall Cost in 30 Seconds

References & Companion Reading

• IS 14458 Part 1:1998 — Guidelines for Retaining Wall for Hill Area (selection of type)
• IS 14458 Part 2:1997 — Design of Retaining/Breast Walls
• IS 456:2000 — Plain and Reinforced Concrete
• IS 1786:2008 — TMT Fe-500D reinforcement
• IS 1893 Part 1:2016 — Seismic design (for Zone IV / V walls)
• IS 1200 Part 5:1982 — Concrete measurement
• RCC Design Suite — for structural design verification
• CPWD DSR 2023 — full SOR with rates by item