Drawing Set · Technical Primer

Reinforced Concrete Cantilever Retaining Wall

Purpose, geometry, soil inputs, stability checks, reinforcement, and failure modes — referenced to Philippine structural practice.

StandardNSCP 2015, 7th Ed.
Issuing BodyASEP
Wall TypeCantilever, RC
Sheets00 – 08

SHEET 01 / 08INTRODUCTION

Holding Back
the Ground

Resists earth pressure by bending, not by mass alone.

Purpose

Retains soil at a higher grade — the stem bends, the footing weight resists.

Why cantilever, not gravity

Steel does the work — far less concrete than a gravity wall.

Economical height

Efficient for 3–6 m retained height.

Typical applications

  • Highway cut & fill (DPWH roads)
  • Bridge abutments & wingwalls
  • Basement / below-grade walls
  • Riverbank & flood-control works
  • Site terracing & building pads
NSCP 2015 Vol. 1, Ch. 2 DPWH DGCS — Highway Structures
SCALE — N.T.S.

SHEET 02 / 08ARCHITECTURAL / STRUCTURAL LAYOUT

Anatomy of
the Wall

TOE HEEL STEM SHEAR KEY (optional) H B B/3
FIG 02.1 — Typical sectionN.T.S.

Component parts

  • Stem — carries earth pressure to the footing
  • Toe — footing ahead of the stem
  • Heel — footing under the backfill
  • Base slab — spreads load to the soil
  • Shear key — optional, adds sliding resistance

Trial proportions

Base width
B
0.5H – 0.7H
Footing thickness
tf
H12 – H10, 300mm min.
Stem thickness, base
ts
≈ H12
Stem thickness, top
ts,top
200–300mm min.
Toe projection
—
≈ B3
Key depth
—
tf10 – tf12
SCALE — N.T.S.

SHEET 03 / 08SOIL PARAMETERS

What the
Ground Tells Us

From a geotechnical investigation — required under NSCP §304.

Required inputs

ParameterSymbolTypical range
Unit weight, backfillγ16 – 20 kN/m³
Friction angleφ28° – 35°
Cohesionc0 (granular)
Bearing capacityqasite-specific
Base friction coeff.μ0.40 – 0.55
Surchargeqe.g. 12 kPa
Unit weight, RCγc23.6 kN/m³

Active pressure coeff.

Ka = 1 − sin φ1 + sin φ

Rankine, level backfill.

Drainage

Design assumes drained backfill — weep holes relieve water pressure.

NSCP 2015, §304
SCALE — —

SHEET 04 / 08STABILITY ANALYSIS

Three Checks
Before One Pour

1 · Overturning

FSOT = ΣMresistingΣMoverturning ≥ 1.5

2 · Sliding

FSslide = μ ΣW + PpPa ≥ 1.5

Pp usually ignored — conservative.

3 · Bearing

e = B2 − x̄ ≤ B6  →  qmax,min = (ΣWB)(1 ± 6eB) ≤ qa

Resultant stays in the middle third.

Pa @ H/3 above base W stem W soil (heel) Pp (often ignored) e middle third q max (toe) q min (heel)
FIG 04.1 — Force system & base pressureN.T.S.
SCALE — N.T.S.

SHEET 05 / 08STRUCTURAL DESIGN

Reinforcing
the Cantilever

main steel — earth face tension — top tension — bottom temp. & shrinkage steel, front face 75mm clear cover (cast against earth) STEM TOE HEEL
FIG 05.1 — Reinforcement, critical sectionsN.T.S.

Design basis

Strength Design, NSCP Ch. 4 (≈ ACI 318).

  • Stem — main steel, earth face
  • Heel slab — tension, top face
  • Toe slab — tension, bottom face
  • Dowels — develop stem steel into footing
Shear at d — NSCP §422 ρmin = 0.0018, Gr. 60
SCALE — N.T.S.

SHEET 06 / 08SAMPLE CALCULATION SUMMARY

One Wall,
Worked in Brief

Illustrative only — not a full design.

Given

H = 4.0 m γ = 18 kN/m³ φ = 30° γc = 23.5 kN/m³ qa = 150 kPa B = 2.6 m (trial)
StepResult
Active coefficientKa = 1−sin30°1+sin30°0.333
Active thrustPa = ½·Ka·γ·H²47.9 kN/m
Point of applicationy = H31.33 m
Total vertical load (trial)ΣW ≈ Wc + Wsoil≈ 178 kN/m
Overturning checkFS = ΣMrΣMo≈ 2.7 > 1.5 OK
Sliding checkFS = μΣWPa≈ 1.8 > 1.5 OK
Bearing checkqmax vs qa≈ 110 < 150 kPa OK
Pa = ½ Ka γ H² = 47.9 kN/m

Reading this result

All three checks clear 1.5 with a 2.6 m trial base.

Not shown here

  • Surcharge & seismic additions to Pa (NSCP §208)
  • Stem, heel, toe bar sizing
  • Full load-combination envelope (NSCP §203)
SCALE — —

SHEET 07 / 08FAILURE MODES

How Walls Fail

Overturning

Undersized base rotates the wall about the toe.

PreventionWiden heel; FS ≥ 1.5.

Sliding

Low base friction pushes the wall sideways.

PreventionShear key; drainage; FS ≥ 1.5.

Bearing failure

Weak soil crushes under the toe; wall tilts.

PreventionGeotech survey; widen footing.

Global / slope

A slip surface passes beneath the whole wall.

PreventionSlope analysis; benching.

Structural

Under-reinforced or corroded steel — stem or footing cracks/fails.

PreventionCorrect bars; cover; QC/QA.

Drainage

Clogged weep holes let water pressure build up behind the wall.

PreventionWeep holes; gravel drain; filter fabric.

SCALE — N.T.S.

SHEET 08 / 08CONCLUSION

Where the Cantilever
Wall Fits

Advantages

  • Economical up to roughly 6–7 m of retained height
  • Far less concrete volume than an equivalent gravity wall
  • Simple formwork and widely understood construction sequence
  • Design method well established under NSCP / ACI 318

Limitations

  • Less efficient beyond ~7–8 m — counterfort walls take over
  • Needs adequately competent bearing soil
  • Performance hinges on drainage detailing and construction QC
  • Thin sections mean rebar placement errors matter more

Recommended applications

Highway cut/fill sections Bridge abutments & wingwalls Basement walls Flood-control / riverbank works Site grading & building pads

References

  • National Structural Code of the Philippines (NSCP), 2015, 7th Ed. — Association of Structural Engineers of the Philippines (ASEP)
  • ACI 318 — Building Code Requirements for Structural Concrete (referenced by NSCP Ch. 4)
  • DPWH Design Guidelines, Criteria and Standards (DGCS) — Highway Structures
  • Das, B.M. — Principles of Geotechnical Engineering
  • McCormac, J.C. & Brown, R.H. — Design of Reinforced Concrete
SCALE — —