IRC 89 : 2018Guidelines for Design and Construction of Soil-Nail Supported Structures

IRC 89 specifies guidelines for design + construction of soil-nail supported structures — soil nailing is a slope stabilisation technique where steel reinforcement bars (nails) are drilled + grouted into the soil mass, creating an in-situ reinforced soil structure. Used for cut-slope stabilisation, retaining wall reinforcement, and embankment toe protection.

Use IRC 89 soil nailing when: - Cut slopes in stable soil that need stabilisation against shallow failure - Excavation support in temporary / permanent applications - Slope steepening of existing embankments - Earth retention as alternative to RCC retaining wall (for moderate height) - Hill road cuts through cohesive / weathered rock - Quarry / mining slope stabilisation

Soil nailing vs alternatives:

| Method | Slope angle | Soil suitability | Cost (per m² wall) | |---|---|---|---| | RCC retaining wall | Vertical | All | High | | Reinforced soil wall (IRC SP 82:2015) | Up to 70° | Granular | Medium-high | | Soil nailing (this code, IRC 89) | Up to 70° | Cohesive / weathered rock | Medium | | Anchored / ground-anchor wall | Vertical | All | High | | Shotcrete (without nails) | Up to 50° | Surface treatment | Low |

Soil nailing combines well with shotcrete facing → modern "soil nail wall" widely used in modern Indian highway + infrastructure projects.

Soil nail components: - Steel rebar: 16-32 mm diameter, Fe 500D / Fe 550D - Length: 0.6-1.0 × wall height (typical 5-15 m) - Spacing: 1.0-1.5 m vertical × 1.0-2.0 m horizontal grid - Inclination: 10-20° below horizontal - Grout: cement grout (1:3 cement-water) at 0.3 MPa pressure - Facing: shotcrete (50-100 mm thick) with welded wire mesh OR steel grid with concrete facing

Design analysis: - Internal stability: nails must resist soil-mass shear; pull-out + tensile capacity - External stability: entire reinforced soil mass against sliding, overturning, bearing - Global stability: deep-seated rotational failure of soil + nail mass + adjacent slope - Surface protection: shotcrete facing prevents soil falls between nails

Acceptance criteria (Clause 7):

| Failure mode | Factor of safety | |---|---| | Internal: nail tensile rupture | 1.5 | | Internal: nail pull-out | 1.5 | | External: sliding | 1.5 | | External: overturning | 2.0 | | External: bearing | 2.0 | | Global slope stability | 1.4 |

Suitability: - Soil with c > 15-20 kPa (cohesive needed for hole stability during drilling) - Stiff clay, weathered rock, lateritic soil - Not suitable: loose granular (drilling collapses), very soft clay, very high water table

Construction sequence: 1. Cut slope to design profile in stages (top-down, 1.5-3 m at a time) 2. Drill + grout nails at design grid + length 3. Install drainage (weep holes, sub-surface drains) 4. Apply shotcrete facing with reinforcement mesh 5. Repeat for next slope stage

Quality control: - Pull-out test on sacrificial nail (typically 1 per 100 nails) to verify capacity - Grout strength test - Shotcrete thickness + reinforcement verification - Long-term monitoring: inclinometer, settlement plates

Cost (typical 2026): - Soil-nail + shotcrete facing: ₹4000-8000 per m² wall - vs RCC retaining wall: ₹6000-12000 per m² wall (4-8 m height) - vs Reinforced soil wall: ₹3500-7000 per m² wall (height-dependent) - Soil nailing competitive for medium-height + cohesive soils

• IRC SP 82:2015 — reinforced soil walls (inextensible).
• IRC SP 102 — reinforced soil walls (extensible / geogrid).
• IRC:5:2015 — bridge design.
• IRC:78:2014 — foundations + substructures.
• IRC:36:2010 — earth embankment construction.
• IRC:77:1979 — hill roads (soil nailing common in hill cut faces).
• IS 456:2000 — RCC code (shotcrete facing).
• IS 1786:2008 — high-strength deformed reinforcement (nail steel).
• IS 8112:1989 / IS 12269:2013 — cement (grout).
• IS 4759:1996 — galvanising (long-term nail protection).
• IS 2720 series — soil testing (slope characterisation).
• IS 6403:1981 — bearing capacity.
• IS 1080:1985 — design of shallow foundations.
• IS 9143 — code of practice for design of foundations on rock.
• BS 8006 — UK code for reinforced soil structures.
• FHWA Soil Nail Design Manual — international reference.

1. Soil nailing in unsuitable soil. Loose granular, very soft clay = drilling collapses; nailing fails. Verify soil suitability. 2. Inadequate cohesion. Soil cohesion c < 15 kPa = drill hole collapses; difficult to grout. Improve via cement injection OR alternative method. 3. Insufficient nail length. Beyond active failure plane needed for anchorage. Per design. 4. Nail spacing too wide. Surface failures between nails; shotcrete inadequate. Per design grid. 5. No pull-out test. Capacity unverified; design not validated. Sacrificial test mandatory. 6. Grout poor quality. Grout strength inadequate; nail-soil bond weak. Verify per IS 4031 cement strength + grout flow tests. 7. No drainage in soil-nail wall. Hydrostatic pressure builds; failure. Provide weep holes + sub-surface drains. 8. Construction sequence wrong. Cut all slope first then nail = collapse during cut. Top-down staged construction essential. 9. No corrosion protection on long-term nails. Steel nails corrode in soil moisture; capacity degrades. Galvanise OR epoxy-coat OR sacrificial coupon design. 10. Inadequate shotcrete facing. Thin / unreinforced shotcrete cracks under soil settlement; surface failures. 75-100 mm with mesh. 11. Sub-surface investigation inadequate. Soil profile uncertain; design assumptions wrong. Detailed investigation needed. 12. No post-construction monitoring. Long-term performance unknown; failure undetected. Inclinometer / settlement plates. 13. Design without seismic check. In Zones IV-V, seismic loads + reduced shear strength affect stability. Per IS 1893 Part 1:2016.

Slope stabilisation strategy cascade:

1. Site investigation — soil profile, water table, slope geometry, instability indicators. 2. Stability analysis — current FS + required FS. 3. Stabilisation method selection: - Drainage improvement (cheap; first option) - Re-grading (flatter slope; needs land) - Surface treatment (vegetation, geotextile, shotcrete) - Soil nailing (this code, IRC 89) — cohesive / weathered rock; medium height - Reinforced soil wall (IRC SP 82, IRC SP 102) — granular fill; tall walls - RCC retaining wall — vertical; high cost; all soils - Ground anchor (tied back) wall — special applications 4. Design (IRC 89:2018): - Internal + external + global stability checks - Material selection (steel, grout, shotcrete) - Drainage design 5. Construction: - Top-down staged - Quality control + pull-out tests 6. Monitoring — long-term displacement / settlement.

IRC 89 is one of the foundational soil-nail codes in India. Modern hill road + cut-slope projects (NH widening through hilly terrain, urban basement excavation in cohesive sites) increasingly use soil nailing — cost-effective + structurally sound for the right soil + geometry.