IRC 45:1972 is the Indian Standard (IRC) for estimation of resistance of soil below maximum scour level — foundations of road bridges. IRC 45 provides the method for calculating lateral soil resistance below the scour line — essential for checking stability of bridge foundations (wells and piles) against overturning due to water current, wind, seismic, and braking forces.
Methods for estimating resistance offered by soil below maximum scour level to lateral loads on bridge foundations — crucial for well and pile foundation stability.
Key reference values — verify against the current code edition / project specification.
| Reference | Value | Clause |
|---|---|---|
| Purpose | Lateral-load resistance of soil below max scour, bridge foundations | Scope |
| Applies to | Well & pile foundations of road bridges | Application |
| Max scour level | Basis from IRC 5 / IRC 78 scour computation | Cross-ref |
| Method | Soil reaction below scour resisting horizontal force/moment | Method |
| Governs | Well/pile grip length & stability | Design |
| Read with | IRC 78 (foundations) / IRC 6 (loads) | Cross-ref |
IRC 45 (1972) provides Recommendations for Estimating the Resistance of Soil Below the Maximum Scour Level in the Design of Well Foundations of Bridges — the IRC's specification for the geotechnical capacity of well (caisson) foundations in scour-prone river beds. It is essential for bridges across major Indian rivers (Ganga, Brahmaputra, Yamuna, Mahanadi, Godavari, Krishna, etc.) where deep well foundations are the standard solution.
Use IRC 45 when you are: - Designing well foundations for river bridges - Computing soil resistance below maximum scour level - Determining founding depth of caissons - Estimating bearing capacity of well foundations - Doing passive pressure analysis for lateral resistance of well foundations - Working on NHAI / state bridge projects crossing major rivers - Rehabilitating existing well foundations (re-rating capacity)
What IRC 45 covers: - Definition of maximum scour level (per IRC:5:2015 + IRC:78:2014) - Soil resistance below scour level (passive + base resistance) - Coefficient of subgrade reaction approach - Lateral resistance of well foundations - Combined bearing + tilt analysis - Quasi-static computation (for design) - Dynamic response (for seismic) - Worked examples of well foundation design
IRC 45 dates from 1972; while the underlying principles remain valid, modern designs typically supplement with: - IRC:78:2014 (current foundations + substructures) - Modern soil mechanics (finite element + numerical methods) - Site-specific hydrogeological + seismic analysis - Modified scour analysis using regime + flood data
Well foundation basics: - A well foundation is a deep cylindrical / rectangular caisson sunk through soft soils + water to a competent founding stratum - Used when scour exceeds the founding depth that direct footings can safely reach (typically depths > 8-10 m below natural ground level) - Standard for major Indian river bridges where alluvial soils + variable scour conditions exist
The critical question IRC 45 answers: Given a well foundation in a scour-prone alluvial river, what is the soil resistance below the maximum scour level that the well can rely upon? This determines: - Required founding depth of the well below scour level - Bearing capacity of the founding stratum - Lateral resistance to wind, seismic, water-current, ice loads - Tilt stability of the well during construction + service
Maximum scour level definition: - Per IRC:5:2015 + IRC:78:2014: - General scour: across river bed - Local scour: around pier obstruction - Maximum scour level (MSL) = general + local scour at design flood (100-year flood typically) - Founding depth = MSL + safety margin (typically 2 m + tip resistance demand)
Soil resistance below MSL — three components:
1. Vertical bearing resistance (base) — at well base: - Bearing capacity from soil shear strength + base area - For granular soil: q_safe = N_q × σ' (effective overburden) × area - For cohesive: q_safe = 5.14 × c_u × area (undrained)
2. Skin friction (sides) — along well periphery below MSL: - α-method or β-method for cohesive soils - β-method or skin friction coefficient for granular soils - Typically 20-40 % of base capacity in well foundations
3. Passive lateral resistance — for lateral loads: - K_p × soil pressure × area - Front pressure resists horizontal load; rear pressure assists in restoring moment - Critical for tall + slender wells subject to horizontal forces
Bearing capacity coefficients (Terzaghi / Vesic — typical): - For granular soil (φ = 25-30°): N_q = 10-20 - For granular soil (φ = 30-35°): N_q = 20-40 - For granular soil (φ = 35-40°): N_q = 40-80 - For cohesive soil (saturated clay): N_c = 5.14
Safe bearing capacity (typical Indian river bed alluvial): - Medium-dense sand below scour level (SPT N 15-30): q_safe = 200-400 kPa - Dense sand (SPT N 30-50): q_safe = 400-700 kPa - Very dense sand / gravel (SPT N > 50): q_safe = 700-1500 kPa - Stiff to hard clay below scour: q_safe = 100-250 kPa - Rock encountered: per rock mechanics; q_safe = 500-3000 kPa depending on rock quality
Side friction (skin) on well periphery: - Granular soil: 10-25 % of base bearing capacity per metre below scour - Cohesive soil: α × c_u (α = 0.3-0.6 depending on consistency) - Typically 20-50 % of base capacity is contributed by side friction
Passive lateral resistance: - K_p × γ' × depth × diameter (per metre of well below MSL) - For granular soil: K_p = 3-5 (depending on φ) - For cohesive (undrained): K_p × c_u ≈ 9-12 × c_u
Coefficient of subgrade reaction (k_h) — for soil-structure interaction analysis: - Granular soil: 5-20 MN/m³ - Soft cohesive: 1-5 MN/m³ - Stiff cohesive: 5-15 MN/m³ - Used in modulus-of-subgrade-reaction method for lateral analysis (Reese, Matlock methods)
Founding depth criteria: - Minimum founding depth = 2 m below MSL (per IRC 78 + IRC 45) - Increase if needed for: - Required bearing capacity - Required passive resistance - Settlement criteria - Specific site conditions - Typical founding depth: 4-8 m below maximum scour level
Tilt stability: - Lateral resistance from passive earth pressure resists tilt moment - Tilt under design wind / seismic + traffic loads: ≤ 1 in 250 (= 0.4 % rotation) - Vertical asymmetry of soil resistance can lead to tilting; analyse + ensure stability
Sinking practice: - Wells sunk by combined dewatering + dredging + skin friction reduction - During sinking: well must remain vertical; tilt corrected continuously - Founding stratum verified by core sampling - Bottom plug + filling with concrete after founding
Construction loads: - Dead load: full self-weight + superstructure + traffic load - Service load: dead + live + impact - Seismic: per IS 1893; well foundations must withstand earthquake without instability - Water current: per IRC:6 — typically 1.5-3 kN/m² for major flood - Ice / debris loads: in cold regions or where applicable
1. Scour analysis under-estimated. Used annual flood; design flood at 100-year is much higher; well founded too shallow. Use design flood (100-year) for MSL; conservative. 2. Geotechnical investigation insufficient. Borehole only at one well location; soil conditions vary; bearing assumption wrong. One borehole per well minimum; correlate. 3. SPT N values used without correction. Field SPT N reported; must be corrected for overburden, hammer efficiency, rod length per IS 2131. Use corrected N for bearing computations. 4. Bearing capacity computed for ideal conditions. Cohesive soil assumed; actually sand with cohesion negligible; bearing capacity over-estimated. Verify soil type + use right formula. 5. Side friction neglected. Well design assumes only base bearing; underestimates capacity by 30-50 %. Include side friction in capacity calculation. 6. No tilt analysis. Lateral loads ignored; well tilts under wind + seismic; impacts deck level. Check tilt stability per IRC 45 + IRC 78. 7. Founding depth less than 2 m below MSL. Per code minimum; sub-spec design; failure during high flood. Strict adherence to minimum founding depth. 8. Construction tolerance not maintained. Well off-vertical during sinking; permanent tilt; load transfer compromised. Real-time monitoring + correction during sinking. 9. Cores not taken at founding level. Founding stratum quality assumed; actually weathered rock or soft pocket; bearing inadequate. Mandatory core verification at well bottom. 10. Plug + filling skipped. Well bottom not concreted; void; bearing transferred incorrectly. Bottom plug + filling concrete mandatory. 11. Long-term settlement not estimated. Founding stratum compressible; consolidation settlement over years; bridge level drops. Estimate + accommodate in design or alternative foundation. 12. Earthquake analysis skipped. Static load + live load only; earthquake forces not included; in seismic zone these dominate. Per IS 1893 + IRC:6. 13. Lateral resistance over-estimated. Passive earth pressure assumed mobilised; in reality only after significant displacement; capacity over-stated. Conservative K_p; verify by lateral load test or analysis. 14. No instrumentation. Long-term performance unmonitored. Settlement plates, tiltmeters on critical bridges; readings periodic. 15. Hydrostatic uplift not checked. During flood, water buoyancy on well + plug; uplift stability may be inadequate. Check uplift; counter-weight or anchorage. 16. Repair / strengthening of existing wells: modern loads exceed original design; well rated for old standards. Re-rate per current loads; strengthen if necessary.
Bridge over major river — IRC 45 touchpoints:
1. Feasibility: Bridge type + foundation type selection. River bed soil + scour assessment. 2. Hydrological study: - Design flood (100-year) - General scour computation - Local scour around piers - Maximum scour level (MSL) established 3. Geotechnical investigation: - Boreholes at each well location (minimum 2 per major bridge well) - Depth ≥ founding depth + 5-10 m additional - SPT every 1-1.5 m - Disturbed + undisturbed samples for lab testing - Cores at competent stratum levels 4. Foundation design (IRC 45 + IRC 78): - Soil resistance below MSL: base bearing + side friction + passive resistance - Founding depth determination based on bearing + lateral capacity - Well dimensions (diameter, depth, wall thickness) - Reinforcement detailing - Stress analysis under all load combinations (vertical, lateral, seismic, current) - Settlement + tilt analysis 5. Construction documentation: - Well sinking method + sequence - Dredging + dewatering plan - Founding stratum verification protocol - Concrete plug + filling specification - Instrumentation plan 6. Tender + BOQ: - Well construction (per cubic metre or per meter depth) - Excavation, dredging, dewatering - Reinforcement + concrete - Foundation testing + verification 7. Construction: - Cofferdam construction - Well casting + sinking - Dredging through well, simultaneous sinking - Founding stratum verification by sampling - Bottom plug + filling - Capping + bridge pier construction - Instrumentation installation 8. Quality control: - Verticality monitoring during sinking (within ± 1:200 of design) - Founding stratum verification (per design SPT N + visual) - Concrete quality at all stages - Instrument readings 9. Pre-opening: - Foundation level + alignment verification - Long-term settlement monitoring baseline - Bridge superstructure construction - Load test (where required) 10. Operations + monitoring: - Annual visual inspection of well + pier - Long-term settlement + tilt monitoring - Underwater inspection every 3-5 years (scour status + foundation visible portion) - Periodic re-rating if traffic loads or design standards change
IRC 45 + IRC 78 + IRC 83 are the foundational geotechnical triad for every major Indian river bridge — invoked on every bridge over the Ganga, Brahmaputra, Yamuna, Krishna, Godavari, Mahanadi, Cauvery, Brahmani, Tapi, Subarnarekha, and dozens of other major rivers.