Bearing Pressure, Settlement and Stability Criteria for Bridge Foundations as per IRC:78 (Part-1)-2024
The design of bridge foundations is governed not only by bearing capacity requirements but also by settlement behavior, contact pressure distribution, and stability against overturning and sliding. IRC:78 (Part-1)-2024 provides detailed provisions regarding allowable bearing pressure, permissible settlements, and safety factors for bridge foundations resting on soil and rock.
This article presents a detailed explanation of Clause 706 covering:
- Allowable bearing pressure
- Settlement criteria
- Differential settlement limits
- Permissible loss of contact
- Factors of safety against instability
- Friction coefficients for foundation design
1. Allowable Bearing Pressure (Clause 706.3.1)
The allowable bearing pressure and settlement characteristics of foundation soil or rock should be determined based on:
- Sub-soil exploration
- Field testing
- Laboratory investigations
- Relevant Indian Standard Codes
The allowable bearing pressure may be calculated as gross pressure at the foundation base without deducting displaced soil weight.
Gross foundation pressure includes the total load divided by foundation area without subtracting the overburden pressure of displaced soil.
2. Bearing Pressure for Foundations Resting on Soil
For open foundations and well foundations resting on soil, IRC specifies that the allowable bearing pressure based on ultimate bearing capacity may be taken using:
Thus, a factor of safety of 2.5 is generally adopted for soil foundations.
3. Bearing Pressure for Foundations Resting on Rock
For foundations resting on rock, allowable bearing pressure depends not only on the strength of intact rock but also on:
- Joints and fissures
- Bedding planes
- Faults
- Weathered zones
- Overall rock mass quality
Factor of Safety for Rock Foundations
Where detailed rock mass analysis is unavailable, the factor of safety based on unconfined compressive strength (UCS) may be taken as:
- Factor of Safety = 6 to 8
The allowable bearing pressure obtained should generally not exceed:
for Load Combination-I unless otherwise justified through detailed investigations.
Intermediate geomaterials such as weathered rock or very soft rock may behave similar to soil and should be treated accordingly in design.
4. Allowable Differential Settlement
Differential settlement is the difference in settlement between adjacent foundations and is extremely critical for bridge performance.
IRC specifies that for simply supported spans:
unless specific provisions are made in the design to accommodate larger settlements.
For structures highly sensitive to settlement, more stringent limits may be adopted based on project requirements.
5. Maximum Allowable Total Settlement
IRC specifies maximum permissible settlement limits for foundations resting on soil due to superstructure loads.
| Foundation Type | Maximum Allowable Settlement |
|---|---|
| Open Foundation | 75 mm |
| Well Foundation | 150 mm |
Well foundations can tolerate comparatively higher settlements because of their large size and rigidity.
6. Minimum Differential Settlement to be Considered in Design
IRC recommends considering minimum differential settlements in substructure and foundation design even if calculated values are smaller.
| Foundation Type | Minimum Differential Settlement |
|---|---|
| Open Foundations Resting on Soil | 15 mm |
| Well Foundations Resting on Soil | 25 mm |
| Pile Foundations Resting on Soil | 1% of Pile Diameter |
| Foundations Resting on Rock | NIL |
These values ensure conservative and realistic structural behavior during design.
7. Permissible Loss of Contact at Foundation Base
For Foundations Resting on Soil
Full contact between foundation base and soil must be maintained under all load combinations.
For Foundations Resting on Rock
In rock foundations, partial loss of contact may occur under extreme loading conditions. In such cases:
- The base area should be reduced to eliminate tensile stress.
- Base pressures should be recalculated.
- Maximum pressure on reduced area should remain within allowable limits.
Minimum Effective Contact Area
| Load Combination | Minimum Contact Area Required |
|---|---|
| Seismic Load Combination or Impact of Barge | 67% of Total Area |
| Other Load Combinations | 80% of Total Area |
Tension should never develop at the soil-foundation interface for foundations resting on soil.
8. Factors of Safety for Stability
IRC specifies minimum factors of safety against:
- Overturning
- Sliding
- Deep-seated failure
| Stability Check | Without Seismic Case | With Seismic and SV Loading |
|---|---|---|
| Against Overturning | 2.0 | 1.50 |
| Against Sliding | 1.50 | 1.25 |
| Against Deep-Seated Failure | 1.25 | 1.15 |
Reduced factors of safety are permitted during seismic events because earthquakes are temporary extreme loading conditions.
9. Friction Coefficient Between Foundation and Soil/Rock
IRC provides typical friction coefficients for stability calculations:
| Interface | Coefficient of Friction |
|---|---|
| Concrete and Soil | 0.5 |
| Concrete and Good Rock | 0.8 |
| Concrete and Fissured Rock | 0.7 |
The friction coefficient is generally represented as:
where:
- μ = Coefficient of friction
- φ = Angle of internal friction
10. Practical Engineering Considerations
- Settlement analysis should include immediate and consolidation settlement.
- Rock mass classification should always be verified through field mapping.
- Bridge foundations near rivers require combined hydraulic and geotechnical assessment.
- Seismic effects should be incorporated in vulnerable zones.
- Differential settlement is often more critical than total settlement.
11. Conclusion
Clause 706 of IRC:78 (Part-1)-2024 provides a comprehensive framework for evaluating bearing pressure, settlement limits, and stability requirements for bridge foundations.
A safe bridge foundation design requires:
- Accurate geotechnical investigation
- Reliable hydraulic assessment
- Proper evaluation of settlement behavior
- Adequate factors of safety
- Careful assessment of rock and soil interaction
Adopting these provisions ensures long-term safety, serviceability, and durability of bridge structures.
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