Detailed Geotechnical Investigation for Bridge Foundations as per IRC:78 (Part-1)-2024

Geotechnical investigation is one of the most important stages in bridge engineering because the safety and performance of the foundation system entirely depend on accurate knowledge of subsurface conditions. IRC:78 (Part-1)-2024 provides comprehensive guidelines regarding detailed investigations, extent of exploration, bore hole requirements, and minimum exploration depths for bridge foundations.

This article explains the provisions of Clause 3 – Detailed Investigation from IRC:78 (Part-1)-2024 in a practical and engineering-oriented manner.

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1. Purpose of Detailed Investigation

Detailed investigation is carried out after preliminary investigations are completed. Based on the preliminary findings, engineers tentatively decide:

• Bridge location
• Type of structure
• Span arrangement
• Type of foundation
• Approximate founding level

After this initial assessment, the scope of detailed investigation is finalized jointly by:

• Design engineer
• Geotechnical consultant
• Exploration team

Main Objective:
To obtain sufficient and reliable geotechnical data required for safe and economical design as well as proper execution of bridge foundations.

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2. Extent of Geotechnical Exploration

IRC specifies that geotechnical exploration should cover:

• The entire length of the bridge
• The influence zone beyond the bridge limits
• Approach embankment influence areas

The investigation should extend approximately:

Twice the depth below the bed level of the last main foundation

to properly assess the effect of approach embankments on end foundations.

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3. Minimum Depth of Subsurface Investigation

For Erodible Soil

In erodible soil conditions, the subsurface investigation should extend:

Minimum 15 m below the anticipated founding level

Why This is Important?
Erodible soils are vulnerable to scour, piping, settlement, and instability. Therefore, deeper exploration is necessary to identify competent bearing strata.

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For Well Foundations

For well foundations, the exploration depth should extend to:

2 times the proposed well diameter OR 15 m below anticipated founding level (Whichever is greater)

This ensures sufficient information regarding:

• Deep bearing strata
• Settlement behavior
• Lateral stability
• Scour resistance

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For Pile Foundations in Hard Rock

For pile foundations terminating in hard rock:

Subsurface exploration depth = 4 times pile diameter below pile tip

This helps confirm:

• Continuity of sound rock
• Absence of weak seams
• Rock quality below founding level

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For Non-Erodible Strata

In non-erodible strata, investigations should conform to the rock classification requirements specified in IRC.

However, if unsuitable or questionable material is encountered, exploration must continue until:

• Firm soil strata are reached, or
• Competent rock is encountered

Important Engineering Principle:
Exploration should never terminate in weak, loose, compressible, or doubtful material.

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4. Minimum Number of Bore Holes

IRC provides minimum bore hole requirements for various structures.

4.1 Bridges

Small Bridges (Length 6 m to 30 m)

• One bore hole at each abutment
• One bore hole at any pier location

Bridges Longer than 30 m

• One bore hole at each foundation location

Bridge Type	Minimum Bore Hole Requirement
Small Bridge (6 m to 30 m)	One at each abutment and one at any pier location
Bridge Length > 30 m	One at every foundation location

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4.2 Culverts

For culverts:

Open trench 3 m deep from lowest bed level

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4.3 Box Type Structures

For box structures:

Open trench 3 m deep at every 10 m interval

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4.4 Retaining Walls

For retaining wall investigations:

One bore hole at every 50 m length

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4.5 Embankments Higher than 6 m

For high embankments:

One bore hole at every 100 m interval

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5. Importance of Proper Geotechnical Investigation

Inadequate investigation can result in:

• Foundation failure
• Excessive settlement
• Scour problems
• Construction delays
• Cost overruns
• Unsafe bridge performance

Field Reality:
Many bridge failures are directly linked to insufficient subsurface investigation and incorrect assessment of soil strata.

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6. Practical Recommendations for Engineers

• Always correlate bore log data with geological mapping.
• Use both field and laboratory testing.
• Perform additional bore holes in variable soil conditions.
• Investigate deeper if weak layers are encountered.
• Maintain accurate bore log records.
• Conduct groundwater observations carefully.

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7. Typical Tests Conducted During Investigation

Field Tests	Laboratory Tests
SPT (Standard Penetration Test)	Grain Size Analysis
CPT (Cone Penetration Test)	Atterberg Limits
Pressuremeter Test	Shear Strength Tests
Plate Load Test	Consolidation Test
Permeability Test	UCS Test for Rock

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8. Conclusion

Clause 3 of IRC:78 (Part-1)-2024 emphasizes that detailed geotechnical investigation is the foundation of safe bridge design. Proper determination of exploration depth, bore hole spacing, and subsurface conditions ensures:

• Reliable foundation design
• Economical construction
• Improved safety
• Long-term structural performance

Bridge foundation failures can largely be avoided through systematic and scientifically planned geotechnical investigations.

Author: Mohan Dangi
Civil Engineer | Geotechnical Enthusiast | Infrastructure Content Creator

Disclaimer: This article is intended for educational and informational purposes only. Engineers should refer to the latest IRC codes, project specifications, and detailed geotechnical reports before finalizing any bridge foundation design or construction activity.