Standard Specifications and Code of Practice for Road Bridges, Section VII — Foundations and Substructure (Revised Revision)

IRC 78: General Definitions & Substructure Components (Key Points)

1. General Definitions (Clause 701)

• Abutment: Structure supporting the ends of a bridge span.
• Pier: Intermediate support between abutments.
• Foundation: The part of the structure that transfers loads to the ground.
• Well Foundation: Large-diameter foundation sunk into the ground or riverbed.
• Pile: Long slender column driven into the soil to support loads.
• Retaining Wall: Structure to retain soil behind it.
• Substructure: Includes piers, abutments, foundations, retaining walls.

2. Substructure Components (Clause 710)

• Piers (710.2): Vertical supports; designed for axial load, bending, shear.
• Wall Piers (710.3): Solid wall-like piers.
• Abutments (710.4): Support bridge ends, resist earth pressure.
• Abutment Pier (710.5): Combination of pier and abutment.
• Wing Walls & Return Walls (710.6): Retain earth at abutment ends.
• Retaining Walls (710.7): Resist lateral soil pressure.
• Pier & Abutment Caps (710.8): Distribute loads from superstructure to piers/abutments.
• Cantilever Caps (710.9): Overhanging caps for load transfer.
• Pedestals (710.10): Support bearings on piers/abutments.

3. Key Formulas & Tables

• Loads & Forces (706.1): Combine dead, live, wind, seismic, and water forces.
• Horizontal Forces at Bearing Level (706.2): Calculate lateral forces due to wind, water current.
• Base Pressure (706.3):
  [ p = \frac{P}{A} \quad \text{(where } P = \text{total load}, A = \text{area of footing)} ]
• Stability Checks: Overturning, sliding, bearing capacity factors per Clause 707 & 708.
• Pile Capacity (Appendix 5): Based on soil-pile interaction and dynamic tests.

4. Important Tables (Refer IRC 78)

• Design parameters for open, well, and

Foundation Design & Scour Protection (IRC 78 Highlights)

1. Design Discharge (Clause 703.1)

• Use design flood discharge considering return periods (e.g., 100-year flood).
• Design discharge influences scour depth and foundation sizing.

2. Depth of Scour (Clauses 703.2 & 703.3)

• Mean Depth of Scour (d_s): Calculated based on flow velocity, sediment size, and hydraulic parameters.
• For gravels, boulders (>2 mm), and clayey beds, no direct formula exists; refer to Appendix-1 for guidelines.
• Maximum Scour Depth: Considered for foundation bottom elevation to prevent undermining.

3. Foundation Depth (Clause 705)

• Depth should exceed maximum scour depth plus safety margin.
• For open foundations, embedment depends on soil bearing capacity and scour.
• For well foundations, embedment is deeper to resist scour and lateral forces.

4. Key Formula for Scour Depth (for non-gravel beds)

[ d_s = K \times y \times \left(\frac{V}{V_c} - 1\right)^m ] Where:

• (d_s) = scour depth
• (K, m) = empirical coefficients
• (y) = flow depth
• (V) = flow velocity
• (V_c) = critical velocity for sediment movement

5. Scour Protection Measures

• Riprap, concrete aprons, sheet piles.
• Design to withstand flow forces and prevent soil erosion around foundation.

Summary Table (Conceptual)

flowchart LR
    A[River Flow] --> B[Calculate Design Discharge]
    B --> C[Estimate Mean Scour Depth]
    C --> D[Determine Max Scour Depth]
    D --> E[Set

IRC 78: Loads, Forces and Their Combinations (Clause 706.1)

Key Load Combinations (Clause 706.1.1)

For design, consider these combinations (G = Dead Load, Q = Live Load, Fwc = Wind, Fp = Prestress, Gb = Buoyancy, Fcf = Centrifugal, Fep = Earth Pressure, Fer = Earthquake, Fr = Rolling, W = Water, Feq = Earthquake force):

Horizontal Forces at Bearing Level (Clause 706.2.1.1)

For simply supported spans with fixed/free bearings on stiff supports:

• Fixed Bearing Horizontal Force:
  [ F_h + \frac{M (R_g + R_q)}{2} ]

• Free Bearing Horizontal Force:
  [ H (R_g + R_q) ]

Where:

• (F_h) = Applied horizontal force
• (R_g) = Reaction at free end due to dead load
• (R_q) = Reaction at free end due to live load
• (H) = Coefficient of friction at movable bearing

Coefficient of Friction (H) Values

Summary Diagram: Load Combinations & Bearing Forces

flowchart TD
    A[Loads & Forces] --> B[Dead Load

IRC 78 Key Points: Footings & Pile Foundations Design

1. Loads & Forces

• Refer Clause 706.1-706.3 for loads, horizontal forces, and base pressure on foundations.
• Combine vertical & horizontal loads per IRC 78 for stability checks.

2. Open Foundations

• Clauses 707.1 to 707.4:
  • Design for bearing capacity & settlement.
  • Special provisions for sloped bed profiles.
  • Construction guidelines ensure proper soil compaction and drainage.

3. Pile Foundations

• Clause 709.1-709.6:
  • Follow IS 2911 for design & construction (Clause 709.1.3).
  • Key design steps:
    • Determine geotechnical capacity (Appendix-5).
    • Structural design of piles & pile caps.
    • Inspection & precautions vary by pile type (driven, bored, etc.).

4. Design Formulae (Appendix-5 Highlights)

5. Pile Cap Design

• Check for bending, shear, and bearing stresses (Clause 709.5).
• Ensure proper anchorage and reinforcement per IS 2911.

Summary Diagram: Pile Foundation Design Flow

flowchart TD
    A[Site Investigation] --> B[Soil Testing & Bearing Capacity]
    B --> C[Select Pile Type]
    C --> D[Geotechnical Capacity Calculation]
    D --> E[Structural Design of Pile]
    E --> F[Design of Pile Cap]
    F --> G[Construction & Quality Control]

References:

• IRC 78: Clauses 706-709, Appendices 5 & 7
• IS 2911: Pile foundation design & construction standards

For detailed formulas and

Key Specifications & Design Guidelines for Well Foundations (IRC 78):

1. Sinking & Seating (Clause 1.5)

• Wells must be sunk by all suitable methods: pneumatic sinking, dewatering, etc.
• Wells should rest evenly on sound rock free from fissures, cavities, weathered zones.
• Embedment depth and extent of seating decided by Engineer-in-charge for safety.
• Provide a sump (shear key) inside well:
  • Depth: 300 mm in hard rock, 600 mm in soft rock.
  • Diameter: 1.5 to 2 m less than inner dredge-hole, minimum 1.5 m.
• Use 6 dowel bars (25 mm dia, deformed), anchored 1.5 m into rock, projecting 1.5 m above.
• Anchor bars grouted in 65 mm dia boreholes with 1:1½ cement mortar.

2. Sinking Procedure (Clause 708.13.1)

• Wells must be sunk true and vertical.
• Start sinking only after steining has cured for 48 hours.
• Maintain detailed records: tilt, shifts, kentledge, dewatering, blasting.
• Follow safety precautions from Appendix-4 (IRC 78).

3. Design Considerations (Clause 705.3)

• Well foundations designed to safely transfer loads to rock strata.
• Ensure adequate embedment for stability against sliding, overturning.
• Check bearing capacity of rock and well dimensions accordingly.

Typical Embedment & Sump Details (Summary Table)

flowchart TD
    A[Start Sinking] --> B[Ensure Steining Cured 48

Pile Caps and Group Pile Design (IRC 78)

Key Specifications:

• Pile Cap Size: Provide minimum 150 mm offset beyond outermost piles (Clause 709.5.1).
• Levelling Course: If pile cap bottom contacts earth, provide 80 mm thick plain cement concrete (Clause 709.5.1).
• Minimum Thickness: At least 1.5 × pile diameter for rigid behavior (Clause 709.5.4).
• Reinforcement: Full anchorage beyond points of stress release; use small diameter bars at corners to avoid cover loss (Clause 709.5.4).

Design Guidelines:

• Follow Appendix-5 for pile capacity under load combination I (Clause 709.3.3).
• Use Clauses 709.3.3 & 709.3.4 for pile group capacity and allowable settlement (Clause 709.3.3).
• Soil classification affects design:
  • Cohesive soil: Su ≤ 0.25 MPa
  • Granular soil: N ≤ 50 blows/0.3 m
  • Intermediate geomaterial

Typical Thickness Formula:

[ t_{min} = 1.5 \times d_{pile} ]

Design Methods:

• Thick slab method
• Strut & Tie method (ensure reinforcement anchorage)

flowchart TD
    A[Pile Group] --> B[Pile Cap]
    B --> C[Minimum thickness = 1.5 × pile diameter]
    B --> D[Minimum offset = 150 mm beyond outer piles]
    B --> E[Levelling course: 80 mm PCC if bottom contacts earth]
    B --> F[Reinforcement anchorage]
    F --> G[Full anchorage length]
    F --> H[Corner bars: small diameter for cover protection]

This ensures safe load transfer and structural integrity per IRC 78.

IRC 78 Key Specifications for Abutments, Piers, Retaining Walls & Substructure:

1. Abutments (Clause 710.4)

• Types: Plain/reinforced concrete or masonry.
• Forms: Solid, buttressed, counterfort, box, spill-through.
• Spill-through abutments analyzed like piers.
• Counterfort abutments treated as T or L type; slab continuous over counterforts.

2. Piers (Clause 710.2 & 710.3)

• Wall piers and column piers.
• Design for vertical & lateral loads.
• Consider horizontal forces at bearing level (Clause 706.2).

3. Retaining Walls (Clause 701.10 & 710.7)

• Design for earth pressure, surcharge, water pressure.
• Stability checks: sliding, overturning, bearing capacity.
• Use soil pressure coefficients from IS 456 or IRC guidelines.

4. Substructure Elements (Clause 710)

• Pier and abutment caps (710.8).
• Cantilever caps (710.9).
• Pedestals below bearing (710.10).

Important Formulas & Tables:

Design Considerations:

• Account for scour depth (Clause 703).
• Use appropriate load combinations (Clause 706.1).
• For well foundations and pile foundations, refer to Clauses 708 & 709 respectively.
• Fill behind abutments and wing walls

Rock Classification for Engineering (IRC 78 - Clause 8.2, Table 2)

Rocks are classified primarily by Unconfined Compressive Strength (UCS) and physical hardness:

Additional Notes:

• Overall rock mass behavior depends on discontinuities (joints, faults, bedding planes) beyond UCS.
• For UCS > 12.5 MPa, or extrapolated SPT N > 500, rock is considered moderately strong or above.
• For UCS between 2.5 and 12.5 MPa, or SPT N between 100 and 500, rock is moderately weak or below.

Summary Diagram of Rock Strength Classification

graph LR
A[Extremely Strong > 200 MPa] --> B[Very Strong 100-200 MPa]
B --> C[Strong 50-100 MPa]
C --> D[Moderately Strong 12.5-50 MPa]
D --> E[Moderately Weak 5-12.5 MPa]

Use this classification for preliminary design and investigation planning, noting that detailed rock mass characterization requires assessment of discontinuities and weathering.

Drainage Arrangements for Abutments and Wing Walls (IRC 78)

• Top Level: Wing/return walls must extend at least 100 mm above embankment top (Clause 710.6.6) to prevent soil erosion by rainwater.

• Drainage Design: Drainage for wing/return walls should follow the abutment drainage system as per Appendix-6 of IRC 78, which typically includes:

  • Provision of weep holes or drain pipes at the back of the walls.
  • Use of filter material (sand/gravel) behind the walls to facilitate drainage.
  • Proper slope to direct surface water away from the structure.

• Backfill: Must comply with Appendix-6 specifications ensuring:

  • Proper compaction.
  • Drainage layer behind walls to avoid hydrostatic pressure buildup (Clause 710.1.4).

• Live Load Surcharge: Return walls should be designed for a live load surcharge equivalent to 1.2 m height of earthfill (Clause 710.6.9).

Typical Drainage Components (from Appendix-6):

flowchart LR
    A[Embankment] --> B[Wing/Return Wall]
    B --> C[Drainage Layer (300 mm granular)]
    C --> D[Weep Holes / Drain Pipes]
    D --> E[Outlet to Drainage Channel]
    B --> F[Filter Material Behind Wall]
    F --> D

Summary: Ensure wing/return walls are raised 100 mm above embankment, provide granular drainage layer with weep holes per Appendix-6, design for 1.2 m earthfill surcharge, and slope diaphragms inward for effective drainage.