The depth to which the riverbed erodes around bridge foundations during floods. IRC 78 uses Lacey's formula to estimate max scour. The foundation must extend well below this depth to remain stable. Under-estimation of scour is the most common cause of bridge collapse.

IRC 78:2014 emphasizes a thorough geotechnical investigation to understand soil strata, bearing capacity, and groundwater conditions. The choice between open, pile, or well foundations depends on these factors, along with economic feasibility, constructability, the type of bridge, expected loads, and local environmental conditions such as scour potential and seismic activity. The aim is to ensure a stable and durable substructure.

IRC 78:2014 provides guidelines for estimating scour depth based on hydraulic factors like discharge, bed material characteristics, and channel geometry. For alluvial channels, empirical formulas are given. The foundation must be designed to extend well below the maximum predicted scour depth to prevent undermining and ensure stability. Protection measures like pitching or aprons are often necessary.

Key parameters include pile diameter, depth, spacing, and material. The design focuses on the ultimate load-carrying capacity (both compression and tension), allowable settlement, and group action effects. IRC 78 specifies minimum concrete grades, reinforcement details, and guidance on calculating skin friction and end bearing resistance based on soil properties and pile type.

Settlement can cause differential movements between foundations, leading to distress in the superstructure. IRC 78:2014 provides permissible settlement values based on foundation type and soil. For open foundations, a maximum settlement of 25 mm is generally allowed. Exceeding these limits can affect the bridge's serviceability and structural integrity, necessitating careful design to minimize differential settlements.

IRC 78:2014 mandates minimum grades of concrete for substructure elements, typically M25 or higher, depending on exposure conditions. Reinforcement detailing, including cover, bar sizes, and spacing, is specified to resist bending, shear, and axial loads. Durability aspects, such as resistance to weathering and chemical attack, are also considered in the material selection and design.

In seismic zones, foundations and substructures must be designed to withstand dynamic lateral forces. IRC 78:2014 requires consideration of seismic coefficients and potential liquefaction of soil. Foundation types and depths are chosen to ensure stability during earthquakes, and substructure elements are designed to accommodate seismic displacements without catastrophic failure. Ductility is an important consideration.

IRC 78:2014 recommends various scour protection measures. These include downstream aprons, pitching with stone or concrete blocks, and aprons extended upstream. The choice and extent of protection depend on the scour depth, flow velocity, and the importance of the structure. Proper design and installation are crucial for the effectiveness of these measures.

Well foundations are typically used for deep foundations in challenging soil conditions. Design involves calculating bearing capacity, stability against tilting and sliding, and the forces during sinking. Challenges include maintaining plumbness during sinking, dewatering for concreting the bottom plug, and ensuring the integrity of the well structure. IRC 78 provides detailed guidance on their design and construction.

The National Highways Authority of India (NHAI) and the Ministry of Road Transport and Highways (MoRTH) mandate the use of IRC codes for all highway and bridge projects under their purview. These codes serve as the standard technical specifications for design and construction, ensuring uniformity, quality, and safety across the national highway network. Compliance is crucial for project approval and funding.

The code provides guidelines and methods for determining the allowable bearing capacity of various soil types. It differentiates between cohesive and cohesionless soils and references standard tests like the Standard Penetration Test (SPT) and laboratory tests. Permissible bearing pressures are given in tables, and designers must ensure that the actual bearing pressure under the foundation does not exceed these allowable values to prevent shear failure and excessive settlement.

Abutments support the bridge ends and retain the approach embankment. IRC 78:2014 specifies design considerations for stability against overturning, sliding, and bearing capacity failure. The type and compaction of backfill material are critical for managing earth pressure; granular, free-draining materials are often preferred. Wing walls are designed to retain the embankment and protect the abutment from erosion.

IRC 78:2014 requires substructure elements to be designed to resist lateral loads, including wind, seismic forces, and braking forces from the superstructure. These loads are applied as forces at the bearing level and transmitted through the substructure to the foundation. The design ensures that the substructure and its foundations can safely withstand these combined loads without failure or excessive deformation.