When: subgrade has adequate bearing (>300 kPa), scour is not a concern (dry nalas, ROBs), foundation depth is manageable (<4m). For river bridges, scour almost always dictates pile or well foundations instead.

1) Bearing capacity (IS 6403), 2) Settlement (<25mm), 3) Overturning stability (FS ≥ 2.0), 4) Sliding stability (FS ≥ 1.5), 5) Base pressure distribution (no tension), 6) Tilt under lateral loads.

Open foundation costs 30-50% less than piles for the same bridge. But it needs good bearing soil at shallow depth. If piles go 15-20m deep, open foundation at 3m is dramatically cheaper.

IRC 83:2018 provides guidelines and provisions for the design and construction of open or direct foundations used in road bridges. It covers essential aspects like determining the bearing capacity of the soil, estimating settlement, checking for tilt and shift, and the design of concrete foundation elements, ensuring the structural integrity and longevity of bridge foundations.

The code outlines methods for determining bearing capacity, primarily based on soil properties like cohesion (c), angle of internal friction (φ), unit weight of soil (γ), and surcharge (q). It references established formulas like Terzaghi's and provides tables for bearing capacity factors (Nc, Nq, Nγ) for various friction angles and footing shapes. Site-specific soil investigation is paramount for accurate parameter determination.

IRC 83:2018 specifies permissible settlement limits to ensure serviceability and prevent undue stress on the bridge superstructure. For shallow foundations, a general limit of 25 mm is often cited, while for critical elements like piers and abutments, stricter limits of 10 mm are typically applied. Table 1 provides specific guidance based on the type of structure and soil.

A reduced Factor of Safety is permissible under seismic conditions, where a value of 1.2 can be used, compared to the general FOS of 3.0 for normal loading. This reflects the dynamic nature of seismic forces and the need for a more performance-based approach in such scenarios, although robust seismic detailing is still required.

The Modulus of Subgrade Reaction (Ks) quantifies the stiffness of the soil foundation and is crucial for analyzing the behavior of the foundation under load, especially for structures like bridge piers. It's used in soil-structure interaction analysis and helps in determining settlements and stresses, particularly when considering the elastic properties of the soil, as detailed in Table 4.

Cl. 9.1 emphasizes the critical need for comprehensive soil investigations. This includes determining soil stratification, physical properties (density, moisture content), engineering properties (shear strength parameters, compressibility), and groundwater levels. The extent and type of investigation depend on the bridge's importance, size, and geological conditions.

The code generally refers to methods involving the elastic theory and consolidation theory. For immediate settlement, elastic properties of the soil are used. For cohesive soils, consolidation settlement, which occurs over time due to the expulsion of pore water, is a critical consideration. The summation of settlements over different layers (Cl. 6.1) is a common approach.

Cl. 8.1 addresses the design of concrete members, focusing on shear and bending moments. It specifies allowable stresses in concrete and requires adequate reinforcement to resist tensile stresses. The thickness of the footing is often governed by shear considerations, especially near the column face.

Ultimate bearing capacity (q_u) is the maximum pressure the soil can withstand without shear failure. Safe bearing capacity (q_s) is the allowable pressure that can be applied to the soil, calculated by dividing the ultimate bearing capacity by a Factor of Safety (FOS). This FOS accounts for uncertainties in soil properties and loading conditions.

Eccentric loading results in non-uniform pressure distribution beneath the footing, leading to higher stresses at one edge and potentially lower stresses or even uplift at the other. The code limits the eccentricity (Cl. 7.1) to B/6 for service loads to ensure that the entire base of the footing remains in contact with the soil and to avoid excessive tilting or rotation.

Fluctuating groundwater levels can significantly impact foundation design. They can reduce the effective stress in the soil, thereby decreasing bearing capacity. In some cases, they can lead to buoyancy or uplift forces, and if the soil is liquefiable, seismic performance can be severely compromised. Therefore, groundwater conditions must be carefully investigated and accounted for.

Cl. 7.1 addresses the criteria for tilt and shift. Tilt refers to the angular rotation of the foundation, while shift refers to the lateral displacement. These are checked to ensure the structural stability and serviceability of the bridge. Excessive tilt can misalign bridge spans and cause distress in the superstructure, while excessive shift can lead to loss of support.