Bridge substructure in sandy soils — best-suited foundation type for flyovers For the construction of flyovers (bridge structures) on sandy soil deposits, which foundation type is generally provided to safely transmit loads and control settlements/scour?

Introduction / Context:
Flyovers and highway bridges impose heavy concentrated loads and must also resist scour, lateral forces, and differential settlement. Choice of foundation depends on subsoil profile and hydraulic conditions. In sandy strata, especially where depth to dense sand varies or scour is possible, deep foundations are preferred.

Given Data / Assumptions:

• Bridge/flyover piers on sand deposits (potentially variable density and groundwater).
• Need for reliable capacity and settlement control under repetitive traffic loads.
• Exposure to potential scour around foundations in open channels or culverts.

Concept / Approach:

For most flyovers, pile foundations (driven precast, cast-in-situ bored, or continuous flight auger) are adopted in sandy soils. Piles transfer loads to deeper dense strata and provide group action and stiffness; they also accommodate construction above traffic. The listed shallow alternatives—strap, raft, combined, or a generic “pier footing”—are typical of building foundations and are seldom used for highway flyover piers on sands due to higher risk of settlement and scour unless exceptional site conditions exist.

Step-by-Step Solution:

Verification / Alternative check:

Bridge design manuals and standard drawings show pile caps with pile groups for piers/abutments on sands; shallow raft/strap/combined footings are unusual for flyovers unless bearing strata are exceptionally competent and scour is absent.

Why Other Options Are Wrong:

• Strap/combined footings are building solutions for closely spaced columns.
• Raft spreads loads but remains shallow and sensitive to scour.
• “Pier footing” as a shallow spread footing is rarely suitable in sandy, potentially erodible conditions.

Common Pitfalls:

• Confusing substructure element (pier) with its foundation type (piles/wells/caissons).
• Ignoring seismic/liquefaction checks for sandy profiles.

Final Answer:

None of these.