Practical Engineering - Why Bridges Don't Sink

Short Summary

Main recommendation: Bridge foundations rely on deep foundations such as driven piles or drilled shafts, which carry heavy loads down to stronger soil or rock and resist sinking through skin friction, end bearing, and resistance to lateral and uplift forces. The video explains how these systems are installed, including underwater, and why they are designed to handle scour and other changing conditions over time.

Key Points

• Bridges concentrate loads into piers or abutments, which sit on soft, often underwater soils that cannot hold heavy weight, so deep foundations are needed to prevent a bearing failure where the foundation sinks.
• Driven piles resist vertical loads through end bearing on firm layers or bedrock and through skin friction along the shaft, which increases as the pile is driven deeper.
• Driven piles are installed by hammering or vibrating, and engineers confirm performance by reaching refusal, where each hammer blow barely moves the pile.
• Piles also resist lateral loads from wind, water, ice, and boats, and uplift from buoyancy or wind; inclined ("battered") piles are used when lateral loads are high.
• Drilled shafts are an alternative to driven piles that cause less noise and vibration, built by drilling a hole, placing rebar, and filling it with concrete, often using casing and a tremie pipe underwater.
• Engineers must predict scour, the erosion of soil around bridge foundations from floods and the structure itself, and design foundations to remain strong over time.

Details Worth Noting

• Concrete instructions: The video does not give step-by-step instructions for a homeowner or builder, but it describes the engineering procedures in sequence. For driven piles: position the pile, drive it with a hammer (dropping a weight from a height) or vibratory head, and keep driving until refusal, recording the blow count per depth. For drilled shafts: drill the hole (using casing in soft or wet soil), clean out the soil, place reinforcing steel, and fill with concrete through a tremie pipe whose end stays below the concrete surface so it does not mix with water and dilute. Keep the casing in place or pull it after the concrete is poured, depending on the project.
• Underwater pours are handled with a tremie because concrete mixed with water loses strength, so the water-to-cement ratio must be preserved.
• Concrete pile sizes can be upwards of 30 inches (75 centimeters) wide, which is hard to drive; steel pipe and H piles are more slender alternatives.
• Drilled shafts can be enlarged at the bottom (reaming) to increase base area, which helps resist sinking and uplift.
• Other deep foundation variations include continuous flight auger piles, stone columns (such as Geopier), helical or screw piles, micropiles, and sheet piles, which are used for walls or barriers.
• Real failures noted: Millennium Tower in San Francisco sank and tilted; in 2004, a Leroy Selmon Expressway pier in Tampa sank 11 feet (over 3 meters) during construction, costing about $90 million to fix and delaying completion by a year.

Bottom Line Deep foundations, whether driven piles or drilled shafts, are what keep bridges stable on soft, underwater ground, and their design must account for both loads and the long-term erosion of the surrounding soil.