1. Piles are quicker to install in clay than a mat foundation
2. A mat foundation spreads loads better but risks excessive settlement in soft clay
3. Piles transfer bridge loads to bedrock, bypassing the weak clay layer
4. Mat foundations require less material but can’t resist lateral forces in clay

Explanation

Explanation: Driven piles extend through soft, compressible clay to reach bedrock, transferring bridge loads to a stable layer and avoiding settlement that could damage the bridge structure. A shallow mat foundation, while effective for spreading loads on firm soil, would settle excessively in soft clay—engineers assess this via settlement calculations (e.g., consolidation theory)—making it unsuitable for heavy bridge loads. Installation time (A) varies by site, and material use (D) isn’t the primary concern—stability is. Lateral resistance (D) matters, but vertical load transfer is key here.

Learning Resources

• Review AASHTO LRFD Bridge Design Specifications, 9th Edition (2020), Section 10 for pile capacity guidelines; CHBDC S6-19, Section 6.8 provides equivalent pile design provisions.
• Study the Oresund Bridge (Denmark-Sweden, 2000) as a case study of piles in deep soft clay.
• Tip: Check geotechnical reports for clay compressibility (Cc, Cr) to estimate settlement.