Civil Engineering Professional Engineer Practice Exam

In terms of stability, how must arch bridges generally be designed in weak soil conditions?

1. Highly flexible

2. Very massive

3. Lightweight and thin

4. Short and wide

The correct answer is: Very massive

In weak soil conditions, arch bridges must generally be designed to be very massive to ensure stability. This is because massive structures can effectively distribute loads and transfer forces to the ground, counteracting the potential for settlement or failure due to the soil's insufficient strength. A massive design includes a sufficient amount of material that can resist lateral forces and tipping moments, which is particularly important when the foundation may not provide adequate support. The weight of the bridge helps to keep it anchored and prevents excessive movement or deformation, which can be critical in weak soil scenarios. The other options do not adequately address the challenges posed by weak soils. For instance, being highly flexible could lead to more dynamic movements that might exacerbate instability. A lightweight and thin design would not provide the necessary resistance against sinking or shifting within weak soil. Similarly, a short and wide design might not offer sufficient structural integrity or load distribution to withstand the specific challenges of weak soil conditions. Therefore, a very massive design is essential for enhancing the overall stability of the arch bridge in such environments.