Last week's answer

Question: When legendary engineer Ralph Modjeski designed the original Huey P. Long Bridge over the Mississippi River in Jefferson Parish, Louisiana, he faced a severe geotechnical dilemma. The Mississippi Delta consists of hundreds of feet of soft, compressible alluvial clay (locally known as "gumbo") with true bedrock buried over 1,000 feet below the surface—far too deep for traditional pneumatic caissons. How did civil engineers successfully anchor the bridge's massive 200 foot tall concrete piers to prevent uneven settlement?

Answer: By sinking open-dredging concrete caissons 170 feet below sea level directly into a thick layer of fine sand, resting the piers on the soil's bearing capacity and massive friction rather than bedrock.

Siems-Helmers Inc. used an approach called the "sand island method" that involved the constructing a large sand pile or island, which was then surrounded by a steel shell in which each pier would stand and then constructing the caisson on top. The caisson was sunk until only about 2 feet could be seen above the sand island, and another 10 feet of concrete layer would be placed on top. 

Sources: American Society of Civil Engineers