Simply bulking up the ground around the piers with loose fill or monolithic concrete would have had the ship run aground; at 9mph (the recorded impact speed) the ship would have slowed to a stop within several yards, and suffered minor damage, maybe would have required being tugged back into clear waters. Running aground puts the majority of impact into a downward force absorbed by the earth. It's hard to build a bridge pier that will withstand 200,000 tons of direct impact, no matter how slowly that's moving.