Why can boulders in the seabed damage steel sheet piles during driving?

When driving steel sheet piles into the seabed, encountering boulders or similar obstacles can cause significant structural damage, such as buckling, below the seabed level.1

This damage occurs because the pile, while appearing to penetrate the ground with little or no damage to its head, is actually being obstructed by the hard material. When a pile strikes a boulder, the resistance prevents further penetration, and the force applied by the pile driver can cause the steel section to deform or buckle underground. This is particularly problematic because such damage is often hidden from view, yet it gravely compromises the pile's efficiency and structural integrity as a retaining quay wall.1

Engineers must rely on the expertise of soil mechanics specialists to assess seabed conditions and mitigate these risks during the design and construction phases of harbors and sea works.1